What is the molecular geometry of a molecule of sulphur dioxide (SO2)?

A buffer is a solution that resists changes in pH when small amounts of an acid or base are added. Buffers work by neutralizing added hydrogen ions (H⁺) or hydroxide ions (OH⁻), thereby maintaining a relatively stable pH. Buffers are made up of a weak acid and its conjugate base or a weak base and its conjugate acid.

• A. It decreases the pH of the solution: This is incorrect because a buffer does not always decrease pH; it resists changes in both directions.
• C. It causes the pH of a solution to become neutral: Incorrect because buffers do not necessarily make a solution neutral; they stabilize pH around a certain value.
• D. It permanently binds hydrogen ions: Incorrect as the binding is reversible, which is essential for maintaining pH balance.

The epididymis is a coiled tube located at the back of each testicle where the sperm mature and are stored until ejaculation. Sperm are produced in the testes and then transported to the epididymis where they undergo maturation and become motile. The epididymis provides a protective environment for the sperm, allowing them to mature and become more resilient to external stressors. During ejaculation, the sperm are transported from the epididymis to the vas deferens and then to the urethra for ejaculation.

Muscle contraction is a complex process that involves the interaction between actin and myosin filaments in the muscle fibers. The sliding of these filaments is initiated by the release of calcium ions from the sarcoplasmic reticulum, a specialized organelle in muscle cells. The calcium ions bind to the protein troponin, which causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on actin. This allows the myosin heads to bind to actin, forming cross-bridges that pull the actin filaments towards the center of the sarcomere, resulting in muscle contraction.

Option a) is incorrect because calcium does not bind to tropomyosin directly, but rather binds to the protein troponin, causing a conformational change in the troponin-tropomyosin complex. Option c) is incorrect because calcium does not activate motor neurons, but rather is released from the sarcoplasmic reticulum in response to an action potential that travels down the motor neuron to the neuromuscular junction. Option d) is incorrect because calcium is required for muscle contraction, not relaxation. The relaxation of muscles after contraction is due to the active transport of calcium ions back into the sarcoplasmic reticulum, which allows the troponin-tropomyosin complex to return to its resting conformation, blocking the myosin-binding sites on actin and ending the cross-bridge cycle.

Diaphragm is responsible for regulating breathing rate and depth. It is a dome-shaped muscle located at the

bottom of the chest cavity that contracts and relaxes to help move air in and out of the lungs.

Isotonic and isometric contractions are two types of muscle contractions that differ in the amount of force produced and the movement of the muscle. In isotonic contractions, the muscle changes length and produces movement, such as lifting a weight. The force generated by the muscle remains constant throughout the movement. Isotonic contractions can be further classified as concentric contractions, in which the muscle shortens as it contracts, and eccentric contractions, in which the muscle lengthens as it contracts.

In contrast, isometric contractions occur when the muscle generates force without changing its length or producing movement. For example, holding a weight in a fixed position without moving it requires an isometric contraction. In an isometric contraction, the force generated by the muscle increases up to a maximum and then remains constant. Isometric contractions can be used to build strength and endurance in the muscle, but they do not produce movement.

The key structural difference between starch and cellulose lies in the type of glucose monomers they contain:

• Starch is composed of alpha-glucose monomers, which are linked by α(1→4) glycosidic bonds.
• Cellulose is composed of beta-glucose monomers, which are linked by β(1→4) glycosidic bonds.

This difference in the orientation of the glucose molecules leads to different structural properties:

• In starch, the alpha-glucose linkage causes the molecules to form a helical, more easily digestible structure.
• In cellulose, the beta-glucose linkage results in straight, rigid chains that form strong fibers through hydrogen bonding, making it difficult for most organisms to digest.

The other options are incorrect:

• A. Incorrect, as cellulose fibrils do have hydrogen bonds, which contribute to its rigid structure.
• B. Incorrect, as both starch and cellulose are made of glucose, not fructose.
• D. Incorrect, both starch and cellulose contain cyclized glucose monomers, but the orientation differs.

The renal vein is responsible for draining oxygen-depleted blood from the kidneys and carrying it back to the heart through the inferior vena cava.

The other options refer to different structures:

• B. Renal Artery: Brings oxygenated blood to the kidneys, not draining it.
• C. Urethra: Transports urine from the bladder to the outside of the body, not involved in blood flow.
• D. Ureter: Carries urine from the kidneys to the bladder, also not related to blood drainage.

The largest organ in the human body by surface area is the skin. It covers the entire external surface of the body and has an average surface area of about 20 square feet in adults.

One of the key differences between skeletal muscles and cardiac muscles is the presence of intercalated discs in cardiac muscle tissue. These discs are specialized structures that facilitate communication and synchronization between cardiac muscle cells, allowing the heart to contract as a unified organ.

The other options are incorrect:

• A. Skeletal muscles are autorhythmic, whereas cardiac muscles are not: This is incorrect because cardiac muscles are autorhythmic; they can generate their own rhythmic contractions. Skeletal muscles require nervous system stimulation to contract.
• C. Skeletal muscles are found in the viscera, whereas cardiac muscles are found in the cranium: This is incorrect; skeletal muscles are primarily associated with the skeleton (attached to bones) and are not typically found in the viscera, while cardiac muscle is found in the heart.
• D. Cardiac muscles are voluntary, whereas skeletal muscles are involuntary: This is incorrect; skeletal muscles are voluntary (under conscious control), while cardiac muscles are involuntary (not under conscious control).

Therefore, the correct distinction is that cardiac muscles contain intercalated discs, while skeletal muscles do not.