photosynthesis
Photosynthesis is a topic that a lot of students don't especially enjoy but the process is one of the most fundamental of all. Without photosynthesis there would be no life on earth. Photosynthesis is the process by which plants and algae (primarily) use the Sun's energy to convert inorganic molecules into large organic compounds. This provides the whole ecosystem with the nutrients it needs to survive.
Remember, the Sun is the source of all energy on Earth. Plants and algae use that energy to produce glucose and oxygen
Remember, the Sun is the source of all energy on Earth. Plants and algae use that energy to produce glucose and oxygen
These are the word and symbol equations for photosynthesis. Make sure you remember them both. Sometimes you might be asked to balance the symbol equation - it's easy to remember as it all involves the number 6! Also, if you know the equation for aerobic respiration then the one for photosynthesis is just the reverse!
Know how the raw materials for photosynthesis (that's water and CO2) get into the leaf. CO2 diffuses in through the stomata, and water enters the plant via osmosis through the root hair cells and then travels to the leaf in the xylem. |
Read more about diffusion and osmosis here
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The products of photosynthesis also need to leave the leaf. Some of the oxygen that is produced is used by the plant in aerobic respiration to produce energy. The remainder exits the leaf through the stomata by diffusion. There are many uses for the glucose (see below). Some of the glucose is transported to other parts of the plant in the phloem.
You can find out more about exchange and transport in plants here
You can find out more about exchange and transport in plants here
Light energy and chlorophyll are essential for photosynthesis. Chlorophyll is a green pigment which is found in the chloroplasts, small organelles in the leaf palisade cell within which photosynthesis takes place. Chlorophyll absorbs light energy and uses it to drive photosynthesis
Adaptations of the leaf for photosynthesis
Make sure you understand how the leaf palisade cell and the leaf are adapted for photosynthesis, and be able to label them. You might be asked what the function is for any of the important parts.
Make sure you understand how the leaf palisade cell and the leaf are adapted for photosynthesis, and be able to label them. You might be asked what the function is for any of the important parts.
Waxy cuticle - waterproofs the leaf
Palisade mesophyll layer - full of regularly spaced palisade cells Palisade cell - packed with chloroplasts (and therefore chlorophyll) to allow photosynthesis to occur. Regular shape to pack lots of cells in Lower epidermis - lower layer of cells in the leaf |
Upper epidermis - transparent to allow light through
Spongy mesophyll layer - full of air spaces which give the cells a large surface area to allow gases to enter and exit the cells Stomata - pores/holes on the bottom side of the leaf that allow gases to diffuse in and out Guard cells - control the opening and closing of the stomata |
There are 4 main adaptations of leaves:
1) they are broad allowing them to absorb as much light as possible
2) they contain chlorophyll (in chloroplasts) to allow them to absorb light
3) they are full of air spaces to allow CO2 and O2 to diffuse into and out of the cells
4) they have veins made of phloem and xylem to bring water to the leaf and remove glucose
1) they are broad allowing them to absorb as much light as possible
2) they contain chlorophyll (in chloroplasts) to allow them to absorb light
3) they are full of air spaces to allow CO2 and O2 to diffuse into and out of the cells
4) they have veins made of phloem and xylem to bring water to the leaf and remove glucose
BBC GCSE Bitesize has a nice revision section on photosynthesis with a handy test so you can see how you're doing
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Limiting Factors
There are 3 factors which can limit the rate of photosynthesis. These are light, temperature and carbon dioxide levels.
Plants tend to grow quicker (and hence photosynthesise faster) in the summer when the weather is warm and there is a lot of sun. The rate of photosynthesis will be limited if light, temperature and/or carbon dioxide are in short supply.
You need to be able to interpret graphs showing the effect of limiting factors on the rate of photosynthesis.
Wherever the rate of photosynthesis increases with an increase of the limiting factor, this means that that factor is limiting (i.e., if you add more of it then you increase photosynthesis). When the rate plateaus, the rate of photosynthesis becomes limited by one of the other 3 factors.
There are 3 factors which can limit the rate of photosynthesis. These are light, temperature and carbon dioxide levels.
Plants tend to grow quicker (and hence photosynthesise faster) in the summer when the weather is warm and there is a lot of sun. The rate of photosynthesis will be limited if light, temperature and/or carbon dioxide are in short supply.
You need to be able to interpret graphs showing the effect of limiting factors on the rate of photosynthesis.
Wherever the rate of photosynthesis increases with an increase of the limiting factor, this means that that factor is limiting (i.e., if you add more of it then you increase photosynthesis). When the rate plateaus, the rate of photosynthesis becomes limited by one of the other 3 factors.
When the rate of photosynthesis plateaus in the graph above, temperature or light become limiting factors (because no matter how much more CO2 you add, the rate of photosynthesis won't increase)
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When the rate of photosynthesis plateaus in the graph above, CO2 or temperature become limiting factors (because no matter how much more light you add, the rate of photosynthesis won't increase)
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The odd one out!
Here, the rate of photosynthesis increases with increasing temperature up to the optimum because the molecules have more kinetic energy. Above the optimum, an increase in temperature will result in the enzymes that control photosynthesis denaturing, and the rate will decrease |
Check out this 'virtual photosynthesis' resource which you can use to investigate the effect of light intensity on the rate of photosynthesis
The diagram on the right shows how pondweed (elodea) can be used to investigate the rate of photosynthesis. This experiment relies on the measurement of oxygen production as a readout of the rate of photosynthesis. Make sure you understand how this works - it comes up often!!
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How plants use glucose
Glucose is the principle product of photosynthesis, and it is vital for the plant's survival. The plant uses it in a number of ways:
Storage as starch - starch is insoluble and so this is how the plant stores glucose (insoluble starch doesn't affect the water balance of the plant like soluble glucose would). Starch is found all over the plant, and it acts as an energy store. Some plants store it in tubers, like potatoes which we eat.
Respiration - aerobic respiration is the process by which cells release energy from glucose and oxygen. This energy can be used by the plant for other processes, such as growth and the production of large molecules
Production of cellulose - cellulose is a complex carbohydrate that gives plant cell walls their structure and strength
Production of amino acids - amino acids are the building blocks of proteins. Plants combine sugars with nitrate ions and other mineral ions from the soil to make amino acids
Production of fats and oils - these acts as another energy store for the plant, and are found in seeds and in the plant cells themselves as lipid droplets
Glucose is the principle product of photosynthesis, and it is vital for the plant's survival. The plant uses it in a number of ways:
Storage as starch - starch is insoluble and so this is how the plant stores glucose (insoluble starch doesn't affect the water balance of the plant like soluble glucose would). Starch is found all over the plant, and it acts as an energy store. Some plants store it in tubers, like potatoes which we eat.
Respiration - aerobic respiration is the process by which cells release energy from glucose and oxygen. This energy can be used by the plant for other processes, such as growth and the production of large molecules
Production of cellulose - cellulose is a complex carbohydrate that gives plant cell walls their structure and strength
Production of amino acids - amino acids are the building blocks of proteins. Plants combine sugars with nitrate ions and other mineral ions from the soil to make amino acids
Production of fats and oils - these acts as another energy store for the plant, and are found in seeds and in the plant cells themselves as lipid droplets
Hydroponics vs. greenhouses/polytunnels
These are ways in which plant growth can be manipulated in order to increase the rate of photosynthesis. If the rate of photosynthesis is increased, plants will flower and fruit quicker, meaning that farmers can harvest their product sooner = MORE MONEY!
All of the manipulations that are made are based on the control of limiting factors
These are ways in which plant growth can be manipulated in order to increase the rate of photosynthesis. If the rate of photosynthesis is increased, plants will flower and fruit quicker, meaning that farmers can harvest their product sooner = MORE MONEY!
All of the manipulations that are made are based on the control of limiting factors
Hydroponics
Computer-controlled high-tech way of growing plants using mineral solutions rather than soil. Advantages: * No soil to dirty the plants * Mineral solutions are perfectly balanced for each type of plant * No ploughing of land required * Fewer staff needed due to computers * Fast turnover = high profits Disadvantages: * Computer controlled = expensive to set up |
Greenhouses
These can be as large as a warehouse, or as small as you might have in your garden. Made of glass. Advantages: * May have lights installed to ensure plants grow all day and night long * Inside the greenhouse is much more controllable than outside * Warmer atmosphere which speeds up growth * CO2 can be pumped in to increase rate of photosynthesis Disadvantages * Not as high yields as hydroponics (but cheaper!) |
In an exam you might be asked what a farmer might consider before spending money on installing greenhouses/hydroponics systems. This will relate to economics: does it cost more than the increase in profits from a higher yield?