All about photosynthesis I
By Joanna George-Johnson & company, Contributor

Students of Herbert Morrisson High cheer as a performer exits the stage during a birthday celebration for the Honourable Louise Bennett-Coverley at the St. James Parish Library, in Montego Bay, recently. - Claudine Housen/staff photographer

Nutrition is the study of nutrients and their relationship to the body. There are two types of nutrition - heterotrophic (hetero meaning other) and autotrophic (auto meaning self).

Photosynthesis is a form of autotrophic nutrition, and is carried out by green plants. This week we take a close look at the process, as well as introduce the anatomy of the leaf.

Photosynthesis is the process by which organisms (in your course of study, plants) produce organic food from inorganic substances with the use of light energy. The reaction is very important in ecosystems, as heterotrophic organisms cannot use the energy directly from the sun. Photosynthesis takes this energy and locks it into the chemical bonds of the organic products of photosynthesis. The energy can be released by the reversal of the photosynthesis reaction, which we will look at in another study.

The photosynthesis equation is:

This equation reveals a few things about photosynthesis:

1. Six molecules of carbon dioxide react with six molecules of water to produce one glucose molecule and six oxygen molecules.

2. The reaction only takes place in the presence of sunlight and chlorophyll.

3. The inorganic substrates on the left (inorganic meaning not made of C, H&O) are converted to the organic product, glucose on the right.

The photosynthesis reaction

Photosynthesis has two main stages - the light reaction and the dark reaction. More accurately, these stages are described as the light dependent and the light independent stages, respectively. Please note that the 'dark' reaction can and does take place in light, but does not require light energy for the reaction to be achieved.

The light (light dependent) reaction stage

• The photosynthetic pigment chlorophyll absorbs light energy.
• The energy absorbed is used to split water molecules into hydrogen and oxygen. This process is called light splitting, or photolysis.

The dark (light independent) reaction stage

The hydrogen produced by the light dependent reaction reacts with carbon dioxide to produce glucose.

The leaf

As you already know, photosynthesis takes place in the leaf. Therefore, to understand photosynthesis better, we must have knowledge of the structure of the leaf.

Parts of the leaf

1. Lamina: The flat green part of the leaf

2. Petiole: The leaf stalk

3. Veins: Also known as the vascular bundles, they are composed of xylem and phloem.

4. Vessels. The xylem is responsible for water transport, while the phloem carries away the organic products of photosynthesis.

5. Epidermis: Outer layer of cells covering the leaf. They do not contain chloroplasts, and as such do not photosynthesise! The epidermis is often covered in waxy cuticle preventing excess water kiss from the leaf.

6. Stoma: A small hole in the leaf epidermis, which allows the entry and exit of substances as necessary. Located on the underside of the leaf.

7. Guard cell: Pair of sausage-shape cell surrounding the stomata. The guard cells open and close the stomata, controlling the entry and exit of substances.

The middle layer of the leaf is call the mesophyll, and is divided into two gross sections - the palisade mesophyll and the spongy mesophyll.

The palisade does most photosynthesis, and its cells are regularly shaped and closely packed. The spongy mesophyll, however, is loosely packed, with large air spaces between the cells.

Questions

1) Why is it important that epidermal cells be transparent?

2) Name the two stages of photosynthesis, explaining what occurs at each stage.

Joanna George-Johnson & company from Ardenne High School