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​:
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​:
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​:
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​:
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​:
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:
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:
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​:

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:
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​:
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​:
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:
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​:
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:
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​:
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​:
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 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 – Catabolic reactions involve the breakdown of large molecules into smaller molecules.

Choice B – Fermentation refers to anaerobic respiration which occurs in the cytoplasm.

Choice C - Anabolic reactions build complex compounds from simpler molecules. In the case of photosynthesis, the reactants carbon dioxide and water are turned into glucose and oxygen gas.

Choice D – Hydrolysis is an example of a catabolic reaction which involves the splitting of water.

Explanation​:
Cell respiration is the controlled release of energy in the form of adenosine triphosphate (ATP) from organic compounds. Cell respiration occurs either in the presence of oxygen or in the absence of oxygen. In the presence of oxygen, the process is known as aerobic cell respiration. In the absence of oxygen, the process is known as anaerobic cell respiration (Choice A).

Anaerobic cell respiration produces a smaller yield of ATP from glucose which is 2 ATP molecules, compared to aerobic cell respiration’s larger yield of ATP, which is approximately 38 ATP molecules.

This is because aerobic cell respiration undergoes the complete metabolic pathway of cellular respiration which involves the reaction of organic compounds such as glucose, lipids or proteins (Choice C) in the presence of oxygen to produce carbon dioxide, water and a large yield of ATP.

On the other hand, anaerobic cell respiration only undergoes glycolysis, the first stage of the metabolic pathway of cell respiration. This involves the incomplete breakdown of glucose in the absence of oxygen to produce lactate, which is also known as lactic acid (Choice D).

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​:
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​:
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 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.