Unformatted text preview: Chapter 10 PHOTOSYNTHESIS
From sunlight to Snickers
*Overview of photosynthesis
*What is light?
*Capturing light energy
*Light dependent reactions
*Light independent reactions
Why is Photosynthesis Important?
*Autotrophs vs. heterotrophs
*What do autotrophs provide heterotrophs with?
*What do heterotrophs provide autotrophs with?
6CO2 + 12 H2O + llight energy →
C6H12O6 + 6O2 + 6H2O
*2 sets of reactions, fig 10.1
*Photo = light dependent reactions
*Synthesis = light independent reactions
Synthesis light *What is light?
*Electromagnetic energy – many different energy levels = spectrum.
*Visible light is a small part of the electromagnetic spectrum, fig 10.4.
*Visible light appears white but contains many different wavelengths (energy
levels) of light that we interpret as different colors.
*If an object absorbs all of the wavelengths (or colors) of light it appears
black, if it reflects all of the colors it appears white.
*If an object appears green, what color is its reflecting? Absorbing?
*If an object appears orange, what color is it reflecting? Absorbing?
*Chloroplasts – a review, fig 10.2.
*Pigments in chloroplasts
*Chlorophyll a and b; absorb blue and red light, reflect greens
*Carotinoids: β carotene, xanthophylls, etc.; absorb blue and green light, 1 reflect yellow/orange/red.
*Chlorophyll b and the carotenoids are accessory pigments. Carotenoids also
protect the plant against UV (sunburn).
*How do pigments capture light energy?
*Photons of light hit a pigment.
*Electrons in pigments absorb the energy and move to a higher energy level
(electron shell), fig 10.9.
*In a test tube the electrons eventually return to ground level and energy is
released as heat and electromagnetic energy = fluorescence.
*How do pigments in plants transform light energy to ATP and NADPH?
*Photosystems organize pigments, fig 10.11
*Antenna complex – accessory pigments absorb light energy, an electron
is excited and transfers the energy to another pigment and another, etc.
*Reaction center – a chlorophyll a molecule is the last to receive the
energy and donates an energized electron to an electron acceptor.
Light Dependent Reactions
*Purple non-sulfur and purple sulfur bacteria have only 1 photosystem and
produce ATP but no NADPH = cyclic photophosphorylation.
*Plants, green algae and cyanobacteria have 2 photosystems and produce ATP
and NADPH = non-cyclic photophosphrylation.
*Photosystem II, fig 10.13
*Chlorophyll a in the reaction center receives energy from accessory
pigments and donates an “energized” electron to pheophytin (PP)
*Pheophytin transfers the electron to plastoquinone (PQ)
*H2O is split and H+ donates an electron to chlorophyll a to replace the one
*Plastoquinone delivers electron to the electron transport chain (cytochrome
*Electron transport chain “pumps” H+ into the thylakoid.
*ATP synthase allows H+ to exit and uses the energy in the H+ to produce
ATP = photophosphorylation.
*Photosystem I, fig 10.14
*Electron from photosystem II is transported by plastocyanin (PC) to
chlorophyll a in photosystem I.
*Electron is reenergized by photons of light.
*Energized electron is transferred to ferredoxin that donates it and a H+ to
NADP – NADPH is reducing power.
*O2 2 Light Independent Reactions – Calvin Cycle
*Calvin Cycle – “reduction” of CO2 to produce sugars.
*Takes place in stroma
*3 Steps, fig 10.19
*Fixation: 3 CO2 combine with 3 ribulose bisphosphate (RuBP, 5 C) in Calvin
Cycle to form 3 6-C molecules.
* These molecules immediately split to form 6 3-C molecules. Enzyme used is
*Reduction: ATP and NADPH from light dependent reactions used to form 6
G3P (glyceraldehyde 3 phosphate).
*Regeneration: 1 G3P exits the cycle and remaining 5 G3P are used to
regenerate the 3 RuBP used in the beginning of the cycle.
*G3P used to produce glucose and fructose. See glycolysis! How many G3P
are needed to produce 1 glucose or fructose molecule?
*Glucose + fructose = sucrose, a transport sugar, that is delivered to other parts
of the plant.
*When glucose production is high, glucose is used to produce a storage
carbohydrate, starch (stored in chloroplasts).
*The enzyme rubisco catalyzes CO2 fixation but also catalyzes oxidation of
RuBP, called photorespiration.
*When CO2 levels are high CO2 fixation dominates.
*When O2 levels are high, oxidation dominates.
*To live, plants must be able to fix CO2.
*How do plants take in CO2?
*Pores, stomata, on underside of leaves open and allow CO2 to enter, fig
10.21. Opening and closing of stomata are controlled by guard cells.
*When stomata are open, H2O iis lost from the leaves.
*During hot dry weather stomata close to prevent H2O loss and CO2 intake
*As CO2 levels decline, photorespiration predominates and sugars are not
*Plants that thrive in hot dry conditions have adapted – C4 pathway. NOTE: the
Calvin cycle is a C3 pathway because it produces a 3-C molecule. A C4
pathway involves a 4-C molecule.
*The C4 pathway is used to store CO2 when the stomata are open and donate
CO2 to the C3 pathway when the stomata are closed.
*2 Methods, fig 10.24
*C4 plants have the C4 pathway and C3 pathway in different cells.
*CAM plants have the C4 and C3 pathway in the same cell.
*C4 plants open stomata for short periods and store CO2 in C4 pathway. 3 *CAM plants open stomata at night and store CO2 in C4 pathway. CO2 is
donated to C3 pathway during the day.
donated 4 ...
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