Chloroplasts

General Information *Chloroplasts are a member of a large group of organelles known as plastids. Plastids are eukaryotes, which means that they contain membrane bound organelles. *Chloroplasts are known as double membrane organelles. *Chloroplasts are around 4-10 micrometres long and 2-3 micrometres wide. *Chloroplasts are found in the mesophyll cells in the leaves of plants. These mesophyll cells are usually packed with 50+ chloroplasts. *This is vital for plants because chloroplasts contain chlorophyll which is the green pigment that absorbs light energy for photosynthesis, a process needed for plant growth and survival. *Chloroplasts can move in cells to maximise the amount of light energy absorbed for photosynthesis. Under low light intensities, they move toward the centre of the cell, covering the whole surface of the cell to maximise their surface area. Under high light intensities, they will move back towards the cell wall, aligning in accordance to the shape of the cell to minimise damage caused by the strength of the light.

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Membranes Stroma Thylakoid

Structure of Chloroplasts *Chloroplasts have a total of two surface membranes which are known as the chloroplast envelope: -Outer membrane. -Inner membrane. *Within the region of the chloroplast envelop, a semi-gel-like fluid known as stroma is found. *Chloroplasts also contain a Thylakoid system. This system is formed by the inner membrane, which is not a continuous membrane, branching to form a system of branching membranes. *The stroma makes up the majority of the volume of the chloroplast and supports the thylakoid system.

Membranes The inner membrane of the chloroplast is the most important to its function. This membrane forms a boarder to the stroma. The inner membrane allows particles to diffuse in and out of the chloroplast, which in turn regulates the activity of the chloroplasts. Also, the inner membrane does not only regulate activity, but also synthesises molecules such as lipids. The outer membrane is located on the outside of the chloroplast. This membrane is semi-permeable and allows only small molecules and ions to diffuse through it.

Stroma The Stroma is, as we know, a semi-gel-like-fluid within the inner membrane of the Chloroplast. The stroma contains a number of products of photosynthesis, a process that takes place in the chloroplast. It contains starch grains and lipid droplets for example, which take up a large amount of space in the stroma. The stroma also supports the Thylakoid system and the circular DNA of the chloroplast. The stroma also occupies the ribosomes of the chloroplast. These ribosomes are usually found in prokaryotic cells, the 70S ribosomes. These ribosomes allow protein synthesis to occur in the chloroplasts, which allows the chloroplasts to make their own proteins. The circular DNA carries the required information for enzymes to be made from these proteins.

Thylakoids In the stroma, there are many flattened sacs known as Thylakoids. A granum is a stack of between 2 and 100 parallel thylakoids, and the plural for granum is grana. Each thylakoid has a membrane called the thylakoid membrane. Within this membrane is the thylakoid lumen which is a liquid that fills the inner part of the thylakoid. Chlorophyll, the pigment we associate with photosynthesis in chloroplasts is found in these thylakoids. The arrangement of the grana in a chloroplast, increases the surface area of the thylakoids, which increases the amount of light absorbed for photosynthesis to take place. Between stacks of grana are stromal lamellae which connect the stacks together. These lamellae act as the backbone of the chloroplasts, keeping each granum a certain distance apart from each other, this in turn maximizes the efficiency of absorbing light energy of each chloroplast.

Chloroplast under an Electron Microscope Granum(Grana) – Stacks of thylakoids. Chloroplast envelope Stromal lamellae YOU MUST BE ABLE TO RECOGNISE THIS AND THE DIFFERENT PARTS TO IT FOR THE EXAM!

Chloroplast Video https://www.youtube.com/watch?v=Y6tjGbBJW3g