Nutrition
1. What This Chapter Covers
Food is needed by all living organisms mainly for growth and repair, and several organisms need it to maintain body temperature as well. The substances taken as food vary enormously, from single-celled organisms like amoeba to multicellular ones like the human being.
Even within the human body, cells require a wide variety of substances as food to carry out their functions. The mode of acquiring food varies from organism to organism, and that variation is what the chapter is organised around.
The textbook's index gives this chapter 10 periods in June and runs it from page 1 to page 25. It opens by asking you to recall two definitions from earlier classes — what autotrophs are and how they get their food, and what heterotrophs are and how they get theirs.
What makes the first half of the chapter worth reading in order is that it is written as a history of experiments. Each raw material of photosynthesis was proved necessary by a specific experiment, and the exam questions are usually about which experiment proved which thing.
2. Autotrophic Nutrition (Textbook 1.1)
Autotrophs are organisms capable of using light energy to synthesize chemical compounds. They acquire nutrients like mineral salts and water from the soil, as well as some gases from the air.
From these very simple substances they produce complex compounds — carbohydrates, proteins and lipids. The compounds produced by autotrophic plants then supply energy to most of the living organisms on earth.
Most of the food we eat is obtained from plants. Even where we depend on animal products, those animals usually depend on plants for their own food. This is why the process that makes it all possible is described in the book as making plants "the universal food providers".
3. Photosynthesis (Textbook 1.2)
Photosynthetic plants contain the green pigment chlorophyll and build up complex organic molecules from simple inorganic ones, using sunlight as an energy source. This is photosynthesis.
It is a very complex process. Several sequential reactions take place in it and intermediate compounds are also formed, which is why scientists worked for a long time simply to write a usable summary equation for it.
The equation used widely is related to the one proposed by C.B. Van Niel in 1931. His statement was that for each molecule of carbohydrate formed, one molecule of carbon dioxide and two molecules of water are required, and along with the carbohydrate one molecule each of oxygen and water are produced.
CO₂ + 2H₂O --(light, chlorophyll)--> CH₂O + H₂O + O₂
Van Niel first worked on purple sulphur bacteria and found that light plays a specific role. Instead of H₂O those bacteria use H₂S as a starting material, and here no oxygen is liberated during photosynthesis — sulphur is released instead. He later observed a similar process in plants.
When glucose was observed to be a product, the equation was written out in full:
6CO₂ + 12H₂O --(light, chlorophyll)--> C₆H₁₂O₆ + 6H₂O + 6O₂
Plants synthesize the smaller and simpler carbohydrates first, and from these build the more complex ones like starch and cellulose. They can also synthesize proteins, fats and other compounds. Animals cannot synthesize carbohydrates at all and depend on plants for them.
Activity 1 — testing a leaf for starch
Take a soft, thin leaf from a plant that is well exposed to sunlight. Boil the leaf in water first. Then transfer it to a test tube containing methylated spirit and boil that tube in a water bath.
Chlorophyll dissolves in the spirit while boiling, and the leaf becomes pale as the chlorophyll is removed. Lift the leaf out carefully with a brush, spread it in a petri dish and add a few drops of iodine or betadine solution.
The presence of starch is indicated by a bluish-black colour. This single test is reused in almost every later experiment in the chapter, so the technique is worth knowing as a technique.
4. Proving the Raw Materials One at a Time
Water (Textbook 1.2.2)
Van Helmont established the role of water by an experiment conducted over a period of five years, studied in class VII. He did not know about photosynthesis at the time; the increase in plant body mass was only later attributed to it.
The book notes that after Van Helmont it took humankind another 300 years to arrive at the present definition of photosynthesis.
Air (Textbook 1.2.3)
Joseph Priestley (1733-1804), in 1770, performed a series of experiments that revealed the essential role of air in the growth of green plants. Photosynthesis was still unknown to scientists then, and oxygen itself was only discovered by Priestley in 1774, the name being coined by Lavoisier in 1775.
Priestley observed that a candle burning in a closed bell jar soon gets extinguished, and that a mouse kept in a closed bell jar would soon suffocate. He concluded that both somehow damage the air inside the jar.
When he placed a mint plant in the same bell jar, the mouse stayed alive and the candle, lit from outside, continued burning. He hypothesized that plants restore the air that breathing and burning use up.
Gaseous exchange occurs in massive amounts through the stomata, usually present in leaves, as long as they are open. Plants also exchange gases through lenticels in the stems and through aerial roots.
Activity 2 — Mohl's half leaf experiment
This one proves specifically that carbon dioxide is necessary. Start with a destarched plant: keep it in the dark for nearly three days so the starch is removed from the leaves.
Take a wide-mouthed transparent glass bottle and put potassium hydroxide (KOH) pellets or solution in it, because potassium hydroxide absorbs carbon dioxide. Insert a split cork in the mouth of the bottle.
Push one leaf of the destarched plant through the split cork so that half the leaf is inside the bottle and the rest stays outside. Keep the plant with the bottle in sunlight for a few hours, then test that leaf for starch as in Activity 1.
The half exposed to atmospheric air and light turns bluish-black. The half inside the bottle, where the KOH has absorbed the carbon dioxide, shows no colour change. That proves carbon dioxide is necessary for photosynthesis.
The two questions the book attaches to this activity are worth answering in your own words: why the plant was kept in the dark first and then in sunlight, and why two leaves had to be tested.
Light (Textbook 1.2.4)
Energy is released when carbon dioxide and water are formed by combining oxygen molecules with carbon and hydrogen. Scientists came to realise that when these reactions run in reverse, the energy released during oxygen formation is reused — meaning plants must be able to absorb energy as they produce oxygen.
The Dutch scientist Jan Ingenhousz (1730-1799) found where that energy comes from. In 1779 he noticed that plants form oxygen only in the presence of light.
Working with the aquatic plant Hydrilla, a submerged rootless water plant, he observed that in bright sunlight small bubbles formed around the green parts, while in the dark they did not. He also found the gas in the bubbles was oxygen.
Engelman, in the early 20th century, detected the point of maximum rate of photosynthesis. He exposed a group of algae to the different colours of light seen in a rainbow, then used oxygen-sensitive bacteria and found that they crowded around the areas illuminated by bright red and blue rays.
Lab Activity — oxygen evolved in the presence of light
Materials: two beakers, two funnels, two test tubes, Hydrilla or another submerged aquatic plant, black paper, a bucket of water, an incense stick and a match box.
Make two identical sets. Keep each setup in a bucket filled with water and invert a water-filled test tube over the stem of the funnel, which retains the water column in the tube. Take the setup out and keep it under sunlight.
Arrange the second set the same way, cover it with black paper or cloth and keep it in the shade. Watch the water level in the test tube.
In the set kept in sunlight the water level gradually falls and gas collects in its place. Test that gas by inserting a glowing match stick or incense stick: it bursts into flame, showing the gas released by the Hydrilla is oxygen. The covered set gives a different amount of gas, which is the comparison the experiment exists to make.
Activity 3 — the black paper design
Take a potted plant and destarch its leaves as before. Cover one leaf with black paper into which a design has been cut, fixed so that light cannot enter the dark part.
Place the plant in sunlight for a few hours, then separate that leaf and test it for starch. Only the parts of the leaf that received light through the cut-out design turn bluish-black with iodine. The design itself appears in starch.
Chlorophyll (Textbook 1.2.5)
Ingenhousz proposed that only green plant parts could carry out photosynthesis. That left real questions: what about plants with variously coloured leaves, what about new leaves that look dark red before turning green, and why can green-coloured animals such as some birds not photosynthesize.
These remained challenges until the green substance could be isolated. In 1817, Pelletier and Caventou obtained an extract of it and named it chlorophyll, meaning green leaf.
Pigments other than chlorophyll — carotenoids and phycobilins — were also found to aid photosynthesis, by passing the energy of sunlight they trap on to chlorophyll.
The first half of the chapter is four experiments and four conclusions. If you can name the scientist, the setup and the one thing it proved, you can answer almost any question the book asks about photosynthesis.
5. Where Photosynthesis Happens (Textbook 1.2.6)
The exact location of the chlorophyll-containing part was not known until another six decades after chlorophyll was discovered. In 1883, Julius Von Sachs observed that chlorophyll is found in organelles within the cell, and these were named chloroplasts.
They are present in large numbers — around 40 to 100 per cell — in parts such as the stomatal guard cells and the ground tissues of the green parts of the plant. A transverse section of a leaf shows them in the palisade and spongy parenchyma.
The book's labelled section of a leaf runs, from the top: cuticle layer, upper epidermis, palisade parenchyma, spongy parenchyma with air spaces, a vascular bundle of xylem and phloem, lower epidermis with guard cells and stomata, and a lower cuticle layer. The palisade and spongy parenchyma together make up the mesophyll.
The book's "Do you know?" box records that if a cell is broken up the chloroplasts break into pieces too, so isolating them is very difficult. It was not until 1954 that Daniel I. Arnon broke up plant cells gently enough to obtain whole chloroplasts for study.
Inside the chloroplast
The chloroplast is a membranous structure consisting of three membranes. The innermost layer forms stacked sac-like structures called grana, believed to be the site where solar energy is trapped.
The intermediary fluid-filled portion is the stroma, believed to be responsible for the enzymatic reactions leading to the synthesis of glucose, which in turn joins together to form starch.
Substances in the chloroplast that capture sunlight are called photosynthetic pigments. Chlorophyll is one such pigment and contains one atom of magnesium. It is similar in structure to the haem of haemoglobin, the iron-containing red pigment that transports oxygen in blood.
Two major kinds of chlorophyll are associated with thylakoid membranes: chlorophyll a, which is bluish-green, and chlorophyll b, which is yellowish-green. Around 250 to 400 pigment molecules are grouped as a light harvesting complex, or photosynthetic unit, in each thylakoid.
Three things happen in the chloroplast during photosynthesis: conversion of light energy to chemical energy, splitting of the water molecule (photolysis of water), and reduction of the carbon dioxide molecule to carbohydrate.
6. Mechanism of Photosynthesis (Textbook 1.3)
Light dependent reaction (1.3.1)
Light plays the key role here. A series of chemical reactions occurs in very quick succession, initiated by light, so the phase is technically the photochemical phase. It takes place in the chlorophyll-containing thylakoids of the grana.
Step I. The chlorophyll, on exposure to light energy, becomes activated by absorbing photons. A photon is a unit of light energy.
Step II. That energy is used in splitting the water molecule to release O₂. This reaction is photolysis — photo meaning light, lysis meaning breaking — discovered by Robert Hill, and therefore also called Hill's reaction.
The hydrogen produced in photolysis is immediately picked up by the special compound NADP (Nicotinamide Adenine Dinucleotide Phosphate) to form NADPH, reduced NADP. Another energy-rich compound, ATP (Adenosine triphosphate), is also formed at the end of the light reaction.
Light independent reaction (1.3.2)
This reaction does not require the presence of light, and in some plants the phase extends past daytime. It is also called the dark reaction, but the book is careful to warn that the term does not mean it occurs at night — only that it does not depend on light.
It takes place in the stroma. Here the hydrogen combines with CO₂, using ATP energy, and produces glucose (C₆H₁₂O₆). The synthesis occurs in a number of steps using special intermediate compounds and enzymes, and the glucose may finally be converted into the storage product starch.
7. Heterotrophic Nutrition (Textbook 1.4)
Organisms that cannot prepare their own food are heterotrophs. Depending on the type and availability of food, they adapt a range of strategies of intake and use.
Some organisms — bread mould, yeast, mushrooms — break down the food materials outside the body and then absorb it. These are saprophytes.
Others derive nutrition from plants or animals without killing them. This parasitic strategy is used by a wide variety of organisms: Cuscuta, lice, leeches and tape worms.
Others take in whole material and break it down inside their bodies. Intake and breaking down depend on the body's structure and function, so the digestive system differs across organisms.
Amoeba and Paramoecium
In Amoeba, food is taken in using temporary finger-like extensions of the cell surface, the pseudopodia, which fuse over the food particle to form a food vacuole. Inside the vacuole, complex substances are broken into simpler ones, which then diffuse into the cytoplasm. Undigested material is moved to the cell surface and thrown out.
Paramoecium is also unicellular but has a definite slipper shape, and food is taken in at a specific spot. Food is moved there by the beating of the cilia covering the whole cell surface, and is ingested at the cytostome. Undigested material is expelled through the cytopyge, the anal pore.
Parasitic nutrition in Cuscuta (1.4.2)
The dodder plant, Cuscuta, is a leafless, twining parasitic plant of the morning glory family Convolvulaceae. The genus contains about 170 twining species, widely distributed through temperate and tropical regions.
Cuscuta contains no chlorophyll; Cuscuta reflexa has been found to have a very little amount of it. It therefore absorbs food through haustoria — root-like structures that penetrate the tissue of a host plant and may kill it.
Its slender, string-like stems may be yellow, orange, pink or brown, its leaves are reduced to minute scales, and its flowers are nodule-like clusters of tiny yellow or white bell-like petals.
The seed germinates and forms an anchoring root, then sends up a slender stem that grows in a spiral until it reaches a host. It twines around the host stem and forms haustoria that penetrate it. Water is drawn from the host's xylem, and nutrients from its phloem.
8. The Human Digestive System (Textbook 1.5)
The alimentary canal is basically a long tube extending from the mouth to the anus, with different parts specialized for different functions. The associated organs and glands are the salivary glands, liver and pancreas.
Passage of food through the gut (1.5.1)
In the mouth, food is cut and crushed by the teeth and mixed with saliva to make a wet, slippery lump called a bolus. This process is mastication.
Saliva is secreted by three pairs of salivary glands and contains mainly the enzyme amylase (ptyalin), which breaks complex carbohydrates into simple ones. Breaking larger complex substances into simple ones with the help of enzymes is digestion. The tongue mixes the food and pushes it on; the lower jaw helps.
The soft food passes down the oesophagus by wave-like peristaltic movements to the stomach. There it is churned with gastric juice and HCl, which makes the medium acidic, and most proteins are broken into smaller molecules by the enzyme pepsin.
Food in this soft, slimy state, with some proteins and carbohydrates already broken down, is called chyme. Ring-like muscles called pyloric sphincters relax to open the passage to the small intestine, and they regulate it so that only small quantities pass at a time.
The small intestine is the longest part of the alimentary canal. Its proximal part, the duodenum, is the site of further digestion of carbohydrates, proteins and fats, and it receives the secretions of the liver and pancreas, which gradually make the interior alkaline.
Fats are digested by first being converted into small globule-like forms with the help of bile juice from the liver — a process called emulsification. Pancreatic juice contains trypsin for proteins and lipase for fats. The intestinal walls secrete intestinal juice (succus entericus), which carries the breakdown further.
Carbohydrate digestion, which started in the mouth and did not occur in the stomach, resumes here as the medium turns alkaline and the enzymes become active again.
Activity 4 — the enzyme chart
| Enzyme or substance | Secreted by | Secreted into | Digestive juice | Acts on | Products |
|---|---|---|---|---|---|
| Ptyalin (salivary amylase) | Salivary glands | Buccal cavity | Saliva | Carbohydrates | Maltose |
| Pepsin | Gastric glands | Stomach | Gastric juice | Proteins | Peptones |
| Bile juice (no enzymes) | Liver | Duodenum | Bile juice | Fats | Emulsification, breaking fats into small globules |
| Amylase | Pancreas | Duodenum | Pancreatic juice | Carbohydrates | Maltose |
| Trypsin | Pancreas | Duodenum | Pancreatic juice | Proteins | Peptones |
| Lipase | Pancreas | Duodenum | Pancreatic juice | Fats | Fatty acids and glycerol |
| Peptidases | Intestinal glands | Small intestine | Intestinal juice | Peptides | Amino acids |
| Sucrase | Intestinal glands | Small intestine | Intestinal juice | Sucrose | Glucose |
The four questions the book attaches to this table are the ones most likely to be lifted into a paper: which enzymes act on carbohydrates, which digestive juice contains no enzymes, what the end products of fat digestion are, and which enzymes act on proteins.
Absorption and defecation
Transport of the products of digestion from the intestine into the blood, through the wall of the small intestine, is absorption. The intestinal wall carries a great number of finger-like projections; the book calls them microvilli and then refers to them as villi, and both words appear in its own text.
They increase the surface area for absorption, and blood vessels and lymph vessels form a network inside them. Products of digestion are absorbed first into the villi and from there into the blood vessels.
After maximum absorption in the small intestine, the rest — which is undigested — passes into the large intestine and is finally expelled through the anus, the last part of the canal. This passage of undigested material out of the body is defecation.
Food that passes out still contains a considerable amount of proteins, fats and carbohydrates, and roughages or fibres of either carbohydrates or proteins.
The book summarises the whole system as four functions: ingestion, taking food into the body; digestion, breaking complex substances into simple ones with specific enzymes; absorption, passage of digested food through the walls of the tract into the circulatory system; and defecation.
9. Health of the Alimentary Canal (Textbook 1.6)
The human alimentary canal usually functions remarkably well considering how badly we treat it. Sometimes it rebels and we feel sick or have indigestion.
Vomiting is the body's method of ridding itself of unwanted or harmful substances from the stomach: the peristaltic movements of the stomach and oesophagus reverse their normal direction and the food is expelled. One of the most common causes is overeating, especially of food high in fat, and it also occurs when we eat something indigestible or poisonous.
A greenish vomit, called bilious or liverish, leaves a bitter taste and is often the result of overeating — the liver cannot cope with the excessive fat and we feel nauseous.
Indigestion is a general term for difficulty in digesting food. The book lists four ways healthy people can usually avoid it: simple, well-balanced meals; avoiding physical exercise soon after eating; drinking adequate water; and taking fibre-rich food to avoid constipation.
A serious condition of indigestion is caused by stomach and duodenal ulcers, which may arise from diet, an infection or habit.
10. Diseases due to Malnutrition (Textbook 1.7)
A balanced diet contains proper amounts of carbohydrates, proteins, vitamins, mineral salts and fats. Two-thirds of the world population is affected by food-related diseases.
Eating food that lacks one or more nutrients in the required amount is malnutrition. Its causes in our country include poor health, wilful starvation, lack of awareness of nutritional values and proper food preparation methods, and socio-economic factors.
Malnutrition is classified three ways: protein malnutrition, calorie malnutrition, and protein-calorie malnutrition.
Kwashiorkor is due to protein deficiency. Body parts become swollen because water accumulates in the intercellular spaces. The symptoms are very poor muscle development, swollen legs, a fluffy face, difficulty in eating, diarrhoea and dry skin.
Marasmus is due to deficiency of both proteins and calories. It generally occurs where there is an immediate second pregnancy or repeated child births. The child is lean and weak with less developed muscles, dry skin and diarrhoea.
Obesity is the opposite case: constant intake of high-calorie food makes a person gain extra fat and become overweight. Obese children may suffer problems related to the cardiovascular system, the kidneys and the gall bladder.
11. Vitamin Deficiency Diseases (Textbook 1.8)
Vitamins are organic substances, micronutrients required in small quantities, and they are not synthesised in the body. We normally get them two ways: through the diet, and from bacteria present in the intestine that synthesise and supply them.
They fall into two groups: water soluble vitamins, which are the B-complex and vitamin C, and fat soluble vitamins, which are A, D, E and K.
| Vitamin | Resources | Deficiency disease | Symptoms |
|---|---|---|---|
| Thiamin (B1) | Cereals, oil seeds, vegetables, milk, meat, fish, eggs | Beri beri | Vomitings, fits, loss of appetite, difficulty in breathing, paralysis |
| Riboflavin (B2) | Milk, eggs, liver, kidney, green leafy vegetables | Glossitis | Cracks around the corners of mouth, red and sore tongue, photophobia, scaly skin |
| Niacin (B3) | Kidney, liver, meat, egg, fish, oil seeds | Pellagra | Dermatitis, diarrhoea, loss of memory, scaly skin |
| Pyridoxine (B6) | Cereals, oil seeds, vegetables, milk, meat, fish, eggs, liver | Anaemia | Hyper irritability, nausea, vomiting, fits |
| Cyanocobalamin (B12) | Synthesised by bacteria present in the intestine | Pernicious anaemia | Lean and weak, less appetite |
| Folic acid | Liver, meat, eggs, milk, fruits, cereals, leafy vegetables | Anaemia | Diarrhoea, loss of leucocytes, problems related to mucus in the intestines |
| Pantothenic acid | Sweet potatoes, ground nuts, vegetables, liver, kidney, egg | Burning feet | Walking problems, sprain |
| Biotin | Pulses, nuts, vegetables, liver, milk, kidney | Nerves disorders | Fatigue, mental depression, muscle pains |
| Ascorbic acid (C) | Green leafy vegetables, citrus fruits, sprouts | Scurvy | Delay in healing of wounds, fractures in bones |
| Retinol (A) | Leafy vegetables, carrot, tomato, pumpkin, papaya, mango, meat, fish, egg, liver, milk, cod liver oil, shark liver oil | Eye and skin diseases | Night blindness, xerophthalmia, cornea failure, scaly skin |
| Calciferol (D), the sunshine vitamin | Liver, egg, butter, cod liver oil, shark liver oil; sun rays stimulate its formation from the subcutaneous fat | Rickets | Improper formation of bones, knock-knees, swollen wrists, delayed dentition, weak bones |
| Tocoferol (E) | Fruits, vegetables, sprouts, sunflower oil | Fertility related disorders | Sterility in males, abortions in females |
| Phylloquinone (K) | Green leafy vegetables, milk, meat, egg | Problems related to blood clotting | Delay in blood clotting, over bleeding |
Two rows in this table are worth noticing. B12 is the one vitamin whose listed source is the intestinal bacteria themselves, and vitamin D is the one the body can form from sunlight acting on subcutaneous fat. Both are asked as one-mark questions.
Key words from the chapter
Glucose, starch, cellulose, chloroplast, grana, stroma, light reaction, dark reaction, heterotrophic nutrition, parasitic nutrition, haustoria, alimentary canal, salivary glands, peristaltic movement, amylase, ptyalin, pepsin, chyme, sphincter, digestion, pancreas, enzymes, villi, bile juice, lipase, fats, liver, emulsification, kwashiorkor, marasmus.
12. Summary
Autotrophic nutrition means taking in simple inorganic materials — minerals and water from the soil, and gases from the air — and synthesizing carbohydrates using the energy of the sun.
Photosynthesis is the process by which living plant cells containing chlorophyll produce food substances, glucose and starch, from carbon dioxide and water using light energy, releasing oxygen as a product. It is summarised as 6CO₂ + 12H₂O --(light, chlorophyll)--> C₆H₁₂O₆ + 6H₂O + 6O₂.
The materials required are light, carbon dioxide, water and the photosynthetic pigment chlorophyll, and each was proved necessary by a separate experiment.
Chloroplasts are the sites of photosynthesis. The light reaction takes place in the grana and the light independent reaction in the stroma. The end products are glucose, water and oxygen.
The three important events in the chloroplast are conversion of light energy into chemical energy, splitting of the water molecule, and reduction of carbon dioxide to carbohydrates.
Heterotrophic nutrition involves taking in complex material prepared by other organisms, and the form it takes depends on the type and availability of the food and how the organism obtains it.
In single-celled organisms food may be taken in by the entire surface, but as complexity increases different parts become specialized for different functions.
Large complex food molecules are broken into simple molecules before being absorbed and used, and that breaking down is digestion. In human beings it happens in stages, with enzymes secreted by glands associated with the alimentary canal, and the digested food is absorbed in the small intestine and sent to all the cells of the body.
