IB Biology Photosynthesis 2015

Topic Four : Photosynthesis
http://sodandmulch.com/wp-content/uploads/2012/03/Oak-Tree.jpg

Essential idea: Photosynthesis uses the energy in sunlight to
produce the chemical energy needed for life.
2.9 Photosynthesis
http://foodphotographyblog.com/wp-content/uploads/2014/07/Hass-Tree-Canopy.jpg
California Avocado Trees

Understandings
Statement Guidance
2.9 U.1 Photosynthesis is the production of carbon
compounds in cells using light energy.
2.9 U.2 Visible light has a range of wavelengths
with violet the shortest wavelength and
red the longest.
2.9 U.3 Chlorophyll absorbs red and blue light
most effectively and reflects green light
more than other colors.
Students should know that
visible light has wavelengths
between 400 and 700
nanometres, but they are not
expected to recall the
wavelengths of specific colours
of light.
2.9 U.4 Oxygen is produced in photosynthesis
from the photolysis of water.
2.9 U.5 Energy is needed to produce
carbohydrates and other carbon
compounds from carbon dioxide.
2.9 U.6 Temperature, light intensity and carbon
dioxide concentration are possible limiting
factors on the rate of photosynthesis.

Applications and Skills
Statement Guidance
2.9 A.1 Changes to the Earth’s atmosphere,
oceans and rock deposition due to
photosynthesis.
2.9 S.1 Drawing an absorption spectrum for
chlorophyll and an action spectrum for
photosynthesis.
2.9 S.2 Design of experiments to investigate the
effect of limiting factors on
photosynthesis.
Water free of dissolved carbon
dioxide for photosynthesis
experiments can be produced
by boiling and cooling water.
2.9 S.3 Separation of photosynthetic pigments by
chromatograph. (Practical 4)
Paper chromatography can be
used to separate
photosynthetic pigments but
thin layer chromatography
gives better results.

Essential idea: Light energy is converted into chemical energy
8.3 Photosynthesis
http://foodphotographyblog.com/wp-content/uploads/2014/07/Hass-Tree-Canopy.jpg
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Understandings
Statement Guidance
8.3 U.1 Light-dependent reactions take place in the
thylakoid membranes and the space inside them.
8.3 U.2 Light-independent reactions take place in the
stroma.
8.3 U.3 Reduced NADP and ATP are produced in the light-
dependent reactions.
8.3 U.4 Absorption of light by photosystems generates
excited electrons.
8.3 U.5 Photolysis of water generates electrons for use in
the light-dependent reactions.
8.3 U.6 Transfer of excited electrons occurs between
carriers in thylakoid membranes.
8.3 U.7 Excited electrons from Photosystem II are used to
contribute to generate a proton gradient.
8.3 U.8 ATP synthase in thylakoids generates ATP using the
proton gradient.
8.3 U.9 Excited electrons from Photosystem I are used to
reduce NADP.
8.3 U.10 In the light-independent reactions a carboxylase
catalyzes the carboxylation of ribulose
bisphosphate.

Statement Guidance
8.3 U.11 Glycerate 3-phosphate is reduced to triose
phosphate using reduced NADP and ATP.
8.3 U.12 Triose phosphate is used to regenerate RuBP and
produce carbohydrates.
8.3 U.13 Ribulose bisphosphate is reformed using ATP.
8.3 U.14 The structure of the chloroplast is adapted to its
function in photosynthesis.
Understandings

Applications and Skills
Statement Guidance
8.3 A.1 Calvin’s experiment to elucidate the
carboxylation of RuBP.
8.3 S.1 Annotation of a diagram to indicate the
adaptations of a chloroplast to its
function.

Photosynthesis: Capturing Energy
2.9 U.1 Photosynthesis is the production of carbon compounds in cells
using light energy.
• Living organisms require
complex carbon
compounds to carry out
life processes and build
the structures in their
cells
• Photosynthesis involves
the conversion of light
energy into chemical
energy (carbohydrates,
lipids, protein and
nucleic acids).
• Chloroplasts absorb light
energy from the sun and
convert this energy into
chemical energy
(glucose) to be used by
the organisms for energy.

Autotrophs and Chemotrophs
• Carbon fixation is the process of building
complex carbon compounds from simple
carbon compounds.
Autotrophs organisms that fix carbon, using carbon
dioxide as a carbon source, and combine it with
water
Photoautotrophs provide nearly all the energy
used by living systems on Earth
using light energy.

Overview of Photosynthesis
• Photosynthesis is a redox reaction:
• Carbon dioxide is reduced to sugar
• Water is oxidized to molecular oxygen
Carbon dioxide + Water + Light Glucose + Oxygen
C6H12O6 + 6O26 CO2 + 6 H2O
Reduction
Oxidation
using light energy.

8.3 U.1 Light-dependent reactions take place in the thylakoid
membranes and the space inside them.
• Double outer membrane
• Thylakoids is the internal
membranes called which
is the location of the light
dependent reaction
• Grana are stacks of
thylakoids
• Stroma cytoplasm that
surrounding the
thylakoids and grana. This
is the location of the light
independent reaction.

2.9 U.2 Visible light has a range of wavelengths with violet the shortest
wavelength and red the longest.
• Light from the sun is composed of a range of wavelengths.
• The visible spectrum is the portion of the electromagnetic spectrum that is visible
to or can be detected by the human eye.
• Electromagnetic radiation in this range of wavelengths (380 to 750 nm) is called
visible light.
• All these wavelengths together form white light, with violet/blue colors having
shorter wavelengths (more energy) and red colors having longer wavelengths (less
energy).
http://www.schome.ac.uk/wiki/images/3/36/EM_spectrum.jpg

2.9 U.3 Chlorophyll absorbs red and blue light most effectively and
reflects green light more than other colors.
http://en.wikipedia.org/wiki/Plant_anatomy#/media/File:03-10_Mnium2.jpg
A Moss named Mnium stellare
• Sunlight is a mixture of different wavelengths of visible light, which we see as
colors.
• The two main colors of light that are absorbed by chlorophyll are red and blue light.
• The main color that is reflected is green light, which is why most leaves look green

Pigments
• A pigment is any substance
that absorbs light. The
color of the pigment
comes from the
wavelengths of light
reflected (in other words,
those not absorbed).
• Chlorophyll is a complex
molecule. Several
modifications of
chlorophyll occur among
plants and other
photosynthetic organisms.
Chlorophyll

Chlorophyll a
• All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and
cyanobacteria) have chlorophyll a.
• Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red
wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths.
• Chlorophyll a is the main photosynthetic pigment in all organisms except bacteria
Accessory pigments
• Accessory pigments absorb energy that chlorophyll a does not absorb.
Accessory pigments include chlorophyll b (also c, d, and e in algae and
protistans), xanthophylls, and carotenoids (such as beta-carotene).

In the absence of equipment use the virtual lab and self-test quiz:
http://www.phschool.com/science/biology_place/labbench/lab4/pigsep.html
Simplified Bioknowledgy protocol based on the SaPS outline:
https://app.box.com/s/i8cc161713atmk7ks5zoex1psyrjir89
Thin layer chromatography for photosynthetic pigments
SAPS have published two (slightly) different protocols:
• http://www.saps.org.uk/secondary/teaching-resources/189-investigation-of-
photosynthetic-pigments-in-green-plants
• http://www.saps.org.uk/secondary/teaching-resources/181-student-sheet-10-
thin-layer-chromatography-for-photosynthetic-pigments
What pigments can you find and identify in a leaf?
Gather leaves of different types and colors and use Thin Layer
Chromatography (TLC) to separate the pigments, including
chlorophyll present in a leaf.
2.9 S.3 Separation of photosynthetic pigments by chromatograph.
(Practical 4)

Light energy converted into chemical energy
• Producers contain
chlorophyll
• Chlorophyll can trap light
energy (photons).
• The chlorophyll convert this
energy into chemical energy.
• The chemical energy is
transferred as bond energy
(electrons)and is transferred
in turn to other chemical
energy stores called
carbohydrates, lipids and
protein.
• These molecules are called
organic molecules.

Partition of Function in the Chloroplast
1. The light-dependent reaction (the harvesting of light) occur
on the thylakoid membrane.
2. The light-independent reaction (carbon fixation reaction or
the formation of carbohydrate), which occurs in the stroma.
2.9 U.5 Energy is needed to produce carbohydrates and other carbon

Photosynthesis occurs in two main phases:
1. Light Dependent Reaction
A. Energy of sun is trapped by chlorophyll molecules (oxidation)
B. ADP is reduced to ATP with NADP+ reduced to NADPH, both are used to fix
carbon.
C. The reaction must have light to take place.
D. This reaction takes place on the thylakoid membranes.
http://3-8photosynthesis.tumblr.com/

8.3 U.2 Light-independent reactions take place in the stroma.
http://www.nature.com/scitable/content/ne0000/ne0000/ne000
0/ne0000/14705803/U1CP4-4_LightDarkRxn_ksm.jpg
• Energy captured from the electron is transferred to NADPH and ATP
and move from the thylakoid into the stroma of the chloroplast.
• Carbon dioxide will be converted into glycerate 3-phosphate (G3P) a
triose phosphate using NADPH and ATP.

8.3 U.3 Reduced NADP and ATP are produced in the light-dependent
reactions.
https://classconnection.s3.amazonaws.com/19/flashcard
s/410019/jpg/cellbio321329367560536.jpg
•At the same time water is split
into oxygen, hydrogen ions and
free electrons are produced:
2H2O 4H+ + O2 + 4e-
(photolysis)
•The electrons then react with a
carrier molecule (NADP),
changing it from its oxidized state
(NADP+) to its reduced state
(NADPH):
NADP+ + 2e- + 2H+ NADPH + H+

http://chm233.asu.edu/reallife/331atp/adp2atp.gif

8.3 U.4 Absorption of light by photosystems generates excited electrons.
http://classroom.sdmesa.edu/eschmid/Lectur40.gif
• Pigments in the thylakoid
membrane absorb light at
certain wavelengths
• The light energy causes
electrons held by pigments to
raise to higher energy states.
This converts the light energy
into a form of chemical energy.
• These excited electrons are
passed from pigment to
pigment until the reach a
molecule called the reaction
center.
• The reaction center pass the
electrons to electron acceptors
in the thylakoid membrane

2.9 U.4 Oxygen is produced in photosynthesis from the photolysis of water.
8.3 U.5 Photolysis of water generates electrons for use in the light-dependent reactions.
• Chlorophyll in the thylakoid membrane
is excited by light absorption.
• Electrons (e-) in the chlorophyll are
energized to an excited state.
• e- captured by primary electron
acceptor
 Redox reaction  e- transfer
 As e- is transferred from one
enzyme to the next it drop to a
ground state
• Photolysis is one of the first step the light
dependent reactions of photosynthesis.
• H2O is split to replace e-  O2 formed
• Water (H2O) is split by photons of light to
produce 4 e- + 4H+ + O2
Photolysis: Splitting a Water Molecule to produce ATP , H+ and O2

Absorption spectra are obtain
from samples of pigment.
Different wavelengths of
light are passed through
and the absorption is
measured using a colorimeter.
This absorption spectra for
chlorophyll shows:
•absorption of blue light
•absorption of red light
•green light is reflected.
2.9 S.1 Drawing an absorption spectrum for chlorophyll and an action
spectrum for photosynthesis

Action Spectrum: Measures the rate of Photosynthesis
•The rate of photosynthesis is
measured at different
wavelengths.
•The maximum rate are at the
blue end and red end of the
visible spectrum.
•The lowest rates are in the
yellow greens.
•Chlorophylls are absorbing blue
and red light well but not green.
spectrum for photosynthesis

spectrum for photosynthesis.
(Edited by Chris Paine)
https://app.box.com/s/88edjbrgbff0febtiyfk8x9l0cdnyshr
https://app.box.com/s/88edjbrgbff0fe
btiyfk8x9l0cdnyshr
http://www.mhhe.com/biosci/genbio/biolink/j_explorations/ch09expl.htm

2.9.U3 Chlorophyll absorbs red and blue light most effectively and reflects green light more than other
colours.

This shows the rate of photosynthesis for all the
wavelengths of light as a % of the maximum possible rate.
%ofthemaximumrateofphotosynthesis

%ofthemaximumrateofphotosynthesis
This shows the absorbance of light by photosynthetic
pigments (here chlorophyll) for all the wavelengths of
light.

Factors that limit the rate of photosynthesis
• The rate of photosynthesis can be affected by light intensity, carbon
dioxide concentration and temperature.
• Under a given set of conditions only one factor will affect the rate of
photosynthesis this factor is at its minimum and is called the limiting
factor.
• As has been shown photosynthesis is a process with many individual steps
or stages.
• The overall rate of photosynthesis is determined by the step that is
proceeding most slowly (rate-limiting step).
2.9 U.6 Temperature, light intensity and carbon dioxide concentration
are possible limiting factors on the rate of photosynthesis.

(a). As the concentration of CO2 is increased the rate of photosynthesis increases.
(b).The concentration of CO2 has saturated the process. The maximum rate of
reaction has been achieved. Further increases in CO2 do not increase the rate.
The rate is now constant.
(c) Note this is the normal concentration of CO2 in the atmosphere.! Therefore this
is often the limiting factor.
Carbon Dioxide (CO2)

2.9.U6 Temperature, light intensity and carbon dioxide concentration are possible limiting factors on
the rate of photosynthesis.
RateofPhotosynthesis
Light intensity
http://i-biology.net/ahl/08-cell-respiration-photosynthesis/8-2-photosynthesis/
When light intensity is increased the rate of
photosynthesis increases therefore it is the
limiting factor at low levels.
At high levels of light intensity further increases have no effect on the rate of
photosynthesis. Therefore light intensity is not the limiting factor, another
factor (e.g. temperature, CO2 concentration, enzymes or chloroplasts working
at maximum efficiency) is limiting photosynthesis.
Light intensity refers to the amount of light, of a given
wavelength, which is available to the plant.

Light intensity
a) As the intensity of light is increased the rate of reaction of photosynthesis
increases. The rate maybe limited by a lack of NADP+
(b) Light intensity has saturated the plants. The rate now remains constant for any
further increase in light intensity.
(c) Note that light intensity to achieve maximum rate of photosynthesis is less
than the intensity of light in summer. Light in not normally a limiting factor

When carbon dioxide concentration is increased
the rate of photosynthesis increases therefore it is
the limiting factor at low concentrations.
Carbon dioxide concentration
Another factor (e.g. temperature, light, enzymes working at
maximum efficiency) is limiting photosynthesis as further
increases in carbon dioxide do not increase the rate of
photosynthesis.
CO2 is a substrate for the
metabolic pathway hence
the relationship is similar to
how enzyme reactions are
limited by substrate
concentration.

Temperature
• The optimum temperature in a temperate climate is about 25° C.
• However. Temperature has many effects on a plant and the graph should be
treated with caution.
• Temperature has just as many effects on respiration, transpiration and
translocation all of which in turn affect photosynthesis.

Photosynthesis is a
metabolic pathway hence
the relationship is similar to
how enzyme reactions are
affected by temperature.
After the optimum
temperature enzymes
denature rapidly
causing a fast
decrease in the rate of
photosynthesis as
temperature increases
further.
As the temperature
approaches the
optimum the enzymes
begin to denature
(active site changes to
become non-functional)
causing the rate of
photosynthesis to
increase more slowly
and eventually peak.
Increases in temperature give
molecules more kinetic energy
causing substrates to collide with
active sites more frequently, this
increases the rate of
photosynthesis
Temperature

2.9.S2 Design of experiments to investigate the effect of limiting factors on photosynthesis.
Placing the plant in a closed space
with water.
CO2 reacts with the water producing
bicarbonate and hydrogen ions, which
increases the acidity of the solution.
Increased CO2 uptake -> increased pH
-> increased rate of photosynthesis.

Aquatic plants can submerged
in water in a closed space with
a gas syringe attached.
Alternatively gas volume can
be measured by displacing
water in an inverted
measuring cylinder or by
simply counting bubbles.
Oxygen probes can be used
with terrestrial plants kept in
closed environments to
measure increases in oxygen
concentration.

Glucose production can be
(indirectly) measured by a
change in a plant's dry
biomass.
starch levels in a plant (glucose
is stored as starch)
can be identified by staining
with iodine solution, this can
be quantitated using a
colorimeter.

8.3 U.5 Photolysis of water generates electrons for use in the light-dependent
reactions.

http://www.guam.net/pub/sshs/depart/science/mancuso/apbiolecture/07_aqpath/image6.gif
• Photosystem II must
replace excited
electrons given away
by chlorophyll
• With the help of an
enzyme in the reaction
center, water
molecules in the
thylakoid space are
split and electrons from
them are given to the
chlorophyll at the
reaction center.
8.3 U.6 Transfer of excited electrons occurs between carriers in thylakoid
membranes.

Protons Build up Inside Thylakoids
8.3 U.7 Excited electrons from Photosystem II are used to contribute to
generate a proton gradient.

8.3 U.8 ATP synthase in thylakoids generates ATP using the proton
gradient.
https://b51ab7d9e5e1e7063dcb70cee5c33cf7f4b7bad8.googledriv
e.com/host/0Bx6hk6AUBHxDc2d4TDJZTFIyMGs/files/Bio%20101/
Bio%20101%20Lectures/Photosynthesis/photosynthesis7.png
• ATP Synthase located in
the thylakoid membranes
allows the protons to
diffuse back down the
concentration gradient to
produce ATP.
• The generation of ATP
using energy released by
the movement of H+ is
called chemiosmosis and
is called
photophosphorylation

8.3 U.8 ATP synthase in thylakoids generates ATP using the proton
gradient.
http://www.uncommondescent.com/wp-content/uploads/2015/02/atpsynthase.gif

8.3 U.9 Excited electrons from Photosystem I are used to reduce NADP.
• A pair of excited electrons e-
pass from the reaction center
of thylakoid into a small
electron transport chain
(ETC).
• At the end of the ETC the
electrons are passed to NADP
in the stroma.
• In addition NADP picks up
two protons (H+) and is
reduced to NADPH.
• NADPH will be used to fix
carbon from carbon dioxide
into a carbohydrate.

Proton motive force generated by:
(1) H+ from water
(2) H+ pumped across by cytochrome
(3) Removal of H+ from stroma when NADP+ is reduced

Animation from Sigma Aldrich:
http://tinyurl.com/5k99sc

Summary of the Light Dependent Reaction
• The light-dependent reactions transform light energy into chemical energy which is
trapped and carried by ATP and NADPH to the Calvin Cycle.
• The light-dependent reactions require chlorophyll and occur in the thylakoid
membranes of the grana of the chloroplast.
• Light energy is also used to split water (Photolysis of water) into:
H2O -----> 2H+ + 2e- + 1/2 O2
• This reaction produces oxygen and provides electrons and Hydrogen for the
reduction of NADP to NADPH (NADP gains H+ and electrons; the water is oxidized
because it loses the H+ and e-)
• The light reactions remove electrons from excited chlorophyll molecules in both
Photosystem I and Photosystem II and pass the higher energy electrons along an
electron transport chain, releasing energy to make ATP (from ADP and P), or
transferring the electrons to NADP.
• The light reactions must occur several times to produce enough ATP and NADPH to
"run" the Calvin cycle

2. The Light Independent Reaction
A. Uses the chemical energy from the LDR (ATP and NADPH2) to fix
atmospheric carbon (CO2)into organic molecules such as glucose.
B. The process does not require light and can occur in both the light and dark
periods.
C. This reaction takes place in the stroma
http://3-8photosynthesis.tumblr.com/

Calvin Cycle: Uses ATP and NADPH to convert CO2 to sugar
• Uses ATP, NADPH, CO2
• Produces 3-C sugar G3P
(glyceraldehyde-3-phosphate)
Three phases:
1. Carbon fixation
2. Reduction
3. Regeneration of RuBP
(CO2 acceptor)
8.3 U.10 In the light-independent reactions a carboxylase catalyzes the
carboxylation of ribulose bisphosphate (RuBp).

The Three phases of
The Calvin Cycle LIR:

Carbon Fixing phase
(carboxylation)
•Adds carbon dioxide to 5C
ribulose bisphosphate
(RuBP)
•Catalyzed into RUBISCO
(one of the most abundant
enzymes on Earth); ribulose
bisphosphate carboxylase

2. Reduction phase
•Citrate is made and broken to
form 2 phosphoglycerate (PGA)
•PGA is rearranged and
phosphorylated by ATP
•NADPH reduces the backbone
further to form glyceraldehyde-3-
phosphate (G3P)
8.3 U.11 Glycerate 3-phosphate is reduced to triose phosphate using
reduced NADP and ATP.

3. Regeneration of RuBP:
– G3P is rearranged,
– & phosphorylated
– With further investment of
ATP…
– To make RuBP, a
bisphosphorylated compound
• Alternatively,
– G3P is shuttled out of the
cycle to produce glucose and
other carbohydrates
elsewhere
8.3 U.12 Triose phosphate is used to regenerate RuBP and produce carbohydrates.
8.3 U.13 Ribulose bisphosphate is reformed using ATP.

8.3 U.14 The structure of the chloroplast is adapted to its function in
photosynthesis.
https://classconnection.s3.amazonaws.com/558/flash
cards/183558/png/picture11328537112859.png
• Outer membrane Consists of
inner and outer phospholipid
bilayers. The membrane helps
increase the concentration of
enzymes, increasing the rates
of reaction inside the
chloroplast.
• Thylakoids A flattened
membrane sac inside the
chloroplast increasing surface
area and concentration of
enzymes, used to convert
light energy into chemical
energy.

8.3 S.1 Annotation of a diagram to indicate the adaptations of a
chloroplast to its function
http://www2.victoriacollege.edu/dept/bio/CoonsWebPages/ch5cell/taL22600_05_11b.jpg

8.3 S.1 Annotation of a diagram to indicate the adaptations of a
chloroplast to its function
• Chloroplast double membrane- Creates a compartment in
which enzymes and other components can be concentrated
• 70S Ribosome allows for the synthesis of proteins
• Stroma Matrix
• Circular DNA source for protein synthesis and mitosis
• Granum stack of thylakoids
• Thylakoids membrane/space increase surface area for light
absorption, which generates electron flow, with the space
providing and area to create a proton gradient

8.3 A.1 Calvin’s experiment to elucidate the carboxylation of RuBP.
http://www.intechopen.com/books/photosynthesis
/the-path-of-carbon-in-photosynthesis-mannosideshttp://bancroft.berkeley.edu/Exhibits/Biotech/Images/3-9lg.jpg

• A timer and a quick acting valve
were used to catch algae at
various stages of the light
independent reaction.
• Hot methanol kills algae; stops
photosynthesis.
• Radioactive carbon (C14) allows
the carbon containing
intermediates to be identified.
• The carbon compounds were
separated at each advancing
stage by chromatography and
identifying (results to the right).
http://5e.plantphys.net/images/ch08/wt0802a.png

http://bancroft.berkeley.edu/Exhibits/Biotech/Images/3-9lg.jpg
Calvin’s experiment analyzed the results using autoradiograms
http://5e.plantphys.net/images/ch08/wt0802a.png
Samples were
taken at
different time
intervals
after exposure
to 14C
After only 5 seconds
there is more labeled
glycerate 3-phosphate
than any other
compound.
This indicates that
glycerate 3-phosphate
is the first product of
carbon fixation
After 30 seconds a
range of different
labeled compounds
occur showing the
intermediate and final
products of the light-
independent reactions

Before designing an carrying out your own
investigation what questions need to be asked
and considerations need to be made?
http://i.ytimg.com/vi/n-YeCeSQS3w/maxresdefault.jpg
http://sbi4u-
photosynthesis.weebly.com/uploads/1/9/2/8/1928
4461/5941741_orig.jpg

http://sbi4u-
4461/5941741_orig.jpg
The independent variable
• Only one limiting factor should be investigated at a time
• The range of values should reflect conditions experienced
by the organism
• The range of values should allow the limiting factor to
range from values that restrict photosynthesis to values
that allow photosynthesis to happen at it’s optimum rate.
• The increments should be sufficiently in size that a trend
can be clearly detected

http://sbi4u-
4461/5941741_orig.jpg
Dependent variable
• An accurate method for measuring the rate of
photosynthesis needs to be used.
• Oxygen production per time unit is recommended.
Leaf discs are a successful and easy way to measure
oxygen generation by leaves
http://www.saps.org.uk/secondary/teaching-resources/284-
investigating-photosynthesis-with-leaf-discs

http://sbi4u-
4461/5941741_orig.jpg
The control variables
• These should include the limiting factors not being
investigated.
• Other key control variables should include any factor that
affects a metabolic pathway controlled by enzymes, e.g. pH.
• Ambient light should be considered as it affects the
wavelength and intensity of light absorbed by the organism.
• The values chosen for the control variables should be close to
their optimum values so that the control variables do not
limit photosynthesis.
(If the control variables limit photosynthesis it may not be possible to
see the impact of the limiting factor being investigated)

Nature of Science: Experimental design - controlling relevant variables in photosynthesis experiments is essential.
(3.1)
http://sbi4u-
4461/5941741_orig.jpg
The control variables - Nature of Science
• Explain why it is essential to control the limiting factors
not being investigated.
• Evaluate which of the identified reasons are the most
important.

2.9.A1 Changes to the Earth’s atmosphere, oceans and rock deposition due to photosynthesis.
http://commons.wikimedia.org/wiki/File:Blue_Marble_Eastern_Hemisphere.jpg
Primordial Earth had a reducing atmosphere that
contained very low levels of oxygen gas (approx. 2%).
Cyanobacteria (prokaryotes) containing chlorophyll first
performed photosynthesis about 2.5 billion years ago.
Photosynthesis creates oxygen gas as a by-product (by the photolysis of water).
Oxygen levels remained at 2% until about 750 million years ago (mya).
From 750 mya until the now there has been a significant rise to 20%.
Oxygen generation also allowed the formation of an ozone layer (O3).
Ozone shielded the Earth from damaging levels of UV radiation. This, in
turn, lead to the evolution of a wider range of organisms.
Iron compounds in the oceans were oxidized:
• The insoluble iron oxides precipitated onto the seabed.
• Time and further sedmentation has produced rocks with
layers rich in iron ore called the banded iron formations.
Oxygen in the atmosphere
lead to the production of
oxidised compounds (e.g.
CO2) in the oceans.

Microfossils of Prokaryotes
• Evident as tiny structures that look very much like cyanobacteria and other
modern-day prokaryotes.
In rocks formed as long ago as ~3.5 bybp
Radiocarbon dating from similar rocks suggest life formed perhaps
even 3.8 bybp
• Stromatolites are layered pillow-like rock structures
 Layers of prokaryotic cells and sediment
 Are plentiful in ancient rocks, formed large reefs
 Found today only in certain limited environments, such as a few hot springs
and in Shark Bay, a Western Australian location
2.9 A.1 Changes to the Earth’s atmosphere, oceans and rock deposition
due to photosynthesis.

Stromatolites
• In the beginning photosynthetic systems may have resembled these
stromatolites at Shark Bay, Western Australia

• Oxygen begins to accumulate about 2.7 bya, known as the “Great Oxidation Event”
– reducing  oxidizing atmosphere
• evidence in banded iron in rocks = rusting
• makes aerobic respiration possible
– photosynthetic bacteria (blue-green algae)
• The oxygen remained at about 2% until about 700 mya. There was then a significant
rise in oxygen until it reached about 20%.
• This lead to a huge increase in species as multicellular organisms evolved

Rust band in the ocean floor
created by the presence of Oxygen

Evidence for O2 production:
• Banded Iron Formations (BIF)
• BIF found in ocean sediments red bands are high in Fe2O3
and Fe3O4 (red bands)- forms when reduced iron reacts
with O2

Formation of Aerobes
• Bacteria which need free oxygen for their survival
• Aerobes came about after onset of photosynthesis
• Critical stage; allowed formation of more complex organisms.
Producing more energy from glucose than anaerobic metabolism
• Out-compete anaerobes for populating the Earth
• Stabilized atmosphere

Bibliography /
Acknowledgments

IB Biology Photosynthesis 2015

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