Chapter 10 Photosynthesis – Part 1

Plan before Thanksgiving Break• Ch. 10 discussion• Quiz on Ch. 9 - Wed., Oct. 28• Quiz on Ch. 10 - TBD• Cellular Respiration & Photosynthesis

Essays due Wed., Nov. 4• Lab 4 (Photosynthesis) and Lab 3 (Mitosis

and Meiosis)• Brief discussion of Ch. 11 and 12

Assignment Nov. 16 - 27

• DATES, CHAPTERS TENTATIVE BUT VERY LIKELY TO BE AS STATED

• Read/review textbook Ch. 13 – 17 on Cellular and Molecular Basis of Genetics

• Take Home Test distributed on Nov. 16

• Take Home Test due Nov. 30

Assignment Dec. 14 – Jan. 4• Read/review textbook Ch. 50 – 55 on

Ecology

• Take Home Test distributed week of Dec. 14

• Take Home Test due Jan. 4, 2010

Photosynthesis in nature• Autotrophs:

biotic producers: photoautotrophs; chemoautotrophs;

obtain organic food without eating other organisms

• Heterotrophs: biotic consumers:

obtain organic food by eating other organisms or their by-products (includes decomposers)

Principles of Energy Harvest (again!)

Photosynthesis

vs.

Cellular respiration

Principles of Energy Harvest -Understand!

Photosynthesis Cellular respiration

Endergonic Exergonic

Products: O2, Products: CO2,C6H12O6 H2O, ENERGY

Reactants: CO2, Reactants: O2, H2O, ENERGY C6H12O6

Chloroplasts Mitochondria

Principles of Energy Harvest

Cell respiration is catabolic

• Breaks down glucose

Photosynthesis is anabolic

• Synthesizes glucose

ESSAY!

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

ESSAY!

• Powered by light, the green parts of plants produce organic compounds and O2 from CO2 and H2O.

• Using glucose as our target product, the equation describing the net process of photosynthesis is:– 6CO2 + 6H2O + light energy -> C6H12O6 + 6O2

• In reality, photosynthesis adds one CO2 at a time:

– CO2 + H2O + light energy -> CH2O + O2

– CH2O represents the general formula for a sugar, (CH2O)n.

Deciphering the equation• One of the first clues to the mechanism of

photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2:

CO2 + 2 H2O -> CH2O + H2O + O2

Did you understand?

• In the photosynthesis reaction,

CO2 + 2 H2O -> CH2O + H2O + O2

Where do each of these components of the products come from?

– Oxygen in glucose?– Oxygen in O2 gas?– Hydrogen in water?– Carbon in glucose?

Deciphering the equation – typical AP Biology Exam question

• One of the first clues to the mechanism of photosynthesis came from the discovery that the O2 given off by plants comes from H2O, not CO2:

CO2 + 2 H2O -> CH2O + H2O + O2

Deciphering the equation – understand this!

Photosynthesis: an overview• Redox process

• H2O is split, and H (e- and H+) is transferred to CO2, reducing it to sugar

• Detect redox reaction with DPIP in Lab 4A

Photosynthesis is a redox reaction• Photosynthesis reverses the direction of electron

flow in respiration.• Water is split and electrons transferred with H+ from

water to CO2, reducing it to sugar.• Chemically: polar covalent bonds (unequal sharing)

are converted to nonpolar covalent bonds (equal sharing).– The reaction is strongly endergonic; light

boosts the potential energy of electrons as they move from water so they can be used to make sugar.

The chloroplast• Eukaryotic organelle• Site of photosynthesis• Pigment: chlorophylls• Plant cell: mesophyll• Gas exchange: stomata• Double membrane (or

is it triple?)• Thylakoids in grana,

stroma

• A typical mesophyll cell has 30-40 chloroplasts, each about 2-4 microns by 4-7 microns long.

• Each chloroplast has two membranes around a central aqueous space, the stroma.

• In the stroma aremembranous sacs, the thylakoids.

Chloroplasts are the size of medium to large bacteria

The chloroplast: 3 membranes?

• Any green part of a plant has chloroplasts.

• However, the leaves are the major site of photosynthesis for most plants.– There are about half a million chloroplasts per

square millimeter of leaf surface.

• The color of a leaf comes from chlorophyll, the green pigment in the chloroplasts.– Chlorophyll plays an important role in the

absorption of light energy during photosynthesis.

Chloroplasts - the sites of photosynthesis in plants

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 10.9

Chlorophyll molecule contains Mg2+

Chloroplasts are found mainly in mesophyll cells forming the tissues in the interior of the leaf.

• O2 exits and CO2 enters the leaf through microscopic pores, stomata, in the leaf.

• Veins deliver water from the roots and carry off sugar from mesophyll cells to other plant areas.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 10.2

Two types of veins• Xylem carries water and minerals from roots to

leaves (and all other parts)

• Phloem carries sugar water from leaves (to all other parts)

Fig. 10.2

Photosynthesis: 2 major steps1. Light reactions

(“photo”) • NADP+ (electron

acceptor nicotinamide adenine dinucleotide phosphate) to NADPH

• Photophosphorylation: ADP ---> ATP

2. Calvin cycle (“synthesis”)

• Carbon fixation: carbon into organics

1. Photosystems: first Ps II, then Ps I

• Light harvesting units ofthe thylakoid membrane• Composed mainly of protein and pigment “antenna complexes”1. “Antenna pigment” molecules are struck by photons (light energy)2. Energy is passed to reaction centers (redox location)3. Excited e- from chlorophyll is trapped by a primary

e- acceptor

Understand the photosystem scheme

Noncyclic electron flowPhotosystem II (P680):

• photons excite chlorophyll e- to an acceptor

• e- are replaced by splitting

of H2O (release of O2)

• e-’s travel to Photosystem I down an electron transport chain

• as e- fall, ADP ---> ATP (noncyclic photophosphorylation)

Noncyclic electron flowPhotosystem I (P700):

• ‘fallen’ e- replace excited e- to primary e- acceptor

• 2nd ETC (Fd~NADP+ reductase) transfers e- to NADP+ ---> NADPH (...to Calvin cycle…)

• These photosystems produce equal amounts of ATP and NADPH

Light reactions animations• http://www.sumanasinc.com/webcontent/anisamples/

majorsbiology/harvestinglight.swf

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio13.swf::Photosynthetic Electron Transport and ATP Synthesis

• http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120072/bio12.swf::Cyclic and Noncyclic Photophosphorylation

The Calvin cycle – the “Dark” Reactions• 3 molecules of CO2 are ‘fixed’

into glyceraldehyde 3-phosphate (G3P):

1. Carbon fixation: each CO2 is attached to RuBP (by “rubisco enzyme”)

2. Reduction: electrons from NADPH reduce to G3P; ATP used up

3. Regeneration: G3P rearranged to RuBP; ATP used; cycle continues

Calvin cycle animations

• http://www.sinauer.com/cooper/4e/animations0305.html

• http://www.cells.de/cellseng/1medienarchiv/Zellfunktionen/Memb_Vorg/Photosynthese/Dunkel_u_Staerke/Calvin-Benson-Zyklus/calvin_lin.htm

Calvin Cycle, net synthesis

• For each G3P (and for 3 CO2)…….Consumption of 9 ATP’s & 6

NADPH (light reactions regenerate these molecules)

• G3P can then be used by the plant to make glucose and other organic compounds

Cyclic electron flow• Alternative cycle when

ATP is deficient• Photosystem I used

but not II; produces ATP but no NADPH

• Why? The Calvin cycle consumes more ATP than NADPH…….

• Cyclic photophosphorylation

Alternative carbon fixation methods

• Hot/dry days; stomata close; CO2 decreases, O2 increases in leaves; O2 added to rubisco; no ATP or food generated

Two Solutions…..1. C4 plants: 2 photosynthetic

cells, bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO2 in mesophyll; new 4C molecule releases CO2 (grasses)

Avoiding Excessive Photorespiration

ESSAY!

Alternative carbon fixation methods

2. CAM plants: open stomata during night, close during day (crassulacean acid metabolism); cacti, pineapples, etc.

Avoiding Excessive Photorespiration

Alternative carbon fixation methods – Another common AP Biology exam question

A review of photosynthesis

• Light, like other form of electromagnetic energy, travels in rhythmic waves.

• The distance between crests of electromagnetic waves is called the wavelength.– Wavelengths of electromagnetic radiation

range from less than a nanometer (gamma rays) to over a kilometer (radio waves).

Light and Photosynthesis - 1

• The electromagnetic spectrum.• The most important segment for life is a narrow

band between 380 to 750 nm, visible light.

• Other light properties are those of a discrete particle, the photon.

• The amount of energy packaged in a photon is inversely related to its wavelength.– Photons with shorter wavelengths pack more energy.

• While the sun radiates a full electromagnetic spectrum, the atmosphere selectively screens out most wavelengths, permitting only visible light to pass in significant quantities.

Light and Photosynthesis -2

• When light meets matter, it may be reflected, transmitted, or absorbed.

– Different pigments absorb photons of different wavelengths.

– A leaf looks green because chlorophyll, the dominant pigment, absorbs red and blue light, while transmitting and reflecting green light.

• A spectrophotometer measures the ability of a pigment to absorb various wavelengths of light.

http://www.youtube.com/watch?v=V1vXCmhWw40&feature=related

– It beams narrow wavelengths of light through a solution containing a pigment and measures the fraction of light transmitted at each wavelength.

– An absorption spectrum plots a pigment’s light absorption versus wavelength.

• Photosynthesis pigments

– Chlorophyll a, the dominant pigment, absorbs best in the red and blue wavelengths, and least in the green.

– Other pigments with different structures have different absorption spectra.

• Collectively, these photosynthetic pigments determine an overall action spectrum for photosynthesis.– An action spectrum measures changes in some

measure of photosynthetic activity (for example, O2 release) as the wavelength is varied.

• The action spectrum of photosynthesis was first demonstrated in 1883 through an elegant experiment by Thomas Engelmann.– In this experiment, different segments of a filamentous

alga were exposed to different wavelengths of light.– Areas receiving wavelengths favorable to photosynthesis

should produce excess O2.

– Engelmann used the abundance of aerobicbacteria clustered along the alga as a measure of O2 production.

• The action spectrum of photosynthesis does not match exactly the absorption spectrum of any one photosynthetic pigment

• Only chlorophyll a participates directly in the light reactions but accessory photosynthetic pigments absorb light and transfer energy to chlorophyll a.– Chlorophyll b, with a slightly different structure than

chlorophyll a, has a slightly different absorption spectrum and funnels the energy from these wavelengths to chlorophyll a.

– Carotenoids can funnel the energy from other wavelengths to chlorophyll a and also participate in photoprotection against excessive light.