Chapter 10 (5)

Chapter 10 notes

Photosynthesis

Photosynthesis In Nature

All life acquires organic compounds for energy and carbon skeletons by one of two ways- Autotrophs: (autos = self, trophos = feed)

- they sustain themselves w/out eating other organisms


- plants are photoautotrophs because they use light as a source of energy- Heterotrophs: (hetero = other, different)

- live on compounds produced by other organisms

Heterotrophs are dependent on photoautotrophs for food and oxygen


Chloroplasts are the site of photosynthesis in plants

All green parts of plants have chloroplasts- color is from chlorophyll, the green pigment in the chloroplasts- chloroplasts are mainly found in the mesophyll, interior tissue of the leaf


- CO2 enters/O2 leaves the leaf through pores called stomata- water is absorbed through the roots

6CO2+6H2O+lightC6H12O6+6O2

Pathways of Photosynthesis

The oxygen produced from photosynthesis comes from the spliting of water, not CO2

- use of oxygen-18 to trace pathway

Hydrogen is extracted from water to make sugar- O2 is a waste product


Photosynthesis is made of two processes, each containing several steps1) Light reaction (AKA light harvesting reaction, light dependent reaction)2) Calvin cycle (AKA dark reaction, carbon fixation reaction)


The light rxn. converts solar energy to chemical energy; occurs in the thylokoid membrane- light absorbed drives a transfer of e- and hydrogen to an acceptor, NADP+

- NADP+ is reduced to NADPH- ATP is generated from ADP through photophosphorylation


The Calvin cycle stores the chemical energy in the form of sugar; occurs in the stroma- CO2 is incorporated w/ other molecules in the chloroplast through carbon fixation- fixed carbon is reduced to carbohydrates


Light is a form of energy known as electromagnetic energy; travels in waves- wavelength is the distance between crests of electromagnetic waves- electromagnetic spectrum: the entire range of radiation


Visible light is the band that ranges from 380 to 750 nm in wavelength- detected as colors by the human eye

The amount of energy is inversely related to the wavelength- shorter the wavelength, greater the energy


In the chloroplasts light is absorbed mainly by chlorophyll a.- blue and red light is primarily absorbed- green is the least absorbent color

Chlorophyll b absorbs light at slightly different wavelengths


Chloroplasts also contain accessory pigments, carotenoids- may broaden the spectrum of colors that can drive photosynthesis- act in photoprotection: they absorb excessive light that might damage chlorophyll


Light Dependent ReactionInside the thylakoid, chlorophyll is

organized w/ proteins into photosystems- light gathering “antenna complex”

Two types of photosystems- PS II, PS I


PS II = P680- best at absorbing light w/ a wavelength of 680 nm

PS I = P700- best at absorbing light w/ a wavelength of 700 nm


Light drives the synthesis of ATP and NADPH by energizing e- in the photosystems

Two possible routes for e- flow- cyclic- noncyclic


Noncyclic electron flow:- predominant route

1) P680 absorbs light, and splits a water molecule

2) e- absorb the energy and then cascade down an ETC from PS II to PS I

3) ATP is formed by noncyclic photophosphorylaton


4) P700 absorbs more light to reenergize the e- so it can pass down another ETC

5) NADP+ is the final e- accepter as it is reduced to NADPH (through redox rxns.)


Cyclic electron flow:- uses PS I, but not PS II- makes ATP, but not NADPH or oxygen

- cyclic photophosphorylation- because the Calvin cycle uses more ATP than NADPH, cyclic e- flow makes up the difference


Chloroplasts and mitochondria both generate ATP through chemiosmosis- both ETC’s pump protons across the membrane during the redox rxns. (transforms redox energy into proton motive force)- both use ATP synthase to couple diffusion to phosphorylation


Differences between oxidative phosphorylation and photophosphorylaton- in mitochondria, high energy e- are from food- in chlorplasts, photosystems capture light energy to drive e- transfer


Difference in the spatial organization- mitochondria: the inner membrane pumps protons from the matrix to the innermembrane space- chloroplasts: thylakoid pumps protons to the thylakoid space from the stroma


Calvin Cycle- uses ATP and NADPH to incorporate CO2 into sugar (does not make glucose but a 3 carbon sugar called G3P)

- 3 steps to the Calvin cycle


Step 1: Carbon fixation- CO2 is attached to RuBP (rxn. is catalyzed by rubisco)- a 6-carbon intermediate is created splits into 2 3-carbon molecules

Step 2: Reduction- ATP and NADPH are used to transform the molecule into G3P


- for every three molecules of CO2, there are 6 molecules of G3P; only 1 is a net gain

Step 3: Regeneration of RuBP- the remaining 5 molecule of G3P are reconverted back into RuBP to continue the Calvin cycle


In hot, dry environments, plants have adapted to maximize photosynthesis, while limiting water loss (transpiration)

Most plants will close the stomata to prevent water loss, but this limits CO2 intake


In most plants, carbon fixation leads to a 3-carbon compound; these plants are called C3 plants

- ex. rice, wheat, soy- in hot weather, the stomata close which deprives the plant of CO2; rubisco can accept O2 instead of CO2


- when oxygen binds to rubisco, photorespiration occurs, not photosynthesis

- process break the resulting compound down into CO2

- photorespiration does not make ATP (like cellular respiration) nor food (like photosynthesis)


Plants have adapted by developing C4 photosynthesis and CAM

C4 plants preface the Calvin cycle w/ an alternate mode of carbon fixation- ex. Sugarcane, corn, grasses- form a 4-carbon compound as its first product


In C4 plants there are 2 types of photosynthetic cells- Bundle-sheath cells are arranged into tightly packed sheaths around the veins; mesophyll cells are more loosely arranged


Step 1: In the mesophyll cells, PEP carboxylase adds CO2 to PEP (phosphoenolpyruvate); has a high affinity for CO2; forms the 4-carbon product

Step 2: 4-carbon compounds move from mesophyll cells to the bundle-sheath


Step 3: Within the bundle-sheath, the 4-carbon compound releases CO2 which then is combined via rubisco

The mesophyll cells pump CO2 into the bundle-sheath so the [CO2] is high enough for rubisco to accept CO2 and not oxygen


CAM plants open their stomata at night, but keep them closed during the day- found in succulent plants- at night the CO2 is incorporated into other organic acids; this mode of carbon fixation is called crassulacean acid metabolism (CAM)


The mesophyll cells of CAM plants store the acids in their vacuoles until they are needed the following day

C4 and cam are similar in that CO2 is first incorporated into an intermediate before entering the Calvin cycle

However, C4 plants have a spatial separation of steps, while CAM is temporal

Chapter 10 (5)

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