Photosynthesis

Trapping Light Energy to Build Carbohydrates

Which organisms are photosynthetic?

Photoautotrophs Plants Algae Some bacteria

What is photosynthesis?

What is photosynthesis? Process by which plants use light energy to build

carbohydrates (food).

Chemical equation:

light + chlorophyll

6CO2 + 6H2O C6H12O6 + 6O2

carbon dioxide + water yields carbohydrate + oxygen gas

in the presence of light & chlorophyll

Why are plants mostly green? Chlorophyll

Green pigment Located in chloroplasts

Are they always green? No!

Think about the leaves in the fall – what happens to them?

Chlorophyll is the most dominant pigment so we see it most, but when it dies out the other pigments show through

Chloroplast Structure Chloroplast Membranes

Two of them Inner and outer

Both composed of a lipid bilayer

Thylakoids Disks of membranes Contain chlorophyll

Grana Stacks of thylakoids

Stroma Fluid inside the chloroplast

http://www.ualr.edu/botany/chloroplast.jpg

http://www.ftexploring.com

A closer look at where it all takes place…

Photosynthesis Reactions Light Reactions

Need light Take place in the thylakoids

Dark Reactions/Calvin Cycle Do not need light Take place in the stroma

Light Reactions

Trap Energy

GOAL: To make NADPH & ATP for the Calvin Cycle

BYPRODUCT: Oxygen gas

Stage 1: Energy is Captured from Sunlight

Chlorophyll absorbs light

Light energy excites chlorophyll electrons

Excited electrons enter the electron transport chain (stage 2B)

Stage 2A: Photolysis Photolysis: An enzyme splits water molecules

2H2O O2 + 4H+ + 4e- Oxygen is released by chloroplasts as a

waste product! Oxygen gas supplies nearly all the oxygen in the

atmosphere

Electrons replace excited electrons in chlorophyll of the thylakoids

Stage 2B: Electron Transport Chain

Excited electrons from chlorophyll and H+ from split water travel through the thylakoids and move to NADP+

Nicotinamide dinucleotide phosphate (you do not need to know this name)

Coenzyme: act as carriers in many biological processes (“the shuttle bus”)

NADPH is formed e- + H+ + NADP+ NADPH NADPH travels to the Calvin Cycle (Stage 3)

ATP is formed and travels to the Calvin Cycle (stage 3) to provide energy

The Products of the Light Reactions

The Light Reactions of Photosynthesis

e

H+

ATP

NADPH

Stage 3: Calvin Cycle

The Dark Reactions

GOAL: To make simple sugars

Carbon Fixation Calvin Cycle consists of carbon fixation

CO2 from the air is converted into a form that is usable by living things

“fixed” into 6 carbon sugars Eventually become glucose and other sugars

Calvin Cycle Reaction Basics Reactants

5-carbon molecule CO2

H from NADPH from Light Reaction’s splitting of H2O

Products Simple sugars

Used to make other carbohydrates

ATP from Light Reactions powers the Calvin Cycle

The Overall Net Result of Photosynthesis

The overall result of photosynthesis is that for every 6 carbon dioxide molecules (plus 6 water) that a plant uses, 1 glucose molecule (plus 6 oxygen) is created.

ELECTRON CARRIERS

Discussion Questions Where do the light reactions take place? What are the products of the light reactions? Where do the chlorophyll replacement electrons

come from? Where do the dark reactions take place? Where does the NADPH in the Calvin Cycle come

from? Where does the ATP in the Calvin Cycle come from? What are the products of the Calvin Cycle? What are chemoautotrophs?