Chap. 6 : Photosynthesis
Photosynthesis – involves a series of
chemical reactions where the products of one
become the reactants of another …. called
biochemical pathways.
I.
Light Absorption (light reactions)
--- occur in chloroplast
A. Composition of Light
ROY G BIV – each color is a certain wavelength
of light. The wavelength decreases left to right.
-- When light hits an object it is either :
reflected, transmitted, or absorbed
( will see it)
(won’t see it)
Ex. Sky is blue because blue light is reflected while
all others are absorbed.
B. Chloroplast Pigments
chlorophyll a -- main pigment
chlorophyll b
accessory pigments – help
chlorophyll a absorb more
carotenoids
wavelengths of light.
II. Electron Transport
A. Photosystem (PS) – cluster of pigments in
thylakoids.
Process of Photosynthesis
Electron replacement into photosystem II
light
water splitting molecule
Photosystem II
PS II
2 H2O
2H2O
4 H+
O2
4 eInside Thylakoid
4 H+ + 4e- + O2
III. Chemiosmosis
-- making of ATP from creating a concentration gradient
of p+( H+). Proton concentration greater inside the
thylakoid than outside in stroma so this creates potential
energy. This energy is used by enzyme ATP synthase
molecule to change ADP to ATP
--- end result of light reactions inside the thylakoids is :
ATP and NADPH molecules
IV. Calvin Cycle (light independent reactions)–
production of organic compounds using energy
stored in ATP and NADPH from the light reactions
A. Steps of Calvin Cycle
- CO is “fixed” – changing the carbon
from CO into a carbon used by organisms
(organic carbon)
- occurs in the stroma of chloroplast
1. CO binds with RuBP (5-carbon sugar)
to create a 6 carbon sugar which
immediately splits into 2 PGA s (3-carbon
molecules)
2
2
2
2. Each PGA gets an ATP and NADPH to energize
PGA making it PGAL. This reduces ATP and
NADPH to ADP and NADP+ which is released to
go back to the light reactions.
3. 5 PGAL get more ATP added to them
and are converted back to RuBP to
continue the Calvin Cycle but one PGAL
will be released to become the organic
compound glucose.
--- It takes 2 cycles through to make 1
glucose
--- Plants that “fix” carbon using only the
Calvin Cycle are called C plants.
s
3
Calvin Cycle (Dark Reactions)
Occurs in stroma of chloroplast
Goes through cycle twice so end result is 1 glucose molecule
(C6H12O6)
*** All 3 parts together ( light dependent /
chemiosmois/ and calvin cycle)
NADPH and ATP form from the light reactions and provide the energy to
fuel the light independent reactions (dark reactions/calvin cycle)
*** Overall reaction for photosynthesis :
6CO2 + 6H2O + light → C6H12O6 + 6O2
B. Alternative Pathways
-- some plants due to their environment (mostly hot and
dry) cannot fix carbon like C3 plants because they would
lose too much water when their stomata(pores in leaf )
opens to take in CO2
1. C4 plants – fix CO2 into a 4-carbon molecule instead of
3-carbon one. Allows stomata to open less therefore
losing less water. Ex. Corn and sugar cane
2. CAM plants
-- open stomata at night and collect
CO2 which they then use during the day in
the Calvin Cycle. Will not grow very big.
Conserves water in hot tropical and desert
areas.
ex. Cacti and pineapple