Life Processes Notes

The processes that together keep an organism alive are called life processes. The maintenance functions go on even when the organism is not visibly active — a sleeping student still respires, circulates blood, and removes wastes.

Living things need a constant input of energy to repair and replace damaged structures and to keep their organisation from breaking down. Energy comes from food, food is broken down using oxygen in most organisms, and materials must be moved in and wastes moved out. So the four life processes usually discussed together are nutrition, respiration, transportation, and excretion.

In single-celled organisms such as Amoeba, the whole surface is in contact with the environment, so exchange of gases and removal of wastes happen by simple diffusion. In large multicellular organisms most cells are not in direct contact with the surroundings, so specialised organ systems are needed.

Autotrophic nutrition is the making of food from simple inorganic substances such as carbon dioxide and water using light energy. Green plants and some bacteria are autotrophs. Heterotrophic nutrition is taking in food made by other organisms; animals, fungi, and most bacteria are heterotrophs.

Photosynthesis occurs mainly in the chloroplasts of leaf mesophyll cells, which contain the green pigment chlorophyll. The three events are: absorption of light energy by chlorophyll, conversion of light energy to chemical energy plus splitting of water into hydrogen and oxygen, and reduction of carbon dioxide to carbohydrate. Stomata on the leaf surface allow gaseous exchange and are guarded by guard cells that open and close by changes in turgidity.

Heterotrophs show different strategies: Amoeba engulfs food by pseudopodia (holozoic), a Plasmodium absorbs nutrients from its host (parasitic), and fungi like bread mould secrete enzymes onto food and absorb the digested material (saprotrophic).

The alimentary canal runs from the mouth to the anus. In the mouth, teeth break food mechanically and salivary amylase (ptyalin) begins the digestion of starch. The food is pushed down the oesophagus by peristalsis.

The stomach wall releases hydrochloric acid (which kills microbes and gives the acidic pH that pepsin needs), the enzyme pepsin (digests proteins), and mucus (protects the stomach lining). In the small intestine, bile from the liver emulsifies fats, pancreatic juice provides trypsin and lipase, and the intestinal juice completes digestion. Absorption occurs through finger-like villi that greatly increase the surface area.

The large intestine absorbs most of the remaining water, and the unabsorbed material is removed as faeces through the anus.

Respiration is the breakdown of glucose to release energy that is stored as ATP. In aerobic respiration glucose is broken down completely using oxygen into carbon dioxide and water, releasing a large amount of energy. In anaerobic respiration, in the absence of oxygen, the breakdown is incomplete.

In our muscles during vigorous exercise, when oxygen supply is short, glucose is converted to lactic acid; the build-up of lactic acid causes cramps. In yeast, anaerobic breakdown gives ethanol and carbon dioxide — this is fermentation, used in baking and brewing.

In humans, air enters through the nostrils, passes down the trachea and bronchi into the lungs, and reaches millions of alveoli. The alveoli have thin walls and a rich blood supply, so oxygen diffuses into the blood and carbon dioxide diffuses out. Haemoglobin in red blood cells carries oxygen; carbon dioxide is mostly carried dissolved in plasma.

The human heart has four chambers — two atria and two ventricles — which prevents oxygenated and deoxygenated blood from mixing. Blood from the body enters the right atrium, goes to the right ventricle, and is pumped to the lungs. Oxygenated blood returns to the left atrium, passes to the left ventricle, and is pumped to the whole body. Because blood passes through the heart twice in one cycle, this is called double circulation.

Arteries carry blood away from the heart and have thick elastic walls to withstand high pressure. Veins carry blood back to the heart, have valves to prevent backflow, and thinner walls. Capillaries are one-cell-thick vessels where exchange of materials with tissues occurs.

Lymph is a colourless fluid that drains extra tissue fluid back into the blood and carries absorbed fats from the intestine. In plants, water and minerals move upward through xylem (pulled up by transpiration), while food made in leaves moves through phloem to all parts by translocation, a process that uses energy.

Excretion removes nitrogenous wastes such as urea from the body. In humans the main excretory organs are the two kidneys, together with ureters, urinary bladder, and urethra. The functional unit of the kidney is the nephron.

In the nephron, blood is filtered in the glomerulus; the filtrate contains glucose, amino acids, salts, urea, and water. As the filtrate flows along the tubule, useful substances — all the glucose, most water and salts — are reabsorbed into the blood. The remaining fluid is urine.

When the kidneys fail, wastes accumulate in the blood and the person may need dialysis, in which blood is passed through a machine with a semi-permeable membrane to remove wastes. Plants excrete in simpler ways: some wastes are stored in leaves that fall off, some as resins and gums in old xylem, and some are released into the soil.