Chapter 11 PhotosynthesisSyllabus Syllabus -- Objectives-- Objectives

Define the term: photosynthesis. Express photosynthesis as a balanced reaction. State the nature of photosynthesis from the syllabus – what are the main

events? State the role & location of chlorophyll. Explain the nature of electron carriage. Identify the sources of light , CO2 & water for photosynthesis. Explain how human intervention can play a role in photosynthesis. Give a detailed account of the stages in photosynthesis: Light Stage 1,

Light Stage 2, Dark Stage. Carry out experiment to show that light is necessary for photosynthesis

Role of Photosynthesis Plants use it to make food Animals get their food from plants by eating them It produces oxygen which is needed in respiration to

release energy It is responsible for forming fossil fuels e.g. oil It removes carbon dioxide from the air

Balanced Equation for Photosynthesis

Key Facts: The sun is the main source of energy for the earth

Autotrophs make their own food and most are green plants

Plants make their food by carrying out photosynthesis

Cells need energy to carry out all the cell activities.

The energy they use is in the form of ATP

ATP (Adenosine Tri Phosphate) is a small package containing energy.

In photosynthesis light is used to make ATP

Learning Check Learning Check

What is photosynthesis?What is photosynthesis?Why is it important to all life?Why is it important to all life?What do plants require for photosynthesis?What do plants require for photosynthesis?What do plants produce in photosynthesis?What do plants produce in photosynthesis?What kind of energy is light energy changed into in What kind of energy is light energy changed into in photosynthesisphotosynthesis

Main Events in Photosynthesis

1. 1. Light is trappedLight is trapped by chlorophyll and provides the energy for by chlorophyll and provides the energy for photosynthesisphotosynthesis

2. 2. Water is split Water is split – sunlight energy is used to split water in half: – sunlight energy is used to split water in half:

3. The 3. The products from splitting waterproducts from splitting water are: are:Protons – passed to a storage pool to use later Protons – passed to a storage pool to use later Electrons – passed to chlorophyllElectrons – passed to chlorophyllOxygen – released out of the leaf or used in respirationOxygen – released out of the leaf or used in respiration

4. Light energises electrons and changes them into high energy electrons. High energy electrons have a lot of energy.

5. Glucose is formed when the high energy electrons, protons Glucose is formed when the high energy electrons, protons from the storage pool and Carbon Dioxide (from the air) from the storage pool and Carbon Dioxide (from the air) combine together in the Calvin Cycle.combine together in the Calvin Cycle.

HH+ + (protons) + e(protons) + e- - (electrons)(electrons) + CO+ CO2 2 C C66HH1212OO66

GlucoseGlucose

Main Events in Photosynthesis Main Events in Photosynthesis

1. Light1. Light

Sunlight is normal source of light for photosynthesis but artificial light e.g. Sunlight is normal source of light for photosynthesis but artificial light e.g. Light bulbs can be used in green houses to grow plants.Light bulbs can be used in green houses to grow plants.

Increasing light can increase growth up to a certain saturation point where Increasing light can increase growth up to a certain saturation point where no more light can be absorbed and photosynthesis will level offno more light can be absorbed and photosynthesis will level off

The light that reaches a plant is trapped by chlorophyll. Chlorophyll is The light that reaches a plant is trapped by chlorophyll. Chlorophyll is found in the chloroplasts of plant cells. Therefore photosynthesis occurs in found in the chloroplasts of plant cells. Therefore photosynthesis occurs in chloroplasts. chloroplasts.

2. Carbon Dioxide

Plants can get their carbon dioxide from either of two sources.

External – most carbon dioxide comes from the air (0.04%)

Internal – Some carbon dioxide used in photosynthesis comes from respiration

Equation for Respiration

Sometimes artificial sources of carbon dioxide are used to stimulate growth eg. Sometimes artificial sources of carbon dioxide are used to stimulate growth eg. burning gas in a green houseburning gas in a green house

3. Water

Water is absorbed from the soil by the roots and travels up to the leaves through the xylem

Learning CheckLearning Check What are the main stages in photosynthesis?What are the main stages in photosynthesis? What is water split into?What is water split into? What happens to each of these products?What happens to each of these products? What is the trapped light energy used for?What is the trapped light energy used for? Where do plants get (a) Light, (b) carbon dioxide and Where do plants get (a) Light, (b) carbon dioxide and

(c) water from?(c) water from? How can humans increase photosynthesis?How can humans increase photosynthesis? Why would humans want to do this?Why would humans want to do this?

Detailed Study of PhotosynthesisDetailed Study of Photosynthesis

Photosynthesis is split into two main stages:Photosynthesis is split into two main stages:

a. The light stagea. The light stage

b. The Dark Stage (Light independent stage).b. The Dark Stage (Light independent stage).Light StageLight Stage

This stage occurs in the chloroplast. It involves electrons and This stage occurs in the chloroplast. It involves electrons and happens very quickly.happens very quickly.

1. Light is absorbed. 1. Light is absorbed.

Light is made up of 7 colours and all the Light is made up of 7 colours and all the

colours of light except green are absorbed colours of light except green are absorbed

by chlorophyll (green is reflected)by chlorophyll (green is reflected)

2. Light energy is transferred to electrons. Pigments are arranged in clusters with a chlorophyll and an electron acceptor. Different pigments absorb different colours and pass the electrons to the chlorophyll that has the electron acceptor attached. The electrons absorb more energy and become high energy electrons. From this point the electrons can enter either of 2 pathways.

Electron Flow Pathway 1:In pathway 1 the high energy electrons move from the electron acceptor molecule to other electron acceptors. They travel in a circle and back to where they started. As the electrons move around the circular path they lose energy. This energy is trapped by ADP and Pi to form ATP.

ADP + energy + P -> ATP + waterBecause light is involved a phosphate group is added to the ADP molecule and the electrons are moving in a circular pathway the process is called cyclic photophosphorylation or cyclic electron flow.

Electron Flow – Pathway 2

In pathway 2 two high energy electrons move from the electron acceptor to other electron acceptors. They do not travel in a circle like in pathway 1. As they move the electrons lose energy and this energy is used to form ATP. Eventually the 2 electrons join to NADP+ and make NADP-

The H+ from the storage pool is now used and stuck to the NADP- to form NADPH.

Because the light is involved, a phosphate is added to the ADP but the electrons don’t move in a circle this is called Non Cyclic Photophosphorylation or Non Cyclic Electron Flow.

NADP+ 2 electrons (2e-) -> NADP-

NADP- + H+ -> NADPH

ATP and NADPH are made as a result of this pathway. Both of these molecules carry energy into the Dark Stage.

End Products of the Light Stage

NADPH, ATP and Oxygen are the end products of the Light Stage.

ATP – will provide energy for reactions in the Dark Stage.

NADPH – Will also provide energy but also the H12 for the glucose molecule

Oxygen – is made from the split water and released from the leaf or recycled in respiration.

The Dark Stage

This stage is also called the light independent stage.

It occurs in the chloroplast and involves enzymes.

The energy carriers (ATP and NADPH) enter the

Dark Stage

And are broken up to release their energy.

ATP + water -> ADP + P + Energy

NADPH -> NADP+ + 2 electrons + H+

The energy released from breaking up ATP

and NADPH drives the Calvin Cycle.

Carbon Dioxide enters the leaf through the stomata (tiny pores on the underside of the leaf). The Calvin Cycle uses the CO2 and mixes it with H from the NADPH.

This step is repeated again and again until C6H12O6 (Glucose) is formed.

The glucose is stored in the leaf as starch and later transported out of the leaf through the phloem tissue for storage or for use in respiration.

Summary of steps in photosynthesis

Summary of steps in photosynthesis

Summary

Light Stage Light energy used to make ATP. Light energy used to produce NADPH from NADP-. Oxygen gas as a by-product. Half of the water used as a hydrogen source is recycled

Dark Stage Carbon dioxide and hydrogen are used to make carbohydrate. The energy to drive this process comes from ATP.

Can You?

Explain the role of ATP Explain the production of ATP from ADP Explain the role of NADP+ in trapping & transferring

electrons & H ions. Explain the Light Stage/Dark Stage State the two-pathway system of electron carriage.

1. Direct to chlorophyll

2. Trapped by NADP+

To investigate the influence of light intensity on the rate of photosynthesisTo investigate the influence of light intensity on the rate of photosynthesis

Method:1. Place a funnel over Elodea, pondweed, in a beaker of pond water

at 25°C. 2. The funnel is raised off the bottom on pieces of blue-tack. This

allows continuous free diffusion of CO2 to Elodea. 3. Invert a test tube full of water over the stem of the funnel to

collect any gas from the Elodea. 4. Place the beaker on a hot plate at 25°C. 5. Maintain and monitor the temperature of the water with a

thermometer.Note: 1. Excess sodium bicarbonate is placed in the water to give a constant

saturated solution of CO2. 2. Place the lamp (the only light source) at a predetermined distance from the plant. 3. Use a light meter to measure the light intensity at this distance. Record the light intensity. 4. Allow the plant five minutes to adjust to the new conditions. 5. Count the number of oxygen bubbles given off by the plant in a five-minute period.

6. Repeat the count twice more and calculate the average of the three readings.

This is the rate of photosynthesis at that particular light intensity. The gas should be checked to prove that it is indeed oxygen — it relights a glowing splint. Repeat at different light intensities by moving the lamp to different distances. Graph the results placing light intensity on the x-axis.