H.
BTEC L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 10: Biological Molecules and Metabolic
Pathways
Assignment 10C: Photosynthesis in plants
Photosynthesis
Photosynthesis can be defined as the process used to produce glucose using
light energy, carbon dioxide and water. The overall chemical equation for this
process is shown below:
6CO2 + 6H2O (+ energy) -> C6H12O6 + 6O2
The glucosecarbon dioxide
produced water
from photosynthesis is usedglucose oxygen
to store energy until plants
release it via respiration- a process used to release energy.
Figure 1- Diagram of the inside of a
The process of photosynthesis occurs in the chloroplasts of plant cells as they
chloroplast
consist of photosynthetic pigments such as
chlorophyll a, chlorophyll b and
carotene- these pigments help to
absorb the light energy required for
photosynthesis and are specifically
located in the thylakoid membrane (see
Figure 1), attached to proteins. The
pigment attached to proteins are
referred to as photosystems. There are
two photosystems used by plants to
absorb light:
- Photosystem I (PSI), absorb light at a wavelength of 700nm
- Photosystem II (PSII), absorb light at a wavelength of 680nm
Both photosystems are involved in the stages of photosynthesis which will be
discussed in detail in the following pages.
The stages of photosynthesis include the light-dependent reaction and the light-
independent reaction, within these reactions different distinct processes occur,
e.g. photophosphorylation- the addition of a phosphate using light energy.
Stages of photosynthesis
Light-dependent reaction
This stage of photosynthesis occurs in the thylakoid membranes of the
chloroplasts and light energy is needed for the stage to start.
In this stage, light energy is absorbed by the photosynthetic pigment,
chlorophyll, in photosystem II. This light energy excites the electrons contained
in the chlorophyll which causes them to be released later from the chlorophyll
molecule- photoionisation of chlorophyll has occurred. The energy from the
photoionisation of chlorophyll is then used for three main things:
- Photophosphorylation, the formation of ATP by adding a phosphate to ADP
using light energy
- Formation of reduced NADP from NADP
1
, H.
BTEC L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 10: Biological Molecules and Metabolic
Pathways
- Photolysis, the splitting of water into protons (H + ions), electrons and
oxygen
This reaction using light energy consists of two types of photophosphorylation:
- Non-cyclic photophosphorylation
- Cyclic photophosphorylation
The process of non-cyclic photophosphorylation forms ATP, reduced NADP and
oxygen using photosystems and electron carriers (proteins that transfer
electrons) which create an electron transport chain- a chain of proteins which
allow excited electrons to flow. Figure 2- Diagram of what happens in the first stage of
non-cyclic photophosphorylation
To start off the process of non-cyclic
photophosphorylation, light energy
is absorbed by photosystem II ((PSII)
and it excites the electrons in the
chlorophyll molecule- as shown in
Figure 2. The electrons increase in
energy so transfer to a higher
energy level. The electrons with a higher energy level are then released from the
chlorophyll molecule and move down the electron transport chain to
photosystem I (PSI).
Figure 3- Diagram of what happens in the second
Following on from this, photolysis occurs to replace the
stage of electrons
non-cyclic in the
photophosphorylation
chlorophyll molecule as the electrons have moved down the electron transport
chain out of photosystem II (PSII)- see Figure 3. In this process, water is split into
protons (H+ ions), electrons and oxygen and can be shown in the following
equation:
H2O -> 2H + + 1/
O2
2
water hydrogen ions oxygen
Figure 4- Diagram of what happens in
The excited electrons lose energy as the third stage of non-cyclic
they move down the electron transport photophosphorylation
chain and the energy from the excited electrons
produces ATP. ATP is produced as the energy lost
from the excited electrons is used to move protons
into the thylakoid, so there is a higher concentration
of protons compared to the stroma- see Figure 4. Figure 5- Diagram of what happens in the last
This creates a proton gradient across the thylakoid membrane. The protons
stage of non-cyclic then
photophosphorylation
move down their concentration gradient into the stroma using the enzyme ATP
synthase. The energy used to move the protons down their concentration
gradient forms ATP by adding inorganic phosphate (P i) to ADP. This formation of
ATP from the flow of electrons down the electron transport chain and the proton
gradient created is known as chemiosmosis which is described by the
chemiosmotic theory.
In the last stage of non-cyclic
photophosphorylation, reduced NADP is formed –
a co-enzyme used in photosynthesis which
2
BTEC L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 10: Biological Molecules and Metabolic
Pathways
Assignment 10C: Photosynthesis in plants
Photosynthesis
Photosynthesis can be defined as the process used to produce glucose using
light energy, carbon dioxide and water. The overall chemical equation for this
process is shown below:
6CO2 + 6H2O (+ energy) -> C6H12O6 + 6O2
The glucosecarbon dioxide
produced water
from photosynthesis is usedglucose oxygen
to store energy until plants
release it via respiration- a process used to release energy.
Figure 1- Diagram of the inside of a
The process of photosynthesis occurs in the chloroplasts of plant cells as they
chloroplast
consist of photosynthetic pigments such as
chlorophyll a, chlorophyll b and
carotene- these pigments help to
absorb the light energy required for
photosynthesis and are specifically
located in the thylakoid membrane (see
Figure 1), attached to proteins. The
pigment attached to proteins are
referred to as photosystems. There are
two photosystems used by plants to
absorb light:
- Photosystem I (PSI), absorb light at a wavelength of 700nm
- Photosystem II (PSII), absorb light at a wavelength of 680nm
Both photosystems are involved in the stages of photosynthesis which will be
discussed in detail in the following pages.
The stages of photosynthesis include the light-dependent reaction and the light-
independent reaction, within these reactions different distinct processes occur,
e.g. photophosphorylation- the addition of a phosphate using light energy.
Stages of photosynthesis
Light-dependent reaction
This stage of photosynthesis occurs in the thylakoid membranes of the
chloroplasts and light energy is needed for the stage to start.
In this stage, light energy is absorbed by the photosynthetic pigment,
chlorophyll, in photosystem II. This light energy excites the electrons contained
in the chlorophyll which causes them to be released later from the chlorophyll
molecule- photoionisation of chlorophyll has occurred. The energy from the
photoionisation of chlorophyll is then used for three main things:
- Photophosphorylation, the formation of ATP by adding a phosphate to ADP
using light energy
- Formation of reduced NADP from NADP
1
, H.
BTEC L3 EXTENDED DIPLOMA IN APPLIED SCIENCE
Unit 10: Biological Molecules and Metabolic
Pathways
- Photolysis, the splitting of water into protons (H + ions), electrons and
oxygen
This reaction using light energy consists of two types of photophosphorylation:
- Non-cyclic photophosphorylation
- Cyclic photophosphorylation
The process of non-cyclic photophosphorylation forms ATP, reduced NADP and
oxygen using photosystems and electron carriers (proteins that transfer
electrons) which create an electron transport chain- a chain of proteins which
allow excited electrons to flow. Figure 2- Diagram of what happens in the first stage of
non-cyclic photophosphorylation
To start off the process of non-cyclic
photophosphorylation, light energy
is absorbed by photosystem II ((PSII)
and it excites the electrons in the
chlorophyll molecule- as shown in
Figure 2. The electrons increase in
energy so transfer to a higher
energy level. The electrons with a higher energy level are then released from the
chlorophyll molecule and move down the electron transport chain to
photosystem I (PSI).
Figure 3- Diagram of what happens in the second
Following on from this, photolysis occurs to replace the
stage of electrons
non-cyclic in the
photophosphorylation
chlorophyll molecule as the electrons have moved down the electron transport
chain out of photosystem II (PSII)- see Figure 3. In this process, water is split into
protons (H+ ions), electrons and oxygen and can be shown in the following
equation:
H2O -> 2H + + 1/
O2
2
water hydrogen ions oxygen
Figure 4- Diagram of what happens in
The excited electrons lose energy as the third stage of non-cyclic
they move down the electron transport photophosphorylation
chain and the energy from the excited electrons
produces ATP. ATP is produced as the energy lost
from the excited electrons is used to move protons
into the thylakoid, so there is a higher concentration
of protons compared to the stroma- see Figure 4. Figure 5- Diagram of what happens in the last
This creates a proton gradient across the thylakoid membrane. The protons
stage of non-cyclic then
photophosphorylation
move down their concentration gradient into the stroma using the enzyme ATP
synthase. The energy used to move the protons down their concentration
gradient forms ATP by adding inorganic phosphate (P i) to ADP. This formation of
ATP from the flow of electrons down the electron transport chain and the proton
gradient created is known as chemiosmosis which is described by the
chemiosmotic theory.
In the last stage of non-cyclic
photophosphorylation, reduced NADP is formed –
a co-enzyme used in photosynthesis which
2
