Life Processes

1. What are life processes?

How do we know something is alive? A running dog or a chewing cow is obviously living, but a sleeping animal and a non-growing plant are alive too. So visible movement is not a reliable test. The real signature of life is invisible — the constant movement of molecules inside the body. Even viruses show no molecular movement until they infect a cell, which is why their "alive" status is debated.

Living things are highly organised structures (organism → organs → tissues → cells → molecules). The environment keeps trying to break this order down. If the order collapses, the organism dies. So living creatures must constantly repair and maintain themselves, and that needs energy and raw materials and a way to move molecules around.

The maintenance jobs that go on all the time — even while we sleep — are the life processes:

• Nutrition — taking in an outside source of energy (food) and raw material to grow.
• Respiration — breaking down food (often using oxygen) to release usable energy.
• Transportation — carrying food, oxygen and wastes from one body part to another.
• Excretion — removing harmful by-products made during these chemical reactions.

In a single-celled organism the whole surface touches the environment, so no special organs are needed — simple diffusion works. In a large multicellular body most cells are deep inside, far from the surface, so diffusion alone is far too slow; specialised tissues and a transport system become essential.

2. Nutrition — autotrophic (photosynthesis)

All organisms need energy and materials but get them differently. Autotrophs (green plants and some bacteria) make their own food from simple inorganic substances — carbon dioxide and water. Heterotrophs (animals, fungi) take in ready-made complex food.

Photosynthesis is the process by which autotrophs build food. They take CO₂ and water and convert them into carbohydrates in the presence of sunlight and chlorophyll (the green pigment inside chloroplasts). Extra carbohydrate is stored as starch — the plant's energy reserve (we store ours as glycogen).

Three events take place during photosynthesis:

• (i) Absorption of light energy by chlorophyll.
• (ii) Conversion of light energy to chemical energy, and splitting of water into hydrogen and oxygen.
• (iii) Reduction of carbon dioxide to carbohydrates.

These steps need not happen one right after another — desert plants take in CO₂ at night and use it during the day.

Raw materials and where they come from: CO₂ enters through tiny pores called stomata on the leaf surface; water comes up from the soil through the roots; nitrogen, phosphorus, iron and magnesium are absorbed from the soil (nitrogen as nitrates/nitrites, or fixed by bacteria).

3. Nutrition — heterotrophic & in single cells

Heterotrophs differ in how they get food, depending on body design:

• Saprophytic — break food down outside the body and then absorb it (fungi like bread moulds, yeast, mushrooms).
• Holozoic — take in whole food and break it down inside (most animals).
• Parasitic — draw nutrition from a living host without killing it (cuscuta/amarbel, ticks, lice, leeches, tapeworms).

4. Nutrition in human beings — the digestive system

The alimentary canal is a long tube from mouth to anus. Different regions are specialised for different jobs (Fig 5.6). Trace the journey of food:

• Mouth (buccal cavity): teeth crush food; the tongue mixes it with saliva from salivary glands. Saliva contains salivary amylase, which breaks starch into simple sugar.
• Oesophagus (food-pipe): rhythmic muscular contractions called peristalsis push food down (these movements happen all along the gut).
• Stomach: a large muscular bag. Its walls churn food; gastric glands release hydrochloric acid (makes the medium acidic, kills germs, activates the enzyme), pepsin (digests protein) and mucus (protects the stomach lining from the acid). A sphincter muscle lets food into the small intestine in small amounts.
• Small intestine: the longest part, tightly coiled. Herbivores have a longer one (to digest cellulose); carnivores have a shorter one. It is the site of complete digestion of carbohydrates, proteins and fats. It receives bile from the liver (makes the food alkaline and emulsifies fats into small globules) and pancreatic juice from the pancreas (contains trypsin for proteins and lipase for fats). Intestinal juice from its own walls finishes the job: proteins → amino acids, carbohydrates → glucose, fats → fatty acids + glycerol.
• Absorption: the inner lining has millions of finger-like villi that greatly increase surface area; they are richly supplied with blood vessels that carry absorbed food to every cell.
• Large intestine: absorbs more water from the undigested material; the remaining waste leaves through the anus, regulated by the anal sphincter.

5. Respiration — aerobic vs anaerobic

Food taken in is used by cells to release energy. The first step is the same in all organisms: glucose (a 6-carbon molecule) is broken down in the cytoplasm into pyruvate (a 3-carbon molecule). What happens next depends on oxygen (Fig 5.8):

• Anaerobic (no oxygen): in yeast, pyruvate → ethanol + CO₂ + energy (this is fermentation). In our muscle cells during sudden hard exercise (lack of oxygen), pyruvate → lactic acid (3-carbon) + energy — the build-up of lactic acid causes muscle cramps.
• Aerobic (with oxygen): in the mitochondria, pyruvate is broken completely into CO₂ + water + energy. This releases much more energy than the anaerobic pathway.

The energy released is immediately used to make ATP — the cell's "energy currency". ATP is made from ADP + phosphate, and when its terminal phosphate bond is broken (using water) it releases about 30.5 kJ/mol to power muscle contraction, protein synthesis, nerve impulses and more.

6. The human respiratory system

Aerobic organisms need a steady supply of oxygen. In plants gases diffuse through stomata and large inter-cellular spaces. Animals have evolved special organs: aquatic animals use gills to take up dissolved oxygen (they breathe faster because water holds little oxygen); terrestrial animals use lungs.

Path of air in humans (Fig 5.9): air enters the nostrils → the nasal passage filters it with fine hairs and mucus → through the pharynx (throat) and larynx → down the trachea (windpipe), which is held open by rings of cartilage so it never collapses → into two bronchi → finer bronchioles → ending in balloon-like alveoli (singular: alveolus).

Why alveoli are perfect for gas exchange: they are very thin-walled and wrapped in a dense net of blood capillaries, giving a huge surface area (about 80 m² if spread out). Here oxygen passes from the alveolar air into the blood, and carbon dioxide passes out of the blood into the air to be breathed out. When we breathe in, the ribs lift and the diaphragm flattens, enlarging the chest cavity so air rushes in. The lungs always keep a residual volume of air so there's time to absorb O₂ and release CO₂.

How oxygen travels: oxygen is carried by the respiratory pigment haemoglobin in the red blood corpuscles; it has a very high affinity for oxygen. Carbon dioxide is more soluble than oxygen, so it is mostly carried dissolved in the blood plasma. (Without haemoglobin, diffusion alone would take ~3 years for oxygen to reach our toes!)

7. Transportation in humans — heart, blood, double circulation

Blood is a fluid connective tissue: a liquid plasma (carries dissolved food, CO₂ and nitrogenous waste) in which cells float. RBCs carry oxygen; platelets plug leaks by helping blood clot at injuries. We need a pump (heart), a network of tubes (vessels), and a repair system (platelets).

The heart is a muscular organ about the size of our fist with four chambers — two upper atria and two lower ventricles. The chambers keep oxygen-rich and CO₂-rich blood from mixing. Step by step (Fig 5.10, 5.11):

• Oxygen-rich blood from the lungs enters the thin-walled left atrium (it relaxes to collect it).
• Left atrium contracts; blood passes to the left ventricle, which then contracts powerfully and pumps blood out through the aorta to the whole body.
• De-oxygenated blood from the body returns through the vena cava to the right atrium.
• Right atrium contracts; blood goes to the right ventricle, which pumps it through the pulmonary arteries to the lungs for oxygenation. Oxygen-rich blood returns via the pulmonary veins.

Ventricles have thicker muscular walls than atria because they must pump blood out to distant organs. Valves stop blood flowing backwards. A wall called the septum separates the two sides.

Double circulation: because the left and right sides are separate, blood passes twice through the heart in one full cycle — once on its way to the lungs (pulmonary circuit) and once on its way to the body (systemic circuit). This keeps oxygenated and de-oxygenated blood completely separate, allowing a highly efficient oxygen supply — important for warm-blooded birds and mammals that need lots of energy to keep body temperature constant. (Amphibians and many reptiles have three-chambered hearts with some mixing; fish have two-chambered hearts and only single circulation.)

Blood pressure: the force of blood against a vessel wall. Pressure during ventricular systole (contraction) is systolic (normal ~120 mm Hg); during ventricular diastole (relaxation) is diastolic (normal ~80 mm Hg). Measured by a sphygmomanometer. High BP (hypertension) is caused by narrowing of arterioles and can rupture a vessel.

8. Transportation in plants — xylem & phloem

Plants have two independent conducting pathways. Because plants don't move and have many dead-celled tissues, their energy needs are low and transport can be slow — but distances can be huge (think tall trees).

Transport of water (xylem): xylem vessels and tracheids of roots, stems and leaves join into a continuous pipe reaching every part. Root cells in contact with soil actively take up ions; this makes the ion concentration higher in the root than in the soil, so water moves in by osmosis, creating root pressure that pushes a column of water upward. But root pressure alone cannot raise water to the top of a tall tree. The main driving force is transpiration — water evaporating from the leaf surface through stomata creates a suction (transpiration pull) that pulls water up the xylem. Transpiration also helps in temperature regulation. (Root pressure matters more at night; transpiration pull dominates by day, Fig 5.12.)

Transport of food (phloem): the soluble products of photosynthesis (mainly sucrose) are moved from leaves to other parts — this is translocation, done by the phloem. Unlike xylem, phloem transport uses energy: sucrose is loaded into phloem using ATP, raising the osmotic pressure so water enters and pushes the material to areas of lower pressure. So phloem can move food both up and down (to roots, fruits, seeds, growing buds) — e.g. in spring, sugar stored in roots travels up to growing buds.

9. Excretion in humans — the kidney & nephron

Excretion is the removal of harmful nitrogenous metabolic wastes (urea, uric acid). The human excretory system (Fig 5.13) has a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys lie in the abdomen on either side of the backbone.

The basic filtering unit of the kidney is the nephron (each kidney has very many). Each nephron works like this (Fig 5.14):

• A cluster of thin-walled blood capillaries called the glomerulus sits inside a cup-shaped Bowman's capsule.
• Blood arriving through a branch of the renal artery is filtered here — water, glucose, amino acids, salts and urea pass into the capsule as filtrate.
• As the filtrate flows along the long coiled tubule, useful substances (glucose, amino acids, salts and most of the water) are selectively reabsorbed back into the blood.
• How much water is reabsorbed depends on how much excess water the body has and how much waste must leave — that's how urine volume is regulated.
• The remaining liquid is urine, which drains through the collecting duct to the ureter → bladder. Urine is stored until the stretched bladder triggers the urge to pass it out through the urethra (under nervous control).

Numbers worth knowing: about 180 L of initial filtrate forms daily, but only about 1-2 L is actually excreted — the rest is reabsorbed.

Artificial kidney (haemodialysis): if kidneys fail, a machine with selectively-permeable tubes suspended in dialysing fluid (same osmotic pressure as blood but free of waste) is used. Blood passes through the tubes and wastes diffuse out into the fluid; the cleaned blood is returned. Unlike a real kidney, dialysis has no reabsorption.

10. Excretion in plants

Plants use very different strategies — they don't have a special excretory organ:

• Oxygen itself is a "waste" of photosynthesis, released through stomata; CO₂ from respiration is also given out.
• Excess water is removed by transpiration.
• Other wastes are stored in cellular vacuoles, or in leaves that later fall off, or as resins and gums (especially in old xylem).
• Some waste is simply excreted into the soil around the plant.

11. NCERT in-text QUESTIONS — answered

Page 81

• Q1. Why is diffusion insufficient for oxygen in multicellular organisms like humans? Because most cells are deep inside the large body, far from the surface; diffusion is far too slow to deliver enough oxygen to all of them, so a fast transport system is needed.
• Q2. What criteria decide whether something is alive? The presence of molecular movement and life processes (nutrition, respiration, transport, excretion) used to maintain its organised structure — not just visible movement.
• Q3. What are outside raw materials used by an organism? Food (carbon source), oxygen, water, and minerals from the environment.
• Q4. What processes are essential for maintaining life? Nutrition, respiration, transportation and excretion.

Page 87

• Q1. Differences between autotrophic and heterotrophic nutrition. Autotrophs make their own food from CO₂ and water using sunlight + chlorophyll (e.g. green plants); heterotrophs take in ready-made complex food and digest it with enzymes (e.g. animals, fungi). Autotrophs need chlorophyll and light; heterotrophs do not.
• Q2. Where do plants get each raw material for photosynthesis? CO₂ from air through stomata; water and minerals from soil through roots; sunlight from the Sun; chlorophyll inside the plant's chloroplasts.
• Q3. Role of acid in our stomach? HCl makes the medium acidic so the enzyme pepsin can act, and it kills germs in food.
• Q4. Function of digestive enzymes? They break complex food into simple, absorbable molecules (starch → sugar, proteins → amino acids, fats → fatty acids + glycerol).
• Q5. How is the small intestine designed to absorb digested food? Its inner lining has millions of finger-like villi that hugely increase surface area; each villus is richly supplied with blood vessels to carry absorbed food away.

Page 91

• Q1. Advantage a terrestrial organism has over an aquatic one for obtaining oxygen? Air contains far more oxygen than water, so terrestrial animals get oxygen more easily and breathe more slowly.
• Q2. Different ways glucose is oxidised to provide energy? Aerobic (in mitochondria, with O₂) → CO₂ + water + lots of energy; anaerobic in yeast → ethanol + CO₂ + energy; anaerobic in muscle (low O₂) → lactic acid + energy.
• Q3. How are oxygen and CO₂ transported in humans? Oxygen is carried by haemoglobin in red blood cells; CO₂ is mostly carried dissolved in the plasma (it is more soluble).
• Q4. How are lungs designed to maximise gas exchange? They branch into millions of tiny thin-walled alveoli covered by blood capillaries, giving a very large surface area (~80 m²).

Page 96

• Q1. Components of the human transport system and their functions? Heart (pumps blood), blood vessels — arteries (carry blood from heart), veins (return blood to heart), capillaries (exchange materials), and blood (carries O₂, CO₂, food, wastes); lymph also helps transport.
• Q2. Why separate oxygenated and deoxygenated blood in mammals and birds? To prevent mixing so a highly efficient oxygen supply is maintained — needed for their high energy demand to keep body temperature constant.
• Q3. Components of the transport system in highly organised plants? Xylem (water and minerals) and phloem (food/products of photosynthesis).
• Q4. How are water and minerals transported in plants? By xylem, upward, driven by root pressure and mainly transpiration pull.
• Q5. How is food transported in plants? By phloem (translocation), using ATP energy, in both upward and downward directions.

Page 98

• Q1. Describe the structure and functioning of nephrons. A nephron has a glomerulus (capillary cluster) inside Bowman's capsule, leading to a long coiled tubule. Blood is filtered at the glomerulus; useful substances and most water are reabsorbed along the tubule; the remaining urine drains to the collecting duct. (See §9.)
• Q2. Methods plants use to get rid of excretory products? Release O₂/CO₂ through stomata, lose water by transpiration, store waste in vacuoles or leaves that fall, store as resins/gums, and excrete into the soil.
• Q3. How is the amount of urine produced regulated? By how much water is reabsorbed in the tubule, which depends on the body's excess water and the amount of dissolved waste to be excreted.

12. NCERT EXERCISES — fully solved

Q1. Kidneys are part of the system for — (c) excretion.

Q2. Xylem in plants is responsible for — (a) transport of water.

Q3. Autotrophic nutrition requires — (d) all of the above (carbon dioxide and water, chlorophyll, and sunlight).

Q4. Breakdown of pyruvate to CO₂, water and energy takes place in — (b) mitochondria.

Q5. How are fats digested? Where? In the small intestine. Bile from the liver makes the food alkaline and emulsifies fats into small globules; then lipase from pancreatic juice breaks them into fatty acids and glycerol.

Q6. Role of saliva in digestion? Saliva wets food for smooth passage; its enzyme salivary amylase begins digesting starch into simple sugar; it also helps in chewing and swallowing.

Q7. Conditions and by-products of autotrophic nutrition? Conditions: carbon dioxide, water, sunlight and chlorophyll. By-products: oxygen (and water).

Q8. Differences between aerobic and anaerobic respiration; organisms using anaerobic. Aerobic: needs oxygen, occurs in mitochondria, completely breaks glucose into CO₂ + water, releases much energy. Anaerobic: no oxygen, occurs in cytoplasm, gives ethanol + CO₂ (or lactic acid), releases little energy. Anaerobic organisms: yeast (and some bacteria).

Q9. How are alveoli designed to maximise gas exchange? They are extremely numerous and thin-walled, balloon-shaped, and wrapped in a dense network of blood capillaries — giving an enormous surface area (~80 m²) for rapid diffusion of O₂ and CO₂.

Q10. Consequences of haemoglobin deficiency? Less oxygen can be carried to the cells, so respiration and energy release fall. This causes anaemia — tiredness, weakness and breathlessness.

Q11. Describe double circulation; why is it necessary? Blood passes through the heart twice in one cycle — once to the lungs (pulmonary) and once to the body (systemic). It is necessary to keep oxygenated and de-oxygenated blood from mixing, ensuring an efficient oxygen supply for high-energy warm-blooded animals.

Q12. Differences between transport in xylem and phloem. Xylem carries water and minerals, only upward, driven by transpiration pull (no energy used), through mostly dead cells. Phloem carries food (sucrose, amino acids), in both directions, using ATP energy, through living cells.

Q13. Compare alveoli and nephrons (structure & function). Both are tiny, very numerous, thin-walled structures surrounded by capillaries that maximise exchange. Alveoli (lungs) exchange gases — O₂ in, CO₂ out — by diffusion. Nephrons (kidneys) filter blood, remove nitrogenous waste as urine and reabsorb useful substances. So both clean the blood, but alveoli handle gases while nephrons handle liquid waste with selective reabsorption.

13. Common mistakes to avoid

• Confusing respiration (cellular release of energy from glucose) with breathing (the physical intake of air) — breathing is only part of respiration.
• Mixing up xylem (water, upward) and phloem (food, both ways) — remember "xylem = water goes up".
• Saying the heart has three chambers in humans — humans have four; three-chambered hearts belong to amphibians/reptiles.
• Forgetting that ventricles have thicker walls than atria (they pump farther).
• Writing that phloem transport is passive — it uses ATP energy; only xylem transport is largely physical.
• Saying urine equals the filtrate — most of the filtrate (water, glucose, salts) is reabsorbed; only a small part becomes urine.
• Balancing the photosynthesis equation wrongly — it is 6 CO₂ + 12 H₂O, not 6 H₂O.

14. Quick revision checklist

• Life processes = nutrition, respiration, transportation, excretion.
• Photosynthesis: 6 CO₂ + 12 H₂O → glucose + 6 O₂ + 6 H₂O (needs sunlight + chlorophyll).
• Digestion enzymes: amylase (starch), pepsin/trypsin (protein), lipase (fat); bile emulsifies fat.
• Glucose → pyruvate; aerobic (mitochondria) = most energy; anaerobic = ethanol or lactic acid.
• Heart = 4 chambers; double circulation; arteries away, veins back, capillaries exchange.
• Xylem = water up (transpiration pull); phloem = food both ways (uses ATP).
• Nephron = glomerulus + Bowman's capsule + tubule; filters then reabsorbs; 180 L filtrate → ~1.5 L urine.