Ch. 7 Photosynthesis
7.6 – 7.9
Light reaction: converting solar
energy to chemical energy
• Sunlight is what type
of energy?
• Electromagnetic
energy (radiation)
• What is a
wavelength?
• Light travels in waves
measured by the
distance between two
adjacent crests
• Visible light only
forms a small percent
of the electromagnetic
spectrum. Can plants
use more than just
visible light for
photosynthesis?
• No, only certain
wavelengths, or
colors of visible light?
• Do we see the wavelengths
absorbed by the plant?
• No. We don’t see those
colors.
• What happens to those other
colors?
• Different pigments in
chloroplasts absorb this light;
(chlorophyll a, absorbs blueviolet and red, & chlorophyll b,
absorbs mainly blue and
orange light, & reflects (looks)
yellow/green,
• What is a carotenoid?
• Another type of
yellow/green pigment
(absorbs blue-green
light) in chloroplasts
that helps protect
plant from excessive
light energy. (absorb
& dissipates energy)
• What is the energy within
a light wavelength called?
• Photons, They’re little
packets, fixed units of
energy in light.
• Since the shorter the
wavelength = more
energy (photons), which
has more energy violet or
red light?
• Violet has nearly twice as
much energy!
• What does that photon
do?
• When a pigment absorbs
a photon, that
chlorophyll’s electron
gains energy (gets
excited & jumps up), and
releases (falls back to
ground) that energy as
heat or light
(aka: fluorescence)
• What is the first step
in the light reaction?
• Solar-powered
electron transfer from
chlorophyll to primary
electron acceptor.
• What makes up a reaction
center?
• Collection of pigment
molecules (chlorophyll a )and
primary electron acceptor.
• What does the reaction center
do?
• Triggers light reaction when
chlorophyll donates an excited
electron to primary e- acceptor,
which passes an e- to electron
transport chain.
• What is the overall
combination of all this called?
• A photosystem
• How many types of
photosystems are there?
• 2. Photosystem I & II.
• How are they different?
• Photosystem I, its
chlorophyll a absorbs red
light best P700, and
Photosystem II’s
chlorophyll a absorbs an
orange/red wavelength of
light best P680.
Electron transport chain to make
ATP, NADPH & 02
• Where do electrons go to when
leaving PS 1?
• Chlorophyll loses 2e- to
Primary e- acceptor which go
down electron transport chain
(ETC) to end up on NADP ->
NADPH
• Are those electrons ever
replaced?
• Yes, PS 2 loses its electrons to
PS 1 passed down via the
ETC.
• What about those
electrons…are they replaced?
• Yes when H2O is split
releasing O2 & supplying
electrons to PS 2.
What is happening in the ETC
• What drives the
transport of hydrogen
ions (H+) across the
thylakoid membrane?
• Arrangements of
electrons being
passes from one
photosystem (protein
ie: blue & purple) to
another.
ATP synthesis in light reactions
• What activates the pumping of
H+ ions out of the stroma?
• The loss of energy each time
an electron goes from one
photosystem to the other.
• Where do the H+ ions go?
• They want to diffuse back in.
• Do they ever get back in?
• Yes, by facilitated diffusion via
ATP synthase, releasing
energy to make ATP from ADP
& P.
• What is this process called?
• Photophosphorylation (ATP
production from initial input of
light energy)