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​:
Ducks and fish are consumers (Choice B, C and D), which is a type of heterotroph, whereas Azolla and rice can carry out photosynthesis and are, therefore, autotrophs (self-feeding).

Explanation:
In arid environments, water availability is often a primary limiting factor. However, soil compaction (Choice C) can exacerbate this limitation by reducing water infiltration and root penetration. High humidity (Choice A), excessive rainfall (Choice B), and low light intensity (Choice D) are less likely to be limiting factors in arid environments.

Explanation:
Structure X is a peripheral protein that is attached to one surface of the cell membrane.

Protein channels span the width of membranes (Choice B) as do transmembrane proteins (Choice A). Cholesterol molecules do not have a globular structure (Choice D).

Explanation:
Water is transported passively via protein channels known as aquaporins.

Cholesterol (Choice A) and glycoproteins (Choice B) are not components responsible for transport while protein pumps (Choice C) carry out active transport which is not utilized in the transport of water.

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.

Explanation​:
Statement I
In the Singer-Nicholson model of the plasma membrane, proteins were believed to move within the phospholipid bilayer. The finding that proteins can move throughout the membrane supported the Singer-Nicholson model and refuted earlier models. Statement I is correct.

Statement II
In an earlier model, proteins were assumed to form one layer above and one layer below the phospholipids. The finding that some membrane proteins were hydrophobic (nonpolar) refuted this model and supported the Singer-Nicholson model, since hydrophobic proteins could not form a continuous layer along the charged phospholipid heads but could be embedded within the nonpolar phospholipid tails. Statement II is correct.

Statement III
In an earlier model, the membrane was assumed to consist of three layers (a layer of lipids sandwiched between two layers of proteins). The finding that the membrane consists of two dark bands separated by a light band was consistent with this earlier model. Statement III is incorrect.

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​:
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 – 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​:
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​:
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 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​:
Water diffuses out of stomata from the inner air space of leaves during transpiration (Option I). Because oxygen is produced during photosynthesis, its concentration is generally higher in the inner air space than the atmosphere which means it diffuses out through the stomata (Option II). Mineral ions (Option III) enter plants through root cells, not through the stomata.

Explanation:
Choice A is correct because there is no compartmentalisation in prokaryotes, hence transcription and translation both take place in the cytoplasm.

Choice B is incorrect because mRNA splicing does not occur in prokaryotes, but does occur in the nucleus of eukaryotic cells. Choice C and D are incorrect because the regulation of transcription and translation of mRNA into a polypeptide chain occurs in both prokaryotes and eukaryotes.

Explanation:
The diagram shows glycerol and 3 fatty acids. When these are joined in a condensation reaction, a triglyceride is formed. Choice D is correct.

Choices A, B and C 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​:
During exhalation, the diaphragm relaxes and lifts upwards to form a dome shape (Statement I). The internal intercostal muscles contract and pull the ribcage inwards and downwards (Statement II).

This causes the volume inside the thorax to decrease which increases pressure inside the thorax (Statement III). Thus the air is forced out of the lungs.

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:
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:
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 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:
Holozoic nutrition refers to the type of nutrition where organisms ingest whole or partially whole food items (Statement II). The ingested food is then broken down by the organism's internal digestive system (Statement III). Therefore Choice B is correct. Saprotrophs (Statement I) are organisms that obtain their nutrients by decomposing dead and decaying organic matter externally and then absorbing the nutrients. Therefore Choices A, C and D are not correct as they all include Statement I.

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 A is correct because a lysosome usually fuses with phagocytic vacuoles containing pathogens or cellular debris, or other vacuoles in the cytoplasm. The enzymes are then released from the lysosome into the vacuole to digest the contents.

Choice B is incorrect because lysosomes contain digestive enzymes that break down cellular components, which involve catabolic reactions, not anabolic reactions. Choice C is incorrect because lysosomes form from vesicles budding off of the Golgi apparatus. Choice D is incorrect because lysosomes are membrane-bound organelles.