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Chloroplasts

Chloroplasts are the site of photosynthesis. Chloroplasts are disc-shaped, 2-10 μm in diameter, about 1 μm thick and are surrounded by a double membrane.The light-dependent stage of photosynthesis takes place in the grana which are stacks of flattened discs called thylakoids. The thylakoids contain the photosynthetic pigment - chlorophyll and other photosynthetic pigments. Some thylakoids are joined to thylakoids in adjacent grana by inter-granal lamellae.The chloroplasts are filled with a fluid matrix called the stroma in which the light-independent reaction takes place. The chloroplast contains its own DNA and 70S ribosomes. The stroma may contain starch grains.

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Specification: AQA Biology (7402), A-level, 3.5.1 Photosynthesis. Topic: Energy for Biological Processes.

Key terms in this lesson

neurotransmitter
A chemical released by a presynaptic neurone that diffuses across a synapse and binds to receptors on the postsynaptic cell, altering its membrane potential.
neuromuscular junction
The synapse between a motor neurone and a skeletal muscle fibre, where acetylcholine triggers depolarisation of the sarcolemma.
hormone
A chemical messenger, secreted by an endocrine gland directly into the blood, that travels to target cells bearing complementary receptors and brings about a response.
insulin
A hormone secreted by the beta cells of the islets of Langerhans in response to a rise in blood glucose concentration, which lowers blood glucose by stimulating cells to take up and use or store glucose.

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