Explanation:
Centrifugation is a technique that primarily separates substances based on their density. When a mixture is centrifuged, particles or components with higher density sediment to the bottom of the tube. Choice A is, therefore, correct.

Centrifugation does not separate substances based on their charge. Electrophoresis is a technique that separates particles or molecules based on their charge. Choice B is, therefore, incorrect. Centrifugation does not directly separate substances based on their chemical composition. Other techniques like chromatography may be more suitable for separating substances by chemical composition. Choice C is, therefore, incorrect. Centrifugation does not separate substances based on their colour. Colour differences might be observed during centrifugation, but they are not the basis for separation. Choice D is, therefore, incorrect.

Explanation:
The bond formed between two glucose molecules to form maltose is an alpha 1-4 linkage. This is a covalent bond between two alpha glucose molecules, where the carbon #1 of one of the glucose molecules forms a bond with the carbon #4 of the other glucose molecule. Choice C is correct.

A hydrogen bond (Choice A) is a weak electrostatic attraction between a hydrogen atom that is covalently bonded to a highly electronegative atom and another electronegative atom in a different molecule or region of the same molecule. For example, between two water molecules. Peptide bonds (Choice B) are covenant bonds that hold amino acids together in polypeptide chains, and phosphodiester bonds (Choice D) are covalent bonds between the ribose sugars and phosphate groups, forming the backbone of DNA and RNA. Choices A, B and D are incorrect.

Explanation:
All the statements are true. A mesocosm is a stable, small representation of an ecosystem and can be used to study how one factor in an ecosystem affects other factors in an ecosystem. In all ecosystems, energy is lost and autotrophs such as algae or green plants generate the energy needed for the ecosystem. Saprotrophs are necessary for nutrient cycling, which is why no external addition of nutrients is generally required. Herbivores are not necessary for mesocosms to be stable as autotroph abundance is also regulated by the availability of resources such as nutrients and light. The only choice that includes all three statements is Choice C. All other choices (A, B and C) are therefore incorrect.

Explanation​:
Choice A - The surface area of a cell affects the rate of diffusion of materials in and out of the cell.

Choice B - A small cell will have a bigger surface area to volume ratio which means that diffusion will be easier and other cell processes will become faster.

Choice C - The volume of the cell affects the rate at which materials are produced.

Choice D - A larger spherical cell will have a low surface area to volume ratio. Therefore, the comparatively large volume of cytoplasm may produce waste products at a rate too high to rely on diffusion through the comparatively small surface area of the cell membrane.

Explanation:
Protein pumps are membrane proteins that pump molecules up their concentration gradient using energy in the form of ATP.

Protein channels (Choice A) carry out passive transport while receptors (Choice D) and peripheral proteins (Choice C) do not transport proteins.

Explanation​:
In prokaryotes, the mRNA produced after transcription is ready for translation and does not undergo splicing. In contrast, eukaryotic pre-mRNA undergoes splicing to remove introns and join exons, producing mature mRNA. Choice C is, therefore, correct.

Both prokaryotes and eukaryotes use DNA as a template for transcription. Choice A is, therefore, incorrect. In prokaryotes, transcription occurs in the cytoplasm, while in eukaryotes, it occurs in the nucleus. Choice B is, therefore, incorrect. Splicing of mRNA occurs in eukaryotes, not prokaryotes. Choice D is, therefore, incorrect

Explanation​:
Choice A - There are still stem cells that can be found in the adult body. For example, stem cells are present in the bone marrow, skin, and liver.

Choice B - The foetal stage of development happens from the ninth week after conception until birth. In this stage, the organs have begun to form, and these developing organs come from differentiated stem cells.

Choice C - Stem cells are characterised by their capacity to divide and differentiate. These cells are similar to a blank canvas with the possibility of becoming any type of cell.

Choice D - Stem cells are yet to undergo specialisation through the process of differentiation.

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 C is correct because the habitat is a place a community, population, species, or organism lives. Choice A refers to an organism's niche, Choice B refers to the ecosystem, and Choice D refers to a community.

Explanation:
Choice C is correct because ribosomes are organelles found on the surface of the rough endoplasmic reticulum and are needed to synthesise polypeptide chains.

The nucleus (Choice A) contains DNA, some of which codes for the polypeptides made by the ribosomes. The role of vesicles (Choice B) is to store substances and to transport substances, including proteins, between cellular compartments. The mitochondria (Choice D) are the organelles responsible for aerobic respiration.

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​:
Veins carry deoxygenated blood from the body back to the heart. As muscles surrounding the veins contract, they push the blood towards the heart and venous valves open to allow the flow of blood. When the muscles relax, the valves close to prevent the blood from flowing backwards in response to gravity. So, venous valves help prevent the backflow of blood away from the heart (Choice B).

The relaxation of skeletal muscles decreases blood pressure in veins and does not play a role in preventing the backflow of blood in veins (Choice A). When the heart relaxes, this decreases blood pressure in arteries, it does not affect blood pressure in veins and plays no role in preventing the backflow of blood in veins (Choice C). Thin vessel walls are most exemplified in capillaries whose walls are one cell thick and allow exchange of substances. Thin walls play no role in preventing the backflow of blood (Choice D).

Explanation​:
Autotrophs are producers (Choice A) which synthesize their own organic compounds. Consumers gain their nutrition by feeding (Choice B) on other organisms. Detritivores consume dead organic matter (Choice C) by ingestion. Saprotrophs break down food outside their bodies (Choice D) by secreting enzymes and then ingesting the digested compounds.

Explanation:
Choice C is correct because Y is the nucleus and is an organelle, whereas X is the cytosol and not an organelle.

Choice A is incorrect because only X, the cytosol, is primarily composed of water and solutes, Y, the nucleus, primarily contains the DNA. Choice B is incorrect because Y, the nucleus, has a specialised function. Choice D is incorrect because Y, the nucleus, is not found in prokaryotes, but both X, the cytosol, and Y, the nucleus, are found in eukaryotes.

Explanation​:
Autotrophs are producers (Choice A, B). They are self-feeding whereas heterotrophs are organisms that gain nutrition by feeding on another organisms (Choice C, D). Detritivores consume dead organic matter by ingestion (Choice C) whereas saprotrophs break down food outside their bodies by secreting enzymes into the environment.

Explanation:
Regular exercise prevents the buildup of fatty plaque in the coronary arteries thereby preventing heart disease (Choice D). Some of the factors that cause the buildup of fatty plaque and occlusion of the arteries causing chest pain are high blood pressure (Choice A), high blood sugar due to diabetes (Choice B) and some genetic factors (Choice C).

Explanation​:
Choice B is correct because disease is a living factor and is, therefore, a biotic factor, not an abiotic one. Sunlight (Choice A), precipitation (Choice C), and pH (Choice D) are all non-living factors and are therefore abiotic.

Explanation​:
Choice C is correct because cellular components are separated using cell fractionation, which requires the ultracentrifuge to separate the cellular components based on density.

Choice A is incorrect because chromatography separates mixtures of solutes, such as pigments, based on adhesion. Choice B is incorrect because gel electrophoresis separates molecules such as nucleic acids based on size, charge or both. Choice D is incorrect because PCR is used to amplify DNA fragments and is not a separation technique.

Explanation:
For oxygen to diffuse into the blood, there must be a higher concentration of oxygen in the alveoli than in the blood (Choice D). This is because gases only diffuse down their concentration gradients.

Similarly, for carbon dioxide to diffuse into the alveoli, there should be a lower concentration of carbon dioxide in the alveoli (Choice C) and a higher concentration of carbon dioxide in the blood (Choice B).

Explanation​:
Choice B is correct because incisors are used for biting chunks of food. Choice A is incorrect because tearing is the role of canines. Choices C and D are incorrect because crushing or slicing is the role of the premolars, and grinding is the role of molars.

Explanation:
Choice A is the correct response because arteries are not non-elastic, instead they do have a degree of elasticity. Choices B, C and D all correctly match the structure and function of each blood vessel.

Explanation​:
Choice C – The statement is false. Phospholipids are a class of lipids which have a hydrophilic “head” and two hydrophobic “tails.” The heads contain a phosphate group and the two tails each contain one fatty acid.

Choice A, B, and D – These statements are all true.

Explanation:
Saprotrophs are decomposers that secrete digestive enzymes onto organic molecules and then absorb the digested nutrients. This is a form of extra-cellular nutrition. Fungi and certain bacteria are examples of saprotrophs. Woodlice or pill bugs are detritivores. Detritivores are decomposers, but they ingest the organic molecules before digesting them into smaller, soluble ones. Choice D is, therefore, correct. Mushrooms and yeast are fungi, and Bacillus subtilis is a bacterium. Therefore Choices A, B and C are all saprotrophs and therefore are incorrect.

Explanation​:
Choice D is correct because line transects can be used to show animal or plant distributions. Choices A, B and C are all factors that could be measured using different sensors that could provide meaning to plant or animal distributions.

Explanation:
The triglyceride molecule consists of one glycerol molecule and three fatty acids.

A phosphate head and two fatty acid tails (Choice A) form a phospholipid, not a triglyceride. One fatty acid and three glycerol molecules (Choice B) or one glycerol molecule and three monosaccharides (Choice D) cannot form a triglyceride.