Chapter 7 Photosynthesis: Using Light to Make Food Lectures by Chris C. Romero, updated by Edward J. Zalisko PowerPoint Lectures for Campbell Essential Biology, Fourth Edition Eric Simon, Jane Reece, and Jean Dickey Campbell Essential Biology with Physiology, Third Edition Eric Simon, Jane Reece, and Jean Dickey
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
LM Plants (mostly on land) PHOTOSYNTHETIC AUTOTROPHS Photosynthetic Protists (aquatic) Photosynthetic Bacteria (aquatic) Forest plants Kelp, a large alga Micrograph of cyanobacteria Figure 7.1
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. Chlorophyll molecules that capture light energy are built into the thylakoid membrane. Stomata are tiny pores in leaves where carbon dioxide enters and oxygen exits.
TEM LM Vein Chloroplast Inner membrane Outer membrane Granum Stroma Thylakoid CO 2 Stomata O 2 Leaf cross section Interior cell Figure 7.2-2
The Overall Equation for Photosynthesis In the overall equation for photosynthesis, notice that: The reactants of photosynthesis are the waste products of cellular respiration. Light energy 6 6 H2O C6H12O6 CO2 Photo- Carbon Water synthesis Glucose dioxide 6 O2 Oxygen gas
In 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.
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
Light H 2 O Chloroplast Light reactions ATP NADPH O 2 Figure 7.3-1
Light H 2 O Chloroplast CO 2 Light reactions NADP + ADP P Calvin cycle ATP NADPH O 2 Sugar (C 6 H 12 O 6 ) Figure 7.3-2
THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY Chloroplasts: Are chemical factories powered by the sun Convert solar energy into chemical energy
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.
Increasing wavelength 10 5 nm 10 3 nm 1 nm 10 3 nm 10 6 nm 1 m 10 3 m Gamma rays X-rays UV Infrared Microwaves Radio waves Visible light 380 400 500 600 700 750 Wavelength (nm) Wavelength = 580 nm Figure 7.4
Chloroplast Light Reflected light Absorbed light Transmitted light Figure 7.6
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
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 yelloworange light reflected from carotenoids.
Animation: Light and Pigments
How Photosystems Harvest Light Energy All chloroplast pigments are built into the thylakoid membrane, where they are organized into lightharvesting complexes. Light behaves as photons, discrete packets of energy. 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.
(a) Absorption of a photon e Excited state Light Photon Chlorophyll molecule Heat Ground state Light (fluorescence) (b) Fluorescence of a glow stick Figure 7.8
A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna. Chloroplast Pigment molecules Thylakoid membrane Figure 7.9-2
A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna. Chloroplast Figure 7.9-1
A photosystem is a group of chlorophyll and other molecules that function as a light-gathering antenna. Chloroplast Pigment molecules Photon Electron transfer Primary electron acceptor Reactioncenter chlorophyll a Reaction center Antenna pigment molecules Thylakoid membrane Transfer of energy Photosystem Figure 7.9-3
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
Primary electron acceptor 2e Light H 2 O Reactioncenter chlorophyll 2e 2 H + + 1 2 O 2 Water-splitting photosystem Figure 7.10-1
Primary electron acceptor Energy to make ATP 2e Light H 2 O Reactioncenter chlorophyll 2e 2 H + + 1 2 O 2 Water-splitting photosystem Figure 7.10-2
Primary electron acceptor Energy to make ATP Primary electron acceptor 2e 2e NADP NADPH 2e Light Light H 2 O Reactioncenter chlorophyll Reactioncenter chlorophyll NADPH-producing photosystem 2e 2 H + + 1 2 O 2 Water-splitting photosystem Figure 7.10-3
The light reactions are located in the thylakoid membrane. An electron transport chain: Connects the two photosystems Releases energy that the chloroplast uses to make ATP
The thylakoid membrane converts light energy to chemical energy of NADPH and ATP To Calvin cycle Light Light NADPH H + ATP H NADP ADP P Stroma Thylakoid membrane Inside thylakoid H 2 O 1 2 Electron transport chain Photosystem Photosystem 2e O 2 H H H H ATP synthase Electron flow H + Figure 7.11
Similarities and Differences between Cellular Respiration and Photosynthesis Electron transport chain pumps hydrogen ions across a membrane. Across the inner mitochondrial membrane in respiration Across the thylakoid membrane in photosynthesis ATP synthase uses energy stored by H + gradient to make ATP. Difference: Food provides the high energy electrons in cellular respiration. Light excited electrons flow down the electron transport chain.
e ATP e e NADPH e e e e Water-splitting photosystem NADPH-producing photosystem Figure 7.12
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
CO 2 (from air) P RuBP sugar Three-carbon molecule P P Calvin cycle Figure 7.13-1
CO 2 (from air) P RuBP sugar Three-carbon molecule P Calvin cycle P ATP ADP P NADPH P NADP G3P sugar Figure 7.13-2
CO 2 (from air) P RuBP sugar Three-carbon molecule P Calvin cycle P ATP ADP P NADPH G3P sugar P P NADP G3P sugar G3P sugar P Glucose (and other organic compounds) Figure 7.13-3
CO 2 (from air) P RuBP sugar Three-carbon molecule ADP P ATP P Calvin cycle P ATP ADP P NADPH G3P sugar P P NADP G3P sugar G3P sugar P Glucose (and other organic compounds) Figure 7.13-4
Evolution Connection: Solar-Driven Evolution C 3 plants: Use CO 2 directly from the air Are very common and widely distributed Examples: soybeans, oats, wheat and rice
On hot dry days, plants close their stomata to reduce their water loss. This also prevents CO2 from entering the leaves. Sugar production decreases. 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
ALTERNATIVE PHOTOSYNTHETIC PATHWAYS C4 Pathway (example: sugarcane) CAM Pathway (example: pineapple) Cell type 1 CO 2 CO 2 Night Four-carbon compound Four-carbon compound Cell type 2 CO 2 CO 2 Calvin cycle Calvin cycle Sugar C 4 plant Sugar CAM plant Day Figure 7.14
Similarities and differences between C 4 and CAM pathways Similarities: Differences: CO 2 is first incorporated into organic molecules before it enters Calvin cycle In C4 plants, the initial steps occur in separate locations In CAM plants, the two steps occur at separate times (night and day) but occur in the same cell
BioFlix: Photosynthesis