Watch this from the start to 3m16s.
The 2 reactions after that is Level 2 bio stuff!
The 2 reactions after that is Level 2 bio stuff!
Photosynthesis is a reaction in the LEAVES of a plant, where water and carbon dioxide become glucose (food for the plant) and oxygen (waste product).
Photosynthesis must occur in the presence of sunlight. Sunlight provides energy for the reaction. Photosynthesis occurs in the chlorophyll - the light-absorbing pigment within chloroplasts (the organelle where PS occurs). |
Materials required for PS
Carbon dioxide
Diffusion is the movement of particles (in this case, carbon dioxide) from a place of high concentration (lots of it) to an area of low concentration (less of it), until the concentrations are equal.
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Water
Osmosis is basically diffusion, but the movement of water and nothing else (ever!). So, osmosis can be described as the movement of water from an area of high concentration to an area of low concentration until the concentrations are equal.
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Materials produced in PS
Glucose (= the goal of PS)
Glucose is the reason photosynthesis happens. Photosynthesis occurs TO PRODUCE GLUCOSE! Once it's made, glucose enters the phloem (food pipe) to be transported around the plant to where it is required. It may also stay in the leaf and be stored as starch until needed.
Glucose is used for several things -
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Glucose exits the leaf via diffusion (into the phloem). The concentration of glucose within the leaf is high when photosynthesis has just occurred, and diffusion involves the movement of molecules from an area of high concentration to an area of low concentration. So the glucose molecules move into the phloem vessels as a result of the concentration difference.
Oxygen
Oxygen is not at all a reason that photosynthesis happens; in fact, it is a waste product! A result of making glucose is that oxygen must also be made but it is released back into the atmosphere because the cell does not require it for this process, so it is eliminated.
However, the plant can use oxygen for respiration (the process that follows photosynthesis) so it's possible some oxygen can be caught in the leaf and not released into the atmosphere - this makes it faster for respiration to occur.
Oxygen exits the leaf through the stomata (the holes on the underside of the leaf) by the process of diffusion. Inside the leaf, where photosynthesis has occurred, there is a high concentration of oxygen whereas in the environment immediately outside the leaf there is a lower concentration in comparison, so the oxygen will move outside.
However, the plant can use oxygen for respiration (the process that follows photosynthesis) so it's possible some oxygen can be caught in the leaf and not released into the atmosphere - this makes it faster for respiration to occur.
Oxygen exits the leaf through the stomata (the holes on the underside of the leaf) by the process of diffusion. Inside the leaf, where photosynthesis has occurred, there is a high concentration of oxygen whereas in the environment immediately outside the leaf there is a lower concentration in comparison, so the oxygen will move outside.
Environmental conditions
and how they can help or hinder photosynthesis...
For photosynthesis to occur at its fastest rate, there needs to be:
If one of these factors is in short supply, the plant may not be able to do photosynthesis at its fastest rate. Photosynthesis may become limited.
- enough light energy (from the sun).
- a warm temperature for the enzyme-controlled reactions to occur.
- plenty of carbon dioxide surrounding the plant.
- plenty of water available for the plant to use!
If one of these factors is in short supply, the plant may not be able to do photosynthesis at its fastest rate. Photosynthesis may become limited.
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Temperature
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Light intensity
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Carbon dioxide concentration
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Water availability
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As temperature increases, so does the rate of photosynthesis. This is because the enzymes catalyzing the reaction are getting more energy from the sunlight as it becomes more intense. This occurs up until a point, the enzymes optimum temperature, where photosynthesis occurs the fastest. Beyond this point, the temperature is too high and causes the enzymes within the chloroplasts to denature. This means they cannot facilitate photosynthesis and its rate will decrease (slow down) until it stops completely.
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As light intensity increases, so does the rate of photosynthesis. This occurs until it reaches a point where, no matter how much light is available, the rate of photosynthesis will increase no more. There could be the light of a million suns shining on that plant but photosynthesis cannot go faster once this point is reached. This is because something else, such as CO2 concentration, water availability or temperature could be limiting the rate.
Often with high light intensity (strong light) comes higher temperatures (think about the temperature during a sunny day vs a cloudy day). |
As the concentration of carbon dioxide increases, so does the rate of photosynthesis. This occurs until it reaches a point where, no matter how much carbon dioxide is available, the rate will increase no more. There could be all the carbon dioxide in the world available to the plant but photosynthesis cannot go faster. This is because something else, such as light intensity, temperature or water availability, could be limiting the rate. Or it could be that all the enzymes are working at maximum speed and can't work any faster!
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The same goes with water availability. If there's a lack of water, that means one of the vital reactants for photosynthesis to occur is not available! This is quite rare because plants have particular adaptations (depending on their environment) to conserve water. Some have deep roots, some have thick cuticles, etc.
Wind can increase the rate of water loss (transpiration) from the leaves, which has the potential to limit water availability.
If I were you, I'd focus on the other three to learn and revise!
Wind can increase the rate of water loss (transpiration) from the leaves, which has the potential to limit water availability.
If I were you, I'd focus on the other three to learn and revise!
Leaf structure
Leaves have a particular structure to make the most of any available sunlight so that photosynthesis can occur at its fastest possible rate.
You need to be able to label parts of the leaf that are labelled A-I in the diagram to the left. Here are the terms you need to be familiar with (in no particular order). Their functions are described further down.
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Leaf adaptations that allow for efficient photosynthesis:
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Palisade mesophyll structure and arrangement
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Spongy mesophyll and air spaces
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Guard cells and stomata
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Upper and lower epidermis, and cuticle
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Vein
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Palisade mesophyll cells are the longest cells in the leaf, packed tightly together in a vertical orientation. They are the cells which contain the highest number of chloroplasts per cell. They are close to the upper surface of the leaf which is where the most sun hits the leaf. Hence why there are lots of cells packed in there, all full of lots of chloroplasts... efficient photosynthesis!
Spongy mesophyll cells are the next layer down - they are round cells that have quite an irregular shape. These cells have a few chloroplasts, just in case any light penetrates through the palisade layer - the light can be captured by the chloroplasts in the spongy layer (it's like a back-up)! These cells are irregularly shaped so that there are air spaces around the cells.
Air spaces within the spongy layer is important. If the spongy layer was packed to the brim (like the palisade layer is), there is no way CO2 can make it to the palisade mesophyll cells fast enough to allow photosynthesis to occur efficiently. Having air spaces in the spongy layer allows CO2 to enter through the stomata and move up through the air spaces and can diffuse into the spongy or palisade cells to be used in PS.
Air spaces within the spongy layer is important. If the spongy layer was packed to the brim (like the palisade layer is), there is no way CO2 can make it to the palisade mesophyll cells fast enough to allow photosynthesis to occur efficiently. Having air spaces in the spongy layer allows CO2 to enter through the stomata and move up through the air spaces and can diffuse into the spongy or palisade cells to be used in PS.
Guard cells are found on the bottom of the leaf and control the opening and closing of the stomata (hole) in the leaf. When there is enough water, they fill the guard cells causing them to swell and change shape - this causes the stomata to be open. When there is a lack of water, the guard cells are flaccid (not swollen and not empty) which causes the stomata (hole) to be closed. When the stomata are open (guard cells full of water), gas exchange (CO2 in and O2 out) is very efficient. However, the leaf is also susceptible to water loss (especially if it is windy). When the stomata are shut (guard cells lack water), there is no space for gasses to move in and out of the leaf, limiting the rate of photosynthesis (as CO2 availability is limited), however water is conserved.
t's a bit of a trade-off!
t's a bit of a trade-off!
This is the upper layer of the leaf - a protective layer. These cells do not have a role in photosynthesis, instead covering the cells involved in PS to protect them from any damage. The cells of the upper epidermis are transparent (see-through) so that light can penetrate these cells and reach the palisade mesophyll cells below without the light losing any intensity. This makes photosynthesis very efficient!
Above the upper epidermis is the waxy cuticle. This is a layer of cells on the very very top surface of the leaf, where the sun hits. Its job is to reduce water loss (it basically blocks evaporation) and it is also water proof so that the leaves don't get catch rainwater, rather it rolls off the leaves. Leaves full of water (as a result of the rain) become heavy and can snap off the tree! Not good for photosynthesis.
The lower epidermis serves the same purpose as the upper epidermis - it is for protection. The guard cells and stomata are also found within this layer of cells.
Above the upper epidermis is the waxy cuticle. This is a layer of cells on the very very top surface of the leaf, where the sun hits. Its job is to reduce water loss (it basically blocks evaporation) and it is also water proof so that the leaves don't get catch rainwater, rather it rolls off the leaves. Leaves full of water (as a result of the rain) become heavy and can snap off the tree! Not good for photosynthesis.
The lower epidermis serves the same purpose as the upper epidermis - it is for protection. The guard cells and stomata are also found within this layer of cells.
The vein(s) within a leaf is where the important vessels are found. Xylem is in the vein and delivers water to (primarily) the palisade mesophyll cells so that photosynthesis can occur. Remember water is essential for PS! Phloem is also present in the vein so when glucose is produced during PS, it can diffuse into the phloem and be transported to where its needed!