What is Photosynthesis? How It Occurs?
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).
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Chloroplasts - the site of photosynthesis - they are an organelle (little organ inside a cell) filled with chlorophyll. Chlorophyll actually absorbs the light from the sun so that photosynthesis can occur!
What Materials are Essential for Photosynthesis?
What materials are essential for photosynthesis? And how do they get into the leaves so photosynthesis can occur?
Carbon Dioxide
Carbon dioxide enters the plant through the leaves. There are special holes on the underside of the leaf called stomata where carbon dioxide can enter. Carbon dioxide enters the leaf by a process called diffusion. |
Water
Water enters the plant through its roots. Once it has entered the roots it travels up the xylem and is delivered to the leaves. Water enters the roots by a process called osmosis. |
Diffusion
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. So, carbon dioxide enters the leaf because there's lots of it outside the leaf (in the atmosphere) and when the stomata (leaf holes) are open, a new area is opened up where there's not much carbon dioxide. The gas moves from the area of high concentration (outside the leaf) to the area of inside the leaf (low concentration). The area inside the leaf will always have a lower concentration of carbon dioxide because when it's in there it gets used up! |
Osmosis
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. Now to relate this to the roots - water moves from a high concentration (soil) to a lower concentration (inside the roots). Water will then enter the xylem vessel (water pipe) and go up to the leaves to be used in photosynthesis. Water can also be lost from the plant through the stomata, so the water concentration inside the plant will always be less than in the soil (unless the soil is all dried out)! Root hairs increase the space available for osmosis to occur, so they help water be absorbed fast! |
What Materials are Made by Photosynthesis?
What materials are made by photosynthesis? And what happens to them afterwards?
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. |
Glucose
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 - To be used in respiration to produce ATP (energy) which can be used for growth, reproduction, etc. No glucose means no energy for life processes to be carried out. This will likely result in plant death! To become new cell parts when required (i.e. growth or repair). It will often turn into cellulose for this - a great material for making cell walls in particular. To be stored for when needed - as ALL the glucose made isn't required immediately! Plants often have a food reservoir (storage system) where glucose is stored as starch (think back to cotyledon/seeds and how they have stored starch in them). The starch can be converted into glucose by enzymes when more glucose is needed. 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. |
What Conditions are Required for Photosynthesis to Occur at its Fastest Rate?
Task: Print out the picture below and annotate it with what enters and exits the plant before/during/after photosynthesis. How does it enter/exit? what happens if something the plant needs is in limited supply?
For photosynthesis to occur at its fastest rate, there needs to be enough light energy (from the sun), a warm temperature for enzyme-controlled reactions to occur, plenty of carbon dioxide surrounding the plant and 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.
If one of these factors is in short supply, the plant may not be able to do photosynthesis at its fastest rate.
Temperature
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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Light intensity
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. |
CO2 Concentration
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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Water Availability
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! |