PHOTOSYNTHESIS
6.1
 The main form of energy from the sun is
in the form of electromagnetic radiation
 Visible radiation (white light) used for
photosynthesis
 Remember : ROY
G. BIV?
The electromagnetic spectrum
 A Red Object absorbs the blue and green wavelengths
and reflects the red wavelengths
Why are
plants
green?
 pigment a compound that absorbs light
 different pigments absorb different wavelengths of white
light.
 chlorophyll is a pigment that absorbs red & blue
light (photons) so green is reflected or transmitted.
 Chlorophyll is located in the thylakoid membranes
So, Plants are green because the green
wavelength is reflected, not absorbed.
2 types of chlorophyll
 Chlorophyll a – involved in light reactions
 Chlorophyll b – assists in capturing light
energy – accessory pigment
 Carotenoids – accessory pigments –
captures more light energy
 Red, orange & yellow
The electromagnetic wavelengths and the
wavelengths that are absorbed by the chlorophyll
During the fall, what
causes the leaves
to change colors?
Fall Colors
 In addition to the chlorophyll
pigments, there are other pigments
present
 During the fall, the green
chlorophyll pigments are greatly
reduced revealing the other
pigments
 Carotenoids are pigments that are
either red, orange, or yellow
Photosynthesis is  conversion of light energy into chemical
energy that is stored in organic compounds
(carbohydrates > glucose)
 Used by autotrophs such as:
 Plants
 Algae
 Some bacteria (prokaryotes)
 glucose - energy-rich chemical produced
through photosynthesis
(carbohydrate)
 C6H12O6
 Biochemical pathway – series of reactions
where the product of one reaction is
consumed in the next
 E.g. photosynthesis product is glucose which is
used in cellular respiration to make ATP
Remember Redox Reaction
reduction/oxidation
The transfer of one or more
electrons from one reactant to
another
Two types:
1. Oxidation is the loss of e2. Reduction is the gain of e-
Oxidation Reaction
The loss of electrons from a
substance or the gain of oxygen.
Oxidation
6CO2 + 6H2O 
C6H12O6 + 6O2
glucose
Carbon
dioxide
Water
Oxygen
Reduction Reaction
The gain of electrons to a
substance or the loss of oxygen.
Reduction
6CO2 + 6H2O  C6H12O6 + 6O2
glucose
Photosynthesis equation
Light energy
6CO2 + 6H2O
C6H12O6 + 6O2
Chlorophyll
Reactants: Carbon dioxide and water
Products: glucose and oxygen which
is a byproduct
Where does
photosynthesis
take place?
Plants
 Mainly occurs in the leaves:
a. stoma - pores
b.mesophyll cells
Mesophyll
Cell
Chloroplast
Stoma
Mesophyll Cell of Leaf
Nucleus
Cell Wall
Chloroplast
Central Vacuole
Photosynthesis occurs in these cells!
Stomata (stoma)
Pores in a plant’s cuticle through which
water vapor and gases (CO2 & O2)
are exchanged between the plant and
the atmosphere.
Stoma
Carbon Dioxide
(CO2)
Guard Cell
Oxygen
(O2)
Guard Cell
Found on the underside of leaves
Chloroplast
Organelle where photosynthesis takes place.
Stroma
Outer Membrane
Inner Membrane
Thylakoid
Granum
Thylakoid stacks are connected together
Parts
chloroplasts – dbl membrane organelle that
absorbs light energy
Thylakoids – flattened sacs contain pigment chlorophyll
Grana (pl: granum) – layered thylakoids (like
pancakes)
Stroma – solution around thylakoids
Stomata – pore on underside of leaf where O2 is
released and CO2 enters
Stroma : chloroplast :: cytosol : cytoplasm
Thylakoid
Thylakoid Membrane
Granum
Thylakoid Space
Grana make up the inner membrane
What do cells
use for
energy?
Energy for Life on Earth
 Sunlight is the ULTIMATE
energy for all life on Earth
 Plants store energy in the
chemical bonds of sugars
 Chemical energy is released as
ATP during cellular respiration
Structure of ATP
 ATP stands for adenosine triphosphate
 It is composed of the nitrogen base
ADENINE, the pentose (5C) sugar
RIBOSE, and three PHOSPHATE groups
 The LAST phosphate group is bonded
with a HIGH ENERGY chemical bond
 This bond can be BROKEN to release
ENERGY for CELLS to use
Removing a Phosphate from ATP
Breaking the LAST PHOSPHATE bond
from ATP, will -- Release ENERGY for cells to use
 Form ADP (adenosine diphosphate)
 Produce a FREE PHOSPHATE GROUP
High Energy Phosphate Bond
FREE PHOSPHATE can be re-attached to
ADP reforming ATP
Process called Phosphorylation
Phosphorylation
Photosynthesis
SUN
1. Light Reaction Produces energy from solar power
(photons) in the form of ATP and
NADPH.
2. Calvin Cycle
 Also called Carbon Fixation or
Carbon Cycle, Uses energy (ATP
and NADPH) from light reaction to
make sugar (glucose).
 3 stages of photosynthesis-
Stages:
 STAGE 1 - LIGHT REACTIONS - energy from
sun is used to split water into H+ an O2
 STAGE 2 – energy is converted to chemical
energy & stored in ATP & NADPH in stroma
 STAGE 3 - CALVIN CYCLE where carbon is
fixed into glucose
Light Reaction (Electron Flow)
 Occurs in the Thylakoid membranes
 2 possible routes for electron flow:
Use Photosystem I and Electron
Transport Chain (ETC) and generate
ATP only
 OR use Photosystem II and
Photosystem I with ETC and
generate O2, ATP and NADPH
Photosynthesis animation
 http://www.mhhe.com/biosci/genbio/biolin
k/j_explorations/ch09expl.htm
ELECTRON TRANSPORT - LIGHT
REACTIONS in 5 steps
Photosystem I and II
 Step 1 – light excites e- in photosystem II
 Step 2 – e- move to primary e- acceptor
Step 3 – e- move along electron transport
chain (etc)
 Step 4 – light excites e- in photosystem I
 Step 5 – e- move along 2nd (etc)
 End – NADP+ combine H+ to make NADPH
Light reaction animation
 http://www.science.smith.edu/department
s/Biology/Bio231/ltrxn.html
Electron transport chain song
 Play the "Come On Down (The Electron
Transport Chain)" song performed by
Sam Reid.
Photolysis –photo-chemical splitting of water
(restoring photosystem II)
Chemiosmosis – synthesis of ATP
 Powers ATP synthesis
 Takes place across the thylakoid
membrane
 Uses ETC and ATP synthase
 H+ move down their concentration
gradient forming ATP from ADP
 Concentration of protons is greater in
thylakoid than stroma
Chemiosmosis
SUN
H+ H+
Thylakoid
(Proton Pumping)
E
T
PS II
PS I
C
H+
H+ H+
H+ H+
H+
ADP + P
H+
H+
high H+
concentration
ATP Synthase
ATP
Thylakoid
Space
low H+
concentration
The Calvin Cycle
6.2
Calvin Cycle  Biochemical pathway in photosynthesis that
produces organic compounds using ATP &
NADPH
 Carbon fixation – carbon atoms from CO2 are
bonded or ‘fixed’ into carbohydrates
 occurs in stroma
Calvin Cycle
Carbon Fixation
C3 plants (80% of plants on earth)
Occurs in the stroma
Uses ATP and NADPH from light
reaction as energy
 Uses CO2
 To produce glucose: it takes 6 turns
and uses 18 ATP and 12 NADPH.




Chloroplast
Outer Membrane
Inner Membrane
STROMA– where Calvin Cycle occurs
Thylakoid
Granum
Calvin Cycle (C3 fixation)
(36C)
6C-C-C-C-C-C
(6C)
6CO2
(unstable)
(30C)
6C-C-C-C-C
RuBP
(30C)
glucose
6C-C-C 12PGA
(36C)
6ATP
6ATP
6NADPH
6NADPH
6C-C-C
6ATP
C3
6C-C-C
(36C)
6C-C-C 12G3P
(6C)
C-C-C-C-C-C
Glucose
Calvin Cycle
Remember: C3 = Calvin Cycle
C3
Glucose
Step 1  CO2 diffuses fr cytosol & combines with
RuBP which splits into pair of PGA
Step 2  PGA gets phosphate gr fr ATP gets
proton fr NADPH to become PGAL
 Reaction produces: ADP, NADP+ &
phosphate to be used again
Step 3  PGAL converts back to RuBP
 Allows Calvin cycle to continue
Alternates:
 C3 plants – use Calvin cycle exclusively
 Form 3-carbon compounds
 C4 pathway – evolved in hot, dry climate
 Form 4-carbon compounds
 Partially close stomata
 E.g. Corn, sugar cane, crabgrass
 CAM – open stomata at night, close in day
 Grow slow, lose less water
 E.g. cactus, pineapple
C4 Plants
 Hot, moist
environments
 15% of plants
(grasses, corn,
sugarcane)
 Photosynthesis
occurs in 2 places:
 Light reaction mesophyll cells
 Calvin cycle - bundle
sheath cells
C4 Plants
Malate-4C sugar
C-C-C-C
Malate
C-C-C-C
Transported
CO2
CO2
C3
glucose
C-C-C
PEP
ATP
Mesophyll Cell
C-C-C
Pyruvic Acid
Bundle Sheath Cell
Vascul
Tissu
CAM Plants
 Hot, dry environments
 5% of plants (cactus and ice
plants)
 Stomates closed during day
 Stomates open during the night
 Light reaction - occurs during
the day
 Calvin Cycle - occurs when CO2 is
present
CAM Plants
Night (Stomates Open)
Day (Stomates Closed)
Vacuole
CO2
C-C-C-C
Malate
C-C-C-C
Malate
C-C-C-C
Malate
CO2
C3
C-C-C
PEP
ATP
C-C-C
Pyruvic acid
glucose
Rate of photosynthesis is
effected by light intensity, CO2 or temperature
 High intensity or high CO2 = high rate
 Growth graph levels off (plateau)
 High temp = initial high rate but peaks
 Rate drops when stomata closes
Recap
 Photosynthesis converts light energy into chemical
energy thru series of biochemical pathways
 Electrons excite in photosystem II – move along
ETC to photosystem I
 electrons are replaced when water is split
 oxygen is byproduct
 ATP synthesized across thylakoid
 Calvin cycle – carbon is fixed & sugar produced
 3 turns produce PGAL (PGAL keeps cycle going
 Other pathways – C3, C4, CAM
References
 www.biologyjunction.com