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 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​:
Heat and pressure from the Earth’s crust decompose the remains of dead plants and animals over millions of years to form oil, natural gas or coal (fossil fuels).

Respiration (Choice B) takes place in a relatively short time as does photosynthesis which requires light to synthesize organic compounds (Choice C). Forest fires (Choice D) often occur naturally and release carbon dioxide into the atmosphere. Global warming has increased the number of wildfires.

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 – 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:
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:
Depolarisation of the presynaptic membrane (Statement II) and the resulting influx of calcium ions into the presynaptic neuron (Statement III) cause the exocytosis (release) of neurotransmitters at the presynaptic membrane.

Statement I is false, as the postsynaptic membrane receives neurotransmitters that are released from the presynaptic membrane. Choices A, C and D are incorrect.

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:
Okapi is a herbivore (II) because it only eats plants. Okapi is a species (III) since it can interbreed and produce fertile offspring.

They are not a hybrid of a giraffe and a zebra (I), even though they look like one.

Explanation:
Choice A is incorrect as it describes how neurotransmitters are released from the presynaptic cleft. Choice C is incorrect as neurotransmitters do not require energy to move across the synaptic cleft. Choice D is incorrect as channels are not present in the synaptic cleft. They are only present in the membranes of neurons.

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​:
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 B is correct because insect-pollinated flowers have coloured petals (Statement II) to attract pollinators and a nectary (Statement I) to reward them, whereas all flowers have a stigma (Statement III). Choice A is incorrect because both statements I and II are correct. Choices C and D are incorrect because the stigma is not adapted for a mutualistic relationship with insects. Only the flower benefits from the presence of a stigma.

Explanation​:
The endocrine system (Statement II) is involved in the fight or flight response through the release of hormones from endocrine glands, such as adrenaline (epinephrine), from the adrenal glands. Adrenaline (epinephrine) is released into the bloodstream and travels throughout the body to target organs and tissues, preparing the body for immediate action. When adrenaline (epinephrine) reaches its target tissues it binds to specific transmembrane receptors on the surface of cells (Statement I). This binding triggers a cascade of intracellular events that ultimately lead to physiological changes associated with the fight or flight response, such as increased heart rate and dilated airways. The autonomic nervous system (Statement III) plays a central role in the fight or flight response. In a frightening situation, the nervous system is activated, leading to the release of adrenaline (epinephrine), increased heart rate, increased blood flow to muscles, and other physiological changes that prepare the body for a rapid response. Choice D is therefore correct.

Choices A, B and C are all incorrect as they do not include all three statements.

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​:
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:
As energy moves from producers to consumers, only about 10% is transferred to the next trophic level. So only about 0.1 % of the energy produced by the producers reaches the tertiary consumers. All other choices (Choice A, B, C) are not correct. Looking at the values in the diagram, producers start with 20 000 units of energy and tertiary consumers receive only 25 units. 25 divided by 20 000 is approximately 0.1%.

Explanation​:
An ecosystem refers to an area where organisms interact with each other and with the physical environment.

A habitat (Choice A) refers to the environment in which a species normally lives. A population (Choice C) is a group of organisms of the same species living in the same area at the same time which can interbreed. A community (Choice D) is a group of populations living and interacting with each other in the same habitat.

Explanation​:
Calories (Choice A) and Kilocalories (Choice B) are measures for energy but do not include area and time which is how productivity is measured in an ecosystem. kJm−2−2 (Choice C) does include the area but not time. To see energy values over a specified time, usually a year, the units kJm−2−2y−1−1 are used.

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:
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:
Neurotransmitters travel freely without energy from an area of high concentration at the presynaptic membrane to a low concentration at the postsynaptic membrane via simple diffusion, Choice C is correct.

Choice A is incorrect, as neurotransmitters do not use a channel to diffuse across the synapse. Choice B is incorrect, as neurotransmitters do not travel against the concentration gradient using energy across the synapse. Choice D is incorrect as although neurotransmitters are released from the presynaptic membrane via exocytosis, they do not travel across the synapse using this method.

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​:
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:
The length of food chains tends to be shorter than 6 organisms, even including parasites and detritivores. When energy is passed from one organism to the next, most of the energy is generally lost as heat which means that less and less energy is available to higher trophic levels.

All other statements are true.