• Chapter 7Photosynthesis

AP Bio Ch 7 Goals Photosynthesis

• Distinguish b/t autotrophic and heterotrophic nutrition.• Distinguish b/t photosynthetic autotrophs and chemosynthetic autotrophs.• Describe the location and structure of the chloroplast.• Explain how chloroplast structure relates to its function.• Write a summary equation for photosynthesis.• Explain the role of REDOX reactions in photosynthesis.• Describe the wavelike behaviors of light.• Describe the relationship b/t an action spectrum & an absorption spectrum.• Explain why the absorption spectrum for chlorophyll differs from the action spectrum for

photosynthesis.• List the wavelengths of light that are most effective for photosynthesis.• Explain what happens when chlorophyll or accessory pigments absorb photons.• List the components of a photosystem and explain their function.• Trace electron flow through photosystems II & I.• Compare cyclic and noncyclic electron flow and explain the relationship b/t these components of

the light reactions.• Summarize the light reactions with an equation and describe where they occur.• Describe important differences in chemiosmosis b/t oxidative phosphorylation in mitochondria &

photophosphorylation in chloroplasts.• Summarize the carbon-fixing reactions of the Calvin cycle & describe changes that occur in the

carbon skeleton of the intermediates.• Describe the role of ATP & NADPH in the Calvin cycle.• Describe what happens to rubisco when the oxygen concentration is much higher than carbon

dioxide.• Describe the major consequences of photorespiration.• Describe two important photosynthetic adaptations that minimize photorespiration.• Describe the fate of photosynthetic products.

Photosynthesis in nature• Autotrophs - biotic producers;

photoautotrophs – E from sunlightchemoautotroph: E from inorganic chemicals

• Heterotrophs - biotic consumers; obtains organic food by eating other organisms or their by-products (includes decomposers)

How do chemoautotrophs do this?

• Sulfur bacteria. Certain colorless bacteria share the ability of chlorophyll-containing organisms to manufacture carbohydrates from inorganic raw materials, but they do not use light energy for this. They secure the necessary energy by oxidizing some reduced substance present in their environment. The free energy released by the oxidation is harnessed to the manufacture of food.

• For example, some chemoautotrophic sulfur bacteria oxidize H2S in their surroundings (e.g., the water of sulfur springs) to produce energy: 2H2S + O2 → 2S + 2H2O; ΔG = -100 kcal

• They then use this energy to reduce carbon dioxide to carbohydrate (like the photosynthetic purple sulfur bacteria). 2H2S + CO2 → (CH2O) + H2O + 2S

The chloroplast• Sites of

photosynthesis• Pigment: chlorophyll• Plant cell: mesophyll• Gas exchange:

stomata• Double membrane• Thylakoids, grana,

stroma• Pigments located in

thylakoid membrane

Overall Reaction

Sun’s E + 6H2O + 6CO2 ------ C6H12O6 + 6O2

Reduced?

Oxidized?

CO2 gets reduced when energized e- from water get added to it, along with H+ making sugar!

Can you feel the goose bumps? Does it not give you chills? Look at Jimmy Pratte; he’s got ‘em!

Nature of Light

• order of colors is determined by wavelength. • The longer the wl of visible light, the more red

the color. • The shorter the wl the more violet

energy of is inversely porportional to the wavelength: longer wavelengths have less energy than do shorter ones.

Pigments

• Light absorbing molecule• color comes from the wl of light reflected ( those not absorbed). • chlorophyll absorbs all wl of visible light except green• Black pigs absorb all wl that strike them. • White pigments/lighter colors reflect all or almost all• All photosynthetic orgs have chloro a. • Accessory pigs absorb E – broaden range

– Chloro b, xanthophylls, carotenoids • Photosynthetic pigs absorb mostly from red & blue wl

xanthophylls

Action & absorption spectrum

• The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons.

• Absorption spectrum shows the wl of light absorbed by various pigments

• Red & blue…

Photosynthesis: an overview

• Redox process• H2O is split, e- (along

w/ H+) are transferred to CO2, reducing it to sugar

• 2 major steps:

1. light reactions (“photo”) √NADP+ (electron acceptor) to NADPH √Photophosphorylation:

ADP ---> ATP

2. Calvin cycle (“synthesis”)

Carbon

fixation: carbon into organics

Photosystems

• Light harvesting units of thylakoid mem

• antenna pigs, rx center chloro a & primary e- acceptor

• Antenna pigs (broaden range of absorption) struck by photons

• e- is passed to rx center

• Excited e- from chloro is trapped by primary e- acceptor

Light Dep Rx Noncyclic electron flow

• Photosystem 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 (photophosphorylation by chemiosmosis)

• Photosystem I (P700): fallen’ e- replace excited e- to primary e- acceptor 2nd ETC (NADP+ reductase) transfers e- to NADP+ ---> NADPH (...to Calvin cycle…)

• These photosystems produce equal amounts of ATP and NADPH

• http://www.youtube.com/watch?v=BK_cjd6Evcw

The Calvin cycle

• 3 molecules of CO2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P)

• Phases: 1- Carbon fixation~ each CO2 is attached to RuBP (rubisco enzyme)

• 2- Reduction~ electrons from NADPH reduces to G3P; ATP used up

• 3- Regeneration~ 5 G3P

rearranged to RuBP; ATP used; cycle continues

http://www.youtube.com/watch?v=mHU27qYJNU0&feature=related

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

• Is Oxygen produced? Why?

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

• Cyclic photophosphorylation

Alternative carbon fixation methods, I• Photorespiration:

hot/dry days; stomata close; CO2 decrease, O2 increase 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)

Alternative carbon fixation methods, II• 2- CAM

plants: open stomata during night, close during day… why?

• (crassulacean acid metabolism); cacti, pineapples, etc.

A review of photosynthesis

Separation of pigments by paper chromatography – Part A of lab

retention factor (Rf value)

Rf value is the ratio between how far the component travels and the distance the solvent travels from a common starting point (the origin).

If one of the sample components moves 2.5 cm up the paper and the solvent moves 5.0 cm, then the Rf value is 0.5.

You can use Rf values to identify different components as long as the solvent, temperature, pH, and type of paper remain the same. In the image below, the light blue shading represents the solvent and the dark blue spot is the chemical sample.

Tape, Measure, ID & place name on it. Staple to lab

Chlorophyll a – bright greeChlorophyll b – olive greenCarotene – orange yellowXanthophyll – yellow (may get 2 different types)

Photo lab part B• Step # 6 only add 1 ml of phosphate buffer to cuvette 5 – it says to add 1 ml

and then a few sentences later has you adding another ml. DON’T. Only add 1 ml.

• positive control: a positive control should give the desired outcome of the experiment, provided that all the reagents and equipment are functioning properly. For example, if your experiment results in the ability of bacteria to grow on a petri plate containing antibiotic, your positive control will be bacteria that are known to carry the appropriate drug resistance marker. Even if none of your experimental bacteria grow, as long as there is growth of the positive control you know that growth was possible.

• negative control: a negative control should be designed to not give the desired outcome of the experiment. In the example above, bacteria which do not carry a drug resistance marker should not be able to grow on a petri plate containing antibiotic. If growth is observed, it is a red flag that something is wrong with the experiment. (What could be one reason for growth?)