Class 10 Science · Chapter 5 NotesLife Processes

Living organisms are highly organised structures made of tissues, cells and molecules. Because of environmental effects, this order tends to break down over time, so organisms must keep repairing and maintaining themselves. These maintenance functions go on even when the organism is doing nothing particular, and the processes that together perform this job are called life processes. Energy is needed for this maintenance, and it comes from outside the body as food. In single-celled organisms the entire surface is in contact with the environment, so simple diffusion is enough for taking in food, exchanging gases and removing wastes. In multi-cellular organisms, most cells are not in direct contact with the surroundings, so diffusion cannot meet the needs of all cells. Specialised tissues take up food and oxygen at one place, creating the need for a transport system, and waste products must be carried away to excretory tissues.

• Life processesnutrition, respiration, transportation and excretion
• Autotrophs make their own food from carbon dioxide and water; heterotrophs depend on complex food made by others
• Diffusion is sufficient in unicellular organisms but not in large multi-cellular organisms

Autotrophs such as green plants and some bacteria use carbon dioxide and water to make carbohydrates in the presence of sunlight and chlorophyll. This process is photosynthesis. The events of photosynthesis include absorption of light energy by chlorophyll, conversion of light energy to chemical energy and splitting of water molecules into hydrogen and oxygen, and reduction of carbon dioxide to carbohydrates. Extra carbohydrate is stored as starch, just as we store energy as glycogen. Carbon dioxide enters leaves through stomata, tiny pores whose opening and closing is controlled by guard cells; water is taken up by roots, and minerals like nitrogen, phosphorus, iron and magnesium come from the soil. Heterotrophs cannot make their own food. Some, like bread moulds, yeast and mushrooms, break down food outside the body and absorb it. Others, like cuscuta, ticks, lice, leeches and tapeworms, live as parasites and derive nutrition without killing the host. In Amoeba, food is taken in by finger-like extensions that form a food-vacuole, while Paramoecium takes food at a fixed spot with the help of cilia.

• Photosynthesiscarbon dioxide + water → carbohydrate in the presence of sunlight and chlorophyll
• Stomatatiny pores on leaves for gaseous exchange; guard cells open and close them
• Enzymes are biological catalysts that break complex food into simpler molecules

The human alimentary canal is a long tube from mouth to anus with specialised regions. In the mouth, teeth crush the food and saliva from salivary glands wets it; salivary amylase breaks down starch into simple sugar. Peristaltic movements of the muscular lining push food forward to the stomach through the oesophagus. Gastric glands in the stomach release hydrochloric acid, the protein-digesting enzyme pepsin and mucus; the acid creates an acidic medium for pepsin, while mucus protects the stomach lining. A sphincter releases food in small amounts into the small intestine, the longest part of the canal, where digestion of carbohydrates, proteins and fats is completed. Bile juice from the liver makes the food alkaline and breaks large fat globules into smaller ones. Pancreatic juice contains trypsin for proteins and lipase for emulsified fats, and intestinal juice finally converts proteins to amino acids, carbohydrates to glucose and fats to fatty acids and glycerol. Villi in the small intestine increase the surface area for absorption. The large intestine absorbs water, and undigested waste leaves through the anus.

• Salivary amylasestarch → simple sugar
• Pepsinproteins → simpler proteins in an acidic medium
• Trypsinproteins; lipase: emulsified fats; bile salts emulsify fats
• Villifinger-like projections that increase surface area for absorption

The food taken in during nutrition is broken down in cells to release energy. In all cases the first step is the breakdown of glucose, a six-carbon molecule, into pyruvate, a three-carbon molecule, in the cytoplasm. In yeast, pyruvate is converted into ethanol and carbon dioxide during fermentation; since this happens in the absence of oxygen, it is called anaerobic respiration. In our muscle cells, when oxygen is lacking, pyruvate is converted into lactic acid, which causes cramps during sudden activity. When oxygen is available, pyruvate is broken down in the mitochondria into carbon dioxide and water; this is aerobic respiration and releases much more energy than the anaerobic pathway. The energy released is used to make ATP from ADP and inorganic phosphate. ATP is the energy currency of the cell and drives endothermic reactions such as muscle contraction, protein synthesis and conduction of nerve impulses. Plants exchange gases through stomata and intercellular spaces; at night carbon dioxide release is the major exchange, while during the day oxygen release is the major event. Aquatic animals breathe faster than terrestrial animals because dissolved oxygen in water is low. In humans, air enters through the nostrils, passes through the throat and into the lungs, ending in alveoli where gases are exchanged. Haemoglobin in red blood corpuscles carries oxygen, while carbon dioxide is mostly transported dissolved in the blood.

• Glucose (6-carbon) → pyruvate (3-carbon) in the cytoplasm
• Anaerobic respiration in yeastpyruvate → ethanol + carbon dioxide
• In musclespyruvate → lactic acid when oxygen is insufficient
• Aerobic respiration in mitochondriapyruvate → carbon dioxide + water + more energy
• ATP is the energy currency; breaking its terminal phosphate linkage releases 30.5 kJ/mol

Blood is a fluid connective tissue with plasma and cells. Plasma transports food, carbon dioxide and nitrogenous wastes in dissolved form, while red blood corpuscles carry oxygen. The heart is a muscular organ with four chambers that keeps oxygen-rich and carbon dioxide-rich blood from mixing. Oxygen-rich blood from the lungs enters the left atrium, passes to the left ventricle, and is pumped to the body. Deoxygenated blood from the body enters the right atrium, goes to the right ventricle, and is pumped to the lungs. Ventricles have thicker walls than atria, and valves prevent backward flow. This double circulation allows an efficient oxygen supply, which is useful for birds and mammals that use energy to maintain body temperature. Arteries carry blood away from the heart under high pressure and have thick elastic walls; veins bring blood back and have valves to keep flow one-way. Capillaries are one-cell thick and allow exchange of materials. Platelets plug leaks by clotting blood. Lymph, a colourless fluid with less protein than plasma, carries digested and absorbed fat from the intestine and drains excess fluid back into the blood.

• Left atrium → left ventricle → body; right atrium → right ventricle → lungs
• Double circulationblood passes through the heart twice in one complete cycle
• Arteriesthick, elastic walls; veins: valves; capillaries: one-cell thick
• Normal blood pressureabout 120 mm Hg systolic and 80 mm Hg diastolic

Plants need a transport system because the distance between soil-contacting roots and chlorophyll-containing leaves can be large. Plants have low energy needs because they do not move and have many dead cells, so they use relatively slow transport systems. Xylem moves water and minerals obtained from the soil, while phloem transports the products of photosynthesis from leaves to other parts. In xylem, root cells actively take up ions, creating a concentration difference that draws water into the root and pushes a column of water upward. This root pressure is more important at night. During the day, transpiration pull becomes the major driving force: evaporation of water from leaf cells creates a suction that pulls water up the xylem. Transpiration also helps in temperature regulation. In phloem, the transport of soluble products of photosynthesis is called translocation. Sucrose is transferred into phloem using energy from ATP, water moves in by osmosis, and the resulting pressure moves material to tissues with less pressure. Translocation occurs in sieve tubes with the help of companion cells, in both upward and downward directions.

• Xylemwater and minerals; phloem: food and other substances
• Transpirationloss of water vapour from aerial parts, creating transpiration pull
• Translocationtransport of soluble products of photosynthesis through phloem

Excretion is the biological process of removing harmful metabolic wastes from the body. Many unicellular organisms remove wastes by simple diffusion from the body surface. In humans, the excretory system includes a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys filter nitrogenous wastes such as urea and uric acid from the blood. The basic filtration unit is the nephron, which has a cluster of thin-walled capillaries associated with the cup-shaped Bowman's capsule. As the filtrate flows along the tubule, useful substances like glucose, amino acids, salts and a major amount of water are selectively reabsorbed. The amount of water reabsorbed depends on how much excess water is in the body and how much dissolved waste must be excreted. Urine passes through the ureter to the urinary bladder and is released through the urethra. Plants use different strategies: oxygen can be considered a waste product of photosynthesis, excess water is removed by transpiration, and other wastes are stored in cell vacuoles, in leaves that fall off, as resins and gums in old xylem, or excreted into the surrounding soil.

• NephronBowman's capsule + coiled tubule; site of filtration and selective reabsorption
• Urine formationfiltration in the kidney, reabsorption of useful substances, storage in the bladder
• Plant excretiontranspiration, storage in vacuoles, falling leaves, resins and gums, release into soil