The Chloroplast

Step 1: Light Dependent

Reactions

Photosynthesis

AHL – Chapter 8.2

Assessment Statements Day 1

� 8.2.1: Draw and label a diagram showing the structure of a choloroplast as seen in an electron micrograph.

� 8.2.2:State that photosynthesis consists of light-dependent and light-independent reactions.

� 8.2.3:Explain the light-dependent reactions.

(include the photoactivation of photosystem II, photolysis of water, electron transport, cyclic and non-cyclic photophosphorylation, photoactivation of photosystem I, and reduction of NADP+)

� 8.2.4 :Explain photophosphorylation in terms of chemiosmosis.

The Choloroplast� A photosynthetic machine!

� All of the stages of photosynthesis occur in the choloroplast.

� Most of the chloroplasts a plant contains are located in the leaves (the photosynthetic factory) of the plant;however, some plants do have chloroplasts in other cells.

� Choloroplasts and mitochondria, represent evidence for the endosymbiotic theory! (remember)

� Double membrane – indicates that perhaps they once needed protection from a potentially hostile environment

� Circular DNA

� 70S ribosomes

The Plastid Family

� There are three types of plastids that occur in plants

� Choloroplasts – green, involved in photosynthesis

� Leucoplasts – are white or “clear” and function as the

energy store-house (in tubers)

� Chromoplasts – are brightly coloured and synthesize

and store large amounts of orange, red, or yellow

pigments.

� All the plastids develop from a common protoplastid.

Review of 3.2

Photosynthesis = light energy from the Sun is used to

transform carbon dioxide and water into energy-rich food

molecules.

6CO2 + 12H2O C6H12O6 + 6H2O + 6O2Carbon

Dioxide

Water

Light

Energy

Glucose Oxygen Water

*Water occurs on both sides because 12 molecules are consumed and 6 molecules are

produced.

Review of core photosythesis

� http://www.abdn.ac.uk/~clt011/flash/samples/photo

syn.swfCellular Respiration = all of the chemical reactions needed to

break down (metabolize) carbohydrates and other molecules

to transfer chemical energy to ATP.

C6H12O6 + 6O2 6H2O + 6CO2 + ATPCarbon

Dioxide

Water

Light

Energy

Glucose Oxygen Energy

Storage

Molecule

What is Photosynthesis?

� Involves over 100 chemical reactions.

� Mostly, an anabolic process (building)

� Focus is to create chemical bonds that can produce organic compounds by using carbon dioxide, water and light energy from the sun.

� The overall process happens in two main stages:

�1. LIGHT-DEPENDENT STAGE� aka. PHOTO stage

�2. LIGHT-INDEPENDENT STAGE� aka. SYNTHESIS stage.

� Cellular Energy is stored as ATP

� Adenosine Tri Phosphate

� ATP contains 2 high energy bonds

How is energy stored in living things?

12

� Adding a phosphate group

to ADP stores energy in

ATP

� Removing a phosphate

group from ATP releases

energy & forms ADP

LoseLoseLoseLose

GainGainGainGain

Chlorophyll’s Role1. Chlorophyll absorbs red light, violet light, and

shades of blue.

2. Converts the absorbed energy into a form that the

synthesis reaction can use.

3. Chlorophyll can only work when it is in an

enzyme filled membrane organelle known as

the chloroplast!

14

Fall Colors

� In addition to the chlorophyll pigments, there are other

pigments present

� During the fall, the green chlorophyll pigments are

greatly reduced revealing the other pigments

� Carotenoids are pigments that are either red, orange, or

yellow

Chloroplast Structure

� Very small – 40 chloroplasts in 1mm.

� Yet, very powerful performing hundreds of reactions

in just 1 second.

� Watery substance called STROMA

� Increased surface area – from the many-folded

thylakoid membrane inside the chloroplast helps

these reactions occur.

http://www.ftexploring.com/photosyn/chloro

plast.html

http://dendro.cnre.vt.edu/forestbiology/phot

osynthesis.swf

� Should include:

� Thylakoid/grana

� Double outer membrane/envelope

� Ribosomes

� Circular DNA

� Lipid/starch granules

� Stroma

Enzymes (hey I thought we were done with that!)

� Proteins that promote or speed up chemical reactions without being used up themselves.

� Chlorophyll is not an enzyme but it can be recycled over and over again.

� The chlorophyll molecule absorbs light photons and then passes the energy onto other molecules.

Light dependent vs. light independent

Light Dependent Light Independent

Need a continual supply

of light

Need light indirectly

Produce intermediate

compounds used in the

light-independent

reactions

Produce organic

compounds such as

glucose and amino acids.

Intermediate compounds

are consumed during

darkness

Photosystem

Light energy behaves as if it

exists in discrete packets

called photons.

Shorter wavelengths of light

have greater energy within

their photons than longer

wavelengths.

These photon packets travel at 300 000 000m/s

The amount of energy in a photon depends on the frequency of light.

The higher the frequency the more energy the photon is able to deliver. (Red photon packets have less energy then violet photon packets)

http://www.stolaf.edu/people/giannini/flasha

nimat/metabolism/photosynthesis.swf

The Light-Dependent Reactions

� Occurs in the thylakoid or grana of the chloroplast (the

poker chips).

� Vocab: stacks of thylakoids are called granum or grana.

� Light supplies the energy for this stage

� Ultimate source of light = Sun!

� Plants have special molecules called pigments that help

absorb light energy travelling to the plant at different

wavelengths.

Light-dependent reactions

� Pigments are organized on the thylakoid membrane

into regions called photosystems.

� Photosystems include chlorophyll, molecules,

assessory pigments and a protein matrix.

� Photosystem contains a reaction center which

includes a pair of cholorphyll molecules, a matrix of

protein, and a primary electron acceptor.

Photosystems on the thylakoid membrane

� Light strikes the chlorophyll and light energy is

transferred to electrons which become excited they

pass on to the ELECTRON TRANSPORT CHAIN

The Light-Dependent Reactions

� What is it?

� A series of proteins embedded within the membrane of

chloroplast

� Electrons bounce from protein to protein

� As this happens, energy is released

� Light enters photosystem - Pigment acts as a collection

area allowing light to combine in the reaction centre

where a continuous supply of water is present.

� Two chlorophyll molecules breakdown the water into

oxygen and hydrogen.

� Producing more oxygen.

Electron Transport Chain

� As electrons are passed from protein to protein, they lose

some energy like losing water in a bucket brigade

Electron Transport Chain

� Electrons are recharged

� They then go through another electron transport

chain

� When they reach the bottom of this chain, they still

have lots of energy left

� Energy from electrons is not wasted.

� Electrons are picked up by a carrier and transported to the

STROMA of the chloroplast.

� The carrier is NADP+

� Energy is stored again then transferred to the stroma

Back to the electron transport chain (ETC)Photoactivation of PSII

� Hundreds of chlorophyll molecules are grouped together in thylakoid membranes – called photosystems

� Cholorophyll in the PSII absorbs light (most efficienct at 680nm) from the sun which excites electrons in the cholorophyll and raises them to higher energy levels.

� Excited electrons are passed from chlorophyll molecule to molecule and eventually to a chain of electron carriers.

� The chlorophyll that has absorbed light is said to be photoactivated.

Photolysis of water

� Once PSII gives away the excited electrons, it must replace them.

� Inside PSII, an enzyme splits water into two electrons, two H+s and an atom of oxygen. The splitting of water is called photolysis.

� Electons replace electrons lost by chlorophyll to the electron transport chain.

� The oxygen is given off as a waste product.

� The H+s remain in the thylakoid interior which lowers the pH and contributes to the chemiosmotic gradient. They are later used to phosphorylate ADP to form ATP.

� Lost electrons are replaced by splitting water

� This is called Photolysis (photo = light; lysis =

splitting)

Recouping the loses

+Water

2H Oxygen

Electron Transport

� Proteins embedded in the thylakoid membrane

transfer energy along a pathway in a series of REDOX

reactions.

� Energy is given up each time an electron passes from

electron carrier to the next.

� Electrons pass from PSII to PSI from PSI to NADP+, and

some energy is used to pump H+s from stroma to

thylakoid interior.

� This contributes to the chemiosmotic gradient and

ATP is produced.

There are two possible pathways for

electron flow:

Cyclic

Photophosphorylation

Non-cyclic

Photophosphorylation

�Energy from excited

electrons is used.

�Electrons flow from PSI to

ETS and back to PSI to

produce ATP only! (NADP is

not reduced).

�NO NADPH+ is produced.

�ONLY INVOLVES PSI

�ATP is produced using

energy from excited

electrons in the PSII.

�There is a one-way flow of

electrons from water to PSII

to ETS to PSI to NADP+

�TWO PRODUCTS RESULT:

NADPH+ H+ and ATP

Photoactivation of PSI

� PSI accepts an electron and can be photoactivated

using the sun’s light (best at 700nm). PSI passes

electrons along a second chain of carriers to NADP+ in

the stroma.

� NADP reductase enables the transfer of the electron

to NADP+

Reduction of NADP+

� NADP+ accepts two excited electrons and a H+ from

the stroma and is reduced to form NADPH.

Let’s Review with a few clips!

� http://dendro.cnre.vt.edu/forestbiology/photosynthe

sis.swf

� http://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter10

/animations.html#

Summary of the light dependent

reactions

1. Chlorophyll in PSII absorbs light energy

2. Electrons are excited and passed along chlorophyll molecules to a primary acceptor (P680) in the centre of PSII.

3. Electrons are passed along to ETC and are replaced by photolysis of water – oxygen is released.

4. As electrons are passed along, energy is given up which is used to synthesize ATP by chemiosmosis.

5. Electrons passed to PSI (P700) which can be photoactivated and pass electrons along a chain of carriers to NADP+ in the stroma.

6. NADP+ accepts two electrons and a hydrogen ion from the stroma and is reduced to form NADPH.

Overall concept….

� The ATP and the NADPH produced in the light-

dependent reactions are the intermediates that are

used in the next stage of photosynthesis to produce

organic molecules!

� All of that and we STILL HAVE TO MAKE GLUCOSE!

IMAGINE!!!!!

HOMEWORK:

� Explain the light-dependent reactions of

photosynthesis (8 marks)

A wee little reading and colouring!

� A break!

� http://faculty.nl.edu/jste/noncyclic_photophosphoryl

ation.htm

� http://tinyurl.com/cluruc

� But first…an animation!

� http://www.science.smith.edu/departments/Biology/

Bio231/ltrxn.html

The “Dark” Reactions

Topic 8.2

Light-Independent Reactions

� Does not require sunlight

� A.K.A. the Calvin Cycle

� Named after Melvin Calvin

� Carbon dioxide is used to form sugars

� Happens in the stroma

� ATP is used to make sugar

HOMEWORK: READ 8.2 on pages 227 to 232 take EXTENSIVE NOTES on light dependent and light independent reactions!!!

Light Independent Reactions

Let’s Talk about this slide, one more time!

Light-Independent Reactions

� The second part of the “photosynthesis story” ..the

let’s make glucose already part!

� aka the Calvin Cycle

� CO2 is converted into glucose in the stroma of the

choloroplast. (do you remember where the stroma is

on the EM?)

� Enzymes are used in a cyclic series of reactions called

the Calvin cycle.

RuBP (ribulose bisphosphate)

carboxylase

� A five carbon sugar called ribulose bisphosphate

(RuBP) is carboxylated using carbon dioxide which

enters the chloroplast by diffusion.

� An enzyme is involved called ribulose bisphosphate

carboxylase (commonly called rubisco).

� CO2 combines with RuBP in the stroma in the

presence of the enzyme to form an unstable six

carbon compound which immediately splits into two

molecules (each with three carbons).

� These smaller molecules are called glycerate 3-

phosphate.

� This process is called “Carbon Fixation”

Reduction of glycerate 3-phosphate (GP)

to triose phosphate (TP)

� The small 3 carbon molecule (glycerate 3-phosphate)

is now reduced using hydrogen supplied by NADPH

and energy from ATP (from the light dependent

reactions). A three carbon sugar called triose

phosphate is produced.

Regeneration of RuBP

� 5/6 of the triose phosphate is used to regenerate

RuBP in order for carbon fixation to continue.

� A series of reactions using ATP will produce 3

molecules of RuBP from 5 molecules of RuBP.

Synthesis of more complex carbs

� 1/6 of the triose phoshpate is used to produce glucose

phosphate when two triose phosphate molecules are

linked together.

� Glucose phosphate is converted into products such as

sucrose, starch, cellulose, amino acids and lipids.

I bet you are ready for an

animation!

� Calvin Cycle animation!

� http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

Summary of photosynthesis

Light Dependent Reactions Light Independent Reactions

Occurs in the thylakoid Occurs in the stroma

Uses light energy to form ATP

and NADPH

Uses ATP and NADPH to form

triose phosphate

Splits water in photolysis to

provide replacement electrons

and H+ and releases oxygen

into the atmosphere.

Returns ADP, Pi and NADP to

the light dependent reaction.

Includes two ETC and

photosystems I and II.

Involves the Calvin cycle.

A Visual of photosynthesis

A Song!� http://www.youtube.com/watch?v=_IV-E68rh18

Chloroplast Structure

� 8.2.6 – explain the relationship between the structure

of the chloroplast and its function.

� Thylakoid has a large surface area for the absorption

of light.

� The space between the thylakoid membranes is small

so H+ ions can accumulate and be used in

chemiosmosis to synthesize ATP.

� The fluid stroma contains the enzymes necessary for

the Calvin cycle.

Chloroplast

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