Life Processes

LIFE PROCESSES are the maintenance jobs a body must keep doing to stay alive, even when it is doing nothing else. The four you must know: NUTRITION, RESPIRATION, TRANSPORTATION and EXCRETION.

Visible movement is NOT a reliable test of life (plants are alive but do not visibly move). The real indicator is that living things constantly repair and maintain themselves, which requires energy.

In very small organisms, diffusion alone can supply every cell. In a large multicellular body, diffusion is far too slow, so specialised organ systems are needed for uptake and transport.

NUTRITION is of two types. AUTOTROPHIC — the organism makes its own food from simple inorganic substances (green plants, some bacteria). HETEROTROPHIC — the organism depends on food made by others (animals, fungi, most bacteria).

PHOTOSYNTHESIS: 6CO₂ + 6H₂O --(sunlight, chlorophyll)--> C₆H₁₂O₆ + 6O₂. It occurs in the chloroplasts, which contain the green pigment CHLOROPHYLL. Raw materials: carbon dioxide, water, sunlight and chlorophyll.

THREE EVENTS of photosynthesis: (1) absorption of light energy by chlorophyll; (2) conversion of light energy into chemical energy AND splitting of water molecules into hydrogen and oxygen; (3) reduction of carbon dioxide into carbohydrate. IMPORTANT: these need not take place one immediately after the other.

STOMATA are tiny pores, mainly on leaves, for gaseous exchange; large amounts of water are also lost through them. Each stoma is bounded by two GUARD CELLS: when water flows IN the guard cells swell and the pore OPENS; when they lose water they shrink and the pore CLOSES. This is how the plant limits water loss.

HETEROTROPHIC nutrition has three types. SAPROPHYTIC — food is broken down OUTSIDE the body by enzymes and then absorbed (bread mould, yeast, mushroom). PARASITIC — food is taken from a living host, harming it (tapeworm, tick, leech, lice, Cuscuta/amarbel). HOLOZOIC — whole food is taken in and digested INSIDE the body (Amoeba, humans).

NUTRITION IN AMOEBA: temporary finger-like extensions called PSEUDOPODIA surround the food particle and trap it in a FOOD VACUOLE, where enzymes digest it; the digested food is absorbed and the undigested residue is egested. PARAMECIUM has a fixed shape, so CILIA sweep food to a fixed spot.

HUMAN ALIMENTARY CANAL, in order: mouth → oesophagus (food pipe) → stomach → small intestine → large intestine → anus. Associated glands: salivary glands, liver, pancreas.

MOUTH: teeth chew (physical breakdown); saliva contains SALIVARY AMYLASE (ptyalin), which begins breaking down starch into sugar. The tongue helps mix the food and the mucus makes it slippery.

PERISTALSIS — rhythmic, wave-like muscular contractions of the alimentary-canal wall that push the food forward. It begins in the oesophagus and continues throughout the canal.

STOMACH — gastric glands release THREE things: (1) HYDROCHLORIC ACID, which makes the medium acidic so pepsin can act, and kills germs; (2) PEPSIN, a protein-digesting enzyme; (3) MUCUS, which protects the stomach's own lining from the acid. (Without mucus the acid would damage the lining — this causes acidity/ulcers.)

SMALL INTESTINE — the LONGEST part of the alimentary canal and the site where digestion is COMPLETED. BILE from the liver makes the acidic food alkaline (so pancreatic enzymes can work) and EMULSIFIES fat — breaking large fat globules into smaller ones. PANCREATIC JUICE supplies TRYPSIN (proteins) and LIPASE (emulsified fats). Intestinal juice completes the job.

FINAL PRODUCTS of digestion: carbohydrates → GLUCOSE; proteins → AMINO ACIDS; fats → FATTY ACIDS + GLYCEROL.

VILLI are thousands of tiny finger-like projections on the inner wall of the small intestine. They greatly INCREASE THE SURFACE AREA for absorption and are richly supplied with blood vessels that carry the absorbed food to every cell.

LARGE INTESTINE absorbs most of the remaining WATER; the undigested residue is egested through the anus. Herbivores eating grass need a LONGER small intestine to digest cellulose; carnivores have a shorter one.

RESPIRATION begins the same way in every cell: GLUCOSE (6 carbon) is broken down in the CYTOPLASM into two molecules of PYRUVATE (3 carbon). What happens next depends on oxygen.

THREE FATES OF PYRUVATE: (a) ABSENCE of oxygen, in yeast → ETHANOL + CARBON DIOXIDE (fermentation/anaerobic respiration); (b) LACK of oxygen in our muscle cells during vigorous exercise → LACTIC ACID (its build-up causes CRAMPS); (c) PRESENCE of oxygen, in the MITOCHONDRIA → CARBON DIOXIDE + WATER, releasing MUCH more energy (aerobic respiration).

The energy released is used to make ATP (adenosine triphosphate) — the ENERGY CURRENCY of the cell. Energy is stored when ADP + phosphate → ATP, and released when ATP is broken down.

RESPIRATION IN PLANTS: gases are exchanged through the STOMATA (and through large INTERCELLULAR SPACES that keep every cell in contact with air); each part — root, stem, leaf — meets its own needs, with little transport of gases between parts. The DIRECTION of net exchange depends on the time of day: AT NIGHT there is no photosynthesis, so CO₂ release is the major exchange; BY DAY the CO₂ made in respiration is used up in photosynthesis, so O₂ release is the major event.

Aquatic animals breathe FASTER than terrestrial animals because the amount of dissolved oxygen in water is much lower than the oxygen in air.

HUMAN RESPIRATORY SYSTEM: nostrils (hair + mucus filter dust) → nasal passage → TRACHEA → two BRONCHI → BRONCHIOLES → ALVEOLI. The trachea is held permanently open by RINGS OF CARTILAGE, which prevent it from collapsing.

ALVEOLI are balloon-like sacs where gaseous exchange happens; their walls carry an extensive network of blood capillaries. Spread flat they would give a surface area of about 80 m². HAEMOGLOBIN (the red pigment in RBCs) carries oxygen; carbon dioxide is more soluble in water and so is carried mostly DISSOLVED IN THE PLASMA.

BREATHING IN: the ribs lift and the diaphragm flattens → the chest cavity becomes LARGER → air is drawn in. Breathing out reverses this. A RESIDUAL VOLUME of air always stays in the lungs so that oxygen absorption can continue between breaths.

HUMAN HEART has FOUR CHAMBERS: right atrium, right ventricle, left atrium, left ventricle. Path: body → VENA CAVA → right atrium → right ventricle → PULMONARY ARTERY → lungs → PULMONARY VEINS → left atrium → left ventricle → AORTA → body. NOTE the two exceptions students are asked about: the PULMONARY ARTERY is the artery that carries DEOXYGENATED blood, and the PULMONARY VEIN is the vein that carries OXYGENATED blood. VENTRICLES have thicker muscular walls than atria because they pump blood out (the left ventricle is thickest of all); VALVES prevent backflow.

DOUBLE CIRCULATION — blood passes through the heart TWICE in one complete cycle of the body. Its purpose is to keep oxygenated and deoxygenated blood SEPARATE, giving an efficient oxygen supply. This is essential for warm-blooded animals (birds and mammals), which need lots of energy to maintain a constant body temperature.

HEARTS OF OTHER VERTEBRATES (a standard 2–3 mark comparison): FISHES have a TWO-chambered heart and SINGLE circulation — blood passes through the heart only ONCE per cycle (heart → gills → body). AMPHIBIANS and most REPTILES have a THREE-chambered heart and tolerate some MIXING of oxygenated and deoxygenated blood, because they are cold-blooded and do not spend energy maintaining a constant body temperature. BIRDS and MAMMALS have FOUR chambers with complete separation and double circulation.

BLOOD VESSELS: ARTERIES carry blood AWAY from the heart, have thick elastic walls (the blood is under high pressure) and no valves. VEINS carry blood TO the heart, have thin walls and VALVES to prevent backflow. CAPILLARIES are only ONE cell thick, which is what allows materials to be exchanged with the surrounding cells.

BLOOD PRESSURE — normal SYSTOLIC (ventricles contract) pressure is about 120 mm Hg and DIASTOLIC (ventricles relax) about 80 mm Hg, measured with a SPHYGMOMANOMETER. Readings around 150/90 mm Hg indicate hypertension (high blood pressure).

PLATELETS help the blood CLOT at a wound. LYMPH is a colourless fluid that leaks out of capillaries, carries digested and absorbed FAT from the intestine, and drains the extra fluid back into the blood.

TRANSPORT IN PLANTS: XYLEM carries WATER and dissolved minerals UPWARD from the roots. At night, ROOT PRESSURE pushes water up; during the day the main driving force is TRANSPIRATION PULL, created as water evaporates from the leaves.

PHLOEM carries FOOD (mainly sucrose) made in the leaves — this is TRANSLOCATION, and it happens in the SIEVE TUBES with the help of adjacent COMPANION CELLS. MECHANISM (the 3-mark answer): sucrose is loaded into the phloem using ENERGY FROM ATP → this raises the OSMOTIC PRESSURE of the tissue → water moves in from the xylem → the pressure built up pushes the material to tissues where the pressure is lower. Because loading can happen at any source, phloem moves food in BOTH directions as the plant needs (e.g. in spring, sugar stored in the root or stem travels to the growing buds).

TRANSPIRATION does three jobs: it helps in the upward movement of water and minerals, it regulates temperature (cooling the plant), and it helps absorb water from the soil.

EXCRETION removes harmful NITROGENOUS WASTES, chiefly UREA. Human excretory system: a pair of KIDNEYS, a pair of URETERS, the URINARY BLADDER and the URETHRA.

NEPHRON is the basic FILTRATION UNIT of the kidney (each kidney has millions). Blood enters a knot of thin-walled capillaries called the GLOMERULUS, which sits inside the cup-shaped BOWMAN'S CAPSULE. Pressure forces water and small molecules out = FILTRATION.

SELECTIVE REABSORPTION — as the filtrate flows along the TUBULE, useful substances (glucose, amino acids, salts and most of the water) are taken BACK into the blood. What remains is urine, which passes through the collecting duct and ureter to the bladder. The amount of water reabsorbed depends on how much excess water the body has and how much dissolved waste must be excreted.

DIALYSIS (artificial kidney) is used when the kidneys fail: blood is passed through tubes in a dialysing fluid that has the same composition as blood plasma EXCEPT that it contains no nitrogenous wastes — so the wastes diffuse out of the blood. There is no reabsorption step in the machine.

EXCRETION IN PLANTS: oxygen is itself a WASTE PRODUCT OF PHOTOSYNTHESIS (respiration CONSUMES oxygen — it never produces it), and along with CO₂ it leaves by diffusion through the STOMATA. Excess water is removed by TRANSPIRATION. Other wastes are stored in cellular VACUOLES, as RESINS and GUMS in old xylem, or are released into the soil — and many wastes go simply by SHEDDING LEAVES.

Photosynthesis

6CO₂ + 6H₂O --(sunlight + chlorophyll)--> C₆H₁₂O₆ + 6O₂

Occurs in chloroplasts. Raw materials: CO₂, water, sunlight, chlorophyll. Products: carbohydrate + oxygen.

Step 1 of respiration (all cells)

Glucose (6C) --[cytoplasm]--> 2 × Pyruvate (3C) + energy

This first step is common to aerobic AND anaerobic respiration, and always happens in the cytoplasm.

Anaerobic — in yeast

Pyruvate --[absence of O₂]--> Ethanol + CO₂ + energy

Fermentation. This is how alcohol is produced.

Anaerobic — in our muscles

Pyruvate --[lack of O₂]--> Lactic acid + energy

Happens during vigorous exercise; the accumulation of lactic acid causes muscle cramps.

Aerobic respiration

Pyruvate --[presence of O₂, in mitochondria]--> CO₂ + H₂O + energy

Releases MUCH more energy than either anaerobic route. Mitochondria are the site.

Energy currency

ADP + Pi ⇌ ATP

ATP is made when energy is released and broken down when the cell needs energy.

Path of blood through the heart

Body → right atrium → right ventricle → lungs → left atrium → left ventricle → body

Blood passes through the heart TWICE per complete cycle = double circulation.

Normal blood pressure

120 / 80 mm Hg (systolic / diastolic)

Measured with a sphygmomanometer; about 150/90 mm Hg indicates hypertension.

Alveolar surface area

≈ 80 m²

The alveoli of both lungs spread flat — this huge area is what makes rapid gaseous exchange possible.

Digestion — end products

carbohydrate → glucose; protein → amino acids; fat → fatty acids + glycerol

Absorbed through the villi of the small intestine into the blood.

Urine formation

Filtration (glomerulus) → Selective reabsorption (tubule) → Urine

Glucose, amino acids, salts and most water are reabsorbed; urea is not.

Direction of transport in plants

Xylem: water + minerals, UPWARD only. Phloem: food, BOTH directions.

Xylem uses root pressure (night) and transpiration pull (day); phloem translocation uses ATP to load sucrose, raising osmotic pressure so water enters and the pressure drives the flow.