Explanation:
Choice C is correct because P is pointing to a disulfide bond, which can only be formed between R groups of amino acids and is a feature of the tertiary structure. The primary structure (Choice A) is the sequence of amino acids in a polypeptide chain. The secondary structure (Choice B) is the formation of alpha-helices or beta-pleated sheets through hydrogen bonding between carboxyl and amino groups of different amino acids. The quaternary structure (Choice D) exists in proteins with more than one polypeptide chain.

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:
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:
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:
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:
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:
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:
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:
The contraction and relaxation of muscles (Choice C) cause bones to move. Cartilage (Choice A) is the smooth tissue covering the end of bones in joints to reduce friction. Ligaments (Choice B) join bone to bone and help keep joints together. Tendons (Choice D) connect muscle to bone, but aren’t responsible for movement.

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:
Choice C is correct because immediately after prophase is prometaphase, during which the nuclear membrane breaks down into small vesicles.

Choice A is incorrect because, during prophase, the nuclear membrane already has existing pores. Choice B is incorrect because the nuclear membrane only reassembles during telophase, which is the last stage of mitosis, and not immediately after prophase. Choice D is incorrect because the mitotic spindle forms between the poles of the cell immediately after prophase.

Explanation:
The disaccharide sucrose is made up of the monosaccharides glucose and fructose. Sucrose forms when glucose and fructose combine in a condensation reaction (a reaction that releases water).

Maltose (Choice A) is a disaccharide made up of two glucose molecules. Lactose (Choice B) is a disaccharide made up of glucose and galactose. Galactose (Choice D) is a monosaccharide, not a disaccharide.

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 protein filament that is thicker and has heads is made of myosin (arrow I). The thinner protein filaments are made of actin, troponin and tropomyosin. Arrow II is pointing to the actin component of the thin filament (Choice D). Tropomyosin is a protein in the thin filament that masks myosin-binding sites (shown as thin black lines). Troponin is also a protein found in thin filaments. It has binding sites for calcium ions (shown as small double circles attached to tropomyosin lines).

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:
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:
Antagonistic muscles in the upper leg of crickets allow a high power output to be generated (Choice D). Strong tendons (Choice A) and a strong exoskeleton (Choice B) are necessary for movement, but do not contribute directly when generating the power that allows significant height to be gained during jumping. Crickets do not have synovial joints (Choice C).

Explanation:
Choice A - Cholesterol is positioned between the phospholipids and maintains the fluidity of the cell membrane.

Choice B - Integral Membrane Proteins are responsible for the transport of materials in and out of the cell as well as other functions such as cell-cell communication.

Choice C - Glycolipids allow cells to recognize each other.

Choice D - Peripheral membrane proteins are involved in maintaining the extracellular matrix and cytoskeleton, in cell-cell communication, and sometimes function as enzymes.

Explanation:
Choice A is correct because microvilli are present, increasing the cell's surface area for absorption. Cells of the proximal convoluted tubule in the nephron require a large surface area for selected reabsorption of substances in the filtrate. Choice B is incorrect because cells lining capillaries are flat (squamous epithelial tissue). Choice C is incorrect because cells of the immune system do not have microvilli. Choice D is incorrect because the function of microvilli is not insulation, and adipose cells do not have microvilli.

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:
Insulin is a hormone whose function is to maintain blood glucose levels (Choice A). Rhodopsin (Choice B) is a pigment responsible for the detection of light. Collagen (Choice C) is a structural protein and Rubisco (Choice D) is an enzyme.

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:
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:
The Golgi apparatus is a membrane-bound organelle that processes, modifies and packages proteins and lipids for secretion or delivery to various cellular compartments. Choice C is, therefore, correct.

The cell wall (Choice A) does provide structural support and protection to plant cells, but it is not an organelle, it is an extracellular structure external to the plasma membrane. The cytoplasm (Choice B) is not an organelle, but its role is to provide a medium for metabolic reactions to occur in. Although anaerobic respiration occurs in the cytoplasm, the cytoplasm does not produce ATP itself.. Lysosomes (Choice D) digest worn-out organelles, pathogens, macromolecules and debris, however, they are membrane-bound organelles.

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.