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:
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:
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:
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:
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:
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:
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 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:
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 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:
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:
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:
A cell membrane is formed of two layers of phospholipids whereby hydrophilic phosphate heads face the cytoplasm of the cell (cell interior) and the cell's environment (exterior) while the hydrophobic lipid tails face each other and form the interior of the membrane.

Having lipid tails pointing away (Choice A) is not a valid choice since lipid tails are hydrophobic and cannot face the cell's aqueous interior and exterior. Proteins are embedded in the membrane and do not line the outer layer (Choice C). The cell membrane is a bilayer and not just a monolayer (Choice D).

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:
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:
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:
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 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 B is correct because Rubisco is the name of the enzyme which is responsible for the fixation of carbon during photosynthesis. Choice A is incorrect as rhodopsin is a pigment found in the retina which is responsible for detecting light . Choices C and D are incorrect as neither are enzymes, with collagen being a structural component of tissues such as skin and insulin being a hormone responsible for regulating blood glucose levels.

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 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 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 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 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 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.