Chapter 5: Life Processes (Biology)

In multicellular organisms, most cells are deep inside the body, far from the air. Diffusion is slow and works only over short distances, so oxygen cannot reach all these cells fast enough. The body is also large and needs a lot of oxygen. So specialised organs (lungs) and a transport system (blood) are needed.

Because most cells are far from the surface and diffusion is too slow over such distances to supply the large amount of oxygen needed.

Visible movement (growth, breathing, walking) is the first sign we use. But some living things show no visible movement, so the real criterion is molecular movement: living things continuously carry out life processes such as nutrition, respiration, transport and excretion to maintain and repair their bodies. If these processes stop, the organism dies.

Ongoing life processes (molecular movements such as nutrition, respiration and excretion); growth and movement are the visible signs.

• Food provides energy for life processes.
• It provides materials for growth, repair and maintenance of the body.
• Oxygen is used to break down food (respiration); water and minerals are needed for body reactions. Plants use CO₂ and water to make food.

To obtain energy and to build and repair the body.

Nutrition, respiration, transportation and excretion (along with control and coordination, growth and reproduction).

Autotrophs make their own food by photosynthesis; heterotrophs depend on food made by others.

• Carbon dioxide: from the air, entering through the stomata of the leaves (aquatic plants use CO₂ dissolved in water)
• Water: from the soil, absorbed by the roots and carried up by the xylem
• Sunlight: from the Sun, absorbed by chlorophyll in the chloroplasts
• Minerals (such as nitrogen, phosphorus, magnesium): from the soil, absorbed by the roots

CO₂ from the air through the stomata, water and minerals from the soil through the roots, and light from the Sun.

The gastric glands secrete hydrochloric acid. It:

• makes the medium in the stomach acidic, which the enzyme pepsin needs to digest proteins;
• kills germs that enter with the food.

(The mucus in the stomach protects its lining from the acid.)

It provides the acidic medium needed by pepsin and kills bacteria in the food.

Digestive enzymes break down large, complex food molecules into small, simple ones that can dissolve and be absorbed into the blood. For example, amylase breaks starch into sugars, pepsin and trypsin break proteins into amino acids, and lipase breaks fats into fatty acids and glycerol.

They break complex food molecules into simpler, soluble ones that can be absorbed.

• It is very long and coiled, giving a long time and large area for absorption.
• Its inner wall has many finger-like projections called villi, which greatly increase the surface area.
• The villi are richly supplied with blood vessels, which carry the absorbed food to every cell of the body.

It is long and has numerous villi with a rich blood supply, which give a huge surface area for absorption.

The amount of oxygen dissolved in water is very low compared with the amount in air. So aquatic organisms such as fish must breathe much faster to get enough oxygen. Terrestrial organisms breathe air, which is rich in oxygen, so they get the oxygen they need with much less effort.

Air has far more oxygen than water, so land animals do not need to breathe as fast as aquatic animals.

In the cytoplasm, glucose (6 carbons) is first broken into pyruvate (3 carbons) in every case. After that:

• Without oxygen, in yeast (fermentation): pyruvate → ethanol + CO₂ + energy
• Lack of oxygen, in our muscles: pyruvate → lactic acid + energy (this causes muscle cramps)
• With oxygen, in mitochondria (aerobic): pyruvate → CO₂ + water + a lot of energy

Anaerobically to ethanol and CO₂ (yeast) or to lactic acid (muscles), or aerobically to CO₂ and water (mitochondria).

• Oxygen is carried mainly by haemoglobin, a red pigment in the red blood cells, which has a high affinity for oxygen. It picks up oxygen in the lungs and releases it in the tissues.
• Carbon dioxide is more soluble in water, so it is carried mostly dissolved in the blood plasma (largely as bicarbonate), from the tissues to the lungs.

Oxygen by haemoglobin in the RBCs; carbon dioxide mainly dissolved in plasma.

In the lungs, the air passage (bronchi) divides into smaller and smaller tubes (bronchioles), which end in tiny balloon-like sacs called alveoli. Millions of alveoli give a very large surface area (about 80 m² if spread out). Their walls are very thin and covered with a dense network of blood capillaries, so gases diffuse quickly.

The lungs contain millions of thin-walled alveoli, surrounded by capillaries, giving a huge area for gas exchange.

• Heart: a muscular pump that pushes blood around the body
• Blood vessels: arteries carry blood away from the heart to the organs; veins bring blood back to the heart; capillaries, with walls one cell thick, exchange materials between blood and cells
• Blood: plasma carries food, CO₂ and nitrogenous wastes; RBCs carry oxygen; WBCs fight infection; platelets help blood clot at a cut
• Lymph: carries digested fat from the intestine and drains excess tissue fluid back into the blood

The heart pumps, the blood vessels carry, and the blood (with lymph) transports materials and protects the body.

Mammals and birds are warm-blooded: they keep a constant body temperature and are very active, so they need a lot of energy and therefore a highly efficient supply of oxygen. Keeping oxygenated and deoxygenated blood separate (in the four-chambered heart) ensures that the body gets fully oxygenated blood.

They need a lot of energy to keep their body temperature constant, so their cells need a fully oxygenated, efficient blood supply.

• Xylem: carries water and minerals from the roots to the other parts of the plant
• Phloem: carries food made in the leaves (and other materials) to all parts of the plant

Xylem and phloem (the vascular tissues).

• The cells of the root take up ions from the soil. This creates a difference in concentration, so water moves into the root and on into the xylem, forming a continuous column of water.
• Transpiration pull: water evaporating from the leaves through the stomata creates a suction that pulls water up the xylem from the roots. This is the main force, especially during the day.
• At night, root pressure helps to push water up.

Through the xylem, by root pressure and mainly by the transpiration pull from the leaves.

Food made in the leaves, such as sucrose, is carried by the phloem; this is called translocation. It uses energy from ATP: sucrose is loaded into the sieve tubes, which raises the osmotic pressure, so water enters and pushes the material to the parts of the plant that need it (roots, fruits, seeds, growing buds). Food can move both upward and downward.

By translocation in the phloem, an active process that uses ATP energy and osmotic pressure.

Structure: the nephron is the basic filtering unit of the kidney (each kidney has about a million). It consists of:

• Bowman's capsule: a cup-shaped structure at the start of the tubule
• Glomerulus: a bunch of thin-walled blood capillaries inside Bowman's capsule
• A long tubule (renal tubule), which leads to a collecting duct

Functioning:

• Filtration: blood enters the glomerulus, and water, glucose, amino acids, salts and urea are filtered into Bowman's capsule.
• Reabsorption: as the filtrate flows along the tubule, useful substances such as glucose, amino acids, salts and most of the water are taken back into the blood capillaries around it.
• What remains is urine, which passes to the collecting duct, then the ureter and urinary bladder.

A nephron filters blood in the glomerulus and Bowman's capsule, then reabsorbs useful substances in its tubule, leaving urine.

• Excess oxygen (from photosynthesis), CO₂ and water vapour are removed through the stomata.
• Many wastes are stored in the vacuoles of cells, or in leaves that later fall off.
• Some wastes are stored as resins and gums, especially in old xylem.
• Some are released into the soil around the roots.

Through stomata, by storing them in vacuoles, leaves that fall, bark, gums and resins, and by releasing them into the soil.

The amount of water reabsorbed in the tubules depends on how much excess water there is in the body and how much dissolved waste must be removed. When we drink a lot of water, less is reabsorbed and more dilute urine is produced; when the body is short of water (in summer or after sweating), more water is reabsorbed and less, concentrated urine is produced. This is controlled by hormones (such as ADH).

By varying how much water the tubules reabsorb, depending on the body's water and waste content.

(c) excretion

(a) transport of water

(d) all of the above

(b) mitochondria

Fats are digested in the small intestine.

• Fats are present as large globules, which are hard for enzymes to act on. Bile from the liver breaks them into tiny droplets (emulsification), giving a large surface area. Bile also makes the medium alkaline.
• Pancreatic lipase (in pancreatic juice) then breaks the emulsified fats into fatty acids and glycerol.
• The intestinal juice completes the digestion, and the products are absorbed in the villi.

In the small intestine: bile emulsifies fats, and lipase (pancreatic and intestinal) breaks them into fatty acids and glycerol.

• Saliva moistens the food, making it easy to chew and swallow.
• It contains the enzyme salivary amylase, which breaks down starch into simple sugar (maltose). So digestion of starch begins in the mouth.

It wets the food, and its amylase begins digesting starch into sugar.

Conditions: the presence of chlorophyll, sunlight, carbon dioxide and water.

Products: carbohydrates (glucose, stored as starch) are the food made; oxygen is released as the by-product (with water).

$6\mathrm{CO}{}_2+12\mathrm{H}{}_2\mathrm{O}\underset{\mathrm{chlorophyll}}{\overset{\mathrm{sunlight}}{\to }}\mathrm{C}{}_6\mathrm{H}{}_{12}\mathrm{O}{}_6+6\mathrm{O}{}_2+6\mathrm{H}{}_2\mathrm{O}$

Chlorophyll, sunlight, CO₂ and water are needed; oxygen is the by-product.

Organisms: yeast, some bacteria (such as those that make curd), tapeworms; our muscle cells also respire anaerobically for a short time during heavy exercise.

Aerobic uses oxygen and releases much energy; anaerobic needs no oxygen and releases little. Examples: yeast, some bacteria, tapeworm.

• They are present in millions, giving a very large surface area.
• Their walls are extremely thin (one cell thick), so gases diffuse quickly.
• They are covered with a dense network of blood capillaries, which keeps a large difference in gas concentration between air and blood.
• They are moist, which helps gases dissolve and diffuse.

They are numerous, thin-walled, moist and richly supplied with capillaries.

Haemoglobin carries oxygen. With too little of it, the blood carries less oxygen to the cells, so less energy is released. The person suffers from anaemia: tiredness, weakness, breathlessness, pale skin and a lower ability to work.

Less oxygen reaches the cells, causing anaemia, with tiredness, weakness and breathlessness.

In each complete cycle, blood passes through the heart twice:

• Pulmonary circulation: deoxygenated blood from the body enters the right atrium, goes to the right ventricle, and is pumped to the lungs, where it picks up oxygen. It returns to the left atrium.
• Systemic circulation: the oxygenated blood goes from the left atrium to the left ventricle, which pumps it to all parts of the body. The deoxygenated blood returns through veins to the right atrium.

Why necessary: it keeps oxygenated and deoxygenated blood completely separate, giving a highly efficient supply of oxygen. Warm-blooded animals such as humans need this to supply the large amount of energy required to keep their body temperature constant.

Blood goes through the heart twice per cycle (heart–lungs–heart–body), which keeps oxygenated and deoxygenated blood separate for an efficient oxygen supply.

Xylem carries water and minerals upwards without ATP; phloem carries food in both directions using ATP.

Both are tiny, numerous and thin-walled, with a rich blood supply, which gives a large surface area for exchange.

See the table above. Both give a large, thin surface with a rich blood supply; alveoli exchange gases while nephrons filter blood.