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All the basic functions performed by living organisms to maintain life are life processes: nutrition, respiration, transport, excretion, control and coordination, and reproduction. Movement (even at the molecular level) is a sign of life, but growth and the use of energy are also criteria. Organisms need energy from outside to do work and to repair and maintain their body structure, and they need raw materials too.
In single-celled organisms, oxygen can reach the cell surface by diffusion. In multicellular organisms like humans, the body is too large and the cells deep in the body are far from the surface, so diffusion alone cannot meet their oxygen needs. They need specialised organs for taking in and moving materials.
Autotrophs make their own food from simple inorganic materials, carbon dioxide and water, using sunlight and chlorophyll. The carbohydrates formed are stored as starch.
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂OOrganisms depend on other organisms for their food.
Amoeba takes food in by pseudopodia (false feet), which surround the food and form a food vacuole. Digestion occurs inside the vacuole and the undigested material is expelled. In Paramecium, cilia sweep the food to a fixed spot where it is taken in.
The alimentary canal is a long tube from the mouth to the anus, with associated glands: salivary glands, liver and pancreas.
The muscular stomach churns the food. Glands in its wall release hydrochloric acid, pepsin (protein-digesting enzyme) and mucus. The acid creates an acidic medium that activates pepsin and kills bacteria; the mucus protects the inner lining from the acid. A sphincter at the exit of the stomach releases food into the small intestine in small amounts.
Unabsorbed food goes to the large intestine, where its walls absorb water. The rest is removed via the rectum and anus. Dental caries (tooth decay) begins when bacteria act on sugar left on the teeth and make acids that damage the enamel.
Glucose is first broken down in the cytoplasm into pyruvate (a three-carbon molecule). What follows depends on the availability of oxygen.
The energy released is stored in the molecule ATP (adenosine triphosphate), the energy currency of the cell. When the cell needs energy, ATP breaks down to release about 30.5 kJ/mol, and this energy drives other reactions, such as muscle contraction and protein synthesis.
The alveoli are tiny balloon-like sacs at the ends of the bronchioles. They give a very large surface area, their walls are thin and surrounded by a network of blood capillaries, so oxygen diffuses into the blood and carbon dioxide diffuses out to be exhaled. During breathing, the ribs move up and out and the diaphragm flattens, increasing the chest volume to draw air in; the reverse pushes air out. The lungs always retain some residual air, so there is enough time for oxygen to be absorbed.
Plants exchange gases by diffusion through stomata in leaves, lenticels in stems and the surface of roots. At night, when photosynthesis does not occur, they take in oxygen and release carbon dioxide; by day the oxygen from photosynthesis is used up in respiration, and the carbon dioxide used.
The heart is a muscular organ, as big as our fist, with four chambers: two upper atria and two lower ventricles. The ventricles have thick walls as they pump blood out. Valves ensure that blood flows in one direction only. The right side receives deoxygenated blood and the left side oxygenated blood; the partition between them prevents mixing.
Blood passes through the heart twice in each complete cycle.
It is necessary because mammals and birds need a lot of energy to maintain body temperature. Separating oxygenated and deoxygenated blood allows a highly efficient supply of oxygen.
The force that blood exerts against the wall of a vessel is blood pressure. It is higher in arteries during ventricular contraction (systolic pressure, about 120 mm of Hg) and lower during relaxation (diastolic, about 80 mm of Hg). It is measured with a sphygmomanometer. High blood pressure can rupture an artery and cause internal bleeding.
Some plasma, proteins and blood cells escape through the pores in the capillaries into the intercellular spaces, forming the colourless fluid lymph. It drains back into larger lymph vessels and then into the veins. It carries digested and absorbed fat from the intestine and drains excess fluid from the extracellular space.
Xylem (tracheids and vessels) carries water and minerals from the roots to the leaves. Water enters the root hair cells by osmosis. Transpiration (loss of water vapour through stomata) creates a suction, the transpiration pull, which draws water up the plant in a continuous column. At night, root pressure due to active uptake of ions helps push water up. In tall trees, transpiration pull is the major driving force.
Phloem (sieve tubes with companion cells) carries the soluble products of photosynthesis, such as sucrose, from the leaves to other parts, including storage organs. This is translocation. It uses energy from ATP and takes place in both directions, upward and downward, according to the needs of the plant.
The system consists of a pair of kidneys, two ureters, a urinary bladder and a urethra. Kidneys filter nitrogenous wastes such as urea and uric acid from the blood.
Each kidney has about a million filtration units, the nephrons. A nephron has a cluster of thin-walled capillaries called the glomerulus enclosed in a cup-shaped Bowman's capsule, and a long tube.
The amount depends on how much excess water is in the body and how much dissolved waste has to be excreted. Water reabsorption is regulated by hormones. If there is a lot of water, much dilute urine is produced.
When the kidneys fail, an artificial kidney can be used. Blood from an artery is passed through tubes with a semi-permeable lining, surrounded by a dialysing fluid with the same osmotic pressure as blood but without nitrogenous wastes. Wastes pass out of the blood by diffusion, and the cleaned blood is returned to a vein. Unlike the kidney, there is no reabsorption.
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In large multicellular bodies most cells are deep inside and far from the surface; diffusion is effective only over very short distances, so it cannot supply oxygen to all cells fast enough. Specialised respiratory and transport systems are needed.
Movement, growth, the ability to carry out life processes (nutrition, respiration, transport, excretion, control and coordination, reproduction), and the need for energy. Movement (even molecular movement) is the most visible sign of life, but movement alone can be misleading (a river moves), so other processes such as growth and responding to stimuli are used too.
For (i) obtaining energy, (ii) building and repairing the body (growth and maintenance), and (iii) making the substances needed for the body's processes.
Nutrition, respiration, transport of materials within the body, and excretion of wastes are essential (along with control and coordination).
Carbon dioxide: from the air, through the stomata. Water: from the soil, absorbed by the roots (with minerals) and carried by the xylem to the leaves. Sunlight: from the Sun, absorbed by chlorophyll in the chloroplasts.
Hydrochloric acid makes the medium acidic, which activates the enzyme pepsin to digest proteins, and kills bacteria that enter with the food.
Digestive enzymes break down complex food molecules into simple, soluble ones that can be absorbed: amylase (starch to sugars), proteases (proteins to amino acids), lipases (fats to fatty acids and glycerol).
It is very long, giving time for digestion and absorption, and its inner wall has numerous finger-like villi that greatly increase the surface area. The villi are richly supplied with blood vessels, which carry the absorbed food to every cell of the body.
Air has much more oxygen than water (the dissolved oxygen is low), so terrestrial organisms need not breathe as fast or spend as much effort to get enough oxygen as aquatic ones, which must pass large volumes of water over their gills.
Glucose is first broken down to pyruvate in the cytoplasm. Then: (i) in the absence of oxygen, to ethanol and CO₂ (yeast), or to lactic acid (muscle cells) – anaerobic respiration; (ii) in the presence of oxygen, to CO₂ and water in the mitochondria with release of much more energy – aerobic respiration.
Oxygen is carried by haemoglobin in the red blood cells, which has a high affinity for oxygen. Carbon dioxide is more soluble in water, so it is mostly transported dissolved in the blood plasma.
Inside the lungs the passage divides into smaller and smaller tubes (bronchi and bronchioles), ending in millions of balloon-like alveoli. The alveoli provide a very large surface area, have thin walls and are surrounded by a rich network of blood capillaries, so oxygen and carbon dioxide are exchanged rapidly.
The heart, the blood and the blood vessels. The heart pumps blood; blood carries oxygen, carbon dioxide, nutrients, wastes and hormones, and fights infection and clots; arteries carry blood away from the heart, veins bring it back, and capillaries exchange materials with the cells. Lymph also helps.
These animals are warm-blooded and need much energy to maintain their body temperature, so they need a high, efficient supply of oxygen. Separating the blood prevents mixing and lets the body receive fully oxygenated blood.
The vascular tissues: xylem (tracheids and vessels), which carries water and minerals, and phloem (sieve tubes and companion cells), which carries food.
The roots absorb water with dissolved minerals. They are carried through the xylem vessels and tracheids to the leaves. The main force is the transpiration pull: loss of water vapour from the stomata creates a suction that pulls up the water column. Root pressure helps at night.
By the phloem, in a process called translocation. Sucrose made in the leaves is loaded into the sieve tubes with the help of ATP energy; it moves to the parts needing it, in both directions, and is unloaded there.
A nephron has a glomerulus (a cluster of thin capillaries) inside a cup-shaped Bowman's capsule, followed by a long tubule that opens into a collecting duct. Blood is filtered in the glomerulus into the capsule. As the filtrate flows through the tubule, useful substances (glucose, amino acids, salts, water) are selectively reabsorbed into the blood capillaries around it. The remaining waste, urea and excess water and salts, is urine, which goes to the collecting duct and the ureter.
Wastes such as oxygen and CO₂ pass out through stomata; excess water by transpiration; other wastes are stored in cell vacuoles, as resins and gums in old xylem, and are shed in falling leaves and bark; some are excreted into the surrounding soil.
It depends on how much excess water is in the body and how much dissolved waste must be excreted. The amount of water reabsorbed in the tubules is controlled by hormones: with excess water, little is reabsorbed and much dilute urine forms; when water is scarce, more is reabsorbed and urine is small and concentrated.
Kidneys filter wastes.
Answer: (c) excretion
Answer: (a) transport of water
Answer: (d) all of the above
Answer: (b) mitochondria
In the small intestine. Bile from the liver makes the food alkaline and emulsifies the fats into small globules; pancreatic lipase and intestinal juice then break the fats into fatty acids and glycerol.
Saliva moistens and softens the food, and its enzyme salivary amylase begins the digestion of starch by breaking it into sugar (maltose).
Necessary: chlorophyll, carbon dioxide, water and sunlight. By-products: oxygen (released through stomata) and water; the main product is carbohydrate.
Anaerobic organisms: yeast and some bacteria (our muscle cells also respire anaerobically for a short time).
They are balloon-like sacs in huge numbers, giving a very large surface area; their walls are very thin (one cell thick) and are surrounded by a dense network of capillaries, so that oxygen diffuses into the blood and carbon dioxide diffuses out very quickly.
Haemoglobin carries oxygen, so a deficiency reduces oxygen supply to the cells, causing less energy release, weakness, tiredness, breathlessness and anaemia.
Blood passes twice through the heart in one complete cycle. Deoxygenated blood from the body enters the right atrium → right ventricle → pulmonary artery → lungs (oxygenated) → pulmonary veins → left atrium → left ventricle → aorta → body. It is necessary because it separates oxygenated and deoxygenated blood, giving an efficient supply of oxygen to meet the high energy needs of warm-blooded animals.