2.2 CELL METABOLISM
2.2.4 Photosynthesis
Photosynthesis is the way green plants use light to make their own food
Role of photosynthesis
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Makes food for plants
Makes food for animals as they eat plants
Makes oxygen for plants and animals
Makes fossil fuels
Represent overall sequence of reactions by a balanced equation
The overall sequence of reactions in photosynthesis can be represented by the equation
chlorophyll
6CO2
+
6H2O
--------------------------->
C6H12O6
+
6O2
Fermoy
Co. Cork
uv light
Remember!
Ms. B. Fennessy
Loreto Secondary School
2.2 CELL METABOLISM
Location of chlorophyll within cells
The green chemical chlorophyll is found in the chloroplasts. The chlorophyll is used to trap the light
energy
Remember!
Identify the sources of light, carbon dioxide and water for photosynthesis in leaf cells.
Light
Light normally comes from the sun. (Artificial light can also be used)
Carbon Dioxide
Carbon dioxide comes from two sources
External source: It diffuses into the plant from the air. It diffuses in through the stomata
Internal source: It is made in the plant by respiration
Water
Water is got from the soil. The roots absorb the water and it passes up through the xylem vessels to the
leaves.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
2.2 CELL METABOLISM
A simple treatment of photosynthesis.
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Chlorophyll in chloroplasts traps sunlight energy.
This trapped energy splits water to release electrons, protons and oxygen.
The electrons are passed to chlorophyll, the protons are released to a general pool of protons,
the oxygen is either released to the atmosphere or it may be used within the cell.
Electrons from chlorophyll are used with protons from the pool of protons to reduce carbon
dioxide to form a carbohydrate Cx(H2O)y
Human intervention: Use of artificial light and carbon dioxide enrichment to promote crop growth in
greenhouses.
Artificial light
Artificial light (electric light bulbs) is used in greenhouses to increase crop growth.
The intensity and duration of the light can be varied
Carbon dioxide enrichment
The air in a greenhouse can be enriched with carbon dioxide by burning gas or
releasing it from cylinders
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
2.2 CELL METABOLISM
H.2.2.8 Role of Adenosine Triphosphate (ATP) and Nicotinamide Adenine
Dinucleotide (NAD/NADP+)
Nature of ATP
ATP (Adenosine Triphosphate) is a complicated chemical made up of adenine, ribose and three
phosphates.
Word structure of ATP
Role of ATP
ATP is a chemical that stores energy in cells
When a cell needs energy ATP releases energy and is changed into ADP (adenosine diphosphate) + P.
When energy is released in a cell it can be stored when ADP and P join to form ATP
*Adenosine triphosphate has a special role in the trapping and transferring of energy for cell activities *
It carries energy from where it is produced to where it is needed
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
2.2 CELL METABOLISM
Role of NAD+/NADP+
NAD+ (Nicotinamide adenine dinucleotide) and NADP+ (Nicotinamide adenine dinucleotide
phosphate) are low energy chemicals.
They are able to accept two high energy electrons (2e-), and a hydrogen ion (H+).
When they break down they release two high energy electrons (2e-) and a hydrogen ion (H+).
*NAD+ and NADP+ have a special role in the trapping and transferring of electrons and hydrogen ions
in cell activities*
In respiration NADH uses its electrons and hydrogen ion to form ATP
In photosynthesis NADPH uses its electrons and hydrogen ion to form glucose.
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork
2.2 CELL METABOLISM
H.2.2.9 Photosynthesis (Extended Study)
Photosynthesis is a two-stage process.
The first stage, driven by light energy, is called the light stage or light-dependent stage.
The second stage, which is dependent on the products of the light stage and does not require light, is
called the dark stage or light-independent stage.
Light Stage
In the light stage, light energy is absorbed and then passed on from one pigment molecule to other
pigment molecules in the chloroplast until it reaches a reaction centre chlorophyll molecule.
From here energised electrons enter two pathways:
Pathway 1: they return directly to chlorophyll releasing their surplus energy for the formation of ATP
Pathway 2: two electrons are trapped by NADP+ forming NADP-.
This leaves the chlorophyll molecules electron deficient.
These are replenished from the splitting of water into 2H+, electrons and oxygen.
The electrons recycle back to the chlorophyll,, the hydrogen ions (H+) enter a proton pool and oxygen
is released to the atmosphere or used in respiration.
The H+ ions from the pool of H+ ions are attracted to NADP– to form NADPH.
Dark Stage
In the dark stage NADPH supplies H+ ions and electrons (reducing power) which are used to convert
CO2 to carbohydrates, Cx(H2O)y.
The energy to achieve this conversion comes from ATP.
ADP and NADP+ return to the light stage to be re-used.
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Both the light stage and the dark stage take place in the chloroplast. However, the light stage
takes place in the grana and the dark stage takes place in the stroma.
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The light stage needs light and can take place only in the light. The dark stage uses molecules
(ATP and NADPH) that were made in the light stage, so it needs the light stage to have taken
place but it does not need light and can take place in the light or dark.
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No enzymes are used in the light stage. The dark stage reactions are controlled by enzymes.
This means that the rate of the dark stage is controlled by temperature.
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End products of light stage are:
o ATP – will provide the energy for the dark stage
o NADP – will supply electrons and H+ ions for the dark stage
o Oxygen – this passes out of leaf to atmosphere or it can be used in respiration
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End products of dark stage are:
o Glucose – used for energy or stored as starch
o ADP, P and NADP+ - reused in the light stage to form ATP and NADPH
Ms. B. Fennessy
Loreto Secondary School
Fermoy
Co. Cork