• Almost all plants are photosynthetic autotrophs,

– 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

THE BASICS OF PHOTOSYNTHESIS

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

WHYWHY ARE ARE PLA PLANTS NTS GREGREEN?EN?

Plant Cells have Green Chloroplasts

The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

• Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

AN OVERVIEW OF PHOTOSYNTHESIS

Carbondioxide

Water Glucose Oxygengas

PHOTOSYNTHESIS

• Chloroplasts absorb light energy and convert it to chemical energy through a serious of reactions that occur in chloroplast.

• The reactions of photosynthesis or two types.

LightReflected

light

Absorbedlight

Transmittedlight

Chloroplast

1.3 Mechanism of Photosynthesis

Types of reactions

Light Reaction

(Hill reaction)Dark Reaction

The reactions of photosynthesis that occur in the presence of light is called light reaction

During this phase the energy required for Carbon fixation is generated.

The reactions of photosynthesis that occur in the absence of light is called dark reaction

.

• To understand photosynthesis let us know the reactions that occur in the presence of light. i:e Oxidation and reduction process

Oxidation:

1. Addition of oxygen atom

2. removal of hydrogen atom

3. removal of electrons from atom

Reduction:

1. Removal of oxygen atom2. Addition of hydrogen atom3. Addition of electrons from atom

1.Light Reactions

• When a substance is oxidised the hydrogen and electrons removed from this substances are transformed to another substance.

• The substance which accepts hydrogen and electrons get reduced.

• The substances which gives electrons are called DONAR and accepts electrons are called ACCEPTOR.

• DH2 + A D + AH2

Reduced Donar

Oxidized acceptor

Oxidized Donar

Reduced Acceptor

• These acceptors which accept electrons are called ELECTRON ACCEPTORS.

• Eg: Ferridoxins Adenine dinucleotide ( NAD)

Nicotinamide adenine dinucleotide phosphate ( NADP)

Plastoquinones

Cytochromes

(Fan in Personal computer)

When an electron acceptor is reduced it may transfer the Hydrogen and electrons to another acceptor molecule

Mechanism of photosynthesis1.During Ps Chlorophyll molecule traps solar energy and gets oxidized

• When light falls on the chlorophyll, energy present in the photons is

absorbed by the chlorophyll molecule.• This energy pushes an electron in the chlorophyll molecule to a

higher energy level.• This electron from chlorophyll is transferred to an electron acceptor

as a result chlorophyll gets oxidized by loosing an electron and the acceptor gets reduced by accepting the electron from chlorophyll.

• Thus the energy present in the photon of the sun light is used to eject an electron from chlorophyll. This is the basic mechanism by which solar energy is trapped by chlorophyll.

• Chlorophyll Chlorophyll* chlorophyll + ẽ

• A + ẽ A--

+ ve

(Low energy state ) (High energy state) (Oxidized) ( Electron)

(Acceptor oxidized) (electron) (Acceptor reduced

2.Chlorophyll splits water molecule and gets back its electron from water

• With the removal of electron, chlorophyll is in oxidation state.• This oxidising power of chlorophyll is used for splitting of water

molecule to liberate electrons.• By accepting these electrons, chlorophyll returns to its original state.• Oxygen is formed when the chlorophyll splits water molecule.• This oxygen escapes in to atmosphere.

• 2 H2O 4 H+ + 4 ẽ - + O2

• 4 ẽ - + 4 chl 4 chl

• 2 H2O + 4 Chl 4 Chl + 4H+ O2

• In this reaction water molecule is split by light activated chlorophyll . Hence this process is called PHOTOLYSIS of water

• Light does not split water molecule directly it acts through chlorophyll molecule in PS II

(Water) (Protons) (Electrons) (oxygen)

(Water) (Chlorophyll oxidised) (Chlorophyll reduced)

3.ATP AND NADPH are formed during photosynthesis

• Protons (derived from splitting of water molecule) are left behind are accumulate in thylokoids.

• When their concentrations becomes sufficiently high they are transported across the thylokoid membrane in to stroma.

• The energy in the movement of protons is used to produce ATP.• The electrons from PSII are taken up by PS I through a serious of

other acceptor molecules in the PS I, these electrons are transferred to NADP to produce NADPH.

• Up to this stage all the reactions occur only when light is present. There fore all these reactions are called LIGHT REACTIONS.

• Oxygen , ATP and NADPH are the end products of light reactions.

Light

NADPH

OH-2 H+

CARBOHYDRATES

Co2

½ O2NADP

Formation of carbohydrates and NADPH

1.3.2 The Dark reaction or Carbon fixation

• Co2 Glucose in a series of reactions that occur in stroma(Chloroplast).

• ATP & NADPH produced in the light reactions are used in these reactions.

• The entire reaction from Co2 to productions of glucose is observed by American scientist Melvin Kelvin.

• So it is called “Calvin Cycle. Calvin received Nobel prize fro this work.

Calvin Cycle 6 Co2 ( 1 mol)

6 Ribulose 1, 5 Di phosphate

6 Hexose Sugar 1,5 Diphosphate

2 PGA

Glyceraldehyde 3 – Phosphate

Glucose

Ribulose – 1, 5 Diphosphate

Corbon Sugar (6 mol) + 2 Phosphates Attached to it

6 molecules of 6 carbon sugar phosphate Is formed

6 HS 1,5 D is highly unstable compound and itBreaks down in to 2 Phosphoglyceric Acid

(Each PGA has 3 Carbon atoms)

ATP & NADPH are produced in Light reaction Are used up at this stage

(3 Carbon atoms)

2 mol of G 3 P (2 x 3=6 C atoms are used to produce 1 mol Glucose ( 6 c atoms)

(Glucose is converted to Starch)

10 mol of G 3 P (10 x 3 = 30 C ) are used to regenerate 6 mol of R 1, 5 D ( 6 x 5 = 30 C atoms)

2 H + OH - 1/2

Water-splittingphotosystem

Reaction-center

chlorophyll

Light

Primaryelectronacceptor

Energyto make

Electron transport chain

Primaryelectronacceptor

Primaryelectronacceptor

NADPH-producingphotosystem

Light

NADP

1

23

How the Light Reactions Generate ATP and NADPH

• The production of ATP by chemiosmosis in photosynthesis

Thylakoidcompartment(high H+)

Thylakoidmembrane

Stroma(low H+)

Light

Antennamolecules

Light

ELECTRON TRANSPORT CHAIN

PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

• 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

AN OVERVIEW OF PHOTOSYNTHESIS

PHOTOSYNTHESIS

• Sunlight provides ENERGY

CO2 + H2O produces Glucose + Oxygen

6CO2 + 6H2OC6H12O6 + 6O2

Steps of Photosynthesis

• Light hits reaction centers of chlorophyll, found in chloroplasts

• Chlorophyll vibrates and causes water to break apart.

• Oxygen is released into air• Hydrogen remains in chloroplast

attached to NADPH

• “THE LIGHT REACTION”

Steps of Photosynthesis

• The DARK Reactions= Calvin Cycle

• CO2 from atmosphere is joined to H from water molecules (NADPH) to form glucose

• Glucose can be converted into other molecules with yummy flavors!

• 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

Photosynthesis occurs in chloroplasts

• The location and structure of chloroplasts

LEAF CROSS SECTION MESOPHYLL CELL

LEAF

Chloroplast

Mesophyll

CHLOROPLAST Intermembrane space

Outermembrane

Innermembrane

ThylakoidcompartmentThylakoidStroma

Granum

StromaGrana

• Chloroplasts contain several pigments

Chloroplast Pigments

– Chlorophyll a – Chlorophyll b – Carotenoids– Xanthophyll

Figure 7.7

Chlorophyll a & b•Chl a has a methyl group •Chl b has a carbonyl group

Porphyrin ring delocalized e-

Phytol tail

Different pigments absorb light differently

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-)

Plants produce OPlants produce O22 gas by splitting H gas by splitting H22OO

• Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons

• The excited electrons are passed from the primary electron acceptor to electron transport chains– Their energy ends up in ATP and NADPH

In the light reactions, electron transport In the light reactions, electron transport chains generate ATP, NADPH, & Ochains generate ATP, NADPH, & O22

• 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

Chemiosmosis powers ATP synthesis in the light reactions

Summary—Light Dependent Reactions

a. Overall inputlight energy, H2O.

b. Overall output ATP, NADPH, O2.

• Animation is of the Calvin Cycle Note what happens to the carbon dioxide and what the end product is.

• Second animation of the Calvin Cycle is very clear and even does the molecular bookkeeping for you.

Light Independent Reactions aka Calvin Cycle

Carbon from CO2 is converted to glucose

(ATP and NADPH drive the reduction

of CO2 to C6H12O6.)

Light Independent Reactions aka Calvin Cycle

CO2 is added to the 5-C sugar RuBP by the enzyme rubisco.

This unstable 6-C compound splits to two molecules of PGA or 3-phosphoglyceric acid.

PGA is converted to Glyceraldehyde 3-phosphate (G3P), two of which bond to form glucose.

G3P is the 3-C sugar formed by three turns of the cycle.

Summary—Light Independent Reactions

a. Overall input CO2, ATP, NADPH.

b. Overall output glucose.

Review: Photosynthesis uses light energy to make food molecules

Light

Chloroplast

Photosystem IIElectron transport

chains Photosystem I

CALVIN CYCLE Stroma

Electrons

LIGHT REACTIONS CALVIN CYCLE

Cellular respiration

Cellulose

Starch

Other organic compounds

• A summary of the chemical processes of photosynthesis

Types of Photosynthesis

C3

C4

CAM

Rubisco: the world’s busiest enzyme!

Competing Reactions

• Rubisco grabs CO2, “fixing” it into a carbohydrate in the light independent reactions.

• O2 can also react with rubisco, inhibiting its active site– not good for glucose output– wastes time and energy (occupies

Rubisco)

Photorespiration

• When Rubisco reacts with O2 instead of CO2

• Occurs under the following conditions:– Intense Light (high O2 concentrations)

– High heat

• Photorespiration is estimated to reduce photosynthetic efficiency by 25%

Why high heat?• When it is hot, plants close their

stomata to conserve water

• They continue to do photosynthesis use up CO2 and produce O2 creates high O2 concentrations inside the plant photorespiration occurs

C4 Photosynthesis

• Certain plants have developed ways to limit the amount of photorespiration – C4 Pathway*– CAM Pathway*

* Both convert CO2 into a 4 carbon intermediate C4 Photosynthesis

Leaf Anatomy

• In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells.

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

Mesophyll cells

Bundle sheath cells

C4 Pathway

• In C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells.

Image taken without permission from http://bcs.whfreeman.com/thelifewire|

C4 Pathway

• CO2 is fixed into a 4-

carbon intermediate

• Has an extra enzyme– PEP Carboxylase that initially traps CO2

instead of Rubisco– makes a 4 carbon intermediate

C4 Pathway

• The 4 carbon intermediate is “smuggled” into the bundle sheath cell

• The bundle sheath cell is not very permeable to CO2

• CO2 is released from the 4C malate goes through the Calvin Cycle

C3 Pathway

How does the C4 Pathway limit photorespiration?

• Bundle sheath cells are far from the surface– less O2 access

• PEP Carboxylase doesn’t have an affinity for O2 allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM Pathway

• Fix CO2 at night and

store as a 4 carbon molecule

• Keep stomates closed during day to prevent water loss

• Same general process as C4 Pathway

How does the CAM Pathway limit photorespiration?

• Collects CO2 at night so that it can be more concentrated during the day

• Plant can still do the calvin cycle during the day without losing water

Summary of C4 Photosynthesis

• C4 Pathway– Separates by

space (different locations)

• CAM Pathway– Separates

reactions by time (night versus day)