Plant Nutrition

Photosynthesis

Plants do not take in food. They make it, from carbon dioxide and water, using light energy trapped by chlorophyll.

The word equation

Photosynthesis makes glucose, a carbohydrate, and releases oxygen. It takes place in chloroplasts, which contain the green pigment chlorophyll.

The role of chlorophyll

Chlorophyll absorbs energy from light and transfers it into energy in chemicals, which is used to build carbohydrates. That is why the reaction does not happen without light, or in parts of a leaf that have no chlorophyll to absorb it.

ExtendedBalanced chemical equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light and chlorophyll)

Six molecules of carbon dioxide and six of water make one molecule of glucose and six of oxygen.

Worked example (raw materials and products): From the word equation, name the raw materials and the products of photosynthesis. Step 1. Raw materials go in before the arrow: carbon dioxide and water. Step 2. Products come after the arrow: glucose and oxygen. Step 3. Light and chlorophyll are the conditions needed, not raw materials. Answer: In: carbon dioxide and water; out: glucose and oxygen.

Products and Mineral Ions

Glucose is not the end of the story. The plant uses it in five ways, and it also needs mineral ions from the soil to make other important substances.

Mineral ions

Plants also need nitrate ions, which are used to make amino acids and so proteins, and magnesium ions, which are used to make chlorophyll.

Worked example (explaining a symptom): A plant grown in soil without magnesium ions has pale yellow leaves and grows poorly. Explain this. Step 1. Magnesium ions are needed to make chlorophyll. Step 2. Without them the leaves cannot make enough chlorophyll, so they are yellow instead of green. Step 3. Less light energy is absorbed, so less photosynthesis occurs and growth is poor. Answer: Not enough chlorophyll.

Testing the Requirements

To show that photosynthesis needs light, chlorophyll and carbon dioxide, test a leaf for starch after changing one requirement at a time.

Testing a leaf for starch

First leave the plant in the dark for a day or two so that its leaves use up their starch. Then, after the experiment, test a leaf: boil it in water to kill it, warm it in ethanol in a hot water bath to remove the chlorophyll, rinse it in water, and add iodine solution. Blue-black shows that starch is present.

Worked example (interpreting a leaf test): A leaf is partly covered with black paper in bright light, then tested for starch. The covered strip stays orange-brown. What does this show? Step 1. The covered strip stays orange-brown, the colour of iodine, so it contains no starch. Step 2. The strip had no light but was otherwise the same as the rest of the leaf. Step 3. So light is needed for photosynthesis. Answer: Light is needed.

Rate of Photosynthesis

Three environmental factors change how fast a plant photosynthesises: light intensity, carbon dioxide concentration and temperature.

Describing the effects

Light intensity and carbon dioxide concentration. As either one increases, the rate of photosynthesis increases, and then it levels off and stays constant.

Temperature. As temperature increases the rate rises to an optimum. Above it, the rate falls quickly, because the enzymes that control photosynthesis are denatured.

Worked example (describing a trend): A pondweed gives off 4 bubbles per minute in dim light and 22 bubbles per minute in bright light. Further brightening gives 23. Describe the pattern. Step 1. From dim to bright light the rate rises from 4 to 22 bubbles per minute. Step 2. With further increase to 23 the rate hardly changes. Step 3. The rate increases, then levels off. Answer: Rises, then levels off.

Measuring Rate and Limiting Factors

An aquatic plant lets you estimate the rate of photosynthesis, because the oxygen it makes appears as bubbles.

Investigating the rate

Place pondweed in water containing a little sodium hydrogencarbonate under a funnel, with a lamp at a measured distance, and count the bubbles of oxygen released in a set time. Change one factor, such as the lamp distance, and keep the others the same. Repeat each condition and calculate a mean.

ExtendedLimiting factors

At any time the rate of photosynthesis is set by the factor in shortest supply: the limiting factor. While light is the limiting factor the rate rises as light increases. When the line levels off, light is no longer limiting, and carbon dioxide or temperature is.

Worked example (identifying the limiting factor): Fig 6.5: at the plateau of the lower line, what is limiting the rate, and how could the rate be increased? Step 1. The rate has levelled off although light is still increasing, so light is no longer limiting. Step 2. The upper line is higher, with high carbon dioxide, so carbon dioxide is limiting the lower line. Step 3. Increasing the carbon dioxide concentration raises the rate. Answer: Carbon dioxide; raise it.

Gas Exchange in an Aquatic Plant

Hydrogencarbonate indicator shows what a plant does to the carbon dioxide in the water around it, in light and in the dark.

The investigation

Put equal amounts of hydrogencarbonate indicator, which starts red, into three tubes. Put a piece of aquatic plant in two of them and none in the third, as the control. Wrap one of the tubes with plant in foil so it is dark, and leave the other in bright light. Leave them for a few hours.

Worked example (reading the indicator): A tube containing pondweed, kept in the dark, turns yellow. What has happened to the carbon dioxide concentration, and why? Step 1. Yellow means more carbon dioxide in the water. Step 2. In the dark there is no photosynthesis, so no carbon dioxide is used. Step 3. The plant still respires and releases carbon dioxide, so the concentration rises. Answer: More carbon dioxide.

Leaf Structure

You must be able to name the tissues in a cross-section of a leaf and say what each one does.

Light enters through the transparent epidermis and is absorbed mainly in the palisade layer. Carbon dioxide enters through the stomata and spreads through the air spaces of the spongy layer. Water arrives in the xylem, and the sucrose made leaves in the phloem.

Worked example (naming from a description): A layer of tall, tightly packed cells just below the upper epidermis contains many chloroplasts. Name it. Step 1. Tall tightly packed cells near the top are palisade cells. Step 2. Many chloroplasts means it is where most photosynthesis happens. Answer: Palisade mesophyll.

How the Leaf Is Adapted

Most leaves are large and thin. Each of these, and each tissue inside, is an adaptation for photosynthesis.

Stomata are mostly on the lower surface, away from the strongest sunlight, so less water is lost by evaporation. Guard cells open and close them, so the plant can take in carbon dioxide and also control water loss.

Worked example (explaining an adaptation): Explain how having most chloroplasts in the palisade layer helps photosynthesis. Step 1. The palisade layer is just below the transparent upper epidermis. Step 2. Light reaches it first and is strongest there. Step 3. Chloroplasts there absorb more light, so more photosynthesis can occur. Answer: More light absorbed.