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​:
Choice D is correct because starch is made of chains of alpha glucose, proteins are made of chains of amino acids and triglycerides are made of fatty acids and glycerol. Choice A is incorrect because starch is not composed of beta glucose and peptides are not the simplest units of proteins. Choice B is incorrect because starch is not composed of beta glucose and triglycerides do not contain glycogen. Choice C is incorrect because peptides are not the smallest unit of proteins and triglycerides do not contain glycogen.

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:
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:
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​:
A potometer is used to measure the rate of transpiration in a plant. When transpiration takes place, water loss occurs because water evaporates inside the leaves and then diffuses out of the leaves. The bubble in the capillary tube will move in the direction of the plant along with the water in the capillary tube which is being absorbed by the plant (Choice C).

Choice B is incorrect because the water and bubble are moving towards the plant, not away from it.

Choice A is incorrect because the water in the reservoir will stay at the same level. The reservoir contains a switch that is used to refill the capillary tube and push the bubble back to the start position. This is done at the end of the experiment to reset the apparatus for the next measurement.

Choice D is incorrect as the water level in the beaker will decrease as water enters the capillary tube and also because of evaporation from the beaker to the atmosphere.

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​:
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​:
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​:
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​:
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 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 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​:
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​:
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 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:
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:
For oxygen to diffuse into the blood, there must be a higher concentration of oxygen in the alveoli than in the blood (Choice D). This is because gases only diffuse down their concentration gradients.

Similarly, for carbon dioxide to diffuse into the alveoli, there should be a lower concentration of carbon dioxide in the alveoli (Choice C) and a higher concentration of carbon dioxide in the blood (Choice B).

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​:
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​:
In prokaryotes, the mRNA produced after transcription is ready for translation and does not undergo splicing. In contrast, eukaryotic pre-mRNA undergoes splicing to remove introns and join exons, producing mature mRNA. Choice C is, therefore, correct.

Both prokaryotes and eukaryotes use DNA as a template for transcription. Choice A is, therefore, incorrect. In prokaryotes, transcription occurs in the cytoplasm, while in eukaryotes, it occurs in the nucleus. Choice B is, therefore, incorrect. Splicing of mRNA occurs in eukaryotes, not prokaryotes. Choice D is, therefore, incorrect

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 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:
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:
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.