PhotosynthesisPhotosynthesis

Photosynthesis

Method of converting sun energy into chemical energy usable by cells

done by autotrophs takes place in specialized

structures inside plant cells called chloroplasts

1. Capturing light energy2. Using captured energy to make ATP and NADP+

(energy carrying co-enzyme like NAD. It is reduced by 2 H atoms to NADPH+ + H+)

3. Using ATP and energy from NADPH to synthesize molecules like glucose.

first 2 steps require sunlight (light dependant) and occur in chlorophyll

last step does not necessarily need sunlight (light independent) and takes place in stroma

3 Steps of Photosynthesis

Light-Dependent Phases

occurs in thylakoid membrane light energy converted to chemical energy of

ATP & NADPH (reduced NADP = NADPH) photosystems (highly organized light capturing

complexes) are found in thylakoid membranes and made of 2 parts:

antenna complex – system of chlorophyll molecules and pigments that transfers energy to reaction centre

reaction centre – protein complex containing chlorophyll a that absorbs energy from antenna complex and raises it to high energy level to start photosynthesis

there are 2 kinds of photosytems (I and II) depending on which wavelength of light chlorophyll a absorbs

Steps in Light Dependent Reactions

1. Photoexcitation: Sunlight hits leaf and some energy passes into stroma

then absorbed by antenna complex of photosystem II and passed along to chlorophyll a.

2. Electron Transport: An electon gets boosted to a higher level and passed

along a ETC (like in cellular respiration) in thylakloid membrane.

This energy boost also causes H2O to split which releases H+ ions and O2 which is vital to other living things.

e- passes through several carriers via redox reactions releasing energy as they pass through the proteins. H+ ions are pumped from stroma into intermembrane space which creates electrochemical gradient.

3. Chemiosmosis: H+ ions move through ATPase complex back into stroma

converting ADP to ATP a process called photophosphorylation. It takes 4 H+ to make 1 ATP.

At same time light of a different wavelength is also striking photosystem 1. e- from P1 pass through another ETC, then move to enzyme NADP reductase that reduces NADP to NADPH which moves to Light Independent Phase.

Reactants: H2O, sunlight

Products: ½ O2, NADPH, ATP

YouTube - Light (Dependant) Reactions of Photosynthesis Animation

2. 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)

occur in stroma don’t require light also known as C3 Cycle

Steps in Light Independent Phase:

1. Carbon Fixation CO2 molecule combines with 5C molecule called

RuBisCO (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. RuBisCO Regeneration G3P phosphorylated by ATP to re-create RuBisCO 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 Fixation

Type of carbon fixation

Stomata open

Advantages Examples

C3 dayUses fewer

ATPMost plants

C4day Faster

photosynthesisCorn, sugar

cane, grasses…

CAMnight Water

efficiency

Succulents, cacti, orchids, pineapple…