IB Biology Review
3.8 & 8.2 : Photosynthesis
Relationship Between
Photosynthesis and Respiration
Products of
photosynthesis
are reactants
in respiration
What is Photosynthesis?
The conversion of light energy to chemical
energy in the chemical bonds of glucose
Which organisms perform photosynthesis?
Autotrophs / Producers
Some bacteria, some protists, plants
Where Does Photosynthesis Occur?
In the chloroplast
What is a chloroplast?
An organelle found only in plants and photosynthetic
protists
absorbs sunlight and uses that energy to drive the synthesis
of glucose from carbon dioxide and water
Where does photosynthesis occur in the chloroplast?
Thylakoid
What is a thylakoid?
A flattened membrane sac inside the chloroplast
What chemical does
photosynthesis need to occur?
Chlorophyll
What is chlorophyll?
The main photosynthetic pigment that absorbs
solar energy in photosynthesis
Two types
Chlorophyll A
Chlorophyll B
both absorb in the blue/violet wavelengths, but at
slightly different wavelengths
Chloroplast Structure
Be able to draw these structures
Outer membrane
Inner membrane
Stroma
Granum
Lumen
Thylakoid
The Reactions of Photosynthesis
Two main sets of reactions
Light Dependent Reactions
The stage of photosynthesis that occurs in the thylakoid
membranes
Converts solar energy to the chemical energy of ATP and
NADPH
Light Independent Reactions / Calvin Reactions
The stage of photosynthesis which combines CO2 and the
chemical energy obtained from the light reactions (ATP and
NADPH)
Creates 3-carbon sugars which are then combined to
produce the 6-carbon glucose
Overview of Photosynthesis
Photosynthesis: Step 1
Light Absorption & Photolysis
Chlorophyll absorbs
sunlight / energy from
proton in
Photosystem II
Electron donated by
donated by the
splitting of water
(photolysis) is given
energy from sunlight
Oxygen (O2) released
Photosynthesis: Step 2
Chemiosmosis, Excitation and
Photophosphorylation
Electron moves through the electron transport chain
Electron gives energy to proteins to pump protons into the
thylakoid space
This proton gradient is used by ATPase to make ATP
Electron goes to photosystem I at end of electron transport
chain
This electron is re-excited
by another burst of light
Electron leaves by
photosystem I
This re-excited electron
is then passed to NADP+
Produces NADPH
Comparison of Chemiosmosis in
Mitochondria and Chloroplasts
In both organelles
Electron transport chains
pump protons (H+) across
a membrane
Protons go from a region
of low H+ concentration
(light gray in this diagram) to
one of high H+
concentration (dark gray)
Protons then diffuse back
across the membrane
through ATP synthase
Produces ATP
Overview of Light
Dependent Reactions
Photosynthesis: Step 3
Calvin Cycle Part 1: Carbon Fixation
Three different carbon molecules from CO2
from air combine with three different RuBP
molecules in stroma of chloroplast
Produces six 3-phosophoglycerate molecules
Photosynthesis: Step 3
Calvin Cycle Part 2: Reduction
Molecule gets phosphorylated by ATP
Molecule is reduced (gains an electron) by NADPH
Becomes G3P
For every 6 G3P
molecules formed,
one leaves the
Calvin Cycle
to make glucose
2 G3P (3 carbon) =
1 glucose (6 carbon)
Photosynthesis: Step 3
Calvin Cycle Part 3: Regeneration of RuBP
The other five G3P
molecules stay in
the Calvin Cycle
Get phosphorylated
by ATP
Regenerates RuBP
Review of Calvin Cycle
RuBP combines with CO2 to
produce 3 phosophoglycerate
After being phosphorylated
by ATP, this resultant
molecule is reduced by
NADPH to become a
molecule called G3P
For every 6 G3P molecules
formed, only one leaves the
Calvin Cycle to make glucose
It takes two G3P (3-carbon)
molecules to make one
glucose (6-carbon) molecule
The other G3P molecules stay
in the Calvin Cycle and are
phosphorylated by ATP to
regenerate RuBP
Review of Photosynthetic Processes
Chloroplast Structure and Function
The thylakoids produce a lot of membrane
surface area within the chlorophlast
Increased surface area increases space for the
light dependent reactions to occur
More light dependent reactions produce more
ATP and NADPH
More ATP and NADPH increase the rate of
glucose production in the Calvin Cycle
What three factors limit
the rate of photosynthesis?
Temperature
Gradual rise in rate
Then sudden drop as proteins denature
Light Intensity
Gradual rise in rate
Followed by a leveling off as maximum rate of
photosynthesis reached
CO2
Gradual rise in rate
Followed by a leveling off as maximum rate of
photosynthesis reached
Absorption Spectrum
A graph showing how well the three
photosynthetic pigments (chlorophyll a and b, and
carotenoids) absorb various wavelengths of light
Shows what waves
of light plants absorb
This graph has a
similar overall
trend as the Action
Spectrum graph
Action Spectrum
A graph that shows how effective different
wavelengths of light are in driving photosynthesis
Shows what waves
of light plants use
for photosynthesis
This graph has a
similar overall
trend as the
Absorption
Spectrum graph
Measuring Photosynthesis
Four main ways
Production of oxygen because
Oxygen is a byproduct of photosynthesis
Measure carbon dioxide uptake because
carbon dioxide is a reactant of photosynthesis
Measure increase in biomass because
products of photosynthesis are used in production of cell
walls and new tissue
Measure electron flow in light reaction using
DPIP as an electron acceptor
Measuring the color change as it is reduced with a
spectrophotometer
IB Exam Question
1. Which two colours of light does
chlorophyll absorb most?
(1 mark)
A.
B.
C.
D.
Red and yellow
Green and blue
Red and green
Red and blue
Correct answer: D
IB Exam Question
2. Explain photophosphorylation
in terms of chemiosmosis. (4 marks)
Electron transport chain (ETC) causes
proton/hydrogen ion pumping;
This creates a high proton gradient inside
thylakoids;
Protons then pass by diffusion into the stroma
through ATP synthase;
ATP synthase catalyses phosphorylation of
ADP to produce ATP;
IB Exam Question
3. What is needed in photosynthesis to convert
carbon dioxide into organic molecules?
(1 mark)
A.
B.
C.
D.
Light and hydrogen from the splitting of
water
Light and oxygen from the splitting of water
ATP and hydrogen from the splitting of
water
ATP and oxygen from the splitting of water
Correct answer: C
IB Exam Question
4. Describe the reactions in the light
independent part of photosynthesis.
Carbon Fixation - RuBP combines with CO2 to
produce 3 phosophoglycerate
After being phosphorylated by ATP, this resultant
molecule is reduced by NADPH to become a
molecule called G3P
For every 6 G3P molecules formed, only one leaves the
Calvin Cycle to make glucose
It takes two G3P (3-carbon) molecules to make one
glucose (6-carbon) molecule
The other G3P molecules stay in the Calvin Cycle and
are phosphorylated by ATP to regenerate RuBP
IB Exam Question
5. Explain how the light-independent
reactions of photosynthesis rely on lightdependent reactions.
(8 marks)
light-independent reaction fixes CO2;
to make glycerate 3-phosphate;
glycerate 3-phosphate / G3P becomes reduced;
phosphoglyceraldehyde / glyceraldehyde 3-phosphate;
using NADPH;
using ATP;
ATP needed to regenerate RuBP;
ATP is made in light-dependent reactions;
light causes excitation of electrons;
flow of electrons causes pumping of protons into thylakoid;
ATP formation when protons pass back across thylakoid
membrane;
electrons are passed to NADP/NADP+;
NADPH produced in the light dependent reactions;
IB Exam Question
6. Describe the relationship between
chloroplast structure and function.
(4 marks)
The thylakoids produce a lot of membrane
surface area within the chloroplast
This increased surface area increases the rate of
light dependent reactions in the electron
transport chain (ETC)
Faster light dependent reactions produce more
ATP and NADPH
These molecules –in turn- increase the rate of
glucose production in the Calvin Cycle
IB Exam Question
7. Explain the reasons for a shape of the
thylakoid membranes in the chloroplast.
(4 marks)
Shape leads to more surface area on the surface
of the thylakoids
This leads to more photosynthetic pigments
and higher rate of light reactions (photosystem
1 and 2)
This leads to more ATP and NADPH produced
for the Calvin Cycle
This leads to more glucose (sugar) produced in
the Calvin Cycle
IB Exam Question
8. Outline the effect of temperature, light
intensity and carbon dioxide concentration
on the rate of photosynthesis.
(6 marks)
light:
rate of photosynthesis increases as light intensity increases;
photosynthetic rate reaches plateau at high light levels;
CO2:
photosynthetic rate rises as CO2 concentration rises;
up to a maximum when rate levels off;
temperature:
rate of photosynthesis increases with increase in temperature;
up to optimal level / maximum;
high temperatures reduce the rate of photosynthesis;
Some of the above points may be achieved by means of annotated
diagrams or graphs.
IB Exam Question
9. Explain three ways in which the rate
of photosynthesis can be measured.
(4 marks)
Production of oxygen because oxygen is a by product
of the reaction;
Measure carbon dioxide uptake because carbon
dioxide is a raw material of the reaction;
Measure increase in biomass; because products are
used in production of cell walls and new tissue;
Measure electron flow in light reaction using DPIP as
an electron acceptor and measuring the color change
as it is reduced with a spectrophotometer
IB Exam Question
10. Explain the reactions involving the use of
light energy that occur in the thylakoids of
the chloroplast.
(8 marks)
Splitting of water/ photolysis provides an electron
Chlorophyll in Photosystem II absorbs light
Electron is raised to a higher energy level (is excited)
Excited electron passes from photosystem II to carriers in
electron transport chain (in thylakoid membrane)
Energy from this excited electron is used to pump hydrogen
ions from stroma into thylakoid space, creating high hydrogen
ion concentration inside thylakoid.
IB Exam Question
11. Pigments are extracted from the leaves of a green
plant. White light is then passed through the
solution of pigments. What effect do the leaf
pigments have on the white light?
(1 mark)
A.
Green wavelengths are absorbed and red
and blue wavelengths are transmitted.
B. Red and blue wavelengths are absorbed and
green wavelengths are transmitted.
C. Blue wavelengths are absorbed and green
and red wavelengths are transmitted.
D. Green and red wavelengths are absorbed
and blue wavelengths are transmitted.
Correct answer: B
IB Exam Question
12. Why is the action spectrum for
photosynthesis similar to the absorption
spectra of photosynthetic pigments? (1 mark)
A.
Photosynthetic pigments have the same optimum
temperature as the enzymes used in
photosynthesis.
B.
Plants absorb the same photosynthetic pigments
for use in photosynthesis.
C.
Only wavelengths of light absorbed by pigments
can be used in photosynthesis.
D.
The amount of energy absorbed by photosynthetic
pigments is equal to the activation energy for
photosynthesis.
Correct answer: C