Life Processes

The chapter opens with a simple question: how do we decide that something is alive? Visible movement is not a safe test, because a sleeping dog or a plant does not move in any obvious way, yet both are alive. What is common to all living things is that they are organised systems whose structures keep breaking down and must be repaired, so every living thing needs a steady supply of energy and building material even when it seems to be doing nothing. The processes that do this maintenance work are called life processes.

NCERT names four of them and the chapter is built around them:

• Nutrition: taking in energy and material from outside. Autotrophs like green plants make their food from carbon dioxide and water; heterotrophs like animals and fungi take ready-made food from other organisms.
• Respiration: breaking down the food inside cells to release energy, usually by oxidation with oxygen taken in from outside.
• Transportation: carrying food, oxygen and wastes from where they are taken in or made to every cell of the body.
• Excretion: removing the harmful by-products of metabolism.

The key idea the examiner looks for is the link between body size and the need for organ systems. In a unicellular organism such as Amoeba, the whole surface of the cell touches the environment, so diffusion is enough to bring in food and oxygen and to throw out wastes. In a multicellular organism most cells are far from the surface, and diffusion is far too slow to supply every cell, so specialised tissues and organs (gut, lungs, heart, kidneys) take over each function. Write this reason whenever you are asked why humans need a respiratory or circulatory system.

Food supplies two things: energy, and the carbon-based material from which the body is built and repaired. Depending on how the carbon is obtained, nutrition is autotrophic (organism makes complex organic food from CO₂ and water, using light energy) or heterotrophic (organism takes complex organic food made by others).

Photosynthesis is the process by which green plants and some bacteria carry out autotrophic nutrition. NCERT writes it as:

6CO₂ + 12H₂O → (sunlight, chlorophyll) → C₆H₁₂O₆ + 6O₂ + 6H₂O

Three events occur, and boards ask for them by name and in order:

1. Absorption of light energy by chlorophyll.
2. Conversion of light energy to chemical energy, and splitting of water molecules into hydrogen and oxygen.
3. Reduction of carbon dioxide to carbohydrates.

The steps need not happen one after the other. Desert plants take up CO₂ at night and store it as an intermediate, then use light energy during the day to reduce it, because their stomata stay closed in the daytime to save water. Carbohydrate not used at once is stored as starch, the plant's internal energy reserve, just as animals store glycogen.

Where does a plant get its raw materials? Water and minerals come from the soil through the roots. CO₂ enters through the stomata, tiny pores on the leaf surface guarded by a pair of guard cells. When water flows into the guard cells they swell and the pore opens; when they lose water they shrink and the pore closes. Since a lot of water is lost through open stomata, the plant opens them only when it needs CO₂. Nitrogen for making proteins is not taken from the air directly; it is absorbed as inorganic nitrates and nitrites from the soil, or as organic compounds made by bacteria from atmospheric nitrogen.

Two NCERT activities and one standard experiment are exam favourites. (i) The variegated leaf (money plant or Croton): the plant is kept in the dark for three days to destarch it, then in sunlight for six hours; the leaf is boiled in water, then in alcohol over a water bath to remove chlorophyll, and tested with iodine. Only the green parts turn blue-black, proving chlorophyll is essential. (ii) Two destarched potted plants under bell jars, one with a watch glass of KOH, which absorbs CO₂; only the leaf from the plant without KOH tests positive for starch, proving CO₂ is essential. (iii) Covering part of a leaf with black paper before exposing it to light shows that sunlight is essential.

Heterotrophs cannot make their own food and depend directly or indirectly on autotrophs. The form of nutrition depends on the type of food and how it is obtained, and NCERT describes three strategies. You should be able to define each and give two examples.

• Saprophytic nutrition: the organism secretes digestive enzymes on to dead and decaying organic matter outside its body, breaks it down there and then absorbs the simple soluble products. Examples: fungi such as bread moulds, yeast and mushrooms, and many bacteria.
• Parasitic nutrition: the organism lives on or inside another living organism (the host) and derives nutrition from it without killing it, usually harming it. Examples: Cuscuta (amarbel) on plants, ticks, lice, leeches and tapeworms on animals.
• Holozoic nutrition: the organism takes in whole food, complex or solid, and breaks it down inside the body. This is the mode of most animals, including humans, Amoeba and Paramoecium. The steps are ingestion, digestion, absorption, assimilation and egestion.

Note the wording in the examiner's answer key. Saprophytes digest outside the body and absorb; holozoic organisms digest inside the body. Parasites take food from a living host; saprophytes from dead matter. A frequent 2-mark question asks for two differences between holozoic and saprophytic nutrition, so keep these two contrasts ready in tabular form.

The chapter also makes a general point: since the food of a heterotroph is complex, it must be broken down into simpler substances before it can be absorbed and used. This breaking down is done by enzymes, biocatalysts that convert complex food into simple, soluble molecules. Different organisms use different enzymes depending on the food they eat and the way they obtain it.

In a single-celled organism the whole surface is in contact with the environment, so food can be taken in over the entire surface. As organisms grow larger and more complex, different parts specialise for different steps of nutrition.

Nutrition in Amoeba is the NCERT example and a regular diagram question. Amoeba takes in food using temporary finger-like extensions of the cell surface called pseudopodia. The pseudopodia fuse over the food particle and enclose it in a food vacuole. Inside the food vacuole, enzymes break the complex food into simpler substances, which then diffuse into the cytoplasm. The undigested material is moved to the surface of the cell and thrown out. This is holozoic nutrition in a single cell: ingestion, digestion, absorption, assimilation and egestion all happen in one cell.

Nutrition in Paramoecium is different because the cell has a definite shape. Food is taken in only at a specific spot, and it is moved to that spot by the movement of cilia, the tiny hair-like structures that cover the entire surface of the cell.

The point the examiner wants is the contrast: Amoeba can take in food at any point on its surface using pseudopodia, while Paramoecium takes food only at a fixed spot with the help of cilia. Pseudopodia also serve Amoeba for locomotion, which is the usual second purpose asked in a diagram question.

For the diagram, draw an irregular cell outline, a nucleus, one or two pseudopodia curving round a food particle, and a food vacuole inside the cytoplasm. Label pseudopodia, food particle, food vacuole and nucleus. Show the stages as three or four small sketches if the question says 'show the steps of nutrition'.

The alimentary canal is a long tube from the mouth to the anus, and different parts of it are specialised for different functions. Learn the passage in order: mouth (buccal cavity) → oesophagus (food pipe) → stomach → small intestine → large intestine → anus.

Mouth. Teeth crush the food and the tongue mixes it with saliva, secreted by the salivary glands. Saliva wets the food so it can be swallowed, and its enzyme salivary amylase breaks starch (a complex carbohydrate) into simple sugar. That is why bread tastes sweet when chewed for a while. The lining of the canal has muscles that contract rhythmically to push food along; this peristaltic movement occurs all along the gut.

Stomach. The muscular walls mix the food with gastric juice from the gastric glands, which contains three things: hydrochloric acid (creates an acidic medium so that pepsin can act, and kills bacteria), pepsin (a protein-digesting enzyme) and mucus (protects the stomach lining from the acid). The exit of the stomach is controlled by a sphincter muscle that releases food in small amounts into the small intestine.

Small intestine. This is the longest part of the canal and the site of complete digestion of carbohydrates, proteins and fats. Its length depends on the diet: herbivores like cows have a longer small intestine to digest cellulose, while carnivores like tigers have a shorter one because meat is easier to digest. Food arriving from the stomach is acidic; bile juice from the liver makes it alkaline so the pancreatic enzymes can act. Bile contains no enzyme, but its salts emulsify fats, breaking large globules into tiny droplets that give the enzyme more surface to act on. The pancreas secretes pancreatic juice containing trypsin (digests proteins) and lipase (breaks down emulsified fats). Glands in the intestinal wall secrete intestinal juice, which finally converts proteins to amino acids, complex carbohydrates to glucose and fats to fatty acids and glycerol.

The inner lining of the small intestine has many finger-like projections called villi, which increase the surface area for absorption; they are richly supplied with blood vessels that take the absorbed food to every cell for building new tissue, repairing old tissue and providing energy. Unabsorbed food passes to the large intestine, where more water is absorbed, and the rest is removed through the anus under the control of the anal sphincter.

Digested food is used as a source of energy inside cells. The food is broken down by oxidation, and this whole process is called respiration. Note the difference the examiner insists on: breathing is the physical process of taking in oxygen and giving out CO₂; respiration is the chemical breakdown of food in cells to release energy.

The first step, common to all organisms, is the breakdown of glucose (a six-carbon molecule) into pyruvate (a three-carbon molecule) in the cytoplasm. What happens to pyruvate next depends on whether oxygen is available, and NCERT gives three pathways. Learn them as a flow chart because it is asked for exactly that way:

• Absence of oxygen (in yeast during fermentation): pyruvate → ethanol + CO₂ + energy. This is anaerobic respiration.
• Lack of oxygen (in our muscle cells during sudden, vigorous activity): pyruvate → lactic acid + energy. The build-up of lactic acid causes muscle cramps.
• Presence of oxygen (in mitochondria): pyruvate → CO₂ + water + energy. This is aerobic respiration and releases much more energy than the anaerobic pathways.

The energy released is used at once to make ATP from ADP and inorganic phosphate. ATP is the energy currency of the cell: when the terminal phosphate bond is broken, about 30.5 kJ/mol of energy is released, and this drives muscle contraction, protein synthesis, conduction of nerve impulses and every other activity of the cell.

For the tabular differences question, the standard three rows are: aerobic needs oxygen while anaerobic does not; aerobic occurs in cytoplasm and mitochondria while anaerobic occurs only in the cytoplasm; aerobic gives CO₂ and water and much more energy while anaerobic gives ethanol and CO₂ (or lactic acid) and much less energy. Examples of organisms using anaerobic respiration: yeast, some bacteria.

Plants exchange gases through stomata and the large intercellular spaces, by diffusion. The direction of diffusion depends on the environment and the need of the plant: at night, when there is no photosynthesis, CO₂ is given out; during the day, the CO₂ from respiration is used up in photosynthesis and oxygen is the major gas released.

Since aerobic respiration needs a continuous supply of oxygen, organisms need a way to take in oxygen and throw out CO₂. Aquatic animals use oxygen dissolved in water, which is far less than the oxygen in air, so they breathe much faster. Fish take in water through the mouth and force it past the gills, where oxygen is taken up by blood. Terrestrial animals take oxygen from the air, and the respiratory organs of all animals share three features: a large surface area, a fine and delicate surface, and a rich blood supply (in animals that transport gases in blood).

The human respiratory system. Air enters through the nostrils; fine hairs and mucus in the nasal passage filter out dust. It passes through the throat (pharynx), then the larynx and the trachea, which is held open by rings of cartilage so that the air passage does not collapse. In the chest the trachea divides into two bronchi, one to each lung, which divide into smaller and smaller bronchioles, ending in balloon-like alveoli. The alveoli provide an enormous surface for gas exchange, about 80 m² in a human, and their walls carry an extensive network of blood capillaries.

Breathing. When we breathe in, the ribs are lifted and the diaphragm flattens, so the chest cavity becomes larger and air is drawn into the lungs and fills the alveoli. Blood brings CO₂ from the body to the alveoli for release, and takes up oxygen from the alveolar air. During the breathing cycle the lungs always retain a residual volume of air, so there is enough time for oxygen to be absorbed and CO₂ to be released.

Transport of gases. Oxygen has low solubility in water, so it is carried by the respiratory pigment haemoglobin in the red blood cells, which has a high affinity for oxygen. CO₂ is more soluble in water and is mostly transported in dissolved form in the blood plasma. A deficiency of haemoglobin means less oxygen reaches the tissues, producing weakness and breathlessness (anaemia).

Diffusion would be enough for a small animal, but in a large body it would take years for oxygen to reach the deepest cells; a circulatory system is therefore needed to carry the gases, which is the link to the next section.

Blood is a fluid connective tissue. The fluid part, plasma, carries food, CO₂ and nitrogenous wastes in dissolved form; the red blood cells carry oxygen, and the blood also carries salts and other substances. Blood needs a pumping organ, a network of tubes to reach every tissue, and a system to repair the network if it leaks.

The heart is a muscular organ about the size of a fist, with four chambers: two upper atria and two lower ventricles. Because both oxygen and CO₂ travel in blood, the heart is built so that oxygen-rich blood does not mix with CO₂-rich blood. Follow the path, since a diagram question usually asks you to label 'the chamber that...':

1. Oxygenated blood from the lungs enters the left atrium (through the pulmonary veins) while it relaxes.
2. The left atrium contracts and the blood passes into the left ventricle, which relaxes to receive it.
3. The left ventricle contracts and pumps blood to the whole body through the aorta.
4. Deoxygenated blood from the body enters the right atrium (through the vena cava) as it relaxes.
5. The right atrium contracts and the blood enters the right ventricle, which pumps it to the lungs through the pulmonary artery to pick up oxygen.

The ventricles pump blood into organs, so they have thicker muscular walls than the atria. Valves between atria and ventricles and at the base of the great vessels ensure that blood does not flow backwards when the chambers contract. A muscular wall, the septum, separates the right and left sides.

Double circulation. Blood passes through the heart twice in one complete cycle: once from body to heart to lungs (pulmonary circulation), and once from lungs to heart to body (systemic circulation). The separation of oxygenated and deoxygenated blood allows a highly efficient supply of oxygen, which birds and mammals need because they use energy constantly to keep their body temperature constant. Amphibians and reptiles have three-chambered hearts and tolerate some mixing; fish have two-chambered hearts, blood is pumped to the gills, oxygenated there and sent directly to the body, so it goes through the heart only once per cycle.

The force with which blood pushes against the wall of an artery is blood pressure: about 120 mm of Hg during ventricular contraction (systolic) and 80 mm of Hg during relaxation (diastolic), measured with a sphygmomanometer. High blood pressure (hypertension) is caused by constriction of arterioles and can rupture an artery.

Arteries carry blood away from the heart to the organs. Because blood leaves the heart under high pressure, arteries have thick, elastic walls. Veins collect blood from the organs and bring it back to the heart. Blood in veins is under low pressure, so they have thinner walls, and they have valves that make sure blood flows in only one direction. On reaching an organ, an artery divides into smaller and smaller vessels until it forms capillaries, whose walls are only one cell thick so that materials can be exchanged between the blood and the surrounding cells. Capillaries join up to form veins that carry the blood away from the organ.

For the standard differences table: arteries carry blood from the heart, veins to the heart; arteries carry oxygenated blood (except the pulmonary artery), veins carry deoxygenated blood (except the pulmonary vein); arteries have thick elastic walls and no valves, veins have thin walls and valves; blood flows under high pressure in arteries and low pressure in veins.

Maintenance by platelets. Any leak in the system reduces the pressure and the efficiency of pumping. The blood has platelet cells, which circulate around the body and plug leaks by helping to clot the blood at the points of injury.

Lymph. Through the pores in the walls of capillaries some plasma, proteins and blood cells escape into the spaces between cells and form tissue fluid or lymph. It is similar to plasma but colourless and contains less protein. Lymph drains into lymphatic capillaries, which join to form larger lymph vessels that finally open into the larger veins. Two functions you must know: lymph carries digested and absorbed fat from the intestine, and it drains excess fluid from the extracellular space back into the blood.

A frequent 3-mark question asks for the function of each component of the human transport system: heart (pumps blood), blood (carries O₂, food, CO₂, wastes), blood vessels (arteries, veins, capillaries as routes), platelets (clotting), lymph (fat transport and drainage). Write one line for each.

Plants are large and their two ends do different jobs: roots take in water and minerals from the soil, leaves make food. Diffusion between neighbouring cells is far too slow to carry raw materials to the leaves and food to the roots, so plants have a transport system made of two conducting tissues: xylem for water and minerals, phloem for the products of photosynthesis. Plants have low energy needs because they do not move, so they can use a slow transport system.

Transport of water. In xylem the vessels and tracheids of roots, stems and leaves are interconnected to form a continuous system of water-conducting channels. Root cells actively take up ions from the soil, which creates a difference in ion concentration between the root and the soil; water moves into the root to balance it, and a steady column of water is pushed upwards. This root pressure is enough for small plants, but not to lift water to the top of a tall tree. There the main force is transpiration pull: water evaporates from the leaf cells through the stomata, creating a suction that pulls water up from the xylem of the roots. So root pressure is important at night, when stomata are closed, and transpiration pull is the major driver during the day.

Transpiration is the loss of water as vapour from the aerial parts of the plant. It (i) helps in the absorption and upward movement of water and dissolved minerals, and (ii) helps in temperature regulation by cooling the leaf. This is the standard answer to 'give three reasons why transpiration is important'.

Transport of food and other substances. The movement of soluble products of photosynthesis through phloem is called translocation. Phloem also carries amino acids and other substances, and delivers them to roots, fruits, seeds and growing organs. Unlike transport in xylem, translocation needs energy. Sucrose is loaded into the phloem tissue (sieve tubes with companion cells) using energy from ATP; this raises the osmotic pressure, water moves in, and the pressure pushes the material to tissues where the pressure is lower. Because this pressure is adjustable, phloem can move material up or down according to the plant's needs, for example carrying sugar stored in the roots up to the buds in spring.

For the differences table: xylem moves water and minerals upwards only from roots to leaves, needs no energy, uses physical forces (transpiration pull); phloem moves food and amino acids in both directions, from leaves to storage organs and back, and uses ATP.

Excretion is the biological process of removing harmful metabolic wastes from the body. Unicellular organisms remove wastes by simple diffusion into the surrounding water; complex multicellular organisms use specialised organs. In humans the excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Urine produced in the kidneys passes through the ureters into the bladder, where it is stored until it is released through the urethra.

How is urine produced? The purpose of making urine is to filter out waste products from the blood. Just as CO₂ is removed in the lungs, nitrogenous wastes such as urea and uric acid are removed from blood in the kidneys. Each kidney contains a very large number of filtration units called nephrons, packed close together. In a nephron, blood from the renal artery enters a cluster of very thin-walled capillaries, the glomerulus, which sits inside a cup-shaped Bowman's capsule. Water and dissolved substances are filtered from the glomerulus into the capsule under pressure. The filtrate then flows along the long coiled tubule of the nephron, where useful substances such as glucose, amino acids, salts and a major amount of water are selectively reabsorbed into the blood. The amount of water reabsorbed depends on how much excess water there is in the body and on how much dissolved waste has to be excreted. The urine that remains drains into the collecting duct and then into the ureter.

The bladder is muscular and under nervous control, so we can usually control the urge to urinate. Since a large volume of filtrate is produced but only about 1.5 litres of urine leave the body each day, the tubule's reabsorption is the key to answering 'why is the volume of urine so much less than the filtrate'.

Artificial kidney (haemodialysis). Kidneys can fail because of infection, injury or restricted blood flow. In dialysis the patient's blood is passed through a series of tubes made of a semi-permeable membrane, suspended in a tank of dialysing fluid whose osmotic pressure is the same as blood but without the nitrogenous wastes. Wastes diffuse from the blood into the fluid, and the cleaned blood is returned to the patient. The function is similar to the kidney, but there is no reabsorption, which is the difference the examiner looks for.

Plants use completely different strategies for excretion from animals, and a 3-mark question regularly asks you to 'list four strategies used by plants for excretion'. Plants do not have organs like kidneys, so they manage their wastes in several simple ways.

• Gaseous wastes. Oxygen is a waste product of photosynthesis and carbon dioxide of respiration. They are removed through the stomata and lenticels by diffusion, and since each is used up by the other process during the day, the net exchange depends on the time of day.
• Excess water is removed by transpiration through the stomata.
• Storage in cellular vacuoles. Many plant waste products are stored in the vacuoles of cells, where they do no harm.
• Storage in leaves that fall off. Wastes are moved into old leaves, which are then shed; shedding leaves is a way of getting rid of accumulated waste.
• Resins and gums. Other wastes are stored as resins and gums, especially in old xylem that no longer conducts water.
• Excretion into the soil. Some waste substances are excreted directly into the soil around the plant through the roots.

The idea worth stating in any answer is that many plant wastes are removed very slowly, because plants have a low rate of metabolism compared with animals and can afford to store wastes for long periods. Notice that the same structures do double duty: stomata release oxygen and water vapour and take in CO₂, and the old xylem that transported water becomes a dump for resins. Two of these examples with the phrase 'stored in' or 'removed through' earns the full three marks.