Nutrition in Plants and Animals

Every living thing needs food to get energy for its life processes, to grow, and to repair its body. The process of taking in food and using it is called nutrition, and the food materials that the body needs — such as carbohydrates, proteins, fats, vitamins, and minerals — are called nutrients. But not all organisms get their food in the same way. The different ways in which living things obtain their food are called the modes of nutrition. There are two broad modes: autotrophic nutrition and heterotrophic nutrition.

In autotrophic nutrition, organisms make their own food from simple raw materials. Such organisms are called autotrophs (auto = self, troph = feeding). The commonest example is green plants, which make food by photosynthesis — using sunlight, water, and carbon dioxide to make sugar, with the help of the green pigment chlorophyll. A few special bacteria make food by chemosynthesis, using energy from chemical reactions instead of sunlight. Because autotrophs make their own food, they are the producers on which all other living things ultimately depend.

In heterotrophic nutrition, organisms cannot make their own food and must obtain it ready-made by feeding on other organisms (plants or animals) or their products. Such organisms are called heterotrophs (hetero = other). All animals, fungi, and many microorganisms are heterotrophs. Heterotrophic nutrition itself takes three main forms, depending on how the food is obtained: holozoic, saprophytic, and parasitic nutrition.

In holozoic nutrition, an organism takes in solid or liquid food, digests it inside its body, and absorbs the useful parts — this is how humans and most animals feed. In saprophytic nutrition, organisms feed on dead and decaying matter, releasing chemicals (enzymes) onto it to break it down and then absorbing it — as fungi like bread mould and many bacteria do. In parasitic nutrition, an organism (the parasite) lives on or inside another living organism (the host) and takes food from it, usually harming the host — as tapeworms, lice, and the plant Cuscuta (dodder) do. So nutrition is either autotrophic (self-feeding) or heterotrophic (feeding on others), and heterotrophs feed in holozoic, saprophytic, or parasitic ways.

Photosynthesis is the process by which green plants make their own food (a sugar called glucose) using carbon dioxide, water, and the energy of sunlight, with the help of the green pigment chlorophyll. It takes place mainly in the leaves, inside tiny structures called chloroplasts that contain chlorophyll. The process also releases oxygen into the air. In simple word form: carbon dioxide + water, in the presence of sunlight and chlorophyll, give glucose + oxygen. Photosynthesis is one of the most important processes on Earth, because it makes food and releases the oxygen that almost all living things need.

Although photosynthesis is one process, it happens in two stages: the light reactions and the dark reactions. The light reactions take place in the parts of the chloroplast that contain chlorophyll and need sunlight directly. In this stage, chlorophyll captures light energy and uses it to split water molecules, releasing oxygen as a by-product and storing the captured energy in special energy-carrying molecules. So the main job of the light reactions is to trap light energy and release oxygen.

The second stage is the dark reactions, also called the Calvin cycle. These reactions are called "dark" not because they need darkness, but because they do not need light directly — they can use the energy already captured in the light reactions. In the dark reactions, the plant takes in carbon dioxide from the air and, using the stored energy, joins it together step by step to build glucose (sugar). So the dark reactions use carbon dioxide and stored energy to make food. Together, the light reactions trap energy and the dark reactions use it to make sugar.

The rate (speed) of photosynthesis depends on several factors. Light intensity is important — more light (up to a point) means faster photosynthesis, because the light reactions can trap more energy. The amount of carbon dioxide matters too — more carbon dioxide allows the dark reactions to make more sugar. Temperature also affects the rate, because the reactions are controlled by enzymes that work best within a certain warmth; too cold or too hot slows them down. The availability of water and the amount of chlorophyll also influence the rate. By understanding these factors, farmers and gardeners can help plants photosynthesise well and grow strongly.

Photosynthesis gives a plant carbohydrates (sugars), but to build proteins, make chlorophyll, and carry out all its life processes, a plant also needs certain minerals. These minerals are simple chemical substances that the plant absorbs from the soil through its roots, dissolved in water. The study of how plants take in and use these minerals is called mineral nutrition. The minerals a plant needs are grouped into two kinds, based on how much of each is required: macronutrients and micronutrients.

Macronutrients are minerals that plants need in large amounts. The most important macronutrients are Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), and Sulphur (S) (along with magnesium). Each has a special role. Nitrogen is needed to make proteins and chlorophyll and is vital for healthy leafy growth. Phosphorus is needed for healthy roots, flowers, and seeds and for energy transfer. Potassium helps the plant make and move sugars and keeps it healthy and resistant to disease. Calcium is needed for strong cell walls, and sulphur is part of certain proteins.

Micronutrients (also called trace elements) are minerals that plants need in only very small amounts, but they are just as essential — without them the plant cannot grow properly. Examples include iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), boron (B), and molybdenum (Mo). For instance, iron is needed for the plant to make chlorophyll, even though only a tiny quantity is required. The key difference between the two groups is quantity: macronutrients in large amounts, micronutrients in tiny amounts.

When a plant does not get enough of a particular mineral, it shows deficiency symptoms — visible signs of poor health that tell us which mineral is missing. For example, a shortage of nitrogen causes yellowing of leaves (especially older leaves) and stunted, poor growth, because chlorophyll and proteins cannot be made properly. A shortage of iron also causes yellowing of young leaves, since chlorophyll cannot form. A lack of phosphorus leads to poor root growth and weak flowering, and a lack of potassium can cause browning at the leaf edges. Farmers add these minerals to the soil through fertilisers to prevent such deficiencies and ensure healthy crops.

Humans show holozoic nutrition: we take in food, break it down, absorb the useful parts, and remove the waste. The food we eat is mostly made of large, complex molecules that the body cannot use directly. Digestion is the process of breaking this food down into small, simple, soluble substances that can be absorbed and used by the body. This work is done by the human digestive system, which is a long tube called the alimentary canal (running from the mouth to the anus) together with helper organs called the digestive glands.

Food first enters the mouth (buccal cavity), where the teeth chew it into smaller pieces and saliva mixes with it, beginning the digestion of starch. The chewed food (now a soft lump) is swallowed and passes down a muscular tube called the oesophagus (food pipe), which pushes it to the stomach by wave-like muscular movements. The stomach is a muscular bag that churns the food and mixes it with digestive juices (including acid and enzymes), turning it into a semi-liquid. Here, the digestion of proteins begins.

From the stomach, the food moves into the small intestine, a long, narrow, coiled tube where most digestion and almost all absorption take place. Here the food is acted upon by juices from the liver and the pancreas, as well as juices made by the intestine itself, which complete the digestion of carbohydrates, proteins, and fats. The liver makes a greenish juice called bile, which helps in the digestion of fats, and the pancreas makes pancreatic juice containing several powerful enzymes. The digested nutrients are then absorbed through the inner wall of the small intestine into the blood.

The undigested food then passes into the large intestine, a wider tube whose main job is to absorb water from the leftover material, making it more solid. The remaining undigested waste (faeces) is stored and finally removed from the body through the anus — a process called egestion. So the human digestive system is a coordinated team of organs: the mouth, oesophagus, stomach, small intestine, and large intestine forming the canal, with the liver and pancreas as glands — together turning food into substances the body can absorb and use.

Digestion is mostly a chemical process — large food molecules are broken into small ones by special chemical helpers called enzymes. An enzyme is a substance (a protein) that speeds up a chemical reaction without being used up itself; it acts as a biological catalyst. In digestion, each enzyme acts on a particular type of food, called its substrate, and changes it into simpler products. Enzymes are specific — each one usually works on only one kind of food (for example, an enzyme that digests starch will not digest protein) and works best in particular conditions of warmth and acidity.

Digestion of food begins in the mouth with the enzyme salivary amylase, present in saliva. Its substrate is starch (a carbohydrate), which it begins to break down into simpler sugars (such as maltose). This is why a piece of bread or rice, if chewed for a while, begins to taste slightly sweet — the starch is being turned into sugar. So in the mouth, salivary amylase starts the digestion of carbohydrates.

In the stomach, the main enzyme is pepsin, which works in the acidic conditions there. Its substrate is proteins, which it begins to break down into smaller pieces (peptides) — the first step of protein digestion. The acid in the stomach helps pepsin to work and also kills many germs in the food. So the stomach is where protein digestion mainly begins, with the help of pepsin.

In the small intestine, digestion is completed with the help of enzymes from the pancreas and the intestine. Two important ones are trypsin and lipase. Trypsin continues protein digestion, breaking proteins (and peptides) down further towards amino acids. Lipase acts on fats, breaking them down into fatty acids and glycerol (helped by bile from the liver, which first breaks the fat into small droplets). By the end, carbohydrates have become simple sugars (glucose), proteins have become amino acids, and fats have become fatty acids and glycerol — small, soluble substances ready to be absorbed.

Once food has been digested into small, simple, soluble substances — glucose, amino acids, fatty acids, and glycerol — these must pass from the digestive system into the body to be used. The process by which these digested nutrients pass through the wall of the intestine into the blood (and lymph) is called absorption. Absorption takes place mainly in the small intestine, which is beautifully designed for this job. After absorption, the nutrients are carried to the body's cells, where they are built into the body or used to release energy — and this final use is called assimilation.

The inner wall of the small intestine is not smooth — it is covered with millions of tiny, finger-like projections called villi (singular: villus). The villi greatly increase the surface area of the intestine, so that far more nutrients can be absorbed at once. Each villus has a very thin wall (just one cell thick), is richly supplied with tiny blood vessels (capillaries), and contains a small lymph vessel. These features make the villi perfect for the quick absorption of digested food into the blood and lymph.

The different nutrients are absorbed in slightly different ways. Glucose (from carbohydrates) and amino acids (from proteins) pass through the thin walls of the villi straight into the blood capillaries, which carry them away. Fatty acids and glycerol (from fats) are mostly absorbed into the lymph vessel inside the villus first, and later reach the blood. Through these routes, the small intestine sends the digested nutrients into the body's transport systems so they can be delivered everywhere they are needed.

The absorbed nutrients are then carried by the blood to every cell of the body, where assimilation takes place. In assimilation, the cells use the nutrients: glucose is broken down in respiration to release energy; amino acids are used to build new proteins for growth and repair; and fats are used to store energy and form parts of cells. Anything not needed at once may be stored (for example, glucose stored in the liver, or fat stored in the body). So the journey of food is completed: digestion breaks it down, villi absorb the small nutrients into the blood, and the body finally assimilates them for energy, growth, and repair.