International A Level Biology / Photosynthesis

Chloroplast structure and function

Edexcel International A Level Biology (YBI11), chapter 14: Photosynthesis, specification point 5.5. Free lesson.

Read the revision notes · International A Level Biology past papers

Chloroplasts contain specialised structures that support photosynthesis. Knowing where each stage happens and how the structures are adapted helps you explain the process and compare chloroplasts with mitochondria.

What you can see inside a chloroplast

The envelope is the double membrane around the chloroplast. Inside it, the stroma is a fluid packed with enzymes. Flattened membrane sacs called thylakoids run through the stroma. They stack into a granum (plural: grana), and intergranal lamellae link one granum to another. A lamella is a thin layer. Starch grains store sugar made in photosynthesis. Small lipid droplets store lipid. The chloroplast has circular DNA and small 70S ribosomes, like a prokaryote. Together, these let it make some of its own proteins. Chlorophyll is in the thylakoid membranes, not in the stroma…

How chloroplast structures suit their roles

The shape and contents of each part help photosynthesis happen. In an explanation, link what a structure is like to how that helps. Thylakoids stacked into grana provide a large membrane surface area for the light-dependent reactions. Thylakoid membranes hold chlorophyll, electron carriers and ATP synthase. Chlorophyll absorbs light, and the reactions on the membranes make ATP and reduced NADP. The stroma contains enzymes, including RUBISCO, for the light-independent reactions. It surrounds the thylakoids, so ATP and reduced NADP do not have far to travel. Intergranal lamellae join grana…

Where the two stages of photosynthesis happen

The location of the light-dependent reactions follows from where their required components are found. The light-independent reactions happen in the stroma. Light-dependent reactions take place on the thylakoid membranes, where chlorophyll, electron carriers and ATP synthase are found. Light-independent reactions take place in the stroma.

How more thylakoids help in shade

Shade leaves have more thylakoids in each granum than leaves in full sun. This gives them more thylakoid membrane, which helps them make use of limited light. More membrane holds more chlorophyll, so more of the dim light is absorbed. More light absorbed means more ATP and reduced NADP are made for the light-independent reactions.

How chloroplasts compare with mitochondria

Both organelles have an envelope, circular DNA and 70S ribosomes. Their inner structures and the fluid inside them differ. A chloroplast has thylakoids stacked into grana. A mitochondrion has an inner membrane folded into cristae. The fluid inside a chloroplast is the stroma; inside a mitochondrion it is the matrix. Starch grains are often present in chloroplasts but are not found in mitochondria.

More lessons in Photosynthesis