photosynthesis process (light and dark reactions)
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Hamza KhanFA16-RO2-019
Cell BiologyPhotosynthesis
Introduction
• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists– Autotrophs generate their own organic matter through
photosynthesis
– Sunlight energy is transformed to energy stored in the form of chemical bonds
(a) Mosses, ferns, andflowering plants
(b) Kelp
(c) Euglena (d) Cyanobacteria
Food Chain
OVERVIEW OF PHOTOSYNTHESIS
Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water
Carbondioxide
Water Glucose Oxygengas
PHOTOSYNTHESIS
• The Calvin cycle makes sugar from carbon dioxide– ATP generated by the light
reactions provides the energy for sugar synthesis
– The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose
LightChloroplast
Lightreactions
Calvincycle
NADP+
ADP+ P
The light reactions convert solar energy to chemical energy◦ Produce ATP & NADPH
Chloroplasts: Sites of Photosynthesis
• Photosynthesis– Occurs in chloroplasts, organelles in certain
plants– All green plant parts have chloroplasts and carry
out photosynthesis• The leaves have the most chloroplasts
• The green color comes from chlorophyll in the chloroplasts
• The pigments absorb light energy
In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts
A chloroplast contains: ◦ stroma, a fluid ◦ grana, stacks of thylakoids
The thylakoids contain chlorophyll◦ Chlorophyll is the green pigment that captures light for
photosynthesis
The location and structure of chloroplasts
LEAF CROSS SECTION MESOPHYLL CELL
LEAF
Chloroplast
Mesophyll
CHLOROPLAST Intermembrane space
Outermembrane
Innermembrane
ThylakoidcompartmentThylakoidStroma
Granum
StromaGrana
Mechanism of Photosynthesis
1) Light-dependent reaction (LIGHT Reaction)◦ Requires light◦ Occurs in chloroplast (in thylakoids)◦ Chlorophyll (thylakoid) traps energy from light◦ Light excites electron (e-)
Kicks e- out of chlorophyll to an electron transport chain Electron transport chain: series of proteins in thylakoid
membrane
Light-dependent reaction (LIGHT Reaction)◦ Energy lost along electron transport chain◦ Lost energy used to recharge ATP from ADP
◦ NADPH produced from e- transport chain Stores energy until transfer to stroma Plays important role in light-independent reaction
◦ Total byproducts: ATP, NADPH, O2
2) Light-independent reaction (Dark Reaction)◦ Does not require light◦ Calvin Cycle
Occurs in stroma of chloroplast Requires CO2
Uses ATP and NADPH as fuel to run Makes glucose sugar from CO2 and Hydrogen
Light Reaction
Cyclic Photophosphorylation Process for ATP generation associated with
some Photosynthetic Bacteria Reaction Center => 700 nm
Ph
oto
n
Ph
oto
n
Water-splittingphotosystem
NADPH-producingphotosystem
ATPmill
Two types of photosystems cooperate in the light reactions
Primaryelectron acceptor
Primaryelectron acceptor
Electron transport chain
Electron transport
Photons
PHOTOSYSTEM I
PHOTOSYSTEM II
Energy forsynthesis of
by chemiosmosis
Noncyclic Photophosphorylation Photosystem II regains electrons by splitting
water, leaving O2 gas as a by-product
The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)
2 H+ + 1/2
Water-splittingphotosystem
Reaction-center
chlorophyll
Light
Primaryelectronacceptor
Energyto make
Electron transport chain
Primaryelectronacceptor
Primaryelectronacceptor
NADPH-producingphotosystem
Light
NADP+
1
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The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane◦ The flow of H+ back through the membrane is harnessed by
ATP synthase to make ATP◦ In the stroma, the H+ ions combine with NADP+ to form
NADPH
Dark Reaction
The Calvin cycle proceeds in three stages: carboxylation, reduction, and regeneration
Carboxylation of the CO2 acceptor, ribulose-1, 5-biphosphate, forming two molecules of 3-phosphoglcerate.
Reduction of 3-phosphoglycerate to form glyceraldehyde-3-phosphate which can be used in formation of carbon compounds that are translocated.
Regeneration of the CO2 acceptor ribulose-1, 5-biphosphate from glyceraldehyde-3-phosphate
Rubisco – the enzyme ribulose biphosphate carboxylase/oxygenase
The affinity of Rubisco for CO2 is sufficiently high to
ensure rapid carboxylation at the low concentration of CO2 found in photosynthesizing cells
The negative change in free energy associated with
carboxylation of RuBP is large so the forward
reaction is favored.
RuBP
Rubisco will also take O2 rather than CO2 and
oxygenate RuBP – called photorespiration.
The rate of operation of the Calvin Cycle can be enhanced by increases in the concentration of its intermediates. That is the cycle is autocatalytic.
A summary of the chemical processes of photosynthesis
Light
Chloroplast
Photosystem IIElectron transport
chains Photosystem I
CALVIN CYCLE Stroma
Electrons
LIGHT REACTIONS CALVIN CYCLE
Cellular respiration
Cellulose
Starch
Other organic compounds
Thank You
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