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Photosynthesis ENVIRONMENTAL PLANT PHYSIOLOGY SESSION 3 Photosynthesis An understanding of the biochemical processes of photosynthesis is needed to allow us to understand how plants respond to changing environmental conditions. This session looks at the details of the physiology and considers how plants can adapt these processes. If you studied the foundation degree with Myerscough previously you will recognise that much of these session notes are a very similar to those provided for the Plant Biology Module. The difference is not in the content but in the level of understanding required! Page Introduction to Photosynthesis 2 First catch your light! 3 The Light Dependent Reaction 7 The Light Independent Reaction 11 Photosynthesis and the Environment 12 Alternative Carbon Fixation Strategies 16 Further Reading 19 LEARNING OUTCOMES You need to be able to describe the biochemical basis of photosynthesis including: • Describing the role of chloroplasts and chlorophyll • Describing the steps of the light reaction and stating its products • Describing the Calvin cycle or light independent reaction and stating its products • Describing the role and position of the electron transport chain • Identify the effects of environmental factors on the rate of photosynthesis and explain how these relate to your field of study • Describe adaptations used by plants to continue photosynthesis in conditions of environmental stress such as hot arid conditions or shade conditions 1 Introduction to Photosynthesis The sun is the main source of energy to all living things. Light energy from the sun is converted to the chemical energy of organic molecules by green plants by a complicated pathway of reactions called photosynthesis. This can be simplified This means that the raw ingredients for photosynthesis are: • Carbon Dioxide • Water • Light Energy Also needed is: Chlorophyll Photosynthesis consists of two main phases: 1) The Light Reaction or Light Dependent Reaction Here light energy absorbed by chlorophyll splits water into oxygen and hydrogen. The oxygen is given off as a by product. The hydrogen is used to produce the energy carrying chemical ATP. The hydrogen then attaches to an NADP molecule forming NADPH. The light reaction can only take place in the presence of light and is thus light dependent. 2) The Dark Reaction or Light Independent Reaction Here the energy in the ATP is used to convert the hydrogen (carried by the NADPH) plus carbon dioxide from the air into simple carbohydrates such as glucose. The Dark Reaction can take place in the absence of light if it has the necessary ATP, NADPH and carbon dioxide, but more often occurs in the light immediately after the Light Reaction. It is therefore more correctly termed the Light Independent Reaction and is sometimes referred to as the Calvin Cycle. Figure 1: The inputs and outputs of the light and dark reactions 2 Where do The Light Dependent and Light Independent Reactions Occur? They happen in the chloroplast. The light dependent reaction occurs on the thylakoid membranes. The light independent reaction occurs in the stroma. Figure 2: Structure of a chloroplast First Catch Your Light! The first step in photosynthesis is to capture some light. So what is light and how do green plants catch it? What Is Light? Light is a form of energy known as electromagnetic radiation. Visible light is part of a wider electromagnetic spectrum (Figure 3). White light is a mixture of light of different colours; the different colours have different wavelengths. The shorter the wavelength the more energy the light has. Figure 3: The Electromagnetic Spectrum 3 Which Wavelengths Of Light Do Plants Need For Photosynthesis? If we measure the rate of photosynthesis of a plant in different wavelengths of light we obtain what is known as an action spectra. This shows that blue and red light work best for photosynthesis, while green is the least effective (Figure 4). Figure 4: Photosynthetic action spectrum of light Question: Which has more energy Blue or Red light? (Answer at bottom of the next page) Photosynthetic Pigments Plants use a number of pigments in the chloroplast to catch light. Different pigments catch or absorb different wavelengths of light. The Chlorophylls Chlorophylls are proteins. They have a complex ring structure with a magnesium atom (with loosely bonded electrons) in the centre and a long hydrocarbon tail (Figure 5). The ring is the part involved in capturing the light energy the hydrocarbon tail is used to attach the molecule to particular proteins within the thylakoid membrane. Chlorophylls absorb blue and red light energy best but chlorophylls a and b have slightly different absorption spectra (Figure 6). Chlorophyll a is blue green and is found in all photosynthetic organisms. Chlorophyll b is yellow green and is found in green plants. Chlorophyll c is found in brown algae. Figure 5: Structure of chlorophylls a & b 4 The Carotenoids These are hydrocarbon compounds which absorb different wavelengths of light from those absorbed by chlorophyll (Figure 6). They transfer any light energy absorbed to the chlorophyll molecules. They can also act to protect the chlorophyll by taking energy away at excessive light intensities. They include a carotene, b carotene and xanthophylls and are various shades of yellow and orange. Figure 6: Absorption spectrum of photosynthetic pigments Light Absorption by Chlorophyll When a chlorophyll molecule absorbs light energy this raises an electron in the outer shell of the magnesium atom to a higher orbit (it is said to be excited). The electron is unstable in this higher orbit and in an isolated chloroplyll molecule will soon (in about a billionth of a second) falls back to its original orbit releasing the energy in the form of fluorescent light (Figure 7). In a chlorophyll molecule in a thylakoid membrane the electron and the energy is passed on to an electron acceptor molecule as the first stage of the light reaction (see following sections on non-cyclic and cyclic photophosphorylation). ENERGY ABSORPTION ENERGY RELEASE Figure 7: Absorption and release of light energy by a magnesium atom in an isolated chlorophyll molecule Answer: Blue light has more energy as it has the shorter wavelength. Blue light has a wavelength of about 400nm whereas red light is around 660-680nm. 5 How are the Photosynthetic Pigments Arranged? The chlorophyll and associated pigments of the thylakoid membrane are organised into groups or photosystems. Each photosystem has an 'antenna complex' consisting of a cluster of a few hundred chlorophyll and carotenoid molecules. When one of the antennae complex molecules absorbs light, the energy is transferred from pigment molecule to pigment molecule (down an energy gradient) until it reaches the reaction centre, a chlorophyll a molecule sitting next to the primary electron acceptor molecule. (This chlorophyll a molecule is no different from all the others except for its position.) Here when the energy excites an electron to a higher orbit and the electron acceptor traps the electron before it returns to its original orbit. Figure 8: An antennae complex There are two types of photosystems in the thylakoid membrane, photosystem I and photosystem II. The two photosystems are made up of different combinations of pigments which mean that photosystem II preferentially absorbs red light at 680nm and photosystem I preferentially absorbs far red light of wavelength about 700nm. The two reaction centres for both are identical chlorophyll a molecules but they are named after the maximum absorption of their respective photosystems as P680 and P700. 6 The Light Dependent Reaction So the light reaction starts with the absorption of light and the photoexcitation of an electron from a chlorophyll molecule. This produces: 1. A chlorophyll molecule with a missing electron 2. An electron acceptor molecule with an extra electron These then feed into the processes (detailed in the next few pages) of: • Non-cyclic photophosphorylation • Photolysis • Cyclic photophosphorylation which produce the two products of the light dependent reaction, ATP and NADPH, which are then used to fuel the light independent (dark) reaction (Figure 9). LIGHT DEPENDENT LIGHT INDEPENDENT REACTION REACTION Figure 9: The inputs and outputs of the light dependent and light independent reactions 7 Non-cyclic Photophosphorylation Non-cyclic photophosphorylation takes the energy from light and produces ATP and NADPH in a number of stages (Figure 10): • At the start of non-cyclic Photophosphorylation, electrons excited from the chlorophyll molecule in photosystem II are passed along an electron transport chain to photosystem I. • As the electrons pass along a series of electron carriers in the thylakoid membranes their energy is used to produce ATP by means of ATP synthase proton pumps. • When an electron reaches the end of the electron transport chain it replaces the electron from the reaction centre chlorophyll, P700, in Photosystem I. • An electron excited from Photosystem I by light energy passes to an electron acceptor and then onto NADP+. The electrons combine with hydrogen ions (from photolysis) to reduce NADP+ to NADPH. The term Photophosphorylation refers to the adding of inorganic phosphate to ADP (phosphorylation) by energy from light (photo) to produce ATP. The electron from Photosystem II (PSII) is not recycled back to Photosystem II returned so this is a non-cyclic process. Figure 10: Diagram to show the stages of non-cylic photophosphorylation 8 The diagram below shows the positioning of the components of non-cyclic photophosphorylation in the thylakoid membrane and is another way to look at the stages of non-cyclic photophosphorylation. Figure 11: Components of and pathway of non-cyclic photophosphorylation (brown arrows denote electron transfer) (diagram from Ridge, I (2002) Plants, Oxford University Press) • Light absorbed by the antenna is funnelled to the reaction centre (R) of photosystem II (PSII). For each photon of light absorbed, an excited electron is transferred to the electron acceptor in PSII and this is replaced by electron from water via the oxygen-releasing complex (see later section on photolysis).
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