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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?
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