Chloroplast Structure

Above: 3D model of a chloroplast

What is a chloroplast ? A chloroplast is a type of organelle.

Chloroplasts are the site of photosynthesis in eukaryotic cells. They are only present in photosynthetic cells e.g. plant cells and algae.

There are no chloroplasts in animal cells or bacteria cells (bacteria cells are also known as prokaryotic cells and do not contain any membrane-bound organelles).

As shown in the diagram of a plant cell, chloroplasts are bigger and fatter than mitochondria. That is why chloroplasts settle first when photosynthetic cells are homogenized and centrifuged. A typical chloroplast has a biconvex shape and a maximum dimension of about 5μm (i.e. 5 micrometers = 0.005 mm, see scientific numbers for more about the units).

  • Although they exist within cells, chloroplasts (and mitochondria) are sometimes referred to as "semi-autonomous organelles" because they contain their own DNA and reproduce independently of the nucleus of the eukaryotic cell in which they are located. The chloroplast's own DNA codes for redox proteins involved in electron transport in photosynthesis
  • Each chloroplast is surrounded by a double-layered membrane i.e. it is enclosed by two membranes separated by an intermembrane space. In green plants both the inner membrane and the outer membrane surrounding chloroplasts are lipid-bilayer membranes.

Simple diagram of the structure of a chloroplast

Diagram of a chloroplast

Above: Diagram of the structure of a chloroplast

Notes about the contents of chloroplasts:

The following table includes comments about each of the structure found inside chloroplasts.

Structure of (or within) chloroplasts:

1.

Chloroplast Envelope:

-

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)

-

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

-

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

-

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

-

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

-

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

7.

Starch granules

-

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

-

Lipid globules are also present in chloroplasts.

9.

Ribosomes

-

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.

Remember : A chloroplast is an organelle (found in plant cells but not animal cells); a chloroplast is not a cell.

For more about cells: the structure of a plant cell, the structure of an animal cell & comparison of plant, animal and bacterial cells. See also very simple information about chloroplasts and photosynthesis (via 3 short YouTube videos).

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