4.7 Plant biology (Part 3)

Photosynthesis, leaf structure and mineral ions — EST 2 BIOLOGY notes: Define photosynthesis, relate leaf structure to function, and explain the need for magnesium and nitrate ions.

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light. Green plants make glucose from carbon dioxide and water, and oxygen is released as a waste product. The reaction requires energy, which is obtained by the pigment chlorophyll trapping light from the Sun.

Chlorophyll is a green pigment found in chloroplasts. It reflects green light (giving plants their colour) and absorbs light energy, transferring it into chemical energy for the synthesis of glucose. Photosynthesis will not occur without chlorophyll.

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy, chlorophyll)

The leaf is adapted for photosynthesis. Carbon dioxide reaches the chloroplasts by diffusion: atmosphere → air spaces around the spongy mesophyll → leaf mesophyll cells → chloroplast.

Structure	Adaptation / function
Wax cuticle	Waterproof layer that reduces water loss by evaporation; thin so it does not block sunlight
Upper epidermis	Thin and transparent so as much light as possible reaches the palisade cells
Palisade layer	Closely packed cells at the top of the leaf full of chloroplasts, maximising light absorption
Spongy mesophyll	Air spaces let carbon dioxide diffuse through the leaf and give a large surface area
Lower epidermis	Contains stomata and guard cells, controlling gas exchange and water loss
Guard cells	Change shape as they gain or lose water, opening or closing the stomata
Stomata	Pores for gas exchange; usually open by day and closed at night; the main site of water loss in transpiration

You should be able to identify these structures on a cross-section diagram and on an electron micrograph of a dicotyledonous leaf.

The carbohydrates made in photosynthesis can be converted into starch, an effective energy store. Plants also need proteins, lipids and nucleic acids (DNA), but as they do not eat, they must make these themselves. Carbohydrates contain carbon, hydrogen and oxygen; proteins also contain nitrogen; chlorophyll contains magnesium and nitrogen. So plants also need mineral ions from the soil.

Ion	Use	Deficiency symptom
Magnesium	Needed to make chlorophyll	Yellowing between the veins of leaves (chlorosis)
Nitrate	A source of nitrogen for making amino acids, which build proteins	Stunted growth and yellowing of leaves

Plants have two types of transport vessel. Xylem vessels transport water and mineral ions from the roots to the stem and leaves. Phloem vessels transport food materials, mainly sucrose and amino acids, from the photosynthesising leaves to non-photosynthesising regions such as the roots and stem. In cross-sections of a root, stem or leaf, the xylem is always on the inside and the phloem is always on the outside.

Learn the functions in pairs: chlorophyll absorbs light; magnesium is needed to make chlorophyll; nitrate is needed to make amino acids; the cuticle reduces water loss; stomata allow gas exchange. Questions often ask for the cause of yellow leaves, where the answer is a lack of magnesium (yellow between the veins) or nitrate (stunted growth and yellow leaves overall).

Finally, note the economic importance of these ideas: farmers add fertilisers containing nitrate and magnesium to improve crop growth, and glasshouses supply light and carbon dioxide to raise yields.

Worked example 1. Explain how the palisade layer is adapted for photosynthesis. [2]

It is at the top of the leaf and its cells are packed with chloroplasts, so they absorb maximum light.

Worked example 2. State the pathway of carbon dioxide into a leaf. [3]

Atmosphere → stomata → air spaces around spongy mesophyll → mesophyll cells → chloroplast, by diffusion.

Worked example 3. Explain why a plant lacking magnesium has yellow leaves. [2]

Magnesium is needed to make chlorophyll; without it leaves yellow between the veins (chlorosis).

Worked example 4. State the function of guard cells. [2]

They change shape to open or close the stomata, controlling diffusion of CO₂ and O₂ and water loss.

Term	Meaning
Photosynthesis	Manufacture of carbohydrates from carbon dioxide and water using light energy.
Limiting factor	A factor in short supply that restricts the rate of a process.
Stomata	Pores in the leaf for gas exchange.
Chlorophyll	The green pigment that absorbs light for photosynthesis.
Guard cell	A cell that opens and closes a stoma.
Palisade mesophyll	A layer of cells with many chloroplasts at the top of the leaf.
Cuticle	Waxy waterproof layer on the leaf surface.

Saying the cuticle is on the lower epidermis only.

Saying stomata are open at night.

Saying plants get all their food from soil.

Saying glucose is stored as glycogen in plants — they store starch.