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

23

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

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