Transcript Photosynthesis - Paisley Grammar School Science Department

Photosynthesis
Learning Outcomes
• Explain absorption, transmission and
reflection of light by a leaf.
Revision from S.G.
• What is the word equation for photosynthesis?
• What is the pigment called that absorbs light
energy and where in a cell do you find it?
• What type of energy is this light energy changed
in to?
• What are some of the different tissues in a leaf
called?
Structure of a leaf
The epidermis
protects the leaf and
is transparent to let
light through.
Palisade mesophyll
cells are closely
packed to absorb
maximum light. They
contain many
chloroplasts. Most
photosynthesis takes
place in the palisade
cells.
Structure of a leaf
Spongy mesophyll
also captures light
and makes food.
Spongy mesophyll
cells have air spaces
between them to
allow movement of
gases.
Veins contain xylem
(top part of vein)
for water transport
and phloem (lower
part of vein) to take
away sugar.
Photosynthesis
• During the process of photosynthesis,
green plants use the light energy, carbon
dioxide and water to make carbohydrates
(e.g. glucose).
• Light energy is “fixed” by the green
pigment called chlorophyll which is found in
the chloroplasts.
Learning Outcomes
• Explain the absorption, transmission, and
reflection of light by a leaf.
• Describe the absorption spectrum of leaf
pigments.
• State that for chlorophyll a and b absorption
occurs in the blue and red light regions of the
spectrum.
• State that the accessory pigments xanthophyll
and carotene absorb light from other areas of the
spectrum and pass energy on to chlorophyll.
Fate of light
• Light hitting a leaf can be:
– Absorbed
– Reflected
– Transmitted
• Most of the light that hits a leaf is
absorbed. Of this absorbed light only a
small part is used in photosynthesis. The
rest is converted to heat and lost.
Light and green leaf
Sunlight 100%
83 % absorbed
12% reflected
NB - only a very
small 4% of
absorbed light is
used in
photosynthesis
5% transmitted
Light
• Light is a form of energy
• When a beam of white light is passed
through a prism, it splits in to different
colours. This is known as the spectrum.
• Each colour of the spectrum has a
different wavelength
The Spectrum
– Blue light has the shortest wavelength
(450nm)
– Red light has the longest wavelength
(650nm)
Fate of Light
• When light strikes a green leaf much of it
is absorbed but only 4% of this is used in
photosynthesis.
• The rest of the absorbed light is
converted to heat and lost.
• Most leaves appear green in colour because
they reflect and transmit green light.
 Black bands = where light energy is absorbed by the leaf.
Therefore it can’t travel through the prism
 Colour = where light is not absorbed (it is either reflected
or transmitted).
Photosynthetic Pigments
• There are 4 photosynthetic pigments
found in a green leaf:
–
–
–
–
Chlorophyll a
Chlorophyll b
Carotene
Xanthophyll
• These pigments can be separated by paper
chromatography.
Photosynthetic Pigments
• Chlorophyll a is the principal pigment as it
is the only pigment that directly brings
about photosynthesis.
• The other accessory pigments absorb light
and pass the energy on to chlorophyll a.
• Chlorophyll a absorbs mainly in the red and
blue regions.
Photosynthetic pigments
• Chlorophyll a contains the element
magnesium.
• Chlorophyll b , carotene and xanthophyll
are described as accessory pigments.
Absorption Spectrum
• This shows the percentage absorption of light by
photosynthetic pigments.
• Since each pigment absorbs different wavelengths
of light it means a larger range of wavelengths can
be absorbed than if there was only one pigment.
Learning Outcomes
• State the relationship between the action
spectrum and the absorption spectrum, in
relation to photosynthetic pigments and
the rate of photosynthesis.
Absorption and Action
spectrum
• An action spectrum shows the
effectiveness of different wavelengths of
light at bringing about photosynthesis.
Action
spectrum of
chlorophyll a
• The action spectrum
and absorption
spectrum follow
similar patterns.
• Certain wavelengths
are used in
photosynthesis.
• Chlorophyll a and b
absorb the red and
blue regions of the
spectrum
• Carotenoids absorb
the green regions
Learning Outcome
• Give an account of the detailed
structure of a chloroplast, locating
the sites for both the light
dependent and independent stages of
photosynthesis.
Chloroplasts
• A chloroplast is bound by a double
membrane and contains a liquid area known
as the stroma and layers of stacked
membranes called the grana.
Grana
• Grana (singular granum) are a coin-like stack of
flattened sacs containing photosynthetic
pigments.
• They have a large surface area of the pigments to
absorb light energy.
• The light dependent stage occurs at the grana.
Chloroplasts
• Stroma is the colourless “background”
material.
• Carbon fixation occurs here.
• There is no chlorophyll but it contains
enzymes and starch grains.
Learning Outcomes
• Describe the photolysis of water (part of
the light dependent stage), identifying the
products and substrates involved.
• Describe how ATP is formed in the lightdependent reaction
• State the importance of ATP and hydrogen
formation in the light dependent stage.
Photosynthesis
• Photosynthesis is the process by which glucose is
synthesised through the reduction of carbon
dioxide.
• The energy needed for this process comes from
light energy.
• Photosynthesis consists of 2 parts: A light
dependent (photochemical) stage and a
temperature dependent (thermochemical) stage.
Light dependent stage
• The light-dependent stage occurs in the
grana of chloroplasts.
• Light enegry is trapped bychlorophyll and
converted into chemical energy.
• This light energy is used to split water
molecules into hydrogen and oxygen. This
is called photolysis of water.
Photolysis
• The oxygen is released as a by-product.
• The hydrogen combines with a hydrogen
acceptor called NADP to form NADPH2.
• At the same time, chlorophyll makes
energy available for the regeneration of
ATP from ADP + Pi (phosphorylation).
Photolysis
• The hydrogen
held by NADPH2
and the energy
held by ATP are
needed for use
in the second
stage of
photosynthesisCarbon fixation.
Learning Outcome
• Describe the calvin cycle (the light
independent stage), identifying its main
substrates and products involved.
• Give an account of the fixation of carbon
dioxide in the Calvin cycle, identifying the
particular roles of RuBP and GP.
Carbon Fixation
• This stage occurs in the stroma of the
chloroplasts.
• It consists of several enzyme controlled
chemical reactions, which take the form of
a cycle (the Calvin cycle).
Carbon Fixation
• A molecule of carbon dioxide enters the
chloroplast by diffusion and joins with a
molecule of 5-Carbon ribulose biphosphate
(RuBP) in the stroma to form a 6-Carbon
molecule.
• This molecule is unstable and rapidly splits
into 2 molecules of 3-Carbon glycerate-3phosphate (GP).
CO2 + 5-C RuBP
6C
2 X 3-C GP
Carbon Fixation
• During the next stage in the cycle, GP is
converted to a 3-Carbon sugar using the
hydrogen temporarily bound to NADP and
some of the energy held in ATP.
• When 2 molecules of this 3C sugar
combine in an enzyme controlled series of
reactions, glucose is formed.
Carbon Fixation
• Photosynthesis is said to be a
reduction reaction because 3C GP is
reduced to form a 3C sugar. It is said
to be reduced because hydrogen is
added to it.
Calvin Cycle
Regenerating RuBP
• The 3 carbon sugars are not all used
to make complex products.
• Some are used to regenerate RuBP
(the carbon dioxide acceptor). This
also requires energy from ATP.
Rate of Photosynthesis
• The rate of photosynthesis can be
measured by the following:
– Evolution of oxygen per unit time
– Uptake of carbon dioxide per unit time
– Production of carbohydrate (as increase
in dry mass) per unit time.
Limiting Factors
• A limiting factor is a factor which holds up
a process because it is in short supply.
• Limiting factors in photosynthesis are:
– Carbon dioxide concentration
– Light intensity
– Temperature
Limiting factors
Limiting Factors
Limiting factors
P temperature
Q carbon dioxide/CO2
R Light intensity
Elodea Bubbler
• The number of oxygen bubbles produced
each minute can be used to measure the
rate of photosynthesis.
• You can change the light intensity etc and
see the effect it has on the rate of
photosynthesis.