So far, all the organelles we have looked at (the nucleus, mitochondria, ribosomes and RER) are found in both animal and plant cells. Today’s organelle, the chloroplasts, are only found in plant cells and some eukaryotic algae.
Their main function is to carry out photosynthesis – the reaction that uses light energy to make glucose and oxygen from carbon dioxide and water. This is an essential first step in the food chain.
Similar to the mitochondria, chloroplasts have a double membrane which is sometimes called the chloroplast envelope. But inside the chloroplasts, things look quite interesting. Disc-shaped thylakoids are stacked on top of each other. A stack of thylakoids is called a granum (plural = grana).
Thylakoid membranes can extend between the grana, and these are called the lamellae (singular = lamella). The thick fluid around all these objects is called the stroma – this contains enzymes needed for photosynthesis. Don’t confuse this with the stomata, which are the pores found in leaves.
There are two main stages of photosynthesis; the light-dependent reaction and the light-independent reaction. As the name suggests, the first stage depends on light. Attached to proteins in the thylakoid membranes there is a pigment called chlorophyll which can absorb light. Chlorophyll is best at absorbing light at the red and blue wavelengths. Green light is reflected back, which is why many plants appear green. The fact that the thylakoids are stacked into grana, and also have membranes extending between grana (the lamellae), means that there is a large surface area of thylakoid membrane to capture light. The energy from light excites electrons in chlorophyll and the light dependent reaction begins. We will cover photosynthesis in more detail in a separate series.
Chloroplasts can also store the useful products of photosynthesis. The photosynthesis reaction produces glucose which can be stored as starch granules. Glucose and other molecules involved in photosynthesis can also be used to produce lipids (fats) which are stored in oil droplets.
One final thing to note about chloroplasts is that they contain their own DNA. It’s thought that they were originally prokaryotic organisms which evolved to live inside another cell, so the small pieces of circular DNA are left over from then.
Summary
- Chloroplasts carry out photosynthesis in plant cells and some algae.
- They have a double membrane, and contain thylakoids stacked into grana which are joined by lamellae. The thick fluid containing these is called the stroma.
- Glucose is stored in starch granules and fats are stored in oil droplets.
- Thylakoid membranes contain chlorophyll which absorbs the light needed to provide energy for photosynthesis.
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