Life Processes

The correct answer is B. Closed stomata reduce the diffusion of carbon dioxide from the air into the leaf. Because carbon dioxide is a raw material for photosynthesis, its reduced availability can lower the rate even when there is enough light.

Open stomata allow carbon dioxide to enter for photosynthesis, but they also let water vapour escape. Closing them reduces water loss, though it also limits gas exchange. This is how stomata help balance gas exchange and water conservation.

The green pigment chlorophyll absorbs light energy. The plant uses this energy to drive photosynthesis and make food.

Leaves make sugars by photosynthesis. Phloem transports these sugars from the leaves to other parts of the plant, including roots, where they can be used or stored.

When water evaporates from the leaves during transpiration, it creates a pull in the xylem. This transpiration pull helps draw water upward from the roots to the leaves.

During photosynthesis, plants use carbon dioxide and water, with light energy, to make glucose. Oxygen is also released in the process.

Amoeba extends pseudopodia around a food particle and engulfs it. Taking food into the organism is called ingestion; the food is then enclosed in a food vacuole.

Muscles in the wall of the alimentary canal contract and relax in waves. These waves, called peristaltic movements, push food forward.

Salivary amylase starts breaking down starch in the mouth. In the stomach, the acidic conditions inhibit this enzyme, so its starch-digesting action becomes limited.

Stomach acid provides the acidic conditions needed for pepsin to digest proteins and also helps kill many microbes. Mucus protects the stomach lining from the acid and digestive enzymes, balancing digestion with protection.

Bile emulsifies large fat globules into smaller droplets; it does not chemically digest fat itself. The smaller droplets provide more surface area for lipase to act on, helping fat digestion.

The small intestine receives bile and pancreatic and intestinal secretions, where much of digestion is completed. Its villi provide a large surface for absorbing digested nutrients into the blood and lymph.

Villi increase the surface area available for nutrient absorption, while alveoli increase the surface area available for gas exchange. In both cases, the larger surface helps exchange occur effectively.

Most digestion takes place in the small intestine. The remaining material passes into the large intestine, where water is absorbed and the faeces become more solid.

Breathing is the physical movement of air into and out of the respiratory system. Cellular respiration is a chemical process in cells that breaks down nutrients, such as glucose, to release usable energy.

In aerobic respiration, oxygen allows glucose to be broken down more completely than in anaerobic respiration. This more complete breakdown releases more energy.

During heavy exercise, muscles need energy rapidly. If oxygen supply cannot meet this demand, some glucose can be broken down anaerobically in muscle cells, producing lactic acid.

Many alveoli provide a large surface area, and their thin walls shorten the distance gases must diffuse. Together, these features support effective oxygen and carbon dioxide exchange.

Haemoglobin carries oxygen in the blood. If haemoglobin is low, less oxygen may reach body tissues, which can reduce aerobic energy production and contribute to tiredness and breathlessness.

The four chambers keep oxygenated blood separate from deoxygenated blood. This helps deliver oxygen-rich blood efficiently to the body, supporting the aerobic respiration needed to meet its high energy demand.

In double circulation, blood passes through the heart between its trip to the lungs and its trip to the body. This keeps the routes separate and allows blood to be sent to the lungs at lower pressure and to the body at higher pressure, supporting effective oxygen delivery.

Arteries carry blood away from the heart under relatively high pressure, so their walls are thick and elastic. Blood flows through veins at lower pressure, and their valves help prevent it from flowing backwards.

Capillaries have very thin walls, which provide a short path for substances such as oxygen and nutrients to move between the blood and surrounding tissues. Arteries have thicker walls adapted to withstand higher pressure.

Some fluid from blood enters the spaces around tissue cells. Lymph collects this excess tissue fluid and helps return it to the bloodstream, supporting fluid balance and circulation.

In humans, the heart provides the pumping force for blood circulation. In plants, transpiration from leaves creates a pull that helps draw water upward through xylem; root pressure can also contribute. Thus, the main driving forces differ.