Photosynthesis takes place inside chloroplasts. You should be able to relate the structure of chloroplasts to how they function. Like mitochondria, chloroplasts are organelles surrounded by two membranes,
called an envelope. They are found in mesophyll cells in leaves. Palisade mesophyll cells contain most
chloroplasts, but they are also found in spongy mesophyll cells. Guard cells also
contain chloroplasts.
» The membranes inside a chloroplast are called lamellae, and it is here that the light-dependent stages take place.
» The lamellae contain chlorophyll molecules, arranged in groups called photosystems. There are two kinds of photosystem, PSI and PSII, each of which contains slightly different kinds of chlorophyll.
» Within the chloroplast are enclosed spaces between pairs of membranes, forming fluid-filled sacs called thylakoids. These are involved in photophosphorylation – the formation of ATP using energy from light. Thylakoids are often arranged in stacks called grana (singular: granum).
» The ‘background material’ of the chloroplast is called the stroma, and this is where the light-independent stage takes place.
» Chloroplasts often contain starch grains and lipid droplets. These are stores of energy-containing substances that have been made in the chloroplast but are not immediately needed by the cell or by other parts of the plant.
Chloroplast pigments
A pigment is a substance that absorbs light of some wavelengths but not others.
The wavelengths that it does not absorb are reflected from it. Chloroplasts contain
several different pigments.
» Chlorophyll is the main pigment contained in chloroplasts. It looks green because it reflects green light. Other wavelengths (colours) of light are absorbed. There are two types of chlorophyll, called chlorophyll a and chlorophyll b.
» Figure 13.3 shows the wavelengths of light absorbed by three of the pigments found in chloroplasts. These graphs are called absorption spectra.
Absorption spectra
If we shine light of various wavelengths on chloroplasts, we can measure the rate
at which they give off oxygen. The resulting graph is called an action spectrum.
Action spectrum
» Chlorophyll a is the most abundant pigment in most plants. Its absorption peaks are 430nm (blue) and 662nm (red). It emits an electron when it absorbs light.
» Chlorophyll b is similar to chlorophyll a, but its absorption peaks are 453nm and 642nm. It has a similar role to chlorophyll a, but is not as abundant.
» Carotenoids are accessory pigments. They are orange pigments that protect chlorophyll from damage by the formation of single oxygen atoms (free radicals). They can also absorb wavelengths of light that chlorophyll cannot absorb, and pass on some of the energy from the light to chlorophyll.
» Xanthophylls are also accessory pigments, capturing energy from wavelengths of light that are not absorbed by chlorophyll.
Practice Questions
Describe the structure of a chloroplast.
- 3 to 10 µm (diameter);
- double membrane;
- ground substance / stroma;
- contains enzymes / named enzyme, e.g. rubisco;
- also, sugars / lipids / starch;
- 70S / AW, ribosomes;
- circular DNA;
- internal membrane system / fluid-filled sacs / thylakoids;
- grana are stacks of thylakoids;
- (grana) membranes hold, photosynthetic pigments / ATP synthase
Relate the structure of the chloroplast to their roles in photosynthesis.
- Are large organelles containing their own DNA and have a double membrane.
- Chloroplasts have a folded inner membrane which gives a greater surface area for biochemical reactions to occur.
- Thylakoid membranes contains pigments/ electron carriers/ enzymes
- Used in cyclic and non – cyclic photophoshorilation/ light dependent reactions
- Stroma (contain enzymes) for the calvin cycle/ dark reaction
- Grana a network of proteins holding pigments into photosynthesis
- Light reactions on thylakoid which contain ATP/ stalked particles
- Membraine system separates the reactions of photosynthesis from other cell reactions
- Stroma fluid which surrounds grana so that products light dependent stage can easily pass into stroma.
- Contain DNA /ribosomes for manufacture of proteins needed for protein synthesis
- Granna are interconnected by membranes called lamella.
Describe the arrangement and location of chloroplast pigments and discuss their effect on absorption spectra.
- chlorophyll a is primary pigment;
- carotenoids / chlorophyll b, is accessory pigment;
- arranged in, light harvesting clusters / photosystems;
- A antenna complex on, grana / thylakoids;
- ref. PI and PII ; A P700 and P680
- primary pigment / chlorophyll a, in reaction centre;
- accessory pigments / carotenoids / chlorophyll b, surround primary pigment;
- light energy absorbed by, accessory pigments / carotenoids / chlorophyll b;
- (energy) passed on to, primary pigment / chlorophyll a / reaction centre;
- chlorophyll a and b absorb light in red and blue/violet region;
- carotenoids absorb light in blue/violet region;
- ref. absorption spectrum peaks;
- diagram of absorption spectrum;
- different combinations of pigments (in different plants) give different spectra
Explain how the palisade mesophyll cells of a leaf are adapted for photosynthesis.
- closely packed — to absorb more incident light ;
- palisade mesophyll near upper surface of leaf — to maximize light interception ;
- arranged at right angles to leaf surface — to reduce number of light absorbing walls;
- cylindrical cells — producing air spaces between cells ;
- air spaces — act as reservoir of carbon dioxide ;
- large surface area — for gas exchange ;
- cell walls thin — so short diffusion pathway ;
- large vacuole — pushes chloroplasts to edge of cell ;
- chloroplasts on periphery — to absorb light more efficiently ;
- large number of chloroplasts — to maximize light absorption ;
- chloroplasts can move within cells — towards light ;
- chloroplasts can move away from high light intensity — to avoid damage ;