Chapter 11 Photosynthesis Syllabus

Chapter 11 Photosynthesis Syllabus -- Objectives n n n n n 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 , CO 2 & 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 n n n 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 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 (Adenosine Tri Phosphate) is a small package containing energy. In photosynthesis light is used to make ATP

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

Main Events in Photosynthesis 1. Light is trapped by chlorophyll and provides the energy for photosynthesis 2. Water is split – sunlight energy is used to split water in half: 3. The products from splitting water are: Protons – passed to a storage pool to use later Electrons – passed to chlorophyll Oxygen – 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 from the storage pool and Carbon Dioxide (from the air) combine together in the Calvin Cycle. H+ (protons) + e- (electrons) + CO 2 C 6 H 12 O 6 Glucose

Main Events in Photosynthesis 1. Light Sunlight is normal source of light for photosynthesis but artificial light e. g. Light bulbs can be used in green houses to grow plants. Increasing light can increase growth up to a certain saturation point where no more light can be absorbed and photosynthesis will level off The light that reaches a plant is trapped by chlorophyll. Chlorophyll is found in the chloroplasts of plant cells. Therefore photosynthesis occurs in 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. burning gas in a green house 3. Water is absorbed from the soil by the roots and travels up to the leaves through the xylem

n n n n Learning Check What are the main stages in photosynthesis? What is water split into? What happens to each of these products? What is the trapped light energy used for? Where do plants get (a) Light, (b) carbon dioxide and (c) water from? How can humans increase photosynthesis? Why would humans want to do this?

Detailed Study of Photosynthesis is split into two main stages: a. The light stage b. The Dark Stage (Light independent stage). Light Stage This stage occurs in the chloroplast. It involves electrons and happens very quickly. 1. Light is absorbed. Light is made up of 7 colours and all the colours of light except green are absorbed 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 + water Because 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 NADPThe 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 (2 e-) -> 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 H 12 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 + 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 CO 2 and mixes it with H from the NADPH. This step is repeated again and again until C 6 H 12 O 6 (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 Light Stage n Light energy used to make ATP. n Light energy used to produce NADPH from NADP-. n Oxygen gas as a by-product. n Half of the water used as a hydrogen source is recycled Dark Stage n Carbon dioxide and hydrogen are used to make carbohydrate. n The energy to drive this process comes from ATP.

Can You? Explain the role of ATP n Explain the production of ATP from ADP n Explain the role of NADP+ in trapping & transferring electrons & H ions. n Explain the Light Stage/Dark Stage n State the two-pathway system of electron carriage. 1. Direct to chlorophyll 2. Trapped by NADP+ n

To 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 CO 2 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 CO 2. 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.

Chapter 11 Photosynthesis Syllabus — Objectives n n