Life Processes
A sleeping dog is still alive. A cactus that has not grown an inch all year is still alive. So what actually separates the living from the non-living, if not visible movement? This NCERT chapter answers with a deeper idea: life is a standing fight against disorder. Every living structure — from a single cell to a human body — is an organised arrangement of molecules, and the environment constantly works to break that order down. To stay alive, an organism must continuously repair and rebuild itself, which means moving molecules around inside itself all the time, even when it looks perfectly still. The processes that keep this maintenance job running are called life processes. This one idea generates the whole chapter. Because energy has to come from outside the body, organisms need nutrition. Because breaking down food efficiently needs oxygen, organisms need respiration. Because food and oxygen are absorbed in one place but needed everywhere in a multicellular body, organisms need transportation. And because chemical reactions leave harmful by-products behind, organisms need excretion. A single-celled organism can handle all of this directly across its own surface; as body size and complexity grow, specialised tissues and organs take over each job, and a transport system becomes essential to connect them. This article follows all four life processes as NCERT builds them: how autotrophs like green plants make their own food by photosynthesis, and how heterotrophs like us digest complex food using enzymes; how aerobic and anaerobic respiration release energy from glucose at different rates; how the human circulatory system and plant vascular tissues move materials around; and how kidneys and plant tissues dispose of waste. Along the way you will meet stomata, alveoli, nephrons, xylem and phloem — different solutions to the very same underlying problem of staying alive.
- Life processes are the maintenance functions — nutrition, respiration, transportation and excretion — that keep an organism's ordered structure intact even at rest.
- Autotrophic nutrition (photosynthesis): , in chloroplasts, CO2 taken in through stomata.
- Heterotrophic nutrition breaks down complex food using enzymes, step by step along the human alimentary canal.
- Respiration always starts the same way: glucose (6C) breaks to pyruvate (3C) in the cytoplasm; aerobic respiration finishes in mitochondria to CO2 and water, releasing much more energy, while anaerobic respiration gives ethanol plus CO2 (yeast) or lactic acid (muscle cramps).
- ATP is the energy currency: its terminal phosphate bond releases about 30.5 kJ/mol when broken.
- The human heart uses double circulation: blood passes through it twice per body cycle, and its four chambers keep oxygenated and deoxygenated blood separate.
- Blood vessels are matched to their job: elastic arteries carry blood away under high pressure, valved veins return it, and one-cell-thick capillaries let materials diffuse into tissues.
- Plants use two tubes: xylem carries water and minerals upward via transpiration pull; phloem carries photosynthesis products using ATP, in either direction.
- Excretion in humans centres on the nephron: it filters blood, selectively reabsorbs glucose, amino acids, salts and water, and passes the rest on as urine — about 180 L filtered daily but only 1-2 L excreted.
- Key idea: all life processes exist to solve one problem — obtaining energy and raw material from outside the body, converting them usefully inside, and removing the resulting waste — so that the organism's molecular order can be continuously repaired and maintained.
Living organisms must continuously perform nutrition, respiration, transportation and excretion because maintaining an organised body against constant breakdown requires a steady inward supply of energy and raw material and a steady outward removal of waste.
What this chapter covers
This chapter answers "what makes something alive, biologically speaking?" and builds outward into four connected systems. It opens by defining life processes as maintenance functions, and why molecular movement is the real signature of life. From there it develops nutrition in two branches: autotrophic (photosynthesis) and heterotrophic (how animals, fungi and parasites digest complex food, worked out for the human alimentary canal). Next comes respiration: the cytoplasm-then-mitochondria pathway, aerobic versus anaerobic respiration, and gas exchange in plants (stomata) and animals (gills or lungs, ending in the human respiratory system and alveoli). Then transportation: the human circulatory system and the plant vascular system (xylem and phloem), and why double circulation matters. Finally excretion: the human excretory system (kidneys, nephrons, ureters, bladder, urethra), and the very different, passive strategies plants use.
The core ideas
What counts as a life process
Movement is the everyday clue we use to decide something is alive, but it is not reliable — a sleeping animal or a non-growing plant is still alive. The deeper criterion biologists use is molecular movement: living structures are highly organised, and this order constantly tends to break down. To counter this, an organism must keep repairing its structures, moving molecules around continuously, even during sleep. The processes that carry out this maintenance are called life processes, and since maintenance needs energy from outside, nutrition exists. In a single-celled organism such as Amoeba, the entire cell surface contacts the environment, so simple diffusion is enough for taking in food, exchanging gases and removing waste. In a multicellular organism, most cells are buried deep inside, so diffusion alone cannot supply them fast enough. This is why complex organisms like humans evolved specialised tissues for nutrition, respiration and excretion, and — crucially — a transportation system connecting these tissues to every other cell.
Autotrophic nutrition: photosynthesis
Autotrophs, mainly green plants and some bacteria, make their own food from simple inorganic raw materials — carbon dioxide and water — using light energy captured by chlorophyll. This process, photosynthesis, happens in three stages that need not occur strictly in sequence: absorption of light by chlorophyll, conversion of that energy to chemical energy along with splitting water into hydrogen and oxygen, and reduction of carbon dioxide to carbohydrates.
where: and are the raw materials, sunlight supplies the energy, chlorophyll (inside chloroplasts) absorbs the light, is glucose (often stored as starch), and is a by-product. Carbon dioxide enters mainly through tiny pores called stomata, opened and closed by guard cells that swell (open) or shrink (close) with their water content. Other raw materials — nitrogen, phosphorus, iron, magnesium — are absorbed from soil by the roots, mostly as inorganic nitrates or nitrites, or nitrogen compounds prepared by soil bacteria.
Heterotrophic nutrition and the human digestive system
Heterotrophs cannot make their own food; they depend on autotrophs and use enzymes to break complex food into simpler, absorbable forms. Strategies vary: bread mould digests food outside the body and absorbs it; parasites such as Cuscuta, ticks and tapeworms derive nutrition from a living host without necessarily killing it. In Amoeba, finger-like extensions engulf food to form a food vacuole; in Paramoecium, cilia sweep food to a fixed spot. In humans, the alimentary canal runs from mouth to anus with regions specialised for each digestive step. Digestion begins in the mouth, where teeth crush food and the tongue mixes it with saliva, containing salivary amylase, which begins breaking starch into simpler sugars. Peristalsis, rhythmic muscular contraction, pushes food along the gut. Food travels via the oesophagus to the stomach, where gastric glands release hydrochloric acid (the medium pepsin needs, and it kills bacteria), the enzyme pepsin, and protective mucus, before a sphincter releases it in small amounts into the small intestine. The small intestine completely digests carbohydrates, proteins and fats, aided by the liver (bile, making the food alkaline and emulsifying fats for enzymes to act on) and the pancreas (trypsin for proteins, lipase for emulsified fats). Intestinal wall glands convert proteins to amino acids, carbohydrates to glucose, and fats to fatty acids and glycerol. Absorption happens through finger-like villi, which increase surface area and carry nutrients to every cell via blood vessels. Undigested food passes into the large intestine, which absorbs water from it, and waste exits through the anus.
Respiration: releasing energy from food
Every organism needs a way to release usable energy from food — respiration. Regardless of oxygen, the first step is the same everywhere: glucose (six-carbon) is broken down in the cytoplasm into pyruvate (three-carbon). What happens next depends on oxygen availability. If oxygen is available, pyruvate moves into the mitochondria and breaks down completely into carbon dioxide and water, releasing a large amount of energy — aerobic respiration. If not, two anaerobic routes exist: in yeast, pyruvate becomes ethanol and carbon dioxide; in human muscle during sudden vigorous activity, it becomes lactic acid instead, whose build-up causes cramps. The energy released synthesises ATP, the cell's energy currency; breaking ATP's terminal phosphate bond releases about .
where: aerobic respiration (mitochondria, needs ) releases far more energy per glucose molecule than either anaerobic route. Plants exchange gases through stomata, direction reversing between day and night; fish extract dissolved oxygen using gills, breathing faster since dissolved oxygen is scarcer; humans use lungs. In the human respiratory system, air enters the nostrils, is filtered and moistened, passes through the throat (kept open by cartilage rings) into the lungs, reaching tiny sacs called alveoli, surrounded by capillaries. Ribs lift and the diaphragm flattens during inhalation, enlarging the chest cavity; the reverse happens on exhalation. Because a large body cannot rely on diffusion alone, haemoglobin in red blood corpuscles binds oxygen strongly in the alveoli and releases it where scarce; carbon dioxide travels mostly dissolved in plasma.
Transportation: the circulatory system
Once food and oxygen are absorbed, both must reach every cell, and waste must be carried away — transportation. In humans, plasma carries digested food, carbon dioxide and nitrogenous waste dissolved, while oxygen is carried by red blood corpuscles, needing a pump (heart), tubes (blood vessels), and a repair mechanism (platelets). The human heart has four chambers, arranged so oxygen-rich and oxygen-poor blood never mix. Oxygenated blood from the lungs enters the left atrium, moves to the left ventricle, and is pumped to the body; deoxygenated blood enters the right atrium, moves to the right ventricle, and is pumped to the lungs. Ventricles have thicker walls than atria, and valves prevent backflow. Since blood passes through the heart twice per body circuit, this is double circulation, giving an undiluted oxygen supply — important for birds and mammals, which constantly maintain body temperature. Amphibians have a three-chambered heart tolerating some mixing; fish have only two chambers, with a single pass per circuit. Blood vessels match their function: arteries carry blood away from the heart under high pressure with thick, elastic walls; veins carry it back under low pressure and rely on valves; capillaries, walls one cell thick, allow direct exchange with cells. Platelets plug leaks and clot blood at injury sites. A separate fluid, lymph, escapes from capillary pores into spaces between cells, carries away excess fluid and absorbed fat, and drains back into the blood.
Transportation and excretion in plants
Plants have far lower energy needs than animals — they do not move, and many tissues are dead cells — so they rely on slower transport systems, even over the large distances in a tall tree. Plants use two tissues: xylem, moving water and minerals upward from the roots, and phloem, moving photosynthesis products to wherever needed — translocation. Water movement in xylem begins at the root, where cells actively take up soil ions, drawing water in by osmosis and pushing a column upward. This root pressure alone cannot explain how water reaches the tops of tall trees; the dominant daytime force is transpiration — evaporation through open stomata — creating suction that pulls water up. Root pressure matters more at night, when stomata are closed. Unlike xylem, phloem translocation actively uses energy: sucrose is loaded in using ATP, raising osmotic pressure so water moves in, pushing material from a high-pressure "source" (leaves) to a low-pressure "sink" (roots, fruits, seeds, growing organs), in either direction. Plants excrete very differently from animals. Oxygen from photosynthesis is itself a waste product released through stomata, along with excess carbon dioxide and water vapour. Because many tissues are dead cells and plants can shed whole structures, other waste is stored in cell vacuoles, deposited in fallen leaves, stored as resins and gums in old xylem, or excreted into the soil.
Excretion in humans: the nephron
Metabolic activity generates nitrogenous waste, such as urea, that must be removed — excretion. The human excretory system has a pair of kidneys, ureters, a urinary bladder and a urethra. Blood is filtered by millions of tiny units called nephrons, each built around a capillary cluster cupped by a coiled tube called Bowman's capsule. As filtrate flows along the tubule, useful substances — glucose, amino acids, salts and most water — are selectively reabsorbed into the blood, depending on how much excess the body needs to shed. What remains becomes urine, travelling via the ureter to the bladder for storage before release through the urethra, under largely voluntary nervous control. A healthy adult's kidneys filter roughly 180 litres daily, yet only 1-2 litres actually leave as urine.
Break down glucose the two ways
The goal is to explain, from the chemistry NCERT gives us, why aerobic respiration is the pathway organisms prefer whenever oxygen is available, and why anaerobic respiration is only a fallback. Both pathways start from the same molecule and the same first step, so the difference in energy output must come from what happens to pyruvate afterwards.
Step 1. Both pathways begin identically. In the cytoplasm, one molecule of glucose (six-carbon) is broken into two molecules of pyruvate (three-carbon). This needs no oxygen and releases little energy, because most of glucose's chemical energy is still locked inside pyruvate.
Step 2. The pathway now branches on oxygen availability. If present, pyruvate enters the mitochondria, the cell's oxygen-using organelles. If absent, pyruvate stays in the cytoplasm and becomes either ethanol plus carbon dioxide (yeast) or lactic acid (oxygen-starved muscle).
Step 3. Inside the mitochondria, aerobic respiration finishes what anaerobic respiration leaves incomplete: pyruvate is broken all the way down into three molecules of carbon dioxide, with water as the other product. Because oxygen is available to accept electrons at the end, pyruvate's bonds break completely, releasing essentially all its remaining energy.
Step 4. Anaerobic respiration stops early. Converting pyruvate to ethanol plus carbon dioxide, or to lactic acid, only rearranges the carbon atoms without breaking them further — most of pyruvate's chemical energy stays locked inside, unreleased, yielding far less usable energy.
Step 5. Both pathways use whatever energy they release to make ATP, which actually powers the cell, since breaking its terminal phosphate bond releases a fixed packet of about 30.5 kJ per mole. Aerobic respiration's larger energy release generates far more ATP — why high-demand organisms rely on it whenever oxygen is available, and why oxygen-starved muscle cramps quickly on lactic acid alone.
Go deeper · Derivation B · 1 minWhy transpiration, not root pressure, drives water to the top of a tall tree
The goal is to work out, using only the physical picture NCERT provides, which force is strong enough to lift a continuous water column to a tree's highest leaves, and why transpiration pull, not root pressure, is the dominant daytime mechanism.
Step 1. Root pressure originates at the root. Root cells actively take up mineral ions from the soil into the xylem, lowering the water potential there. Water moves in to even out this difference, and as more enters from below, it pushes the existing column upward — a push from the bottom.
Step 2. Root pressure alone is limited. It is generated only at the base, and a push at one end of a long, narrow column must work against the column's full weight and friction all the way up. For a tall tree, this push is nowhere near strong enough to raise water reliably to the topmost leaves.
Step 3. A second force operates from the top, at the leaves. Water vapour continuously evaporates from leaf cells, escaping mainly through open stomata — transpiration. As a leaf cell loses water, it draws replacement water from the neighbouring xylem vessel, which draws from the vessel below it, down to the roots.
Step 4. This chain forms a continuous suction, and because water molecules cling together in the narrow xylem vessels, the suction is transmitted as tension all the way down to the roots. This transpiration pull hauls water upward even in very tall trees, and is strongest during the day, when stomata are open.
Step 5. Putting the two together explains the daily pattern: at night, stomata close and transpiration is minimal, so root pressure dominates; during the day, transpiration pull takes over, moving water and minerals from roots to leaves.
Worked examples
Board level - identifying life processes and matching structures to functions
Classify each of the following as primarily nutrition, respiration, transportation or excretion, and name one specialised structure involved: (a) a leaf taking in carbon dioxide, (b) blood carrying digested food to a muscle cell, (c) a nephron removing urea from blood, (d) alveoli exchanging oxygen and carbon dioxide.
Step 1. A leaf taking in carbon dioxide is autotrophic nutrition; the structure is the stoma, controlled by guard cells.
Step 2. Blood carrying digested food to a muscle cell is transportation, moving absorbed material to a muscle; the structures are blood vessels, particularly capillaries.
Step 3. A nephron removing urea from blood is excretion, since urea is nitrogenous waste; the structure is Bowman's capsule, where filtration begins.
Step 4. Alveoli exchanging gases is respiration, supplying oxygen for aerobic glucose breakdown and removing the carbon dioxide produced; the structure is the alveolus, whose thin, capillary-rich wall maximises diffusion area. Answer: (a) nutrition, stomata; (b) transportation, blood vessels/capillaries; (c) excretion, nephron; (d) respiration, alveoli.
Board level - reading NCERT's own worked reasoning on stomatal exchange
During the day, a plant's leaves are photosynthesising steadily and also respiring. Explain whether the net direction of oxygen and carbon dioxide exchange through the stomata will differ between day and night, and why.
Step 1. At all times, a plant cell respires, consuming oxygen and producing carbon dioxide, continuously, day and night.
Step 2. During the day, photosynthesis consumes far more carbon dioxide, and produces far more oxygen, than respiration uses or generates. Respiration's carbon dioxide is entirely used up by photosynthesis, so the net stomatal exchange becomes a release of surplus oxygen.
Step 3. At night, photosynthesis stops, but respiration continues; with nothing consuming it, the carbon dioxide from respiration now dominates. Answer: net oxygen release dominates during the day, and net carbon dioxide release dominates at night.
Board HOTS / foundation JEE-NEET - double circulation and energy demand
Explain, using the chapter's reasoning, why birds and mammals — which maintain a constant, high body temperature — need a four-chambered heart with complete separation of oxygenated and deoxygenated blood, while fish, which have only a two-chambered heart, survive perfectly well without this separation.
Step 1. Maintaining a constant body temperature, as birds and mammals do, requires continuously generating heat through metabolism, needing a high, steady rate of aerobic respiration throughout the body.
Step 2. Aerobic respiration stays efficient only with a well-oxygenated blood supply; if oxygen-rich blood mixes with oxygen-poor blood, every cell receives blood with lower oxygen concentration, diluting the supply everywhere.
Step 3. A four-chambered heart, with the left side entirely oxygenated and the right entirely deoxygenated, prevents this mixing, so tissues receive blood that is as fully oxygenated as possible — supporting the sustained aerobic respiration constant-temperature maintenance demands.
Step 4. Fish are ectothermic and do not need continuous metabolic heat; their modest, steady oxygen demand is met by a two-chambered heart with a single pass per circuit, without the added complexity of chamber separation. Answer: complete separation of oxygenated and deoxygenated blood supports the sustained aerobic respiration constant-temperature animals need, while fish's lower, steady demand is met by a simpler two-chambered heart. Timing note: about three to four minutes for a full written answer. The key move examiners look for is connecting "constant body temperature" to "sustained aerobic respiration" to "need for undiluted oxygen" to "chamber separation" — stating the conclusion without this chain loses marks even if correct.
Common traps
Confusing "breathing" with "respiration" is the single most common error: breathing is only the mechanical gas-exchange step; respiration is the entire chemical process, including glucose breakdown, and continues even during sleep. Students often reverse xylem and phloem — xylem moves water and minerals upward, mostly one-way; phloem moves photosynthesis products in either direction, using ATP. Another slip is treating "aerobic" and "anaerobic" as fixed properties of an organism rather than a pathway: human muscle normally respires aerobically but switches temporarily to lactic acid during vigorous exercise. Students also describe the kidney as simply "removing water," when its job is selective — most filtered water and useful substances are reabsorbed, and only excess water and genuine waste leave as urine. Finally, many answers call arteries "oxygenated" and veins "deoxygenated" as a blanket rule — this reverses for the pulmonary vessels, since artery and vein are defined by direction relative to the heart, not oxygen content.
You should be able to…
- Explain why molecular movement is the reliable indicator of life.
- Write the balanced equation for photosynthesis, and the roles of chlorophyll, stomata and guard cells.
- Describe how food is digested and absorbed through the alimentary canal, naming the key enzyme or secretion at each stage.
- Trace the two branches of glucose breakdown from pyruvate, and why aerobic respiration releases more energy.
- Describe the human respiratory system and how alveoli maximise gas exchange.
- Explain double circulation, and why oxygenated and deoxygenated blood must stay separate.
- Compare arteries, veins and capillaries, and the roles of platelets and lymph.
- Explain how xylem uses transpiration pull and phloem uses ATP-driven translocation.
- Describe a nephron's structure and function, and what selective reabsorption means.
- Compare how plants and humans dispose of metabolic waste.
Ask the tutor: Want a walk-through comparing the alveolus and the nephron structurally, since NCERT explicitly asks students to compare them?
Why can't a multicellular organism like a human just rely on diffusion for oxygen and food the way Amoeba does?
Diffusion works well only over very short distances, since it relies on random molecular motion and slows down enormously with distance — if human oxygen transport depended on diffusion alone, it is estimated it would take about three years for an oxygen molecule to travel from the lungs to the toes. In Amoeba, every part of the cell is close to the surface, so diffusion across the surface is fast enough. In a human, most cells are buried deep inside the body, far from any exchange surface, so diffusion alone cannot deliver materials fast enough to keep them alive. This is exactly why complex organisms evolved a dedicated transportation system that actively pumps blood to every cell, rather than waiting for slow, passive diffusion.
What's the actual difference between breathing and respiration, since people use the words interchangeably?
Breathing is the physical, mechanical process of moving air in and out of the lungs, something you can see, hear, and even briefly control, like holding your breath. Respiration is the broader biological process by which a cell breaks down glucose to release usable energy, whether or not oxygen is involved, and it happens continuously inside every living cell, including cells that never touch air directly. Breathing simply supplies the oxygen respiration needs and removes the carbon dioxide it produces; respiration itself is the actual energy-releasing chemistry happening at the cellular level, in the cytoplasm and mitochondria. Breathing is a small, visible part of the much larger, continuous, invisible process of respiration.
If plants make their own food by photosynthesis why do they also need to respire and use oxygen?
Photosynthesis and respiration serve different purposes, and a plant needs both. Photosynthesis manufactures glucose (food) using carbon dioxide, water and light — but making the food is not the same as using it for energy. Like humans metabolising their food, a plant's own cells must respire, breaking down the glucose photosynthesis produced, to release energy for growth, repair and all other cellular activity. Respiration happens in every living plant cell, all the time, regardless of whether photosynthesis is happening, which is why plants take in oxygen and release carbon dioxide at night, when photosynthesis has stopped but respiration continues.
Why does the small intestine need to be so long and coiled up, and does it differ between animals?
The small intestine has two demanding jobs in sequence: complete chemical digestion of carbohydrates, proteins and fats using bile and pancreatic and intestinal enzymes, then absorption of the resulting simple molecules through millions of villi lining its wall. Both jobs need time and surface area, which a longer intestine provides, while extensive coiling lets this length fit inside a compact abdominal cavity. The length differs by diet: herbivores, eating cellulose-rich grass that is hard to digest, need a notably longer small intestine, while carnivores like tigers, eating meat that digests more easily, have a shorter one.