PHOTOSYNTHESIS TOPIC 3.8 and 8.2

3.8: Objectives State that photosynthesis involves the conversion of light energy into chemical energy. State that light from the Sun is composed of a range of wavelengths (colours). State that chlorophyll is the main photosynthetic pigment. Outline the differences in absorption of red, blue and green light by chlorophyll. State that light energy is used to produce ATP, and to split water molecules (photolysis) to form oxygen and hydrogen. State that ATP and hydrogen (derived from the photolysis of water) are used to fix carbon dioxide to make organic molecules. Explain that the rate of photosynthesis can be measured directly by the production of oxygen or the uptake of carbon dioxide, or indirectly by an increase in biomass. Outline the effects of temperature, light intensity and carbon dioxide concentration on the rate of photosynthesis.

Photosynthesis: Light energy is converted into chemical energy. Sugar molecules are produced by using CO 2 and H 2 O SL-HL

General formula of photosynthesis

What is light? Light is a form of the electromagnetic radiation produced by the Sun. Which of these colors does chlorohyll absorb and use for photosynthesis?

Where does photosynthesis take place in eukaryotic cell? SL-HL Where does photosynthesis take place in prokaryotic cell?

Chlorophyll SL-HL 3.8.3

SL-HL

ENGELMAN’S EXPERIMENT Which wavelengths are used in photosynthesis?

He used filamentous algae in a fresh water pond. He shone fine beams of white light and light of different wavelengths onto different parts of of the algae. He used aerobic bacteria as oxygen indicator. Where algal cells produce oxygen, bacteria migrated to certain of the illuminated regions. Bacteria accumulated around red and blue light region but not around green light. ENGELMAN’S EXPERIMENT 1882

What happens in photosynthesis?

Two main steps in photosynthesis 1- Light dependent reactions: Light is used to produce ATP by photophosphorylation. At the same time light is used to split water into oxygen and hydrogen (photolysis). These hydrogen protons are used in the second step to make sugar. Oxygen is given out. Oxygen is the waste product of the photosynthesis. 2- Sugars are built up from carbon dioxide. Carbon dioxide from atmosphere and ATP and hydrogen from light dependent reactions are used to make sugar.

How can we measure the rate of photosynthesis?

Either by measuring the amount of reactants. Or by measuring the amount of products.

Factors Affect Rate of Photosynthesis a) CO 2 concentration: The average CO 2 content of the air is 0.04%. As long as there is no limiting factors an increase in the CO 2 levels up to 0.5% usually result in an increase in the rate of photosynthesis. Above 0,1% can damage the leaves.

b) Light Intensity: It is directly proportional with light intensity. The graph levels off at one point because the photosynthetic pigments have become saturated with light and some other factors (limiting factors) like availability of CO 2, or amount of pigments..etc.

c) Temperature: Changes in the temperature has little effect on the light dependent reaction, WHY? Because, they are driven by light energy, not heat! However, the reactions of the Calvin Cycle are catalyzed by enzymes which are sensitive to temperature changes. Besides enzyme activity temp. increase is a stress condition in which plants secrete abscisic acid that leads stomata closure.

TOPIC 8.2 HL PHOTOSYNTHESIS

1- Draw and label a diagram showing the structure of a chloroplast as seen in electron micrographs. 2- State that photosynthesis consists of light-dependent and light- independent reactions. 3- Explain the light-dependent reactions. 4- Explain photophosphorylation in terms of chemiosmosis. 5- Explain the light-independent reactions. 6- Explain the relationship between the structure of the chloroplast and its function. 7- Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants. 8- Explain the concept of limiting factors in photosynthesis, with reference to light intensity, temperature and concentration of carbon dioxide. 8.2: Objectives

8.2.1

8.2.2

SL-HL

WHAT IS THE SOURCE OF OXYGEN ?

Redox in photosynthesis HL

8.2.3 LIGHT DEPENDENT REACTIONS a.Cyclic photophosphorylation b.Non-cyclic photophosphorylation

8.2.3 Light dependent reactions

Light Reactions ATP production takes place by photophosphorylation. There are 2 kinds of photophosphorylation. a.Cyclic p. b.Non-cyclic p.

Cyclic photophosphorylation Light energy is absorbed by chlorophyll a. Light energy moves electrons of chl a to higher energy level. When these electrons return to chl a they release energy which is used to make ATP by chemiosmosis.

Photosynthetic pigments fall into 2 categories. Primary pigments: chlorophyll a (with different absorption peaks) Accessory pigments: other forms of chlorohyll a, chlorophyll b, and caretenoids. They are arranged in light harvesting clusters: PHOTOSYSTEM The primary pigments are said to act as reactions centers.

Photosystem I; is arranged around a molecule of chlorophyll a with peak absorption at 700 nm (p700) Photosystem II; is based on a molecule of chlorophyll b with a peak absorption of 680 nm (p680)

Each photosystem contains molecules of pigments which serve as light trapping antenna. When light energy is absorbed by one of the pigments, it is bounced around other pigments of the photosystem until it reaches to chl.a which is the reaction center of the photosystem

Cyclic photophosphorylation Electrons of chl.a is excited by light energy. These electrons are taken and transferred by ETS proteins. During transfer of electrons ATP is produced by chemiosmosis. Electrons return to their original chl.

Cyclic photophosphorylation

Non-cyclic photophosphorylation

Electrons of chl.a do not return to chl a.They are given to NADP to form NADPH. Missing electrons of chl.a are replaced by elecrons that come from chl.b. Missing electrons of chl.b are replaced by elecrons that come from water. Both ATP and NADPH2 are produced.

chemiosmosis

When light energy is absorbed by chl.a e - of the chl. are boosted to a higher energy level The e -’ s are transferred by a primary e acceptor protein of the thylakoid membrane. The e - passes along the ETS of the thylakoid membrane As electron travels down this chain, the energy they released is used to pump protons (H + ) from stroma into thylakoid space

A proton gradient is established accross the membrane When the protons pass into the stroma through a special protein (ATP synthase). ATP is produced For each molecules of ATP produced, 2 e’s must travel down the ETS

Non-cyclic photophosphorylation Both chl a (photosystem I) and b (photosystem II) are used. Both chls absorb light energy and their electrons move to higher energy level. These energized electrons are transferred through ETS. During the transfer of electrons energy is released which is used to produce ATP by chemiosmosis. At the same time water molecules are broken down into H and O 2 by using light energy (photolysis)

CHEMIOSMOSIS 8.2.4

During transfer of electrons of chl a and b on ETS energy is released. This energy is used to accumulate H ions in the thylokoid space. As a result a H ion concentration gradient is generated (thylokoid side has more H + than stroma site. These H ions try to diffuse the stroma site. While they diffuse to the stroma site through a protein called ATP synthase ATP is produced.

RELATIONSHIP BETWEEN STRUCTURE OF CHLOROPLAST PHOTOSYNTHESIS 8.2.6

LIGHT INDEPENDENT REACTIONS CALVIN CYCLE 8.2.5

ACTION SPECTRUM and ABSORPTION SPECTRUM The action spectrum shows the rate of photosynthesis at different wavelengths. The absorption spectrum shows how strongly the pigments absorb at different wavelengths. The shorter the wavelength, the more energy it contains. During photosynthesis the light energy is converted into chemical energy. The absorbed light excites electrons in the pigment molecules and the energy can be passed on to be used by the plant.

WHAT ARE THE LIMITING FACTORS IN PHOTOSYNTHEIS? Temperature Carbon dioxide Light intensity Water Number of stomata Surface area of the leaf