Life Processes
Digestion in humans
Food travels through the alimentary canal in this order: mouth, pharynx, oesophagus (peristalsis pushes food), stomach, small intestine, large intestine, anus. Digestion is chemical breakdown by enzymes into absorbable molecules: carbohydrates to glucose, proteins to amino acids, fats to fatty acids and glycerol.
| Region / gland | Secretion | What it does |
|---|---|---|
| Mouth (salivary glands) | Saliva with salivary amylase | Starch to sugar (maltose) |
| Stomach | Gastric juice: HCl, pepsin, mucus | Pepsin digests protein; acid makes the medium acidic and kills germs; mucus protects the wall |
| Liver | Bile | Makes food alkaline; emulsifies fats (no enzyme) |
| Pancreas | Pancreatic juice: trypsin, lipase, amylase | Trypsin digests proteins, lipase digests emulsified fats, amylase starch |
| Small intestine | Intestinal juice | Completes digestion; villi absorb the products |
| Large intestine | None of note | Absorbs water; rest leaves as faeces |
The small intestine is the longest part of the canal and its inner wall has finger-like villi that increase the surface area for absorption; each villus has a network of blood vessels. Its length varies with diet: it is longer in herbivores, which must digest cellulose, than in carnivores.
Respiration: releasing energy from food
Respiration releases energy from glucose. The first step is in the cytoplasm: glucose (6-carbon) is broken to pyruvate (3-carbon). After that the path depends on oxygen. With oxygen (aerobic, in mitochondria) pyruvate is broken to CO₂, water and a large amount of energy. Without oxygen (anaerobic): in yeast, pyruvate gives ethanol + CO₂ + energy (fermentation); in our muscles, pyruvate gives lactic acid + energy, and the build-up causes cramps. Aerobic respiration gives much more energy than anaerobic.
The energy is stored as ATP (the cell's energy currency): ADP + inorganic phosphate + energy gives ATP, and when ATP is split to ADP the energy (about 30.5 kJ/mol) is released for contraction, protein synthesis and conduction of nerve impulses.
| Type | Where | End products |
|---|---|---|
| Aerobic | Mitochondria (after cytoplasm) | CO₂ + H₂O + energy |
| Anaerobic in yeast | Cytoplasm | Ethanol + CO₂ + energy |
| Anaerobic in muscle | Cytoplasm | Lactic acid + energy |
The human air passage is nostrils, pharynx, larynx, trachea (kept open by cartilage rings), bronchi, bronchioles and alveoli, the thin-walled balloon-like sacs where gases are exchanged. During inhalation the ribs lift and the diaphragm flattens, enlarging the chest and drawing air in. Haemoglobin in red blood cells carries oxygen; CO₂ is more soluble in water and is carried mostly dissolved in blood. Aquatic animals breathe faster than land animals because water holds far less dissolved oxygen than air. Plants exchange gases through stomata (leaves), lenticels (woody stems) and root surfaces by diffusion.
Transport in humans and plants
The human heart is a muscular organ with four chambers: two upper atria and two lower ventricles, kept apart so oxygenated and deoxygenated blood do not mix. The path of blood is: body, vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta, body. Blood passes through the heart twice in one complete circuit, which is double circulation. The left ventricle has the thickest wall because it pumps blood to the whole body. Valves prevent backflow. Fishes have a two-chambered heart (single circulation), amphibians and most reptiles three-chambered, birds and mammals four-chambered.
| Vessel / fluid | Feature | Carries |
|---|---|---|
| Artery | Thick, elastic wall; no valves | Blood away from heart (pulmonary artery carries deoxygenated) |
| Vein | Thin wall; valves | Blood to heart (pulmonary vein carries oxygenated) |
| Capillary | One cell thick; exchange with tissues | Joins artery to vein |
| Plasma | Liquid part of blood | Food, CO₂, urea, salts, dissolved |
| Lymph | Colourless, fewer proteins | Digested fat from intestine; drains tissue fluid |
Blood pressure is measured with a sphygmomanometer. A normal adult has a systolic (ventricle contracting) pressure of about 120 mm Hg and diastolic (relaxed) about 80 mm Hg; the difference between them (the pulse pressure) is therefore 120 − 80 = 40 mm Hg. Platelets help blood clot at a cut. In plants, xylem (vessels and tracheids, dead cells) carries water and minerals upward, driven mainly by transpiration pull, the suction caused by loss of water vapour from leaves; root pressure helps at night. Phloem (sieve tubes with companion cells, living) carries the prepared food (translocation), in both directions, and uses ATP energy.
Excretion in humans and plants
Excretion removes harmful metabolic wastes. The human excretory system is a pair of kidneys, two ureters, a urinary bladder and a urethra. The functional unit of the kidney is the nephron: a cluster of thin-walled capillaries called the glomerulus, cup-shaped Bowman’s capsule, and a long tubule. The urine path is kidney, ureter, bladder, urethra.
- Filtration in the glomerulus: water, glucose, amino acids, salts and urea pass into Bowman’s capsule.
- Selective reabsorption in the tubule: useful substances (glucose, amino acids, salts, much water) return to blood.
- The remaining fluid, urine, collects in the collecting duct and passes to the ureter; its quantity depends on how much excess water and how much dissolved waste (mainly urea) is to be removed.
Worked example: an adult’s kidneys form about 180 litres of filtrate a day, but only 1 to 2 litres leave as urine. If 1.5 L of urine is passed, the amount reabsorbed is 180 − 1.5 = 178.5 L. When the kidneys fail, an artificial kidney (haemodialysis) cleans the blood: blood flows between membranes bathed in a dialysing fluid with the same osmotic pressure as blood but no nitrogenous waste, so urea diffuses out.
Plants excrete differently: oxygen and CO₂ leave by stomata, excess water by transpiration, and other wastes are stored in cell vacuoles, in old xylem (as gums and resins), or shed in falling leaves and bark; some plants give latex.
Key takeaways
- Plants make food by photosynthesis: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O; the oxygen released comes from water and CO₂ enters through stomata.
- Digestion: mouth digests starch (amylase), stomach proteins (pepsin, in HCl), small intestine all three food groups; bile is not an enzyme, it emulsifies fat.
- Glucose is split to pyruvate in the cytoplasm; aerobic respiration finishes in mitochondria, anaerobic gives ethanol + CO₂ (yeast) or lactic acid (muscle). ATP is the energy currency.
- Four-chambered heart and double circulation: vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta.
- Xylem carries water upward (transpiration pull); phloem carries prepared food in both directions using ATP. Normal blood pressure is about 120/80 mm Hg.
- The nephron filters, reabsorbs selectively and forms urine; about 180 L filtrate a day gives 1 to 2 L urine; haemodialysis is the artificial kidney.
Practice questions (16)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
During photosynthesis in a green leaf, the oxygen gas that is released into the air comes from the splitting of which substance?
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Answer: A — Water absorbed by the roots
The light reaction splits water into hydrogen and oxygen, so the released oxygen comes from water. Carbon dioxide is the tempting answer because it is the gas taken in, but its carbon and oxygen end up in glucose. Glucose and starch are products, not sources.Which organism is correctly matched with its mode of nutrition?
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Answer: C — Bread mould: saprophytic nutrition
Bread mould is a fungus that feeds on dead and decaying matter, which is saprophytic nutrition. Amoeba is holozoic (it engulfs food), Cuscuta is a parasite on a living host, and a tapeworm is a heterotrophic parasite. The Cuscuta pairing is the tempting error, since holozoic and parasitic are often swapped.Using the balanced equation of photosynthesis, 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O, how many molecules of carbon dioxide are needed to make 3 molecules of glucose?
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Answer: B — 18
One glucose molecule needs 6 CO₂ molecules, so 3 glucose molecules need 3 × 6 = 18 CO₂. The count 6 is the figure for one glucose, and 36 wrongly multiplies by 12, the number of water molecules in the equation.Consider these statements about digestion in humans. P: Bile contains enzymes that digest fats. Q: Pepsin acts in the acidic medium of the stomach. R: Salivary amylase acts on starch. S: Villi are present in the large intestine. Which statements are correct?
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Answer: B — Q and R only
Q and R are correct. P is wrong because bile has no enzymes: it emulsifies fat and makes the medium alkaline. S is wrong because villi line the small intestine, where absorption occurs; the large intestine mainly absorbs water.Food leaving the stomach is acidic, but the enzymes of the pancreatic juice need an alkaline medium. Which secretion makes the food alkaline in the small intestine?
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Answer: D — Bile from the liver
Bile from the liver is alkaline and also emulsifies fat, so the medium suits trypsin and lipase. Pepsin is an enzyme that needs acid, and hydrochloric acid only adds to the acidity. Saliva acts earlier, in the mouth.When yeast respires without oxygen, which products are formed along with energy?
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Answer: A — Ethanol and carbon dioxide
In yeast, pyruvate is converted to ethanol and CO₂ (fermentation). Lactic acid is the product in animal muscle, and carbon dioxide with water is the product of aerobic respiration, which is the usual swap.After a sudden sprint, the leg muscles of an athlete ache and cramp. This is caused by the build-up of which compound?
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Answer: C — Lactic acid, formed from pyruvate in muscle cells
With too little oxygen, muscle cells convert pyruvate to lactic acid, and its build-up causes cramps. Ethanol is the yeast route, not the animal one. Carbon dioxide is carried away in blood, and urea is a nitrogenous waste unrelated to cramps.Which sequence correctly gives the path taken by inhaled air in a human being before it reaches the alveoli?
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Answer: B — Nostrils, pharynx, larynx, trachea, bronchi, bronchioles
Air passes nostrils, pharynx, larynx, trachea, bronchi, bronchioles and then alveoli. The trachea splits into the two bronchi, so it must come before them, and the larynx lies above the trachea. The other orders swap adjacent organs.Consider these statements. P: Aquatic animals breathe faster than land animals because water holds less dissolved oxygen than air. Q: Most carbon dioxide is carried in the blood by haemoglobin. R: Woody stems exchange gases through lenticels. S: Anaerobic respiration in muscle takes place inside mitochondria. Which statements are correct?
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Answer: D — P and R only
P and R are correct. Q is wrong because haemoglobin carries oxygen, while carbon dioxide, being more soluble in water, is carried mostly dissolved in blood. S is wrong because anaerobic breakdown of pyruvate occurs in the cytoplasm; only the aerobic stage is in mitochondria.Blood returning from the lungs to the heart first enters which chamber?
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Answer: C — Left atrium
The pulmonary veins bring oxygenated blood into the left atrium. The right atrium receives deoxygenated blood from the body through the vena cava. Ventricles are the pumping chambers and receive blood from the atria, not directly from the veins.Which blood vessel carries deoxygenated blood away from the heart?
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Answer: A — Pulmonary artery
The pulmonary artery leaves the right ventricle carrying deoxygenated blood to the lungs. The vena cava also carries deoxygenated blood, but it brings it towards the heart. The aorta takes oxygenated blood away and the pulmonary vein brings oxygenated blood in.A healthy adult has a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg. By how much does the pressure in the arteries rise when the ventricles contract?
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Answer: D — 40 mm Hg
The rise from the relaxed to the contracting phase is 120 − 80 = 40 mm Hg. The figure 200 comes from wrongly adding the two readings, and 120 or 80 is just one of the readings, not the difference.Consider these statements about transport in plants. P: Xylem is made of dead cells, vessels and tracheids. Q: Phloem translocation uses energy from ATP. R: Sucrose is carried upward by transpiration pull. S: Xylem carries prepared food in both directions. Which statements are correct?
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Answer: C — P and Q only
P and Q are correct. R and S swap the tissues: transpiration pull acts on water in xylem, while prepared food such as sucrose moves in phloem, in both directions.Which of the following statements about the heart and circulation is NOT correct?
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Answer: B — The left ventricle is thinner-walled than the right as it serves only the lungs
The statement about the left ventricle is false: it pumps blood to the whole body through the aorta, so it has the thickest wall. The right ventricle supplies only the lungs. The other three statements are standard facts.In the formation of urine in a nephron, which is the correct order of events?
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Answer: A — Filtration in the glomerulus, selective reabsorption in the tubule, collection in the collecting duct
Blood is first filtered in the glomerulus into Bowman’s capsule, then useful substances such as glucose and water are reabsorbed along the tubule, and the remaining fluid collects in the collecting duct before going to the ureter. Reabsorption cannot precede filtration, because there is nothing to reabsorb until the filtrate exists.The kidneys of an adult form 180 litres of filtrate in a day, and 99% of this is reabsorbed. How much urine is passed in that day?
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Answer: C — 1.8 L
If 99% is reabsorbed, 1% remains: 1% of 180 L = 1.8 L of urine, which agrees with the usual 1 to 2 L a day. The figure 178.2 L is the amount reabsorbed (99% of 180), the quantity the question does not ask for, and 0.18 L comes from taking 0.1%.