photosynthesis overview
- energy for all life on Earth ultimately comes from photosynthesis
- oxygenic photosynthesis is carried out by:
- cyanobacteria
- 7 groups of algae
- all land plants - chloroplasts
- anoxygenic photosynthesis is carried outby certain bacteria
chloroplast
Thylakoid membrane
internal membrane
- contains chlorophyll and other photosynthetic pigments
- pigments clustered into photosystems
Chloroplast
Grana
stacks of flattened sacs of thylakoid membrane
Chloroplast
Stroma lamella
conncect grana
Chloroplast
Stroma
semiliquid surrounding thylakoid membranes
Light-dependent reactions
- require light
- capture energy from sunlight
- make ATP and reduce NADP+ to NADPH
Carbon fixation reactions or light-independent reactoins
- does not require light
- use ATP and NADPH to synthesize organic molecules from CO2
Pigments
- molecules that absorb light energy in the visible rang
- light is a form of energy
- Photon:a partile of light
- acts as a discrete bundle of energy
- energy content of a photon is inversely proportional to the wavelength of the light
- Photoelectric effect:removal of an electron from a molecule by light
absorption spectrum
- when a photon strikes a molecule, its energy is either:
- lost as head
- absorbed by the electrons of the molecule
- absorption spectrum:range and efficiency of photons molecule is capable of absorbing
2 types of pigments used in green plant photosynthesis
- Chloropylls
- Carotenoids
Chlorophyll a
- Main pigment in plants and cyanobacteria
- Only pigment that can act directly to convert light energy to chemical energy
- Absorbs violet-blue and red light
Chlorophyll b
- Accessory pigment or secondary pigment absorbing light wavelengths that chlorophyll a does not absorb
What is the structure of chlorophyll?
porphyrin ring
* complex ring structure with alternating double and single bonds
* magnesium ion at the center of the ring
* photons excite electrons in the ring
* electrons are shuttled away from the ring
Action spectrum
- relative effectiveness of different wavelengths of light in promoting photosynthesis
- corresponds to the absorption specturm for chlorophylls
Carotenoids
- carbon rings linked to chains with alternating single and double bonds
- can absorb phontons with a wide range of energies
- also scavenge free radicals - antioxidant
Phycobiloproteins
- important in low-light ocean areas
photosystem organization
Antenna Complex
- hundreds of accessory pigment molecules
- gather photons and feed the captured light energy to the reation center
photosystem organization
Reaction Center
- 1 or more chlorophyll a molecules
- passes excited electrons out of the photosystem
More in Depth
Antenna Complex
- also called light-harvesting complex
- light-harvesting complexes consist of a web of chlorophyll molecules linked together and held tightly in the thylakoid membrane by a matrix of proteins
- energy and not electrons are transferred
More in Depth
Reaction Center
- transmembrane protein: pigment complex
- when a chlorophyll in the reaction center absorbs a photon of light, an electron is excited to a higher energy level
- ** Light-energized electron** can be transferred to the primary electron acceptor, reducing it
- Oxidized chlorophyll then fills its electron “hole” by oxidizing a donor molecule
- water acts as weak electron donor releasing O2
Light-Dependent Reactions
Thylakoid Reaction
- Primary Photoevent
- photon of light is captured by a pigment molecule - Charge Separation
- energy is transferred to the reaction center; an excited electron is transferred to the an acceptor molecule - Electron Transport
- electrons move through carriers to reduce NADP+ - Chemiosmosis
- produces ATP
Cyclic Photophosphorylation
- in sulfur bacteria, only one photosystem is used
- generates ATP via electron transport
- anoxygenic photosynthesis
- excited electron passed to electron transport chain
- generates a proton gradient for ATP synthesis
Oxygenic Photosynthesisin Chloroplast
- photosystem I:
- functions like sulfur bacteria
- photosystem II:
- functions like non-sulfur bacteria
- can generate an oxidation potenital high enough to oxidize water
- the 2 photosystems carry out a noncyclic transfer of electrons that is used to generate both ATP and NADPH
Photosystem I Job
transfers electrons ultimatley to NADP+, producing NADPH
* electrons lost from photosystem I are replaced by electrons from photosystem II
