Topic 5: On the Wild Side
Photosynthesis
Photosynthesis reaction
Photosynthesis combines carbon dioxide with water to form glucose and oxygen. The glucose is
used in respiration to produce energy while the oxygen is released from the plant through the
stomata.
Chloroplast structure
Photosynthesis takes in the chloroplasts of plant cells. Chloroplasts contain fluid-filled sacs called
thylakoids. Thylakoids are stacked up like pancakes to form structures which we call grana. Each
granum is connected together by pieces of thylakoid membrane called lamellae. The gel-like
substance which surrounds the thylakoids is called the stroma.
The thylakoids within the chloroplast provide a large surface area to allow as much light to be
absorbed as possible. Within the thylakoid membrane are photosystems which consist of a pigment
molecules attached to proteins. The pigment is what gives plants their colour and includes
, chlorophyll a, chlorophyll b and carotene. Plants contain two photosystems, called photosystem I
and photosystem II. PSI absorbs light at a wavelength of 700 nm while PSII absorbs light at a
wavelength of 680 nm.
The overall process of photosynthesis can be split into two stages: the first one stage is known as the
light-dependent reaction and (unsurprisingly) requires light to get going. The second is called the
light-independent reaction which doesn’t need light but does need the products that were generated
in the first stage.
Light-Dependent Reaction (LDR)
The LDR takes place in the thylakoid membranes of the chloroplasts. It takes place in the following
stages:
1. Light energy is absorbed by PSII (even though PSII is involved before PSI, it was discovered
afterwards - hence the confusing naming system). Light excites electrons within PSII and
causes them to move into a higher energy state. The electrons are passed onto a series of
electron carriers within the electron transport chain to PSI.
2. The electrons which have been lost from PSII need to be replaced. This happens through the
photolysis of water - light energy causes a water molecule to split apart and release hydrogen
ions, electrons and oxygen. The electrons from water replace the electrons lost from PSII.
3. As the electrons move along the electron transport chain, they move from high to low energy.
The energy lost by the electrons is used to pump hydrogen ions from the stroma into the
thylakoids. This generates a proton gradient across the thylakoid membrane.
4. Protons flow down their concentration gradient through ATP synthase. The energy from the
movement of protons is used to phosphorylate ADP to ATP (photophosphorylation) in a
process called chemiosmosis.
5. Light is absorbed by PSI causing another electron to become excited and be passed along the
rest of the electron transport chain.
6. The electron is passed onto NADP to form reduced NADP (NADPH). NADPH is an electron
carrier which transfers electrons from one molecule to another.
7. The ATP and reduced NADP move into the stroma for the next stage of photosynthesis, the light
independent reaction.
Photosynthesis
Photosynthesis reaction
Photosynthesis combines carbon dioxide with water to form glucose and oxygen. The glucose is
used in respiration to produce energy while the oxygen is released from the plant through the
stomata.
Chloroplast structure
Photosynthesis takes in the chloroplasts of plant cells. Chloroplasts contain fluid-filled sacs called
thylakoids. Thylakoids are stacked up like pancakes to form structures which we call grana. Each
granum is connected together by pieces of thylakoid membrane called lamellae. The gel-like
substance which surrounds the thylakoids is called the stroma.
The thylakoids within the chloroplast provide a large surface area to allow as much light to be
absorbed as possible. Within the thylakoid membrane are photosystems which consist of a pigment
molecules attached to proteins. The pigment is what gives plants their colour and includes
, chlorophyll a, chlorophyll b and carotene. Plants contain two photosystems, called photosystem I
and photosystem II. PSI absorbs light at a wavelength of 700 nm while PSII absorbs light at a
wavelength of 680 nm.
The overall process of photosynthesis can be split into two stages: the first one stage is known as the
light-dependent reaction and (unsurprisingly) requires light to get going. The second is called the
light-independent reaction which doesn’t need light but does need the products that were generated
in the first stage.
Light-Dependent Reaction (LDR)
The LDR takes place in the thylakoid membranes of the chloroplasts. It takes place in the following
stages:
1. Light energy is absorbed by PSII (even though PSII is involved before PSI, it was discovered
afterwards - hence the confusing naming system). Light excites electrons within PSII and
causes them to move into a higher energy state. The electrons are passed onto a series of
electron carriers within the electron transport chain to PSI.
2. The electrons which have been lost from PSII need to be replaced. This happens through the
photolysis of water - light energy causes a water molecule to split apart and release hydrogen
ions, electrons and oxygen. The electrons from water replace the electrons lost from PSII.
3. As the electrons move along the electron transport chain, they move from high to low energy.
The energy lost by the electrons is used to pump hydrogen ions from the stroma into the
thylakoids. This generates a proton gradient across the thylakoid membrane.
4. Protons flow down their concentration gradient through ATP synthase. The energy from the
movement of protons is used to phosphorylate ADP to ATP (photophosphorylation) in a
process called chemiosmosis.
5. Light is absorbed by PSI causing another electron to become excited and be passed along the
rest of the electron transport chain.
6. The electron is passed onto NADP to form reduced NADP (NADPH). NADPH is an electron
carrier which transfers electrons from one molecule to another.
7. The ATP and reduced NADP move into the stroma for the next stage of photosynthesis, the light
independent reaction.
