植物的营养

Photosynthesis 光合作用 is the process by which plants make carbohydrates 碳水化合物 from simple raw materials 原料, using energy 能量 from light. It is how plants feed themselves.

The word equation

In words: plants take in carbon dioxide 二氧化碳 and water, and make glucose 葡萄糖 and oxygen 氧气. This reaction can only happen in the light, and only where there is chlorophyll 叶绿素.

(Supplement) The balanced chemical equation is:

What chlorophyll does

Chlorophyll is a green pigment 色素 found in the chloroplasts 叶绿体. It traps light and transfers the energy from light into energy stored in chemicals. That chemical energy is then used to build carbohydrates.

What the plant does with the glucose

Minerals from the soil

Plants also take up mineral ions 离子 from the soil through their roots:

• nitrate ions 硝酸根离子 — needed to make amino acids 氨基酸 (and so proteins).
• magnesium ions 镁离子 — needed to make chlorophyll.

A plant short of nitrate grows poorly with weak stems; a plant short of magnesium has yellow leaves (it cannot make enough chlorophyll).

The rate of photosynthesis

The rate 速率 of photosynthesis depends on three things:

• light intensity 光照强度 — more light gives a faster rate, up to a point.
• carbon dioxide concentration 浓度 — more carbon dioxide gives a faster rate.
• temperature 温度 — a warmer temperature is faster, until it gets so hot that the enzymes are denatured.

(Supplement) At any moment, the one factor that is in shortest supply holds back the rate. This is called the limiting factor 限制因素. For example, on a dull day light intensity is usually the limiting factor; on a bright day carbon dioxide may be.

Worked example. On a graph of rate against light intensity, the line rises steeply at first and then levels off flat. Which factor is limiting on each part? On the rising part, adding light raises the rate, so light intensity is the limiting factor there. On the flat part, adding more light changes nothing, so light is no longer limiting - something else is, usually carbon dioxide concentration or temperature. The test is always the same: if adding more of a factor speeds the reaction up, that factor was the one holding it back.

(Supplement) Reading two curves. Exam graphs often show the same experiment at two carbon dioxide concentrations, as in the figure.

• Where the two curves are on top of each other, extra carbon dioxide does nothing, so carbon dioxide is not limiting there. Light is.
• Where a curve is flat, more light does nothing, so light is not limiting there.
• Moving from the lower curve to the higher one only changed carbon dioxide. The rate went up, so carbon dioxide was the limiting factor on the lower curve's flat part.

Common mistakes.

• Saying "light is limiting" on the flat part of the curve. Light is limiting only where adding more light still raises the rate.
• Saying the rate falls when the curve levels off. A flat line means the rate stays constant at its highest value.
• Saying high temperature "kills" photosynthesis. Above the optimum the rate falls because the enzymes are denatured.
• Forgetting that the plant also respires. In dim light, a plant can take in no carbon dioxide overall, because respiration releases as much as photosynthesis uses.

Investigating photosynthesis

To show that a plant needs light, chlorophyll and carbon dioxide, you test a leaf for starch after taking one of them away. A control 对照 keeps every other condition the same, so the test is fair. (First leave the plant in the dark to use up its starch.) A leaf with green and white parts shows starch only in the green parts, which have chlorophyll.

You can also watch gas exchange 气体交换 in a water plant using hydrogencarbonate indicator 指示剂, which changes colour as the carbon dioxide level changes. In the light the plant takes in carbon dioxide for photosynthesis; in the dark it gives out carbon dioxide from respiration.

A leaf is well adapted 适应 for photosynthesis. Most leaves are broad, with a large surface area 表面积 to catch plenty of light, and thin, so that gases and light quickly reach all the cells.

The middle of the leaf, the mesophyll 叶肉, is where most photosynthesis happens. These are the main parts of a leaf:

• Learn the word equation exactly: carbon dioxide + water → glucose + oxygen (in light, with chlorophyll).
• Chlorophyll is in the chloroplasts; it does not get used up — it transfers light energy.
• Link each factor to the rate: more light, more carbon dioxide, warmer (but not too hot) → faster, until a limiting factor stops further increase.
• For leaf structure, always link the part to its job: broad and thin to catch light; palisade cells full of chloroplasts; stomata and air spaces for gas exchange.
• Nitrate ions → amino acids; magnesium ions → chlorophyll.