Download Photosynthesis Outline | Date: Energy conversions Autotrophs o

Photosynthesis Outline | Date:
Energy conversions
Autotrophs
o
Producers
o
Photoautotrophs
Heterotrophs
o
Consumers
Plants
Structures
o
Leaves
o
Stomates
o
Roots
o
Chloroplasts

Double membrane

Stroma

Thylakoids

Chlorophyll
Pigments
o
Light
o
Types

Chlorophyll
Central atom: magnesium

Carotenoids
Photosynthesis
Reaction
Oxygen comes from split water molecules
o
Provides electrons
Two stages
o
Light reaction
o
Calvin cycle
Redox reactions
o
Oxidation
o
Reduction
o

Splitting water releases electrons and H+

These bond to CO2, reducing it to sugar

Electrons increase the potential energy

Light provides extra free energy
OIL RIG
Light reaction
o
Thylakoids
o
Solar  chemical energy
o
Net products: NADPH, ATP, oxygen
o
Main events

Light absorbed

Electrons transferred from H 2O to NADP+, forming NADPH

Water is split, O2 released

o
o
ATP is generated through photophosphorylation
Light

Electromagnetic energy made up of photons

Pigments absorb light of different wavelengths

Absorption spectrum

Action spectrum
Photosystems

Groups of pigment molecules in thylakoids; absorbs photons
Light-harvesting complex: chlorophyll and carotenoid
o
Photons excite the electrons of chlorophyll
Reaction center: chlorophyll only
o
Conversion of light  chemical energy

Thylakoids contain two photosystems, named photosystem I and photosystem II

Linear (noncyclic) electron flow
Predominant route
Overview
o
Sunlight energizes electrons, generating ATP as they are passed PS II to PS I
o
Excited again in PS I, and transferred from NADP +  NADPH
o
NADPH and ATP are used in the Calvin cycle
Specific steps
o
1: PS II absorbs light, an electron is excited and lost, and chlorophyll now has an
“electron hole”
o
2: Enzyme splits H2O into 2 H+, 2 electrons, and O. The O combines with another
O atom, forming atmospheric O2.
o
3: Original excited electron passes from PS I to PS II through an electron
transport chain (similar to cell respiration). ATP is made through chemiosmosis.
o
4: Energy from electron transfer pumps protons into the thylakoid space, creating
a buildup (gradient) of H+. H+ ions diffuse through ATP synthase, generating
ATP to be used in the Calvin cycle.
o
5: The electron from PS II ends up in PS I, which has just lost an electron due to
light energy.
o
Excited electrons are passed to another electron transport chain, converting
NADP+ into NADPH. This will also be used in the Calvin cycle.

Cyclic electron flow
PS I only – uses a short circuit of linear electron flow by cycling the electrons back to
their original PS I starting point
Creates equal ATP and NADPH, but the Calvin cycle requires more ATP
Electrons are rerouted back to the electron transport chain from PS I to produce more
ATP
Uses chemiosmosis to produce ATP, but does not make NADPH, oxygen, and does not
use water

Chemiosmosis
The way chloroplasts and mitochondria generate ATP
Basic steps:
o
Electron transport chain uses flow of electrons to pump H+ across thylakoid
membrane.
o
A proton-motive force is created within the thylakoid space that is used by ATP
synthase to convert ADP  ATP (phosphorylation). It is generated in:

H+ from water

H+ pumped across the membrane

Removal of H+ from the stroma when NADP+  NADPH
Dark reaction (Calvin cycle)
o
o
Overview

Occurs in stroma

Carbon fixation: CO2 from air is used in organic molecules

Uses fixed carbon, NADPH, and ATP from light reactions to form new sugars

Actual product is glyceraldehyde 3-phosphate (G3P), which then forms glucose/sugar
Steps

Carbon fixation: 3 CO2 + RuBP (ribulose bisphosphate, a 5-carbon sugar)
Catalyzed by rubisco (enzyme – RuBP carboxylase)

Reduction: 6 ATP and 6 NADPH used to produce 1 net G3P (glyceraldehydes 3-phosphate)
One G3P leaves to be used by the plant cell (two are needed to form glucose)

Regeneration: 3 ATP regenerate RuBP
Remaining 5 G3P become starting molecules, and the cycle continues
o
Endergonic Reaction

The formation of one net G3P requires 9 ATP and 6 NADPH, both replenished by light reactions

One of the 6 total G3P that are made is a net gain, and will be used for biosynthesis or energy
Photosynthesis Alternatives
Problem with photosynthesis and C3 plants
o
CO2 and H2O enter and exit via stomates (stomata)
o
Hot, dry climate – close stomates, less sugar because of low CO2
o
Rubisco binds to O2 in place of CO2, causing breakdown (oxidation) of RuBP (starting molecule)

Low RuBP = low energy and carbon = photorespiration
This can drain up to 50% of the carbon fixed by the Calvin cycle
Adaptations to arid climates – metabolic adaptations reduce photorespiration
o
C4 plants
o
CAM plants
C3
Description
Enzyme
C4
CAM