Photosynthesis Ch. 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). 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 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. C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy enzymes 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)

Leaf Anatomy

Photosynthesis: redox process Oxidation-reduction reaction: –Oxidation-loss of electrons from one substance –Reduction-addition of electrons to another substance

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 tumn-colours-are-se-8810.jpg

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 CO 2 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; O 2 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 O 2

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; O 2 released Calvin Cycle produces: ADP and NADP+ that are used by the light reactions; glucose produced

Carbon enters the cycle in the form of CO 2 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 CO 2 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 CO 2 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

Calvin Cycle

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 O 2 to RuBP rather than CO 2, since CO 2 can’t enter the plant (photorespiration) This yields no sugar molecules or ATP

Special Adaptations that Save Water C4 Plants=plants that incorporate CO 2 before the Calvin cycle Different plant anatomy: –Bundle-sheath cells— thylakoids not stacked –Calvin cycle confined to chloroplasts of bundle- sheath cells –Mesophyll cells loosely arranged

C4 Plants 1.In the mesophyll, CO 2 is added to phosphenolpyruvate (PEP) to form oxaloacetate (4-carbon compound) PEP carboxylase-high affinity to CO 2 and no affinity for O 2, thus no photorespiration possible 2.Oxaloacetate converted to malate (4-carbon compound)

C4 Plants 3.Mesophyll export malate through plasmodesmata to bundle-sheath cells Malate releases CO 2, which is then fixed by rubisco in the Calvin cycle Process minimizes photorespiration and enhances sugar production by maintaining a CO 2 concentration sufficient for rubisco to accept CO 2 rather than oxygen

Crassulacean acid metabolism (CAM) CAM plants=succulent plants that open their stomata primarily at night and close them during the day (opposite most plants) At night, CO 2 is taken in by open stomata and incorporated into a variety of organic acids. Organic acids stored in vacuoles of mesophyll cells until morning, when stomata close

During daytime, light reactions supply ATP and NADPH for the Calvin cycle. At this time, CO 2 is released from the organic acids made the previous night and is incorporated into sugar in the chloroplast

Are similar in that CO 2 is first incorporated into organic intermediates before it enters the Calvin cycle Differ in that the initial steps of carbon fixation in C4 plants are structurally separate from the Calvin cycle; in CAM plants, the two steps occur at separate times Regardless of whether the plant uses C3, C4, or CAM pathway, all plants use the Calvin Cycle to produce sugar from CO 2 The CAM and C4 pathways:

How Photosynthesis Moderates Global Warming Photosynthesis has an enormous impact on the atmosphere. –It swaps O 2 for CO 2.

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 CO 2.

Global Warming Greenhouse gases (CO 2, CH 4, 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 water

References Unless otherwise noted, pictures are from Essential Biology with Physiology, 2 nd edition. Campbell, Reece, and Simon. (2007).