Chapter 7 - Photosynthesis

• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists

– They generate their own organic matter through photosynthesis..

(a) Mosses, ferns, andflowering plants

(b) Kelp

(c) Euglena(d) Cyanobacteria

Meso-phyll

Vein

Stomata

Leaf cross section Mesophyll cell

Chloroplast

Innermembrane

Outermembrane

Grana Stroma Thylakoid Thylakoidspace

• Photosynthesis occurs in the chloroplasts, which:– Are found in the cells of the mesophyll– Contain stroma, a thick fluid– Contain thylakoids, membranous sacs in stacks

called grana..

• The reactants and products of the reaction..

The Overall Equation for Photosynthesis

Lightenergy

Carbondioxide

WaterPhoto-

synthesis Glucose Oxygengas

• Photosynthesis is a two-step process:– Light reactions – require sunlight energy

to split water molecules into 2H and O2

– Calvin Cycle (dark reactions) – hydrogen generated from the light reactions is used to reduce CO2 to generate sugar – does not require sunlight• Leaves absorb CO2 through stomata..

LightChloroplast

Lightreactions

Calvincycle

NADP

ADP+ P

• Chlorophyll molecules absorb photons– Electrons in the pigment gain energy

– The energy is released and used..

Excitedstatee

Heat

Light

Photon

Light(fluorescence)

Chlorophyllmolecule

Groundstate

1 2

(a) Absorption of a photon

(b) fluorescence of isolated chlorophyll in solution

(c) Fluorescence of a glow stick

• The light reactions in the thylakoid membrane

2NADP+ 2NADPH2

– 2H2O O2 ..

Light Light

To Calvin Cycle

Stroma

Thylakoidmembrane

Electron transport chainATP

synthase

Inside thylakoid

1/2

NADPADP + P

Light Reactions

• The Calvin cycle– Functions like a sugar factory within a

chloroplast– Regenerates the starting material with each turn

6NADPH2 6NADP + 12H

6CO2 C6H12O6

The Calvin Cycle: Making Sugar From Carbon Dioxide

Input

3 CO2

6 3-PGA

6

6 ADP + 6

6

6 NADP

6 G3P

Glucose andother compoundsOutput

5 G3P

3

3 ADP

3 RuBP

Calvincycle

1 G3P

• Photosynthesis has an enormous impact on the atmosphere– It swaps CO2 in the atmosphere for O2

Why do we need the O2?..