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​:
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​:
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:
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:
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​:
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​:
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:
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​:
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:
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​:
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​:
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:
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​:
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 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 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:
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 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 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​:
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​:
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​:
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​:
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.