CBSE Class 10 Science Advanced Biology Nutrition Notes Set 01

Nutrition

Nutrition and nutrients

Nutrition covers two things: the intake of nutrients by an organism, and the way the organism then uses them. Nutrients are the carbohydrates, fats, proteins, minerals, vitamins and water that an organism gets from its surroundings. They serve as a source of energy and as the building material for tissues and organs, and so keep all the metabolic activities of the body going.

A. Modes of Nutrition

On the basis of whether an organism can make its own food, nutrition is of two kinds.

(a) Autotrophic nutrition

Auto means self and trophic means food. An autotroph builds its own food: inorganic molecules (CO₂ and H₂O) are changed into organic compounds such as carbohydrates. Green plants are the common example. Autotrophs are of two types:

1. Photoautotrophs use light energy to make food, for example plants, algae and the purple and green bacteria.

2. Chemoautotrophs use energy obtained from chemical reactions, for example nitrifying, iron, sulphur and methane bacteria.

(b) Heterotrophic nutrition

Hetero means different. A heterotroph cannot make food and gets it from other plants or animals. All animals are heterotrophs, and so are most bacteria, fungi and non-green plants such as yeast. Heterotrophs are grouped by the kind of food they eat:

Heterotrophs are also grouped by the way they feed:

1. Holozoic. The organism takes in solid food. Digestive enzymes break the large food molecules into small soluble ones, which are absorbed and used, and the part that is not digested or absorbed is thrown out (egestion). Examples: amoeba and human beings.

2. Saprotrophic. Sapro means rotten. The organism gets food from dead and decaying matter, on which it pours out enzymes that change the complex compounds into simple ones. Examples: fungi and bacteria.

3. Parasitic. A parasite lives on or inside another living organism (the host) and gets its food from it. The parasite gains and the host is harmed. Examples: tapeworm, Plasmodium (malaria), Ascaris (roundworm), Enterobius (pinworm), Taenia, Fasciola (liver fluke), fungi such as Albugo, Phytophthora and Puccinia, and the flowering plant Cuscuta (amarbel). Cuscuta has neither chlorophyll nor leaves. It sends sucking roots (haustoria) into the phloem of its host and takes food from there.

4. Symbiotic (mutualism). Two organisms live together in such a way that both gain. For example, Escherichia coli in the human intestine makes vitamin B₁₂, which we use, and in return it gets simple food from our intestine.

Nutrition may also be studied according to where it occurs: nutrition in plants and nutrition in animals.

B. Nutrition in Plants

Plants are autotrophs, so they are the producers of a food chain. Their green pigment, chlorophyll, traps solar energy and changes it into the chemical energy of food. This process is photosynthesis.

(a) Photosynthesis

Meaning. The word means 'building up by light'. Photosynthesis is the making of organic compounds such as glucose from the inorganic molecules CO₂ and H₂O, by the cells of green plants that contain chlorophyll, in the presence of sunlight.

How a green plant makes its food by photosynthesis

Equation:

\[ 6CO_2 + 12H_2O \xrightarrow[\text{Chlorophyll}]{\text{Sunlight}} C_6H_{12}O_6 + 6O_2 + 6H_2O \]

Essential requirements of photosynthesis

1. Sunlight. The sun is the source of energy. Plants use the visible region of the spectrum, which spans roughly 400 nm to 700 nm (the figure marks 390 nm to 780 nm). This region is white light, a mixture of seven colours of different wavelengths (VIBGYOR).

Photosynthesis is fastest in red light and slowest in green light, because the green parts of the plant reflect the green light.

2. Chlorophyll. This is the green pigment of the chloroplast. It is present in the largest quantity in green leaves and also in other green aerial parts. There are six kinds: chlorophyll a, b, c, d, e and bacteriochlorophyll, of which a and b are the commonest. Plants also have other pigments:

3. Carbon dioxide. Land plants take in CO₂ from the air through stomata, tiny pores on the leaf surface that also serve for gas exchange and transpiration. Each stoma is guarded by two kidney-shaped guard cells. Aquatic plants take the CO₂ dissolved in water through the general body surface, and so they photosynthesise more than land plants.

Stomata: open and closed

4. Water. Roots absorb water from the soil by osmosis, and a tissue called xylem carries it to the leaves.

Compensation point. This is the light intensity at which the CO₂ used in photosynthesis equals the CO₂ given out in respiration. It is reached when light is weak, in the early morning and in the evening.

Site of photosynthesis

In prokaryotes (such as bacteria) it takes place in lamellar chromatophores, and in eukaryotes in the chloroplast, the green plastid, which is the main site in higher plants. Plastids are of three kinds according to their pigments:

1. Leucoplasts are colourless, are found in underground parts and store food: aleuroplasts store protein, elaioplasts store oil and amyloplasts store starch.

2. Chromoplasts carry a colour other than green and occur in aerial parts.

3. Chloroplasts carry the green pigment chlorophyll.

Internal structure of a chloroplast

A chloroplast is enclosed by two membranes and has two parts inside:

Grana. A granum is a pile of flat membranous sacs called thylakoids. A cell has about 40 to 60 grana, and a granum has 50 or more thylakoids. Chlorophyll lies in the thylakoid membrane and traps solar energy in packets called photons (quanta). Grana are joined to one another by channels, the stroma lamellae (Fret's channels).

Stroma. This is the colourless, protein-rich matrix in which the grana are embedded. It holds the enzymes of the dark reaction.

Mechanism of photosynthesis

Photosynthesis takes place in two stages.

(I) Light reaction (photochemical reaction, also called Hill's reaction after Robert Hill). Site: grana. It needs light and water, and produces ATP, NADPH₂ and oxygen. It has three steps:

1. Photo-excitation of chlorophyll. A chlorophyll molecule absorbs photons and is raised to a higher energy level, so that it gives up an electron.

2. Photolysis of water (also called photo-oxidation of water), which needs Mn²⁺ and Cl⁻ ions:

4H₂O → 4H⁺ + O₂ + 4e⁻ + 2H₂O

Oxygen is released as a by-product, and the H⁺ ions reduce NADP: 2NADP + 4H⁺ → 2NADPH₂.

3. Photophosphorylation. ATP is made in the quantasomes, using inorganic phosphate: ADP + iP → ATP.

(II) Dark reaction (thermochemical reaction, or the Calvin–Benson cycle after Melvin Calvin and Benson). Site: stroma. It needs CO₂, NADPH₂, ATP and enzymes, and the product is glucose. It has three steps:

1. Carboxylation. CO₂ is taken up by an acceptor: RuBP in C₃ plants, or PEP in C₄ plants. The enzymes are RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) and PEPCase (phosphoenolpyruvate carboxylase) respectively.

2. Synthesis of glucose from the CO₂ that has been captured.

3. Regeneration of RuBP, so that the cycle can continue.

Factors affecting photosynthesis

1. Light. Plants usually use sunlight, but marine algae can photosynthesise even in moonlight, and plants can also use artificial light. The rate is highest in red light and lowest in green light. Very strong light lowers the rate because it damages (denatures) chlorophyll.

2. Temperature. The best range is 25 °C to 30 °C.

3. Carbon dioxide. CO₂ is the first limiting factor, as air contains only 0.03 to 0.1% of it.

4. Oxygen. A higher concentration of oxygen lowers the rate.

5. Chlorophyll. The rate is directly proportional to the chlorophyll content. Etiolated cells do no photosynthesis, and a variegated leaf photosynthesises only where it has chlorophyll.

Significance of photosynthesis

It is a boon to nature and to human beings: 1. it produces food, and 2. it controls the composition of the atmosphere and purifies the air (CO₂ is used and O₂ is given out).

Activities related to photosynthesis

1. To show that chlorophyll is necessary. Take a croton plant (a variegated one), whose leaves are green in some parts and white in others. The green part has chlorophyll and the white part has none.

Steps: keep the plant in complete darkness for about three days to remove the starch from the leaves (destarching). Then expose it to strong sunlight for 3 to 4 days. Pluck a leaf, boil it briefly in water, and then heat it in alcohol until the green colour is removed. Rinse the pale leaf in warm water to soften it and add a few drops of iodine.

Observation: the part that was originally white does not turn blue-black, so it has no starch. The part that was green turns blue-black, so it has starch.

Conclusion: photosynthesis to make starch needs chlorophyll.

2. To show that CO₂ is necessary (Moll's half-leaf experiment). Destarch a potted plant that has long leaves by keeping it in the dark for two or three days. Insert half of one leaf into a bottle through a split cork. The bottle contains concentrated potassium hydroxide (KOH) solution, which absorbs all the CO₂ in it. Put the setup in the sun and test the leaf for starch with iodine after 3 to 4 hours.

Moll's half-leaf experiment: CO₂ is needed for photosynthesis

Observation: only the part of the leaf outside the bottle, which received CO₂, turns blue-black. Conclusion: photosynthesis does not take place without CO₂.

3. To show that sunlight is necessary. Destarch a potted plant by keeping it in the dark for two to three days. Wrap a strip of aluminium foil (or black paper) tightly around the middle of one leaf on both sides, while it is still on the plant, so that light cannot enter from the sides. Cover only a small patch of the leaf. Leave the plant in the sun for 3 to 4 days, then test the leaf for starch.

Light is needed for photosynthesis: foil experiment

Observation: the starch test is positive only in the part that was exposed to light, and negative under the foil. Conclusion: sunlight is necessary for photosynthesis.

C. Nutrition in Animals

Animals are heterotrophs and depend on plants for food, directly or indirectly. Some are parasitic or saprotrophic, but most are holozoic. The steps of holozoic nutrition are the same from a unicellular animal to a multicellular one:

1. Ingestion: taking in the food.

2. Digestion: breaking large, complex molecules into simpler, smaller, soluble ones.

3. Absorption: the digested food passes through the wall of the intestine into the blood or body fluid.

4. Assimilation: the body cells take up the absorbed food.

5. Egestion: the undigested matter is thrown out.

Digestion is regulated by hormones of some endocrine glands. The alimentary canal first appeared in flatworms (Platyhelminthes), but in them there was only a mouth. A coiled and well-developed canal appeared in annelids and continues up to mammals.

(a) Nutrition in a unicellular animal: Amoeba

Amoeba is a one-celled animal that lives in water. Its nutrition is holozoic and it feeds by phagocytosis (cell eating). The steps are:

1. Ingestion. The one cell does all the work. It puts out pseudopodia round the food particle and encloses it in a food vacuole, which is like a temporary stomach.

2. Digestion. Digestive enzymes pass from the cytoplasm into the food vacuole and split the food into small soluble molecules.

3. Absorption. The digested food diffuses into the cytoplasm, and the vacuole disappears.

4. Assimilation. The absorbed food gives energy through respiration and is used for growth and reproduction.

5. Egestion. The undigested material is expelled from the cell.

Nutrition in Amoeba

(b) Nutrition in a higher animal: human beings

Humans have an advanced digestive system made of the alimentary canal, a long hollow tube of several organs, and various digestive glands.

1. Mouth. A small slit through which the food is taken in.

2. Buccal cavity. The mouth opens into this chamber. Its roof is the palate. On its floor lies the muscular tongue, which is used for chewing, swallowing, tasting and speech, and carries papillae with taste buds. The teeth cut, chew and grind the food, which is physical (mechanical) digestion. The jaws have four types of teeth:

Dental formula (for half of the upper jaw over half of the lower jaw):

Alimentary canal of a human being along with the digestive glands

3. Oesophagus (food pipe). It carries food from the mouth to the stomach. Its walls are very muscular and no digestion happens in it. The walls contract and relax in waves, called peristaltic movement, which pushes the food into the stomach and onward through all parts of the canal.

4. Stomach. A J-shaped bag on the left side of the abdomen. Branched, tubular glands in its inner wall secrete gastric juice.

5. Small intestine. The longest part of the canal, about 6.5 m in an adult. It is called 'small' only because it is narrow. It has three regions: duodenum, jejunum and ileum. The duodenum, the first part, receives secretions of the liver and pancreas. The inner wall carries many finger-like projections (villi, with microvilli on their cells), which increase the surface area for absorption.

6. Large intestine. The small intestine opens into it, and the undigested food moves on to the anus through the rectum. Its three parts are the caecum, colon and rectum.

Digestive glands

These glands secrete the enzymes or hormones that help in digestion.

1. Salivary glands. Three pairs open into the buccal cavity. Saliva has the enzyme salivary amylase (ptyalin), which partly digests starch into maltose, so digestion of starch begins in the mouth itself. The food stays only a short time there, so the digestion is incomplete.

Starch \( \xrightarrow{\text{salivary amylase}} \) Maltose

2. Gastric glands. Food is mixed and churned in the stomach for some 3 to 4 hours. The gastric juice contains hydrochloric acid, the enzyme pepsin and mucus. The acid makes the juice acidic, and in this medium pepsin starts to digest proteins into smaller molecules. Mucus protects the stomach wall from the acid. So protein digestion starts in the stomach.

Protein \( \xrightarrow{\text{pepsin}} \) Peptones + Peptides

Functions of HCl: it activates the inactive pepsinogen into pepsin, and it kills the bacteria that enter with the food.

3. Liver. The largest gland. It makes bile, which has bile juice and bile pigments, is alkaline, has no enzymes, and is stored for the time being in the gall bladder. Bile helps in the digestion and absorption of fat. Functions of bile: it makes the acidic food from the stomach alkaline so that the pancreatic enzymes can act, and its salts break fats into small globules (emulsification), which makes it easier for enzymes to digest them.

4. Pancreas. It lies below and parallel to the stomach and pours pancreatic juice into the small intestine. The juice contains pancreatic amylase (digests starch), trypsin (digests proteins) and lipase (digests emulsified fats). The pancreas also makes two hormones, insulin and glucagon, so it has both exocrine and endocrine functions. Bile and pancreatic juice enter the duodenum through a common duct (the hepato-pancreatic duct).

5. Intestinal glands. They secrete intestinal juice (succus entericus) and mucus.

Digestion in the small intestine

From the stomach the partly digested food enters the duodenum. Bile and pancreatic juice are alkaline, so fats are emulsified there, the remaining proteins are digested by trypsin, and starch by pancreatic amylase. The food then moves to the ileum, where the intestinal juice completes the digestion. The final products are:

Absorption. The digested molecules are small enough to pass through the wall of the small intestine, which has blood capillaries, into the blood. The small intestine is the main region of absorption. For fats, lymph capillaries called lacteals take up the products.

Assimilation. The blood carries the dissolved food to all parts of the body, where it becomes a part of the cells and is used for energy, growth and repair. The food that is not needed at once is stored in the liver as the carbohydrate glycogen.

Egestion. What cannot be digested passes into the wider large intestine, whose walls absorb most of its water, so it becomes almost solid. It is stored for a time in the rectum and then passed out through the anus as faeces (stool). The act of removing faeces is egestion or defaecation.

Some Additional Points

1. Herbivores such as the cow, which eat grass, have a longer small intestine so that the cellulose can be digested completely, while carnivores such as the tiger have a shorter one.

2. Rabbits and rats cannot digest and absorb cellulose fully in one passage of food. They therefore eat their own faeces, which are rich in undigested cellulose. This habit is called coprophagy.

3. An opossum has the greatest number of teeth (50).

4. Dysphagia means difficulty in swallowing. Diastema is the gap between the incisors and premolars in a rabbit.

5. Reverse peristalsis (anti-peristalsis) in the stomach and duodenum causes vomiting.

6. Whales and sea cows have no salivary glands. The tusks of an elephant are modified incisors.

7. The stomach of ruminants such as cow, buffalo, sheep and goat has four chambers: rumen, reticulum, omasum and abomasum.

8. Endosymbiosis: this is the theory that mitochondria and chloroplasts were once free prokaryotes that, millions of years ago, came to live inside other cells and became their organelles.

9. Carbon fixation: the use of ATP and NADPH to convert CO₂ into carbohydrates, which occurs in the chloroplast.

10. Fatty acids are the basic units of many lipids (fats), for example triglycerides.

11. Enzymes are proteins that speed up biological reactions without being used up, so they act as catalysts. They are made in living cells and may work inside or outside the cell.

12. Peristalsis is an involuntary, wave-like movement along hollow tubes with circular and longitudinal muscles, such as the intestines.