Chapter 4 - Bioenergetics (Part 2)
Chapter 4 - Bioenergetics (Part 2)
Chapter 4 - Bioenergetics (Part 2)
5 Mechanism of photosynthesis
Dark Reactions
Light independent reactions do not require direct energy of sunlight, I may occur
during the day time and are called dark reactions
Melvin Calvin in 1950 investigated the sequence of photosynthesis, therefore,
the series of reactions taking place are called Calvin-Benson Cycle
Dark reaction occurs in the stroma of chloroplast
Calvin-Benson cycle is composed of three stages:
1. Carbon fixation
2. Reduction
3. Regeneration of RuBP
4.1.5 Mechanism of photosynthesis
1. Carbon fixation
A 5-carbon sugar called Ribulose Biphsphate (RuBP) already present in stroma reacts
with CO2 to form 6 carbon compound
This 6 carbon compound is unstable, so it splits into two molecules of 3-carbon
compound called Phosphoglycerate (PGA)
This process is accelerated by an enzyme called Rubisco (Ribolose biohosphate
carboxylase) which is the most common protein in the nature
Since the carbon that was part of CO2 is now part of an organic molecule so this
reaction is called Carbon Fixation
2. Reduction
PGA formed in the previous step (Carbon Fixation) is reduced into
phosphoglyceraldehyde (PGAL) in this step
The products of the light reaction i.e. NADPH and ATP are used in this process
Each molecule of PGA receives energy from ATP and hydrogen from NADPH of light
reaction to form PGAL and water
ADP and NADP return back to light reaction
3. Regeneration of RuBP
In this stage, RuBP is regenerated to continue the cycle
Out of every six PGAL molecules formed in the previous step, one molecule leaves the
cycle to be used by plant for making glucose. The other five PGAL molecules are recycled
to regenerate 3 molecules of five carbon RuBP
A. Preparatory phase
Preparatory phase is phosphorylation of glucose by two ATP molecule. Phosphorylation is the chemical
addition of a phosphoryl group to an organic molecule
ATP transfers energy and phosphate to glucose forming glucose 6-phosphate and itself convers to ADP
Glucose-6 Phosphate is isomerised into fructose -6 phosphate
Fructose -6 phosphate reacts with another ATP molecule forming fructose 1-6 biophosphate and ADP
Fructose 1-6 biophosphate splits into 3 Carbon phospho-glyceraldehyde (PGAL) and dihydroxy acetone
phosphate (DAP)
With the above splitting this phase is completed
4.2.1 Aerobic respiration
B. Oxidative Phase
In oxidative phase first two Hydrogen atoms are removed from 3-PGAL and transferred to a molecule of NAD, a co-enzyme.
So, PGAL is oxidized to PGA and NAD is reduced to NADH. The reaction is accompanied by the addition of phosphate group.
The resultant molecules are 1-3 biphosphoglycerate
2PGAL + 2Pi -------------→ 2BPGA + 2H2
2NAD + 2H2 -------------→ 2NADH2
Each molecule of 1-3 biphosphoglycerate transfers high energy phosphate to ADP forming ATP molecule and itself changes to
3 phosphoglycerate
1-3 biphosphoglycerate + ADP -------------→ 3 phosphoglycerate + ATP
In the next step 3 phosphoglycerate converts into 2- phosphoenol pyruvate (PEP) with the elimination of one water molecule
2PGA -------------→ 2PEP + 2H2O
In the last step , phosphoenol pyruvate (PEP) reacts with ADP forming an ATP and pyruvic acid.
2PEP + 2ADP -------------→ 2 pyruvic acid and 2ATP
4.2.1 Aerobic respiration
Cells mostly degrade glucose to release energy but cells can oxidize and degrade
other food molecules such as proteins and fats to release energy
Proteins are broken down into amino acids. Amino group is removed from amino
acids forming ammonia and the remaining molecules enter the Kreb’s cycle
Fat is hydrolysed into glycerol and three fatty acids.
Glycerol is converted to PGAL which enters the process of respiration into
glycolytic pathway
Each fatty acid is degraded into two carbon fragments acetyl groups which enters
the Kreb’s cycle
4.2.2 Anaerobic respiration
4.2.2 Anaerobic respiration
Lactic acid fermentation occurs in muscle cells of humans and in many microorganisms.
In the first step glucose is broken down into pyruvic acid which is basically glycolysis. In
the second step, pyruvic acid is reduced by NADH2 into lactic acid. Here compared to 36
ATP molecules of aerobic process, only 2 ATP molecules are produced
4.2.2 Anaerobic respiration