A2. Photosynthesis and the Light Dependent Reaction Flashcards
Chloroplasts
Photosynthesis takes place in the ________________of __________cells. Chloroplasts are ________, _____________organelles surrounded by a _________ _____________. Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called ________(singular = _________). The grana are linked together by bits of _______________membrane called _____________(singular = __________).
Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts are small, flattened organelles surrounded by a double membrane. Thylakoids (fluid-filled sacs) are stacked up in the chloroplast into structures called grana (singular = granum). The grana are linked together by bits of thylakoid membrane called lamellae (singular = lamella).
Chloroplasts contain photosynthetic pigments (e.g. ______________ _, _____________ _and ____________). These are coloured substances that absorb the light energy needed for photosynthesis. The pigments are found in the thylakoid membranes
-they’re attached to proteins. The __________and __________is called a photosystem. There are two photosystems used by plants to capture light energy. Photosytem I (or PSI) absorbs light best at a wavelength of ____ nm
Chloroplasts contain photosynthetic pigments (e.g. chlorophyll a, chlorophyll b and carotene). These are coloured substances that absorb the light energy needed for photosynthesis. The pigments are found in the thylakoid membranes
-they’re attached to proteins. The protein and pigment is called a photosystem. There are two photosystems used by plants to capture light energy. Photosytem I (or PSI) absorbs light best at a wavelength of 700 nm
Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel-like substance called the _______. It contains __________, _____and ___________ ______. Carbohydrates that are produced by photosynthesis but not used straight away are stored as _______ ______in the _________.
Contained within the inner membrane of the chloroplast and surrounding the thylakoids is a gel-like substance called the stroma. It contains enzymes, sugars and organic acids. Carbohydrates that are produced by photosynthesis but not used straight away are stored as starch grains in the stroma.
Redox reactions
Redox reactions are reactions that involve ____________ and ____________. They occur in photosynthesis (and in respiration) so it’s really important that you get your head round them:
- If something is reduced it has ___________electrons (e), and may have __________hydrogen or _____oxygen.
- If something is oxidised it has _____electrons, and may have ______ hydrogen or _________oxygen.
- Oxidation of one molecule always involves ___________of another molecule.
Redox reactions
Redox reactions are reactions that involve oxidation and reduction. They occur in photosynthesis (and in respiration) so it’s really important that you get your head round them:
- If something is reduced it has gained electrons (e), and may have gained hydrogen or lost oxygen.
- If something is oxidised it has lost electrons, and may have lost hydrogen or gained oxygen.
- Oxidation of one molecule always involves reduction of another molecule.
Coenzymes
A coenzyme is a molecule that ____ the __________of an __________. They work by transferring a ____________group from one molecule to another. A coenzyme used in photosynthesis is NADP. NADP transfers hydrogen from one molecule to another this means it can __________(give hydrogen to) or _________(take hydrogen from) a molecule.
A coenzyme is a molecule that aids the function of an enzyme. They work by transferring a chemical group from one molecule to another. A coenzyme used in photosynthesis is NADP. NADP transfers hydrogen from one molecule to another this means it can reduce (give hydrogen to) or oxidise (take hydrogen from) a molecule.
The stages of photosynthesis are …
- The light-dependent reaction
- The light-independent reaction
- The light-dependent reaction
Light energy is _____________by _____________(and other photosynthetic pigments) in the photosystems. The ________ _________ __________the _____________in the _______________, giving them ________ _________, which eventually causes them to be ____________from the ____________ _________. This process is called __________________. The chlorophyll molecule is now a _____________charged ion.
Some of the energy from the released electrons is used to add a _____________group to _____to form ATP, and some is used to _________NADP to form ___________ _______. ATP transfers energy and reduced NADP transfers _____________to the _______ _______________ __________. During the process, _____ is ___________to _,
- The light-dependent reaction
Light energy is absorbed by chlorophyll (and other photosynthetic pigments) in the photosystems. The light energy excites the electrons in the chlorophyll, giving them more energy, which eventually causes them to be released from the chlorophyll molecule. This process is called photoionisation. The chlorophyll molecule is now a positively charged ion.
Some of the energy from the released electrons is used to add a phosphate group to ADP to form ATP, and some is used to reduce NADP to form reduced NADP. ATP transfers energy and reduced NADP transfers hydrogen to the light-independent reaction. During the process, H₂O is oxidised to O,
- The light-independent reaction (the Calvin cycle)
As the name suggests, this reaction doesn’t use light energy directly. (But it does rely on the products of the light-dependent reaction.) It takes place in the ________of the chloroplast-see Figure 3. Here, the ____and ___________ _____from the light-dependent reaction supply the energy and hydrogen to make glucose from CO2.
- The light-independent reaction (the Calvin cycle)
As the name suggests, this reaction doesn’t use light energy directly. (But it does rely on the products of the light-dependent reaction.) It takes place in the stroma of the chloroplast-see Figure 3. Here, the ATP and reduced NADP from the light-dependent reaction supply the energy and hydrogen to make glucose from CO2
.
Tip: The light-independent reaction can take place in the ____. However, it needs the products of the light- dependent reaction, (___and ___________ ______) so in reality it only continues for a little while after it gets dark.
Tip: The light-independent reaction can take place in the dark. However, it needs the products of the light- dependent reaction, (ATP and reduced NADP) so in reality it only continues for a little while after it gets dark.
Figure 3: How the light-dependent and light-independent reactions link together in a chloroplast.
The light-dependent reaction
In the light-dependent reaction, the energy resulting from the photoionisation of chlorophyll is used for three things:
- Making ATP from ADP and inorganic phosphate. This is called photophosphorylation-it’s the process of adding phosphate to a molecule using light.
- Making reduced NADP from NADP.
- Splitting water into protons (H’ ions), electrons and oxygen. This is called photolysis—it’s the splitting (lysis) of a molecule using light (photo) energy.
Non-cyclic photophosphorylation
Non-cyclic photophosphorylation produces ____, ___________ ______ and ___________. To understand the process you need to know that the photosystems (in the ____________ ______________) are linked by __________ __________. Electron carriers are __________ that ____________ _____________. The photosystems and electron carriers form an ____________ _____________ _________- a chain of proteins through which ____________ electrons flow. There are several processes going on all at once in non-cyclic photophosphorylation.
Non-cyclic photophosphorylation
Non-cyclic photophosphorylation produces ATP, reduced NADP and oxygen. To understand the process you need to know that the photosystems (in the thylakoid membranes) are linked by electron carriers. Electron carriers are proteins that transfer electrons. The photosystems and electron carriers form an electron transport chain - a chain of proteins through which excited electrons flow. There are several processes going on all at once in non-cyclic photophosphorylation-they’re shown separately in the diagrams below.
Non-cyclic photophosphorylation
1. ________ ________ _________ ___________in ______________
Light energy is ____________by ____. The light energy ________ ___________in _________________. The electrons move to a higher energy level (ie. they have more energy). The high-energy electrons are ____________from the ______________and move down the __________ ____________ _______to ____.
- Light energy excites electrons in chlorophyll
Light energy is absorbed by PSII. The light energy excites electrons in chlorophyll. The electrons move to a higher energy level (ie. they have more energy-see Figure 4). These high-energy electrons are released from the chlorophyll and move down the electron transport chain to PSI.
Figure 4: Light energy excites electrons in PSI, moving them to a higher energy level.
Non-cyclic photophosphorylation
2. Photolysis of water produces ________, ___________and _________.As the __________ _________from ______________ leave _____to move down the ____________ _____________ ________, they must be replaced. Light ________splits _________into __________(H+ ions), ___________and ___________-this is _____________.
The reaction is: H2O —— 2H + 1/2O2
- Photolysis of water produces protons, electrons and oxygen As the excited electrons from chlorophyll leave PSII to move down the electron transport chain, they must be replaced. Light energy splits water into protons (H+ ions), electrons and oxygen-this is photolysis.
The reaction is: H,O2H+0,
Non-cyclic photophosphorylation
3. Energy from the excited electrons makes ATP
The _________ ___________lose ________as they move down the ___________ _____________ _______.This energy is used to transport _________(H+ ions) into the ______________so that the ____________has a _________ _____________of _________than the _________. This forms a ________gradient across the ____________ ______________. __________move down their _______________ gradient, into the stroma, via the __________ ____
_____________, which is embedded in the _______________membrane. The energy from this movement combines ____and _________ ______________(P) to form ___.
- Energy from the excited electrons makes ATP
The excited electrons lose energy as they move down the electron transport chain. This energy is used to transport protons (H+ ions) into the thylakoid so that the thylakoid has a higher concentration of protons than the stroma. This forms a proton gradient across the thylakoid membrane. Protons move down their concentration gradient, into the stroma, via the enzyme ATP synthase, which is embedded in the thylakoid membrane. The energy from this movement combines ADP and inorganic phosphate (P) to form ATP.
Figure 5: The excited electrons lose energy as they pass down the electron transport chain.
Non-cyclic photophosphorylation
4. Energy from the ___________ __________generates ___________ _____.
Light ________is absorbed by ____, which _________ the ___________again to an even __________ energy level. Finally, the electrons are transferred to _______, along with a ________(H+ ion) from the _______, to form ____________ ______
- Energy from the excited electrons generates reduced NADP
Light energy is absorbed by PSI, which excites the electrons again to an even higher energy level. Finally, the electrons are transferred to NADP, along with a proton (H+ ion) from the stroma, to form reduced NADP
Cyclic photophosphorylation
Cyclic photophosphorylation produces ____ and only uses ____. It’s called ‘cyclic’ because the electrons from the _____________ molecule aren’t passed onto NADP, but are passed back to ____via __________ _________. This means the electrons are recycled and can repeatedly flow through PSI. This process doesn’t produce any ___________ ____ or oxygen-it only produces small amounts of ____.
Cyclic photophosphorylation
Cyclic photophosphorylation produces ATP and only uses PSI. It’s called ‘cyclic’ because the electrons from the chlorophyll molecule aren’t passed onto NADP, but are passed back to PSI via electron carriers. This means the electrons are recycled and can repeatedly flow through PSI. This process doesn’t produce any reduced NADP or oxygen-it only produces small amounts of ATP.
Figure 6: Light energy excites electrons in PSI to an even higher energy level.
