8.2 Photosynthesis

Topic 8 Cell Respiration & Photosynthesis

Photosynthesis 8.2.1 Draw and label a diagram showing the structure of a

chloroplast as seen in electron micrographs.

8.2.2 State that photosynthesis consists of light-dependent and light-independent reactions.(These should not be called “light” and “dark” 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+)

Photosynthesis 8.2.4 Explain photophosphorylation in terms of

chemiosmosis.

8.2.5 Explain the light-independent reactions.(Include the roles of ribulose bisphosphate (RuBP) carboxylase, reduction of glycerate 3-phosphate (GP) to triose phosphate (TP), NADPH + H+, ATP, regeneration of RuBP, and subsequent synthesis of more complex carbohydrates)

Photosynthesis 8.2.6 Explain the relationship between the structure of the

chloroplast and its function.

(Limit this to the large surface area of thylakoids for light absorption, the small space inside thylakoids for accumulation of protons, and the fluid stroma for the enzymes of the Calvin cycle)

Photosynthesis 8.2.7 Explain the relationship between the action spectrum

and the absorption spectrum of photosynthetic pigments in green plants.A separate spectrum for each pigment (chlorophyll a, chlorophyll b, and so on) is not required.

8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.

Chloroplast Structure Chloroplasts have an envelope, inner and outer membranes. Inside, thylakoid membranes form stacks called grana (granum – sing) Thylakoids provide a large surface area for light absorption. Thylakoids also have a small internal space. Chloroplasts have smaller ribosomes (70S) and circular DNA. The fluid filling the chloroplast is called the stroma. It contains the

enzymes for the Calvin Cycle.

Chloroplast Structure

Ref: Biology for the IB Diploma, Allott

Chloroplast Structure

Ref: Biology for the IB Diploma, Allott

Photosynthesis Photosynthesis is the process that plants, algae and some

bacteria use to produce all of the organic compounds they need.

Photosynthesis is a metabolic pathway, consisting of many chemical reactions.

Photosynthesis consists of; The light dependent reactions

produce intermediate compounds for the light independent reactions

The light independent reactions where glucose, amino acids and other organic compounds are

made.

Light Dependent Reactions Chlorophyll absorbs light and the energy from the light

raises an electron in the chlorophyll molecule to a higher energy level. An excited electron The chlorophyll is photoactivated

Chlorophyll is located in the thylakoid membrane and it is arranged in groups of hundreds of molecules called photosystems.

There are two types of photosystem: Photosystem I Photosystem II

Light Dependent Reactions These occur in the thylakoid membranes Photosystem II absorbs light energy. This energy is used to excite electrons from chlorophyll to a higher

energy level. The excited electron from photosystem II is passed along a chain of

electron carriers in the thylakoid membrane. The electrons lost from photosystem II are replaced by splitting water

– photolysis. Oxygen gas is released and the protons can either combine with NADP+ at the end of the pathway or be pumped into the thylakoid interior.

As the electrons are passed from one carrier to the next, they give up some of their energy. One of these is cytochrome, which acts as a proton pump to pump protons into the thylakoid interior

Light Dependent Reactions The protons then diffuse back through ATP synthase,

making ATP. This is called non-cyclic photophosphorylation. The coupling of electron transport to ATP synthesis is by

Chemiosmosis (like in the mitochondrion). The electrons are then passed to photosystem I, where

light energy is absorbed to boost them to a higher level. The electrons are passes through a short chain of carrieres

to NADP+, in the stroma, forming NADPH + H+.

Light Dependent Reactions Alternatively the electrons can fall back through carriers

to photosystem I. As the electrons flow along a chain of carriers they cause

pumping of protons across the thylakoid membrane. A proton gradient is formed and this allows production of

ATP by ATP synthase. ATP synthesised in this way is called cyclic

photophosphorylation.

Ref: Biology for the IB Diploma, Allott

Ref: IB Biology Higher Level, OSC

Photophosphorylation This is the synthesis of ATP using the energy from light.

The principle is the same as for oxidative phosphorylation in the mitochondria. Electrons flow through the carriers, including photosystem II

and I As the pass through the cytochrome carrier, protons are pumped

into the interior of the thylakoid. The thylakoid interior is small to increase the concentration of

protons. The protons then flow back by diffusion down a concentration

gradient into the stroma through ATP synthase ATP is generated from ADP + P

Photophosphorylation

Ref: IB Biology Higher Level, OSC

Cyclic and Non-cyclic Photophosphorylation The difference between cyclic and non-cyclic

photophosphorylation is that with non-cyclic, the electrons continue on the end in NADP+, whereas in cyclic they are returned to the chlorophyll in photosystem I.

The cyclic pathway has TWO advantages: It provides additional ATP needed to drive the light independent

reactions. The Calvin Cycle requires ATP and NADPH + H+ in the ratio of 3:2. Non-cyclic photophosphorylation gives a ratio of 1:1.

It provides ATP that can be used for other processes such as stomatal opening by guard cells.

Light Independent Reactions The light independent reactions are driven by the ATP

and NADPH + H+ made in the light dependent reactions. Both sets of reactions continue at the same time. The light independent reactions occur in the stroma. It is a cyclic system called the Calvin Cycle.

Ref: Biolgy, Campbell 7th ed.

Light Independent Reactions Carbon Fixation:

CO2 is an essential substrate in the Light In-dependent reactions.

It enters the chloroplast by diffusion. At the start of the Calvin cycle, CO2 combines with ribulose

bisphosphate (RuBP), a five carbon compound. This is a carboxylase reaction and the enzyme is called rubisco. The product of this reactions is a 6 carbon compound which

immediately splits to form TWO molecules of glycerate 3-phosphate.

Light Independent Reactions Carbohydrate Synthesis:

Glycerate 3-phosphate is converted into a carbohydrate by a reduction reaction.

Hydrogen is supplied by NADPH. Energy is also required in the form of ATP. Both NADPH and ATP were synthesised in the Light

Dependent Reactions. Gylcerate 3-phosphate is reduced to a 3 carbon sugar, triose

phosphate. 2 triose phosphate molecules are joined together to form

glucose phosphate. Condensation reactions join the glucose phosphate molecules

together to form Starch.

Light Independent Reactions Not all of the triose phosphate sugar molecules are used

to produce glucose. Some are used to resynthesise ribulose bisphosphate

(RuBP). In fact 5/6 of the triose phosphate molecules are used to

resynthesise RuBP. Only 1/6 of the triose phosphate molecules produce is use

to make glucose phosphate. Thus, 2 turns of the calvin cycle produce 1 molecule of

glucose phosphate.

Ref: IB Biology Higher Level, OSC

Ref: Biology for the IB Diploma, Allott

Action Spectrum of Photosynthesis A spectrum is a range of wavelengths of electromagnetic radiation

(the radiation emitted from the sun). The spectrum of visible light is the range of wavelengths from

400nm to 700 nm. Each wavelength is colour of light:

400-525 violet-blue 525-625 green-yellow 625-700 orange-red

The efficiency of photosynthesis is not the same in all wavelengths. The Action Spectrum shows the efficiency of the different

wavelengths of light that are used in photosynthesis.

Action Spectrum of Photosynthesis

The Absorption Spectrum of Photosynthesis The Absorption Spectrum of Photosynthesis shows how

much of a particular wavelength of light that are absorbed by the photosynthetic pigments.

The two most common pigments are chlorophyll a and chlorophyll b.

The action spectrum and absorption show strong similarities: Greatest absorption in violet-blue range. Also a high level of absorption in the red range. The lowest absorption in the yellow-green range.

There is a close correlation between the action spectrum and the absorption spectrum.

Limiting Factors There are 3 main factors that can affect the rate of

photosynthesis: Light intensity Temperature Concentration of Carbon dioxide.

Changes to one of these factors can affect the rate of photosynthesis.

The factor that is nearest to its minimum at any one time is called the Limiting Factor and this is the one that affects the rate of photosynthesis.

Rate limiting steps Different factors limit different steps in photosynthesis. Identify the limiting factor for:

1. Photolysis

2. Carbon fixation

3. Photophosphorylation

4. Reduction of NADP

5. Regeneration of RuBP

Why the light independent stage is not really light independent The light independent reactions rely on products of the

light dependent reactions. Without light there is no way to regenerate ATP and

NADPH Without ATP and NADPH GP cannot be converted to TP Without ATP RuBP cannot be regenerated from TP

IBO guide: 8.2.1 Draw and label a diagram showing the structure of a

chloroplast as seen in electron micrographs.

8.2.2 State that photosynthesis consists of light-dependent and light-independent reactions.(These should not be called “light” and “dark” 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+)

IBO guide: 8.2.4 Explain photophosphorylation in terms of

chemiosmosis.

8.2.5 Explain the light-independent reactions.(Include the roles of ribulose bisphosphate (RuBP) carboxylase, reduction of glycerate 3-phosphate (GP) to triose phosphate (TP), NADPH + H+, ATP, regeneration of RuBP, and subsequent synthesis of more complex carbohydrates)

IBO guide: 8.2.6 Explain the relationship between the structure of the

chloroplast and its function.

(Limit this to the large surface area of thylakoids for light absorption, the small space inside thylakoids for accumulation of protons, and the fluid stroma for the enzymes of the Calvin cycle)

IBO guide: 8.2.7 Explain the relationship between the action spectrum

and the absorption spectrum of photosynthetic pigments in green plants.A separate spectrum for each pigment (chlorophyll a, chlorophyll b, and so on) is not required.

8.2.8 Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide.