extended answer biology

Describe the metabolic events of germination in a starchy seed. 9.4 (5)

The absorption of water is followed by the formation of gibberellin in the embryo's cotyledon. This stimulates the production of amylase which catayese the breakdown of starch into maltose. This maltose then diffuses to the embryo for energy release and growth.

Outline the importance of acid conditions in the stomach. 6.1 (3)

Acid conditions in the stomach favor some hydrolysis reactions and help to control pathogens in ingested food. The acid conditions within the stomach convert the inactive pepsinogen to pepsin. It also leads to the denaturing of proteins, exposing the polypeptide chains so that the enzyme pepsin can hydrolase the bonds within the polypeptides.

Outline the role of condensation and hydrolysis in the relationship between amino acids and polypeptides. 2.4 (4)

Amino acids are linked together by condensation reactions to form polypeptides, this forms a molecule of water, and a new peptide bond between the two amino acids. Hydrolysis is the breaking of a peptide bond between two amino acids in a polypeptide chain. This requires the addition of water.

Distinguish between aerobic and anaerobic respiration

Anaerobic respiration occurs in the cytoplasm. In contrast aerobic respiration only occurs in the presences of oxygen, has high yield of ATP, and occurs in the mitochondria.

Describe how, in microscope images, blood vessels can be identified as arteries, capillaries and veins. 6.2 (3)

Blood vessels can be identified as arteries, capillaries, or veins by looking at their structure. Arteries have a thick wall as narrow lumen, veins have a thin wall and a wide lumen. While capillaries have thin walls with one layer of cell.

Heart rate is affected by the hormone epinephrine. The action of epinephrine is mediated by a chemical called a second messenger. Explain the mechanism of action of a second messenger. D.5 (3)

Binding of hormones to membrane receptors activates a cascade reaction mediated by a second messenger inside the cell. In the case of epinephrine, when it reaches the liver, it binds to G-protein couple receptor. This activates the G-protien which uses GTP as an energy source to activate the enzyme adenylyl cyclase, which converts ATP to cAMP a common second messenger. cAMP then activated another enzyme which is responsible to breakdown and inhibit glycogen synthesis.

Compare simple diffusion with facilitated diffusion as mechanisms to transport solutes across membranes. 1.4 (6)

Both simple and facilitated diffusion move particles across membranes without the use of ATP, i.e. they are passive forms of transport. Simple diffusion can only occur when particles diffuse from an area of high concentration to an area of low concentration. Similarly in facilitated diffusion particles pass through channels from areas of high concentration to areas of low concentration. Simple diffusion can only happen if the phospholipid bilayer is permeable to particles, that means non polar particles such was oxygen can diffuse easily. In contrast facilitated diffusion allows ions and particles that can not easily diffuse through phospholips, to get through the plasma membrane using channels. Channels are narrow holes with walls that consist of proteins. The chemical properties and diameter of wall ensure that only one type of particle passes through.

The pumping of blood is a vital process. Explain the roles of the atria and ventricles in the pumping of blood. 6.2, D.4 (4)

Each side of the heart has two chambers, a ventricle that pumps blood out into the arteries and an atrium that collects blood from the veins and passes it to the ventricle. The passing of blood between arteries and veins is regulated by valves. Ventricles pump blood at high pressure because of their thicker muscular walls. Left ventricle pumps oxygenated blood to body tissue and right ventricle pumps deoxygenated blood to the lungs to become oxygenated.

Describe the lock and key model of enzyme activity and how the induced fit model extends it. 2.5 (5)

Enzymes have an active site to which specific enzymes bind. The shape and chemical properties of the active sites and substrate match each other. This allows the specific substrate to bind and prevents other substances from binding. The induced fit model acts as an extension to the lock and key model, and refers to the conformational change of the active site to provide a better and more efficient fit for the substrate to bind with the active site.

Outline the role of hydrolysis in the relationship between monosaccharides, disaccharides and polysaccharides 2.3 (4)

Hydrolysis is the breaking of bonds through the addition of water. In the case of carbohydrates the hydrolysis reaction results in the breaking of glycosidic bonds in a polysaccharides which is a chain of monosaccharides. Repeatedly breaking these bonds using hydrolysis results in disaccharides and eventually monosaccharides.

Explain the process involved in the Krebs cycle. 8.2 (8)

In the krebs cycle the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers (NAD+ and FAD). For every turn of the cycle which requires 1 acetyl CoA from the previous link reaction, the production of reduced NAD occurs three times, decarboxylation occurs twice which produces two molecules of CO2, and the reduction of FAD occurs once. Once molecule of ATP is also generated.

Explain the propagation of electrical impulses along a neuron including the role of myelin. 6.5 (7)

Nerve impulses are action potential propagated along the axons of neurons. Myelinated nerve fibers transmit nerve impulses much more rapidly than unmyelinated nerve fibers. The propagation of the action potential happens because the sodium ion movements that depolarize one part of the neuron trigger depolarization in the neighbouring part of the neuron. This is due to the movement of sodium ions and the difference in concentration outside and inside the axon. Inside the axon there is a higher sodium ion concentration in the depolarized part of the axon so sodium ions diffuse along inside the axon to the neighbouring part that is still polarized. These movements are called local currents. This causes the potential to rise from the resting potential of -70mV to about -50mV. Sodium channels are voltage gated and only open when the negative -50mV is reached. Outside the axon the concentration gradient is in the opposite direction so sodium ions diffuse from the polarized part back to the part that has just depolarized.

Some prokaryotes cause infectious diseases which stimulate the body's immune system. Outline the principles that form the basis of immunity. 6.3 (6)

Our immunity is made up of two systems. The skin and mucus membranes form a primary defense against pathogens that cause infectious diseases. The skin acts as a barrier keeping out pathogens. While mucus also acts as a physical barrier; pathogens and harmful particles are trapped in it and either swallowed or expelled. IT also has antiseptic properties because of the presence of the antibacterial enzyme lysozyme. If pathogens have passed the physical barriers of the skin and mucus they encounter the second line of defense in the form of white blood cells. Ingestion of pathogens by phagocytic white blood cells gives non-specific immunity to diseases this is the process of phagocytosis. Furthermore the production of antibodies by lymphocytes in response to particular pathogens gives specific immunity. Antibodies can either make a pathogen more recognizable to phagocytes so they are more readily engulfed, or prevent viruses fro docking to host cells to that they cannot enter the cells. Some lymphocytes become memory cells, meaning they are long lived immunity specific to the pathogen.

Outline the role of phloem in the active translocation of food molecules. 9.2 (5)

Phloem transported organic compounds throughout the plant. The transport of organic solutes in a plant is called translocation. Phloem links parts of the plants hat need a supply of and other solutes (sinks) to parts that have a surplus (sources). Sources can become sinks and vis versa, which is why the active translocation in the phloem can occur in both directions. Phloem loading is the active transport, used to load organic compounds into phloem sieve tubes at the source and moves them to the sink. This requires a concentration gradient of the substances to be transported for example sucrose, and a proton gradients, which generates energy in the form of ATP which is then used to carry sucrose into the companion cell-sieve tube complex.

Describe how the rate of photosynthesis can be measured 2.9 (table on 137) (6)

Photosynthesis can be measured in many ways as it involves the production of oxygen, the uptake of carbon dioxide and an increase in biomass. For example, aquatic plants release oxygen bubbles during photosynthesis and so these can be collected and measured. The uptake of carbon dioxide is more difficult to measure so it is usually done indirectly. When carbon dioxide is absorbed from water the pH of the water rises and so this can be measured with pH indicators or pH meters. Finally photosynthesis can be measured through an increase in biomass. If batches of plants are harvested at a series of times and the biomass of these batches is calculated, the rate increase in biomass gives an indirect measure of the rate of photosynthesis in the plants.

Outline the role of auxin in phototropism 9.3 (7)

Phototropism is the directional growth of plants in response to the external stimuli of light. Plants use hormones to control the growth of stems and roots, both rate and the direction of growth are controlled. Auxin is the hormone responsible for cell growth rate by changing the pattern of gene expression. Auxin efflux pumps can set up concentration gradients of auxin in plant tissue. Higher concentrations of auxin on the shadier side of the stem cause greater growth on this side, so the stem grows in a curve towards the source of the brighter light. The leaves attached to the stem will therefore receive more light and be able to photosynthesize at a greater rate.

Outline the four levels of protein structure. 2.4 (4)

Primary structure refers to the sequence and number of amino acids in a given polypeptide. Secondary structure refers to the refers to the joining of polypeptides through condensation reactions to form a polypeptide. Tertiary structure refers to the further folding of polypeptides stabilized by interactions of their R groups.

Distinguish between the structure of prokaryotes and eukaryotes 1 (8)

Prokaryotes have a simple cell structure without compartments, while eukaryotes have a compartmental cell structure, meaning they are divided by single or double membranes into compartments called organelles. Prokaryotic cells just have one undivided space, filled with cytoplasm. Eukaryotes have a nucleus that contains chromosomes, while prokaryotes do not have a nucleus and only contain naked DNA. Prokaryotes have a cell wall outside the cell membrane, which eukaryotic cells do not. Prokaryotes divide by binary fission while eukaryotic cells divide by mitosis.

Some proteins in membranes act as enzymes. Outline enzyme-substrate specificity 2.5 (6)

Some proteins act as enzymes. The tertiary structure i.e. the shape of the protein is determined by its primary structure i.e. the sequence of amino acids. This characteristic shape can give rise to specific active site on the protein. Enzymes have an active site to which specific enzymes bind. The shape and chemical properties of the active sites and substrate match each other. This allows the specific substrate to bind and prevents other substances from binding in a "lock-key" model. The induced fit model acts as an extension to the "lock-key" model, and refers to the conformational change of the active site to provide a better moe efficient fit for the substrate to bind with the active site.

Explain how electrical signalling in the heart leads to ventricular contraction. D.4 (4)

The contraction of the heart is systole and the relaxation of the heart is referred to as diastole. These processes are controlled by the SA node which is the pacemaker of the heart. Because gap junctions allow electric charges to flow freely between cells, the contraction which originated in the SA node spreads very rapidly across the entire atrium as if it were one cell. This causes the atria to undergo systole. Signals from the SA node that cause contraction cannot pass directly from atria to ventricles. Therefore the signal from the SA node reaches the atrioventricular (AV) node, which causes signals to spread throughout the heart via specialized heart muscles called the Pukinje fibers. When this occurs the ventricles undergo systole.

Explain the control of blood glucose concentrations in humans. 6.6, D.5 (8)

The hypothalamus monitors and signals if blood glucose levels are too high or too low to the pancreas. Insulin and glucagon are recreated by alpha and beta cells in the pancreas to control blood glucose concentration if there is a deviation from homeostatic conditions. The pancreas is made up of two glands the endocrine glandular tissue, which sercreates digestive enzymes and the endocrine tissue called islets of langerhans, which secrete hormones directly into the bloodstream. These two hormones sercreated by the islets of langerhans are alpha and beta cells. Alpha cells synthesize and secrete glucagon if the blood glucose level falls below the set point, which break down glycogen into glucose in targets cells located in the liver, thereby increasing the concentration of glucose in the blood. Beta cells synthesize insulin and secrete it when the blood glucose concentration rises above the set point. This hormone stimulates uptake of glucose by tissue, and also stimulates the conversion of glucose to glycogen. There by reducing the concentration of glucose in the blood.

Outline the control of milk secretion by oxytocin and prolactin. D.5 (3)

The production and secretion of milk by maternal mamary glands following birth is called lactation, and is controlled and regulated by oxytocin and prolactin. Prolactin is responsible for the development of the mammary glands and the production of milk. It is recreated by the anterior pituitary in response to the release of PRH from the hypothalamus. Oxytocin is responsible for the release of milk from mammary glands. It is produced in the hypothalamus. Oxytocin is responsible for the release of milk from mammary glands. It is produced in the hypothalamus and recreated by neurosecretory cells that extend into the posterior pituitary.

Explain how the light-independent reaction of photosynthesis relies on the light-dependent reaction. 8.3 (5)

The products of the light dependent reactions, which occur in the thylakoids are reduced NADP and ATP serve as energy sources for the light independent reactions. In the calvin cycle of the light independent reactions, requires endergonic reactions to be coupled to the hydrolysis of ATP and the oxidation of reduced NADP the calvin cycle is the final step of photosynthesis, a rubisco catalysis the carboxylation of RuBP to form 2 glycerate 3-phosphate then reduced NADP and ATP from the light dependent reactions are needed to form 2 triose phosphate. 2 triode phosphate is used to regenerate RuBP and produce carbohydrates.

Explain how the properties of water, that are essential to living things, arise from the dipolar nature of water. 2.2 (8)

The properties of water are it adhesive, cohesive, multiple thermal properties, and solvent properties. Water is dipolar due to the unequal sharing of electrons in water molecules, the hydrogen atoms have a partial positive charge and oxygen has a partial negative charge. This leads to strong bonds called hydrogen bonds between water molecules. Water molecules are cohesive, meaning they stick to each other, this is useful for water transport in plants. Water molecules are adhesive meaning they can form hydrogen bonds between other polar molecules, this is useful in leaves where water adheres to cellulose molecules in cell walls. Once of water thermal properties is that it has a high specific heat capacity, caused by its hydrogen bonds restricting movement of water molecules, meaning a high temperature is needed to break these hydrogen bonds. This is useful for aquatic organisms as water temperature remains relatively stable in comparison to air or land. The solvent property of water is that water forms hydrogen bonds with other polar molecules and forms solvent shells which separate them from each other, thereby dissolving them. This allows cytoplasm to be a complex mixture of substances, therefor allowing metabolism to occur.

Explain the role of chemoreceptors in the regulation of ventilation rate. D.6 (3)

The rate of ventilation is controlled by the respiratory control centre in the medulla oblongata. If an increase in blood carbon dioxide or a drop in blood pH is detected, the chemoreceptors in the carotid artery and the aorta send a message to the medulla oblongata. This causes and increase in ventilation rate, which is an increase of gas exchange.

Explain the roles of structures in the kidney that maintain the water balance of the blood in humans. 11.3 (8)

The ultrastructure of the glomerulus and bowman's capsule facilitate ultrafiltration, which is the separation of particles differing in size from water, in glomerular filtrate. The pressure in capillaries in the glomerulus in the kidney, is very high and their walls are very permeable, so the volume of fluid out is higher than the volume of fluid in. This pressure mechanism acts as the selective pressure of ultrafilirtion. Particles must pass through the tree parts of ultrafiltration system fenestration, the basement membrane, and podocytes to become part of the glomerular filtrate. Then glomerular filtrate passes through the proximal convoluted tubule, which selectively reabsorbs useful substances by active transport, to the loop of hence. Which is a tube consisting of descending limb that carries filtrate into the medulla of the kidney, and an ascending limb that brings it back out to the cortex. The loop of hence maintains hypertonic conditions in the medulla, by creating a concentration gradient of solute. After leaving the loop of hence the filtrate travels to the distal convoluted tubule, then to the collecting duct where osmoregulation occurs. Osmoregulation is the keeping of the relative amount of water and solutes in balance by the kidney. The collecting duct carries the filtrate back through the cortex and medulla to the renal pelvis where it is excreted as urine.

Explain the process of aerobic respiration in a cell starting at the end of glycolysis and including oxidative phosphorylation.8.2 (8)

Two pyruvate from glycolysis are decarboxylated and oxidized in the mitochondria in the presence of oxygen. The first step is the link reaction which occurs in the mitochondrial matrix. IN the link reaction the pyruvate is converted into acetyl coenzyme A, with the help of coenzyme A and electron carried NAD+. The next is the the krebs cycle with also occurs in the mitochondrial matrix, the oxidation of acetyl groups is coupled to the reduction of hydrogen carriers. For every turn of the cycle, the production of reduced NAD occurs three times, decarboxylation occurs twice and the reduction FAD occurs once. Also one molecule of ATP is generated. The Final step of aerobic respiration os oxidative phosphorylation, because ADP is phosphorylated to produce ATP, using energy released by oxidation. During his process NAD and FAD are oxidized in the electron transport chain. The transport of electrons between carriers in the electron transport chain is coupled to proton pumping, which causes a gradient of protons to build up in the inter membrane space. The protons diffuse through ATP synthase to generate ATP is chemiosmosis. Oxygen is needed to bind with the free protons to form water to maintain the hydrogen gradient, by acting as the final electron acceptor in the transport chain.

Explain the mechanism of ventilation in humans 6.4 (8)

Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in the alveoli and blood flowing in adjacent capillaries during gas exchange. Gas exchange is when humans take in oxygen used in cell respiration and breathe out carbon dioxide produced in the process. It happens by diffusion between air in the alveoli and blood flowing in the adjacent capillaries. The gasses only diffuse because there is a concentration gradient, created by ventilation. The concentration gradient is made up because the air in the alveolus has a higher concentration of oxygen and lower concentration of carbon dioxide than the blood in the capillary.

Explain the reasons for the ineffectiveness of antibiotics in the treatment of viral diseases 6.3 (4)

Viral diseases cannot be treated using antibiotics because they lack metabolism. Viruses are nonliving and can only reproduce when they are inside a living cell. They use the chemical processes of living host cell, instead of having a metabolism of their own. They do not have their own means of transcription of protein synthesis and they rely on the host cell's enzymes for ATP synthesis and other metabolic pathways. These processes cannot be targeted by drugs as the host cell would also be damaged.

Explain how humans release energy from digested foods to make it available for processes in cells 6.1 (7)

When humans consume food through their moths, where starches are digested with the help of enzymes and lubricants. Food then passes down to the stomach through the esophagus with the help of peristalsis. Once in the stomach churning and mixing with secreted water and acid kills bacteria in food and starts digestion of proteins. The food is then moved to the small intestine for the final stages of digestion of lipids, carbohydrates, proteins and nucleic acids with the help of enzymes such as lactase and exopeptidase, and the absorption of nutrients occurs. Absorption is the process of taking substances int cells and the blood carried out by the vili, which line the walls of the small intestine.Absorbed nutrients enter, in the absence of oxygen they undergo anaerobic respiration in the cytoplasm, and in the presence of oxygen they enter the mitochondria and undergo aerobic respiration. The process of cellular respiration produces ATP, which humans use as energy.

Outline how William Harvey changed the understanding of blood flow around the human body. 6.2 (4)

William Harvey discovered the circulation of the heart acting as a pump. Harvey demonstrated that blood flow through the larger vessels is unidirectional, with valves that prevent back flow. He also showed that the ate of flow through major vessels is too high for blood to be consumed in the body after being pumped out by the heart, therefore it must return to the heart and travel through the system again. He also showed blood is pumped out by arteries and returns in the veins.

Outline the use of models to investigate the transport of water in xylem 9.1 (3)

models of water transport in xylem using simple apparatus including blotting or filter paper, porous pots and capillary tubing. Ports pots can be used to model evaporation from leaves. Capillary tubes can be used to investigate cohesion between water molecules against other substances. Filter paper is used to investigate the adhesive properties of water.