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PhotosynthesisCh. 7
Ms. Haut
• All cells need energy to carry out their activities• All energy ultimately comes from the sun• Photosynthesis—process in which some of the
solar energy is captured by plants (producers) and transformed into glucose molecules used by other organisms (consumers).
6CO2 + 6H2O C6H12O6 + 6O2
Light energy
enzymes
Basics of Photosynthesis
• Glucose is the main source of energy for all life. The energy is stored in the chemical bonds.
• Cellular Respiration—process in which a cell breaks down the glucose so that energy can be released. This energy will enable a cell to carry out its activities.
C6H12O6 + 6O2 6CO2 + 6H2O + energyenzymes
Basics of Photosynthesis
Basics of Photosynthesis
• Autotroph—organisms that synthesize organic molecules from inorganic materials (a.k.a. producers)– Photoautotrophs—use light
as an energy source (plants, algae, some prokaryotes)
• Heterotroph—organisms that acquire organic molecules from compounds produced by other organisms (a.k.a. consumers)
http://www.flatrock.org.nz/topics/animals/assets/conscious_animal.jpg
Leaf Anatomy
Photosynthesis: redox process
• Oxidation-reduction reaction: – Oxidation-loss of electrons from one
substance– Reduction-addition of electrons to another
substance
6CO2 + 6H2O C6H12O6 + 6O2
Light energy
enzymes
oxidation
reduction
Releases e- (and H+ ions)
gains e- (and H+ ions)
6CO2 + 6H2O C6H12O6 + 6O2
Light energy
enzymes
oxidation
reduction
Releases e- (and H+ ions)
gains e- (and H+ ions)
Light energy
enzymes
Light energy
enzymes
oxidation
reduction
oxidation
reduction
Releases e- (and H+ ions)
gains e- (and H+ ions)
A Photosynthesis Road Map
– Photosynthesis is composed of two processes:
• The light reactions convert solar energy to chemical energy.
• The Calvin cycle makes sugar from carbon dioxide.
Figure 7.4
The Nature of Sunlight– Sunlight is a type of energy called radiation
• Or electromagnetic energy.– The full range of radiation is called the electro-magnetic
spectrum.– Light may be reflected, transmitted, or absorbed when it
contacts matter
Chloroplasts: Nature’s Solar Panels
• Chloroplasts absorb select wavelengths of light that drive photosynthesis.
• Thylakoids trap sunlight
Photosynthetic Pigments
• Pigments-substances that absorb light (light receptors)
• Wavelengths that are absorbed disappear• Wavelengths that are transmitted and reflected as
the color you see
http://image.guim.co.uk/Guardian/environment/gallery/2007/nov/02/1/[email protected]
Plant Pigments
• Chlorophyll a – absorbs blue-violet and red light, thus appears green
• Accessory pigments• Absorb light of varying wavelengths and
transfer the energy to chlorophyll a • Chlorophyll b-yellow-green pigment• Carotenoids-yellow and orange pigments
Photosynthesis: 2 stages
1. Light reactions—convert light energy to chemical bond energy in ATP and NADPH
• Occurs in thylakoid membranes in chloroplasts
2. Calvin Cycle—carbon fixation reactions assimilate CO2 and then reduce it to a carbohydrate
• Occurs in the stroma of the chloroplast• Do not require light directly, but requires
products of the light reactions
Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released
Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced
How Photosystems Harvest Light Energy
• Photosystem: assemblies of several hundred chlorophyll a, chlorophyll b, and carotenoid molecules in the thylakoid membrane
– form light gathering antennae that absorb photons and pass energy from molecule to molecule
– Photosystem I—specialized chlorophyll a molecule, P700
– Photosystem II—specialized chlorophyll a molecule, P680
Light Reactions
• Light drives the light reactions to synthesize NADPH and ATP• Includes cooperation of both photosystems, in which e- pass continuously from water to NADP+
1. When photosystem II absorbs light an e- is excited in the reaction center chlorophyll (P680) and gets captured by the primary e- acceptor.
• This leaves a hole in the P680
2. To fill the hole left in P680, an enzyme extracts e- from water and supplies them to the reaction center
• A water molecule is split into 2 H+ ions and an oxygen atom, which immediately combines with another oxygen to form O2
3. Each photoexcited e- passes from primary e- acceptor to photosystem I via an electron transport chain.
• e- are transferred to e- carriers in the chain
4. As e- cascade down the e- transport chain, energy is released and harnessed by the thylakoid membrane to produce ATP • This ATP is used to make glucose during Calvin cycle
5. When e- reach the bottom of e- transport chain, it fills the hole in the reaction center P700 of photosystem I.
• Pre-existing hole was left by former e- that was excited
6. When photosystem I absorbs light an e- is excited in the reaction center chlorophyll (P700) and gets captured by the primary e- acceptor.
• e- are transferred by e- carrier to NADP+ (reduction reaction) forming NADPH
• NADPH provides reducing power for making glucose in Calvin cycle
Chemiosmosis• Energy released from ETC is
used to pump H+ ions (from the split water) from the stroma across the thylakoid membrane to the interior of the thylakoid.– Creates concentration gradient
across thylakoid membrane– Process provides energy for
chemisomostic production of ATP
Light reactions produce: ATP and NADPH that are used by the Calvin cycle; O2 released
Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced
• Carbon enters the cycle in the form of CO2 and leaves in the form of sugar (glucose)
• The cycle spends ATP as an energy source and consumes NADPH as a reducing agent for adding high energy e- to make sugar
• For the net synthesis of this sugar, the cycle must take place 2 times
The Calvin Cycle: Making Sugar from Carbon Dioxide
The Calvin Cycle: Carbon Fixation
1. 3 CO2 molecules bind to 3 molecules of ribulose bisphosphate (RuBP) using enzyme, RuBP carboxylase (rubisco)• Produces 6
molecules of 3-phosphoglycerate (3-PGA)
The Calvin Cycle: Reduction
2. 6 ATP molecules transfer phosphate group to each 3-PGA to make 6 molecules of 1,3-diphosphoglycerate• 6 molecules of
NADPH reduce each 1,3-bisphosph. to make 6 molecules of glyceraldehyde 3-phosphate (G3P)
The Calvin Cycle: Regeneration of RuBP
3. One of the G3P exits the cycle to be used by the plant the other 5 molecules are used to regenerate the CO2 acceptor (RuBP): 3 molecules of ATP are used to convert 5 molecules of G3P into RuBP3
• 3 more CO2 molecules enter the cycle, following the same chemical pathway to release another G3P from the cycle.
• 2 G3P molecules can be used to make glucose
The Calvin Cycle: Regeneration of RuBP
Special Adaptations that Save Water
• C3 Plants=plants that only use Calvin Cycle to fix carbon
• During dry conditions C3 plants conserve water by closing stomata
• Plants then fix O2 to RuBP rather than CO2, since CO2 can’t enter the plant (photorespiration)
• This yields no sugar molecules or ATP
How Photosynthesis Moderates Global Warming
• Photosynthesis has an enormous impact on the atmosphere.– It swaps O2 for CO2.
http://www.destination360.com/asia/malaysia/images/s/borneo-rainforest.jpg
How Photosynthesis Moderates Global Warming
• Greenhouses used to grow plant indoors– Trap sunlight that warms the air inside.
• A similar process, the greenhouse effect,– Warms the atmosphere.
– Is caused by atmospheric CO2.
Global Warming
• Greenhouse gases (CO2, CH4, CFC’s) are the most likely cause of global warming, a slow but steady rise in the Earth’s surface temperature.– Destruction of forests may
be increasing this effect.– Combustion of fossil fuels
Global Warming Consequences
• Polar ice caps melting• Rise in sea level and
flooding of current coastline– New York, Miami, Los
Angeles underwater
• Change in types of plants—more adapted to warmer temps. and less waterhttp://i.treehugger.com/images/2007/10/24/melting%20ice-jj-002.jpg
References
• Unless otherwise noted, pictures are from Essential Biology with Physiology, 2nd edition. Campbell, Reece, and Simon. (2007).
