What are life processes?
Even a sleeping organism is busy: its cells keep repairing themselves, and its body keeps moving molecules around. The processes that together maintain life are life processes: nutrition, respiration, transportation and excretion.
Nutrition
Autotrophic nutrition
Green plants make carbohydrates by photosynthesis:
6CO₂ + 12H₂O → (sunlight, chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O
It happens in three steps:
Chlorophyll absorbs light energy.
Light energy is converted to chemical energy, and water splits into hydrogen and oxygen.
Carbon dioxide is reduced to carbohydrates.
Gas exchange happens through stomata, whose opening is controlled by guard cells: they swell with water to open the pore and shrink to close it. Plants also take up nitrogen as nitrates and nitrites from the soil.
Desert plants take in CO₂ at night, when their stomata can open without much water loss, and store it as an intermediate to use in daylight.
Heterotrophic nutrition in humans
Region | Juice and enzyme | Action |
|---|---|---|
Mouth | saliva: salivary amylase | starch → maltose (a sugar) |
Stomach | gastric juice: pepsin, HCl, mucus | pepsin digests proteins; HCl makes the medium acidic for pepsin and kills germs; mucus protects the lining |
Small intestine | bile from the liver | makes the medium alkaline and emulsifies fats into small globules |
pancreatic juice: trypsin, lipase, amylase | trypsin digests proteins; lipase breaks emulsified fats | |
intestinal juice | completes digestion: proteins → amino acids, carbohydrates → glucose, fats → fatty acids and glycerol |
The inner wall of the small intestine has finger-like villi, which increase the surface area for absorption and are richly supplied with blood vessels. The large intestine absorbs extra water, and the anal sphincter regulates the exit of waste.
Respiration
Glucose is first broken into a 3-carbon molecule, pyruvate, in the cytoplasm. What happens next depends on oxygen:
Pathway | Where | Products |
|---|---|---|
without oxygen, in yeast (fermentation) | cytoplasm | ethanol + CO₂ + energy |
lack of oxygen in muscle cells | cytoplasm | lactic acid + energy (causes cramps) |
with oxygen (aerobic) | mitochondria | CO₂ + H₂O + much more energy |
The energy is stored as ATP (adenosine triphosphate), the energy currency of the cell.
Human respiratory system. Air passes through the nostrils (where hairs and mucus filter it), pharynx, larynx and trachea (kept open by rings of cartilage), then the bronchi and bronchioles, ending in tiny balloon-like sacs called alveoli. The alveoli give an enormous surface area for gas exchange; spread out, they would cover about 80 m².
Haemoglobin, the respiratory pigment in red blood cells, has a high affinity for oxygen and carries it to the tissues. CO₂ is more soluble in water and is mostly carried dissolved in the blood.
Aquatic animals breathe faster than land animals, because water holds much less dissolved oxygen than air.
Transportation
Human circulation
The heart is a muscular organ with four chambers: two atria (upper) and two ventricles (lower). The ventricles have thicker walls, since they pump blood out to organs.
Deoxygenated blood from the body enters the right atrium and passes to the right ventricle, which pumps it to the lungs.
Oxygenated blood from the lungs enters the left atrium and passes to the left ventricle, which pumps it to the whole body.
Blood passes through the heart twice in each complete cycle: double circulation. Keeping oxygenated and deoxygenated blood separate gives a highly efficient oxygen supply, needed by warm-blooded birds and mammals. Valves stop blood flowing backwards.
Vessel | Carries blood | Walls |
|---|---|---|
Arteries | away from the heart, under high pressure | thick, elastic |
Veins | towards the heart | thinner, with valves |
Capillaries | connect arteries and veins; exchange materials with cells | one cell thick |
Platelets help blood clot at a cut. Lymph (tissue fluid) carries digested fats from the intestine and drains excess fluid back into the blood.
Blood pressure is the force blood exerts on the walls of a vessel. Normal is about 120/80 mm Hg (systolic/diastolic), measured with a sphygmomanometer.
Transport in plants
Tissue | Transports | How |
|---|---|---|
Xylem | water and minerals from the roots upward | root pressure (mainly at night) and the suction created by transpiration (mainly in the day) |
Phloem | food (sucrose) from leaves to other parts, in both directions | translocation, using energy from ATP |
Transpiration, the loss of water vapour from leaves, pulls water up the plant and helps control its temperature.
Excretion
In humans, the kidneys remove nitrogenous wastes such as urea from the blood. Each kidney has about a million filtering units called nephrons. In each nephron:
Blood is filtered in a cup-shaped Bowman's capsule around a cluster of capillaries, the glomerulus.
Useful substances such as glucose, amino acids, salts and much of the water are reabsorbed in the tubule.
The remaining urine passes through the ureters to the urinary bladder and out through the urethra.
If the kidneys fail, an artificial kidney (haemodialysis) can filter the blood.
In plants, oxygen and water vapour leave through stomata. Other wastes are stored in leaves that later fall, in old xylem as resins and gums, or in vacuoles.
Summary
Photosynthesis makes food; human digestion breaks it into absorbable molecules using enzymes.
Respiration releases energy as ATP; aerobic respiration in mitochondria releases the most.
Double circulation in a four-chambered heart separates oxygenated and deoxygenated blood.
Xylem carries water; phloem carries food. Nephrons filter blood to make urine.