Overview: The Process That Feeds the BiospherePhotosynthesis is the process that converts solar energy into chemical energyDirectly or indirectly, photosynthesis nourishes almost the entire living world

Figure 8.1. How can sunlight, seen here as a spectrum of colors in a rainbow, power the synthesis of organic substances?

*

Autotrophs (auto- means self, and trophos means feeder) sustain themselves without eating anything derived from other organismsAutotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic moleculesAlmost all plants are photoautotrophs, using the energy of sunlight to make organic molecules

Heterotrophs (hetero- means other) obtain their organic material from other organismsHeterotrophs are the consumers of the biosphereAlmost all heterotrophs, including humans, depend on photoautotrophs for food and O2

*

Photosynthesis occurs in plants, algae, certain other protists (informal term applied to any eukaryote that is not a plant, animal, or fungus most protists are unicellular but some are multicellular), and some prokaryotesThese organisms feed not only themselves but also most of the living world

*

Concept 8.1: Photosynthesis converts light energy to the chemical energy of food

*

Chloroplasts: The Sites of Photosynthesis in PlantsChlorophyll = green pigment within chloroplasts ( a type of plastid)

Chloroplasts - 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

Chlorophyll is in thylakoid membranes which may be stacked in columns called grana (pl.), granum (sing.)

Chloroplasts also contain stroma, an interior fluid

2014 Pearson Education, Inc.

*

Tracking Atoms Through Photosynthesis: Scientific InquiryPhotosynthesis is a complex series of reactions that can be summarized as the following equation

6 CO2 12 H2O Light energy C6H12O6 6 O2 6 H2O

*

The Splitting of Water provides a source of electrons and protons (H+), and gives off O2 as a by-productEnzymes in chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product

Figure 8.4. Tracking atoms through photosynthesis. The atoms from CO2 are shown in pink, and the atoms from H2O are shown in blue.

*

The Two Stages of Photosynthesis: A PreviewPhotosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)The light reactions (in the thylakoids):Split H2ORelease O2Reduce the final electron acceptor, NADP, to NADPHUse chemiosmosis to power the addition of a phosphate group to ADP, thereby generating ATP a process called photophosphorylation

*

Thylakoid membranes (green) are sites of the light reactions, whereas the Calvin cycle occurs in the stroma (gray) (Fig. 8.5)

The light reactions use solar energy to make ATP and NADPH, which supply chemical energy and reducing power, respectively, to the Calvin cycle.

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

The Calvin cycle forms sugar from CO2, using ATP and NADPH

Figure 8.5. An overview of photosynthesis.An overview of Photosynthesis: cooperation of the light reactions and the Calvin cycle

*

Concept 8.2: The light reactions convert solar energy to the chemical energy of ATP and NADPHChloroplasts are solar-powered chemical factoriesTheir 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 waves, which are disturbances of electric and magnetic fields

Wavelength = 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 (with a fixed quantity of energy), called photons

Fig. 8.6. The electromagnetic spectrum. White light is a mixture of all wavelengths of visible light. A prism can sort white light into its component colors by bending light of different wavelengths at different angles. (Droplets of water in the atmosphere can act as prisms, forming a rainbow. Visible light drives photosynthesis.

*

Photosynthetic Pigments: The Light ReceptorsPigments are substances that absorb visible light

Different pigments absorb different wavelengths

Wavelengths that are not absorbed are reflected or transmitted

Leaves appear green because although chlorophyll absorbs violet-blue and red light it reflects and transmits green light

Figure 8.7. Why leaves are green: interaction of light with chloroplasts. The chlorophyll molecules of chloroplasts absorb violet-blue and red light (the colors most effective in driving photosynthesis) and reflect or transmit green light. This is why leaves appear green.

*

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

Accessory pigments include chlorophyll b and a group of pigments called carotenoids

Figure 8.9aAn Absorption Spectrum is a graph plotting a pigments light absorption versus wavelength

*For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files.

An Overview of PhotosynthesisLight Reactions: Are carried out by light-absorbing complexes (photosystems) in the thylakoid membranes Electron Transport Chains and Chemiosmosis convert light energy to the chemical energy of ATP and NADPHSplit H2O and release O2 to the atmosphere

Calvin Cycle Reactions: Take place in the stroma Use ATP and NADPH to convert CO2 to the sugar G3P Return ADP, inorganic phosphate, and NADP+ to the light reactions

A Photosystem: A Reaction-Center Complex Associated with Light-Harvesting ComplexesA photosystem consists of a reaction-center complex (a protein complex holding a special pair of chlorophyll a molecules) surrounded by light-harvesting complexesThe pair of chlorophyll a molecules in the reaction-center complex are special because of their location and association with other molecules and proteins The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

Figure 8.12

*

The reaction-center complex also contains a molecule capable of accepting e-s and becoming reduced called the primary electron acceptor (called pheophytin)

The primary electron acceptor (Fig. 8.12) accepts excited electrons from chlorphyll a 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

Fig. 8.12 (a) How a photosystem harvests light. When a photon strikes a pigment molecule in a light-harvesting complex, the energy is passed from molecule to molecule until it reaches the reaction-center complex. Here, an excited electron from the special pair of chlorophyll a molecules is transferred to the primary electron acceptor.

*

1) 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 P6802) Photosystem I (PS I) is best at absorbing a wavelength of 700 nm. The reaction-center chlorophyll a of PS I is called P700

There are two types of photosystems in the thylakoid membraneFig. 8. UNO5. How linear electron flow during the light reactions generates ATP and NADPH. The gold arrows trace the current of light-driven electrons from water to NADPH.

*

Linear Electron Flow: involves the flow of electrons through both photosystems to produce ATP, NADPH and oxygen using light energyFig. 8. UNO5. How linear electron flow during the light reactions generates ATP and NADPH. The gold arrows trace the current of light-driven electrons from water to NADPH.

*

2014 Pearson Education, Inc.

Comparison of chemiosmosis in mitochondria and chloroplastsAlthough the spatial organization of chemiosmosis differs slightly between chloroplasts and mitochondria, its easy to see the similarities in the two.Chemiosmosis: uses energy stored in a proton (H+) ion gradient across a membrane to drive cellular work, such as ATP synthesis

In both kinds of organelles, electron transport chains pump protons (H+) across a membrane from a region of low H+ concentration (light gray in the diagram right) to one of high H+ concentration (dark gray) (Fig. 8.15 right)

The protons then diffuse back across the membrane through ATP synthase, driving the synthesis of ATP

Concept 8.3: The Calvin cycle uses the chemical energy of ATP and NADPH to reduce CO2 to sugarThe Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle

Unlike the citric acid cycle, the Calvin cycle is anabolic

The Calvin cycle builds sugar from smaller molecules (ie., CO2) by using ATP and the reducing power of electrons carried by NADPH (both ATP and NADPH are generated by the light reactions)

Figure 8.UN03

*

The Calvin Cycle has 3 phases6 P iNADPHInput 3ATPCalvinCycleas 3 CO2RubiscoPhase 1: Carbon fixationPhase 2: ReductionPhase 3: Regenerationof RuBPG3POutputGlucose and other organiccompoundsG3PRuBP3-Phosphoglycerate1,3-Bisphosphoglycerate6 ADP6666P3PPP6 NADP6P5PG3PATP3 ADP33PP1PPFig. 8.17-3. The Calvin CyclePhase 1: Carbon fixation: The Calvin Cycle incorporates each CO2 molecule, one at a time, by attaching it to the 5C sugar ribulose bisphosphate (RuBP) using the enzyme Rubisco. The product of the reaction is a 6C intermediate so unstable that it immediately splits in half, forming 3-Phosphoglycerate.

cell spends3 ATP to regenerateRuBP5C sugarPhase 2: Reduction.Involves the reduction and phosphorylation of 3-phosphoglycerate to a 3Csugar Glyceraldehyde-3 phosphate (G3P)Phase 3: Regeneration of RuBP.Involves the rearrangement of Glyceraldehyde-3 phosphate (G3P) to regenerate the initial CO2 receptor, ribulose bisphosphate (RuBP), a 5C sugar.1 G3P molecule exits the cycle (later used to form glucose)6 molecules of G3P generated

Evolution of Alternative Mechanisms of Carbon Fixation in Hot, Arid ClimatesAdaptation to dehydration is a problem for land plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesisOn hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesisThe closing of stomata reduces access to CO2 and causes O2 to build upThese conditions favor an apparently wasteful process called photorespiration

*

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 compoundPhotorespiration decreases photosynthetic output by consuming ATP, O2, and organic fuel and releasing CO2 without producing any ATP or sugar

*

C4 plants minimize the cost of photorespiration by incorporating CO2 into a 4C compound (instead of the normal C3 compound)

An enzyme (PEP carboxylase) in the mesophyll cells has a high affinity for CO2 and can fix carbon even when CO2 concentrations are low (due to closed stomata)

These 4C compounds are exported to bundle-sheath cells (which are packed around veins of the leaf), where they release CO2 that is then used in the Calvin cycle

Some plants, including cacti and 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

C4 Plants vs. CAM Plants

*

*

*

*

*

*

*

*

*

*

*

*

*

*

*For the Cell Biology Video Space-Filling Model of Chlorophyll a, go to Animation and Video Files.*

*

*

*

*

*

*

*