Essays on organisms, populations and the environment

Question 1

Write an essay on the following topic.

The importance of controlling water loss in organisms.

Accepted Answer

Terrestrial insects and xerophytic plants show a compromise between efficient gas exchange and limiting water loss. Insects have a large surface area to volume ratio, so they could dry out quickly. Their exoskeleton is covered by a waterproof waxy cuticle, gas exchange takes place inside the body through the tracheal system, and the spiracles can be closed by valves when the insect is inactive; hairs around the spiracles trap moist air. Xerophytes such as marram grass have a thick waxy cuticle, rolled leaves, stomata sunk in pits and hairs. These trap a layer of humid air next to the stomata, reducing the water potential gradient between the inside of the leaf and the air, so less water vapour diffuses out. Controlling water loss is important because an insect or plant that lost water faster than it could replace it would die from dehydration. However, spiracles and stomata must open for oxygen and carbon dioxide to diffuse in, so water loss can only be reduced, not prevented.

The waxy cuticle works because of the properties of lipids. Like triglycerides, waxes contain long hydrocarbon chains that are non-polar. Water molecules are polar, so they are not attracted to these chains and cannot easily pass through them. A layer of lipid on the surface of a leaf or an insect's exoskeleton is therefore hydrophobic and waterproof. This is important because it allows organisms to live on land in dry air, where evaporation would otherwise remove water from every exposed surface; plants of dry habitats have much thicker cuticles than plants of shaded, humid places.

In plants, some water loss is unavoidable and is even useful. Transpiration, the evaporation of water from mesophyll cells and its diffusion out through the stomata, creates tension in the xylem, and cohesion between water molecules, due to hydrogen bonds, pulls a continuous column of water up from the roots. This transpiration stream carries mineral ions such as nitrate to the leaves and supplies water for photosynthesis. Controlling the rate of loss is therefore a balance. When water is scarce, the guard cells lose water and the stomata close, so transpiration falls and the plant does not wilt; but closed stomata also reduce the uptake of carbon dioxide, so the rate of photosynthesis falls. Some desert plants open their stomata only at night, when it is cooler and the air is more humid, and store the carbon dioxide they absorb as an organic acid, which releases carbon dioxide for the Calvin cycle during the day while the stomata are closed. This greatly reduces water loss.

Mammals control the water lost in urine by osmoregulation. When the water potential of the blood falls, for example after sweating, osmoreceptors in the hypothalamus detect the change and the posterior pituitary releases more antidiuretic hormone (ADH). ADH binds to receptors on the cells of the distal convoluted tubule and collecting duct and makes them more permeable to water, because more water channel proteins (aquaporins) are inserted into their membranes. The loop of Henle maintains a high concentration of sodium ions in the medulla, so the tissue fluid there has a low water potential, and water leaves the collecting duct by osmosis and is reabsorbed into the blood. A small volume of concentrated urine is produced. This is important because it returns the water potential of the blood to normal by negative feedback, so cells do not lose water by osmosis, which would disrupt their metabolism. Desert mammals such as the kangaroo rat have very long loops of Henle, which produce a steeper gradient in the medulla, so they produce very concentrated urine and can survive with almost no drinking water.

Behaviour can also control water loss. Woodlice lose water easily because their exoskeleton lacks a thick waxy layer. They show a kinesis, a non-directional response in which the speed of movement depends on the intensity of the stimulus: in dry air they move faster, so they are likely to leave the dry area quickly, and in humid air they slow down, so they spend more time there. This is important for their survival because it keeps them in damp places, such as under logs and stones, where evaporation from their bodies is slow; these places also hide them from predators.

Water loss is also important in succession. Pioneer species, such as lichens on bare rock or marram grass on sand dunes, can colonise only because they limit water loss in dry, exposed conditions with little soil to hold water; marram grass, for example, has rolled leaves with stomata sunk in pits. When they die, saprobionts decompose them and the organic matter forms humus, which increases the depth of the soil and its ability to retain water. The environment becomes less hostile, so species that are less able to control water loss can colonise, and these may out-compete the pioneers, changing the community step by step towards a climax community. Because the availability of water changes at each stage, adaptations for controlling water loss determine which species can survive at each stage, and conserving habitats such as sand dunes depends on understanding and managing this succession.

Question 2

Write an essay on the following topic.

The importance of efficiency in living organisms and ecosystems.

Accepted Answer

Energy transfer between trophic levels is inefficient, and this shapes ecosystems. Of the chemical energy in the food an animal ingests (I), some is lost in faeces and urine (F) and much is released in respiration (R) and lost as heat, so its net production is N = I - (F + R). Often only about 10% of the energy at one trophic level becomes biomass at the next, because not all of each organism is eaten or digested and because respiratory losses are large. This is important because it limits food chains to a few trophic levels and means that far more people can be fed from an area of crops than from livestock fed on those crops. Farmers increase the efficiency of energy transfer to humans by simplifying food webs, using herbicides and pesticides so that less energy is lost to weeds and pests, and by reducing respiratory losses in livestock, for example by keeping animals indoors in warm conditions with restricted movement, so that more of their food energy goes into growth. These methods raise concerns about animal welfare.

Photosynthesis itself captures only a few per cent of the light energy reaching a plant, because some light is reflected or passes through the leaf, much is of wavelengths that chlorophyll does not absorb, and energy is lost as heat during the reactions. Efficiency also depends on limiting factors: when carbon dioxide concentration or temperature limits the light-independent reaction, the ATP and reduced NADP made in the light-dependent reaction cannot be used as fast as they are produced. This is important because the gross primary production of an ecosystem sets the energy available to every other organism in it; increasing the efficiency of photosynthesis in crops, for example by raising carbon dioxide concentration in glasshouses, increases yield.

Aerobic respiration releases energy from glucose far more efficiently than anaerobic respiration. Glycolysis alone gives a net gain of only two molecules of ATP per glucose, whereas the link reaction, the Krebs cycle and oxidative phosphorylation, in which protons that have been moved across the inner mitochondrial membrane diffuse back through ATP synthase, produce many more. Even so, only roughly a third of the chemical energy in glucose is conserved in ATP; the rest is released as heat. Breaking the oxidation of glucose into many small steps is important because energy is released in small amounts that can be used to make ATP, rather than all at once. In endotherms such as mammals, the heat helps to maintain a constant body temperature, but it means that mammals convert a smaller proportion of their food into new biomass than ectotherms such as fish, which use less of their food in respiration.

Gas exchange in fish is made efficient by counter-current flow. Water flows over the gill lamellae in one direction while blood flows through them in the opposite direction, so at every point the blood meets water with a higher oxygen concentration than itself. A diffusion gradient is maintained along the whole length of the lamella, and around 80% of the oxygen in the water can be absorbed, compared with about 50% if the water and blood flowed in the same direction, when their concentrations would become equal. This efficiency is important because water contains much less oxygen than air and is dense, so moving it over the gills uses energy; extracting as much oxygen as possible from each volume of water supports the high rate of respiration of active fish.

Myelination makes the conduction of nerve impulses both faster and more efficient. In a myelinated motor neurone, Schwann cells wrap the axon in a myelin sheath, which insulates it, so depolarisation can occur only at the nodes of Ranvier between the Schwann cells. The action potential therefore jumps from node to node (saltatory conduction), which is much faster than in a non-myelinated axon of the same diameter, where depolarisation must pass along the whole membrane. Because ions cross the membrane only at the nodes, fewer sodium and potassium ions move during each impulse, so less ATP is needed by the sodium-potassium pump to restore the resting potential. This is important because rapid impulses allow fast responses, such as a reflex that withdraws a hand from a hot object, at a low cost in energy.

The digestion of proteins in mammals is efficient because enzymes act in sequence. Endopeptidases hydrolyse peptide bonds within polypeptide chains, producing many shorter chains and so more ends; exopeptidases then hydrolyse the peptide bonds at the ends of the chains, releasing dipeptides and single amino acids; and dipeptidases bound to the membranes of the microvilli of the ileum hydrolyse dipeptides into amino acids. The amino acids are therefore produced right at the surface where they are absorbed, by co-transport with sodium ions. This is important because it increases the rate of digestion and the proportion of protein that is absorbed, so fewer amino acids and less chemical energy are lost in faeces, improving the efficiency with which an animal uses its food.

Question 3

Write an essay on the following topic.

The importance of defence mechanisms in organisms.

Accepted Answer

Mammals defend themselves against pathogens by non-specific and specific mechanisms. Phagocytes recognise pathogens as foreign by their antigens and engulf them by phagocytosis, enclosing them in a phagosome; lysosomes fuse with the phagosome and release lysozymes, which hydrolyse the pathogen. The phagocyte then presents the pathogen's antigens on its cell-surface membrane, becoming an antigen-presenting cell. In the cellular response, helper T cells with complementary receptors bind to the presented antigens and are activated; they stimulate cytotoxic T cells, which kill infected body cells, as well as phagocytes and B cells. In the humoral response, a B cell with a complementary antibody on its surface is selected (clonal selection), divides by mitosis and forms plasma cells, which secrete antibodies, and memory cells. Antibodies bind to antigens and cause agglutination of pathogens, making phagocytosis easier, and memory cells allow a faster and larger secondary response if the same antigen is met again, which is the basis of vaccination. This defence is important because it can destroy almost any pathogen and gives long-term immunity; its importance is shown by HIV, which destroys helper T cells, so that people with AIDS die from infections that the immune system would normally overcome.

The simple reflex is a defence against physical damage. When a receptor in the skin is stimulated by a harmful stimulus such as a hot object, impulses pass along a sensory neurone to a relay neurone in the spinal cord and then along a motor neurone to a muscle, which contracts and withdraws the hand. Because only three neurones are involved and the brain does not have to make a decision, the response is very rapid and automatic. This is important because the tissue is removed from the harmful stimulus before serious damage occurs, which also reduces the chance of pathogens entering through damaged skin. Other reflexes, such as blinking when an object approaches the eye and coughing when particles enter the airways, protect delicate tissues in the same way.

Cells also defend the body against cancer. Tumour suppressor genes code for proteins that slow down the cell cycle, repair damaged DNA or cause cells with damaged DNA to die by apoptosis. One such protein, p53, accumulates when DNA is damaged and stops the cell cycle until the damage is repaired, or triggers apoptosis if it cannot be repaired. If a tumour suppressor gene is inactivated by a mutation, or by increased methylation that prevents its transcription, this defence is lost, so cells with damaged DNA continue to divide uncontrollably and may form a malignant tumour. The immune system provides a second line of defence, because cytotoxic T cells can recognise and destroy some cancer cells that display abnormal antigens. Tumour suppressor genes are therefore important because they stop mutations, which arise spontaneously during DNA replication, from producing cancers.

Bacteria also have defences, and natural selection makes them more common. A random mutation can produce an allele that gives resistance to an antibiotic, for example by coding for an enzyme that breaks the antibiotic down. When the antibiotic is used, susceptible bacteria are killed but resistant bacteria survive and reproduce by binary fission, passing on the allele, which can also be transferred to other bacteria on plasmids. This directional selection increases the frequency of the resistance allele in the population. It is important for the bacteria, because their defence allows them to survive, but harmful to humans, because antibiotics, one of our most important defences against bacterial disease, become ineffective, as with MRSA. Many antibiotics are themselves chemical defences of other microorganisms: the fungus Penicillium produces penicillin, which prevents bacteria from building their cell walls and so reduces competition from bacteria.

RNA interference (RNAi) is a defence against viruses in many eukaryotes. Many viruses produce double-stranded RNA when they replicate inside a host cell. In plants and insects, an enzyme cuts this into short pieces called small interfering RNA (siRNA). One strand of each siRNA combines with a protein complex and binds, by complementary base pairing, to viral mRNA, which is then cut up, so the viral proteins cannot be translated. This is important because it stops the virus from replicating; the same mechanism can be used by scientists to silence specific genes, and it is being used to make crop plants resistant to viral diseases.

Blood clotting is another defence. When a blood vessel is damaged, platelets stick to the exposed collagen and, together with the damaged tissue, release clotting factors. These start a cascade of reactions in which prothrombin is converted into the enzyme thrombin, a step that needs calcium ions; thrombin then hydrolyses the soluble plasma protein fibrinogen to form insoluble fibrin. Fibrin forms a mesh of fibres that traps red blood cells and platelets, forming a clot. This is important because it prevents the loss of blood, which would lower blood pressure and the delivery of oxygen to tissues, and seals the wound so that pathogens cannot enter.

Question 4

Write an essay on the following topic.

The ways in which the environment influences the phenotype of organisms.

Accepted Answer

The phenotype of an organism is the expression of its genetic constitution and its interaction with the environment. Many characteristics, such as height in humans or the yield of a crop, show continuous variation because they are controlled by many genes and are also strongly affected by environmental factors, such as diet or the supply of light, water and mineral ions. Two plants with identical genotypes, produced by cloning, may grow to very different sizes if one lacks nitrate, which it needs to make amino acids and nucleotides, or magnesium, which it needs to make chlorophyll. In the same way, the average height of people in many countries has increased over a few generations as nutrition has improved, a change far too rapid to be explained by changes in allele frequencies. This is important because natural selection acts on phenotypes, and because plant and animal breeders, and doctors studying disease, need to separate genetic and environmental effects, for example by comparing identical twins brought up in different families.

Epigenetics is a mechanism by which the environment alters gene expression without changing the base sequence of DNA. Environmental factors such as diet, stress or toxins can lead to increased methylation of DNA, in which methyl groups are added to cytosine bases, or to decreased acetylation of histones. Methylated DNA prevents transcription factors from binding, and decreased acetylation makes the chromatin more condensed, so in both cases transcription of the gene is inhibited. These epigenetic changes are passed on to daughter cells when a cell divides, and some may be passed to offspring. This is important because it explains why identical twins become more different as they get older, and why studies of people whose mothers experienced famine while pregnant found differences in DNA methylation and in their risk of disease decades later.

Environmental factors also influence the development of cancer. Mutagenic agents such as ultraviolet light and carcinogens in tobacco smoke increase the rate of gene mutation, and if mutations inactivate tumour suppressor genes or convert proto-oncogenes into oncogenes, cells divide uncontrollably. Abnormal methylation can have the same effect: hypermethylation of tumour suppressor genes prevents their transcription, and hypomethylation of oncogenes activates them. Increased concentrations of oestrogen, for example produced by fat tissue in the breast after the menopause, are linked with the development of some breast cancers. Much of the evidence shows correlations between environmental factors and cancer, which do not on their own prove a causal relationship, but this knowledge is important because changing the environment, for example by not smoking and avoiding sunburn, reduces the risk of cancer.

Type II diabetes shows the interaction of genes and environment in a physiological phenotype. Some people inherit a higher susceptibility, but the condition usually develops in people who are obese, have a diet high in refined sugars and take little exercise. Their body cells, particularly in muscle and the liver, become less responsive to insulin because their insulin receptors respond less, so less glucose is taken up and converted to glycogen and the blood glucose concentration remains high. Unlike type I diabetes, in which the beta cells do not produce insulin, type II diabetes can often be controlled by changing the environment: a controlled diet, weight loss and regular exercise. This is important because the same genotype may or may not lead to disease, depending on lifestyle.

In plants, the direction of growth is determined by the environment. In a shoot lit from one side, IAA moves to the shaded side and stimulates cell elongation there, so the shoot bends towards the light. In roots, IAA accumulates on the lower side, where a high concentration inhibits cell elongation, so the root grows downwards. Genetically identical seedlings therefore develop different shapes in different light conditions. Seedlings grown in the dark become tall, thin and yellow, because they elongate rapidly and do not make chlorophyll, whereas plants grown in shade often have larger, thinner leaves than plants of the same species in full light. This is important because such responses allow plants to adjust their phenotype to make the best use of the light available for photosynthesis.

Temperature can influence phenotype through its effect on enzymes. In Himalayan rabbits and Siamese cats, an allele codes for a temperature-sensitive form of tyrosinase, an enzyme needed to make the dark pigment melanin. At normal body temperature the tertiary structure of this enzyme is changed, so its active site no longer binds its substrate and no pigment is made; in the cooler extremities, such as the ears, nose, paws and tail, the enzyme is active and the fur is dark. If a patch of fur on the back is shaved and kept cold, the new fur grows dark. This is important because it shows clearly that the same allele can produce different phenotypes in different environments, through the effect of temperature on the tertiary structure of a protein.

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