Nutrition in Plants

All living things need food to stay alive. Food gives organisms the energy to carry out their life activities, the materials to grow and repair their bodies, and the substances needed to stay healthy and resist disease. The process of taking in food and using it for energy, growth, and repair is called nutrition, and the substances in food that the body uses are called nutrients (such as carbohydrates, proteins, fats, vitamins, and minerals).

Living organisms obtain their food in two main ways, called modes of nutrition. Organisms that can make their own food from simple raw materials are said to follow the autotrophic mode of nutrition ("auto" means self, "trophic" means feeding). Green plants are the most important autotrophs: they make their own food using sunlight, in a process called photosynthesis. Because they prepare food for themselves and, indirectly, for all other living things, green plants are also called producers.

Organisms that cannot make their own food and depend on other organisms for it follow the heterotrophic mode of nutrition ("hetero" means other). Animals, fungi, and most microorganisms are heterotrophs, as they take in food made by plants or other organisms. They are also called consumers, because they consume the food produced by others. Humans, too, are heterotrophs.

This distinction is very important for understanding life on Earth. Green plants form the foundation of almost all food chains, since they capture the Sun's energy and store it in food. Every other organism — whether it eats plants directly or eats animals that ate plants — ultimately depends on the food made by green plants. In this chapter we focus on how plants, especially green plants, obtain their nutrition.

The process by which green plants make their own food is called photosynthesis. The word means "making (synthesis) with the help of light (photo)". In photosynthesis, green plants use sunlight to combine two simple raw materials — carbon dioxide and water — to make a sugar called glucose (a carbohydrate, which is the plant's food) and release oxygen as a by-product. This remarkable process feeds the plant and supplies oxygen to nearly all life on Earth.

The two raw materials come from the plant's surroundings. Carbon dioxide is taken in from the air through tiny pores on the leaves called stomata. Water is absorbed from the soil by the roots and carried up to the leaves. The energy needed to drive the process comes from sunlight, which is captured by a green pigment called chlorophyll present in the leaves. The process can be summed up by the simple word equation: carbon dioxide + water → (in the presence of sunlight and chlorophyll) → glucose + oxygen.

The glucose made in photosynthesis is the plant's food. Some of it is used immediately for energy, and the rest is stored, often after being converted into starch, in the leaves and other parts of the plant. We can show that a green leaf has made starch by the iodine test: when iodine solution is added to a leaf that has been making food, the starch present turns blue-black, proving that photosynthesis has occurred.

Photosynthesis is one of the most important processes in nature. It provides food not only for the plant itself but, directly or indirectly, for all other living things, and it releases the oxygen that animals and humans need to breathe. It also removes carbon dioxide from the air. Because of photosynthesis, green plants are truly the food-makers and life-supporters of our planet.

Photosynthesis can take place in any green part of a plant, but it occurs mainly in the leaves, which are specially designed for the job. A leaf is sometimes called the "food factory" of the plant, because it has all the features needed to capture sunlight and make food efficiently. Understanding the structure of a leaf helps explain how it carries out photosynthesis so well.

Leaves are perfectly suited to their role. They are usually broad and flat, giving a large surface area to catch as much sunlight as possible. They are green because they contain plenty of chlorophyll, the pigment that absorbs light. They have a network of veins that carry water to the leaf and carry away the food made. Most importantly, the surface of the leaf has countless tiny pores called stomata (singular: stoma), through which gases pass in and out: carbon dioxide enters for photosynthesis, and oxygen and water vapour leave. Each stoma can open and close, controlled by guard cells on either side of it.

For photosynthesis to take place, certain conditions must be present. There must be sunlight to provide energy, chlorophyll to capture that light, carbon dioxide from the air, and water from the soil. If any one of these is missing — for example, if a leaf has no chlorophyll, or is kept in darkness, or is denied carbon dioxide — photosynthesis cannot occur. This can be shown by experiments: a leaf kept in the dark, or a part of a variegated (partly white) leaf that has no chlorophyll, fails the starch test, while a green leaf in sunlight passes it.

The leaf's clever design shows how plant structure is matched to function. Its broad shape captures light, its chlorophyll absorbs energy, its stomata exchange gases, and its veins transport materials — all working together so the leaf can make food for the whole plant. This is why the leaf is rightly called the food factory of the plant.

Most plants are green and make their own food by photosynthesis, but not all plants are fully self-feeding. Some plants have unusual modes of nutrition because they cannot make all their food themselves. These special modes include parasitic, insectivorous, saprophytic, and symbiotic nutrition, each an interesting adaptation to the plant's way of life.

A parasitic plant obtains its food from another living plant, called the host, usually harming it. A common example is the Cuscuta (Amarbel or dodder), a yellow, leafless, non-green plant that twines around a host plant and sends sucker-like roots into it to draw out food and water. Because Cuscuta has no chlorophyll, it cannot photosynthesise and must live as a parasite at the host's expense.

An insectivorous plant is a green plant that can photosynthesise but grows in soil poor in nitrogen, so it traps and digests insects to obtain this missing nutrient. The famous pitcher plant has a leaf shaped like a deep jug (pitcher) with a lid; when an insect lands inside, it slips down and is digested by juices in the pitcher. The Venus flytrap is another well-known insectivorous plant. These plants are partly autotrophic (they make food) and partly heterotrophic (they get nitrogen from insects).

Two more modes complete the picture. A saprophytic plant or fungus feeds on dead and decaying matter, releasing chemicals onto it to digest it and then absorbing the nutrients; common fungi and mushrooms are saprophytes, and they are important because they help decompose dead remains and recycle nutrients in nature. In symbiotic nutrition, two different organisms live together and both benefit: for example, in a lichen, an alga (which photosynthesises and makes food) lives together with a fungus (which provides shelter and water), helping each other. These varied modes show how plants and plant-like organisms have adapted to obtain food in many different ways.

Plants take in water and dissolved minerals from the soil through their roots, and they use these along with carbon dioxide and sunlight to make food and to build their bodies. Among the minerals plants need, nitrogen is especially important, because it is needed to make proteins and other vital substances for growth. However, as plants grow season after season, they keep removing these nutrients from the soil, so the soil's supply of nutrients must be replenished (restored), or the soil becomes poor and crops grow badly.

There is plenty of nitrogen in the air — it makes up most of the atmosphere — but plants cannot use nitrogen directly from the air. The nitrogen must first be changed into a form the plant can absorb through its roots, such as nitrates. Nature has a clever way of doing this. Certain bacteria called Rhizobium live in the root nodules of leguminous plants (the pulse family, such as gram, peas, beans, and groundnut). These bacteria take nitrogen from the air and fix it into a usable form in the soil — a process called nitrogen fixation — in exchange for food and shelter from the plant. This is another example of a symbiotic relationship.

Farmers use this natural process to keep their soil fertile. By practising crop rotation — growing a leguminous crop (which, with Rhizobium, restores nitrogen) in between other crops — they naturally replenish the soil's nitrogen without expensive chemicals. In addition, farmers add manure (rotted plant and animal waste) and fertilisers (manufactured nutrient mixtures) to the soil to restore nitrogen and other minerals. Manure is especially good because it also improves the soil's structure and is eco-friendly.

Replenishing soil nutrients is essential for healthy crops and good harvests. By understanding that plants need minerals such as nitrogen, that they cannot use atmospheric nitrogen directly, and that Rhizobium bacteria fix nitrogen for leguminous plants, farmers can keep the soil fertile through crop rotation, manure, and fertilisers. This connects plant nutrition directly to agriculture and the food we all depend on.