Photosynthesis
Topic 3.8
Key points…..

Photosynthesis involves conversion of
LIGHT energy (usually from the sun) into
CHEMICAL energy (in food).
(Microsoft Clip art)

The overall equation:
Carbon Dioxide + Water + Light → Glucose + Oxygen
6CO2 + 6H2O + Energy → C6H12O6 + 6O2
Sunlight consists of WHITE light but is actually
made up of a range of wavelengths (colours)
including red, blue and green.
http://mirror-in-bom1.gallery.hd.org/_exhibits/natural-science/prism-and-refraction-of-light-into-rainbow-AJHD.jpg
http://quarknet.fnal.gov/quarknet-summer-research/QNET2010/Astronomy/em_spectrum.jpg
More key points….
Pigments in the leaves can absorb light.
The main photosynthetic pigment is
CHLOROPHYLL. (There are a few others
too, such as the carotenoids.)
 Chlorophyll absorbs red and blue light
well, but reflects green light. This is why
leaves appear green.
 Some of the energy absorbed by
chlorophyll is used to produce ATP.

Absorption Spectra
show how much light
is taken in by
pigments.
In chlorophyll this
light energy is used to
drive photosynthesis.
This is shown in the
Action Spectra
which shows how
much Photosynthesis
reactions take place
http://www.life.illinois.edu/govindjee/paper/fig5.gif


Some of the energy absorbed by chlorophyll is
used to split water molecules into oxygen and
hydrogen.
This is called PHOTOLYSIS.
H+
H
H
O


H+
Light
Energy
O2-
The hydrogen is used later on in the process
of photosynthesis.
The oxygen is released as a waste product.

Lets look at the equation for Photosynthesis
again…….
6CO2 + 6H2O + Energy → C6H12O6 + 6O2




Where do these “C’s” come from?
Where do these “O’s” come from?
What about these “O’s”?
Where do this “H’s” come from?
The carbon is used to make glucose
initially, and a range of organic
substances.
 Conversion of carbon in a gas (CO2) to
carbon in a solid (glucose) is called
CARBON FIXATION.
 Carbon fixation involves the use of
hydrogen from photolysis and energy
from ATP.

The leaf is the
organ
responsible for
photosynthesis.
So how do all
the necessary
ingredients for
Photosynthesis
meet up in the
leaf?
http://s2.hubimg.com/u/746729_f520.jpg


Carbon dioxide is absorbed into
the leaf via the stoma / stomata.
Oxygen leaves by the stoma.
http://www.emc.maricopa.edu/faculty/farabee/biobk/leafstru.gif

Water
comes up
from the
roots via
tubes
called
Xylem

Glucose that
is produced
is taken to
the rest of
the plant in
the phloem
tubes.
Look at the picture of the leaf structure on the
previous slide – then fill in the labels on this one
 A- Air space
http://www.bioclix.org/images/leaf_2.jpg

B- Spongy
mesophyll

C- Vascular
bundle

D- Palisade layer

E- Stomata
Your task:
 In
the next 3 slides think about what
you would have to do to measure the
Rate (speed) of Photosynthesis.
Draw how you would set up the
apparatus (equipment) for the 3
different ways of measuring the rate
mentioned.
Measuring rate of Photosynthesis

Production of oxygen
Aquatic plants (eg. Elodea) give off bubbles of
oxygen during photosynthesis. These
bubbles can either be counted or collected
and the volume measured to give an
indication of rate.
Production of Oxygen Experiment
Measuring rate of photosynthesis 2

Uptake of carbon dioxide
Leaves absorb carbon dioxide from the air or
water, but this is difficult to measure directly.
CO2 is an acidic gas, so when it is removed
from water by photosynthesis, the pH rises.
This can be measured with indicators and pH
meters.
Uptake of Carbon Dioxide Experiment
Measuring rate of photosynthesis 3

Increase in biomass
Biomass is the biological mass of
organisms. If batches of plants are
harvested at different times, and the
biomass is determined, the rate of increase
in biomass gives an indirect measure of
rate of photosynthesis.
Increase in Biomass Experiment
What affects how fast Photosynthesis
occurs? Why and how does it affect it?



Amount of CO2 and H2O.
If you don’t have raw materials………..
Light Intensity
You need energy to make the chemical
reactions happen…….
Temperature
Photosynthesis needs enzymes……..
Draw and annotate graphs to explain each.
Production of oxygen set up


http://moodle.schoolnet.lk/file.php/21/assets/images/ks4/elodea.gif
You could either
count the number
of bubbles in a set
amount of time or
measure the
volume of gas
produced over a
longer period of
time.
Sodium Hydrogen
Carbonate provides
CO2 to the plant.
Back
Uptake of Carbon Dioxide set up

http://c.photoshelter.com/img-get/I0000xQykazfHOUU/s
Put an aquatic plant in
the water with a known
amount of CO2
dissolved in it. Monitor
the pH levels (using a
pH probe) over time to
measure CO2 uptake.
pH should increase with
time. Rate of change of
pH is a measure of
Rate of P/S.
Back
Increase in Biomass set up
Weigh the dry aquatic plant, carry out the
experiment and weigh the dry plant again.
% change in biomass over time can be a
measure of Rate of P/S.
(Microsoft Clip art)
Back