Life Processes in Plants

A growing plant shows clear signs of change over time: new leaves appear, its stem thickens, and its height steadily increases. Since a plant can't actually eat anything, what is driving all of this growth?

Here's one tidy way to find out: grow three identical young saplings of the same fast-growing plant, in identical pots of soil, but treat each one differently. Give the first plant both full sunlight and regular watering. Give the second plant full sunlight but no water at all. Give the third plant regular watering but keep it permanently in the dark. After about two weeks, the differences are striking. The first plant, with both sunlight and water, grows the tallest, with the most leaves and a healthy green colour. The second plant, denied water entirely, typically wilts badly and may die, regardless of how much sunlight it received — proving that sunlight alone cannot substitute for water. The third plant, kept in the dark despite regular watering, survives but grows poorly, with a thin, weak stem and pale, yellowish leaves, since it has no sunlight to properly make use of.

Put together, this simple comparison shows that a plant's growth genuinely depends on both water and sunlight working together — removing either one noticeably damages growth, even when the other is provided in full.

Most leaves are green because they contain a pigment called chlorophyll, which absorbs sunlight efficiently and uses that captured light energy to manufacture the plant's food, storing the surplus as starch, a carbohydrate built for long-term storage. This is exactly why leaves are often described as the plant's "food factories."

You can actually prove a leaf contains starch using a neat chemical trick. First, the leaf is boiled briefly to soften its tissue, then dipped in alcohol (heated carefully, never over an open flame, since alcohol is highly flammable) until its natural green colour fades away completely — this step is purely to make any colour change easy to see afterward, not to produce the starch itself. Once the leaf is pale and colourless, a few drops of iodine solution are added. If starch is present, the leaf turns a deep blue-black colour; if there's no starch, it stays unchanged.

This test becomes especially revealing when applied to a leaf that naturally has both green and pale (non-green) patches. If one such leaf comes from a plant kept in sunlight and an identical one comes from a plant kept in complete darkness for a day or two, testing both leaves shows something striking: on the sunlit plant's leaf, only the green patches turn blue-black — the pale patches stay unchanged, since they lack enough chlorophyll to make starch. On the leaf from the plant kept in the dark, not even the green patches turn blue-black at all, because without light, no starch could be produced in the first place, chlorophyll or not. Together, this tells us that both chlorophyll and sunlight are genuinely necessary ingredients for a leaf to make starch — neither one is enough on its own.

Chlorophyll and sunlight aren't the whole story — a leaf also needs carbon dioxide from the air to make starch. This can be shown by first letting a potted plant sit in darkness for a couple of days so it uses up any starch already stored in its leaves (this is called destarching). One leaf on this destarched plant is then partly sealed inside a container holding a chemical that absorbs carbon dioxide from the air, while the rest of that same leaf stays outside, exposed to ordinary air. After leaving the whole arrangement out in bright daylight for several hours, only the part of the leaf that still had access to carbon dioxide tests positive for starch with iodine — the part sealed away from carbon dioxide does not, even though it had the exact same sunlight and chlorophyll as the rest of the leaf. This confirms that carbon dioxide is a required raw material for photosynthesis, not an optional extra.

Photosynthesis also releases something back into the surroundings. If a water-dwelling green plant is kept submerged in a container of water, placed in sunlight, and arranged so that any gas it releases gets trapped and collected in an upside-down test tube, bubbles of gas visibly collect there over time. If a glowing splint (a thin stick with a tiny ember) is inserted into this collected gas, it bursts into a brighter flame — a reliable chemical test confirming the gas is rich in oxygen. This shows that oxygen is specifically released during photosynthesis, not just "some gas."

Putting it all together, photosynthesis can be summarised by one word equation:

Carbon dioxide + Water (using light energy, captured by chlorophyll) → Glucose + Oxygen

The glucose produced is a simple carbohydrate. Some of it is used immediately as an energy source, while the rest is converted into starch for storage. Gases involved in this whole process — carbon dioxide entering, oxygen leaving — pass in and out of the leaf through tiny pores called stomata, found mainly on the leaf's lower surface.

Roots absorb water, along with dissolved minerals from the soil, but that water still needs to reach every leaf, flower, and growing shoot, often quite far above ground. This upward journey happens through a specialised tube-like tissue called the xylem, which runs continuously through the roots, stem, and leaves.

You can actually watch this pathway in action with a simple demonstration: place the freshly cut stem of a young plant (ideally one with white flowers) into water mixed with a few drops of red ink. Within a day, the red colour becomes visible climbing up through the stem and even reaching the leaves and petals, revealing the otherwise hidden path water takes through the plant. Dissolved minerals travel upward through this very same xylem pathway, alongside the water.

Food works in the opposite direction. Since leaves are where photosynthesis actually happens, the sugars produced there need to reach every other living part of the plant, including parts that may be far from any leaf at all, like roots growing underground. This job belongs to a second, separate tube-like tissue called the phloem, which distributes food throughout the plant and allows some of it to be stored in parts such as roots or seeds for later use.

It's easy to assume that plants only ever take in gases and never release any waste gas of their own — but that isn't true. Soaked, germinating seeds (like moong beans) can be placed on damp cotton inside a sealed flask connected by a tube to a test tube of limewater, with the whole set-up left undisturbed in the dark for a day. Afterward, the limewater turns noticeably milky — a clear chemical sign that extra carbon dioxide has built up inside the flask. Since this set-up was kept in complete darkness, photosynthesis (which needs light) could not have been happening at all, which means this carbon dioxide can only have come from one place: the seeds themselves, respiring.

Plant respiration follows the exact same word equation as it does in animals:

Glucose + Oxygen → Carbon dioxide + Water + Energy

This energy is what the plant actually uses for its own growth, repair, and everyday functioning — it isn't optional or occasional. The key distinguishing point is timing and location: respiration happens constantly, in every living plant cell, day and night, in both green and non-green parts (including roots, which never see sunlight at all). Photosynthesis, by contrast, only happens in chlorophyll-containing parts, and only while light is available. So at night, or deep underground in the roots, only respiration is taking place — photosynthesis simply isn't possible there.