Uses of light and electron transmission microscopes 光學顯微鏡和電子透射顯微鏡的用途
Structure, functions and locations of cell organelles: cell membrane, mitochondrion, chloroplast, nucleus, ribosome, cytoplasm, cytosol, vacuole and cell wall 細胞器的結構、功能和位置：細胞膜、粒線體、葉綠體、細胞核、核醣體、細胞質、細胞膜、液泡和細胞壁
Application of high salt or sugar concentrations in food preparation 在食品製作中使用高濃度鹽或糖
Labelled diagrams of plant and animal cells 動植物細胞標籤圖
Labelled diagrams of cell membrane, mitochondrion and chloroplast 細胞膜、粒線體和葉綠體的標籤圖
Labelled diagram of light microscope 標籤的光學顯微鏡示意圖
Practical Activities 實踐活動
Be familiar with the use of the light microscope 熟悉使用光學顯微鏡
Prepare and examine slides of one plant cell and one animal cell ( and 準備並檢查一個植物細胞和一個動物細胞的切片（ 和
Conduct any activity to demonstrate osmosis 進行任何演示滲透的活動
Microscopy 顯微鏡
The light microscope is used to observe and magnify prepared slides. The slide is placed on the stage and the fine and coarse adjusters are used to focus the image produced in the eyepiece. The light microscope can be used to view living specimens although its magnification is limited. 光學顯微鏡用於觀察和放大準備好的玻片。將載玻片放在載物台上，使用微調器和粗調器對目鏡中產生的影像進行聚焦。儘管光學顯微鏡的放大倍率有限，但仍可用於觀察活體標本。
The Transmission Electron Microscope (TEM) is capable of much higher magnifications, even to view the structure of DNA molecules. The TEM is not capable of providing images of living organisms. 透射電子顯微鏡（TEM）的放大倍率要高得多，甚至可以觀察DNA 分子的結構。 TEM 無法提供生物體的圖像。
Figure 2.1 The light microscope 圖2.1 光學顯微鏡
Magnification 放大倍率
The magnification of an object is the product of the magnification of the eyepiece and the objective lens. 物體的放大倍率是目鏡和物鏡放大倍率的乘積。
Eyepiece Lens 目鏡透鏡
Objective Lens 物鏡
Magnification 放大倍率
Mandatory activity 強制性活動
To make slides of animal and plant cells and observe under a microscope 製作動物和植物細胞的切片，並在顯微鏡下進行觀察
Preparation of the slide 載玻片的製備
A very thin slice (thin enough for light to pass through) of the specimen is placed on a clean glass slide with a drop of water. 在乾淨的玻璃載玻片上滴一滴水，將標本切成很薄的薄片（薄到光線可以通過）。
A cover slip is placed over the specimen (carefully, to prevent the entry of any air bubbles). 將蓋玻片蓋在標本上（注意防止氣泡進入）。
Staining is done to highlight different structures in the specimen.
Iodine is added to onion cells to highlight the nucleus and starch grains.
Methylene blue is used to highlight the nucleus and cytoplasm of cheek cells.
Precautions using the microscope
Never touch electric light sources, as they can cause burns.
When focusing at low power , use the coarse adjuster before finishing focusing with the fine adjuster.
At high power ( ), always bring the lens and stage to near contact while viewing from the side. Focusing is then done using the fine adjuster to move the slide and lens away from one another while observing through the eyepiece.
Treat all dyes as poisonous.
Mandatory activity
To prepare a slide of an animal cell
Use the blunt end of a match to scrape cells off the inside of the cheek.
Gently spread the cells (to avoid crushing) on a clean glass slide and allow drying in the air to fix the cells onto the slide.
Add methylene blue for one minute and gently rinse off using dripping water. This stains the nucleus and the cytoplasm.
Add a cover slip carefully, at an angle, to avoid any air bubbles.
Mandatory activity
To prepare a slide of a plant cell
Peel a single layer of cells from an onion.
Place the sample onto a slide with a drop of water, carefully, to avoid air bubbles.
Add some iodine to stain the nucleus and the vacuole.
Add a cover slip carefully, at an angle, to avoid any air bubbles.
Drawings of each type of cell as seen under the light microscope are shown in fig. 2.2.
Onion Cells
Figure 2.2 Plant and animal cells using the light microscope
Cell ultrastructure
With the higher magnification possible using an electron microscope, many different cell organelles can be identified (fig. 2.3).
Figure 2.3 Plant cell
Organelle
Structure
Function
Nucleus
Surrounded by a double membrane
with many pores. Contains
chromosomes (DNA) in the form of
chromatin
Stores genetic information
(DNA) and controls all
activities in the cell
Ribosome
Made of protein and RNA
Protein synthesis
Mitochondrion
Double membrane with inner foldings
of cristae (see fig. 2.4)
Krebs cycle and hydrogen
carrier systems of aerobic
respiration occur here
Cell Membrane
A phospholipid bilayer with protein
(see fig. 2.6)
Acts as a selectively
permeable barrier
Cytosol
Liquid portion of the cytoplasm
outside the nucleus
Provides a liquid medium for
enzymes
Cytoplasm
Matrix surrounding the nucleus in
which organelles are suspended
Support for organelles
Organelle
Structure
Function
Organelles Only Present in Plant Cells
Chloroplast
Double membrane with inner folds
of lamellae and grana containing
chlorophyll (see fig. 2.5)
Carries out photosynthesis
Large Vacuole
Single-membrane fluid-filled cavity
Contains food stored as cell sap
Cell Wall
Fully permeable wall of cellulose
Provides strength and support
Figure 2.4 Mitochondrion
Figure 2.5 Chloroplast
Figure 2.6 Cell membrane
Movement through cell membranes
The cell membrane is a selectively permeable barrier. A number of different mechanisms are used to transport materials across membranes.
Osmosis and diffusion
Osmosis is the movement of water from a high concentration to a low concentration across a selectively permeable membrane. No energy is required for osmosis.
Plant root hairs absorb water from the soil by osmosis.
Application of Osmosis in the Food Industry
The high sugar concentration in jam acts as a preservative, as it prevents the growth of micro-organisms. The sugar draws water from the cells of the microorganisms by osmosis. This prevents their growth and reproduction.
Diffusion is the movement of a substance from its region of high concentration to its region of low concentration along a diffusion gradient. Diffusion does not require energy.
Oxygen gas in the lungs moves across the alveoli into the blood capillaries by diffusion.
Turgor and plasmolysis
Many plants use lignified xylem cells (wood) to support their aerial parts.
Non-woody green plants do not have this support.
Non-woody plants pack their cells and vacuoles with water by osmosis.
Cells in this condition are said to be fully turgid.
If a green plant is not watered for a number of days, the cells lose their water and the plants wilt due to the loss of turgidity and support (see fig. 2.7).
Cell plasmolysed
Cell fully turgid
Figure 2.7 Turgor
Mandatory activity
Conduct any activity to demonstrate osmosis
Soak two 50 cm strips of Visking (dialysis) tubing in water for ten minutes to soften them.
Tie a knot at one end of each strip.
Half fill one strip with an 80 per cent sucrose solution and the other with distilled water.
Tie a second knot at the open end of each strip and rinse each under the tap.
Dry each strip with paper towels.
Place each strip on a balance and record their mass.
Observe each strip and record their turgidity.
Place each strip in separate labelled beakers of distilled water and leave for 15 minutes (see fig. 2.8).
Remove the strips and dry as before.
Record the mass of each and their turgidity.
Repeat points 8 to 10 twice more.
Expected results
The mass and turgidity of the Visking tubing with sucrose solution should increase with time. The Visking tubing with distilled water (the control) should show no change.
Important points to remember
The Visking tubing acts as a semi-permeable barrier. The smaller water molecules can pass through, but sugar molecules cannot.
The distilled water in the beaker is the high concentration of water. The sucrose solution in the Visking tubing is the low concentration of water. This difference is what causes osmosis to occur.
Figure 2.8 Osmosis in an artificial cell
Eukaryotic and prokaryotic cells
Animal, plant and fungi cells all have their nuclear material (DNA) stored in a nuclear membrane sac. Such cells are known as eukaryotic.
Bacteria are described as prokaryotic organisms. They are much smaller than eukaryots and their nuclear material is not bound by a membrane but spread around the cytoplasm.
Differences between Eukaryotic and Prokaryotic Organisms
Eukaryotic
Membrane-bound nucleus separating it from cytoplasm
Larger cell with many membrane-bound organelles
Cell wall, if present, made of cellulose or chitin
Prokaryotic
Nuclear material spread around organism in ring shapes
Smaller with no membrane-bound organelles
Cell wall made of protein
2011 Q14 (c) HIGHER LEVEL
(c) (i) State the precise location of the cell membrane in plant cells.
(ii) With what type of cell do you associate membrane-bound organelles?
(iii) What corresponding term is used to describe bacterial cells?
(iv) The cell membrane is described as being selectively permeable. What does this mean?
(v) Why is diffusion alternatively known as passive transport?
(vi) Osmosis may be described as 'a special case of diffusion'. Explain why.
(vii) Describe, with the aid of a labelled diagram, how you demonstrated osmosis in the laboratory.
(viii) Name the structure by which Amoeba gets rid of excess water that has entered by osmosis.
LEAVING CERT MARKING SCHEME
(c) (i) Immediately inside the cell wall
(ii) *Eukaryotic
(iii) *Prokaryotic
(iv) Only some substances are allowed through
(v) No (or little) energy (or ATP) required
(vi) Movement of water or (osmosis) requires a membrane
(vii) Diagram: Container +2 solutions separated by a membrane
Effect of temperature and pH ranges on enzyme activity (with graphs)
Applications of immobilised enzymes
Induced fit theory of enzyme action
Optimum activity of enzymes under specific conditions of pH and temperature
Heat denaturation of proteins (enzymes)
Practical Activities
Investigate the effect of pH on enzyme activity
Investigate the effect of temperature on enzyme activity
Investigate the effect of heat denaturation on enzyme activity
Prepare one enzyme immobilisation and examine its application
Each type of enzyme is made up of a long chain of amino acids with a secondary structure that provides it with a unique folded shape. This shape provides the enzyme with a specific active site.
IMPORTANT DEFINITIONS
Enzymes: Biological catalysts, protein in nature, and control metabolic reactions.
Active site: The point on an enzyme of temporary attachment to the substrate. It is specific.
Denatured enzyme: Loss of activity due to an irreversible change in enzyme structure (active site). It is caused by heat, pH change, etc.
Enzyme saturation: Enzyme functioning at a maximum rate under specific circumstances. The rate cannot increase, even if more substrate is added, unless enzyme concentration is increased.
Substrate: A substance that attaches to the enzyme at the active site. It is converted to product(s) and released.
Factors affecting enzyme activity
pH
Most enzymes function in a near neutral (see fig. 2.9).
Outside this range, most enzymes tend to denature, i.e. the active site changes shape and it can no longer function.
An exception is the enzyme pepsin, found in the stomach. It can function at a pH of 1.5 .
Figure 2.9 Graph of pH vs enzyme activity rate
Temperature
Enzymes can only function in a fluid environment.
In solid ice an enzyme has an activity rate of zero.
The rate of enzyme action increases as the temperature increases up to a limit of around (for warm-blooded animals). Above that temperature the enzyme becomes denatured.
Plant enzymes often have an optimum temperature of around (see fig. 2.10).
Figure 2.10 Graph of temp vs enzyme activity rate
Enzyme action (How an enzyme works)
The induced fit theory of enzyme action describes how the enzyme slightly changes shape so that its active site perfectly fits the substrate.
The enzyme and substrate temporarily bind together, forming the enzyme-substrate complex.
The binding action converts the substrate to products, which are then released.
The enzyme is now free to repeat the process (see fig. 2.11).
Figure 2.11 Enzyme action
Mandatory activity
To investigate the effect of pH on the rate of enzyme activity
Chop a celery stallk (source of enzyme catalax) into pieces of equal size.
Using a balance, weigh out 5 g of celery.
Add the celery to a graduated cylinder.
Add of buffer solution ( ) to the same graduated cylinder.
Add one drop of washing-up liquid to the graduated cylinder using a dropper.
Using a micro pipette, add of hydrogen peroxide to a test tube.
Place the graduated cylinder and the test tube in a water bath at (see fig. 2.12).
Leave for ten minutes to adjust to the new temperature.
Carefully pour the hydrogen peroxide into the graduated cylinder.
Note and record the volume in the cylinder immediately.
Read the volume again after five minutes and record.
Subtract the initial volume from the final volume to calculate the volume of foam formed. Record the result.
Repeat the above procedures for buffer solutions of pH 4 and 10.
A graph should be drawn of enzyme activity (volume of foam after two minutes) against pH . Place pH on the horizontal axis.
Expected results
The greatest enzyme activity (volume of foam per unit time) would be expected around . Enzyme activity would decrease in buffer solutions at the extremes of the pH scale, due to enzyme denaturation.
Figure 2.12 Setting up the experiment
Important points to remember
Enzyme catalase (from celery), substrate hydrogen peroxide, products water and oxygen gas.
Independent or manipulated variable , by using different buffer solutions.
Dependent variable rate of enzyme activity, measured by recording volume of foam produced per unit time ( .
Controlled variables:
(i) Enzyme concentration: Using equal weights of similarly sized chopped celery.
(ii) Substrate concentration: Using equal volumes of the same hydrogen peroxide solution.
(iii) Temperature: Using a water bath and thermometer.
Control experiment: Same as above with celery that has been boiled in water for 15 minutes. (This denatures the enzyme, catalase.)
Mandatory activity
To investigate the effect of temperature on the rate of enzyme activity
Chop a celery stalk into pieces of equal size.
Using a balance, weigh out 5 g of the celery.
Add the celery to a graduated cylinder.
Add of buffer solution to the same graduated cylinder.
Add one drop of washing-up liquid to the graduated cylinder using a dropper.
Using a micro pipette, add of hydrogen peroxide to a test tube.
Stand the cylinder and the boiling tube in a water bath with a mixture of ice and water for ten minutes. This will cool the contents to (see fig. 2.13).
Carefully pour the hydrogen peroxide into the graduated cylinder.
Note and record the volume in the cylinder immediately.
Read the volume again after five minutes and record.
Subtract the initial volume from the final volume to calculate the volume of foam formed. Record the result.
Repeat the procedures above using water baths at temperatures of and .
Draw a graph of enzyme activity (volume of foam after two minutes) against temperature. Put temperature on the horizontal axis.
Expected results
The greatest enzyme activity (volume of foam per unit time) would be expected around a temperature of . Enzyme activity would decrease at higher temperatures due to heat denaturation. At (ice) enzyme activity ceases.
Figure 2.13 Setting up the experiment
Important points to remember
Enzyme catalase (from celery), substrate hydrogen peroxide, products water and oxygen gas.
Independent or manipulated variable temperature, using water bath and thermometer.
Dependent variable rate of enzyme activity, measured by recording volume of foam produced per unit time .
Controlled variables:
(i) Enzyme concentration: Using equal weights of similarly sized chopped celery.
(ii) Substrate concentration: Using equal volumes of the same hydrogen peroxide solution.
(iii) pH : Using the same buffer solution.
Control experiment: Same as above with celery that has been boiled in water for 15 minutes. (This denatures the enzyme, catalase.)
Mandatory activity
To investigate the effect of heat denaturation on catalase activity
Place 5 g of chopped celery into a boiling tube.
Place the boiling tube in a boiling water bath for ten minutes.
Remove the boiling tube and allow it to cool.
Add of buffer solution to a graduated cylinder.
Using a dropper, add one drop of washing-up liquid.
Add the denatured celery to the graduated cylinder.
Add of hydrogen peroxide to a test tube.
Place the test tube and the graduated cylinder in a water bath at (see fig. 2.14).
Record the presence or absence of foam.
Repeat the procedure from step 4 using 5 g of 'live' unheated celery.
Expected results
The 'boiled celery' should show no activity, due to the heat denaturation of catalase. The 'live' unheated celery should produce bubbles of oxygen.
Figure 2.14 Setting up the experiment
Enzyme immobilisation
This describes a procedure to extract useful enzymes, usually from micro-organisms.
After removal, the enzymes are then stabilised so that they can be repeatedly used to catalyse chemical reactions.
The enzymes can then produce large quantities of useful products.
Biotechnology
Techniques using micro-organisms, or their enzymes, to produce useful products in medicine or industry.
Advantages of enzyme immobilisation
Easy recovery of enzymes for reuse.
Easy harvesting of products (no enzyme contamination).
Greater enzyme stability.
DEFINITIONS
Continuous flow bioprocessing involves the regular addition of nutrients to immobilised enzymes or micro-organisms in a bioprocessor (bioreactor). This produces a continuous flow of product.
Batch processing is the addition of a fixed amount of nutrients to microorganisms at the beginning of a process. On completion the product and micro-organisms are separated from the mix.
Applications of enzyme immobilisation
The production of fructose for use in canned drinks. Fructose is sweeter than glucose or sucrose, so it is preferred for sweetened drinks. The enzyme glucose isomerase converts glucose to fructose. This enzyme is difficult to produce, so it is reused through enzyme immobilisation.
To make lactose-free milk. Many people are lactose intolerant. They cannot produce the enzyme lactase that breaks lactose down. Immobilised lactase is used to remove lactose from milk.
Clarification of fruit juices. Fruit juices contain binding carbohydrates called pectins. These can make the juices more viscous and cause cloudiness. Immobilised pectinase is used to remove pectins from fruit juices.
Production of vinegar. Bacteria are immobilised to convert ethyl alcohol and oxygen to acetic acid or vinegar.
Diagnostic reagents. Dipsticks with the immobilised enzyme glucose oxidase can be used to test for glucose concentrations in blood samples.
Mandatory activity
To prepare an enzyme immobilisation and examine its application
Add 0.4 g of sodium alginate to of water.
Mix 2 g of yeast in of distilled water and leave for five minutes.
Prepare 1.4 per cent calcium chloride solution and place in a tall beaker.
Mix the alginate solution and the yeast suspension and draw the mixture into a syringe.
From a height of about 10 cm , release the mixture from the syringe, one drop at a time, into the calcium chloride. Each drop will form a bead (see fig. 2.15).
Leave the beads to harden for 20 minutes.
Filter the beads, wash with distilled water and place into a separating funnel (A) (see fig. 2.16).
Mix another 2 g of yeast with of distilled water.
Pour this mixture into a second separating funnel (B).
Make a 100 ml solution of 1 per cent sucrose and heat to .
Pour of this solution into each of the separating funnels.
Immediately dip Clinstix strips into samples taken from each separating funnel.
Remove the strips, leave for ten seconds, and compare both strips with the colour card supplied.
Repeat the test every 30 seconds.
Figure 2.15 Enzyme immobilisation
Figure 2.16 Free and immobilised yeast
Expected results
The samples released from the separating funnel with the 'free' yeast are more turbid (cloudy) due to the presence of the yeast cells.
The 'free' yeast cells break down the sucrose at a faster rate than the immobilised cells.
Important points to remember
Enzyme enzymes in yeast cells, substrate sucrose, products glucose.
Independent or manipulated variable = yeast cells in beads and free yeast cells.
Dependent variable rate of glucose formation.
Controlled variables:
(i) Enzyme concentration: Equal amounts ( 2 g ) of yeast cells.
(ii) Substrate concentration: Equal volumes of the same sucrose solution.
(iii) Temperature: Sucrose solutions at .
Note
The darker pink or more purple the strip, the greater the quantity of glucose present.
The use of Clinstix is a quantitative test for glucose. The colour changes indicate the quantity of glucose present.
The enzymes in the yeast cells break down the disaccharide sucrose to its monosaccharides.
2015 Q7 (c) HIGHER LEVEL
(c)(i) In relation to an investigation you carried out into heat denaturation of an enzyme, answer the following:
Name the enzyme you used.
Name the enzyme's substrate.
Name the product(s) formed.
(ii) How did you denature the enzyme?
(iii) How did you know that the enzyme had been denatured?
(iv) Why are buffers needed when carrying out experiments with enzymes in school?
LEAVING CERT MARKING SCHEME
(c)
c)
1. Catalase
2. Hydrogen peroxide
3. Oxygen (and water)
or
Pepsin (or protease)
Protein
Peptides (or amino acids)
or
Amylase
(or diastase)
Starch
Maltose
(ii) Boil or heat to high temperature ( )
(iii) Negative result for named test for product or positive result for named test for substrate [must match enzyme or produce in c(i) above]
(iv) To maintain (a constant) pH or to vary pH
Photosynthesis
Definitions
Photosynthesis
Anabolic reaction
Phosphorylation
Photophosphorylation
Reduction (of NADP)
Outline
Photosynthesis and its role in nature
Balanced equation of photosynthesis
Sources of light, carbon dioxide and water in the leaf
Adaptations of the leaf for photosynthesis
Formation of oxygen gas
Formation of carbohydrate
Location of chlorophyll in cells
Use of artificial light, controlled carbon dioxide and temperature levels, in greenhouse cultivation
The roles of chlorophyll, carbon dioxide and the splitting of water in photosynthesis
Detailed structure and functions of the energy molecules
NADH and NADPH
ADPIATP
Light stage I (cyclic) and light stage II (non-cyclic) of the light dependent stages
Photolysis of water/NADPH and ATP formation
Dark stage and the roles of ATP and NADPH
Practical Activity
Investigate the influence of light intensity or carbon dioxide concentration on the rate of photosynthesis
Role of photosynthesis in nature
Photosynthesis provides food (energy) for plants.
All animals depend directly or indirectly on plant photosynthesis for their food (energy).
Photosynthesis removes carbon dioxide from the air and produces oxygen gas for aerobic respiration in both plants and animals.
Photosynthesis can be summarised by the balanced chemical equation:
sunlight energy
Word equation for photosynthesis
carbon dioxide + water glucose + oxygen
Adaptations of the leaf for photosynthesis
Large surface area to capture light energy
Very thin to facilitate gaseous exchange
Stomata on the lower epidermis to control gaseous exchange
Large numbers of chloroplasts in the palisade layer at the upper surface to capture light
Leaf veins of xylem and phloem for transport
Figure 2.17 Transverse section through a leaf
Energy in the cell
Organisms must store energy so that it is available when required. The only means of storing energy in the cells is in chemical form, that is, in chemical bonds.
ATP
Adenosine triphosphate (ATP) is a molecule that stores energy in cells. When a cell requires energy, ATP is converted to ADP +P , releasing energy. Energy from respiration is stored when ADP +P join to form ATP. This process is called phosphorylation (see fig. 2.18).
ATP
Adenosine triphosphate is made up of adenine (amino acid), ribose (sugar) and three phosphate molecules. The energy is stored in the electrons of the chemical bonds linking the phosphate molecules.
NADH
NADH (Nicotinamide Adenine Dinucleotide) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate) are two molecules also involved in the storage of energy in the cell. They store and transfer hydrogen ions, energy and electrons for metabolism.
Figure 2.18 Energy in cells
Biochemistry of photosynthesis
The process of photosynthesis results in the splitting of water, releasing:
Oxygen (used for respiration or released to the atmosphere)
Hydrogen and electrons (which are combined with carbon dioxide gas to form carbohydrate)
The process of photosynthesis can be divided into two stages.
Light stage, which is dependent on the presence of light and occurs in the grana of the chloroplast.
Dark stage, which is light-independent (goes on night and day) and occurs in the stroma of the chloroplast.
1. Light stage
This occurs in two parts:
(a) Light stage I (cyclic)
(b) Light stage II (non-cyclic).
Light stage I
Light energy trapped by chlorophyll is passed to an electron.
The electron moves to an electron acceptor and is passed through a number of carriers.
Energy is released at each step and used to form ATP.
The electron finally returns to the chlorophyll. See fig. 2.19.
Figure 2.19 Light stage I
Light stage II
Light energy trapped by chlorophyll is passed to two electrons.
The two electrons leave the chlorophyll and move to an electron acceptor.
Both electrons join with NADP (nicotinamide adenine dinucleotide phosphate).
The NADP then causes to split (photolysis), releasing oxygen, two electrons and two ions.
The oxygen can be used for respiration or released as waste.
The electrons are passed through carriers, releasing energy to form ATP.
The hydrogen atoms released when water is split reduce NADP to NADPH. See fig. 2.20.
Figure 2.20 Light stage II
2. Dark stage
ATP and NADPH produced in the light stage are used.
is reduced to form glucose and then starch.
The ADP and NADP produced are reused in the light stage (see fig. 2.21).
Figure 2.21 Dark stage
Mandatory activity
To investigate the effect of light intensity on the rate of photosynthesis
Cut a fresh stem of Elodea at an angle with a wet blade (this facilitates the release of bubbles) and place in a test tube with a solution of excess sodium bicarbonate.
Place the test tube in a beaker of water with a thermometer. This is to check that the temperature does not vary.
Set a lamp beside the beaker. Set the distance from the lamp to the test tube with the Elodea at 15 cm . Record the light intensity at the test tube using a light meter (see fig. 2.22).
Allow five minutes for the Elodea to equilibrate to the new conditions.
Measure the rate of photosynthesis by counting the number of bubbles of oxygen gas produced per unit time (usually five minutes).
Record the result, repeat the count and calculate an average.
Move the lamp to new positions ( and 60 cm ). Repeat steps 3 to 6 .
Draw a graph of the rate of bubble production against light intensity. Put light intensity on the horizontal axis.
Expected results
The rate of photosynthesis (number of bubbles of oxygen produced per minute) increases as light intensity increases. At the highest light intensities, the rate of photosynthesis may level off due to enzyme saturation.
Figure 2.22 To investigate the effect of light intensity on the rate of photosynthesis
Important points to remember
Independent or manipulated variable light intensity.
Dependent variable rate of photosynthesis (measured by recording the number of bubbles of oxygen produced per unit time).
Controlled variables:
(i) Temperature: Using a water bath and thermometer.
(ii) concentration: Using a saturated solution of sodium bicarbonate.
Greenhouse cultivation
The knowledge we have of the requirements and optimal conditions for photosynthesis gave rise to the development of greenhouses for certain types of crop. A greenhouse can artificially control:
light type and intensity
temperature
carbon dioxide concentrations and mineral levels.
This helps protect the crop and ensures maximum productivity.
Light
Light is probably the most important factor affecting photosynthesis. Small increases in light intensity can sharply increase the rate of photosynthesis. In the greenhouse, light can be provided for longer periods artificially. It is important to provide light of the correct wavelengths in greenhouses to maximise photosynthesis.
Temperature
All metabolic reactions in plants are controlled by enzymes. Plant enzymes generally function best at a temperature of . Controlled heating can ensure an optimum temperature for plant growth.
Carbon dioxide
The levels of in the air can also be controlled. Tomato plant yields can be increased if the levels are raised from the normal 0.03 per cent in air to 1 per cent in the atmosphere inside the greenhouse.
2016 Q11 (b) HIGHER LEVEL
(b) Answer the following questions from your knowledge of photosynthesis.
(i) Where in plant cells does the process take place?
(ii) Name a substance which absorbs light energy for the process.
(iii) In which pathway of the light stage is oxygen produced?
(iv) Outline how this oxygen is produced.
(v) Give one fate of this oxygen.
(vi) What is the fate of the carbon in the carbon dioxide used in the dark stage?
(vii) Give one reason why a suitable temperature is necessary for the dark stage to occur.
(viii) Aquatic plants such as Elodea are particularly suitable for investigating photosynthesis. Suggest a reason for this.
LEAVING CERT MARKING SCHEME
11.(b) (i) Where: Chloroplast
(ii) Substance: Chlorophyll
(iii) Pathway: Pathway 2
(iv) produced: Water split / using light (energy) OR Photolysis / of water
(v) Fate of : Released (to the atmosphere) or (used in) respiration
(vi) Fate of carbon: Makes carbohydrate (or named carbohydrate)
(viii) Why Elodea: Bubbles (of oxygen visible for counting)
Respiration
Definitions
Respiration
Catabolic reaction
Anaerobic respiration
Aerobic respiration
Glycolysis
Outline
Role of respiration in organisms
Balanced equation of aerobic respiration
Two-stage process of aerobic respiration/location of stages in the cell
Anaerobic respiration in animals and plants
Fermentation and role in industry
Differences between aerobic and anaerobic respiration
Glycolysis, pyruvic acid and ATP formation
Products of anaerobic respiration in plant and animal cells
Acetyl Co A/Krebs cycle, and NADH formation
Electron transport chain/ATP production and role of
Practical Activity
Prepare and show the production of alcohol by yeast
Respiration is the chemical breakdown of food to release energy. Respiration occurs in every cell in the body.
Respiration is a catabolic chemical reaction. In the process, complex biomolecules are broken down to simple inorganic molecules, releasing energy.
There are two types of respiration.
Aerobic respiration is the release of energy from food that requires the presence of oxygen.
Anaerobic respiration is the release of energy from food that does not require the presence of oxygen.
Equation of aerobic respiration
Chemical equation
C}\mp@subsup{\textrm{C}}{6}{}\mp@subsup{\textrm{H}}{12}{}\mp@subsup{\textrm{O}}{6}{}+6\mp@subsup{\textrm{O}}{2}{}\longrightarrow6\mp@subsup{\textrm{CO}}{2}{}+6\mp@subsup{\textrm{H}}{2}{}\textrm{O}+\mathrm{ energy
Word equation
glucose + oxygen \longrightarrow carbon dioxide + water + energy
Equation of anaerobic respiration in plants (Fermentation)
Word equation
glucose \longrightarrow ethanol + carbon dioxide + energy
Biochemistry of cellular respiration
Aerobic cellular respiration occurs in three stages.
Anaerobic glycolysis: Occurs in the cytoplasm and does not involve oxygen.
Krebs or citric acid cycle: Occurs in the lumen of the mitochondrion and oxygen is necessary.
Electron transport chain or oxidative phosphorylation: Occurs on the cristae of the mitochondrion and oxygen is necessary.
Anaerobic glycolysis
This is the breakdown of glucose to pyruvic acid, with ATP being produced.
In plants, if no oxygen is present, the pyruvic acid is converted to ethyl alcohol (ethanol) and carbon dioxide. This process is called fermentation.
In the absence of oxygen, animal cells convert the pyruvic acid to lactic acid.
Krebs cycle
If oxygen is present, the pyruvic acid is converted to acetyl co-enzyme A (acetyl co A), releasing a molecule of carbon dioxide.
The acetyl co A enters the lumen of the mitochondrion and then Krebs cycle begins.
Hydrogen (in the form of NADH ) and are produced.
Electron transport chain
The products of the Krebs cycle (NADH) enter the cristae of the mitochondria to begin the electron transport chain.
The hydrogen, electrons and energy stored in NADH are used to produce ATP.
Finally, oxygen combines with hydrogen to produce the waste .
Aerobic respiration is summarised in fig. 2.23.
Figure 2.23 Aerobic respiration
Differences between aerobic and anaerobic respiration
Aerobic Respiration
Anaerobic Respiration
Oxygen necessary
Oxygen not necessary
Occurs in mitochondria
Occurs in cytoplasm
Large amount of energy produced
Small amount of energy produced
End products are
End products are lactic acid or ethyl alcohol and
Mandatory activity
To prepare and show the production of alcohol by yeast
Add of 10 per cent glucose to each of two conical flasks.
Add 5 g of yeast to one and mix thoroughly.
Label the second flask with no yeast 'control'.
Add a fermentation lock half-filled with water to both of the flasks (see fig. 2.24).
Place both flasks in an incubator at for 24 hours.
To test for alcohol
Test the contents of each flask as follows:
Filter the mixture from the conical flask into a beaker.
Remove of the filtrate and place in a test tube.
Add of potassium iodide solution.
Add of sodium hypochlorite solution.
Heat gently for five minutes in a warm water bath.
Figure 2.24 Setting up the experiment
Expected results
A positive result for alcohol is the formation of a yellow colour or yellow crystall.
A negative result is no yellow colour or crystal formation.
Important points to remember
The fermentation lock allows gas (carbon dioxide) to escape from the flask but prevents the entry of air or contaminants into the flask. This provides anaerobic conditions.
The incubator at provides the optimum temperature for the yeast cells (enzymes) to respire.
Substrate glucose, products carbon dioxide and ethanol.
2016 Q11 (c) HIGHER LEVEL
(c) Answer the following questions from your knowledge of respiration.
(i) Name the 3 -carbon molecule that is an intermediate compound in both aerobic and anaerobic respiration.
(ii) What name is given to the biochemical pathway by which this intermediate compound is produced?
(iii) What happens to the intermediate compound referred to in (i) above when oxygen is available and used in the breakdown of glucose? In your answer, refer to:
Krebs cycle.
Electron transport system.
(iv) What is produced from the intermediate compound referred to in (i) above when oxygen is not available
in muscle?
in yeast?
LEAVING CERT MARKING SCHEME
(c) (i) 3-carbon intermediate: *Pyruvate or *pyruvic acid
(ii) Pathway: *Glycolysis
(iii) Oxidative fate of pyruvate: Converted to Acetyl (Co- Enzyme A) or enters mitochondrion
or Kreb's cycle: produced / ATP produced / NADH produced
or electron transport system: protons (or ions) combine with or electrons ( ) combine with / to form water / energy to ADP and to make ATP
Any one further point from 1 or from 2.
(iv) 1. Anaerobic product in muscle: *Lactic acid or *lactate
Anaerobic product in yeast: *Ethanol (and carbon dioxide)
3
Cell Continuity
Definitions
Cell continuity
Haploid
Diploid
Mitosis
Cell cycle
Cancer
Meiosis
Outline
Interphase
Differences between mitosis and meiosis
Significance of mitosis and meiosis
Cell cycle
Two possible causes of cancer
Detailed description and diagrams of stages of mitosis
Cell division
Cell division is essential to all living things.
It allows a multicellular organism to replace worn or damaged cells.
It is also the basis of reproduction in every organism.
Chromosomes, made up of DNA and protein, carry the genetic code in the nucleus from one generation to the next. In any one individual it is vital that this genetic code is copied faithfully from one cell generation to the next. This is what is known as cell continuity.
Haploid and diploid cells
A haploid cell has half the full complement of chromosomes . The chromosomes do not exist in pairs. Haploid cells are usually either spores or gametes produced during sexual reproduction.
A diploid cell has the full complement of chromosomes (2n) existing in pairs. All somatic cells (body cells not involved in reproduction) are diploid.
Mitosis
Mitosis is a form of cell division where one cell divides to form two cells, each identical to the parent cell. Mitosis occurs in somatic cells (cells not involved in reproduction).
Significance of mitosis
In multicellular organisms, genes are faithfully transmitted from one cell generation to the next.
Unicellular organisms use mitosis for reproduction to produce genetically identical offspring.
Stages of mitosis
Figure 2.25 Mitosis in a cell with a diploid chromosome number
Interphase
This is a stage between cell divisions when the cell carries out normal activities.
To prepare for cell division at the end of interphase:
The cell builds up a store of energy.
Nuclear material (to build new chromosomes) replicates.
Cell organelles replicate.
The chromosomes are not yet individually visible and form a mass of chromatin.
Prophase
Chromosomes thicken and become visible.
Homologous chromosomes lie together in pairs.
Each chromosome forms an identical copy of itself and is joined to the original at the centromere. The pair are called sister chromatids.
The centriole replicates and they move to opposite sides of the cell.
The centrioles leave a trail of spindle threads.
Finally, the nuclear membrane breaks down.
Metaphase
The chromosomes (sister chromatids) lie in a straight line across the middle of the cell, attached to the spindle threads by the centromere.
Anaphase
The spindle threads contract, separating the sister chromatids.
Telophase
A nuclear membrane forms around each set of chromosomes and a cell membrane forms to create two new cells, each with a diploid chromosome number identical to the parent cell.
Cell cycle
This is the sequence of events that occurs between one cell division and the next in mitosis. It can be divided into three main stages:
In interphase the cell grows and carries out its functions. At the end of interphase the chromosomes replicate, forming sister chromatids. Interphase takes up 90 per cent of the cell cycle.
Mitosis takes place, forming two new nuclei.
The cytoplasm finally divides, forming two new daughter cells. The process is called cytokinesis.
Cancer
Cancer is a general term to describe a disorder in the body's growth. Cancer cells fail to respond to normal controls on their multiplication and enlargement. The growth of cancer cells results in a tumour, which crowds out healthy cells. There are two types of tumour. A benign tumour grows slowly and the adverse effects are usually to simply apply physical pressure to surrounding tissues. A malignant tumour consists of rapidly growing cells that invade and can destroy other tissues. When malignant tumours invade the blood or lymphatic systems, they can spread to other parts of the body.
Lung cancer
Cigarette smoking has been directly linked to lung cancer. Cigarette smoke contains carcinogens, which change or mutate genes that control cell division and development in cells. The most common form of death from cancer in males is lung cancer.
Skin cancer
Skin cancer can be caused by ultraviolet (UV) radiation from sunlight. The UV rays penetrate skin cells, causing mutations in the DNA. With the deterioration of the protective ozone layer, a significant increase in the incidence of skin cancer can be expected.
Meiosis
A second form of cell division can occur. It is known as meiosis or reduction division.
Meiosis is cell division where one cell divides to form four cells, each with half the number of chromosomes of the parent cell.
Significance of meiosis
It is a mechanism of producing gametes (spores in higher plants) with half the number of chromosomes of the parent cell, so at fertilisation the full complement is restored.
It is a means of producing changes in genotype, leading to variation in the offspring.
4 Cell Diversity - Tissues, Organs and Systems
Definitions
Tissue (and examples)
Organ (and examples)
Organ system (and examples in animals and plants)
Outline
Applications of tissue culture in plants and animals
When organisms evolved to a multicellular state, their cells became specialised. Cells no longer needed to be capable of carrying out all activities to maintain life. Division of labour meant that different cells or groups of cells could carry out specialised functions. Multicellular organisms could organise similar cells into tissues.
Tissues
Examples of animal tissues are:
connective tissue, the skin, for protection
A tissue is a group of similar cells working together. Cells in tissues are more efficient than cells working individually.
muscle tissue to move body parts
Organs consist of a group of different tissues working together.
Organ systems
The kidney is an organ that works with other organs such as the bladder, renal arteries and veins, and the ureters and urethra. Together, these form the excretory system. This system's function is to rid the body of the wastes of metabolism.
The digestive system is made up of the gut with accessory organs such as the pancreas and the liver. The digestive system carries out the physical and chemical digestion of food in the body.
Tissue culture
Whole plants can be cultured asexually from very small pieces of tissue extracted from a parent plant. The process can be called micropropagation, tissue culture or cloning. By this process a single plant can produce thousands of genetically identical offspring.
Advantages of tissue culture in plants
Advantageous genetic characteristics can be faithfully passed to all the offspring.
Enormous numbers of offspring can be produced.
The timing of development can be controlled.
Rare plants can be reproduced easily.
Disadvantages
All offspring are susceptible to the same diseases and pests, which increases the rates of transmission.
Long-term micropropagation can lead to plants becoming sterile.
Tissue culture in animals
Tissue culture in animals involves the removal of unspecialised stem cells from body tissues or developing embryos. Under the right conditions, in a nutrient solution, these cells can be stimulated to develop into different body tissue cells.
Applications of tissue culture in animals
Tissue culture in animals is used for:
The development of cells to act as hosts in the culture of viruses to produce viral vaccines.
The culture of skin tissues to help repair burn injuries.
The growth of bone tissue used in reconstructive surgery.
The culturing of organs which will not lead to patient rejection after transplant.
Genetics
Definitions
Genetics
Alleles
Chromosome
Dihybrid cross
Dominant
Fertilisation
Gamete
Gene
Genotype
Heterozygous
Homozygous
Incomplete dominance
Monohybrid cross
Phenotype
Recessive
Outline
Genetic Crosses
Carry out a genetic cross showing parent genotype, gamete genotype, F1 genotype and F1 phenotype
Sex Chromosomes and (Sex Determination)
Gregor Mendel and his crosses that led to his two laws
State and explain the law of segregation
State and explain the law of independent assortment
Dihybrid cross (showing parent genotypes, gamete genotypes, F1 and F2 genotypes and phenotypes)
Define linkage and sex linkage: The effects of linkage on Mendel's law of independent assortment
Sex linkage and sex-linked crosses
Important definitions
Alleles: Different genes that control the same trait and have the same locus on homologous chromosomes, e.g. and .
Chromosome: A threadlike structure in the nucleus, made up of DNA and protein,
Genetics is the study of heredity, that is, the transfer of characteristics or traits from one generation to the next. Modern genetics is concerned with the study of genes. containing genes.
Dihybrid cross: Genetic cross where two characteristics (pairs of genes) are studied, e.g. TtYy TTYY.
Dominant: The gene that is expressed in the phenotype of the heterozygous condition, e.g. Tt has a tall stem, T is dominant.
Fertilisation: The fusion of two haploid gametes to form a diploid zygote.
Gamete: A haploid sex cell capable of fusion (fertilisation).
Genes: These are units of heredity, made of DNA, that control characteristics in an organism.
Genotype: The genetic make-up of an organism, e.g. Rr.
Heterozygous: An organism that has two different genes controlling the same trait, e.g. Tt.
Homozygous: An organism that has two identical genes controlling the same trait, e.g. TT or tt.
Incomplete dominance: When neither allele is completely expressed in the phenotype of the heterozygous condition, e.g. in cattle the gene codes for red coat, the gene codes for white coat. In the heterozygous condition, codes for roan coat colour.
Monohybrid cross: A genetic cross where only one characteristic or trait (pair of genes) is studied, e.g. .
Phenotype: Observable characteristics (traits) in an organism, determined by its genes and the genes' interaction with the environment.
Recessive: An allele whose expression in the phenotype is masked by a dominant allele, e.g. Tt has a tall stem, is recessive.
Gregor Mendel
Mendel was a very careful worker who planned his experiments on a large scale. He recognised that by taking a large number of separate measurements he could eliminate chance effects. He chose the pea plant to study because:
it had several very sharply contrasting characteristics (for example, short stem, tall stem)
it did not have intermediate forms (no incomplete dominance).
Mendel's first experiment was to cross a tall-stem (pure breeding - homozygous) pea plant (TT) with a short-stem plant (tt). He gathered the seeds produced and planted them. He then crossed two of the F1 generation (see fig. 2.26).
The chromosome diagram shows the location of genes on chromosomes in the nucleus of each plant cell.
On observation of the results of his first experiment Mendel formed his First Law, the Law of Segregation.
Mendel's Law of Segregation: Traits are controlled by pairs of factors (genes). Only one of any pair can enter a gamete.
Mendel then studied the inheritance of two characteristics at a time. He crossed a plant homozygously dominant for two characteristics, tall stem and yellow seeds (TTYY), and a plant doubly recessive for the same characteristics (ttyy). He planted the seeds produced and then crossed two of the new offspring (see fig. 2.27).
A dihybrid cross between two plants with genotypes of TTYY and ttyy is carried out below:
First Cross
This cross will produce sixteen possible options in the F2 genotype. The easiest way to carry out the cross accurately is to use a Punnet square.
Figure 2.27 Mendel's Law of Independent Assortment
Genotypes
Gametes
TY
Ty
tY
ty
TY
TTYY
TTYy
TtYY
TtYy
Ty
TTYy
TTyy
TtYy
Ttyy
tY
TtYY
TtYy
ttYY
ttYy
ty
TtYy
Ttyy
ttYy
ttyy
phenotype
9 Tall + Yellow
3 Tall + Green
3 Short + Yellow
1 Short + Green
From these results, Mendel formulated his second law - the Law of Independent Assortment.
Mendel's Law of Independent Assortment: When gametes are formed, either of a pair of alleles can enter a gamete with either of another pair.
Linkage and the effects on Mendel's Second Law
When Mendel formulated the Law of Independent Assortment, he had studied two traits that were controlled by pairs of genes that were on different homologous pairs of chromosomes.
Non-allelic genes (genes that control different traits) that are found on the same chromosome are said to be linked.
Gametes formed when genes are not linked
Non-homologous chromosomes
T
Gamete genotype
Ty
Gametes formed when genes are linked
If the genes were linked, his results would have been different.
Linked genes do not follow Mendel's Law of Independent Assortment. The gametes formed are very different (see fig. 2.28).
Consider the situation if an individual with a genotype TtYy is crossed with an individual with a genotype ttyy and the genes are linked. The chromosome diagrams and the cross are shown in fig. 2.29 .
Chromosome diagrams
Parent genotype
Gamete genotype
genotype
phenotype
Figure 2.29 Genes are linked
Sex chromosomes
In humans, the diploid number of chromosomes is 23 pairs.
Twenty-two pairs of chromosomes are autosomes that control almost all characteristics except sex determination.
One pair of chromosomes determines the sex of the individual and these are known as the sex chromosomes.
There are two different types of sex chromosome, the chromosome (which is the larger of the two) and the Y chromosome.
An individual with a genotype of XX is female, while XY is male.
Figure 2.30 Sex determination
Note: The male gamete determines the sex of the child.
Sex linkage
Genes located on the X chromosome that control characteristics other than the sex of the individual are called sex-linked genes.
Examples of such genes are the gene for red/green colour blindness and the gene for haemophilia.
Example of sex linkage
In Drosophila melanogaster (fruit fy) the gene for eye colour is sex-linked. The gene for red eye, , is dominant to the gene for white, r . A white-eyed male is crossed with a heterozygous red-eyed female . The genotypes and phenotypes of the offspring produced are shown in fig. 2.31
Figure 2.31 Sex linkage
2015 Q10 (a)/(c) HIGHER LEVEL
(a) (i) Which famous 19th-century biologist is regarded as 'the father of genetics'?
(ii) In genetics, what is meant by segregation?
(iii) Give an example of a sex-linked characteristic in humans.
(c) Unlike the situation in humans, maleness in birds results from the presence of the chromosome pair in the fertilised egg and femaleness results from the XY pair. In a particular bird species, green plumage is dominant to yellow plumage and long tail (L) is dominant to short tail (I). The gene for plumage colour is linked to the gene for tail length.
Study the genotypes of the above bird species shown in the diagrams below and in your answer book match the correct genotype to each of the descriptions (i) to (vi). A diagram may match more than one of the descriptions.
(i) A female that is heterozygous in respect of plumage colour and tail length.
(ii) A male that can produce only one type of gamete.
(iii) The individual that can produce the greatest number of different gametes.
(iv) A male, all of whose offspring will have long tails.
(v) A female, all of whose offspring will have green plumage.
(vi) A male that is homozygous in respect of plumage colour and tail length.
(vii) In your answer book, write out the genotypes of the gametes that bird D can produce.
LEAVING CERT MARKING SCHEME
10. (a) (i) Mendel ..... (3)
(ii) Separation of homologous chromosomes or separation of alleles ..... (3)
(iii) Haemophilia or (red-green) colour blindness ..... (3)
(c) (i) ..... (3)
(ii) ..... (3)
(iii) ..... (3)
(iv) *C ..... (3)
(v) *B ..... (3)
(vi) *C ..... (3)
(vii) glX / glY ..... (2[3])
Chromosome and DNA
Definitions
DNA
Hydrogen bonds
Codon
Triplet code
Anticodon
RNA
Coding and non-coding DNA
DNA profiling
Genetic testing and screening
Genetic engineering
Transcription
Translation
Outline
DNA structure (including diagram)
Base pairing A-T and C-G
DNA replication
RNA and the function of mRNA
Differences between DNA and RNA
Protein synthesis (DNA, mRNA, ribosome, triplet code, specific amino acid sequence, correctly folded protein)
Coding and non-coding DNA
DNA profiling, stages involved and two applications
Genetic testing and genetic screening
Process of genetic engineering and three applications
Non-nuclear inheritance, i.e. DNA present in mitochondria and chloroplasts
Detailed structure of DNA including deoxyribose, purine and pyrimidine bases and hydrogen bonds
Protein synthesis including tRNA and rRNA
Practical Activity
Isolate DNA from plant tissue
DNA structure
Chromosomes are made up of deoxyribonucleic acid (DNA) and protein. DNA is the substance that carries the genetic code. It is made up of chains of single units called nucleotides.
A DNA nucleotide consists of:
sugar (deoxyribose)
phosphate molecule
nitrogen-containing base.
There are four different types of base:
adenine
thymine
cytosine
guanine
Adenine and guanine are purine bases, while thymine and cytosine are pyrimidine bases.
Figure 2.32 DNA double helix
DNA has a double helix shape (see fig. 2.32). As you can see from the diagram, it is a ladder-like structure that has been twisted in opposite directions at either end.
The deoxyribose and the phosphate form the uprights of the ladder and the rungs are pairs of the nitrogen-containing bases.
The deoxyribose and phosphate strands are anti-parallel (run in opposite directions).
A purine base can only link to a pyrimidine base due to size restrictions. Adenine and thymine can only be paired together, similarly only guanine and cytosine can be paired.
The base pairs in DNA are held together by hydrogen bonds, which are bonds of electrical attraction.
DNA replication
This is the means by which chromosomes (DNA) can form identical copies of themselves.
DNA replication begins when:
The hydrogen bonds holding the base pairs together break.
The strands of the parent DNA then separate.
The DNA double helix unwinds.
Each strand of the DNA now acts as a template.
Nucleotides, with specific bases from the cytoplasm, match the free bases on each of the parent strands of DNA.
This process produces two DNA helices identical to the first. One half of each double helix contains the original parent strand (see fig. 2.33).
Figure 2.33 DNA replication
DNA and protein synthesis
Specific enzymes control all chemical reactions in the body. Enzymes are proteins made up of a defined sequence of amino acids.
DNA codes for amino acids, and their correct sequence, through a triplet code.
The nitrogen-containing bases (A, T, G and C) in DNA are arranged in threes along the double helix.
Each triplet codes for one specific amino acid.
DNA sends its code for an enzyme to the ribosome by messenger RNA, or mRNA.
Ribosomes, in the cytoplasm, make the proteins.
Protein synthesis
The whole process of protein synthesis occurs in two stages: transcription and translation.
1. Transcription
When a particular enzyme is needed by a cell, the portion of DNA in the nucleus that codes for it unwinds, exposing its bases.
A strand of mRNA is produced from RNA nucleotides, mirroring the DNA code.
When complete, the strand of mRNA separates from the DNA and moves to a
ke
point
The nucleotides on mRNA are arranged in a triplet code forming codons. Each codon codes for a particular amino acid.
tRNA also has a triplet code of nucleotides that match the codons of mRNA. Each matching triplet is known as an anticodon. ribosome (rRNA) in the cytoplasm.
2. Translation
At the ribosome, the mRNA code is matched by nucleotides of transfer RNA ( RNA). Each RNA molecule carries a specific amino acid.
The tRNA carries the amino acids in the correct sequence to the ribosome.
The amino acids are then linked together in strict order, producing the protein (enzyme), which then assumes its unique folded shape.
Differences between DNA and RNA
DNA
Structure
Function
Location
Deoxyribose is the sugar
Double helix shape
Base pairing
Has the base thymine instead
of uracil
Codes for genotype
Nucleus
RNA
Ribose is the sugar
Single helix
No base pairing
Has the base uracil instead of
thymine
mRNA carries code from
nucleus to rRNA
(ribosomes); tRNA
transports amino acids
Cytoplasm
(HL) Non-nuclear inheritance of DNA
Many scientists believe that mitochondria and chloroplasts evolved from forms of bacteria. Through evolution they became assimilated into larger-celled organisms. The two organisms then formed a mutualistic (symbiotic) relationship, giving rise to plant and animal cells. Mitochondria and chloroplasts are unique as organelles in that they:
contain their own DNA
can replicate themselves in the cell.
Mandatory activity
To isolate DNA from plant tissue
Note: The extra information in brackets below is not required when describing the procedure.
Add 3 g of salt to of washing-up liquid in a beaker and bring up to 100 with distilled water. (Salt reduces the attraction between protein and DNA. Washing-up liquid breaks down phospholipid membranes in the cell.)
Chop some onion into very small pieces and add to the beaker. (Chopping breaks down the cell walls and allows cytoplasm to leak out.)
Put the beaker in a water bath at for exactly 15 minutes. (High temperature denatures enzymes harmful to DNA. Any longer than 15 minutes, the DNA itself would break down.)
Cool the mixture by placing in a large beaker of ice water for five minutes. (Slows the activity of any remaining enzymes harmful to DNA.)
Place the mixture in a blender for three seconds. (Blending further breaks down cell walls and membranes. Any longer than three seconds shreds the DNA.)
Filter the mixture into a second beaker. (Removes cellular debris.)
Place of the filtrate into a test tube.
Add 3 drops of protease solution and mix gently. (Breaks down proteins associated with DNA.)
Trickle of ethanol from the freezer down the side of the test tube.
Leave for a few minutes to settle. (Cold ethanol draws water from , condensing it.)
Gently stir with a glass rod.
Expected result
White mucus-like DNA forms at the interface of the ethanol and the filtrate.
DNA profiling
Humans have 23 pairs of chromosomes in the nucleus of every cell in the body (with the exception of gametes). A single chromosome can have up to 4,000 genes, which code for different traits. It is known that 90 per cent of DNA does not code for any gene or protein in the body.
Coding DNA refers to sections of DNA that make up genes. They code for an enzyme or protein.
Non-coding DNA describes sections of DNA, between genes, that do not code for an enzyme or protein. They are often referred to as 'junk DNA'.
The sections of non-coding DNA often have repeating nucleotide sequences in sections called hypervariable regions. The number and length of these nucleotide sequences vary between individuals, but are similar in related individuals.
Forensic scientists use DNA profiling to compare DNA from hair, saliva, blood or semen found at the scene of a crime.
The procedure for DNA profiling is
DNA profiling or DNA fingerprinting
uses the repeating nucleotide sequences of non-coding DNA to produce a pattern of bands for comparison of individuals. outlined in fig. 2.34.
Tissue sample
DNA extracted
DNA fragmented
Separation of
DNA fragments
Separation of double strands
Hybridisation
Fragment distribution analysed
Tissue sample is obtained from source.
Solvents used to separate DNA from proteins.
Enzymes called restriction endonucleases digest the DNA, breaking it into fragments at specific points.
Fragments are separated on the basis of size using gel electrophoresis.
The sample is immersed into an alkali to separate DNA into single strands.
Labelled nucleotide sequences of specific code, called probes, are added. These match certain parts of the core nucleotide sequences and pair up with them.
An X-ray film is placed over the nucleotides and the marked sections with the probes appear as dark bands.
Patterns of banding from different samples are compared.
Applications of DNA profiling
DNA profiling can be used to:
prove the parentage of a child
detect criminals guilty of violent crimes
confirm pedigree in animals.
Genetic screening and testing
Genetic screening is the use of laboratory procedures to test a large number of individuals to identify those who may have or may pass on a genetic disorder.
Example: Amniocentesis is the testing of the cells in the amniotic fluid around the foetus for genetic disorders such as Down's syndrome.
Genetic testing describes the laboratory procedures used to investigate an individual suspected of having a high risk of a genetic disorder, based on family history or a positive screening test.
Example: The testing for the genetic disorder responsible for cystic fibrosis.
Genetic engineering and applications
Genetic engineering is a process where genes from one organism are introduced into the genome (DNA) of an unrelated organism, usually micro-organisms.
The micro-organisms with the new genes are replicated and used to create large quantities of useful chemicals.
Note: The process is often referred to as recombinant DNA technology.
Genetic engineering involves the following steps.
Locating a specific gene in a donor cell.
Isolation of the gene.
Insertion of the gene into the DNA that has been removed from a micro-organism.
Transferring the DNA and new gene back into the micro-organism.
Replicating the micro-organism and harvesting the chemicals produced due to the new gene.
The process is summarised in fig. 2.35 .
Figure 2.35 Genetic engineering
Applications of genetic engineering
There is enormous demand for the hormone insulin to treat insulin-dependent diabetes. This disease used to be treated by using insulin obtained from the pancreas of cattle and pigs. Subtle differences in the forms of insulin stimulated antibody responses in some humans. Genetic engineering is now used to isolate the human gene for insulin production. The gene is inserted into a host bacterium to produce large quantities of human insulin.
Genetically modified plants have an advantageous gene inserted into their DNA which is passed on to future generations. Characteristics such as disease and insect resistance have been introduced into food crops. The improved plant has greater yields.
In animals, genetic engineering has been used to increase meat and milk yields in cattle.
In microorganisms, bacteriophage can be genetically engineered to kill antibioticresistant bacteria.
2014 Q7 (a)/(b) HIGHER LEVEL
(a) (i) What is the chemical composition of a chromosome?
(ii) What is meant by the term junk DNA?
(b) (i) In relation to the isolation of DNA from a plant tissue, explain why you used each of the following:
Washing-up or similar liquid.
Sodium chloride.
Protease.
Freezer-cold ethanol.
LEAVING CERT MARKING SCHEME
(a)
(a) (i) DNA and protein
(ii) Non-coding (DNA)
(b)
(b) (i) 1. To breakdown the (cell) membrane(s)
To cause the DNA to clump
To breakdown (or remove or digest) the protein in the chromosomes
To bring the DNA out of solution or to make the DNA visible or to separate the DNA
2015 Q10 (b) HIGHER LEVEL
(b) Write notes on each of the following topics in relation to nucleic acids. In each case your notes should contain three points. Do not give diagrams in your answers.
(i) Complementary base pairs.
(ii) Codons.
(iii) Transcription.
LEAVING CERT MARKING SCHEME
(b) (i) (Two bases joined by) hydrogen bonds / purine with pyrimidine / Cytosine with Guanine / Adenine with Thymine in DNA / Adenine with Uracil in RNA or Thymine replaced by Uracil in RNA
(ii) Sequence(c) of three bases / on DNA / on mRNA or on TRNA / (each codon) codes for one amino acid/that codes for a start (or stop)
(iii) mRNA is formed / using a (single) strand of DNA / (DNA acts) as a template (or described) / in nucleus / (catalysed by) RNA polymerase
Evolution
Definitions
Fossil
Evolution
Natural selection
Variation
Mutation
Species
Outline
Causes of variation in both sexual reproduction and mutations
Examples of evidence to support evolution
A fossil is the remains of a once-living plant or animal.
Evolution is a process where organisms that now exist have descended from different ancestors. It is brought about by mutations (genetic changes). Improved characteristics are then passed on to offspring by natural selection.
Natural selection (Darwin and Wallace)
Darwin and Wallace developed the theory of natural selection, based on their observations, to explain the process of evolution.
Their observations included the following:
Organisms generally produce large numbers of offspring.
Population numbers tend to remain constant as survival rates are low.
Variation occurs between individual members of any species.
Natural selection is the way in which organisms become better adapted to their environment due to a mutation.
The genes for the improved characteristics can then be passed on to their offspring.
Natural selection is thought to play a large role in the process of evolution.
Inherited variations are the basis of natural selection and evolution.
Variation in a species over long periods can give rise to new species. This process is known as speciation.
Evidence for evolution
There are different sources of evidence that support the theory of evolution. One source is the study of comparative anatomy.
Comparative anatomy
This is a comparison of bone structure in the forelimb of very diverse animals such as the whale, mole, bat and human. They all have a pentadactyl limb arrangement of bones (see fig. 2.36).
Figure 2.36 Pentadactyl limb
Key point
Limbs of various animals that have the same basic structure but different functions are known as homologous structures.
Different functions for limbs with similar structures are examples of adaptative radiation.
Variation of species
A species is defined as a group of similar individuals of common ancestry that can interbreed to produce fertile offspring.
Different characteristics found in the individuals of a species are described as variations.
Variations can occur due to:
the shuffling of genes that occurs, during meiosis, to produce gametes
mutations.
Mutations
A mutation is a change in the nucleotide sequence of DNA that can alter the genotype of an organism.
Mutations are inherited if they are present in a gamete.
Causes: Mutations can occur with exposure to mutagens such as X-rays, radioactivity and chemicals called carcinogens.
Gene and chromosome mutations
A gene mutation is caused by a change in the base (nucleotide) sequence of the DNA in a gene. This can alter the protein it produces.
Example: Cystic fibrosis is caused by a gene mutation.
A chromosome mutation is due to a change in a chromosome structure or a change in the number of chromosomes present in an organism.
Example: Down's syndrome is caused by the presence of an extra chromosome.
2016 Q14 (a) HIGHER LEVEL
Answer any two of (a), (b), (c).
(a) (i) Explain the term species.
(ii) What term is used to describe the differences which exist between individuals of a species?
(iii) The differences referred to in (ii) form the basis of evolution by natural selection.
Explain the term evolution?
Outline the role of natural selection in evolution.
(iv) Explain the term mutation.
(v) Give one example each of a disorder caused by:
Gene mutation.
Chromosome mutation.
(vi) Give one cause of the differences referred to in (ii) above, other than mutation.
LEAVING CERT MARKING SCHEME
(a) (i) Species: A group of organisms capable of interbreeding to produce fertile offspring.
(ii) Intraspecific differences: *Variation
(iii) 1. Evolution: Genetic changes (in populations) / in response to environment / over time / giving rise to new species
Role of natural selection: Better adapted survive / reproduce / adaptation is inherited / adaptation (becomes) more common
(iv) Mutation: A change in DNA (or gene or chromosome or genetic material)
(v) 1. Gene mutation disorder. Sickle cell anaemia or any valid example
Chromosome mutation disorder. Down's syndrome or any valid example
(vi) Cause of variation: Sexual reproduction or meiosis or formation of gametes or fertilisation of gametes or independent assortment
2012 Q6 HIGHER LEVEL
(a) In genetics, what is meant by the term variation?
(b) Variation can result from mutation. Name one other cause of variation.
(c) Name two types of mutation.
(d) Name two agents responsible for increased rates of mutation.
(e) Briefly explain the significance of mutation in relation to natural selection.
LEAVING CERT MARKING SCHEME
(a) Differences (within a population or within a species or between individuals)
(b) Sexual reproduction or meiosis or independent assortment or environmental
(c) (i) Gene (mutation)
(ii) Chromosome (mutation)
(d) (i) Example 1
(ii) Example 2
(e) New phenotypes or new types or new features / Better adapted or survival of the fittest (or advantageous) or less well adapted (or disadvantageous)
