Nutrition in Plants

Think about how you get energy: you eat food. Now think about a plant. It never moves to find a meal, yet it grows taller and stronger every week. So where does its food come from?

The answer is that a plant makes its own food. It takes simple raw materials — carbon dioxide from the air and water from the soil — and, using light energy, builds them into a sugar called glucose. This process is called photosynthesis.

Because plants make their own food, we call them producers. Every food chain starts with a producer. Animals, including you, are consumers — we cannot make food, so we depend on producers.

Two ideas to hold onto:

• Photosynthesis is a chemical reaction — small molecules are joined into a bigger one.
• It needs energy, and that energy comes from light, usually sunlight.

Once you understand photosynthesis, you understand where almost all the energy in living things first comes from.

Photosynthesis can be summed up in one tidy word equation.

In words: carbon dioxide + water → glucose + oxygen.

The things on the left are the raw materials. The things on the right are the products.

• Glucose is a sugar. The plant uses it for energy in respiration, or builds it into starch, cellulose and other tissue.
• Oxygen is released as a waste product — but it is the gas that almost every other living thing needs to breathe.

Notice that light energy sits above the arrow, not on either side. That is because light is needed for the reaction but is not a substance that gets used up.

Most photosynthesis happens in the leaves. A leaf is shaped to do this one job extremely well.

• A leaf is broad and flat, giving a large surface area to catch light.
• It is thin, so gases do not have far to travel inside it.
• The palisade layer near the top is packed with chloroplasts to trap the most light.
• Tiny holes called stomata in the lower surface let carbon dioxide in and oxygen out.
• Air spaces in the spongy layer let gases move easily to and from every cell.

Every feature points to the same idea: structure suited to function.

Photosynthesis needs energy, and that energy arrives as light. But light cannot be used directly — it must first be trapped. That is the job of chlorophyll.

Chlorophyll is a green pigment found inside the chloroplasts. It absorbs light energy and passes it on to power the reaction that joins carbon dioxide and water into glucose.

Here is a neat clue: chlorophyll absorbs red and blue light strongly, but it reflects green light. The green light bouncing back is exactly why leaves look green to your eyes.

Think about what happens with no chlorophyll:

• A part of a leaf with no chlorophyll (a pale or white patch) cannot photosynthesise.
• This is why a variegated leaf — green and white — only makes food in its green areas.

Chlorophyll also explains why most plants grow towards a window: more light means more energy for photosynthesis, so the plant grows where it can capture the most.

Photosynthesis does not always run at the same speed. Its rate depends on the conditions around the plant. A condition that is in short supply and holds the process back is called a limiting factor.

The three main limiting factors are:

• Light — more light means more energy, so the rate rises. In dim light, light limits the rate.
• Carbon dioxide — it is a raw material. With little carbon dioxide, the rate stays low.
• Temperature — a warmer plant works faster, up to a point. Too hot, and the rate falls because the reaction is damaged.

On the graph, the line rises while light is the limiting factor, then levels off (a plateau) once a different factor takes over.

Photosynthesis gives a plant glucose — but glucose alone is not enough to build a whole healthy plant. To make proteins, chlorophyll and other substances, a plant also needs mineral nutrients.

These minerals are dissolved in soil water and are absorbed by the roots, especially the root hair cells.

The key minerals to know are:

• Nitrates — needed to make proteins, which the plant uses for healthy growth.
• Magnesium — needed to make chlorophyll, the green pigment.
• Phosphates — needed for healthy roots and energy transfer inside cells.

If a mineral is missing, the plant shows clear signs:

• A shortage of nitrate leads to weak, stunted growth.
• A shortage of magnesium leads to yellow leaves, because the plant cannot make enough chlorophyll.

This is why farmers and gardeners add fertilisers — they replace the minerals that growing crops take out of the soil.

Understanding plant nutrition now connects to many topics ahead:

• Food chains and energy flow — every food chain starts with a producer that photosynthesises.
• Respiration — plants use the glucose they make to release energy, just as you do.
• Gas exchange — the oxygen plants release and the carbon dioxide they take in link to breathing in all living things.
• Ecology and the environment — plant growth, deforestation and the carbon cycle all build on photosynthesis.

Whenever a later topic mentions energy entering living things, trace it back — it almost always begins with a plant and a leaf catching the light. Take your time with this idea; it is one of the biggest in all of biology.