The Light-Dependent & Light-Independent Reactions Photosynthesis The Light-Dependent & Light-Independent Reactions

Photosynthesis & Light Review What is the visual spectrum of light composed of? How do pigments capture light energy? What is the difference between the absorbance spectrum of pigments and the action spectrum of photosynthesis?

Main Concepts Discussed: Chloroplast Structure Light-dependent Reactions Light-independent Reactions

Chloroplast The chloroplast is the site of photosynthesis. Chloroplasts are concentrated in the mesophyll tissue found in the leaf. Where is the mesophyll?

Chloroplast – the Photosynthetic Organelle in Plant Cells

Chloroplast Structure Chloroplasts have two membranes. The space inside the inner membrane is called the stroma. http://t2.gstatic.com/images?q=tbn:ANd9GcRV6sYLX5jc7lyktses_8UoW9ySx9GkvBDrXx3iMBbSaNuR-8pTjw

Chloroplast Structure In the stroma they have a set of membraneous sacs called thylakoids. Thylakoids have an inner lumen called the thylakoid space. Stacks of thylakoids are called grana (granum is singular). http://t2.gstatic.com/images?q=tbn:ANd9GcRV6sYLX5jc7lyktses_8UoW9ySx9GkvBDrXx3iMBbSaNuR-8pTjw

SEM of a Chloroplast

Identify Chloroplast Parts b. d. c. http://t2.gstatic.com/images?q=tbn:ANd9GcS-EO4gcGRpmUaZ9532fsBsfeqLvoq-t777FdNHH0rhKK8UMnXqWA

Photosynthetic Processes The processes of photosynthesis take place in various parts of the chloroplast. Before we talk about those, we need to take another look at the overall photosynthesis process and its inputs and outputs.

Photosynthesis Photosynthesis is a Redox reaction. Is photosynthesis an endergonic or an exergonic reaction? Is it anabolism or catabolism?

Two Stages of Photosynthesis Light-dependent reactions (during day-time only, occur in the thylakoid membrane) Light-independent reactions, or the Calvin Cycle (could occur both day and night, in the stroma), These two stages are linked together by NADPH and ATP.

Overview: Light-dependent Reactions Require light and occur only during the day in nature. They take place in the thylakoid membrane of the chloroplast. Light reactions involve a) Splitting of water to produce oxygen (photolysis) b) Energy production (ATP) c) Reduction of NADP+ to NADPH. Light reactions produce NO organic molecules! They furnish the energy that eventually powers the food-making machinery of photosynthesis.

Overview: Light-independent Reactions Are independent of light and may occur during the day or the night. Take place in the stroma of the chloroplast. Involves the production of ‘food’ (stored energy) as glucose and other organic molecules by using CO2, ATP, and NADPH.

Overview of the Overviews NADPH is produced by the light-dependent reactions and provides high-energy electrons for reduction in the light-independent reactions. ATP from the light-dependent reactions provides chemical energy that powers several of the steps of the light-independent reactions. The light-independent reactions usually run in the daytime when light reactions power the cycle’s sugar assembly.

Light-dependent Reactions Details:

Photosystems Capture Solar Power We said in the previous presentation that when a pigment absorbs a photon, one of the pigment’s electrons gains energy. The electron gains enough energy to bump up into a higher shell. Normally, the electron then drops back into its original shell, releasing the energy it had gained.

Photosystems Capture Solar Power In the light-dependent reactions, pigments are arranged into photosystems. In the photosystems, excited electrons get transferred to other molecules, instead of dropping down to their lower energy level. As we talked about in respiration, transfer of electrons to other molecules also transfers their energy.

Photosystems Capture Solar Power The pigments in photosystems transfer electrons to a primary electron acceptor. The solar-powered electron transfer from chlorophyll (pigments) to the primary electron acceptor is the first step in the light-dependent reactions.

Visible Light Drives the Light Reactions As we saw in the absorbance spectrum, pigments can only absorb certain wavelengths of light Therefore, the light reactions of photosynthesis only use certain wavelengths of visible light. Chlorophyll a absorbs mainly blue-violet and red light. Chlorophyll b absorbs mainly blue and orange light and reflects (appears) yellow-green.

Pigments of the Chloroplast Chlorophyll a directly participates in the light- dependent reactions. Chlorophyll b and carotenoids (other pigments) do not directly participate in light-dependent reactions. They broaden the range of light the plant can use Then, these pigments convey the light energy they absorb (their excited electrons) to chlorophyll a, which puts the energy to work.

Basic steps of a Light Reaction Light reaction involves the following basic steps: 1) Absorption of light 2) Excitation of Chlorophyll a and emission of electron 3) Formation of ATP and NADPH via the electron transport chain

Photosystem I There are two types of Photosystems, (Photosystem I and Photosystem II) occuring within the thylakoid membrane. In Photosystem I, chlorophyll a molecule, P700, is the reaction center. This molecule absorbs light of 700 nanometers wavelength (red light). Its excited state emits electrons which are accepted by the primary electron acceptor and passed down an electron transport chain, eventually reducing NADP+ to NADPH.

Photosystem II In Photosystem II, chlorophyll a molecule, P680, is the reaction center. This molecule absorbs light of 680 nanometers wavelength (orange-red light) – which has a higher energy level. Its excited state (due to light absorption) emits electrons that are accepted by a primary electron acceptor and passed down an electron transport chain, eventually replenishing the lost electrons from P700.

Photosystem II The electron transport chain in Photosystem II enables the chloroplast to make ATP by chemiosmosis. H20 is split into H+ (protons), electrons and Oxygen gas. These electrons replenish the lost electrons from P680

Photosystems I and II

Electron Transport Chains: Noncyclic Electron Flow Key events in the light reactions: The absorption of light energy The excitation of electrons by that energy The formation of ATP and NADPH using energy made available by the cascade of energized electrons down electron transport chains

Noncyclic Electron Flow and the Light Reactions In the diagram, we can see that both photosystems absorb light energy and excited electrons pass from the reaction-center chlorophylls to the primary electron acceptors Then each primary electron acceptor is oxidized as it donates high energy electrons to the first electron carrier of an electron transport chain.

Photosystems I and II

Noncyclic e- Flow and the Light Rxns Additional redox reactions then shuttle the electrons from one electron carrier molecule to the next down an energy cascade. At each step in the cascade, the electrons lose energy, some of which ends up being captured and temporarily stored in either ATP or NADPH molecules.

Photosystems I and II

Summary of Light Rxns Every molecule of NADPH formed in the light reactions requires 2 electrons from photosystem 1. NADPH, ATP, and O2 are the products of the light reactions! ATP is made by chemiosmosis - which we will look at next!

Chemiosmosis Powers ATP Synthesis in Light Reactions DOES THIS SOUND FAMILIAR? Energy released during electron flow drives the transport of hydrogen ions (H+) across the thylakoid membrane Electron transport in the chloroplast drives chemiosmosis the same way it does in the mitochondria

Chemiosmosis The two photosystems and electron transport chains are located within the thylakoid membrane of a chloroplast. The photosystems are arranged in such a way that energy released during electron flow drives the transport of hydrogen ions (H+) across the thylakoid membrane, into the thylakoid lumen.

Chemiosmosis The pumping of H+ creates a huge concentration gradient of H+ across the thylakoid membrane. The only way for the H+ to go down its concentration gradient is through ATP synthase ATP synthase uses the kinetic energy of the H+ flow to create ATP from ADP and Pi. In photosynthesis, the chemisomotic production of ATP is called photophosphorylation because the initial energy input is light energy.

Light-dependent Reactions: Video We’ll watch this 5-min video clip to see how the molecules of the photosystems and electron transport chains work together.

Comparison of Chemiosmosis At its heart, the chemiosmosis of both cellular respiration and photosynthesis is very similar.

Cyclic Electron Flow Noncyclic Electron Flow provides BOTH NADPH and ATP for the Calvin cycle The light-independent reactions need more ATP than Noncyclic e- Flow can provide, SO – Cyclic Electron Flow occurs to provide extra ATP to the light-independent reactions.

Cyclic Electron Flow

Cyclic e- Flow In cyclic e- flow, only Photosystem I is used. Photoexcited electrons from photosynthesis can be returned from ferredoxin (Fd) to P700 chlorophyll through the cytochrome complex and plastocyanin (Pc) This provides additional ATP for the Calvin cycle but does not produce NADPH This production of ATP is called cyclic photophosphorylation, to differentiate from noncyclic photophosphorylation.

Review How, specifically, does light energy drive the light- dependent reactions? What are the products of the light-dependent reactions? Describe how chemiosmosis works in the light- dependent reactions.

Light-independent Reactions Details

Light-independent Reactions: The Calvin Cycle Take place during the day and sometimes the night in the stroma of the chloroplast. The inputs are: CO2 (from the air – an oxidized form of carbon) ATP and NADPH ( both generated by the light reactions) Using carbon from CO2 , energy from ATP, and high energy electrons from NADPH, the light- independent reactions construct an energy rich sugar molecule, glyceraldehyde 3-phosphate (G3P). G3P is a reduced form of carbon.

The Calvin Cycle The plant cell can use G3P to make glucose or other organic molecules as needed. Any excess of glucose is converted into starch – stored in roots, tubers and fruits. To make a molecule of G3P, the cycle must incorporate the carbon atoms from 3 molecules of CO2.

glycerate-3-phosphate triose phosphate (TP)

The Calvin Cycle: Summary For a net gain of 1 G3P molecule: 9 ATPs were consumed 6 NADPHs were oxidized The G3P that left the cycle is the starting material for synthesis of other compounds (e.g. glucose, starch and other more complex carbohydrates)

Review: Think back to the light-dependent and light-independent reactions. What are the big ideas from each section? How are these two reaction series connected to each other?

Adaptations of the Mitochondrion and the Chloroplast for Function Both mitochondria and chloroplasts’ structures are adapted for their functions. Explain three ways each is structurally suited to its purpose.