Aug 4, 2013

Photosynthesis (SPM Level)

Photosynthesis – takes place in the chloroplast
There two main stages :  

i) light reaction and 
ii) dark reaction
Light reaction occurs only in the presence of light. 

Dark reaction occurs during day and night.
Light reaction - occurs in grana.
Chlorophyll captures light energy - excites the electrons to higher energy levels.
Electrons then leave the chlorophyll.
Light energy is also used to split water molecule into hydrogen ions and hydroxyl ions
This is known as photolysis of water.
Hydrogen ions combine with electrons released by chlorophyll to form hydrogen atoms.
The energy from excited electrons is used to form adenosine triphosphate (ATP).
At the same time, hydroxyl ion loses an electron to form hydroxyl group.
This electron is then received by chlorophyll.
The hydroxyl groups then combine to form water and gaseous oxygen.
Oxygen is released into the atmosphere and used for cellular respiration. 
The ATP molecules provide energy while the hydrogen atoms provide reducing power for the dark reaction.

Dark reaction - also known as Calvin cycle.
Occurs in stroma

Hydrogen atoms are used to fix carbon dioxide into a series of reactions catalysed by photosynthetic enzymes.
Carbon dioxide is reduced into glucose.
Glucose monomers undergo condensation to form starch which will stored temporarily as starch grains in the chloroplast.
Light reaction - occurs in the grana (that contained chlorophyll) - takes place in the presence of sunlight and chlorophyll - chlorophyll absorbs light; then it becomes activated and this energy is used to :
i) produce energy in the form of – ATP (used for dark reaction)
ii) split up water molecules (photolysis) into hydroxyl ions(OH-) and hydrogen ions (H+) - oxygen is released; but hydrogen enters dark reaction.
Dark reaction (Light independent reaction) - takes place in the stroma -ATP combined with hydrogen atoms (from the light reaction) are used to reduce carbon dioxide to form glucose.
Glucose produced –
i) converted to starch (stored),
ii) transformed - sucrose ; transported to other parts
iii) synthesis of cellulose
iv) converted to amino acids and fatty acids