By Mary ReyesLauren Dobry

& Haleigh Rousseau

Basic InformationPhotosynthesis involves the conversion of :

light energy chemical energy

Light from the sun is composed of a range of wavelengths (colors).

Chlorophyll is the main photosynthetic pigment.Light energy is used to produce ATP, and to split

water molecules (photolysis) to form oxygen and hydrogen.

ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules.

Generally, cell respiration is a catabolic process whilst photosynthesis is an anabolic one

Absorption of RED, BLUE, and GREEN Light by Chlorophyll

When a plant leaf is hit by sunlight, the BLUE and RED wavelengths of light are absorbed by chlorophyll and used for photosynthesis. However, almost all the energy of the GREEN wavelengths is reflected and not absorbed. Therefore, photosynthesis cannot occur when the leaf is hit solely by a green wavelength.

A chart showing the Action and Absorption Spectrums. BLUE, VIOLET, and RED light are most effective during the process of photosynthesis, since chlorophyll is the predominant photosynthetic pigment. Notice the similarities between the Action and Absorption Spectrums. The most absorbed wavelengths are the most active during photosynthesis, while the most reflected wavelengths are the least active.

Effect of changing light intensityThere is a positive correlation between light

intensity and photosynthesis until the enzymes and other factors are already working at their maximum rate. When this occurs, the rate of photosynthesis plateaus.

Effect of changing temperatureAs the temperature increases, the rate of

photosynthesis and the incidence of molecular collisions also increases. However, at a certain temperature, the enzymes and other proteins become denatured and the rate of photosynthesis drops tremendously until it becomes non-existent.

Effect of changing CO² concentrationThere is a positive correlation between

carbon dioxide concentration and photosynthesis, but the rate of reaction reaches a plateau unless light or temperature is also increased. Aquatic plants can alter the pH of the water during photosynthesis, raising it, due to CO² absorption.

Structure of the ChloroplastUnlike respiration, where some of the steps occur outside of the mitochondrion, all of the photosynthetic process occurs within the chloroplast. The chloroplast contains a double membrane and is about the same size as a bacterial cell. Like the mitochondrion, it contains its own DNA and 70S ribosomes. The DNA of the chloroplast is in the form of a ring. Besides DNA and ribosomes, the interior of the chloroplasts includes the grana, thylakoids, and stroma.

A granum is made up of numerous thylakoids stacked like a pile of coins. The thylakoids are flattened membrane sacs with components necessary for the absorption of light, which is the first step in the process of photosynthesis. The fluid stroma is similar to the cytosol of the cell. It occurs outside the grana but within the double membrane. Stroma contains many enzymes and chemicals necessary to complete the process of photosynthesis. Like mitochondria, chloroplasts are capable of reproducing independently of the cell.

Chloroplast Structure

Function Allowed

Extensive membrane surface area of the

thylakoids

Allows greater absorption of light by

photosystems

Small space (lumen) within the thylakoids

Allows faster accumulation of protons to create a concentration

gradient

Stroma region similar to the cytosol of the cell

Allows an area for the enzymes necessary for

the Calvin cycle to work

Double membrane on the outside

Isolates the working parts and enzymes of the

chloroplast from the surrounding cytosol

The First Stage of PhotosynthesisThis stage is a set of reactions typically

referred to as the light-dependent reactions.

During this stage, chlorophyll (and other photosynthetic pigments) absorb light energy and convert it into a form of chemical energy, specifically ATP.

Light energy is also used to accomplish a reaction called photolysis, during which a water molecule is split into its component elements: hydrogen and oxygen.

The oxygen that is split away due to the photolysis of water is typically released from the plant as a waste product.

Here is an animation that explains the specifics of the light-dependent reactions: http://www.youtube.com/watch?v=eY1ReqiYwYs

Respiration Chemiosmosis

Photosynthesis Chemiosmosis

Electron Transport Chain in membranes of cristae

Electron Transport Chain in membranes of thylakoids

Energy released when electrons are exchanged from one carrier to another

Energy released when electrons are exchanged from one carrier to another

Released energy used to actively pump hydrogen ions to intermembrane space

Released energy used to actively pump hydrogen ions to thylakoid

space

Hydrogen ions come from matrix Hydrogen ions come from stroma

Hydrogen ions diffuse back into matrix through channels of ATP

synthase

Hydrogen ions diffuse back into stroma through channels of ATP

synthase

ATP synthase catalyses the oxidative phosphorylation of ADP to ATP

 ATP synthase catalyses the phosphorylation of ADP to ATP

The Second Stage of Photosynthesis

The second stage is a series of reactions collectively known as the light-independent reactions or the Calvin Cycle.

ATP and hydrogen are used as forms of chemical energy to convert carbon dioxide and water into useful organic molecules for the plant.

Carbon dioxide is one of the few molecular forms of carbon that is considered to be inorganic.

Glucose, a typical product of photosynthesis, is an organic molecule. It requires six inorganic carbon dioxide molecules to form one glucose molecule.

This conversion of an inorganic form of an element to an organic form is known as fixation.

Therefore, photosynthesis can be described as a series of reactions in which carbon dioxide and water are fixed into glucose, and oxygen is produced as a by-product.

Fixation requires energy that comes directly from the ATP and hydrogen created in the first stage of photosynthesis.

Ultimately, the energy can be traced back to sunlight.

It is also important to note that glucose is only one of the many possible organic molecules that can form from photosynthesis.

Here is an animation that explains the specifics of the Calvin Cycle:

http://www.youtube.com/watch?v=mHU27qYJNU0&NR=1

Light-dependent Reaction

Light-independent

ReactionOccurs in the

thylakoidsOccurs 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 to release oxygen to the atmosphere

Returns ADP, inorganic phosphate

and NADP to the light-dependent

reaction

Includes two electron transport chains and photosystems I and II

Involves the Calvin cycle

• This is an alternate way creating ATP during the light-dependent reaction. It only occurs when light is not the limiting factor of photosynthesis, and when there are large amounts of NADPH in the chloroplasts.

• Light energized electrons from photosystemI go to the cytochrome complex and finish the last leg of the electron transport train.

• ATP is then created via chemiosmosis • NADPH is not created because the electrons

do not go through the second electron transport chain

Which two colors of light does chlorophyll absorb most?

The answer

was D

GOOD JOB

How is the proton gradient generated in chloroplasts during photosynthesis?

The answer

was A

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Plants living in water can cause a pH change in the water when they carry out photosynthesis. What is the pH change and what causes it?

The answer

was B

GOOD JOB

Why is the action spectrum for photosynthesis similar to the absorption of photosynthetic pigments?

The answer

was C

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What is the advantage of having a small volume inside the thylakoids of the chloroplast?

The answer

was A

GOOD JOB

Which diagram represents the process of cyclic photophosphorylation?

The answer

was B

GOOD JOB

Which graph shows the effect of increasing carbon dioxide concentration on the rate of photosynthesis?

The answer

was B

GOOD JOB

Which line shows an action spectrum of photosynthesis?

The answer

was A

GOOD JOB

What is occurring at X during the light-dependent reaction in photosynthesis?

The answer

was A

GOOD JOB