Photosynthesis
Adrian Whyte and Joanna Johnson, Contributor

Members of the Queen's School debating team receive their award at the 2005 National Schools' Debate Competition held at the LOJ Auditorium, New Kingston. The competition was won by St. Mary High school. - Carlington Wilmot Photo

PLANTS ARE known in ecology as primary producers. The reason for this is that they are able to produce or synthesis their own food, they are also able to make enough for other organisms as well.

Photosynthesis is the process that green plants use to make their own food. Using carbon dioxide and water with energy from sunlight plants are able to synthesise carbohydrates, namely glucose, which can later be converted to starch and other organic substances.

The general formula for the process of photosynthesis is:

	Sunlight
Carbon dioxide + water		Glucose + Oxygen
	Chlorophyll
	Sunlight
6CO2 + 62O		C6H12O6 + 6O2
	Chlorophyll

Photosynthesis is an extremely complicated process, but for ease of understanding, we normally break it down into two parts.

* Light reaction: Here is where the energy harnessed by chlorophyll from the sun is used to split water molecules in a reaction called photolysis. Water is split into hydrogen and oxygen. This part of photosynthesis must occur in the presence of light. (Although sunlight is the light energy of choice this can happen under a light bulb with the right amount of energy.)

* Dark reaction: The hydrogen and energy from the dark reaction along with the carbon dioxide is used to synthesise glucose. N.B. Although called the dark reaction, it can occur in the light, it just does not need light to happen.

REQUIREMENTS FOR PHOTOSYNTHESIS AND THEIR SOURCE

* Carbon dioxide: This gas simply diffuses into the leaf through the stomata, which are on the undersize of the leaf.

* Water: The water a plant needs comes through its roots from the soil via a process known as osmosis (Hence the reason for
watering your plants). The water then travels up the stem towards the leaf through specialised cells known as xylem.

* Sunlight: This provides the energy for the process. It simply penetrates through the transparent epidermis into the chloroplast of the mesophyll cells (Light sources other than sunlight can be used to power the process as long as the light source has the correct wavelength).

THE FATE OF THE GLUCOSE THAT IS MADE

Glucose made from the process of photosynthesis may be used for several different things.

* It may be used directly by the plant to provide energy.

* Some will be converted to sucrose for transportation to various parts of the plant that do not photosynthesise.

* Some of the glucose will be converted to starch to be stored for future use. Some of which form the fruits of some plants.

* Some of the glucose can also be combined with other substances to form other nutrients needed by the plant, such as proteins and lipids, needed by the plants.

QUESTIONS:

1. Why is glucose converted to sucrose to be transported?

2. Why is starch used as the storage molecule?

ANSWERS:

1. A good transport molecule must be relatively unreactive, small and soluble. Although glucose is smaller than sucrose and more soluble it is too reactive and therefore is not a suitable transport molecule. The glucose will get involved with reactions that it should not if it were the transport molecule.

2. A good storage molecule should have the following characteristics. It must be insoluble, take up little space and relatively unreactive. Glucose, therefore, could not fit these criteria for a number of reasons.

1) It is too reactive and will get involved in reactions it should not and could cause problems.

2) It is soluble in water and will, therefore, change the concentration of the cell contents and this could damage the cell because it could lead to excessive osmosis.

3) it will not be compact enough and thus will take up too much space. For these reasons starch is the preferred storage molecule.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.