Chloroplast Structure

1.

Chloroplast Envelope:

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Each chloroplast is enclosed (surrounded by) a chloroplast envelope consisting of three layers:

1. The outer membrane is a phospholipid membrane
2. The intermembrane space
3. The inner membrane is a phospholipid membrane

Overall the chloroplast envelope is semi-permeable.
It is permeable to glucose molecules and certain ions including Fe²⁺ and Mg²⁺, and oxygen and carbon dioxide.

2.

Stroma (chloroplast matrix)

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The chloroplast matrix is called the stroma and contains enzymes that catalyze the light-independent reactions of photosynthesis.

3.

Thylakoids
each thylakoid has a lumen

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Thylakoids are also referred to as thylakoid membranes. They are disc-shaped structures that are the sites of light absorption at which the light-dependent reactions of photosynthesis take place. The region within the membrane forming each thylakoid (by enclosing the contents of the thylakoid) is called the lumen of the thylakoid. Either on the surface of, or embedded within, thylakoids are:

• chlorophyll molecules - on the surface of thylakoids. (Chlorophyll is green and plentiful in chloroplasts in plant cells, hence many plants are also green! Although chlorophyll is the main pigment in chloroplasts there are also other pigments - different pigments absorb different wavelengths of light.)
• accessory pigments
• enzymes
• electron transport systems

The light absorbing molecules within thylakoid membranes are arranged in photosystems. Thylakoids are also the sites at which ATP synthesis occurs within chloroplasts.

4.

Grana (plural),
singular - Granum

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Thylakoids are arranged in stacks called grana (plural).
A single granum is a stack of several thylakoids one on top of another. As shown in the diagram and model above, there are many such grana within each chloroplast.

5.

Lamellae (plural)
singular - Lamella

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As shown above, stromal lamellae connect two or more grana to each other. In this way the lamellae act as a "skeleton" of the chloroplast, maintaining efficient distances between the grana, thereby maximizing the overall efficiency of the chloroplast.

6.

Circular DNA

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Each chloroplast contains one or more molecules of small circular DNA.

7.

Starch granules

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Starch exists in chloroplasts in the form of tiny lumps called "granules" or sometimes "grains". These are present because they are the (insoluble) storage carbohydrate product of photosynthesis.

8.

Lipid globules

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Lipid globules are also present in chloroplasts.

9.

Ribosomes

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Chloroplasts contain the smaller type of ribosomes (i.e. "70S ribosomes"), which is the same type as those freely distributed around the cytoplasm of prokaryotic cells.