CHAPTER 6NUTRITION

6.11 THE MECHANISM OF PHOTOSYNTHESIS

The Parts of Chloroplast in relation to Photosynthesis

During photosynthesis, carbon dioxide (CO2) and water (H2O) form new bonds, generating glucose (C6H12O6) and releasing oxygen using light energy which is absorb by chloroplast in the green parts of plants.

2. Photosynthesis occur in chloroplasts of palisade mesophyll, spongy mesophyll and guard cell.

ChloroplastThe chloroplast has double membraneThe cytoplasm of the chloroplast is called stromaThe dark reaction of photosynthesis takes place in the stromaChloroplasts contain membranous structures which are piled in stacks called grana (*singular: granum)

Grana contain the light tapping pigment chlorophyll. Chlorophyll is the most abundant photosynthetic pigment in plants. Grana are embedded in a gel-like matrix called stroma.

Structure of the chloroplast

The light reaction and dark reaction of photosynthesis

Light ReactionDuring the light reaction, chlorophyll absorb light energy which excites the electrons from the chlorophyll molecules. In the excited state, the electrons leave the chlorophyll molecules. The energy from the excited electrons is transformed to chemical energy and used to form high energy molecules of adenosine triphosphate (ATP), which is required for the dark reaction.

The light energy trapped by the chlorophyll is used to split water molecules into hydrogen ions (H+) and hydroxyl ions (OH). This reaction is known as photolysis of water.Each hydrogen ion then combines with an electron released by the chlorophyll to form hydrogen atom. The hydrogen atom will become a reducing agent for the dark reaction.

light24H2O chlorophyll 24H+ + 24OH water hydrogen hydroxylmolecules ions ions

24H+ + 24e 24Hhydrogen hydrogenions atoms

Each hydroxyl ion releases its electron to the chlorophyll to form a hydroxyl group. This maintains the neutrality of the chlorophyll. The hydroxyl groups then combine to form water and oxygen. Some of the oxygen is used by the plant for aerobic respiration, but most of its is released into the atmosphere.

24OH 24OH + 24ehydroxyl hydroxyl (given back to ions group chlorophyll)

24OH 12H2O + 6O2hydroxyl water oxygen (releasegroups to the air)

Therefore, the light reaction is an energy-capturing reaction responsible for the production of chemical energy in the form of ATP and reducing agents in the form of hydrogen atoms required for the dark reaction.

Dark ReactionThe dark reaction also known as light independent reaction. It takes place in the stroma of the chloroplast which is rich in enzymes. In the presence of ATP, carbon dioxide absorbed into the leaf will combine with the hydrogen atoms from the light reaction to form glucose.

6CO2 + 24H 6(CH2O)+ 6HOcarbon hydrogen glucose water dioxide atoms

6(CH2O) C6H12O6

(CH2O) is the basic unit of glucose. Six units of (CH2O) will combine to form a molecule of glucose. The reaction between the light reaction of photosynthesis is shown in the figure.

Light Reaction(granum)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Dark Reaction(stroma)

chlorophyllWater moleculesHydrogen ionsOxygen diffuses out of the leafHydrogen atoms, ATPCarbon dioxidediffuses into the leafGlucose

Glucose produced during photosynthesis will be converted immediately to starch to be stored temporarily in the leaf cells or may be transformed into sucrose to be transported to other parts of the plant. Some glucose will be also used in the synthesis of cellulose for the cell wall.

Some of the simple sugars produced can also be converted to amino acids and fatty acids to be used in protein synthesis and fat synthesis respectively.

A schematic diagram of the light reaction and dark reaction in photosynthesis. 24H2OChlorophyll24H+24e24H24OH24OH12H2O6O26H2O6(CH2O)6CO2C6H12O6StarchSucroseAmino acidsFatty acids24eLight reactionDark reaction

Chemical equation of photosynthesis

light6H2O + 6CO2 chlorophyll C6H12O6 + 6O2 Water carbon glucose oxygen dioxide

Comparison between light reaction and dark reaction in photosynthesis

Prepared by,Group 6

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