Download OCR A Level Biology B Learner resource

Teacher Resource 1 – Photosynthesis
This activity requires full concentration and participation of all class members.
1.
What does a plant need to carry out photosynthesis?
Light, carbon dioxide, water
2.
What is produced by photosynthesis?
Glucose and oxygen
3.
What do humans use these products for?
Cell respiration
4.
The chemical equation for photosynthesis:6CO2 + 6H2O  C6H12O6 + 6O2
Compare that to the equation for respiration:C6H12O6 + 6O2  6CO2 + 6H2O + energy
5.
Why are we dependent on plants?
They produce the food and oxygen we need to produce ATP.
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Overview of Process of Photosynthesis:•
Carbon dioxide and water are taken in by plants
•
Plants absorb light energy and convert it to a usable form- ATP
•
Energy is used to “fix” carbon dioxide into sugar molecules (chemical energy)
•
Sugar is converted to starch and stored for use by the plant, and by animals when they
eat plants.
Chloroplasts
6.
Draw and label a chloroplast:-
Chloroplasts are the organelles in the leaf where photosynthesis takes place. They are
positioned in leaves to harvest the maximum amount of light energy.
They contain chlorophyll molecules – a pigment that absorbs sunlight
A system of flattened membranous vesicles – the thylakoids - are located throughout the
chloroplast. These are surrounded by a liquid based matrix – the stroma.
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Photosynthesis consists of two stages –
 The light-dependent stage which takes place on the thylakoid membranes where
chlorophyll is situated.
 The light-independent stage (the Calvin Cycle) which takes place in the stroma
containing enzymes which control the complex reactions.
What happens in the light-dependent reactions:Two useful products are formed by the light-dependent stage – ATP and NADPH
These are needed to drive the light-independent stage.
7.
Using the sorting activity, produce a summary of the reactions that occur during the lightdependent stage.
The light-dependent reaction – the production of ATP and NADPH
Photons of light are absorbed by chlorophyll a molecule in photosystem II.
This causes 2 electrons from chlorophyll a to become excited and rise to a higher energy
level.
The excited electrons are picked up by electron acceptors and passed through a series of
electron carriers releasing energy and then passed to photosystem I.
The energy released is used to pump protons from the stroma across the thylakoid
membranes into the thylakoid space producing a proton gradient.
The protons flow back through an ATP synthase channel, producing ATP from ADP and Pi.
Light energy also causes water to split – photolysis.
2e-, 2H+and O2 are produced. The O2 is released as a by-product.
The electrons produced by photolysis replace the electrons lost by the chlorophyll a
molecule in photosystem II.
Light hits chlorophyll in photosystem I causing electrons to become excited and rise to a
higher energy level.
These electrons pass down a series of electron carriers and energy is released. The final
electron acceptor is NADP+. NADP+ joins with H+ to produce NADPH using energy from
the electron carriers.
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The light-independent reactions:•
The reactions take place in the stroma of the chloroplasts and form a metabolic
pathway known as the Calvin Cycle.
•
These reactions do not need light but do need the products of the light dependent
reactions. Because of this they cannot go on for long in the dark as the ATP and
NADPH run out.
•
The light independent reactions consist of a cycle which uses up carbon dioxide, ATP
and NADPH.
•
It produces carbohydrate molecules.
8.
What happens in the light-independent reactions:-
9.
Use the sorting activity to produce your own summary.
The light-independent reaction
CO2 combines with RUBP (5C). The reaction is catalysed by Rubisco.
The 6C molecule produced is unstable and breaks down to produce two molecules of
GP (3C) – glycerate phosphate.
ATP provides the energy and NADPH provides the H to reduce GP to triose phosphate
(TP)
Most of the TP is used to regenerate RUBP so the cycle can continue.
Some TP is converted into molecules such as glucose.
Some TP is modified to produce lipids and proteins.
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Products of photosynthesis
During photosynthesis carbon dioxide is ‘fixed’ producing glycerate phosphate (GP) and triose
phosphate (TP). GP and TP can then be used to make carbohydrates, lipids and proteins
within a plant.
Complete the following table to explain how GP or TP is converted into carbohydrates, lipids
and proteins:Organic
molecule
Diagram of molecule
Production
mechanism
carbohydrate
Diagram of glucose
(monosaccharide)
Two molecules of
TP join to form six
carbon sugars, eg
glucose
carbohydrate
(polysaccharide)
Molecules of
glucose join
together by
condensation
reactions
Diagram of amylose and amylopectin
Amylose – α 1-4
glycosidic bonds
forming a coil
(20% of starch)
Amylopectin - α
1-4 glycosidic
bonds and some
α1-6 glycosidic
bonds so
structure is
branched (80%)
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glycerol
TP can be
converted into
glycerol
Fatty acids
GP is converted
to acetyl coA
which can then be
used to form fatty
acids
lipids
The fatty acids
and glycerol are
combined via
ester bonds to
produce lipids
Amino acid
GP is converted
into amino acids
by the addition of
nitrates and (in
some cases)
sulphates
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Protein
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The amino acids
are joined by
peptide bonds to
produce proteins
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