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​:
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:
When organisms respire, they lose energy in the form of heat. This form of energy cannot be recovered and is finally lost from the system.

Initially, during photosynthesis, energy is captured from light (Choice A). Carbon dioxide is not a form of energy (Choice C), and chemical energy (Choice D) is held in the bonds of organic molecules made during photosynthesis.

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​:
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​:
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:
The active site of an enzyme is specific to a particular substrate. All other statements are true (Choices A, C and D).

Explanation​:
Choice A - Antibodies are globular proteins synthesized by plasma B cells. The genes associated with antibody synthesis are expressed in plasma B cells.

Choice B - Macrophages do not synthesize antibodies and therefore do not express these genes. Macrophages are large phagocytic cells and present antigens to helper T cells.

Choice C - Helper T cells do not synthesize antibodies. They stimulate B cells to divide by mitosis to produce plasma B cells and memory B cells.

Choice D - Red blood cells do not have a nucleus and therefore do not have the genes for antibody synthesis.

Explanation​:
The all-or-none law states that an action potential will only be transmitted if a threshold potential is reached, and the size of the action potential does not vary. Choice A is correct. Choice B is incorrect as if the threshold potential isn’t reached, then no action potential is triggered. Choice C is incorrect because if the threshold potential is not reached, then an action potential is not triggered and so neurotransmitters are not released at the synaptic cleft. Choice D is incorrect as an action potential only moves in one direction.

Explanation​:
Choice C is correct because smallpox was the first human disease eradicated by vaccination. Choice A is incorrect because polio has not been fully eradicated. Choice B is incorrect because monkeypox has not been eradicated. Choice D is incorrect because there is no vaccine for bubonic plague.

Explanation​:
Inhibitors prevent enzymes from catalysing reactions and therefore decrease the rate of enzyme catalysed reactions.
Inhibitors do not activate enzymes, thereby increasing the rate of reaction (Choice A).
Inhibitors do not help the substrate bind to the active site (Choice B). In fact they usually prevent the substrate from binding to the active site.
Inhibitors decrease the rate of reaction, they do not speed it up (Choice C).

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:
Neonicotinoids bind irreversibly to acetylcholine receptors at cholinergic synapses so that acetylcholine (a neurotransmitter) is unable to bind and an action potential cannot be generated.
Choices B, C and D are incorrect as they do not describe how neonicotinoids work.

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 D is correct because a tropism is any growth response of a plant towards or away from a stimulus. Choice A is incorrect because this is the definition for only one type of tropism, geotropism. Choice B is incorrect because day length does not cause a plant to grow in one direction. Choice C is incorrect because this is a specific definition for phototropism whereas tropism in general is a growth response towards or away from a stimulus, and not only light.

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.

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​:
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:
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​:
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 A - Cells of multicellular organisms have different functions. They interact with one another to perform more complex functions such as responding to the environment, movement and homeostasis.

Choice B - Cells of multicellular organisms communicate with each other. These cells communicate to coordinate the activities of organs, tissues and other cells.

Choice C - Cells interact with one another forming complex structures and functions that any one cell type alone could not perform

Choice D - Cells have different structures based on the cell’s specialised roles.

Explanation​:
Choice B is correct because a protein called testis determining factor (TDF) stimulates the development of embryonic gonads as testes. Testosterone causes the development of the testes (Choice A), stimulates sperm production (Choice C) and stimulates the development of secondary male characteristics (Choice D).

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 glycolysis (Choice A), glucose is broken down into pyruvate and this happens in the cytoplasm. Glycolysis is incorrect because pyruvate is only produced at the end of glycolysis and is not converted to acetyl CoA.

Pyruvate produced during glycolysis enters the mitochondria where it is converted to acetyl CoA during the link reaction. (Choice B)

The Link reaction (Choice B) is the reaction that bridges glycolysis with the next stage which is the Krebs cycle (Choice C). Hence, the Krebs cycle is incorrect because acetyl CoA is the reactant and needs to be produced prior to entering the Krebs cycle.

The stage after the Krebs cycle is known as oxidative phosphorylation (Choice D) which involves the synthesis of ATP.

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.