Chloroplast structure and function

The chloroplast is an organelle found in plant cells and is the site of photosynthesis, where light energy is used to make glucose from carbon dioxide and water.

Under the electron microscope a chloroplast appears as a lens-shaped body bounded by two membranes. Inside, a system of flattened membrane sacs sits in a fluid matrix.

The membranes hold the light-absorbing pigments; the fluid holds the enzymes. This division of labour is the reason the chloroplast is examined so often in Form 4 Chapter 2 and again in Form 5 when photosynthesis is studied in detail.

Parts and functions

How structure suits function

The thylakoid membranes are arranged into stacks called grana, which gives a large surface area packed with chlorophyll molecules to absorb as much light as possible. The stroma surrounding the grana is a fluid-filled space containing the enzymes needed to fix carbon dioxide into glucose, so the light-absorbing and sugar-making steps of photosynthesis happen close together inside one organelle.

Chloroplasts are also concentrated in cells near the leaf surface, such as palisade mesophyll cells, where they receive the most light.

An examiner marking a structure-to-function answer looks for a feature, a consequence and a link to photosynthesis. The five adaptations below follow that pattern.

• Stacked thylakoids in grana provide a very large membrane surface area, so more chlorophyll can be held and more light energy trapped per chloroplast.
• A double membrane keeps the enzymes and pigments of photosynthesis separate from the reactions of the cytoplasm, so the concentration of reactants inside can be kept high.
• The stroma is a fluid, so carbon dioxide and enzymes can move freely and collisions between enzyme and substrate happen quickly.
• Grana and stroma lie side by side, so the products of the light-dependent reactions on the thylakoids pass directly to the enzymes in the stroma with no long transport distance.
• Chloroplasts are small and present in large numbers rather than as one large body, and in some plants they can shift position inside the cell, so they can be arranged to catch light from whichever direction it arrives.

Related processes

Photosynthesis is the process the chloroplast exists for. In the light-dependent stage, chlorophyll on the thylakoids absorbs light and uses the energy to split water, releasing oxygen and producing energy carriers.

In the light-independent stage, enzymes in the stroma use those carriers to reduce carbon dioxide to glucose. Some of that glucose is stored as starch grains inside the chloroplast, which is why the iodine test on a leaf turns blue-black only in the green parts that were exposed to light.

The chloroplast is also compared with the mitochondrion in Chapter 2. Both have a double membrane, both contain their own enzymes, and both are involved in energy conversion, but the chloroplast builds glucose using light while the mitochondrion breaks glucose down in aerobic respiration to release energy.

A plant cell needs both: photosynthesis by day supplies the glucose that respiration uses at every hour.

Gas exchange in the leaf feeds the chloroplast. Carbon dioxide enters through the stomata, diffuses through the air spaces of the spongy mesophyll and dissolves into the cells before reaching the stroma.

Oxygen produced on the thylakoids leaves by the same route. So the chloroplast's activity is linked to stomatal opening, to diffusion and to transpiration, all of which appear in later chapters.

Common labelling errors

Worked question

Question. Diagram 1 shows the structure of a chloroplast as seen under an electron microscope. Structure P is a stack of flattened membranes and region Q is the fluid surrounding it.

(a) Name structures P and Q. (b) State the role of Q in photosynthesis.

(c) Explain how the arrangement of P is suited to its function. (d) A student counted more chloroplasts per cell in the palisade mesophyll than in the spongy mesophyll of the same leaf.

Explain this observation.

Model answer. (a) P is a granum (stack of thylakoids); Q is the stroma. (b) The stroma contains enzymes that catalyse the light-independent reactions, in which carbon dioxide is fixed and converted into glucose.

(c) The thylakoids are stacked, which gives a large surface area in a small volume; this surface holds a large amount of chlorophyll, so more light energy is absorbed for the light-dependent reactions. (d) Palisade cells lie directly beneath the upper epidermis, so they receive the highest light intensity; having more chloroplasts lets them absorb more light and carry out most of the leaf's photosynthesis, whereas spongy cells are deeper, receive less light and have more air spaces for gas exchange.

Source:SRC-DSKP-EN

Frequently asked questions