Life Processes

The basic functions that an organism must carry out to maintain and sustain life are called life processes. They include nutrition, respiration, transportation and excretion (along with control & coordination and reproduction). These processes keep the body working, repair worn-out parts and provide energy — even when the organism appears to be at rest.

Nutrition is the process of taking in food and using it for energy, growth and repair. Based on how they obtain food, organisms show two main modes:

• Autotrophic nutrition — the organism makes its own food from simple inorganic substances. Green plants and some bacteria are autotrophs.
• Heterotrophic nutrition — the organism depends on others for food. Animals and fungi are heterotrophs.

Green plants make food by photosynthesis: using carbon dioxide and water, in the presence of sunlight and the green pigment chlorophyll, they produce glucose and release oxygen.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll)

The main events are: (1) absorption of light energy by chlorophyll, (2) conversion of light energy to chemical energy and splitting of water into hydrogen and oxygen, and (3) reduction of carbon dioxide to carbohydrate. Gases enter and leave the leaf through tiny pores called stomata, whose opening and closing is controlled by guard cells. The glucose made is often stored as starch.

In heterotrophic nutrition organisms take in food made by others. It is of three main types: saprotrophic (feeding on dead, decaying matter, e.g. fungi), parasitic (feeding on a living host, e.g. Cuscuta, ticks) and holozoic (taking in whole food and digesting it inside the body, e.g. humans, Amoeba).

In humans, nutrition is holozoic and takes place in the alimentary canal (digestive tract) with the help of associated glands. The journey of food:

• Mouth — teeth chew the food; saliva (from salivary glands) contains the enzyme salivary amylase that begins digesting starch into sugar.
• Oesophagus (food pipe) — pushes food to the stomach by wave-like muscular movements called peristalsis.
• Stomach — gastric glands release hydrochloric acid (kills germs, makes the medium acidic), the enzyme pepsin (digests proteins) and mucus (protects the stomach lining).
• Small intestine — the main site of digestion and absorption. It receives bile from the liver (emulsifies fats) and pancreatic juice from the pancreas (trypsin for proteins, lipase for fats, amylase for starch). Its own juice completes digestion. The inner wall has finger-like villi that greatly increase surface area for absorption of digested food into the blood.
• Large intestine — absorbs water; the undigested waste (faeces) is removed through the anus (egestion).

Respiration is the process of breaking down food (glucose) to release energy for the body’s use. The energy is stored in molecules of ATP (adenosine triphosphate), the ‘energy currency’ of the cell. Respiration is of two types:

• Aerobic respiration — takes place in the presence of oxygen, in the mitochondria. Glucose is completely broken down into carbon dioxide and water, releasing a large amount of energy.
• Anaerobic respiration — takes place without oxygen. In yeast, glucose → ethanol + CO2 (fermentation); in our muscle cells during heavy exercise, glucose → lactic acid, releasing less energy. The build-up of lactic acid causes muscle cramps.

In humans, air is taken in through the respiratory system: nostrils → nasal cavity → trachea (windpipe, kept open by rings of cartilage) → two bronchi → bronchioles → tiny air sacs called alveoli in the lungs. The alveoli have thin walls and a rich network of blood capillaries, where exchange of gases occurs: oxygen diffuses into the blood and carbon dioxide diffuses out to be breathed out. Oxygen is carried in the blood by the red pigment haemoglobin in red blood cells.

In plants, gas exchange occurs through stomata (leaves), lenticels (stems) and the root surface; there is no special transport of gases as in animals.

Transportation carries materials (food, oxygen, carbon dioxide, wastes) to and from cells. In humans, this is done by the circulatory system — the heart, blood and blood vessels.

• The heart has four chambers: two upper atria and two lower ventricles. The right side handles deoxygenated blood (to the lungs) and the left side handles oxygenated blood (to the body). Because blood passes through the heart twice in one full cycle, it is called double circulation, and the oxygenated and deoxygenated blood do not mix — this is efficient for warm-blooded animals.
• Blood vessels: arteries carry blood away from the heart (thick walls, high pressure); veins carry blood back to the heart (have valves to prevent back-flow); capillaries are very thin vessels where exchange of materials with cells occurs.
• Blood contains plasma, red blood cells (carry O2 via haemoglobin), white blood cells (fight disease) and platelets (help clotting). Lymph is another fluid that helps transport and immunity.

In plants there are two transport tissues: xylem carries water and minerals upward from the roots to the leaves; the pull is created mainly by transpiration (loss of water vapour from leaves). Phloem carries food (made in leaves) to all parts of the plant; this movement is called translocation and requires energy.

Excretion is the removal of harmful nitrogenous and other metabolic wastes from the body. In humans, the excretory system consists of a pair of kidneys, two ureters, a urinary bladder and the urethra.

• The kidneys filter waste (mainly urea) from the blood and produce urine.
• The basic filtering unit of the kidney is the nephron. Each kidney has about a million nephrons. In a nephron, blood is filtered in a cup-shaped structure (Bowman’s capsule around a ball of capillaries, the glomerulus); then, as the filtrate passes along the tubule, useful substances such as glucose, amino acids, salts and most water are reabsorbed back into the blood. The remaining liquid is urine.
• Urine passes through the ureters to the urinary bladder, where it is stored, and is removed through the urethra.

When the kidneys fail, wastes build up in the blood. They can be removed by an artificial process called dialysis using a dialysis machine (artificial kidney).

In plants, excretion is simpler. Plants get rid of excess water by transpiration; oxygen and carbon dioxide are released through stomata; and many waste products are stored in leaves that fall off, in bark, or as gums, resins and latex. Some wastes are also released into the surrounding soil.