Photosynthesis and its equation

Last time

Before we start: last time, in Limiting factors and mineral ions for plants, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Food from light

Every bite of food you eat started as sunlight trapped by a green plant. Plants make their own food, glucose, from carbon dioxide and water, using light energy. That is photosynthesis, and it also gives us the oxygen we breathe.

The definition

Photosynthesis is the process by which plants synthesise carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide from the air and water from the soil. The products are glucose and oxygen.

The word equation

Here is the word equation. Carbon dioxide plus water gives glucose plus oxygen. Light energy and chlorophyll are written over the arrow, because they are needed but are not reactants.

The balanced equation

The balanced symbol equation. Six C O two plus six H two O gives C six H twelve O six plus six O two. Check it: six carbons, twelve hydrogens and eighteen oxygens on each side.

Chlorophyll

Chlorophyll is a green pigment found in chloroplasts. It absorbs light energy and transfers it into chemical energy, stored in glucose. It reflects green light, which is why leaves look green.

Where it happens

Photosynthesis happens in the chloroplasts, mostly in palisade mesophyll cells near the top of the leaf. Carbon dioxide enters through the stomata and diffuses to the cells. Water arrives from the roots through the xylem.

What happens to the glucose

It is used in respiration to release energy. It is turned into starch for storage, and into cellulose for cell walls. It is turned into sucrose to be carried in the phloem, and into nectar to attract insects. And with nitrate ions it is used to make amino acids, and then proteins.

Testing a leaf for starch

To show a leaf has photosynthesised, test it for starch, because glucose is stored as starch. Boil the leaf in water to kill it, then in ethanol to remove the chlorophyll, then rinse and add iodine. Blue-black means starch is present. Orange-brown means no starch.

Destarching and controls

Before an experiment, keep the plant in the dark for two days to use up its starch. This is destarching. Then cover part of a leaf with foil. Only the uncovered part turns blue-black. This shows light is needed for photosynthesis.