4.1. Photosynthesis Light-Dependent Reactions

Transcription

1 4.1 Photosynthesis Light-Dependent Reactions

2 Photosynthesis Each year, Canada s boreal forest convert 12.5 million tonnes of carbon into energy-rich compounds for billions of organisms

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4 Photosynthesis A series of metabolic pathways 2 main steps: 1. Light-dependent reactions: Energy ATP and NADPH 2. Light-independent reactions CO 2 + ATP + NADPH sugar

5 Light-Dependent Reactions

6 Leaf Structure Water and carbon dioxide used to make glucose Water enters through roots and is transported to leaves Carbon dioxide enters through stomata in the leaves

7 Chloroplast Where photosynthesis takes place Thylakoid disks stack to form grana Stroma contains catalytic enzymes

8 Light Energy Light is absorbed as photons Each photon carries a specific amount of energy With the right amount of energy, electrons can jump up to an upper energy level

9 Photon Each wavelength is associated with a certain amount of energy in its photons Longer wavelength less energy An atom in a plant can only absorb photons that have an amount of energy that is exactly equal to the difference between two energy levels

10 Leaf colours - Pigments

11 Photosynthetic Pigments Absorb certain wavelengths of visible light and pass on to other compounds Absorb different combinations of colours

12 Photosynthetic Pigments Chlorophyll a and b: main plant pigment reflects green light Carotenoids: accessory plant pigments reflect yellow, orange, and red light Absorbance spectrum: a graph that shows the relative amounts of light of different wavelengths that a compound absorbs

13 Photosystems Protein based complex Composed of cluster of pigments that absorb light energy of many wavelengths Located on the thylakoid membrane

14 Photosystem Antenna complex: Pigments that capture photon (chlorophyll b, carotenoids, etc) transfer energy to reaction centre Reaction centre Made up of a pair of chlorophyll a molecules and proteins With energy received, 2 electrons are excited, jump up in energy level and captured by an electron carrier

15 Photosystems I and II Chloroplasts in plants and algae use two photosystems that work together to convert light energy into chemical energy. 1. Photosystem I (P700) : reaction center that absorbs wavelengths of 700 nm 2. Photosystem II (P680) : reaction center that absorbs wavelengths of 680 nm

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17 Light-Dependent Reactions Step 1 P680 molecule absorbs a photon in its antenna complex Transfers energy to reaction center an electron is excited (this can occur 200 times a second) electron acceptor takes the electrons (P680 P680 + ) P680 + has a strong attraction for electrons and pulls them from water. TWO H 2 O molecules are split and transfer FOUR electrons to the reaction center of P680 to replenish it. Oxygen gas is formed and released FOUR H + remain in the thylakoid space.

18 Light-Dependent Reactions Step 2 Electron carrier transfers electrons to a series of increasingly electronegative complexes (electron transport system a series of REDOX reactions) Energy released by the redox reactions is used by b 6 -f complex to pump a H + from the stroma into the thylakoid space creates an electrochemical gradient.

19 Light-Dependent Reactions Step 3 P700 absorbs photons in the antenna complex. Energy is transferred until it reaches the reaction center and excites another 2 electrons. An electron carrier captures the 2 excited electrons and carries it to the enzyme NADP reductase. NADP + undergoes a REDOX reaction with the NADP reductase and captures the electrons to become NADPH. The missing electrons from P700 are replaced by the 2 electrons from P680.

20 Light Dependent Reaction - Summary Electron Transport System

21 Making ATP by Chemiosmosis Photophosphorylation: using photons to drive the phosphorylation of ADP to ATP through chemiosmosis Similar to aerobic respiration B 6 -f complex releases a large amount of H + into the thylakoid space electrochemical gradient Thylakoid membrane impermeable to H + H + must pass through ATP synthase Oxidative phosphorylation to produce ATP

22 Noncyclic Photophosphorylation Z scheme Unidirectional Generates 1 NADPH and 1 ATP Ratio not sufficient for light-independent reactions Light independent reaction requires 3 ATP : 2 NADPH

23 Cyclic Photophosphorylation Only Photosystem I is used Electrons return to P700 through the b 6 -f complex Produces H + gradient produces more ATP No NADPH or O 2 produced