DNA & Meiosis

Individual nucleotides of DNA are made of 3 components:

• A sugar called deoxyribose
• A phosphate group
• An organic base belonging to one of two different groups, either a single-rung base (pyrimidines) which are cytosine and thymine or a double rung base (purines) which are adenine and guanine.

The deoxyribose sugar, phosphate group and organic base are combined, as a result of condensation reactions to give a single nucleotide (mononucleotide). Two joined together from a dinucleotide and many joined together are called a polynucleotide.

The base always contains nitrogen and carbon.

Made up of two strands of nucleotides. They are joined together by hydrogen bonds formed between 2 bases.

The phosphate and deoxyribose molecules alternate to form the uprights and the organic bases pair together to form the rungs.

Adenine always pairs with thymine by means of 2 hydrogen bonds and guanine always pairs with cytosine by means of 3 hydrogen bonds.

The ratio of adenine and thymine to guanine and cytosine vary from species to species.

The base and sugar join with a glycosidic bond whilst the phosphate and sugar join with an ester bond. Both require a condensation reaction to occur.

• It is very stable and can pass to generations without change.
• It's 2 seperate strands are joined only with hydrogen bonds which allow them to seperate during replication.
• It is a large molecule and carries an immense amount of information.
• By having the base pairs in the helical cylinder the information is protected from being corrupted.

Genes are sections of DNA that contain the coded info for making polypeptides. The coded information is in the form of a specific sequence of bases. Polypeptides combine to make proteins and so genes determine the proteins of an organism. As enzymes control chemical reactions they are responsible for an organism's developments and activities.

In prokaryotic cells (bacteria) the DNA molecules are smaller; they form a circle and are not associated with protein molceules; they therefore do not have chromosomes.

In eukaryotic cells the DNA molecules are larger; they form a line and occur in association with proteins to form chromosomes.

Chromosomes are only visible as distinct structures when a cell is dividing. The rest of the time they are widely dispersed throughout the nucleus. When visible they appear as 2 threads joined at a single point. Each thread is called a chromatid. The DNA in chromosomes is held in position by proteins. The length of DNA is highly coiled and folded. It is coiled and looped and further coiled before being packed into the chromosome.

Homologous chromosomes: when one of each pair is derived from the chromosomes provided by the mother and the other derived from the father. These are known as homologous pairs and the total number is referred to as the diploid number. During meiosis, halving the number of chromosomes is done in a manner which ensures each daughter cell receives one chromosome from each homologous pair.

Each gene exists of 2, or occasionally more different forms. Each of these forms is called an allele. Each individual inherits one from each parent. Any differences in the base sequence of an allele may result in a different sequence of amino acids being coded for.

The division of the nucleus of cells occurs in one of 2 ways. Either through mitosis which produces 2 daughter nuclei with the same number of chromosomes as the parent cell and as each other. The other way is through meiosis which produces 4 daughter nuclei, each with half the number of chromosomes as the parent cell.

Meiosis is necessary in sexual reproduction when 2 gametes fuse to give rise to new offspring. if each gamete has a full set of chromosomes then the cell they produce has double the number.

Meiosis involves 2 nuclear divisions that normally occur one after the other. In the first division the homologous chromosomes pair up and their chromatids wrap around each other. Equivalent portions of these chromatids may be exchanged through crossing over. By the end of this stage the homologous pairs have seperated with one chromosome from each pair going into one the 2 daughter cells. In the second division the chromatids move apart. At the end 4 cells have been formed.

During meiosis 1, each chromosome lines up along its homologous partner. They then arrange themselves randomly in a line. The one that goes into the daughter cell depepnds on how they are lined up. Since they are lined up randomly the combination of chromosomes that goes into the daughter cell is also random. This is called independant segregation.

In crossing over the chromatids of each pair twist around each other, tensions are created during this and portions break off. These portions rejoin with chromatids of its homologous partner and the equivalent portions are exchanged.

Gene- a section of DNA that codes for a polypeptide.

Locus- the position of a gene on a chromosome or DNA molecule.

Allele- one of the different forms of a particular gene.

The greater number of alleles that all members of a species possess, the greater the genetic diversity of the species which increases the likelihood that a species will be able to adapt to environmental change. This is due to a wider range of alleles and characteristics. There is a greater probability that some individual will possess a characteristic that suits it to the new onvironmental conditions.

Selective breeding is also known as artificial selection which involves selecting individuals with desired characteristics and using them to parent the next generation. Offspring that dont exhibit the desired characteristics are killed or prevented from breeding therefore the variety of alleles in the population is deliberately restricted to a small number of desired alleles.

The founder effect occurs when individuals colonise a new region. They carry with them only a small fraction of the alleles of the population. These alleles may not be representative. The new population that develops from the few colonisers will show less genetic diversity than the population they came from.

Genetic bottlenecks is when populations may sometimes suffer a drop in numbers. This may be due to things such as a volcanic eruption or interference by man. The few survivors will possess a smaller variety of alleles than the original population and as they breed and become re-established the genetic diversity will remain restricted.

Some ethics of selective breeding are that it is interfering with nature, animal rights and welfare, if it is beneficial to the environment and the cost.

Primary structure- 4 polypeptide chains

Secondary structure- Each of the polypeptide chains are coiled into a helix.

Tertiary structure- Each polypeptide chain is folded into a precise shape.

Quarternary structure- All four polypeptides are linked to form a spherical molecule. Each polypeptide is associated with a haemoglobin group which contains an iron ion which combines with a single O2 molecule making 4 O2 molecules carried by a single haemoglobin group.

The role of haemoglobin is to transporrt oxygen. To be efficient at transpoting oxygen it must readily associate with oxygen at the surface where gas exchange takes place and it must readily dissociate from oxygen at tissues requiring it.

In the presence of carbon dioxide, the new shape of haemoglobin binds more loosely to oxygen which makes haemoglobin release its oxygen.

Haemoglobin with a high affinity for oxygen- Take up oxygen more easily but release it less rapidly.

Haemoglobin with a low affinity for oxygen- Take up oxygen less easily but release it more readily.

Scientists found a correlation between the type of haemoglobin in an organism and factors in which it lived or its metabloic rate. Explanations are an organism in an environment woth little oxygen requires haemoglobin that readily combines with oxygen and an organism with a high metabolic rate needs to release oxygen readily into its tissues.

The process when haemoglobin combines with oxygen is called loading or associating.

The process when haemoglobin releases its oxyggen is called unloading or dissociating.

When haemoglobin is exposed to different partial pressures of oxygen it doesn't absorb it evenlt. At low concentrations of oxygen the four polypeptides of the haemoglobin molecule are closely united and so it's difficult to absorb the first oxygen molecule. The graph tails off at very high oxygen concentrations because haemoglobin is almost saturated with oxygen. The further to the left the curve is the greater the affinity for oxygen and vice versa.

Haemoglobin has a reduced affinity fro oxygen in the presence of CO2. The greater the concentration the more readily the haemoglobin releases its oxygen.

At the gas exchange surface the level of CO2 is low because it diffuses across the exchange surface and is expelled. The affinity for oxygen is increased which means that oxygen is readily loaded by haemoglobin.In respiring tissues the level of CO2 is high. The affinity for oxygen is reduced which means oxygen is readily unloaded from haemoglobin into the muscle cells.

Loading, transport & unloading of O2:

The higher thhe rate of respiration, the more CO2 the tissues readily produce, the lower the pH, the greater the haemoglobin shape changes, the more readily oxygen is unloaded, the more oxygen is available for respiration.

It is a polyysaccharide found in parts of a plant in the form of small grains. It forms a large component of food. It is made up of chains of alpha glucose monosaccharides linked by glycosidic bonds formed by condesation reactions. The unbranched chain is wound into a tight coil that makes the molecule very compact.

It is suited for energy storage because it is insoluble and doesn't draw water into cells, it doesn't easily diffuse out of cells, it is compact so a lot can be stored into a small space and when hydrolysed it forms alpha glucose which is easily transported and readily used in transpiration.

Glycogen is similar to starch but has shorter chains and is more highly branched. In animals it is stored as small granules mainly in the muscles and liver.

Made up of monomers of beta glucose. The main reasons for differences in the structure and function is that in the beat glucose units the positions of the -H group and the -OH group is above the ring which means to form glycosidic links the molecule must be rotated 180 degrees.

It has straight unbranched chains that run parallel to one another allowing hydrogen bonds to form cross-linkages between adjacent chains.

The cellulose molecules are grouped to form microfibrils which are arranged in parallel groups called fibres.

It is a major component of plant cells and provides rigidity. The wall also prevents the cell from bursting as water enters by osmosis. It exerts an inward pressure that stops further influx of water.

The main features thhat suit a leaf palisade cell to photosynthesis include long, thin cells that form a layer to absorb light, chloroplasts that arrange themselves to collect a maximum amount of light and a large vacuole that pushes cytoplasm and chloroplasts to thhe edge of the cell.

Chloroplasts vary in shape and size but are disc shaped. Their main features include the chloroplast envelope, the grana where the first stage of photosynthesis occurs and the stroma where the second stage of photosynthesis occurs.

Chloroplasts are adapted to their function in the following ways:

The granal membranes provide a large SA for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis.

The fluid of the stroma possesses all enzymes needed to carry out the second stage of photosynthesis.

Chloroplasts contain DNA and ribosomes so they can quickly manufacture proteins needed for photosynthesis.

Cell wall consists of polysaccharides. There is a thin layer called the middle lamella which marks the boundary between adjacent cell walls and cements them together.

The functions of the cellulose cell wall are to provide mechanical strength and to allow water to pass along it.

Plant cells have a cellulose cell wall surounding the cell, chloroplasts present in large numbers, large single central vacuole filled with sap and starch grains that are used for storage.

Animal cells only have a cell-surface membrane surrounding the cell, no chloroplasts, if vacuoles are present they are small and scattered and glycogen granules which are used for storage.

Cell division occurs in 2 main stages:

Nuclear division where the nucleus divides through either mitosis and meiosis and cell division which follows nuclear division and is the process where the whole cell divides.

In semi-conservative replication there must be 4 requirements:

The four types of nucleotide, each with their bases must be present.

Both strands of the DNA molecule must act as a template for the attachement of these nucleotides.

The DNA polymerase is needed to catalyse the reaction.

A source of chemical energy is required to drive the process.

Takes place as follows:

The DNA helicase breaks the hydrogen bonds linking the base pairs.

The double helix seperates into its 2 strands and unwinds.

Each exposed polynucleotide acts as a template for complementary nucleotides to attach.

Energy is used to activate the nucletoides.

The nucleotides are joined by DNA polymerase to form the missing polynucleotide strand on each of the 2 original poolynucleotide strands.

Each of the new DNA molecules contains one of the original DNA strands.

Nuclear division can take place by either mitosis or meiosis: Mitosis produces two daughter nuclei that have the same number of chromosomes as the parent cell. Meiosis produced four daughter nuclei, each with half the chromosomes of the parent cell.

This is the division of the nucleus of a cell that results in each of the daughter cells having an exact copy of the DNA of the parent cell. Except in the event of a mutation.

Mitosis is always preceded by a period during which the cell is not dividing. This is called interphase. It is a period of considerable cellular activity that included the replication of DNA.

1)  Prophase in which the chromosomes become visible and the nuclear envelope disappears.

2)  Metaphase in which the chromosomes arrange themselves at the centre of the cell.

3)  Anaphase in which each of the two threads of a chromosome migrates to an opposite pole.

4)  Telophase in which the nuclear envelope reforms.

Mitosis is important because it produces daughter cells that are identical to the parent cells.

- Growth: when 2 haploid cells fuse together, the diploid cell has all the genetic info needed to resemble its parents, all cells grown from this need to possess the same info.

- Differentiation: these cells change to give groups of specialised cells. The different cell types each divide by mitosis to give tissues made up of identical cells.

- Repair: if cells are damaged or die it’s important that the new cells have an identical structure and function to those lost.

The cell cycle takes place in three stages:

1)      Interphase: this occupies most of the cell cycle and is known as the resting phase because no division takes place.

(a)  First growth phase when proteins from which cell organelles are synthesised are produced.

(b)  Synthesis phase when DNA is replicated.

(c) Second growth phase when organelles grow and divide and energy stres are increased.

2)    Nuclear division when the nucleus divides either into two or four.

3)    Cell division which follows nuclear division and is the process where the whole cell divides into two or four.

Cancer is a group of around 200 diseases caused by a growth disorder of cells. It’s the result of damage to the genes that regulate mitosis and the cell cycle. This leads to uncontrolled growth of cells and a tumour develops and expands in size. Cancers are most commonly found in the lungs, prostate gland, breast and ovaries.

The treatment often involves blocking some part of the cell cycle. In this way the cycle is disrupted and cell division and cancer growth ceases. Drugs used to treat cancer disrupt the cycle by:

- Preventing DNA from replicating

- Inhibiting the metaphase stage by interfering with the spindle formation.

The drugs also disrupt the cell cycle of normal cells.

Single-celled organism performs all essential life functions inside the boundaries of a single cell. They cannot be totally efficient at all functions because each function requires a different type of cellular structure. All cells in an organism are initially identical and as it matures, each cell takes on its own individual characteristics that suit it to the function it will perform when it’s mature. Each cell becomes specialised in structure to suit it to its role (cell differentiation).

Examples of tissues include:

Epithelial tissues: these are found in animals and are sheets of cells that line the surface of organs. They often have a protective or secretory function and there are many types including those made up of thin, flat cells that line organs where diffusion takes place.

Xylem: this occurs in plants and is made of a number of cell types. It’s used to transport water and mineral ions through the plant and gives mechanical support.

An organ is a combination of tissues that are coordinated to perform a range of functions.

In animals for example the stomach is made up of tissues such as:

 Muscle to churn and mix stomach contents.

Epithelium to protect the wall and produce secretions.

Connective tissue holds other tissues.

 In plants a leaf is made up of the following tissues:

Palisade mesophyll made of leaf palisade cells that photosynthesize.

Spongy mesophyll for gaseous diffusion.

Epidermis to protect the leaf and allow gaseous diffusion.

 Phloem to transport organic materials away from the leaf.

Xylem to transport water and ions into the leaf.

The digestive system: Digests and processes food, is made up of organs including the salivary glands, oesophagus, stomach etc.

 The respiratory system: Used for breathing and gas exchange. It’s made up of organs that include the trachea, bronchi and lungs.

The circulatory system: Pumps and circulates blood. It is made up of organs that include the heart, arteries and veins.

Examples of things which need to be interchanged between an organism and its environment include:

Respiratory gases, nutrients, excretory products, heat

 This exchange can take place in two ways:

Passively by diffusion and osmosis and actively by active transport

 To overcome the problem organisms have evolved some of the following features:

A flattened shape so no cell is far from the surface and by specialised exchange surfaces with large area to increase the surface area to volume ratio.

Gas exchange in single-celled organisms:

Oxygen is absorbed by diffusion across their body surface.

Carbon dioxide from respiration diffuses out across their body surface.

 Gas exchange in insects

The problem for all terrestrial organisms is that water easily evaporates from the surface of their bodies and they become dehydrated.

To reduce water loss, terrestrial organisms exhibit two features:

 Waterproof coverings rigid outer skeleton in insects and a small surface area to volume ratio to minimise the area over which water is lost.

These features mean insects can’t use their body surface to diffuse respiratory gases. They have therefore developed an internal network of tubes called tracheae. These are supported by strengthened rings to prevent them from collapsing. The tracheae divide into smaller tubes called tracheoles. The tracheoles extend through the body tissues.

Respiratory gases move in and out of the tracheal system in two ways:

Along a diffusion gradient: During respiration, oxygen is used and the concentration at the ends of the tracheoles decreases. This creates a diffusion gradient that causes gaseous oxygen to diffuse from the atmosphere along the tracheae and tracheoles to cells.

Carbon dioxide is produced during respiration which creates a diffusion gradient in the opposite direction. This causes gaseous carbon dioxide to diffuse along the tracheoles and tracheae from cells to the atmosphere.

Ventilation: The movement of muscles in insects can create mass movements of air in and out of the tracheae.

Fish have a waterproof, gas-tight outer covering. They also have a small surface area to volume ratio. Their body surface is not adequate to supply and remove their respiratory gases and so they have developed a specialised internal gas exchange surface.

Structure of the gills

The gills are located in the body of the fish behind the head. They are made up of gill filaments. These are stacked up in a pile and at right angles to the filaments are gill lamellae, which increase surface area of the gills. Water is taken in through the mouth, forced over the gills and through an opening on the side of the body. The flow of water over the gill lamellae and the blood within them are in opposite directions which is known as counter current flow.

The counter current exchange principle

- Blood is already well loaded with oxygen meets water which has its max concentration of oxygen. This means diffusion of oxygen from water to blood takes place.

- Blood with little or no oxygen meets water which has had most but not all of its oxygen removed. Diffusion of oxygen from water to blood takes place.

Structure of a plant leaf and gas exchange

There is a short diffusion pathways and a plant leaf has a very large surface area to volume ratio.

Most gaseous exchange occurs in leaves which show the following adaptations for quick diffusion:

Thin, flat shape that provides a large surface area.

Stomata, mostly in the lower epidermis.

Interconnecting air-spaces that occur throughout the mesophyll.

Stomata

These are minute pores which occur mainly in the leaves. Each stoma is surrounded by a pair of special cells (guard cells). These can open and close the stomatal pore. Therefore they can control the rate of gaseous exchange.

Why large organisms need a transport system

With increasing size, the surface area to volume ration decreases so the needs of the organism cannot be met by the body surface.

A specialist exchange surface is needed to absorb nutrients and respiratory gases and to remove excretory products. These are located in specific regions of the organism.

A transport system is required to take materials from cells to exchange surfaces and vice versa.

Whether of not there is a specialised transport medium and whether or not It’s circulated by a pump depends on 2 factors:

The surface area to volume ratio.

How active the organism is.

Features of transport systems

- A suitable medium to carry materials (normally liquid based on water).

- Form of mass transport.

- A closed system of tubular vessels that contains the transport medium and forms a branching network to distribute it.

- A mechanism for moving the transport medium within the vessels.

   (a) Animals use muscular contraction.

   (b) Plants rely on physical processes.

- A mechanism to maintain the mass flow in one direction.

- A means of controlling the flow of the transport medium.

Arteries carry blood away from the heart into the arterioles    - Tough outer layer- resists pressure changes. - Muscle layer- contract and control the flow of blood. - Elastic layer- maintains blood pressure by stretching and springing back. - Thin inner lining- prevents friction and allows diffusion. - Lumen- central cavity of the blood vessel where blood flows.  Artery structure related to function - Muscle layer is thicker than veins: Smaller arteries can be constricted and dilated to control the volume of blood passing through. - Elastic layer is thicker than veins: The wall is stretched at the beat of the heart and springs back when the heart relaxes. This helps maintain a high pressure. - Overall thickness of the wall is large: Resists the vessel bursting under pressure. - There are no valves: Because blood is under constant high pressure.

Arterioles are smaller arteries that control blood flow from arteries to capillaries. Same layered structure as arteries.   Arteriole structure related to function   Muscle layer is thicker than arteries: The contraction of this layer allows constriction of the lumen. This restricts the flow of blood and controls its movement.  Elastic layer is thinner than arteries:  Because blood pressure is lower.  

Capillaries are tiny vessels that link arterioles to veins. Same layered structure as arteries.   

- Walls consist of lining layer: For short diffusion distance. - Numerous and highly branched: Large surface area. - Narrow diameter: Permeate tissues. - Lumen is narrow: Reduces diffusion distance. - Spaces between lining: So white blood cells can escape.

Veins carry blood from capillaries back to the heart. Same layered structure as arteries.      - Muscle layer is thin: Because veins carry blood away from tissues. - Elastic layer is thin: Because low pressure of blood will not cause them to burst. - Thickness of wall is thin: As pressure is low. - There are valves: To prevent backflow.    

Tissue fluid contains glucose, amino acids, fatty acids, salts and oxygen. It’s formed in the blood plasma.

Blood pumped around the heart creates hydrostatic pressure at the arterial end of the capillaries which forces tissue fluid out of blood plasma.

The outward pressure is opposed by:

Hydrostatic pressure of tissue fluid outside capillaries and lower water potential of the blood.

The pressure is only enough to force small molecules out leaving cells and

proteins in the blood. This is called ultra filtration.

Return of tissue fluid to the circulatory system.

- The loss of the tissue fluid from the capillaries reduces the hydrostatic pressure inside.

- By the time the blood reaches the venous end the hydrostatic pressure is less than the tissue fluid outside of it.

- Tissue fluid is forced back into the capillaries.

-Osmotic forces pull water back into the capillaries.

Not all the tissue fluid can return to the capillaries so the remainder is carried back via the lymphatic system. This is a system of vessels that begin in the tissues. The contents of the lymphatic system are moved by:

- Hydrostatic pressure

- Contraction of body muscles.

Each root hair is a long, thin extension of a root epidermal cell. They are efficient surfaces for the exchange of water and mineral ions because:

- They provide a large surface area as they are long.

- They have a thin surface layer.

The apoplastic pathway:

As water is drawn into endodermal cells, it pulls more water along due to the cohesive properties of the water molecules. This creates a tension that draws water along the cell walls. The mesh-like structure of the cellulose cell walls has water-filled spaces so there is little resistance to the pull of water.

The symplastic pathway:

This takes place across the cytoplasm as a result of osmosis. The water passes through the cell walls along the plasmodesmata. Each plasmodesma is filled with cytoplasm.

1)      Water entering by osmosis increases the water potential of the r.h.c.

2)    The r.h.c has a higher w.p than the first cell in the cortex.

3)    Water moves from the r.h.c to the first cell by osmosis.

4)    The first cell has a higher w.p than its neighbour in the stem.

5)    Water moves into the neighbour.

6)    The neighbour has a higher w.p so it moves to the 3^rd cell.

7)    The loss of water from the first cell lowers its water potential causing more water to enter.

8)    A water potential gradient is set up which carries water along the cytoplasm to the endodermis.

When the water reaches the endodermis the waterproof band making up the Casparian ***** prevents it progressing. Water is then forced into the protoplast where it joins water that’s arrived by the symplastic pathway.

Active transport of salts is most likely to allow water into the xylem. This requires energy and takes place along carrier proteins.

The active transport of mineral ions creates lower water potential in the xylem. Water moves into the xylem by osmosis which creates a force to move water up the plant (root pressure).

The humidity of the atmosphere is less than of the air spaces next to the stomata. If the stomata are open, water vapour molecules diffuse out into surrounding air. Water lost from the air spaces is replaced by evaporating water from cell walls of the mesophyll cells.

Movement of water across the cells of a leaf

Water is lost from mesophyll cells by evaporation to the air spaces of the leaf. This is replaced by water reaching the mesophyll cells from the xylem by either the apoplastic or symplastic pathways. In the case of the symplastic pathway, the water movement occurs because:

- Mesophyll cells lose water to air spaces.

- The cells have lower water potential so water enters.

- Loss of water from neighbouring cells lowers their water potential.

- They take in water from their neighbours by osmosis.

The two main factors responsible for the movement of water up the xylem are cohesion-tension and root pressure. The cohesion-tension theory operates as follows:

1)      Water evaporates from leaves by transpiration.

2)    Water molecules form H bonds and stick together (cohesion).

3)    Water forms a continuous pathway across mesophyll cells and down the xylem.

4)    As water evaporates from the mesophyll cells more molecules are drawn up.

5)    Water is pulled up the xylem (transpirational pull)

6)    Transpirational pull puts xylem under tension.

 Transpirational pull is passive so it doesn’t require energy. The xylem vessels are dead so can’t actively move the water.

Leaves have a large surface area to absorb light and stomata allow inward diffusion of CO2. Both features result in loss of water. Transpiration is not essential because osmosis could achieve this.

Materials e.g. mineral ions, sugars and hormones are moved around the plant. The water carrying them is carried up the plant by transpirational pull.

· Light: Stomata are the openings where CO2 diffuses. Photosynthesis only occurs when there’s light so stomata are open and close in the dark. When open water leaves into the atmosphere. Increase in light increases the rate of transpiration.

· Temperature: This affects how much water air can hold and the speed at which water molecules move. A rise in temperature increases kinetic energy and speed of water molecules therefore increasing evaporation. It also decreases the amount of water air can hold.

· Humidity: This is the measure of the number of water molecules in the air. Humidity affects the water potential gradient between the air outside and inside the leaf. When air outside has high humidity the gradient is reduced and rate of transpiration is lower.

· Air movement: As water diffuses through stomata, it collects as vapour around them on the inside of the leaf. The water potential is therefore increased which reduces the water potential gradient between the atmosphere and air spaces. The transpiration rate is therefore reduced.

· Thick cuticle: The thicker the cuticle, the less water can escape. Many evergreen plants for example holly have thick cuticles to reduce water loss especially during winter.

· Rolling up of leaves: Most leaves have their stomata confined to the lower epidermis. The rolling of leaves protects the lower epidermis from the outside and helps trap a region of still air. The region becomes saturated with water vapour so there is no water potential gradient between the inside and outside. Plants such as marram grass roll their leaves.

· Hairy leaves: A thick layer of hairs on leaves help trap moist air next to the leaf surface. The water potential gradient between inside and outside is reduced therefore less water is lost through transpiration. One type of heather plant has this.

· Stomata in pits or grooves: These also trap moist air and reduce the water potential gradient. Pine trees use this modification.

· Reduced surface area to volume ratio of the leaves: By having leaves that are small and roughly circular in cross-section e.g. pin needles, the rate of water loss can be reduced.

The definition of a species is not easy, but members of a single species have certain things in common:

- They are similar to one another but different from other species: They have very similar genes and closely resemble each other physically and biochemically. They have similar patterns of development and occupy the same ecological niche.

- They are capable of breeding: This means when a species reproduces, any of the genes of the genes of its individuals can be combined and belong to the same gene pool.

  Over 200 years ago the Swedish botanist Linnaeus devised a common system of naming organisms. Its features are as follows:

- It’s a universal system based on Greek or Latin names.

- The first name, the generic name, denotes the genus to which the organism belongs.

- The second name, the specific name, denotes the species where the organism belongs.

- The names are printed italics or if handwritten underlined.

- The first letter of the generic name is in upper case but the specific name in lower case and if the specific name is not known and can be written as ‘sp’.

 Grouping species together- the principles of classification

The theory and practise of biological classification is called taxonomy.

There are two forms of biological classification:

- Artificial classification: Divides according to differences such as colour, size, number of legs etc. These are analogous characteristics where the have the same function but not the same evolutionary origins.

- Natural classification:

a) Based on evolutionary relationships between organisms and ancestors.

b) Classifies species into groups sharing features derives from ancestors.

c) Arranges into a hierarchy.

Each group is called a taxon. The groups are positioned in a hierarchal order called taxonomic ranks. The largest group is called a kingdom within each kingdom the largest groups are known as phyla. The ranks are Kingdom, Phylum, Class, Order, Family, Genus and Species.

Phylogeny

The phylogeny of an organism reflects the evolutionary branch that led up to it. The

phylogenetic relationships of different species are represented by a tree like diagram called

a phylogenetic tree.

- Species are not fixed forever and change and evolves over time.

- Within a species is a lot of variation.

- Many species are extinct.

- Some species rarely reproduce.

- Members of different groups of the same species may be isolated and never meet or interbreed.

- Groups of isolated organisms may be classified as a different species.

- Some species are sterile.

One way to determine similarities between the DNA of different organisms is to use DNA hybridisation.

This depends upon a particular property of the DNA double helix. When DNA is heated,

its double strand separates into two complementary strands. When cooled, the bases on

each strand recombine with each other to reform the original double strand.

1)    DNA from 2 species is extracted, purified and cut. DNA from one species is labelled with a radioactive or fluorescent marker andxed with the other DNA.

2)  Mixture of DNA is heated to separate the strands. It's cooled to allow the strands to combine.

3)  Some double strands that reform will be made of one strand from each species (hybridisation). New strands are called hybrid strands (50% labelled).

4) Hybrid strands are separated out and temp is increased.

5) At each temp stage the degree that the 2 strands are linked is measured.

6)  If the species are closely related they will share complementary bases.

7) More H bonds linking them in the hybrid strand.

8)e greater the amount of H bonds the stronger the hybrid strand.

9) The stronger the strand the higher the temp to separate it.

10) The higher the temp where it splits, the more related the 2 species are.

· Serum albumin from species A is injected to B.

· Species B produces antibodies specific to antigen sites on the albumin from A.

· Serum is extracted from B that contains antibodies specific to antigens on albumin for A.

· Serum from B is mixed from serum from blood of a third species C.

· The antibodies respond to their corresponding antigens on the albumin of C.

· The response is the formation of a precipitate.

· The greater the number of similar antigens, the more precipitate formed and the closer they are related.

Courtship behaviour helps ensure that mating is successful by enabling individuals to:

- Recognise members of their own species: To ensure that mating only takes place between members of the same species.

- Identify a mate that is capable of breeding: because both partners need to be sexually mature, fertile and receptive to mating.

- Form a pair bond: that will lead to successful mating and rising of offspring.

- Synchronise mating: so that it takes place when there is the maximum probability of the sperm and egg meeting.

Courtship behaviour is used by males to determine whether a femal is at the receptive stage. If she responds with the appropriate behavioural response it continues and is likely to result in the production of offspring. During courtship, animals use signals to communicate with mates and members of their own sex. Males carry out actions which acts as a stimulus to the female, who responds with a specific action of her own.

Mutations are changes in DNA that result in different characteristics. They arise in many ways for example, some bases may be added, deleted or replaced during replication. Any differences in the base sequence of a DNA molecule results in a different amino acid sequence being coded for which will lead to a differet polypeptide. This means a different protein which can disrupt the metabolic pathway leading to production of other substances.

This occurs when one bacterial cell transfers DNA to another and takes place as follows:

- One cell produces a thin projection that meets another cell and forms a thin conjugation tube between the 2 cells.

- The donor cell replicates one of its small circular pieces of DNA (plasmid).

- The circular DNA os broken to make it linear before it passes along the tube to the recipient cell.

- Contact between the cells is brief leaving time for only a portion of the donor’s DNA to be transferred.

- The recipient cell acquires new characteristics from the donor cell.

One way they work is to prevent bacteria from making normal cell walls.

In bacterial cells, water constantly enters by osmosis. This entry of water would normally cause the cell to burst- osmotic lysis. It doesn’t burst because of the cell wall that surrounds all of the bacterial cells. The wall is made of tough material that isn’t easily stretched. As water enters the contents expand and push against the cell wall.

Certain antibiotics kill bacteria by preventing them forming cell walls. They inhibit the synthesis and assembly of the important peptide cross-linkages in the bacterial cell walls. This weakens the walls making them unable to withstand pressure. As a result they are unable to prevent water entering and osmotic lysis occurs.

The gene for penicillinase and hence antibiotic resistance is passes from one generation to the next by vertical gene transmission.

The allele for the resistance is carried on the circular loops of DNA called plasmids. These can be passed from cell to cell by conjugation. Resistance can find its way into other bacterial species by horizontal gene transmission.

Horizontal gene transmission can lead to certain bacteria accumulation DNA that gives them resistance to a range of antibiotics.

Antibiotic resistance is on the increase for a number of reasons:

- Antibiotics are used to treat minor ailments whose symptoms are trivial/short lived.

- Antibiotics are sometimes used to treat viral diseases; they may help prevent the development of secondary bacterial infections.

- Patients do not always complete the course of antibiotics.

- Patients stockpile unused antibiotics.

- Doctors accept patients’ demands for treatments.

- Antibiotics are used in the treatment of minor ailments in animals.

- They are used to prevent disease among intensively reared animals.

Biodiversity is the term used to describe variety in the living world. It refers to the number and variety of living organisms in a particular area and has three components:

· Species diversity: The number of different species and the number of individuals of each species within one community.

· Genetic diversity: The variety of genes possessed by the individuals that make up one species.

· Ecosystem diversity: The range of different habitats in a particular area.

One measure of biodiversity is species diversity and has 2 components:

- The number of different species in a given area.

- The proportion of the community that is made up of one species.

One way of measuring species diversity is to use an index calculated as follows: 

D=   N ( N-1)

∑n (n-1)

Where:

d= species diversity index

N= total number of organisms of all species

n= total number of organisms of each species

∑= the sum of

Impact of agriculture

As natural ecosystems develop them become complex communities with many individuals of a large number of different species. Agricultural ecosystems are controlled by humans.

Farmers select species for particular qualities meaning the number of species, and genetic variety of alleles they possess is reduced.  Any particular area can only support a certain amount of biomass.

Impact of deforestation

As forests form layers between the ground and tops of trees there are numerous habitats available. Different species are adapted to living in these habitats and species diversity is high.

The most serious consequence is loss of biodiversity. Up to 50000 species are lost each year.