Chapter 7 Photosynthesis: Using Light to Make Food Laura Coronado Bio 10 Chap 7

Biology and Society: Green Energy – Wood has historically been the main fuel used to: Cook Warm homes Provide light at night – Industrialized societies replaced wood with fossil fuels. – To limit the damaging effects of fossil fuels, researchers are investigating the use of biomass (living material) as efficient and renewable energy sources. Laura Coronado Bio 10 Chap 7

Figure 7.0 Laura Coronado Bio 10 Chap 7

Biology and Society: Green Energy – Fast-growing trees, such as willows: Can be cut every three years Do not need to be replanted Are a renewable energy source Produce fewer sulfur compounds Reduce erosion Provide habitat for wildlife Laura Coronado Bio 10 Chap 7

THE BASICS OF PHOTOSYNTHESIS – Photosynthesis: Is used by plants, some protists, and some bacteria Transforms light energy into chemical energy Uses carbon dioxide and water as starting materials – The chemical energy produced via photosynthesis is stored in the bonds of sugar molecules. – Organisms that use photosynthesis are: Photosynthetic autotrophs The producers for most ecosystems Laura Coronado Bio 10 Chap 7

Plants (mostly on land) Photosynthetic Protists (aquatic) PHOTOSYNTHETIC AUTOTROPHS Photosynthetic Bacteria (aquatic) Micrograph of cyanobacteriaKelp, a large alga Forest plants LM Figure 7.1 Laura Coronado Bio 10 Chap 7

Chloroplasts: Sites of Photosynthesis – Chloroplasts are: The site of photosynthesis Found mostly in the interior cells of leaves – Inside chloroplasts are membranous sacs called thylakoids, which are suspended in a thick fluid, called stroma. – Thylakoids are concentrated in stacks called grana. – The green color of chloroplasts is from chlorophyll, a light-absorbing pigment. – Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits. Laura Coronado Bio 10 Chap 7

Leaf cross section Stomata Vein CO 2 O2O2 Interior cell LM Stroma Granum Thylakoid Chloroplast Outer membrane Inner membrane TEM Figure Laura Coronado Bio 10 Chap 7

Carbon dioxide 6 O 2 6 CO 2 6 H 2 O C 6 H 12 O 6 Water Glucose Photo- synthesis Oxygen gas Light energy Figure 7.UN1 Laura Coronado Bio 10 Chap 7

Photosynthesis – Sunlight provides the energy – Electrons are boosted “uphill” and added to carbon dioxide – Sugar is produced – During photosynthesis, water is split into: Hydrogen Oxygen – Hydrogen is transferred along with electrons and added to carbon dioxide to produce sugar. – Oxygen escapes through stomata into the atmosphere. Laura Coronado Bio 10 Chap 7

A Photosynthesis Road Map – Photosynthesis occurs in two stages: The light reactions convert solar energy to chemical energy The Calvin cycle uses the products of the light reactions to make sugar from carbon dioxide Laura Coronado Bio 10 Chap 7

Light H2OH2O O2O2 Chloroplast Light reactions NADPH ATP Calvin cycle CO 2 NADP + ADP P Sugar (C 6 H 12 O 6 ) Figure Laura Coronado Bio 10 Chap 7

THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY – Chloroplasts: Are chemical factories powered by the sun Convert solar energy into chemical energy Laura Coronado Bio 10 Chap 7

The Nature of Sunlight – Sunlight is a type of energy called radiation, or electromagnetic energy. – The full range of radiation is called the electromagnetic spectrum. Laura Coronado Bio 10 Chap 7

Visible light Wavelength (nm) Radio waves Wavelength = 580 nm Micro- waves Gamma rays Infrared UV X-rays 10 –5 nm 10 –3 nm10 3 nm 1 nm 10 6 nm 1 m1 m 10 3 m Increasing wavelength Figure 7.4 Laura Coronado Bio 10 Chap 7

The Process of Science: What Colors of Light Drive Photosynthesis? – Observation: In 1883, German biologist Theodor Engelmann saw that certain bacteria tend to cluster in areas with higher oxygen concentrations. – Question: Could this information determine which wavelengths of light work best for photosynthesis? – Hypothesis: Oxygen-seeking bacteria will congregate near regions of algae performing the most photosynthesis. Laura Coronado Bio 10 Chap 7

The Process of Science: What Colors of Light Drive Photosynthesis? – Experiment: Engelmann: Laid a string of freshwater algal cells in a drop of water on a microscope slide Added oxygen-sensitive bacteria to the drop Used a prism to create a spectrum of light shining on the slide – Results: Bacteria: Mostly congregated around algae exposed to red-orange and blue-violet light Rarely moved to areas of green light – Conclusion: Chloroplasts absorb light mainly in the blue-violet and red-orange part of the spectrum. Laura Coronado Bio 10 Chap 7

Bacterium Wavelength of light (nm) Light Number of bacteria Algal cells Prism Microscope slide Figure 7.5 Laura Coronado Bio 10 Chap 7

Light Chloroplast Reflected light Absorbed light Transmitted light Figure 7.6 Laura Coronado Bio 10 Chap 7

Chloroplast Pigments – Chloroplasts contain several pigments: Chlorophyll a: – Absorbs mostly blue-violet and red light – Participates directly in the light reactions Chlorophyll b: – Absorbs mostly blue and orange light – Participates indirectly in the light reactions Laura Coronado Bio 10 Chap 7

Carotenoids Absorb mainly blue-green light Participate indirectly in the light reactions Absorb and dissipate excessive light energy that might damage chlorophyll The spectacular colors of fall foliage are due partly to the yellow-orange light reflected from carotenoids. Laura Coronado Bio 10 Chap 7

Figure 7.7 Laura Coronado Bio 10 Chap 7

How Photosystems Harvest Light Energy – Light behaves as photons, discrete packets of energy. – A photosystem is a group of chlorophyll and other molecules that function as a light- gathering antenna. – Chlorophyll molecules absorb photons. Electrons in the pigment gain energy. As the electrons fall back to their ground state, energy is released as heat or light. Laura Coronado Bio 10 Chap 7

Light (b) Fluorescence of a glow stick Photon Heat Light (fluorescence) Ground state Chlorophyll molecule Excited state e–e– (a) Absorption of a photon Figure 7.8 Laura Coronado Bio 10 Chap 7

Chloroplast Thylakoid membrane Pigment molecules Antenna pigment molecules Primary electron acceptor Reaction- center chlorophyll a Photon Electron transfer Reaction center Photosystem Transfer of energy Figure Laura Coronado Bio 10 Chap 7

How the Light Reactions Generate ATP and NADPH – Two types of photosystems cooperate in the light reactions: The water-splitting photosystem The NADPH-producing photosystem – The light reactions are located in the thylakoid membrane. Laura Coronado Bio 10 Chap 7

Primary electron acceptor Water-splitting photosystem Light H2OH2O 2 H Reaction- center chlorophyll 2e – – O2O2 + + Electron transport chain Energy to make ATP Primary electron acceptor 2e – Light NADPH-producing photosystem Reaction- center chlorophyll 2e – NADPH NADP  Figure Laura Coronado Bio 10 Chap 7

– An electron transport chain: Connects the two photosystems Releases energy that the chloroplast uses to make ATP Laura Coronado Bio 10 Chap 7

Light H2OH2O Thylakoid membrane 2e – O2O2 ATP NADP  Light Stroma Inside thylakoid Photosystem Electron transport chain NADPH ADP  P ATP synthase To Calvin cycle Electron flow HH HH HH HH HH HH HH Figure 7.11a Laura Coronado Bio 10 Chap 7

Water-splitting photosystem e – Photon ATP NADPH e – e – e – e – e – Photon e – NADPH-producing photosystem Figure 7.12 Laura Coronado Bio 10 Chap 7

THE CALVIN CYCLE: MAKING SUGAR FROM CARBON DIOXIDE – The Calvin cycle: Functions like a sugar factory within the stroma of a chloroplast Regenerates the starting material with each turn Laura Coronado Bio 10 Chap 7

ATP NADPH Calvin cycle ADP  P NADP  P P P P P P ADP  P ATP G3P sugar Three-carbon molecule G3P sugar RuBP sugar CO 2 (from air) Glucose (and other organic compounds) Figure Laura Coronado Bio 10 Chap 7

Evolution Connection: Solar-Driven Evolution – C 3 plants: Use CO 2 directly from the air Are very common and widely distributed – C 4 plants: Close their stomata to save water during hot and dry weather Can still carry out photosynthesis – CAM plants: Are adapted to very dry climates Open their stomata only at night to conserve water Laura Coronado Bio 10 Chap 7

Sugar C 4 Pathway (example: sugarcane) C 4 plant CAM plant Sugar Calvin cycle Calvin cycle Day Cell type 1 Four-carbon compound Night Four-carbon compound Cell type 2 CAM Pathway (example: pineapple) ALTERNATIVE PHOTOSYNTHETIC PATHWAYS CO 2 Figure 7.14 Laura Coronado Bio 10 Chap 7

Carbon dioxide 6 O 2 6 CO 2 6 H 2 O C 6 H 12 O 6 Water Glucose Photo- synthesis Oxygen gas Light energy Figure 7.UN1 Laura Coronado Bio 10 Chap 7

Light reactions CO 2 O2O2 H2OH2O (C 6 H 12 O 6 ) NADPH Light Sugar ATP ADP P NADP  Calvin cycle Figure 7.UN2 Laura Coronado Bio 10 Chap 7

Light reactions CO 2 O2O2 H2OH2O (C 6 H 12 O 6 ) NADPH Light Sugar ATP ADP P NADP  Calvin cycle Figure 7.UN3 Laura Coronado Bio 10 Chap 7

Light H2OH2O O2O2 Chloroplast Light reactions NADPH ATP Calvin cycle CO 2 NADP + ADP P Sugar (C 6 H 12 O 6 ) Stack of thylakoids Stroma Figure 7.UN5 Laura Coronado Bio 10 Chap 7

acceptor Water-splitting photosystem Photon H2OH2O 2 H Chlorophyll 2e – O2O2 + + Electron transport chain ATP NADPH-producing photosystem Chlorophyll NADPH NADP + e – 2e – – – e – acceptor ADP Photon 2 1 Figure 7.UN6 Laura Coronado Bio 10 Chap 7

NADPH Calvin cycle ADPP NADP  P ATP G3P CO 2 Glucose and other compounds Figure 7.UN7 Laura Coronado Bio 10 Chap 7