6. Chemistry of Photosynthesis

The Chemistry of Photosynthesis

lightenergy

water + carbon dioxide glucose + oxygen

Photosynthesis is the process by which organic molecules are synthesised by the reduction of carbon dioxide. The energy required for this comes from light energy. Light energy is absorbed by photosynthetic pigments.

Photosynthesis consists of two parts:• A Light-dependant stage called

photolysis• A Temperature-dependant stage

called carbon-fixation or The Calvin Cycle

Photolysis

PhotolysisLight energy is used to spit molecules of water into hydrogen and oxygen. This happens in the grana of choloroplast

Light energy splits water into hydrogen and oxygen – “photolysis”

Hydrogen combines with a hydrogen acceptor NADP to form NADPH2

Energy released regenerates ATP from ADP and Pi – “photophosphorylation”

The hydrogen carried by NADPH2 and the energy held by ATP are essential for carbon-fixation

Top Tip and Handy Hint

NADP is the hydrogen acceptor in Photosynthesis, don’t confuse with NAD in respiration

What you should know...

Carbon FixationThis temperature-dependant stage of

photosynthesis consists of several enzyme-controlled reactions forming a cycle called the Calvin Cycle.

CO2

(1C)

Unstable 6C compound

Glycerate Phosphate (GP) 3C

NADPH2

NADP

Triose phosphate (3C)

Glucose (6C)

Ribulose Bisphosphate (RuBP) 5C

Carbon dioxide entering the chloroplast combines with 5-carbon ribulose bisphosphate (RuBP)

Occurs in the Stroma This molecule is unstable and quickly

splits into two 3-carbon molecules of glycerate-3-phosphate

ATP and hydrogen (in NADPH2) from photolysis are used to produce 3-carbon triose phosphate sugar

Some of this is then converted to glucose (6 carbons)

Some triose phosphate are used to regenerate RuBP

Conversion of triose phosphate (3C) to RuBP (5C) requires energy from ATP

We need to know the number of carbon atoms in the key molecules:

Key Molecule Number of Carbon Atoms

Carbon dioxide

GP

RuBP

Glucose

We need to know the number of carbon atoms in the key molecules:

Key Molecule Number of Carbon Atoms

Carbon dioxide 1

GP 3

RuBP 5

Glucose 6

Summary:

Rate of photosynthesisThe rate of photosynthesis can be

calculated by measuring: Rate of oxygen production (counting

bubbles) Rate of CO2 uptake Production of carbohydrate (increase

in dry mass)

Limiting Factors= a factor that limits the rate of a

reaction by being in short supply

The rate of photosynthesis can be limited by:

Light Intensity Carbon Dioxide concentration Temperature

e.g.1 Plant kept in low light intensity (Graph ABC):

Between points A and B, rate of photosynthesis is limited by CO2 concentration

Between points B and C, rate of photosynthesis is limited by light intensity

e.g.2 Plant kept in higher light intensity (Graph ADE):

In Graph ADE, CO2 concentration is the limiting factor for a greater part of the graph (A-D, compared to A-B at low light intensity)

Light intensity only becomes the limiting factor at point D