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CHAPTER 10
PHOTOSYNTHESIS
LEAF STRUCTURE

stomata – pores in
lower epidermis of leaf


mesophyll – inner-leaf
tissue


gas exchange
most chloroplasts
located in these cells
veins (phloem & xylem)


phloem carries sugars
away from leaves
xylem carries water to
leaves
CHLOROPLAST STRUCTURE


stroma – inner fluid
thylakoids –
interconnected
membranous sacs


contains chlorophyll
grana – stacks of
thylakoids
PHOTOSYNTHESIS SUMMARY
6 CO2 + 6 H2O + light energy  C6H12O6 + 6 O2
involves redox reactions
 2 stages:

 light
reactions – convert solar energy to chemical
energy stored temporarily in ATP & NADPH
 Calvin cycle – conversion of CO2 into glucose using
the energy stored in ATP & NADPH
VISIBLE (WHITE) LIGHT
a mixture of wavelengths 380-750 nm
 when passed thru a prism, separates into the
colors of the rainbow

PIGMENTS
absorb visible light
 some wavelengths (colors of light) are not
absorbed but reflected or transmitted– this is
the color we see

 (ex)
leaves look green
because the pigment
chlorophyll reflects and
transmits green light
SPECTROPHOTOMETER
a machine that determines the wavelengths of
light absorbed by a pigment by measuring the
percent transmittance of each color of light
 allows us to create an absorption spectrum

ABSORPTION SPECTRUM
FOR PHOTOSYNTHETIC PIGMENTS


chlorophyll a absorbs blue-violet & red light best
chlorophyll b absorbs blue & orange light best
EXCITED PIGMENTS – KEY TO LIGHT RXNS
pigments become excited when they absorb light
because absorption of a photon of light boosts an
electron to a higher energy level
 if the energy is not captured, the electrons will
quickly fall back to their ground state & the energy
is released as heat
 sometimes light is also
emitted as the electrons fall
back down to their ground
state – the resulting afterglow is called fluorescence

PHOTOSYSTEMS
light-harvesting complexes in the thylakoid
membranes consisting of pigment molecules
bound to proteins
 two types: photosystem II (P680) & photosystem I
(P700)
 create a greater surface area for absorbing light
 increase the range of wavelengths that can be
absorbed by the plant (due to presence of
chlorophyll a, chlorophyll b, and carotenoids)
 have two parts: reaction center (chlorophyll a +
primary electron acceptor) surrounded by lightharvesting complexes

light strikes pigment
molecules in lightharvesting complexes
 energy is passed from
one pigment molecule to
another until it reaches
chlorophyll a in the
reaction center
 this excites an electron in
chlorophyll a which is
picked up by the primary
electron acceptor

ELECTRON FLOW

non-cyclic electron flow
flow of electrons from PS II  PS I  NADP+
 produces ATP and NADPH for the Calvin cycle


cyclic electron flow
cycling of electrons within PS I; does not involve PS II
 produces additional ATP needed for Calvin cycle


NADPH concentration regulates which type occurs

high [NADPH] stimulates cyclic electron flow to balance
out NADPH & ATP levels
NONCYCLIC ELECTRON FLOW
CYCLIC ELECTRON FLOW
CHEMIOSMOSIS

ATP is made during the light reactions using the
same process (chemiosmosis) that makes ATP
during oxidative phosphorylation of cellular
respiration
SUMMARY OF THE LIGHT REACTIONS
light strikes PS II causing electrons in chlorophyll a
in the reaction center to become excited
 the excited electrons are picked up by the primary
electron acceptor & passed down an electron
transport chain which drives the synthesis of ATP
by chemiosmosis
 meanwhile, light strikes PS I causing electrons in
chlorophyll a in the reaction center to become
excited
 the excited electrons are picked up by the primary
electron acceptor & passed down an electron
transport chain to NADP+ which is reduced to
NADPH

SUMMARY OF THE LIGHT REACTIONS

cont.
the electrons lost from the chlorophyll a
molecules in each photosystem are restored as
follows:
 electrons
are restored to PS II by the splitting of
H2O which produces O2 as a by-product
 electrons are restored to PS I by the electron
transport chain that follows PSII (PS I is the final
electron acceptor for this ETC)
CALVIN CYCLE
carbon fixation – CO2 is attached to a molecule
called RuBP by the enzyme rubisco forming an
unstable six-carbon compound that immediately
splits into two three-carbon compounds called 3PGA
 ATP & NADPH drive the conversion of 3-PGA to
G3P
 for every three CO2 that enter the cycle, six G3P
are made – one leaves the cycle and five are
recycled to regenerate RuBP (requires ATP)
 glucose is made from the G3P that leaves the
Calvin cycle

PHOTORESPIRATION





plants that perform the steps of photosynthesis previously
discussed are called C3 plants
C3 plants use CO2 directly from the air by opening their
stomata
this can be a problem on hot, dry days when plants close
their stomata to reduce water loss because when stomata
are closed, no CO2 can enter the leaf & no O2 can get out
the O2 build-up causes photorespiration, a process in which
rubisco adds O2 to RuBP in the Calvin cycle instead of CO2
photorespiration does not produce glucose like
photosynthesis or ATP like cellular respiration so it is
basically a wasteful process
ADAPTATIONS TO PHOTORESPIRATION

C4 photosynthesis
 PEP carboxylase (which has a higher affinity for CO2
than rubisco & no affinity for O2) combines CO2 with
PEP to make oxaloacetate which is converted to
malate and stored in the bundle-sheath cells
 CO2 is released in the bundle-sheath cells and
enters the Calvin cycle
 this adaptation is used in hot regions with intense
sunlight where stomata partially close during the day
 examples of plants that use this adaptation are corn
& sugarcane
C4 PHOTOSYNTHESIS
ADAPTATIONS TO PHOTORESPIRATION

cont.
CAM photosynthesis
 plants take in CO2 at night and store it in organic
acids
 CO2 is released during the day for use in the Calvin
cycle when light is available for the light reactions
 this adaptation is used by plants that live in
extremely arid environments like deserts
 examples of plants that use this adaptation are
cacti, pineapples, & succulents (water-storing plants)
C4 PHOTOSYNTHESIS

spatial separation of steps
COMPARISON
CAM PHOTOSYNTHESIS

temporal separation of steps