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

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:
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:
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:
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:
Facilitated diffusion is a passive process that allows the movement of molecules across a cell membrane with the help of transport proteins. It does not require energy input (ATP) because molecules move down their concentration gradient. Therefore Choice B is correct.

Sodium-potassium pumps (Choice A) actively transport sodium ions out of cells and potassium ions into cells against their concentration gradients. This process requires energy in the form of ATP. Endocytosis (Choice C) is an active process by which cells engulf substances by invagination of the cell membrane to form a vesicle around them. This process requires energy in the form of ATP. Anabolic reactions (Choice D) involve the synthesis of complex molecules from more simple ones. These reactions typically require energy input in the form of ATP.

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:
A positive feedback mechanism is a physiological process in which a response amplifies the initial stimulus, leading to an accelerated and often self-reinforcing outcome. The dilation of the cervix during childbirth stimulates the release of the hormone oxytocin. Oxytocin then brings about further uterine contractions, which brings about further cervix dilation. This is, therefore, an example of positive feedback, and Choice D is correct.

A negative feedback mechanism is a regulatory mechanism in which a response counteracts and reverses the initial stimulus, maintaining stability and homeostasis in the system. Choices A, B, and C are all examples of negative feedback mechanisms.

Explanation:
Photolysis is the splitting of water molecules in the presence of light to produce electrons, hydrogen ions and oxygen. This is part of the light-dependent reaction of photosynthesis.

Chemiosmosis (Choice B) happens in both photosynthesis and cellular respiration. In both processes, ATP is formed from ADP and inorganic phosphate. Hence, the products are not electrons, hydrogen ions and oxygen molecules.

The Calvin cycle (Choice C) is part of the light-independent reaction of photosynthesis. It produces carbon compounds and does not require light.

The electron transport chain (Choice D) is a component of both photosynthesis and cellular respiration. In photosynthesis, it takes place after photolysis which is when water molecules are split, producing electrons, hydrogen ions and oxygen. Photolysis needs to take place first before these electrons can move down the electron transport chain.

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:
-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:
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:
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:
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:
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:
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 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:
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 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 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:
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.