Overview: The Process That Feeds the Biosphere

                  Photosynthesis is the process that converts solar energy into chemical energy

                  Directly or indirectly, photosynthesis nourishes almost the entire living world

                  Autotrophs sustain themselves without eating anything derived from other organisms

                  Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules

                  Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules

                  Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes

                  These organisms feed not only themselves but also most of the living world

                  Heterotrophs obtain their organic material from other organisms

                  Heterotrophs are the consumers of the biosphere

                  Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

                  The Earth’s supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago

                  In a sense, fossil fuels represent stores of solar energy from the distant past

 

Concept 10.1: Photosynthesis converts light energy to the chemical energy of food

                  Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

                  The structural organization of these cells allows for the chemical reactions of photosynthesis

 

Chloroplasts: The Sites of Photosynthesis in Plants

                  Leaves are the major locations of photosynthesis

                  Their green color is from chlorophyll, the green pigment within chloroplasts

                  Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf

                  Each mesophyll cell contains 30–40 chloroplasts

                  CO2 enters and O2 exits the leaf through microscopic pores called stomata

                  The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana

                  Chloroplasts also contain stroma, a dense interior fluid

 

Tracking Atoms Through Photosynthesis: Scientific Inquiry

                  Photosynthesis is a complex series of reactions that can be summarized as the following equation:

 

The Splitting of Water

                  Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

 

Photosynthesis as a Redox Process

                  Photosynthesis reverses the direction of electron flow compared to respiration

                  Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced

                  Photosynthesis is an endergonic process; the energy boost is provided by light

 

The Two Stages of Photosynthesis: A Preview

                  Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)

                  The light reactions (in the thylakoids)

             Split H2O

             Release O2

             Reduce NADP+ to NADPH

             Generate ATP from ADP by photophosphorylation

 

                  The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH

                  The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

 

Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH

                  Chloroplasts are solar-powered chemical factories

                  Their thylakoids transform light energy into the chemical energy of ATP and NADPH

 

The Nature of Sunlight

                  Light is a form of electromagnetic energy, also called electromagnetic radiation

                  Like other electromagnetic energy, light travels in rhythmic waves

                  Wavelength is the distance between crests of waves

                  Wavelength determines the type of electromagnetic energy

                  The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation

                  Visible light consists of wavelengths (including those that drive photosynthesis) that produce colors we can see

                  Light also behaves as though it consists of discrete particles, called photons

 

Photosynthetic Pigments: The Light Receptors

                  Pigments are substances that absorb visible light

                  Different pigments absorb different wavelengths

                  Wavelengths that are not absorbed are reflected or transmitted

                  Leaves appear green because chlorophyll reflects and transmits green light

                  A spectrophotometer measures a pigment’s ability to absorb various wavelengths

                  This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

                  An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength

                  The absorption  spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

                  An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

                  The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann

                  In his experiment, he exposed different segments of a filamentous alga to different wavelengths

                  Areas receiving wavelengths favorable to photosynthesis produced excess O2

                  He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production

                  Chlorophyll a is the main photosynthetic pigment

                  Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis

                  Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

 

Excitation of Chlorophyll by Light

                  When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable

                  When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence

                  If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

 

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting Complexes

                  A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes

                  The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

                  A primary electron acceptor in the reaction center accepts excited electrons and is reduced as a result

                  Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

                  There are two types of photosystems in the thylakoid membrane

                  Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm

                  The reaction-center chlorophyll a of PS II is called P680

                  Photosystem I (PS I) is best at absorbing a wavelength of 700 nm

                  The reaction-center chlorophyll a of PS I is called P700

 

Linear Electron Flow

                  During the light reactions, there are two possible routes for electron flow: cyclic and linear

                  Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy

                  A photon hits a pigment and its energy is passed among pigment molecules until it excites P680

                  An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)

                  P680+ is a very strong oxidizing agent

                  H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

                  O2 is released as a by-product of this reaction

                  Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I

                  Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane

                  Diffusion of H+ (protons) across the membrane drives ATP synthesis

                  In PS I (like PS II), transferred light energy excites P700, which loses an electron to an electron acceptor

                  P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via the electron transport chain

                  Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)

                  The electrons are then transferred to NADP+ and reduce it to NADPH

                  The electrons of NADPH are available for the reactions of the Calvin cycle

                  This process also removes an H+ from the stroma

 

Cyclic Electron Flow

                  Cyclic electron flow uses only photosystem I  and produces ATP, but not NADPH

                  No oxygen is released

                  Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

                  Some organisms such as purple sulfur bacteria have PS I but not PS II

                  Cyclic electron flow is thought to have evolved before linear electron flow

                  Cyclic electron flow may protect cells from
light-induced damage

 

A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

                  Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy

                  Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP

                  Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities

                  In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix

                  In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma

                  ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

                  In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH

 

Concept 10.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugar

                  The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

                  The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH

                  Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde 3-phospate (G3P)

                  For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO2

                  The Calvin cycle has three phases

             Carbon fixation (catalyzed by rubisco)

             Reduction

             Regeneration of the CO2 acceptor (RuBP)

 

Concept 10.4: Alternative mechanisms of carbon fixation have evolved in hot, arid climates

                  Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis

                  On hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesis

                  The closing of stomata reduces access to CO2 and causes O2  to build up

                  These conditions favor an apparently wasteful process called photorespiration

Photorespiration: An Evolutionary Relic?

                  In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)

                  In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two-carbon compound

                  Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

                  Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2

                  Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

                  In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle

 

C4 Plants

                  C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

                  This step requires the enzyme PEP carboxylase

                  PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low

                  These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

                  In the last 150 years since the Industrial Revolution, CO2 levels have risen greatly

                  Increasing levels of CO2 may affect C3 and C4 plants differently, perhaps changing the relative abundance of these species

                  The effects of such changes are unpredictable and a cause for concern

 

CAM Plants

                  Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

                  CAM plants open their stomata at night, incorporating CO2 into organic acids

                  Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

 

The Importance of Photosynthesis: A Review

                  The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

                  Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

                  Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

                  In addition to food production, photosynthesis produces the O2 in our atmosphere