Photosynthesis Part 2

Chloroplasts• In green plants,

photosynthesis occurs within organelles called chloroplasts.

• Chloroplasts contain photosynthetic membranes arranged in flattened sacs called thylakoids which contain chlorophyll.

• Thylakoids are arranged in stacks called grana.

• The regions between the grana are called stroma.

2 Types of Photosynthetic Reactions

The Light Reactions• Takes place in the

presence of light (during the daylight hours).

• Light supplies the energy for these reactions to occur.

The Dark Reactions• These reactions occur

without light (during the night).

• However, the dark reactions depend upon the high energy chemical products produced during the light reactions.

Light and Dark Reactions Overview

The Light Reactions• Begin when chlorophyll in the thylakoids of

the chloroplast absorb light.

• The chlorophyll molecules in these thylakoid membranes are packaged into two photosytems.

-Photosystem I

-Photosytem II

• Each photosystem contains hundreds of chlorophyll molecules.

• Each chloroplast in a plant cell can contain millions of these photosystems!

The Light Reactions and Photosystems• Photosystems I and II are linked together in

structure and function.

• When photosystem II absorbs light, electrons are passed to an electron transport chain.

• This chain uses the electrons’ energy to make ATP.

• At the end of the chain electrons are passed to photosystem I.

• When photosystem I absorbs light, its electrons are passed to an electron carrier molecule NADP to make NADPH2

The Light Reactions and Photosytems• NADP is similar to the electron carrier NAD+

that we saw in cell respiration.• Photosystem II provides photosystem I with a

continuous supply of electrons for these reactions.

• When photosystem II absorbs light and loses electrons, it replaces the lost electrons by removing electrons from water (oxidizing water).

• As a result Oxygen is produced.• Here you can see directly how water is

necessary and oxygen is produced!

The Light Reactions and Photosystems• In light, both photosystems absorb light at

the same time.

• Photosystem I uses absorbed light to generate NADPH2 by reducing NAD+ .

• Photosystem II uses absorbed light to generate ATP via an electron transport chain.

• These two high energy products, ATP and NADPH2 are used to power the remaining photosynthetic reactions – the dark reactions

The Light Reactions and Photosystems

Dark Reactions

Dark Reactions and the Calvin Cycle• These dark reactions occur in the stroma of the

chloroplast.• CO2 which diffuses into the stroma from the air

outside is fixed, or used to make glucose.• The CO2 diffuses through tiny in pores in the

leaf called stomates or stomata.• The incorporation of CO2 into an organic

compound during photosynthesis is called Carbon fixation.

• Carbon Fixation: Occurs through a series of enzyme controlled reactions called the Calvin Cycle.

Calvin Cycle and Carbon Fixation• The starting and ending compound in the Calvin

cycle is a 5-Carbon sugar called Ribulose Biphosphate or RuBP.

• The cycle begins when CO2 reacts with RuBP producing 2 molecules of a 3 carbon compound called Phosphoglycerate or PGA.

• ATP produced from photosystem II, and NADPH2 produced from photosytem I in the light reaction are used to convert each PGA molecule to a molecule of phosphoglyceraldehyde or PGAL (G3P)

Calvin Cycle and Carbon Fixation

• Most of the PGAL (G3P)is used to make more RuBP so the cycle can continue.

• For every 6 molecules of CO2 that react in the cycle, 12 molecules of PGAL (G3P) are formed.

• 10 of these PGAL (G3P) molecules are used to make RuBP while 2 PGAL (3 carbon) molecules react to form 1 glucose molecule (6 carbon)

Calvin Cycle and Carbon Fixation