Explanation​:
The digestion of starch is a catabolic reaction where starch molecules are broken down into maltose by hydrolysis.

Peptide formation from amino acids (Choice A), starch synthesis from glucose (Choice B), and DNA synthesis from nucleotides (Choice D) are all examples of anabolic reactions.

Explanation:
Choice A is correct because zoonotic diseases are diseases that can be transmitted from one species to another, such as from chickens to humans. Choice B is incorrect because zoonotic diseases do not only affect domesticated animals. Choice C is incorrect because zoonotic diseases are not only caused by viruses. Choice D is incorrect because zoonotic diseases do not only affect humans.

Explanation​:
Photosynthesis (Choice B) is the process by which carbon dioxide in the air is converted to carbon compounds in green plants.

Respiration releases carbon dioxide (Choice A), and decomposition by decomposers (Choice C) also releases carbon dioxide. Transpiration is the process by which water is drawn up the stem of a plant to the leaves where it evaporates (Choice D) and is not involved directly in the carbon cycle.

Explanation​:
Choice D is correct because the gland X is the pineal gland and it produces melatonin. Choice A is incorrect because leptin is produced by adipose tissue. Choice B is incorrect because thyroxine is produced by the thyroid gland. Choice C is incorrect because progesterone is produced by ovaries.

Explanation​:
-50mV is the threshold potential that must be reached for voltage-gated sodium channels to open.
Choice A is incorrect, as -70 mV is the resting potential of a neuron. Choice C is incorrect as at +30 mV, voltage-gated potassium channels are triggered to open. Choice D is incorrect as at 0mV the voltage-gated sodium channels are already open.

Explanation​:
In the light-dependent reaction, NADP is the final electron acceptor in Photosystem I and this is reduced to form NADPH.

CO22​ is fixed by the enzyme Rubisco and is not an electron acceptor (Choice A).

ATP is synthesized by ATP synthase using the proton gradient in the thylakoids and is not an electron acceptor (Choice C).

Oxygen (Choice B) is the terminal electron acceptor in aerobic cell respiration, but not in photosynthesis.

Explanation​:
Thin-layer chromatography is a method that can be used to separate pigments extracted from a leaf. Paper chromatography uses paper with a line drawn approximately 1 cm from one end, which is called the baseline (Choice D). This is where a drop of leaf extract is placed. Then the paper is immersed in a solvent with the level of solvent being right below the baseline. This solvent will then move up the paper to separate the pigments found in the leaf extract.

The coloured spots show the distance moved by each pigment (Choice C). The solvent will then move further up and will eventually stop moving. The mark where the solvent stops is known as the solvent front and shows the distance moved by the solvent (Choice B).

In order to find the Rf value for a given pigment, the distance moved by the pigment is divided by the distance moved by the solvent (Choice A).

Explanation​:
Both metabolic pathways are catalysed by a series of different enzymes. Pathway 1 proceeds in a linear manner, and the end product is D. Pathway 2 is a cycle where the various products, such as A, are constantly regenerated.
Pathway 1 is a chain reaction and Pathway 2 is a cyclical reaction, so Choices A and D are not correct.
The end product for Pathway 1 is D, and the products keep cycling in Pathway 2, so Choice C is not correct.

Explanation​:
Both, the brewing of beer and baking of bread, involve anaerobic yeast fermentation. In the brewing of beer, yeast feeds on glucose and respires anaerobically to produce ethanol, carbon dioxide and energy. The ethanol produced is the alcohol found in beer. Similarly, with the baking of bread, yeast respires anaerobically to produce ethanol, carbon dioxide and energy. However, the ethanol that is produced evaporates during baking.

Carbon dioxide (Choice A) is a product present in both the brewing of beer and the baking of bread. Carbon dioxide is soluble in beer and is found in small amounts. It is responsible for the rising of the bread.

Likewise, water (Choice D) is also a substance present in both the brewing of beer and the baking of bread. In the brewing of beer and in the baking of bread, water is added to the mixture.

In addition to yeast fermentation, anaerobic respiration can also occur in humans and other animals. This is known as lactic acid fermentation. Glucose is not broken down completely and forms lactic acid (Choice B) in the absence of oxygen.

Explanation​:
Aquatic ecosystems take in inorganic carbon when carbon dioxide dissolves in water. Dissolved carbon dioxide can combine with water to form carbonic acid (H22​CO33​) which dissociates into hydrogen ions (H

) and hydrogen carbonate ions (HCO33​

). So, inorganic carbon is present as both dissolved carbon dioxide and hydrogen carbonate ions in aquatic ecosystems.

Carbon dioxide, not carbon monoxide (Choice B) is the form of inorganic carbon that autotrophs can use. Methane (Choice C and D) is a form of organic carbon, not inorganic carbon.

Explanation​:
The mass of enzyme added remains the same throughout because the enzyme acts as a catalyst and does not get used up during the reaction. All other choices (Choices A, B and C) are used in obtaining the rate of an enzyme-catalysed reaction since they involve measuring the substrate used or product formed over a specified amount of time.

Explanation​:

The stage that takes place in the cytoplasm is glycolysis where glucose is converted to pyruvate (Choice D).

Choice A is the Krebs cycle where acetyl CoA is oxidized in the mitochondrial matrix. Choice B is the link reaction where pyruvate gets converted to acetyl CoA. The link reaction occurs before the Krebs cycle. Choice C is the electron transport chain where most of the ATP is produced and takes place on the cristae of the mitochondria.

Explanation​:
Cell respiration is the process where energy is released from organic compounds to form adenosine triphosphate (ATP). ATP consists of the nitrogenous base adenine, a ribose sugar and three phosphate groups. These three components contain the elements oxygen, hydrogen, phosphorus, carbon and nitrogen (Choice D). Even though ATP is made up of these five elements, they are not products of hydrolysis.

ATP is the immediately available source of energy in cells and fuels cell processes. This takes place when ATP undergoes hydrolysis and the energy which is stored in the phosphate bond of ATP is released. The phosphate bonds that are broken lead to the formation of adenosine diphosphate (ADP), inorganic phosphate and a release of energy.

Hydrolysis is a catabolic reaction of breaking down large molecules into small molecules. Water is required to split apart ATP, hence both water and ATP are the reactants in a hydrolysis reaction and not products (Choice C).

The main stages of cellular respiration are glycolysis, the Krebs cycle and the electron transport chain. In glycolysis, glucose is broken down to pyruvate (Choice A) which then moves on to the Krebs cycle and electron transport chain before generating a high yield of 38 ATP.

Explanation:
Letter X indicates the cell body, which contains the cytoplasm and the nucleus. Choice A is correct.

An axon is a long, thin projection of a neuron that carries action potentials away from the cell body, Choice B is incorrect. Dendrites are multiple specialised extensions branching from a neuron’s cell body or dendron, responsible for receiving incoming signals from other neurons or sensory receptors, Choice C is incorrect. Nerve fibres are a general term for any long, slender projection from a cell body that is capable of transmitting electrical signals - it can be an axon or dendron, Choice D is incorrect.

Explanation​:
Oxidative decarboxylation involves oxidation and decarboxylation. During the link reaction, the oxidative decarboxylation of pyruvate produces a two-carbon acetyl group.

The phosphorylation of glucose (Choice B) occurs during glycolysis before the oxidative decarboxylation of pyruvate and not after. Phosphorylation is a reaction where a phosphate group is added which makes the molecule less stable.

In the electron transport chain, both NADH and FADH22​ release their hydrogens (protons) and electrons which leads to the transport of electrons and movement of these protons (Choice C). Electron transport and proton movement are some of the stages of oxidative decarboxylation of pyruvate, hence it does not happen right after.

Hydrogen ions are then used to power ATP synthase, producing ATP which is the final stage of cellular respiration (Choice D). Hence it does not happen right after the oxidative decarboxylation of pyruvate.

Explanation:
The enzyme lactase breaks down the substrate lactose into glucose and galactose.

Glucose and galactose are products of the reaction (Choices A and B).

Fructose is not a substrate for the enzyme lactase (Choice D).

Explanation​:
Glycolysis occurs in the cytoplasm of the cell. Then, in aerobic respiration, the product of glycolysis (pyruvate), then is transported into the mitochondrion, where it enters the link reaction. The link reaction takes place in the fluid part of the mitochondrion, the matrix. The product of the link reaction then enters the Krebs cycle, which also occurs in the matrix. The final stage, the electron transport chain, occurs in the membranes of the cristae. Choice B is, therefore, correct.

Choices A, C and D are incorrect as they give one or more incorrect locations.

Explanation:
Reaction X represents the synthesis of a more complex compound from simpler molecules and therefore is a depiction of an anabolic reaction.

Reaction X cannot be catabolic (Choice A) since a more complex molecule is produced. Catabolism results in simpler products compared to the reactants. Reaction X cannot represent all metabolic reactions (Choice C) since metabolic reactions include both anabolic and catabolic reactions while reaction X represents only anabolic reactions. Reaction Y represents a catabolic reaction and not an anabolic reaction (Choice D).

Explanation​:
ATP from cell respiration is an immediate source of energy for metabolic processes.

Neurons transmit information in the form of electrical impulses (Statement I). This transmission can only happen when there is a difference in concentrations of Na

and K

ions across the axonal membrane of a neuron. Energy in the form of ATP is required for the Na

/K

pump to restore the ionic balance after the voltage-gated response.

Osmosis (Statement II) does not require ATP. It is a passive transport process where water can move through a partially permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

The condensation reaction of a polypeptide (Statement III) is the joining of amino acids together to form a polypeptide. It is an anabolic reaction where ATP is required to join amino acids together to form a peptide bond, and water is produced.

Explanation:
Almost all enzymes are globular proteins and differ from fibrous proteins in that they are sensitive to pH and temperature and are mostly water soluble.

Fibrous proteins (Choice A), unlike globular proteins, are long and narrow, less sensitive to changes in pH and temperature and are generally insoluble in water. Enzymes are not lipids (Choices B, D). Enzymes tend to be hydrophilic, not hydrophobic since they are mostly soluble in water.

Explanation​:
Saltatory conduction is when electrical impulses jump between nodes of Ranvier (the gaps between sections of the myelin sheath that surrounds axons).
Choice B is incorrect because it is a description of synaptic transmission.
Choice C is incorrect because it is a description of how the resting potential is maintained.
Choice D is incorrect as it describes how an action potential travels down an unmyelinated neuron.

Explanation​:
Choice B is correct because in phototropism auxin efflux pumps move auxin to the shady side of the stem, which stimulates growth and results in the bending of the stem towards the light source. Choice A is incorrect because auxin moves downwards in stems. Choice C is incorrect because auxin efflux pumps move auxin to the shady side of a stem. Choice D is incorrect because auxin does not move into flowers.

Explanation​:
Choice D is correct because cAMP binds to inactive PKA after being formed to activate PKA.

Choice A is incorrect because the enzyme adenyl cyclase (AC) acts on ATP to form cAMP. This has to take place first to form cAMP. Choices B and C are incorrect because these two reactions take place before cAMP is formed. A ligand such as epinephrine binds to the G protein-coupled receptor, resulting in guanosine triphosphate (GTP) binding to the G protein to activate it.

Explanation​:
A community is a group of populations living and interacting in a shared habitat (biotic factors). The abiotic factors for this community have not been described but could be the depth of the water, light intensity and the nutrients available for phytoplankton.

A population (Choice A) is a group of organisms of the same species living in the same area at the same time which can interbreed. A habitat (Choice C) refers to the environment in which a species normally lives. An ecosystem (Choice D) is a group of populations living and interacting in a common habitat and includes both biotic and abiotic factors.

Explanation:
Choice A is correct because GTP, guanosine triphosphate, needs to bind with the G protein and replace the GDP to activate the G protein.

Choice C is incorrect because GDP is attached to an inactive G protein, which is then replaced by GTP to activate the G protein. Choices B and D are incorrect because cAMP is the secondary messenger that gets formed from ATP in reactions that take place after the G protein is activated.