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Chapter 7

BIOL 150B Chapter Notes - Chapter 7: Photosystem, Photosystem I, Accessory Pigment

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David Creasey

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1. Photosynthesis: using light energy to make food molecules from CO & water
light + HO + CO O + sugar
2. Photoautotrophs: use light to synthesize organic molecules from CO & water
are self-sufficient at producing their own organic molecules
3. Adaptations of leaves for photosynthesis
flattened to provide lots of surface area
thin to allow sunlight to reach chloroplasts inside
epidermis: layer or transparent cells on the upper & lower surfaces of the
leaf to protect the inner parts, while still allowing sunlight to enter
cuticle: waxy covering of the epidermis to reduce evaporation
stomata: pores in the leaves in which CO enters & O exits
mesophyll: layers of cells inside the leaf containing many chloroplasts
chloroplasts: site of photosynthesis, found mainly in mesophyll cells
-surrounded by a double membrane
-stroma: dense fluid inside chloroplast surrounding the grana
-thylakoids: interconnected membraneous sacs
-grana: stacks of thylakoid sacs
-chlorophyll: pigment found in the thylakoid membrane
the waste product O is created from the reactant HO (not the CO)
4. Visible light: (to humans..!) from 380nm wavelength (violet) to 750nm
wavelength (red) on the electromagnetic spectrum
photon: an individual packet of energy
5. Photosynthetic pigments in chloroplasts capture light
chlorophyll a: absorbs blue, violet & red reflects green
-the key light-capturing molecule in chloroplasts
-participates directly in light reactions
chlorophyll b: absorbs blue & red-orange reflects yellow-green
-conveys energy into chlorophyll a
-accessory pigment
carotenoids: absorbs violet, blue & green reflects yellow-orange
-conveys energy to chlorophyll a
-accessory pigment
6. Photosystem: the light harvesting unit
reaction centre: chlorophyll a & primary electron acceptor of the
electron transport chain
-Photosystem reaction centre II: P680 form of chlorophyll a
occurs 1st, but was discovered 2nd (II)
-Photosystem reaction centre I: P700 form of chlorophyll a
occurs 2nd, but was discovered 1st (I)
light harvesting complexes: composed of chlorophyll b & carotenoids
7. Electron-transport chain: consists of a series of electron-carrier molecules
embedded in the thylakoid membrane
8. 2 stages of photosynthesis:
(1) Light reactions (photo part)
converts solar energy to chemical energy, releasing O as waste
occurs in association with the thylakoid membranes
HO is split apart & O is released as a by-product
i) light is absorbed by a photosystem
ii) chlorophyll a loses an electron
splitting HO continuously replaces the electrons that
are boosted out of Photosystem II by light energy
iii) primary electron acceptor captures the electron & passes it
to the first molecule of the electron transport chain
iv) the electron travels from molecule to the next, losing
energy as it goes
the lost energy is used to pump H ions against its
concentration gradient into the thylakoids
there is a build up of potential energy from H ions
which will eventually be used to produce ATP
(v) the energy-depleted electron leaves electron transport
chain II & enters the reaction centre of photosystem I
where it replaces the electron ejected when the light strikes
the photosystem
(vi) the electron then travels along electron transfer chain I,
again losing energy as it goes
(vii) eventually the electron reaches the terminal electron
acceptor NADP, which forms NADHP after picking up 2
energetic electrons (same as NADH, only with a P group)
(viii)chemiosmosis: enzyme ATP synthase produces ATP
by allowing H ions to flow down their concentration
gradient across the thylakoid membrane, using this energy
to combine ADP + P
(2) Calvin cycle (synthesis part)
assembles sugar molecules using CO & the energy-containing
products that were produced during light reactions (ATP & NADHP)
i) carbon fixation: combines carbon from CO with RuBP
rubisco: an enzyme that combines 3 CO molecules
with 3 RuBP (5-carbon) molecules = forming 3
unstable 6-carbon molecules that immediately split in
half = making 6 PGA (3-carbon) molecules
-is a very large macromolecular complex
-found in all photosynthetic organisms
-is the most abundant protein on the planet
ii) G3P synthesis
energy donated by ATP & NADPH rearrange the 6
PGA molecules into 6 G3P (3-carbon) molecules
1 G3P molecule exits the cycle
iii) RuBP regeneration
the remaining 5 G3P molecules regenerate back into 1
RuBP to restart the cycle
iv) 2 G3P molecules can be combined to form 1 glucose (6-
carbon) molecule
within the thylakoid membrane, the overall path of an electron is:
photosystem II electron transport chain II
photosystem I electron transport chin I NADHP
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