Explanation​:
Choice D is correct because larger cells do not generally have fewer transport proteins. Choice D is also correct because as cell size increases, cytoplasmic volume increases, increasing the cell's metabolic demands which increases the cell’s need to exchange substances. Because cytoplasmic volume increases more rapidly than the surface area of the cell membrane, the surface area to volume ratio decreases. This means that as cell size increases, the cell membrane's surface area is soon unable to excrete waste and intake nutrients rapidly enough.

Choice A and C are incorrect as they include the statement that fewer transport proteins are present in the cell membranes of larger cells. Choice B is incorrect as it does not include the statement that increases in cytoplasmic volume increase metabolic demands

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:
Cholesterol is a steroid, not a transport channel.

Reducing membrane fluidity (Choice A), increasing membrane flexibility (Choice B), and reducing permeability to hydrophilic substances (Choice D) are all roles of cholesterol in cell membranes.

Explanation:
Choice A is correct because dense bones would make the duck heavier, therefore making it harder for it to move.

Choice B is incorrect, as wings enable the duck to fly. Choice C is incorrect, as webbed feet aid the duck in swimming more efficiently. Choice D is incorrect as the oily coating on feathers helps to waterproof the duck and keep it light so it does not become waterlogged. This helps the duck to swim and fly more easily.

Explanation:
Sieve plates (Statement I) and plasmodesmata (Statement II) are features found in phloem. Sieve plates possess pores which allow continual flow of sap through the sieve tubes. Plasmodesmata are small channels which connect the sieve tube elements to the adjacent companion cells. A high lignin content (Statement III) is found in xylem vessels. It provides support to withstand hydrostatic pressure, and is also impermeable to water.

Explanation​:
Obligate anaerobes are single-celled organisms that will not survive if oxygen is present. Facultative anaerobes are organisms that can either respire aerobically or anaerobically. Obligate aerobes can only respire if oxygen is present and cannot survive in conditions low in oxygen. Therefore Choice A is correct.

Choices B and D are incorrect because obligate anaerobes cannot respire when oxygen is present. Choice C is incorrect as obligate anaerobes require anoxic conditions, and obligate aerobes cannot carry out anaerobic respiration.

Explanation​:
Blood plasma (Choice D) is the liquid component of blood and is composed of water which contains proteins, ions, and dissolved nutrients.
Red blood cells (Choice A) transport oxygen.
Platelets (Choice C) are responsible for the process of blood clotting.
White blood cells (Choice B) are part of the immune system.

Explanation​:
The sinoatrial node is a region of specialized cardiac muscle cells found in the wall of the right atrium. The sinoatrial node acts as a pacemaker and is involved in controlling heart rate (Choice A).
The opening of the atrioventricular valves occurs when there is an increase in pressure inside the atria. Blood flows to the ventricles when these valves are open. This action does not control the heart rate (Choice B).
Pulmonary circulation is when blood flows into the lungs from the right ventricle and is returned to the left atrium after it is oxygenated, and does not control the heart rate (Choice C).
The opening of the semilunar valves occurs when the pressure in the ventricles increases (Choice D). This does not control heart rate.

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​:
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​:
When two monosaccharides are joined together to form a disaccharide, water is produced, therefore it is a condensation reaction. Choice A is correct.

Choice B is incorrect as hydrolysis uses water to break bonds, in this case converting a disaccharide into a monosaccharides. Choice C is incorrect as catabolism breaks down large molecules into smaller molecules. Choice D is incorrect as oxidation means adding oxygen or losing electrons.

Explanation​:
Choice D is correct because zebras run away rather than towards predators. Choices A, B and C are incorrect because zebras move to find mates, escape grassland fires and find sources of food.

Explanation​:
Choice C correctly describes the cell type adapted for gas exchange in the lungs. Choice A is incorrect because although it is a correct description of type II pneumocytes, these cells are not adapted for gas exchange. Choice B is incorrect because type I pneumocytes do not contain many secretory vesicles. Choice D is incorrect because type II pneumocytes are not extremely thin.

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​:
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​:
Statements I and III are correct. Statement II is incorrect because only some joints (knee and elbow) allow movement in one direction; there are other synovial joints that allow movement in additional directions (hip and shoulder).

Explanation​:
Sucrose is actively loaded into phloem at sites called sources, which are regions of the plant where photosynthesis occurs (such as the leaves) or where nutrients are stored (such as in storage organs). Sucrose is unloaded from phloem tubes in sinks, which are parts of the plant that can’t produce organic compounds themselves, such as the roots. The movement of sucrose in the phloem is bidirectional, meaning it can be transported upwards or downwards, depending on the location of the sink. This relates to Choice A.

Choice B refers to sugar being transported from sink to the source which is incorrect. Choices C and D refer to sugar transport being down only, which is incorrect.

Explanation​:
Amino acids (Choice B) can form polypeptides through the formation of peptide bonds as a result of condensation reactions. Fatty acids (Choice A) are involved in the formation of lipids and are not monomers. Proteins (Choice C) are composed of one or more polypeptides and are polymers. Nucleotides (Choice D) are the building blocks of DNA and RNA.

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​:
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 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:
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:
The release of Calcium ions from the sarcoplasmic reticulum initiates the process of muscle contraction (Choice A). Potassium ions (Choice B) and sodium ions (Choice C) are involved in nerve transmission and raising the membrane potential of muscle fibres to bring about contraction, but they do not bind to the sarcomere. Hydrogen ions (Choice D) do not diffuse from the sarcoplasmic reticulum.

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:
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.