Brent Cornell

   
Chloroplasts are the ’solar energy plants’ of a cell – they convert light energy into chemical energy

• This chemical energy may be either ATP (light dependent) or organic compounds (light independent)
• Only photosynthetic tissue possess chloroplasts (e.g. is present in leaves but not roots of plants)

Chloroplasts are thought to have once been independent prokaryotes that were internalised by eukaryotes via endosymbiosis

• They have a double membrane structure (due to vesicular coating as part of the endocytotic process)
• They have their own DNA (circular and naked) and ribosomes (70S)
• Their metabolic processes are susceptible to certain antibiotics

The structure of the chloroplast is adapted to the function it performs:

• Thylakoids – flattened discs have a small internal volume to maximise hydrogen gradient upon proton accumulation 
• Grana – thylakoids are arranged into stacks to increase SA:Vol ratio of the thylakoid membrane
• Photosystems – pigments organised into photosystems in thylakoid membrane to maximise light absorption
• Stroma – central cavity that contains appropriate enzymes and a suitable pH for the Calvin cycle to occur
• Lamellae – connects and separates thylakoid stacks (grana), maximising photosynthetic efficiency

Structure and Function of a Chloroplast

   
Electron micrographs of a chloroplast may differ in appearance depending on where the cross-section occurs

Typically, chloroplast diagrams should display the following features:

• Usually round in appearance with a double membrane exterior
• Flattened discs (thylakoids) arranged into stacks (grana), connected by lamellae
• Internal lumen of thylakoids is very small (allows for a more rapid generation of a proton motive force)
• Ribosomes and chloroplast DNA are usually not visible at standard resolutions and magnifications
• Starch granules may be visible and will appear as dark spots within the chloroplast

Chloroplast Diagrams

⇒  Click on the diagram to show / hide labels