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Chapter 7
PHOTOSYNTHESIS
A. LightVisible light makes up only a small portion
of the electromagnetic spectrum.
Sunlight consists of:
4% Ultraviolet (UV) radiation
44% Visible light 52% Infrared (IR)
radiation
Overview of PhotosynthesisPhotosynthesis- process by which plants, algae and some microorganisms harness solar energy to make biochemicals.
Occur in organelles – chloroplasts
Two stages – light reaction and carbon reaction
The products of photosynthesis, glucose and other carbohydrates – photosynthate.
Characteristics of Visible Light:• is a spectrum of colors ranging from
violet to red• consists of packets of energy called
photons • photons travel in waves, having a
measurable wavelength (λ) λ = distance a photon travels during a complete vibration [measured in nanometers (nm)]
A photon’s energy is inversely related to its wavelength......the shorter the λ, the greater the
energy it possesses.Which of the following photons
possess the greatest amount of energy?
Green photons λ = 530nm
Red photons λ = 660nm
Blue photons λ = 450nm
What happens to light when it strikes an object?
• reflected (bounces off)
Only absorbed wavelengths of light function in photosynthesis.
• transmitted (passes through)
• absorbed
B. Photosynthetic PigmentsMolecules that capture photon
energy by absorbing certain wavelengths of light.
1. Primary pigments• Bacteriochlorophyll - green
pigment found in certain bacteria.• Chlorophylls a & b - bluish green
pigments found in plants, green algae & cyanobacteria.
Chlorophyll a is the dominant pigment in plant cells.
2. Accessory Pigments• Carotenoids - red, orange, yellow pigments
found in plants, algae, bacteria & archaea.• Xanthophylls – red and yellow pigments
found in plants, algae & bacteria.• Fucoxanthin –brown pigment found in brown
algae, diatoms, & dinoflagellates• Phycoerythrin - red pigment found in red
algae.• Phycocyanin - blue pigment found in red
algae & cyanobacteria.• Bacteriorhodopsin – purple pigment found in
halophilic archaeaEach pigment absorbs a particular range
of wavelengths.
Light- form of energy- exists as photons. Photons possess different wavelengths that represent different energy levels.
Different wavelengths seen as different colors. Pigment molecules possess different abilities to absorb wavelengths and appear as different colors.
Chlorophyll is the major pigment molecule and appears as green.
Plants and other photosynthetic species use different pigments to absorb different wavelengths and use light more efficiently.
C. ChloroplastsSites of photosynthesis in plants &
algae.Concentrated in mesophyll cells of
most plants.
Chloroplast structure:
• Stroma - gelatinous matrix; contains ribosomes, DNA & various enzymes.
• Thylakoid - flattened membranous sac; embedded with photosynthetic pigments.
Chloroplasts – type of plastid- unique organelles with multiple layers which increase surface area to improve efficiency.
Chlorophyll is imbedded within the membrane layers in complexes that maximize the absorption and transduction of energy.
D. Photosynthesis
Occurs in two stages:• Light reactions - harvest photon
energy to synthesize ATP & NADPH. • Carbon reactions (Calvin cycle) -
use energy from light reactions to reduce CO2 to carbohydrate.
6CO2 + 12H2O C6H12O6 + 6O2 + 6H2O
Overview of Photosynthesis
Molecules in thylakoid membrane capture sunlight energy and transfer energy to molecules of ATP and NADPH. Enzymes of caebon reactions use this energy to capture CO2 abd build glucose.
1. Light Reactions• require light• occur in thylakoids of chloroplasts• involve photosystems I & II (light
harvesting systems).
Photosystems contain antenna complex that captures photon energy & passes it to a reaction center.
Light Reactions of Photosynthesis
ATP Production by Chemiosmotic Phosphorylation
•Light reactions of photosynthesis boost electrons into higher energy levels.
•Transfer them to the carriers NADPH and ATP for use in the cell.
•Additional energy is used to pump hydrogen ions into lumen of thylakoids- establishing gradient called proton motive force.
•Protons escape through a membrane-bound ATP synthase – uses the energy release to phosphorylate ATP (chemiosmotic phosphorylation)
•Electrons – ultimately replaced by converting water to protons and oxygen.
2. Carbon Reactions (Calvin cycle; C3 cycle) • do NOT require light (occur in both
darkness & light as long as ATP & NADPH are available)
• occur in stroma of chloroplasts• require ATP & NADPH (from light
reactions), and CO2
Calvin Cycle
Plants that use only the Calvin cycle to fix carbon are called C3 plants.
Ex. cereals, peanuts, tobacco, spinach, sugar beets, soybeans, most trees & lawn grasses.
Carbon fixation uses energy from ATP and NADPH to convert gaseous carbon dioxide to organic molecules such as glucose.
The enzyme system constantly recycles its components, forming Calvin cycle.
Key enzyme – rubisco attaches carbon to the carrier ribulose bisphosphate.
E. PhotorespirationProcess that counters
photosynthesis.Occurs when stomata close under
hot, dry conditions:• O2 levels in plant increase• CO2 levels in plant decrease
Under these conditions, rubisco fixes O2 (rather than CO2).Thus, PGAL is NOT produced.
F. C4 and CAM Photosynthesis
Adaptations that allow certain plants to conserve water and reduce photorespiration at higher temperatures.
1. C4 Photosynthesis
C4 plants reduce photorespiration by physically separating the light reactions and Calvin cycle.
Leaf anatomy of a C4 plant
C4 Photosynthesis:
• Light reactions occur in chloroplasts of mesophyll cells.
• Calvin cycle occurs in chloroplasts of bundle sheath cells.
2. CAM PhotosynthesisCAM plants reduce photorespiration
by acquiring CO2 at night.
Night:• mesophyll cells fix
CO2 as malic acid• malic acid is stored
in vacuoles.
Day:• malic acid releases
CO2 which enters Calvin cycle.
Malic acid
Inefficiency of rubisco causes photorespiration.
To live in hot climates, plants adept at manipulations that reduce photorespiration.
C4 plants use a intermediate to separate the light and carbon reactions from each other within different cell types.
Resulting in higher carbon dioxide concentration within bundle-sheath cells – reduce photorespiration.
CAM plants fix carbon at night when temperatures are lower and water loss is less of a problem.