Life Processes — Notes

Living organisms are highly organised; keeping this order (maintenance) needs energy and raw materials all the time, even at rest. Molecular movement inside cells is a basic sign of life.

The basic life processes are nutrition, respiration, transportation and excretion.

In single-celled organisms the whole surface is in contact with the surroundings, so diffusion is enough. In large multicellular organisms most cells are far from the surface, so diffusion is too slow and specialised tissues and organ systems are needed.

Autotrophs (green plants, some bacteria) make carbohydrates from carbon dioxide and water using sunlight and chlorophyll — photosynthesis. Excess carbohydrate is stored as starch.

Events of photosynthesis: (i) absorption of light energy by chlorophyll; (ii) conversion of light energy to chemical energy and splitting of water into hydrogen and oxygen; (iii) reduction of carbon dioxide to carbohydrates. Desert plants take in carbon dioxide at night and use it during the day.

Stomata are tiny pores, mostly on the lower surface of leaves, for exchange of gases. Guard cells swell with water and open the pore; when they lose water the pore closes, reducing water loss.

Raw materials: carbon dioxide from air (through stomata); water and minerals (nitrogen, phosphorus, iron, magnesium) from soil through roots. Nitrogen is used to make proteins.

Experiments: a variegated leaf shows that chlorophyll is necessary (starch only in green parts); a plant with potassium hydroxide in a closed jar shows that carbon dioxide is necessary; covering part of a leaf shows that light is necessary. Plants are destarched in the dark first; leaves are boiled in alcohol to remove chlorophyll before the iodine test.

Heterotrophs depend on food made by others: saprophytic (fungi such as bread mould, mushrooms, yeast), parasitic (Cuscuta, ticks, lice, tapeworm), holozoic (Amoeba, humans). Amoeba engulfs food with pseudopodia into a food vacuole; Paramoecium uses cilia.

Human alimentary canal: mouth → oesophagus → stomach → small intestine → large intestine → anus. Food is moved by peristalsis.

• Mouth: teeth chew; tongue mixes; salivary amylase converts starch into sugar.
• Stomach: gastric glands release hydrochloric acid (acidic medium, kills germs), pepsin (proteins) and mucus (protects the lining). A sphincter controls exit of food.
• Small intestine (longest part): bile from the liver (stored in the gall bladder) makes the food alkaline and emulsifies fats; pancreatic juice has trypsin (proteins) and lipase (emulsified fats); intestinal juice completes digestion — proteins → amino acids, carbohydrates → glucose, fats → fatty acids and glycerol. Villi greatly increase the surface for absorption into blood.
• Large intestine: absorbs water; the anal sphincter controls removal of waste. Herbivores have a longer small intestine than carnivores, to digest cellulose.

In the cytoplasm, glucose (6-carbon) is broken into pyruvate (3-carbon) with a little energy. Then:

• Without oxygen (yeast): pyruvate → ethanol + carbon dioxide + energy (fermentation).
• Lack of oxygen (muscle cells): pyruvate → lactic acid + energy (causes cramps).
• With oxygen (mitochondria): pyruvate → carbon dioxide + water + much energy (aerobic). Energy is stored as ATP, the energy currency of the cell, and used for muscle contraction, protein synthesis and nerve impulses.

Plants exchange gases by diffusion through stomata, stem openings and roots; by day photosynthesis uses carbon dioxide so net oxygen is released; at night carbon dioxide is released. Aquatic animals breathe faster because dissolved oxygen is low.

Human respiratory system: nostrils (hairs, mucus filter air) → pharynx → larynx → trachea (rings of cartilage stop it collapsing) → bronchi → bronchioles → alveoli. Breathing in: ribs lift and diaphragm flattens, enlarging the chest cavity. Alveoli give a large surface (about 80 m²) with thin walls and many capillaries; oxygen is carried by haemoglobin in red blood cells; carbon dioxide mostly dissolved in blood. Residual volume of air remains in the lungs.

Blood: plasma (carries food, carbon dioxide and nitrogenous wastes in dissolved form), red blood cells (haemoglobin carries oxygen), white blood cells, and platelets (clot blood at wounds).

Heart: four chambers. The right atrium receives deoxygenated blood from the body (vena cava) → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle (thickest wall) → aorta → body. Valves prevent backflow. Blood passes through the heart twice per circuit: double circulation. Separation of oxygenated and deoxygenated blood gives an efficient oxygen supply, needed by birds and mammals, which maintain a constant body temperature. Fish have 2 chambers; amphibians and many reptiles 3.

Blood vessels: arteries (thick, elastic walls; carry blood away from the heart), veins (thinner walls, valves; carry blood to the heart), capillaries (walls one cell thick; exchange with cells).

Blood pressure: normal about 120/80 mm of mercury (systolic/diastolic), measured with a sphygmomanometer; high blood pressure (hypertension) is caused by constriction of arterioles and can rupture vessels.

Lymph: tissue fluid that drains into lymphatic vessels; carries digested and absorbed fat from the intestine and drains excess fluid back into the blood.

Xylem carries water and minerals from roots to leaves. Root cells take up ions, so water enters the root and is pushed up (root pressure, important at night). In the day, evaporation of water from leaves (transpiration) creates a suction (transpiration pull), the main force moving water up. Transpiration also helps absorption of minerals and regulates temperature.

Phloem carries food (sucrose), amino acids and other substances — translocation — in any direction, to roots, fruits, seeds and growing parts. It uses energy from ATP: sugar loaded into phloem draws in water, and the pressure moves the material.

Human excretory system: a pair of kidneys, ureters, urinary bladder and urethra. Each kidney has about a million nephrons: a glomerulus (cluster of capillaries) inside Bowman's capsule, then a long tubule that drains into a collecting duct.

• Filtration in the glomerulus: water, glucose, amino acids, salts and urea pass into the capsule.
• Selective reabsorption in the tubule: glucose, amino acids, salts and much water return to the blood (about 180 L of filtrate a day gives only 1–2 L of urine).
• Urine is stored in the bladder and released through the urethra (under nervous control).

Haemodialysis (artificial kidney): blood passes through long semi-permeable tubes in a dialysing fluid with the same osmotic pressure as blood but no nitrogenous wastes; wastes diffuse out; there is no reabsorption.

Plants: oxygen and water vapour leave through stomata; wastes stored in leaves are lost when leaves fall; wastes stored in vacuoles; gums and resins stored in old xylem; some wastes released into the soil.