Photosynthesis

Plants and other photoautotrophs use the following reaction to make food:

6 CO2 + 6 H2O + Energy C6H12O6 + 6 O2

In plants, the reactions take place in the chloroplast:

cross-section of a leaf

chloroplast

• The energy in the equation is provided by light from the sun.

• light strikes the leaf of a plant and is absorbed by pigments (called chlorophylls) in the chloroplast.

• the chlorophylls absorb certain wavelengths of light, and as a result, photosynthesis occurs best at these wavelengths.

The chlorophyll molecules are embedded in the thylakoid membrane, arranged into a network called a photosystem.

Chlorophyll molecules are called antenna pigments because they anchor themselves into the membrane here…

and stick out to absorb light energy using electrons in a porphyrin ring (up top).

Photosynthesis takes place in 2 stages:

1. The light dependent reactions - where energy from the sun is converted into chemical energy (in the form of ATP and NADPH - a mobile electron carrier).

2. The light independent reactions - where the energy made in stage 1 is used

to make organic compounds (glucose) from CO2

photosynthesis movie

The Light Dependent Reactions

• 3 parts:

• a photon is absorbed by a chlorophyll electron

• electron transport - the excited electron is moved through membrane carriers, pumping H+ to create a gradient and eventually reduces NADP+ to NADPH.

• H+ moves through ATP synthase to drive the formation of ATP from ADP.

Photosystems

• the primary light harvesting units of chloroplasts.

• in the chloroplast, chlorophyll molecules are found in clusters called photosystems.

• a photosystem consists of an antenna complex and a reaction centre - embedded in the thylakoid membrane.

• an antenna pigment absorbs a photon and transfers the energy from pigment to pigment until it reaches a chlorophyll a molecule in the reaction centre.

• the chlorophyll a molecule transfers this electron to a primary electron acceptor.

Electron Flow and Chemiosmosis

• Two photosystems (1 and 2) are used to make ATP and NADPH.

• photosynthesis starts when a photon strikes PS II.

• Z-protein splits water into 2 H+ , O2 and electrons.

• protons are pumped into the lumen (middle) of the thylakoid and eventually make ATP.

• the electrons leave the ETC by reducing NADP+ to form NADPH.

Light Independent Reactions (the Calvin Cycle)

• ATP and NADPH generated in light reactions used to fuel the reactions which take CO2 and break it apart, then reassemble the carbons into glucose.

• called carbon fixation: taking carbon from an inorganic molecule (atmospheric CO2) and making an organic molecule out of it (glucose)

• occurs in the stroma• don’t require light• also known as C3 Cycle

An overview – Don’t copy this – we’ll use the handout I already gave you.

Steps in Light Independent Phase:

1. Carbon Fixation • An enzyme, RuBisCo, combines a CO2 molecule

with a 5C molecule called RuBP (ribulose biphosphate) to produce 2, 3C molecules called 3-PGA (3-phosphoglycerate).

2. Reduction Reactions • 3-PGA phorphorylated by ATP then reduced by

NADPH to produce G3P (glycerate 3-phosphate).3. RuBP Regeneration • G3P phosphorylated by ATP to re-create RuBP to

restart the cycle. For every 2 G3P, 1 molecule glucose is removed from cycle, so need 6 CO2 to create 1 glucose.

Other Methods of Carbon FixationType of carbon fixation

Stomata open

Advantages Examples

C3 dayUses fewer

ATPMost plants

C4day

Faster photosynthe

sis

Corn, sugar cane,

grasses…

CAMnight Water

efficiency

Succulents, cacti, orchids, pineapple…