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​:
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​:
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​:
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 B is correct because Rubisco is the name of the enzyme which is responsible for the fixation of carbon during photosynthesis. Choice A is incorrect as rhodopsin is a pigment found in the retina which is responsible for detecting light . Choices C and D are incorrect as neither are enzymes, with collagen being a structural component of tissues such as skin and insulin being a hormone responsible for regulating blood glucose levels.

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 C is correct, as ATP is short for adenosine triphosphate. As the name tells us, it has three phosphate groups, represented by the three circles attached to the central ribose sugar. The rectangle represents the adenine.

Choice A is a cholesterol molecule, Choice B is a glucose molecule, and Choice D is a molecule of ADP (adenosine diphosphate), which is the molecule that results when ATP loses its third phosphate molecule, releasing energy.

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

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:
A respirometer is a device that is used to measure the rate of respiration. A basic feature of a respirometer is a sealed container containing the living organism and an alkaline substance such as soda-lime (Choice A) or potassium hydroxide to absorb the carbon dioxide gas, which is the product of respiration.

The rate of respiration is measured by a reduction in the volume of oxygen due to the oxygen consumed by the living organism. The reduction in the volume of oxygen inside the sealed container causes a reduction in pressure inside the container. This causes the syringe to move which is how oxygen consumption is measured (Choice B). The rate of respiration can then be calculated based on the amount of oxygen consumed over a period of time.

An oxygen gas sensor (Choice C) can also be used to measure the reduction in the amount of oxygen. However, the experimental setup for this question does not include a sealed bio chamber bottle that is designed to fit the gas sensor probe. Since this experimental setup used a bung to seal the container, removing it to place an oxygen gas sensor probe inside the flask will not provide an accurate measurement of the amount of oxygen consumed.

When using a respirometer, the temperature is a controlled variable and should be kept constant (Choice D) to avoid changes to the volume of oxygen gas. Temperature also affects the rate of respiration. There should not be more than one independent variable being tested in an experimental setup.

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​:
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​:
Autotrophs are producers and form the base of food chains. They carry out photosynthesis and get their energy from the Sun.

The most abundant organisms (Choice A) are usually the producers but there are exceptions. Energy always enters the food chain via the autotrophs, not consumers (Choice C).

The flow of energy (Choice D) is from producers to primary to secondary to tertiary consumers

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​:
Since the experiment is about testing the temperature; the temperature would be the independent variable, not the controlled variable. A water bath would be used in this experiment to set up different increments of the independent variable.

When investigating the effect of temperature on photosynthesis, the other limiting factors (concentration of carbon dioxide and light intensity) must be controlled. The light intensity can be controlled by placing the test tube containing the Elodea at a constant distance from the same light source for each trial (Choice C) to make sure that the light intensity is the same for all trials.

Using the same volume and concentration of sodium hydrogen carbonate solution (Choice B and Choice D)ensures that the source of carbon dioxide is controlled, and it is not a limiting factor.

Explanation:
The statement for Choice B is incorrect, because longitudinal muscle does not stop the bolus from moving backwards, this is the role of the circular muscles. Choice A, Choice C and Choice D are correct statements because the longitudinal and circular muscles do work together, longitudinal muscle does move the bolus along the alimentary canal and the bolus is mixed with enzymes as a result of peristalsis.

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​:
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​:
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​:
In glycolysis, ATP is first needed to supply inorganic phosphate, which is attached to the glucose molecule to form a phosphorylated sugar molecule, fructose 6-phosphate. ATP is needed again to supply a second inorganic phosphate which is attached to this already phosphorylated sugar molecule to form fructose 1,6-bisphosphate.

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​:
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​:
A second infection by a pathogen triggers a much faster and more potent immune response, resulting in the production of greater levels of antibodies (Choice B), when compared to primary infection. Primary infection causes a smaller increase in antibody numbers; this is represented by Y (Choice A). Infection by a different pathogen (Choice C) would cause a raise in antibody numbers similar to levels in point Y. Antibiotics do not increase antibody numbers (Choice D).

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.