Cells and organisation, Year 8: how plants feed

A limiting factor

HintThink of the slowest worker on a production line.

The whyA plant can only photosynthesise as fast as its scarcest need allows. Supplying more of anything else makes no difference until that shortage is fixed.

Temperature

HintGreenhouses are heated because of it.

The whyPhotosynthesis is a set of chemical reactions, and these run slowly when it is cold. That is why plants grow very little in winter.

Light

HintFind the one thing in the description that is in short supply.

The whyThe plant has enough warmth and enough carbon dioxide, so the dim light sets the pace. Brighter lamps would speed it up.

It increases

HintWith plenty of light and warmth, this gas was the thing in shortest supply.

The whyAir holds very little carbon dioxide, so on a bright, warm day it is usually the limiting factor. Growers add more to raise their yield.

Nitrates (a source of nitrogen)

HintFarmers add this to fields in fertiliser.

The whyProteins contain nitrogen, which glucose does not. Plants get it by absorbing nitrate ions through their roots.

Magnesium

HintIt is a metal; a plant short of it has yellow leaves.

The whyEach chlorophyll molecule has a magnesium atom at its centre, so the plant cannot make the green pigment without it.

It cannot make enough chlorophyll

HintThink about what gives a healthy leaf its green colour.

The whyWithout the green pigment the leaf looks yellow and absorbs less light, so the plant also makes less food and grows poorly.

To build other substances, such as proteins

HintGlucose contains only carbon, hydrogen and oxygen.

The whyProteins need nitrogen and chlorophyll needs magnesium. These elements are not in glucose, so they must come in through the roots.

Xylem

HintIt is made of long, hollow, dead tubes.

The whyXylem is made of hollow tubes running from root to leaf, a little like very thin drinking straws.

Phloem

HintUnlike the water tubes, it can carry its load both up and down the stem.

The whySugar made in the leaves is needed everywhere, from the root tips to the growing buds, so this tissue carries it in both directions.

Upwards, from roots to leaves

HintThink about where water enters the plant and where it evaporates away.

The whyWater is absorbed by the roots and lost as vapour through the leaves. This pulls a continuous stream up through the xylem.

Xylem

HintThe dye was carried along with the water.

The whyThe dye dissolves in the water and is drawn up with it, so it stains the water-carrying tubes and nothing else.

Iodine solution

HintIt is orange-brown in the bottle.

The whyA leaf stores some of the glucose it makes as starch, so finding starch shows that photosynthesis has been taking place.

Blue-black

HintMuch darker than the orange-brown it starts as.

The whyIf the leaf stays orange-brown, there is no starch in it, as happens when a plant has been kept in the dark for a couple of days.

To remove the green chlorophyll

HintThink about what would hide the result of the iodine test.

The whyEthanol dissolves the pigment out and leaves the leaf pale, so a blue-black result shows up clearly.

In a hot water bath, with no flame nearby

HintThe Bunsen burner is turned off first.

The whyA tube of ethanol is stood in a beaker of very hot water. Ethanol boils at a lower temperature than water, so this is hot enough.