Question 1

What is the difference between positive and negative feedback?

Accepted Answer

Negative feedback reverses a change to maintain a stable internal environment, like when blood glucose rises and insulin is released to lower it. Positive feedback amplifies a change, moving the system away from its set point; it's less common but important in processes like childbirth (oxytocin release) and blood clotting. In A-Level Biology, you need to know that negative feedback is the main homeostatic mechanism, while positive feedback is usually harmful unless it's part of a specific process that needs to be completed quickly.

Question 2

How does the kidney produce concentrated urine?

Accepted Answer

The kidney produces concentrated urine using the loop of Henle, which creates a concentration gradient in the medulla. The descending limb is permeable to water but not salts, so water leaves by osmosis. The ascending limb is impermeable to water but actively transports sodium and chloride ions out, making the medulla interstitial fluid very salty. In the collecting duct, antidiuretic hormone (ADH) increases water permeability, allowing water to be reabsorbed into the salty medulla, concentrating the urine. This process is vital for water conservation.

Question 3

Why is ATP called the universal energy currency?

Accepted Answer

ATP (adenosine triphosphate) is called the universal energy currency because it stores and transfers energy in all living cells. When ATP is hydrolysed to ADP and inorganic phosphate, it releases a small, manageable amount of energy (about 30.6 kJ/mol) that can be used directly for cellular processes like muscle contraction, active transport, and biosynthesis. ATP is continuously regenerated through respiration and photosynthesis, making it a renewable source of energy that couples exergonic and endergonic reactions.

Question 4

What is the role of NAD in respiration?

Accepted Answer

NAD (nicotinamide adenine dinucleotide) is a coenzyme that acts as a hydrogen carrier in respiration. It accepts hydrogen atoms (reducing to NADH) during glycolysis and the Krebs cycle, and then delivers them to the electron transport chain in oxidative phosphorylation. This transfer of electrons is crucial for creating a proton gradient that drives ATP synthesis. Without NAD, the Krebs cycle would stop, and ATP production would plummet. NAD is regenerated from NADH when it donates electrons, allowing it to be reused.

Question 5

How do plants adapt to low light conditions for photosynthesis?

Accepted Answer

Plants adapted to low light, like those on a forest floor, often have larger, thinner leaves with more chloroplasts to capture more light. They may also have a higher concentration of chlorophyll and other pigments (like carotenoids) to absorb a broader spectrum of light. Some plants adjust their leaf angle to maximise light capture. In terms of biochemistry, they may have a higher rate of the light-dependent reactions relative to the Calvin cycle, and they often use C3 photosynthesis, which is more efficient in low light than C4 or CAM.

Question 6

What is the difference between aerobic and anaerobic respiration?

Accepted Answer

Aerobic respiration uses oxygen to fully oxidise glucose to carbon dioxide and water, producing up to 38 ATP per glucose. It occurs in the cytoplasm and mitochondria, involving glycolysis, the link reaction, the Krebs cycle, and oxidative phosphorylation. Anaerobic respiration occurs without oxygen and only produces 2 ATP per glucose via glycolysis, followed by fermentation. In animals, pyruvate is reduced to lactate (lactic acid) to regenerate NAD, while in plants and yeast, it's converted to ethanol and carbon dioxide. Anaerobic respiration is less efficient but allows ATP production when oxygen is scarce.

Module 5: Communication, homeostasis and energy

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Related Topics in OCR A-Level Biology

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