Comprehensive responses to 14 potential essay titles that haven't come up on the new AQA Biology specification yet, including 8 fully-developed model essays and 6 additional essay plans. Every essay contains a paragraph of off-the-specification content to help you achieve the top band and score 25/...
1. The importance of responses to the internal and external environment in organisms
The control of blood water potential and osmoregulation is an example of a physiological response to
both the internal and external environment. When a change in the environment, such as drought,
renders water unavailable, or a hot day increases water lost via the evaporation of sweat, a person
becomes dehydrated. This lowers the water potential of their blood (a change in the internal
environment), causing water to move out of osmoreceptors in the hypothalamus by osmosis. The
hypothalamus produces more ADH as a result, so more ADH is then released by the posterior
pituitary. ADH binds to receptors on the surface of cells lining the walls of the collecting duct,
stimulating the addition of aquaporins into the cell surface membrane. This increases the
reabsorption of water by osmosis in the collecting duct, thereby maintaining blood water potential
within restricted, homeostatic limits. It is important that decreases in blood water potential are
responded to, as low blood water potential can cause water to leave the tissues by osmosis down its
water potential gradient, increasing blood volume, and so blood pressure. This risks damaging the
endothelium of capillaries and the walls of blood vessels, increasing the chances of an aneurysm.
Osmoregulation is also important because water is also important in hydrolysis reactions, as a
metabolite, and as a solvent within the body; if too much is lost in the urine, then these reactions
could not occur.
Another homeostatic mechanism that allows animals, this time, to respond to changes in food
availability is the control of blood glucose. When food is scarce (a change in the external
environment), blood glucose levels decrease (a change in the internal environment). In response,
alpha cells in the Islets of Langerhans in the pancreas secrete glucagon, which binds to receptors on
the cell surface membranes of liver cells. This changes the shape of the enzyme adenylate cyclase on
the inside of the membrane. Adenylate cyclase converts ATP to cyclic AMP, which acts as the ‘second
messenger’ by activating protein kinase enzymes, which in turn activate a cascade of enzyme-
controlled reactions that lead to the break down of glycogen into glucose (glycogenolysis). Glucose is
then released into the bloodstream down its concentration gradient, thereby increasing blood
glucose levels. This response is important because, if levels of blood glucose get too low, then the
rate of respiration will decrease as there is less phosphorylation of glucose in glycolysis, so less triose
phosphate is produced and oxidised to pyruvate. There is less pyruvate for the link reaction, so less
acetate is produced, slowing down the rate of the Krebs cycle and, therefore, also the electron
transport chain in oxidative phosphorylation. The result is that less ATP is made and less energy is
released for anabolic reactions, protein synthesis, cell division and active transport, which could lead
to cell death.
The control of heart rate is an example of how the autonomic nervous system contributes to
responses to the internal and external environment. If a change in the external environment, such as
the appearance of a predator, dictates that we need to run away swiftly, this increases our rate of
respiration and therefore increases the concentration of carbon dioxide in the blood (a change in the
internal environment). Lower blood pH is detected by chemoreceptors in the aorta and carotid
arteries, which send more impulses to the medulla, which in turn sends more impulses along the
sympathetic nervous system to the SAN. This increases heart rate, which ensures that carbon dioxide
is removed from the respiring tissues and exhaled, maintaining blood pH within normal limits. This is
important for ensuring that enzymes in the blood continue to function. Low pH is associated with an
increase in the concentration of H+ in the blood, which can break hydrogen and ionic bonds in the
,tertiary structure of enzymes, leading to denaturation and slowing down important metabolic
reactions.
Plants, as well as animals, respond to changes in their environment. Tropisms are responses via
growth to a directional stimuli. Stems of plants exhibit positive phototropic responses. IAA, a plant
growth hormone, is produced in the shoot tip and then diffuses down the plant shoot. If sun light is
coming from a single direction, IAA moves towards the shaded side, where it stimulates cell
elongation. The shaded side grows faster than the unshaded side, causing growth towards the light
source. This response is important because it increases the exposure of the leaves to sunlight,
thereby increasing the likelihood that a photon of light will be absorbed by chlorophyll, exciting two
electrons to a higher energy level, causing them to enter the electron transport chain. The
movement of electrons along carrier proteins releases energy to pump protons across the thylakoid
membrane. Protons then move back across the membrane down an electrochemical gradient by
facilitated diffusion through ATP synthase, catalysing the reaction of ADP and Pi to form ATP. Protons
and electrons released from the photolysis of water, the rate of which is also increased by increased
exposure to sunlight, combine to reduce NADP to NADPH. ATP and NADPH are important in the
reduction of GP to triose phosphate in the light independent reaction.
Wound healing can be considered a homeostatic process because it restores an animal’s internal
environment to its undamaged state, in response to an external stressor that has caused a cut or
graze. Following a cut, damage to collagen causes the release of soluble factors to encourage
inflammation, by promoting migration of immune cells to the area of damage. This is important
because immune cells encourage the secretion of matrix molecules synthesised within cells to
rapidly repair damaged tissues. The extra-cellular matrix (the non-cellular component of all tissues
and organs in the body) provides both structural integrity and a foundation for immune cell
migration across the matrix to the injury site to mediate wound closure. This prevents wounds
becoming infected with bacteria, which could be life-threatening. Fibronectin, produced by
fibroblasts in response to endothelial injury, is a fibrous protein which, in its plasma form, is crucial in
helping blood clots to form at the site of the wound, preventing excessive bleeding.
, 2. The importance of energy transfers within and between organisms
The light dependent reaction of photosynthesis is an example of the transfer of light energy (in the
form of photons) to chemical energy (in the form of ATP) within plants. Light energy from the sun
hits a molecule of chlorophyll in the palisade mesophyll of plant leaves and, if it is the correct
wavelength, is absorbed. This excites two electrons to a higher energy level, causing them to enter
the electron transport chain. Electrons are passed along carrier proteins, releasing energy to pump
protons across the thylakoid membrane. Protons then move back across the thylakoid membrane
down an electrochemical gradient by facilitated diffusion through ATP synthase, catalysing the
reaction of ADP and Pi to form ATP. This is important because ATP and NADPH – the formation of
which relies on light energy from the sun – are required to reduce GP to triose phosphate in the light
independent reaction, and ATP is needed to regenerate RuBP (which allows the Calvin cycle to
continue). 1/6 of the carbon stored in triose phosphate is used to synthesise new organic molecules,
such as glucose, which is a highly important respiratory substrate.
Receptors, such as the Pacinian corpuscle (a sensory neurone wrapped in layers of lamellae), are an
example of energy transfers within animals. Mechanical energy (in the action of pressing one’s
fingertip against a hard surface, for example) is transferred to electrical energy in the form of a
generator potential and then, if the threshold is reached, an action potential. By converting
mechanical pressure to nervous impulses, the Pacinian corpuscle acts as a transducer. In the Pacinian
corpuscle, pressure deforms the lamellae, opening the stretch-mediated sodium ion channels,
causing an influx of sodium ions into the sensory neurone down their electrochemical gradient,
establishing a generator potential. If enough sodium ions diffuse into the neurone, the threshold
potential is reached and an action potential is generated. Mechanical energy is thereby transferred
to electrical impulses that are then propagated along sensory neurones to the central nervous
system (CNS) for processing. This is important because it allows animals to detect stimuli, including
dangerous changes in their environment. For example, running away from predators. In another
instance, touching a very hot surface leads to the reflex action of pulling one’s hand away, preventing
damage to the body’s tissues.
During active transport, chemical energy released from ATP is transferred to kinetic energy in the
form of a conformational shape change of a carrier protein in the phospholipid bilayer of a cell. A
molecule binds to a complementary binding site on a specific carrier protein – as does ATP. ATP is
hydrolysed to ADP and Pi, and when the Pi binds to the protein, it causes it to change shape,
releasing the bound molecule to the other side of the membrane. Pi is then released from the carrier
protein (and it used to resynthesise ATP), causing it to revert back to its original position in the
membrane, ready for the process to be repeated. The transfer of chemical energy to kinetic energy in
active transport is important because it allows substances to be moved against their concentration
gradient. In neurones, this allows sodium potassium pumps to maintain resting potential by actively
transporting 3 Na+ out of the axon and 2 K+ into the axon, thereby establishing a potential difference
of -70mV between the inside and outside of the neuron. The neuron relies on this potential
difference to generate an action potential in response to a stimulus.
Sliding filament theory provides another example of the transfer of chemical to kinetic energy in the
body. The hydrolysis of ATP provides the chemical energy for myosin heads to ‘bend’, once attached
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