06 Lecture Ppt

Photosynthesis Produces Food and Releases Oxygen

6.1 Photosynthesizers are autotrophs that produce their own food Photosynthesis converts solar energy into the chemical energy of a carbohydrate Photosynthesizers are the base of almost every food chain in the world Autotrophs and heterotrophs (consumers) use organic molecules from photosynthesis

Figure 6.1 Photosynthetic organisms

6.2 In plants, chloroplasts carry out photosynthesis Photosynthesis occurs in green plant parts Carbon dioxide enters leaves via stomata Water and carbon dioxide diffuse into chloroplasts , the photosynthetic organelles Chloroplasts contain the chlorophyll and other pigments that absorb solar energy

Figure 6.2 Leaf structures specialized for photosynthesis

6.3 Photosynthesis is a redox reaction that releases O 2 Oxidation is the loss of electrons and reduction is the gain of electrons Because they always occur together, the entire reaction is a redox reaction Photosynthesis is a redox reaction in which hydrogen atoms are transferred from water to carbon dioxide

APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 6.4 Experiments showed that the O 2 released by photosynthesis comes from water In 1930 C.B. van Niel used the oxygen isotope O 18 to track the path of oxygen production in photosynthesis

6.5 Photosynthesis involves two sets of reactions The light reactions Only occur when solar energy is available Chlorophyll molecules absorb solar energy to energize electrons used in ATP production The Calvin cycle reactions CO 2 is taken up and reduced to a carbohydrate that can be converted to glucose ATP and NADPH from light reactions are needed

Figure 6.5A Overview of photosynthesis

First, Solar Energy Is Captured

6.6 Light reactions begin: Solar energy is absorbed by pigments Solar energy can be described in terms of its wavelength and its energy content Most common pigments chlorophylls a and b and carotenoids are capable of absorbing various portions of visible light

Figure 6.6A The electromagnetic spectrum includes visible light Figure 6.6B Absorption spectrum of photosynthetic pigments

APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 6.7 Fall temperatures cause leaves to change color Chlorophyll is not very stable and in the fall, sufficient energy to rebuild chlorophyll is not available Chlorophyll in leaves disintegrates, and we begin to see yellow and orange pigments in the leaves

6.8 Solar energy boosts electrons to a higher energy level Photosystem I (PS I) & Photosystem II (PS II) consist of antenna molecules and a reaction center Antenna molecules absorb light and pass energy to the reaction center In reaction center excited electrons are passed to electron acceptors

Figure 6.8 A general model of a photosystem

6.9 Electrons release their energy as ATP forms Electron transport chain - a series of carriers that pass electrons from one to another Each electron transfer releases energy that is ultimately used to make ATP ATP synthase complexes - hydrogen ions flow through, providing energy for ATP synthesis

Figure 6.9 High-energy electrons (e − ) release energy as they pass down an electron transport chain

6.10 During the light reactions, electrons follow a noncyclic pathway

6.11 The thylakoid membrane is organized to produce ATP and NADPH Chemiosmosis - ATP production is tied to an H + gradient across a membrane

Figure 6.11 Organization of a thylakoid

Second, Carbohydrate Is Synthesized

Fig 6.12 Calvin cycle reactions

6.12 The Calvin cycle uses ATP and NADPH from the light reactions to produce a carbohydrate Calvin cycle reduces CO 2 from the atmosphere to produce carbohydrate CO 2 Fixation – CO 2 combines with RuBP with the help of RuBP carboxylase CO 2 Reduction – CO 2 is reduced to a carbohydrate

RuBP Regeneration – every three turns of the Calvin cycle, five molecules of G3P reform three molecules of RuBP

6.13 In plants, carbohydrate is the starting point for other molecules Figure 6.13 Fates of G3P

C 3 , C 4 , and CAM Photosynthesis Thrive Under Different Conditions

6.14 C 3 photosynthesis evolved when oxygen was in limited supply In C 3 plants the first detectable molecule after CO 2 fixation is a C 3 molecule 3PG Common in plants where temperature and rainfall are moderate Under water stress, stomata close, limiting CO 2 and reducing efficiency O 2 competes with CO 2 for the active site of RuBP carboxylase, and less C 3 is produced Figure 6.14 Carbon dioxide fixation in C3 plants as exemplified by these wildflowers

6.15 C 4 photosynthesis boosts CO 2 concentration for RuBP carboxylase C 4 plant - the first detectable molecule following CO 2 fixation is a four carbon molecule Avoid the uptake of O 2 by RuBP carboxylase by increasing the amount of CO 2 available to the enzyme

Figure 6.15A Anatomy of a C 3 plant compared to a C 4 plant

Figure 6.15B Carbon dioxide fixation in C 4 plants as exemplified by corn

6.16 CAM photosynthesis is another alternative to C 3 photosynthesis CAM – crassulacean-acid metabolism Partitioning based on time CAM plants open their stomata only at night; during the day, the stomata are closed, conserving water

Figure 6.16 Carbon dioxide fixation in CAM plants as exemplified by pineapple

APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 6.17 Destroying tropical rain forests contributes to global warming Tropical rain forests contribute greatly to uptake of CO 2 because they are the most efficient of all terrestrial ecosystems CO 2 in our atmosphere traps radiant heat from the sun and warms the world Deforestation in tropical rain forests accounts for 20–30% of all carbon dioxide in the atmosphere Burning also removes trees that would ordinarily absorb CO 2

Figure 6.17 Global warming: Past trends and future predictions

Connecting the Concepts: Chapter 6 General equation for photosynthesis 6CO 2 + 6H 2 O -> C 6 H 12 O 6 + 6O 2 Two separate sets of reactions Light reactions (in thylakoid membrane) Absorb solar energy and produce NADH and ATP, which are provided to the Calvin cycle to reduce CO 2 Calvin cycle reactions (in stroma) Reduce carbon dioxide to a carbohydrate C 4 and CAM are alternative means of supplying RuBP carboxylase with CO 2 while limiting its exposure to oxygen. Photosynthesis keeps the biosphere functioning by supplying energy to all organisms Organisms use cellular respiration, which occurs in the mitochondria, to get energy out of carbohydrates

06 Lecture Ppt

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