Cells and organisation, Year 8: how plants feed

A limiting factor

提示Think of the slowest worker on a production line.

为什么A 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

提示Greenhouses are heated because of it.

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

Light

提示Find the one thing in the description that is in short supply.

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

It increases

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

为什么Air 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)

提示Farmers add this to fields in fertiliser.

为什么Proteins contain nitrogen, which glucose does not. Plants get it by absorbing nitrate ions through their roots.

Magnesium

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

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

It cannot make enough chlorophyll

提示Think about what gives a healthy leaf its green colour.

为什么Without 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

提示Glucose contains only carbon, hydrogen and oxygen.

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

Xylem

提示It is made of long, hollow, dead tubes.

为什么Xylem is made of hollow tubes running from root to leaf, a little like very thin drinking straws.

Phloem

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

为什么Sugar 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

提示Think about where water enters the plant and where it evaporates away.

为什么Water is absorbed by the roots and lost as vapour through the leaves. This pulls a continuous stream up through the xylem.

Xylem

提示The dye was carried along with the water.

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

Iodine solution

提示It is orange-brown in the bottle.

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

Blue-black

提示Much darker than the orange-brown it starts as.

为什么If 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

提示Think about what would hide the result of the iodine test.

为什么Ethanol 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

提示The Bunsen burner is turned off first.

为什么A 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.