1. Photosynthesis
the process by which light energy is converted to chemical bond energy and carbon is fixed into organic compounds. The general formula is:

2. Where does the energy to synthesise ATP come from?
Catabolic (breakdown) reactions
Redox (reduction/oxidation) reactions
The main way in which ATP is synthesised is by the removal of hydrogen atoms from intermediate compounds in a metabolic pathway
When two hydrogen atoms are removed from a compound, they are picked up by a HYDROGEN CARRIER or ACCEPTOR
We say the hydrogen carrier is reduced
Electrons from the hydrogen atoms are passed along carriers (Electron Transport Chain)
When a component of the chain receives one of the hydrogen atoms, we say it is REDUCED
When a component passes an electron on, we say it is OXIDIZED
Each of these redox reactions releases a small amount of energy and this energy is used to synthesise ATP

3. PHOTOSYNTHETIC PIGMENTS-absorb light energy and use it to provide energy to carry out photosynthesis
Plants contain two major groups of pigments, the chlorophylls and carotenoids.
Chlorophyll a whose double bonds play a critical role in the light reactions & are a source of the electrons that flow through the electron transport chains during photosynthesis.)
Chlorophyll b are green and absorb all wave­lengths of light in the red, blue, and violet range.

4. PHOTOSYNTHETIC PIGMENTS cont.-
Carotenoids are yellow, orange, and red. They absorb light in the blue, green, and violet range.
Xantho­phyll is a carotenoid with a slight chemical variation.

5. Chloroplasts stroma, where the light-independent reactions occur.
grana, where the light reactions occur.
The grana consist of layers of membranes called thylakoids- the site of photosystems I and II.
The chloroplast is enclosed by a double membrane.

6. Two main processes of photosynthesis
light dependent or light reactions-use light energy directly to produce ATP that powers the light-independent reactions
light independent reactions-consist of the Calvin cycle, which produces sugar.
To power the production of sugar, the Calvin cycle uses the ATP formed during the light reactions.
Both reactions occur only when light is present.

7. PHOTOSYSTEMS
Light-harvesting complexes in the thylakoid membranes of chloroplasts. (a few hundred in each thylakoid)
Consists of a reaction center containing chlorophyll a and a region containing several hundred antenna pigment molecules that funnel energy into chlorophyll a.
Two types cooperate in the light reactions of photosynthesis, PS I and PS II. (named in the order that they were discovered)
PS II operates first, followed by PS I.
PS I absorbs light best in the 700 nm range; also called P700.
PS II absorbs light best in the 680 nm range; also called P680.

8. LIGHT-DEPENDENT REACTIONS—THE LIGHT REACTIONS
Light is absorbed by the photosystems (PS II & I) in the thylakoid membranes and electrons flow through electron transport chains.
there are two possible routes for electron flow: noncyclic and cyclic photophosphorylation.

9. Noncyclic Photophosphorylation
During N.P.- electrons enter two electron transport chains, and ATP and NADPH are formed. The process begins in PS II and proceeds through the following steps:
Photosystem II—P680. Energy is absorbed by P680. Electrons from the double bonds in the head of chlorophyll a become energized and move to a higher energy level. They are captured by a primary electron acceptor.

10. Photosystem II—P680 (cont.)
Photolysis. Water gets split apart, providing electrons to replace those lost from chlorophyll a in P680.
into two electrons, two protons, and one oxygen atom.
Two oxygen atoms combine to form one 02 molecule, which is released into the air as a waste product of photosynthesis.

11. Photosystem II- P680 (cont.)
Electron transport chain- Electrons from P680 pass along an electron transport chain consisting of plastoquinone (PQ), a complex of two cytochromes and several other proteins, and ultimately end up in P700 (PSI).
This flow of electrons is exergonic and provides energy to produce ATP by chemiosmosis.
Because this ATP synthesis is powered by light, it is called photophosphorylation.

12. Chemiosmosis
The process by which ATP is formed during the light reactions.
Protons that were released from water during pho­tolysis are pumped by the thylakoid membrane from the stroma into the thyla­koid space (lumen). ATP is formed as these protons diffuse down the gradient from the thylakoid space, through the ATP-synthase channels, and into the stroma.
The ATP produced here provides energy for the Calvin cycle.
NADP becomes reduced when it picks up the two protons that were released from water in P680. Newly formed NADPH carries hydrogen to the Calvin cycle.
photosystem II

13. Photosystem I—P700
Energy is absorbed by P700. Electrons from the head of chlorophyll a become energized and are captured by a primary electron receptor.( similar to the way it happens in P680.)
Two differences are:
the electrons that escape from chlorophyll a are replaced with electrons from photosystem II, instead of from water.
This electron transport chain contains ferrodoxin and ends with the production of NADPH, not ATP.

14. Cyclic Photophosphorylation
The purpose of cyclic photophosphorylation is to produce ATP. No NADPH is pro­duced, and no oxygen is released.
The production of sugar during the Calvin cycle consumes enormous amounts of ATP, periodically, the chloroplast runs low on ATP. When it does, the chloroplast carries out cyclic photophosphorylation to replenish the ATP levels.
Cyclic electron flow takes photo-excited electrons on a short-circuit pathway. Electrons travel from the P680 electron transport chain to P700, to a primary elec­tron acceptor, and then back to the cytochrome complex in the P680 electron transport chain