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Chapter 10: Photosynthesis Essential Knowledge 2.a.1 – All living systems require constant input of free energy (10.1-10.3). 2.a.2 – Organisms capture and store free energy for use in biological processes (10.1-10.3). Photosynthesis Process by which plants use light energy to make food A reduction process that makes complex organic molecules from simple molecules Ps General Equation 6 CO2 + 6 H2O C6H12O6 + 6 O2 Requires: Chlorophyll Light ♦Notice: This is VERY similar to the cell resp eq. Ps: a redox process Hydrogens are added to Carbons-reduces CO2 to sugar Water is split; becomes source of hydrogens Complex covalent bonds made Requires energy Ps Has two chemical reactions: 1. Light Reaction 2. Dark Reaction Names are from “light” as a requirement, not where or when they occur Light A form of electromagnetic radiation/energy Visible light has the right energy for use in Ps Travels in waves Too Hot Too Cold Just Right Detected by our eye-hence the term visible Action Spectrum Not all colors are useable to the same degree for Ps Red and Blue light - absorbed and used in Ps Green light - reflected or transmitted Comment In oceans, red light is lost or filtered out early because it has lower energy Only blue light which has higher energy can reach the lower depths Result Many deep ocean fish are bright red in color. Why? They can’t be seen because there is no red light to reflect their color Photosynthesis Pigments Pigment: absorb light Different pigments absorb different wavelengths Types: Chlorophylls (a and b) Accessory Pigments (carotenoids) Chlorophylls Have C,H,O,N and Mg Two major types: a, b Molecule has a lipophilic tail (allows it to dissolve into membranes) Contains Mg in a reaction center. Accessory Pigments Absorb light energy and transfer it to chlorophyll Various shades of yellow/orange Function: photoprotection Ex: Carotene (orange) Xanthophyll (yellow) Fall Leaf Colors Chlorophyll breaks down N and Mg salvaged and moved into the stem for next year Accessory pigments remain behind, giving the various fall leaf colors Parts of Plant: Ps function Stomata Roots Mesophyll Veins Site of Ps Chloroplast Contain chlorophyll pigment All green parts of plants contain chloroplasts Most are found in leaves Why? Larger surface area Chloroplast Structure Double outer membrane Inner membrane folded and stacked into grana Stroma - liquid that surrounds the thylakoid membranes Photosystems Collection of pigments that serve as a light trap Made of chlorophyll and the accessory pigments Two types known: PSI & PSII Parts of Photosystem 1) Rxn center/e- acceptor Chlorophyll uses light energy to move e- to next energy level 2) Pigment Light 3) absorption, photon acceptor Light-harvesting complex Proteins, contain pigment Light Reaction Same thing as Noncyclic Photophosphorylation Location - grana of the chloroplast Function - to split water and produce ATP and NADPH (provide chem energy for next cycle) Light Reaction Requirements: Light Water ADP + Pi NADP+ Products: O2 ATP NADPH Chemiosmosis Model The chloroplast produces ATP in the same manner as the mitochondria in Rs Light energy is used to pump H+ across a membrane When the H+ diffuses back, ATP is generated Chemiosmosis H+ are pumped into the thylakoid space ATP and NADPH are made when the H+ diffuse into the stroma Both Rs and Ps use ETC to pump H+ Dark Reactions How plants actually make food (carbohydrates) Don't require light directly to run Also known as the Calvin cycle or C3 Ps Calvin cycle/Dark reaction – builds sugars from smaller molecules Function - use ATP and NADPH to build food from CO2 Location - stroma of the Calvin Cycle chloroplast movie Anabolic Rubisco Ribulose BisPhosphate Carboxylase Enzyme that adds CO2 to an acceptor molecule Most important enzyme on earth Catalyzes first step of dark rxn C3 Ps C3 plants are plants that DO use Rubisco enzyme “normally” Keep stomata open during day MOST plants are C3 plants When you “think of Ps”, you think of a C3 plant performing Ps Adaptive value: Require less Less enzyme activity, no specialized organs/anatomy Photorespiration When Rubisco accepts O2 instead of CO2 as the substrate Generates no ATP Actually uses ATP Decreases Ps output by as much as 50% Photorespiration May reflect a time when O2 was less plentiful and CO2 was more common Important to evolution of autotrophs Photo=occurs in light Respiration=consumes oxygen while producing CO2 Alternate Ps Methods 1. C4 Ps 2. CAM Ps Intro to C4 and CAM Remember: MOST plants are C3 plants that performs Ps “normally” Both are adaptations to arid conditions CAM plants “idle” and save energy while C4 plants go through Ps faster C4 Ps Uses a different enzyme to initially capture CO2 Separates CO2 capture from carbon fixation into sugar Still uses C3 Ps to make sugar, but only does so in the bundle sheath cells C4 Ps Found in 19 plant families Characteristic of hot regions with intense sunlight Examples - sugarcane, Bermuda grass, crab grass C3 Ps Can vs use photorespiration Shade to full sun High water use Cool temperatures Slow to moderate growth rates Cool season crops C4 Ps No Photorespiration Full sun only Moderate water use Warm temperatures Very fast growth rates Warm season crops CAM plants Found in plants from arid conditions (where water stress is a problem) Avoid H2O stress by keeping stomates closed during the day Generally have slow growth Ex: pineapple, cacti, orchids CAM Plants, cont. Store CO2 as C4 acid During the day, the acid is broken down and CO2 is made into sugar. Ps:Rs Ratios Reflect a plant’s balance in making food and using food 1. Ps > Rs, energy available for growth and reproduction 2. Ps = Rs, no growth, but don’t die either 3. Ps < Rs, death by starvation Comments - Ps:Rs Rs happens 24 hours a day Ps only in light Plants overwinter on stored food when Ps > Rs If Ps < Rs, best solution is to increase the amount of light Factors Affecting Ps 1. Light - quantity and quality 2. Temperature - too hot or too cold 3. CO2 - often limits C3 plants 4. Minerals - especially NPK and Mg Importances of Ps 1. Food - either directly or indirectly comes from plants 2. Oxygen in the air 3. CO2 balance 4. Plant products 5. Life on Earth Summary Identify the basic equation of photosynthesis. Identify the main reactions of photosynthesis. Recognize the function, location, requirements and products of the photosynthesis reactions. Recognize the role of light and pigments in the photosynthesis process. Recognize the effects and importance of photorespiration. Recognize various types of photosynthesis and their importance. Identify the biological and economic importance of photosynthesis. Exclusion Statements You do NOT need to memorize the steps in the Calvin Cycle, the structure of the molecules, and the names of the enzymes (except ATP synthase).
