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Photosynthesis
•
•
All energy on earth comes from the sun.
We depend on:
1. Plants
2. Algae (underwater plants)
3. Cyanobacteria (photosynthetic bacteria-unicellular)
– To provide this energy to us!
– These organisms obtain energy
directly from the sun and are called
autotrophs
– We are heterotrophs-we obtain
energy by eating other organisms
A little bit about Cyanobacteria
• The endosymbiotic theory proposes that an
ancestor of cyanobacteria was engulfed by
an ancestor of today’s eukaryotic cell and
gave rise to plant cells
• Algal and plant cells contain chloroplasts
(primarily in the mesophyll and guard cells)
Leaf Structure
and
Photosynthesis
The leaf is perfectly designed for
maximum function
• Ground tissue: function = photosynthesis
palisade and spongy mesophyll have many
Chloroplasts and air spaces enable efficient
gas exchange.
• Vascular tissue: function = support and transport
» xylem and phloem
• Dermal tissue separates the inside of the leaf
from the air outside the plant.
• Epidermal cells
• waxy cuticle
• Guard cells for stomata
Structure of a leaf
Photosynthesis
primarily occurs in
chloroplasts of
leaves
Lilac (Syringa)
• Leaves are thin to maximize surface
area exposed to sunlight.
• It also limits the distance that gases
need to travel to the chloroplasts
Plant structure
• Obtaining raw materials
– sunlight
• leaves = solar collectors
– CO2
• stomates = gas exchange
– H2O
• uptake from roots
– nutrients
• N, P, K, S, Mg, Fe…
• uptake from roots
stomate
transpiration
gas exchange
Plant Structure: Leaves
• Stomata
– Stomata = pores in epidermis used for gas exchange
• CO2 into cell for photosynthesis
• H2O out of leaf by evaporation to facilitate transpiration
(process by which water pulled up plant)
– Guard cells = epidermal cells that open and close
stomata
• Stomata open when guard cells
swell with water
• Stomata close when guard cells
collapse together (shrivel)
Stomates
Stomates
• Stomata are open in the daytime and closed at night
• Light activated proton pumps in guard cell
membranes cause K+ to move from neighbouring
epidermal cells into guard cells-this results in H2O
moving into guard cells (by osmosis) causing them to
swell.
• Increased turgor pressure causes them to open.
• As the concentration of sucrose in guard cells
decreases in the evening, water moves out of the
cells and stomata close.
Stomata typical of dicots
Potato (Solanum)
Stomata typical of monocots
Maize (Zea)
Scanning electron microscope images
leaves
Chloroplasts
cross section
of leaf
absorb
sunlight & CO2
CO2
chloroplasts
in plant cell
chloroplast
chloroplasts
contain
chlorophyll
make
energy & sugar
chloroplast
H+
Plant structure
ATP
+
+ H+ H H+
+
H
H
+ H+ H+ H+
+
H
H
thylakoid
• Chloroplasts
– double membrane
– stroma
outer membrane
inner membrane
• fluid-filled interior
– thylakoid sacs
stroma
– grana stacks (sing. granum)
• Thylakoid membrane
contains
– chlorophyll molecules
– electron transport chain
– ATP synthase
• H+ gradient built up within
thylakoid sac
thylakoid
granum
Chloroplasts
• Have 3 membranes
• The third membrane is
called the thylakoid.
• The thylakoid is folded
and looks like stacks of
coins called granum
• The stroma is the space
surrounding the grana
Chloroplasts
• Chlorophyll molecules
are embedded in the
thylakoid membrane
• Act like a light “antenna”
• These molecules can
absorb sunlight energy.
Image from Biology 11: College Preparation. Pg 73. Nelson,
Toronto. 2003.
Chlorophyll
• There are several types of chlorophyll
• Chlorophyll a (blue-green) and chlorophyll b
(yellow-green)
• Both contain a porphyrin ring attached to a
long H-C tail or phytol tail (hydrophobic to
anchor into membrane)
• Porphyrin rings are also found in the
cytochromes of ETC
Chlorophyll
• The ring has a Mg atom in the centre and
alternating single/double bonds
• Electrons in the porphyrin ring absorb light
energy and begin photosynthesis
• Chlorophyll a differs in that it has a methyl
group (-CH3) at position –R while chlorophyll
b contains an aldehyde group (-CHO)
• Chlorophyll a is the primary light-absorbing
pigment.
Overview
Overall Reaction
What is the equation for photosynthesis? Similar
to what?
carbon dioxide + water  glucose (simple sugar) +
oxygen
CO2 (g) + H2O (l) + light energy [CH2O]+ O2(g)
6CO2 + 12H2O + light energy  C6H12O6 + 6O2 + 6H2O
How are they different?
3.1 Homework
• P.145
• Q 1-7
3.2
Light energy and photosynthetic
pigments
Overview
• There are 3 distinct stages to photosynthesis;
1. Capturing light energy
2. Using this energy to make ATP and reduced
NADP+ (energy shuttling coenzyme-similar to
NAD+)
3. Using the free energy of ATP and the reducing
power of NADPH + H+ to synthesize organic
compounds such as glucose, from CO2
Light (dependent)Reactions
(Stages 1 and 2)
•
Happen ONLY in
sunlight on the
thylakoid
membrane
1. Light is absorbed
by chlorophyll
molecules
2. The energy
generates
molecules of ATP
Light Independent ReactionsCalvin Cycle-Carbon Fixation
(formerly the “dark reactions”)
•
Happen in sunlight, and in the dark.
–
Hence “independent of light”
1. ATP generated by sunlight drives the Calvin Cycle.
2. Monosaccharides (eg. glucose) are manufactured in the
cycle.
3. Monosaccharides are used to “build” polysaccharides (eg.
Starch).
Summary
Image from: Biology 11: College Preparation. Pg 74. Nelson, Toronto. 2003.
Photosynthesis
• Light reactions
– light-dependent reactions
– energy conversion reactions
• convert solar energy to chemical energy
• ATP & NADPH
• Calvin cycle
It’s not the
Dark Reactions!
– light-independent reactions
– sugar building reactions
• uses chemical energy (ATP & NADPH) to
reduce CO2 & synthesize C6H12O6
Absorption of
Light by
Chlorophyll
Pigments of photosynthesis
• Chlorophylls & other pigments
– embedded in thylakoid membrane
– arranged in a “photosystem”
• collection of molecules
– structure-function relationship
A Look at Light
• The spectrum of color
V
I
B
G
Y
O
R
Light: absorption spectra
• Photosynthesis gets energy by absorbing
wavelengths of light
– chlorophyll a
• absorbs best in red & blue wavelengths & least in green
– accessory pigments with different structures absorb
light of different wavelengths
• chlorophyll b, carotenoids, xanthophylls
Why are
plants green?
What
wavelengths of
light do you think
plants use the
least in
photosynthesis?
•Chlorophyll A and B
absorb light mostly in
the red and blue
regions of the
spectrum
•Carotene and
xanthophyll absorb
light from other
regions and pass the
energy to chlorophyll
Light absorption by
chlorophyll
Photosynthetic pigments are arranged as
“photosystems”
Photosystems of photosynthesis
• 2 photosystems in thylakoid membrane
– collections of chlorophyll molecules
– act as light-gathering molecules
– Photosystem II
reaction
• chlorophyll a
• P680 = absorbs 680nm
wavelength red light
center
– Photosystem I
• chlorophyll b
• P700 = absorbs 700nm
wavelength red light
antenna
pigments
Measuring light absorption
• A spectrometer is used to measure
the amount of absorption at each
wavelength
• An action spectrum shows the rate of
photosynthesis at different light
intensities
Brown seaweeds
• subjected to low light intensities
• Chlorophyll C instead of chlorophyll B
• fucoxanthin
-very wide absorption spectrum
-absorbs light in parts of the
spectrum where chlorophyll is less
efficient
Red seaweeds
• Have chlorophyll D
• Phycoerythrin
Green seaweeds
• Found in shallow
water
• Do not have
fucoxanthin
• Does not have
phycoerythrin
Accessory Pigments
• Chlorophyll is not the only lightabsorbing molecule in chloroplasts
• Accessory pigments like orange
carotenoids (beta-carotene) and yellow
xanthophylls help absorb other light
energy which may damage chlorophyll
Changing colours
• During spring and summer leaves
appear green because of the high
concentration of chlorophyll
• As temperatures cool, chlorophyll is
broken down and we see the oranges
and yellows and even reds
(anthocyanin)
Practice Questions
• P.154
• Q 1-3, 6-8