Life Processes

Sugars made in leaves or released from storage are transported through phloem from a source to a sink, such as a growing or storage region. Active loading of sugars into phloem uses energy, which is why translocation requires energy.

Egestion is the removal of undigested food from the digestive tract. Excretion is the removal of metabolic wastes, such as urea and carbon dioxide, produced by the body's cells. They remove different kinds of waste.

Filtration in a nephron allows wastes to enter the filtrate, but useful substances such as glucose and much of the water also enter it. Reabsorption returns needed substances and water to the blood; without it, they could be lost in urine.

When kidneys cannot filter the blood adequately, wastes and excess water and salts can build up. During dialysis, these substances pass from the blood across a semipermeable membrane into a cleansing fluid, helping clean the blood.

Plants can store some wastes in leaves, bark, resins or gums. When leaves or other old parts are shed, some of the stored wastes are removed with them. Plants also use other routes, such as diffusion, to dispose of some wastes.

Nutrition supplies food to the body, but cells need respiration to release usable energy from that food. If respiration is impaired, less usable energy becomes available even when nutrition is normal.

The products of one process can be used in the other: photosynthesis makes food and oxygen from carbon dioxide and water, while aerobic respiration uses food and oxygen and releases carbon dioxide and water. Thus, matter is recycled between the processes; energy flows rather than being recycled.

In a multicellular organism, many cells are far from the body surface. Diffusion is effective over short distances but too slow to supply all these cells quickly, so specialised transport systems are needed.

A unicellular organism is small and its single cell is in contact with the surrounding environment. Materials therefore travel only short distances, allowing diffusion to meet many of its exchange needs.

Oxygen diffuses from the air in the alveoli into nearby blood capillaries. The circulatory system carries it to cells, where it is used in aerobic respiration to release energy from food.

Cells produce carbon dioxide during respiration, and the blood carries it to the lungs for removal. If too much remains in the body, it can make the blood more acidic and disturb acid–base balance.

Digestion breaks food into simpler substances, but these nutrients still need to be absorbed into the blood and transported to cells throughout the body. Digestion alone does not deliver them to the cells.

Digestion makes food molecules small enough for absorption. If they are not absorbed into the blood, cells cannot receive these nutrients to use as fuel in cellular respiration, so energy supply can fall.

Organisms obtain food or, in the case of plants, make it. Food that needs digestion is broken into simpler substances; cells then break down food molecules during cellular respiration, releasing usable energy.

Metabolic processes produce waste products. If these wastes build up, they can harm cells or interfere with their normal activities. Excretion removes such wastes and helps maintain a stable internal environment.

Xylem transports water from the roots to the leaves. Since water is a raw material for photosynthesis, a blockage that reduces its supply to leaves can limit the rate of food production.

Phloem transports sugars and other food made in leaves to parts of the plant, including roots. If this transport is impaired, roots may receive less food for growth and energy.

Digestion makes nutrients available, and transport delivers nutrients and gases to cells. Cells use these materials in respiration to release energy, so the processes are interdependent.

Cells produce waste products, and the transport system carries some of them in the blood to organs that remove them. If excretion stops, these wastes cannot be removed and may accumulate in the blood.

The body needs nutrients and energy, must transport materials to cells, and must remove wastes. Together, these processes help maintain a stable internal environment.

During photosynthesis, plants use light to make carbohydrates from carbon dioxide and water. Oxygen is released as a by-product, so its release is associated with this food-making process.

Bile salts emulsify large fat droplets into smaller ones; bile does not digest fat chemically. This increases the surface available for lipase, the fat-digesting enzyme, to act on.

Filtration in the nephron lets small substances pass from the blood into the filtrate. This includes wastes, but also useful substances and water. Selective reabsorption returns needed substances to the blood.

Cells need materials from nutrition and gases delivered by transport, and they release energy through respiration. Excretion then removes wastes produced by the body's activities. This shows how the processes work together.

Living organisms need materials and energy to carry out life activities. Their life processes make substances available and transport them where needed, while excretion removes wastes and helps maintain internal balance.