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​:
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​:
Ducks and fish are consumers (Choice B, C and D), which is a type of heterotroph, whereas Azolla and rice can carry out photosynthesis and are, therefore, autotrophs (self-feeding).

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:
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:
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​:
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:
Choice A is correct because there is no compartmentalisation in prokaryotes, hence transcription and translation both take place in the cytoplasm.

Choice B is incorrect because mRNA splicing does not occur in prokaryotes, but does occur in the nucleus of eukaryotic cells. Choice C and D are incorrect because the regulation of transcription and translation of mRNA into a polypeptide chain occurs in both prokaryotes and eukaryotes.

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​:
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​:
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 – 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:
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​:
Statement II is incorrect as both starch and glycogen are made up of $\alpha -$ glucose molecules (Choices B and D). Choice A is incorrect as it only includes one correct statement (statement I).

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​:
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​:
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 correct because a lysosome usually fuses with phagocytic vacuoles containing pathogens or cellular debris, or other vacuoles in the cytoplasm. The enzymes are then released from the lysosome into the vacuole to digest the contents.

Choice B is incorrect because lysosomes contain digestive enzymes that break down cellular components, which involve catabolic reactions, not anabolic reactions. Choice C is incorrect because lysosomes form from vesicles budding off of the Golgi apparatus. Choice D is incorrect because lysosomes are membrane-bound organelles.

Explanation​:
Choice B is correct because incisors are used for biting chunks of food. Choice A is incorrect because tearing is the role of canines. Choices C and D are incorrect because crushing or slicing is the role of the premolars, and grinding is the role of molars.

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​:
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:
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​:
The process of air entering the lungs is known as inhalation. The term $\text{“}$inspiration$\text{”}$ is used instead of inhalation sometimes. In order for inhalation to take place, the pressure inside the lungs must be lower compared to atmospheric pressure. To decrease the pressure inside the lungs, the external intercostal muscles must contract and become shorter to pull the ribs up and out, increasing the volume inside the thorax.

Choice A is incorrect because abdominal wall muscles relax during inhalation. Lengthening of the abdominal muscles can contribute to inhalation by pushing the abdominal wall out which increases the volume inside the thorax.

Choice B is incorrect because when the diaphragm relaxes it forms a dome shape and therefore reduces the volume of the thorax. This occurs during exhalation.

Choice D is incorrect because internal intercostal muscles relax and lengthen during inhalation. They contract and shorten during exhalation.

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.