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- Define photosynthesis as the process by which plants manufacture carbohydrates from raw materials using energy from light.
We have another definition to learn here! Simply memorise it:
Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light.
- State the word equation for photosynthesis: carbon dioxide + water –> glucose + oxygen, in the presence of light and chlorophyll
Again, simply memorise this point!
- State the balanced equation for photosynthesis
- Explain that chlorophyll transfers light energy into chemical energy in molecules, for the synthesis of carbohydrates
Photosynthesis is an energy-consuming process.
For plants to power this process, the energy from sunlight is absorbed by chlorophyll and converted into a form that plants can use – chemical energy. This chemical energy is what fuels photosynthesis.
- Outline the subsequent use and storage of the carbohydrates made in photosynthesis
Carbohydrates made during photosynthesis are used for:
- Energy from respiration can be used for a variety of things like
- Active transport
- Producing cellulose (a critical structural component in plant cell walls). Cellulose is a polymer made of many glucose subunits. It provides plant cell walls with strength and rigidity – in fact, about 50% of wood is cellulose. You would also need energy from respiration to help power the synthesis of cellulose. Note: you might make cellulose because you’re trying to thicken cell walls, make more cells, increase cell sizes, or repair damaged cell walls.
Other uses of glucose include:
- Converting it to sucrose, so that they can be transported to different parts of the plant through the phloem. This is so parts of the plant that can’t photosynthesise/ can’t photosynthesise as much as other parts of the plant, receive enough carbohydrates. Some parts of the plant may not be able to photosynthesise as they do not contain any chlorophyll, e.g. flowers or roots. Another reason they may not be able to photosynthesis is that they do not receive any sunlight, e.g. the roots.
- Making proteins: Glucose and nitrates are used to form amino acids, which can then be used to create different proteins. Proteins are important: they are needed for:
- Making enzymes – enzymes catalyse almost all the metabolic processes in plants and animals. Without enzymes, these reactions will not be able to occur at all, or will not happen fast enough to sustain life. This makes enzymes extremely important.
- Making hormones – many hormones are made of proteins. Hormones regulate many processes – e.g. it is a hormone that controls which direction a plant grows.
- Growth – proteins are used in many structural components of plants and plant cells. Without proteins, it would be impossible to increase cell size/ increase the number of cells in a plant, so a plant would not be able to grow.
- Cell and tissue repair – as proteins are an important structural component of plants, if a part of a plant is damaged, it likely needs proteins to help fix it.
Carbohydrates are stored as:
- Starch – when in a plant, glucose is often converted to starch and then stored in plant cells. Starch is a large polymer made of glucose subunits.
- Oils – glucose may be converted to an oil. Oils are very good at storing energy because they have a lot of chemical bonds, so they can store a lot of chemical potential energy. This is especially important in seeds – oils are a huge way of energy storage in seeds.
- Investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls
Take a potted plant with variegated leaves (leaves that have both green and white patches, like the leaf on the left) and destarch the plant by keeping it in complete darkness for two days (about 48 hours).
Place it in sunlight for a few days, so that it can form some new starch. Finally, perform the starch test on one of the leaves (add a few drops of iodine to the leaf.)
The green parts (i.e. the parts with chlorophyll) will turn blue-black, and the white parts will be orange-brown. This shows that starch is only formed where chlorophyll is present. Hence, photosynthesis can only occur in the presence of chlorophyll.
Destarch a plant.
Cut out a strip of opaque black paper and clip it a section of one of the leaves, as shown.
Leave the plant in sunlight for a few days.
Perform the starch test and observe.
The areas that turn blue-black (and hence contain starch) are the areas exposed to sunlight, and the orange-brown area was the section covered by paper. This shows that light is necessary for photosynthesis.
Destarch two potted plants.
Cover both plants in transparent plastic bags; place a petri dish of sodium hydrogencarbonate in one, and a petri dish of soda lime in the other (as shown in the diagram). Sodium hydrogencarbonate gives off carbon dioxide, and soda lime absorbs carbon dioxide from the air.
Leave these two plants in sunlight for a day (at least 6 hours). Perform the starch test on a leaf from each plant.
You will find that the leaf from the plant with sodium hydrogencarbonate turns blue-black, and the leaf from the plant with soda lime turns orange-brown. This shows that carbon dioxide is necessary for photosynthesis.
- Investigate and describe the effect of varying light intensity and temperature on the rate of photosynthesis (e.g. in submerged aquatic plants)
This experiment is a little bit more complicated, so I’ll write out the full procedure for you guys!
- Metre rule
- Gas syringe (not necessary)
- 400cm3 beaker
- Test tube containing dilute sodium hydrogencarbonate solution
- An aquatic plant, e.g. Canadian pondweed
- Cut the stem of a pondweed that has been well illuminated and is hence, producing bubbles. Place the stem upside down in the test tube. Place the test tube in a beaker of water (this water prevents the temperature varying too much – water has a high specific heat capacity), and note the temperature. This temperature should be checked at regular intervals to make sure that it remains constant – add hot water to increase the temperature and cold water to lower it.
- Attach the gas syringe, if you have one.
- Place the set up in a dark room; if you don’t have one, darken the room as much as possible (turn off all the lights, draw any curtains and blinds, etc.) and place a lamp 10cm away from the beaker.
- Allow the plant to adjust to the light intensity – this is apparent when the plant produces bubbles at a constant rate. If you have a gas syringe, attach the tube over the opening of the test tube, and measure the volume of gas produced over five minutes. Otherwise, simply count the number of bubbles produced over 5 minutes, and divide by 5 (to gain the bubbles/ min)
- Repeat steps 3 and 4, changing the distance between the lamp and beaker each time. Use regular intervals, e.g. 10cm, 20cm, 30cm, 40cm, 50cm and 60cm. Record your results. Light intensity is inversely proportional to the square of the distance, so doubling the distance quarters the intensity.
- Plot a graph of your recorded values.
You should find that, as the distance increases, the volume of gas collected/ number of bubbles produced per minute falls. As the process of photosynthesis gives off oxygen gas, we can infer that the more gas is given off, the more photosynthesis is occurring.
So, this shows that the rate of photosynthesis falls with falling light intensity. We can rewrite this statement to show that the rate of photosynthesis increases with increasing light intensity.
However, after a certain point, as the light intensity increases, the rate of photosynthesis will no longer increase; as it has reached a maximum
To measure the effect of temperature on photosynthesis, you can use the same experimental setup. Except this time, instead of changing the light intensity, you change the temperature of the warm water bath that the test tube with the plant is in.
Now to explain the results:
In the region (a), you will notice that as you increase the temperature, the amount of gas collected increases. So the rate of photosynthesis increases. This continues until photosynthesis reaches its maximum rate – the graph flattens out. This is section (b)
The temperature at (b) is the optimum temperature for photosynthesis.
After (b), as you continue to increase the temperature, the rate of photosynthesis dramatically falls. This is because the enzymes and proteins involved in photosynthesis start to denature, so they can no longer function. This region of the graph is (c).
- Identify the chloroplasts, cuticle, guard cells and stomata, upper and lower epidermis, palisade mesophyll, spongy mesophyll, vascular bundles, xylem and phloem in leaves of a dicotyledonous plant
Structure of a dicot leaf:
- Describe the significance of the features of a leaf in term of functions, to include:
- Palisade mesophyll and distribution of chloroplasts – photosynthesis
- Stomata, spongy mesophyll and guard cells – gas exchange
- Xylem for transport and support
- Phloem for transport
It is mostly the mesophyll cells that contain chloroplast. Of these mesophyll cells, the palisade mesophyll cells contain the most chloroplast. Therefore, these are the cells the perform the most photosynthesis. Also, note that the palisade cells are closer to the top of the cells, and so receive more sunlight than the spongy mesophyll. Palisade mesophyll cells are also packed tightly together, so as many cells as possible can receive a high amount of sunlight.
Guard cells are the two cells surrounding a stoma. It is the gap between to guard cells that form a stoma (the plural of stoma is stomata). Guard cells control whether the stoma is open or close.
Stomata are present to allow gases to diffuse into and out of the leaf. Stomata are mostly present on the lower epidermis of a leaf; however, all plant surfaces that are exposed to the air have stomata. The leaf has the most, as this is where photosynthesis occurs, so plenty of carbon dioxide is required from the air. Of this, the underside has more, so that the rate of water loss (due to transpiration) won’t be as high.
Remember, carbon dioxide and water are needed for photosynthesis. When the availability of water is extremely low, guard cells become flaccid (less stiff), so they close. This prevents carbon dioxide from diffusing in through the stoma, halting photosynthesis. This stops the plant cells from using up too much of what little water is left.
When water is freely available, the guard cells are turgid, forcing them apart and allowing the stoma to open. This allows carbon dioxide to diffuse in, allowing photosynthesis to occur.
The spongy mesophyll is named ‘spongy’ because there are many intercellular air spaces between the mesophyll cells in this layer, giving the layer a spongy texture. These air gaps allow gases to diffuse all around the leaf – so the mesophyll cells (especially the palisade mesophyll) can receive plenty of carbon dioxide. The intercellular air spaces in spongy mesophyll also make it easier for gases the diffuse in from the stoma and reach the other leaf cells. The air spaces also make it easier for gases from other leaf cells to diffuse and find their way out through the stoma.
When plants photosynthesis, they require CO2. This diffuses in through the stomata, through intercellular airspaces present between the spongy mesophyll, and into the photosynthesising cells.
Now, as for the vascular bundles: these contain the xylem and phloem, usually encased in an endodermis which can be one to several cells thick. In a dicot leaf, the xylem is typically present above the phloem.
Xylem vessels are long continuous tubes made up of dead cells that transport water and mineral ions from the root to different parts of the plant.
Water in the soil first diffuses into a root hair (the long finger-like process on a root hair cell), by osmosis. It then diffuses across the root cortex and into the xylem. Note: the root cortex is made up of the cells under the epidermis (the outer layer of cells of the root) and outside the xylem.
Xylem vessels are very strong as they have a woody material called lignin deposited in their cell walls. Therefore, they also act as structural support for the plant.
Phloem vessels transport assimilates (substances made by the plant itself) from a source (a place where these assimilates are produced, e.g. a leaf) to a sink (a place where assimilates are used or stored, e.g. the stem, root).
- Describe the importance of:
- Nitrate ions for making amino acids
Proteins are made up of amino acids. Each amino acid has at least one amine group (-NH2), and plants get the nitrogen for this amino acid synthesis from nitrate ions. Protein synthesis is vital for plants to stay alive – proteins make up enzymes, hormones, are used for growth and repair, etc.
- Magnesium ions for making chlorophyll
Magnesium forms the central ion in a chlorophyll molecule. Chlorophyll is essential for photosynthesis.
- Explain the effects of nitrate ion and magnesium ion deficiency on plant growth
Nitrate ions are required to make proteins. Growth involves cell division or just a general increase in cell size. This means more proteins! So a deficiency of nitrate ions will result in stunted plant growth. It also causes leaves to turn yellow.
Less magnesium means less chlorophyll, which in turn means less photosynthesis. This means that the plant won’t have enough energy for growth.
Notes submitted by Sarah
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