Life Processes

We still call a sleeping dog alive, because it is breathing. Visible movement is not enough. A virus shows no molecular movement of its own until it infects a cell. That is why there is a debate about viruses.

If repair stops, the body order breaks. These repairs are the life processes. Energy enters from outside as food — that is nutrition. Many organisms take in oxygen to break food down — that is respiration. In a multicellular body not every cell touches the outside, so transport is needed. Removing harmful waste is excretion.

Green plants and some bacteria are autotrophs. They build food from CO2 and water. Animals and fungi are heterotrophs. They break complex food with enzymes.

Chlorophyll absorbs light. Light energy becomes chemical energy and water splits into hydrogen and oxygen. Carbon dioxide is then reduced to carbohydrate. The three steps need not finish in one instant. Desert plants take CO2 at night, make an intermediate, and use the energy of light by day.

The green dots in a leaf section are chloroplasts. Chlorophyll sits in them.

The tiny pores on a leaf are stomata. Most of the gas exchange of photosynthesis passes through them. Water can also be lost here, so the pores close when CO2 is not needed. If the guard cells swell, the pore opens. If they shrink, it closes. Gas exchange also happens across stems and roots.

Roots take water from the soil. Nitrogen, phosphorus, iron and magnesium also come from the soil. Nitrogen is needed to make proteins. It arrives as nitrate or nitrite, or the plant takes compounds that bacteria have made from nitrogen in the air.

Fungi, such as bread mould, yeast and mushrooms, digest food outside the body and then absorb it. Cuscuta, ticks, lice, leeches and tapeworms feed on another organism without killing it. This is parasitic nutrition.

Amoeba gathers food with temporary feet and makes a food vacuole. Digested food diffuses into the cytoplasm, and the leftover is thrown out at the surface. Paramecium has a fixed shape. Cilia move food to one fixed spot.

Teeth crush the food. Saliva wets it so the soft lining of the canal is not scratched. The tongue turns the morsel. Muscles of the alimentary canal contract in a wave and push the food forward. This is peristalsis.

The food pipe carries food to the stomach. Gastric glands release hydrochloric acid, pepsin and mucus. The acid makes the medium for pepsin and kills germs in the food. Mucus protects the wall from the acid. If that cover fails, the burning of acidity follows.

A sphincter at the stomach exit lets food into the small intestine in small amounts. Herbivores that eat grass have a long small intestine, because cellulose digests slowly. A carnivore such as a tiger has a shorter intestine.

Food from the stomach is acidic. Pancreatic enzymes need an alkaline medium. Bile from the liver makes the medium alkaline and breaks large fat globules into small ones, the way soap breaks up oil. In pancreatic juice, trypsin digests protein and lipase digests fat. Intestinal juice turns proteins into amino acids, carbohydrates into glucose, and fats into fatty acids and glycerol.

Finger-like villi on the inner wall of the small intestine increase the area. They carry blood vessels, which take digested food to every cell. The large intestine absorbs water. The remaining waste leaves through the anus. Bacteria on teeth make acid from sugar. Under plaque, saliva cannot neutralise the acid. Brushing after food removes the plaque.

Glucose is a six-carbon molecule. The first step is in the cytoplasm: glucose breaks into three-carbon pyruvate.

If oxygen is present, pyruvate becomes carbon dioxide and water in the mitochondria. This is aerobic respiration. Much more energy is released. Without oxygen, in yeast, pyruvate becomes ethanol and CO2. This is anaerobic respiration, or fermentation. If oxygen is short in our muscle, pyruvate becomes lactic acid. That acid causes cramp after a sudden run.

The energy released makes ATP from ADP and phosphate. Breaking the terminal phosphate of ATP releases about 30.5 kJ/mol. ATP is the energy currency of the cell. C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

Land animals use the oxygen of the air. Hairs and mucus in the nostrils trap dust. Rings of cartilage in the throat stop the air passage from collapsing. Inside the lungs the tubes end in balloon-like alveoli. Spread out, the alveoli would cover about 80 square metres.

When we breathe in, the ribs lift and the diaphragm flattens. The chest cavity grows and air is drawn in. Blood releases CO2 into the alveolus and picks up oxygen. A little air always stays in the lungs, the residual volume, so that gas exchange has time.

At night a plant mainly sends CO2 out. By day the CO2 of respiration is used in photosynthesis and oxygen is released. Tobacco and gutkha harm the mouth, lungs and heart. Smoking destroys the cilia, so dust and germs reach the lungs.

The heart is a muscular organ about the size of a fist. Oxygenated blood from the lungs enters the left atrium, then the left ventricle, and the ventricle pushes it through the body. Deoxygenated blood from the body enters the right atrium, moves to the right ventricle, and the ventricle sends it to the lungs. The ventricle wall is thick, because it must push blood far. Valves stop blood from flowing backward.

In one cycle blood passes through the heart twice. This is double circulation. A fish heart has two chambers and the blood passes once. Animals such as the frog have a three-chambered heart and some blood mixes. Birds and mammals hold their own body temperature, so they need a separate and full supply of oxygen.

Arteries carry blood away from the heart and have thick, elastic walls. Veins bring blood back and have valves. A capillary wall is one cell thick. Platelets clot blood at a wound and stop the leak. Normal systolic pressure is about 120 mm Hg and diastolic pressure about 80 mm Hg. Lymph is colourless, has less protein, and carries digested fat from the intestine.

Plants manage with less energy, but a tall tree has a long distance to cover. Xylem carries water and minerals from root to leaf. Root cells take up ions actively and water follows. This root pressure matters more at night. By day the real pull is transpiration.

Phloem carries the soluble products of photosynthesis, amino acids and other substances. It moves in both directions. Sucrose enters the sieve tube using energy from ATP, osmotic pressure rises and water comes in. In spring the store in the root moves up toward the buds.

Humans have a pair of kidneys, ureters, a urinary bladder and a urethra. Nitrogenous waste such as urea is separated from the blood in the kidney. Each kidney has a huge number of nephrons. A cluster of capillaries drops filtrate into the cup-shaped Bowman capsule.

The first filtrate contains glucose, amino acids, salts and a lot of water. Along the tubule these are selected and returned to the blood. How much water returns depends on spare water in the body and on the dissolved waste. A healthy adult forms about 180 litres of filtrate a day, and only one or two litres of urine. In an artificial kidney the tubes are semi-permeable and the dialysing fluid has no nitrogenous waste. There is no reabsorption.

Plants lose spare water by transpiration. Other wastes go into vacuoles, into leaves that fall, into old xylem as resin and gum, or into the surrounding soil. The oxygen of photosynthesis is also a waste in one sense.