Photosynthesis
Curriculum expectations
C3.2 explain the chemical changes and energy conversions associated with the process of photosynthesis (e.g., carbon dioxide and water react with sunlight to produce oxygen and glucose)
C3.3 use the laws of thermodynamics to explain energy transfer in the cell during the processes of cellular respiration and photosynthesis.
C3.4 describe, compare, and illustrate (e.g., using flow charts) the matter and energy transformations that occur during the processes of cellular respiration (aerobic and anaerobic) and photosynthesis, including the roles of oxygen and organelles such as mitochondria and chloroplasts.
C3.3 use the laws of thermodynamics to explain energy transfer in the cell during the processes of cellular respiration and photosynthesis.
C3.4 describe, compare, and illustrate (e.g., using flow charts) the matter and energy transformations that occur during the processes of cellular respiration (aerobic and anaerobic) and photosynthesis, including the roles of oxygen and organelles such as mitochondria and chloroplasts.
photosynthesis presentation
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Content of this page is adapted from:
Di Giuseppe, M., Vavitsas, A., Ritter, B., Fraser D., Arora, A., and Lisser, B. (2003) Biology 12 Textbook, p. 138-167. Nelson Thomson Learning, Toronto.
Di Giuseppe, M., Vavitsas, A., Ritter, B., Fraser D., Arora, A., and Lisser, B. (2003) Biology 12 Textbook, p. 138-167. Nelson Thomson Learning, Toronto.
Which organisms perform photosynthesis?
Organisms which are able to perform photosynthesis are called photoautotrophs. These organisms obtain energy from light to convert inorganic materials into organic materials for use in cellular functions such as biosynthesis and respiration.
Examples of such organisms include:
Examples of such organisms include:
- plants
- algae
- protists
- cyanobacteria
overview of photosynthesis
Photosynthesis is the process through which light energy (any light, not necessarily sunlight) is converted into chemical energy.
This process is comprised of two sets of reactions:
1. The Light-Dependent Reactions (aka Light Reactions) - the photo part
2. The Light-Independent Reactions (aka Calvin Cycle) - the synthesis part
The overall reaction equation is shown below. However, from the breakdown above, you can see how that not all reactants are used at once, and not all products are given off at once. This equation does not give any indication of the complex steps that take place in this process.
This process is comprised of two sets of reactions:
1. The Light-Dependent Reactions (aka Light Reactions) - the photo part
- Requires H2O, chlorophyll, and light energy
- Produces O2, ATP, and NADPH
2. The Light-Independent Reactions (aka Calvin Cycle) - the synthesis part
- Requires ATP, NADPH, and CO2
- Produces glucose (sugars), ADP+Pi, and NADP+
The overall reaction equation is shown below. However, from the breakdown above, you can see how that not all reactants are used at once, and not all products are given off at once. This equation does not give any indication of the complex steps that take place in this process.
The diagram below is a very good representation of an overview of photosynthesis.
All reactions involved in photosynthesis take place inside the chloroplasts: light reactions in the thylakoid membrane, and Calvin cycle in the stroma.
All reactions involved in photosynthesis take place inside the chloroplasts: light reactions in the thylakoid membrane, and Calvin cycle in the stroma.
Photosynthesis is an important process, because directly or indirectly, it nourishes almost the entire living world.
- Makes energy-rich organic molecules (glucose) from energy-poor inorganic molecules (CO2 and H2O)
- It is the start of all food chains & webs.
- It also makes oxygen.
Where does it all happen?
In the leaves, photosynthesis happens in mesophyll cells found in the palisade layer and the spongy layer. Upper epidermal cells are colourless and transparent in order to maximize the amount of light reaching mesophyll cells in these layers. The stomata allow the exchange of oxygen, CO2, and water vapour with the atmosphere. Guard cells have chloroplasts and can also perform photosynthesis.
Specifically, photosynthesis takes place inside the chloroplasts of these cells. So where do chloroplasts come from?
The Endosymbiotic Theory:
Photo credit:
http://www.origin-of-mitochondria.net/?attachment_id=114
Because of this, chloroplasts continue to contain their own DNA to this day, and whereas DNA in a plant cell nucleus is linear, chloroplast DNA is often circular like that of bacteria!
The Endosymbiotic Theory:
- An ancestor of cyanobacteria was engulfed by an ancestor of today’s eukaryotic cells.
- They formed a symbiotic relationship – eukaryote offered protection, cyanobacteria offered food.
- Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria.
Photo credit:
http://www.origin-of-mitochondria.net/?attachment_id=114
Because of this, chloroplasts continue to contain their own DNA to this day, and whereas DNA in a plant cell nucleus is linear, chloroplast DNA is often circular like that of bacteria!
Looking into more detail at the structure of a chloroplast, we can see that it has a double membrane. The space between the inner membrane and the outer membrane is called the intermembrane space. Inside the chloroplast, there are stacks of thylakoids which are called grana (sg. granum). Unstacked thylakoids are called lamellae (sg. lamella). The inner part of the chloroplast is also filled with a fluid called stroma. Finally, the space inside thylakoids is called lumen. The thylakoid membrane is a lipid bilayer with chlorophyll molecules embedded.
light as a source of energy
Out of all the total solar energy that reaches Earth:
- 60% is lost to the atmosphere.
- 40% reaches plants
- Out of this 40%, only 5% is used in photosynthesis.
Light is a form of electromagnetic radiation.
Light travels in wave packs as photons (aka quanta).
Photon wavelength is inversely proportional to energy (the shorter the wavelength, the “bluer” the light, the higher the energy.
T.W. Engelmann’s Experiment (1882):
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T.W. Engelmann's findings match up with the absorption spectrum of chlorophyll a and chlorophyll b.
Photosynthesis happens more rapidly in the blue and red ends of the spectrum.
Green light is not absorbed; it is reflected, and that’s why plants look green.
Photosynthesis happens more rapidly in the blue and red ends of the spectrum.
Green light is not absorbed; it is reflected, and that’s why plants look green.
Some photosynthetic pigments:
Anthocyanins are found in vacuoles; the others are found in chloroplasts.
Chlorophyll a and b are the most common pigments.
- Chlorophyll a (reflects blue-green)
- Chlorophyll b (reflects yellow-green)
- Carotenoids (reflect yellow-orange)
- Xanthophylls (reflect yellow)
- Anthocyanins (reflect red, violet, blue)
Anthocyanins are found in vacuoles; the others are found in chloroplasts.
Chlorophyll a and b are the most common pigments.
Chlorophyll molecules are composed of two parts: a porphyrin ring head, and a hydrocarbon chain tail.
The porphyrin ring is where the loose, excitable electrons are found. Hydrocarbon chain is hydrophobic and it anchors the chlorophyll molecule in the lipid bilayer of the thylakoid membrane. The two most common types of chlorophyll are chlorophyll a and chlorophyll b. The difference between the two is that at position R (as indicated in the diagram to the right), chlorophyll a contains a methyl group (CH3) and chlorophyll b contains an aldehyde group (CHO). |
capturing light energy
Photosystems consist of:
The main photosynthetic pigment is Chlorophyll a. Two types involved are:
Other accessory pigments found in the antenna complex are:
A photosystem is just a set of chlorophyll and accessory pigment molecules close to each other. They absorb light and transfer the energy from molecule to molecule and finally to the reaction centre (chlorophyll a). The electron is then stripped away by a molecule called the “primary electron acceptor”. The primary electron acceptor is reduced and the reaction centre chlorophyll a molecule is oxidized
The two photosystems work together to start the process of photosynthesis.
Photosystem I was discovered first, but Photosystem II occurs first.
- Antenna complex - captures light first.
- Reaction centre - a chlorophyll a molecule.
The main photosynthetic pigment is Chlorophyll a. Two types involved are:
- Chlorophyll a p680
- Chlorophyll a p700
Other accessory pigments found in the antenna complex are:
- Chlorophyll b - broader spectrum used for photosynthesis
- Carotenoids - absorb excessive light that would damage chlorophyll
A photosystem is just a set of chlorophyll and accessory pigment molecules close to each other. They absorb light and transfer the energy from molecule to molecule and finally to the reaction centre (chlorophyll a). The electron is then stripped away by a molecule called the “primary electron acceptor”. The primary electron acceptor is reduced and the reaction centre chlorophyll a molecule is oxidized
The two photosystems work together to start the process of photosynthesis.
- Photosystem I - contains p700 chlorophyll a (absorption peaks at 700nm red light).
- Photosystem II - contains p680 chlorophyll a (absorption peaks at 680nm red light).
Photosystem I was discovered first, but Photosystem II occurs first.
So what happens when a chlorophyll molecule interacts with light energy (photons)?
Photoexcitation:
- Before light strikes the molecule, electrons are at ground state
- A photon of light hits.
- Electron excited to higher energy state
- Electron falls back down to ground state and gives off a photon of energy (flourescence) and some heat.
Isolated chlorophyll molecules fluoresce when separated from the photosynthetic membrane in which they are normally embedded.
If illuminated in bright white light, an isolated solution of chlorophyll will fluoresce, giving off red light and heat
Light-dependent reactions
PSI: A photon of 700nm hits, and transduction happens. One of its electrons is pulled away by the primary electron acceptor, and it’s then sent to a “shuttle molecule” (this is a missing step in the diagram below). The electron is then sent to Ferredoxin which is the first electron receptor. Ferredoxin pulls the electron harder from the shuttle molecule. Ferredoxin containing the electron and NADP+ are both substrates of an enzyme called NADP reductase. Once these are both attached, the electron can be transferred from Ferredoxin to NADP+. But NADP+ is not an electron carrier, so it can only take the electron if the third active site is filled by an H+. 1NADP+ + 2H+ + 2electrons -> 1NADPH (this should be called NADPH2). The result is NADPH, which goes to the Calvin Cycle. But PSII needs to happen first in order for p700 to get its electrons back to replenish them.
PSII: Similarly, photons hit p680, electrons are lost to the primary electron acceptor, and are then lost to Plastoquinone. This is a shuttle molecule which moves to the active site of the cytochrome b6-f complex. Plastoquinone needs a H+ atom to shuttle over, and so it moves H+ from the stroma into the lumen. The cytochrome b6-f complex dumps the H+ because it only takes the electron. Plastocyanin is connected to the cytochrome b6-f complex and PSI, so the electron gets transferred back to PSI. This happens twice because NADPH needs 2 electrons.
Splitting H2O: PSII is attached to an enzyme molecule which splits water into protons and electrons (called water-splitting complex). This Z-protein is the only enzyme known that is able to break down water; this process takes a lot of energy. So, water is the source of all the electrons involved in photosynthesis, and this is why we need to water our plants. The oxygen molecules are released into the atmosphere for us to breathe.
ATP synthesis: ATP synthase or ATPase is a transmembrane protein which allows H+ to pass through, and the energy gets stored in the bonds of ATP. ATP then goes to the Calvin Cycle.
PSII: Similarly, photons hit p680, electrons are lost to the primary electron acceptor, and are then lost to Plastoquinone. This is a shuttle molecule which moves to the active site of the cytochrome b6-f complex. Plastoquinone needs a H+ atom to shuttle over, and so it moves H+ from the stroma into the lumen. The cytochrome b6-f complex dumps the H+ because it only takes the electron. Plastocyanin is connected to the cytochrome b6-f complex and PSI, so the electron gets transferred back to PSI. This happens twice because NADPH needs 2 electrons.
Splitting H2O: PSII is attached to an enzyme molecule which splits water into protons and electrons (called water-splitting complex). This Z-protein is the only enzyme known that is able to break down water; this process takes a lot of energy. So, water is the source of all the electrons involved in photosynthesis, and this is why we need to water our plants. The oxygen molecules are released into the atmosphere for us to breathe.
ATP synthesis: ATP synthase or ATPase is a transmembrane protein which allows H+ to pass through, and the energy gets stored in the bonds of ATP. ATP then goes to the Calvin Cycle.
The Z-diagram on the right shows pathways and energy changes of non-cyclic electron flow in a graph form.
You can clearly see from this diagram that the flow of electrons is linear, or non-cyclic.
You can clearly see from this diagram that the flow of electrons is linear, or non-cyclic.
The Z-diagram on the right shows pathways and energy changes of cyclic electron flow in a graph form.
Cyclic electron flow is a non-sustainable pathway.
If Ferredoxin is close to the cytochrome b6-f complex, it interacts with cytochrome b6-f instead of NADP reductase. If the electrons are passed to cytochrome b6-f, they then get transported to Plastocyanin and back to PSI. So, no NADPH is made but we still get ATP. This will lead to the plant dying because it can’t get through the Calvin cycle and it can’t make glucose. No oxygen is released.
Cyclic electron flow is a non-sustainable pathway.
If Ferredoxin is close to the cytochrome b6-f complex, it interacts with cytochrome b6-f instead of NADP reductase. If the electrons are passed to cytochrome b6-f, they then get transported to Plastocyanin and back to PSI. So, no NADPH is made but we still get ATP. This will lead to the plant dying because it can’t get through the Calvin cycle and it can’t make glucose. No oxygen is released.
Light-independent reactions
Melvin Calvin (1911-1997) researched and determined the details of this cycle in the early 1960's.
He received a Nobel prize in Chemistry in 1961.
The Calvin Cycle needs:
The Calvin Cycle is divided into three phases
1. Carbon fixation: Ribulose-1,5-bisphosphate (RuBP) is a 5-carbon chain with two phosphates attached. This molecule has to be present in the stroma for the Calvin cycle to start, and it comes from the parent cell after division. CO2 comes from the atmosphere through the stomata and it is dissolved in water. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a large, slow enzyme compared to other enzymes, so the cell needs to make lots of them (there are millions in each cholorplast). Rubisco is the most common enzyme in the world. It has 3 active sites: one big one where RuBP attaches, one where CO2 attaches, and one where water attaches for a hydrolysis reaction (which breaks down the 6-carbon chain into 2 PGA). This helps the C from CO2 attach to the RuBP to make a 6-carbon chain. Now this is very unstable and immediately makes 2 PGA’s (1 PGA = 3-carbon chain).
2. Reduction reactions: The energy of ATP is used and ATP is broken into ADP+Pi. The phosphate goes to PGA (low energy) to make 1,3-BPG (high energy). This happens twice and so it uses 2 ATP. Now NADPH gives the H+, the Pi of the 1,3-BPG (acid) comes off and it makes two G3P (aldehyde). You need two NADPH here. To make an aldehyde from an acid, you need H+. G3P holds a lot of energy because it took the H+ from NADPH (NADPH loses energy in this process). Most things in plants can not use G3P in this form, so some G3P makes glucose with the help of an enzyme called G3P-carboxylase, through a condensation reaction.
3. Regeneration of RuBP: We can not just take the two G3P’s and link them together to make glucose because RuBP needs to be regenerated, or else the cycle does not continue. In reality, we begin with 3 RuBP and 3 CO2; it makes 6 PGA and use 6 ATP; it makes 6 ADP and 6 1,3 BPG; it makes 6 G3P, 6 Pi, 6 NADP+ using 6 NADPH. One G3P exits the cycle, gets attached to the enzyme that makes glucose; but, you need 2 G3P to make one glucose. The other 5 G3P go through a series of reactions that put together the 3 RuBP but they need the Pi from 3 ATP. This whole thing happens twice to make 1 glucose from 2 G3P.
- ATP (from the light reactions)
- H atoms from NADPH (from the light reactions)
- CO2 (from the environment)
The Calvin Cycle is divided into three phases
1. Carbon fixation: Ribulose-1,5-bisphosphate (RuBP) is a 5-carbon chain with two phosphates attached. This molecule has to be present in the stroma for the Calvin cycle to start, and it comes from the parent cell after division. CO2 comes from the atmosphere through the stomata and it is dissolved in water. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a large, slow enzyme compared to other enzymes, so the cell needs to make lots of them (there are millions in each cholorplast). Rubisco is the most common enzyme in the world. It has 3 active sites: one big one where RuBP attaches, one where CO2 attaches, and one where water attaches for a hydrolysis reaction (which breaks down the 6-carbon chain into 2 PGA). This helps the C from CO2 attach to the RuBP to make a 6-carbon chain. Now this is very unstable and immediately makes 2 PGA’s (1 PGA = 3-carbon chain).
2. Reduction reactions: The energy of ATP is used and ATP is broken into ADP+Pi. The phosphate goes to PGA (low energy) to make 1,3-BPG (high energy). This happens twice and so it uses 2 ATP. Now NADPH gives the H+, the Pi of the 1,3-BPG (acid) comes off and it makes two G3P (aldehyde). You need two NADPH here. To make an aldehyde from an acid, you need H+. G3P holds a lot of energy because it took the H+ from NADPH (NADPH loses energy in this process). Most things in plants can not use G3P in this form, so some G3P makes glucose with the help of an enzyme called G3P-carboxylase, through a condensation reaction.
3. Regeneration of RuBP: We can not just take the two G3P’s and link them together to make glucose because RuBP needs to be regenerated, or else the cycle does not continue. In reality, we begin with 3 RuBP and 3 CO2; it makes 6 PGA and use 6 ATP; it makes 6 ADP and 6 1,3 BPG; it makes 6 G3P, 6 Pi, 6 NADP+ using 6 NADPH. One G3P exits the cycle, gets attached to the enzyme that makes glucose; but, you need 2 G3P to make one glucose. The other 5 G3P go through a series of reactions that put together the 3 RuBP but they need the Pi from 3 ATP. This whole thing happens twice to make 1 glucose from 2 G3P.
Glossary terms used throughout the text above
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1,3 BPG
Absorption spectrum ATP ATP synthase Calvin Cycle Chlorophyll Chloroplast Cyclic electron flow Cytochrome b6-f complex Endosymbiosis Epidermis Ferredoxin Fluorescence |
G3P
Glucose Granum Guard cells Hydrolysis Lamella Light Light Reactions Lumen Mesophyll cells NADP reductase NADPH Non-cyclic electron flow |
Palisade layer
PGA Photoautotroph Photoexcitation Photons Photosynthesis Photosystem I Photosystem II Photosystems Pigments Plastocyanin Plastoquinone Porphyrin Primary electron acceptor |
Reaction centre
RuBP Rubisco Spongy layer Stomata Stroma Thylakoid Transduction Water-splitting complex |