PHOTOSYNTHESIS
1. Photosynthesis as an energy transfer process
1.1 Chloroplast
Photosynthesis is the process by which plants use energy from sunlight to produce glucose from
carbon dioxide and water. This takes place in the chloroplast.
Each chloroplast is surrounded by a double-membrane envelope (phospholipid bilayer)
Chloroplasts are filled with a fluid known as the stroma -site of light-independent stage of
photosynthesis and CO2, enzymes and sugars are dissolved in this.
A separate system of membranes is found in the stroma – site of light-dependent stage of
photosynthesis
o The membrane contains the pigments, enzymes and electron carriers required for the light-
dependent reactions
o This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids
– stack up to form grana – creates large surface area for photosynthesis
o The pigment molecules are arranged in a photosystem to absorb maximum light and
transfer the energy and electrons to the next pigment until it reaches reaction centre)
The stroma also contains small 70S ribosomes, a loop of DNA to code for proteins and starch grains
There are two groups of pigment molecules:
o Chlorophyll: Chlorophyll a (yellow-green) and chlorophyll b (blue-green) - absorb
wavelengths in the blue-violet and red regions of light spectrum
o Carotenoids: B carotene (orange) and Xanthophyll (yellow) – absorb wavelengths in the blue-
violet region of light spectrum
, 1.2 Stages of photosynthesis
Light-dependent stage of photosynthesis, occurs in thylakoids:
ATP is produced (from ADP and Pi by ATP synthase in a process
called photophosphorylation (ADP + Pi → ATP)
+
Photophosphorylation uses the proton (H ) gradient generated
by the photolysis of water
Energy from ATP and hydrogen from reduced NADP (accepts
hydrogen ion from photolysis of water) are passed from the
light-dependent stage to the light-independent stage of
photosynthesis
Light-independent stage of photosynthesis, occurs in stroma:
The energy and hydrogen are used during the light-independent reactions (known collectively as the Calvin
cycle) to produce complex organic molecules, including carbohydrates like starch, sucrose, and cellulose
1.3 Photophosphorylation: cyclic and non-cyclic
Photophosphorylation is the process by which ATP is synthesised form ADP and Pi. It is part of the light-
depended stage of photosynthesis.
Two group of photosystems is involved
o Photosystem 1 – absorbs light of wavelength 700nm
o Photosystem 2 – absorbs light of wavelength 680 nm
Cyclic Phosphorylation
Involves photosystem I (PSI) only
Light is absorbed by photosystem I and passed to primary pigment (P700) – electron there is excited to higher
energy level and emitted - photoactivation
This excited electron is transported via a chain of electron carriers - electron transport chain (passed back to
the chlorophyll molecule in photosystem I -cyclic)
This provides energy to transport protons (H+) from the stroma (outside thylakoid membrane) to the thylakoid
lumen via a proton pump
A proton concentration gradient created – provides energy which is used by ATP synthase to synthesise ATP –
chemiosmosis
The ATP then passes to the light-independent reactions. Forms 2 ATP
1. Photosynthesis as an energy transfer process
1.1 Chloroplast
Photosynthesis is the process by which plants use energy from sunlight to produce glucose from
carbon dioxide and water. This takes place in the chloroplast.
Each chloroplast is surrounded by a double-membrane envelope (phospholipid bilayer)
Chloroplasts are filled with a fluid known as the stroma -site of light-independent stage of
photosynthesis and CO2, enzymes and sugars are dissolved in this.
A separate system of membranes is found in the stroma – site of light-dependent stage of
photosynthesis
o The membrane contains the pigments, enzymes and electron carriers required for the light-
dependent reactions
o This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids
– stack up to form grana – creates large surface area for photosynthesis
o The pigment molecules are arranged in a photosystem to absorb maximum light and
transfer the energy and electrons to the next pigment until it reaches reaction centre)
The stroma also contains small 70S ribosomes, a loop of DNA to code for proteins and starch grains
There are two groups of pigment molecules:
o Chlorophyll: Chlorophyll a (yellow-green) and chlorophyll b (blue-green) - absorb
wavelengths in the blue-violet and red regions of light spectrum
o Carotenoids: B carotene (orange) and Xanthophyll (yellow) – absorb wavelengths in the blue-
violet region of light spectrum
, 1.2 Stages of photosynthesis
Light-dependent stage of photosynthesis, occurs in thylakoids:
ATP is produced (from ADP and Pi by ATP synthase in a process
called photophosphorylation (ADP + Pi → ATP)
+
Photophosphorylation uses the proton (H ) gradient generated
by the photolysis of water
Energy from ATP and hydrogen from reduced NADP (accepts
hydrogen ion from photolysis of water) are passed from the
light-dependent stage to the light-independent stage of
photosynthesis
Light-independent stage of photosynthesis, occurs in stroma:
The energy and hydrogen are used during the light-independent reactions (known collectively as the Calvin
cycle) to produce complex organic molecules, including carbohydrates like starch, sucrose, and cellulose
1.3 Photophosphorylation: cyclic and non-cyclic
Photophosphorylation is the process by which ATP is synthesised form ADP and Pi. It is part of the light-
depended stage of photosynthesis.
Two group of photosystems is involved
o Photosystem 1 – absorbs light of wavelength 700nm
o Photosystem 2 – absorbs light of wavelength 680 nm
Cyclic Phosphorylation
Involves photosystem I (PSI) only
Light is absorbed by photosystem I and passed to primary pigment (P700) – electron there is excited to higher
energy level and emitted - photoactivation
This excited electron is transported via a chain of electron carriers - electron transport chain (passed back to
the chlorophyll molecule in photosystem I -cyclic)
This provides energy to transport protons (H+) from the stroma (outside thylakoid membrane) to the thylakoid
lumen via a proton pump
A proton concentration gradient created – provides energy which is used by ATP synthase to synthesise ATP –
chemiosmosis
The ATP then passes to the light-independent reactions. Forms 2 ATP
