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  1. Define photosynthesis as the fundamental process by which plants manufacture carbohydrates from raw materials using energy from light.

It’s a pretty self-explanatory point:

Photosynthesis is the fundamental process by which plants manufacture carbohydrates from raw materials using energy from light.

  1. Explain that chlorophyll traps light energy and converts it into chemical energy for the formation of carbohydrates and their subsequent storage.

Photosynthesis is an energy consuming process.

For plants to power this process, the energy from sunlight is absorbed by chlorophyll and converted in to a form that plants can use – chemical energy. This chemical energy is what fuels photosynthesis.

  1. State the word equation for the production of simple sugars and oxygen.

Carbon dioxide + Water à Glucose + Oxygen

  1. State the balanced equation for photosynthesis in symbols

6CO2 + 6H2O à C6H12O6 + 6O2

(write light above the arrow and chlorophyll below, to show that these are two necessary components of the reaction.)

  1. Investigate the necessity for chlorophyll, light and carbon dioxide for photosynthesis, using appropriate controls.

Chlorophyll:

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 (ie. 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, and hence, photosynthesis can only occur in the presence of cholorophyll.


Light
:

Destarch a plant. Cut out a strip of opaque black paper and clip it to 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.

Carbon dioxide:

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 CO2 is necessary for photosynthesis.

  1. Investigate and state the effect of varying light intensity 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!

Apparatus

  • Lamp
  • Metre rule
  • Stop watch
  • Timer
  • Gas syringe (not necessary)
  • 400cm3 beaker
  • Thermometer
  • Test tube containing dilute sodium hydrogencarbonate solution
  • An aquatic plant, e.g. Canadian pondweed

Method

 

  1. 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 so that it is constant – add hot water to increase the temperature and cold water to lower it.
  2. Attach the gas syringe, if you have one.
  3. 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.
  4. 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 5 minutes. Otherwise, simply count the number of bubbles produced over 5 minutes, and divide by 5 (to gain the bubbles/min)
  5. Repeat steps 3 and 4, with varying distances such as 30cm, 40cm, 50cm, etc. and record your results. Light intensity is inversely proportional to the square of the distance, so doubling the distance quarters the intensity.
  6. 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. This shows 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.

  1. Describe the intake of carbon dioxide and water by plants.

Carbon dioxide:

Carbon dioxide diffuses in through the stomata (singular is stoma) on the lower epidermis of the leaf.

 

Water:

Water in the soil first diffuses into a root hair, by osmosis. It then diffuses across the root cortex and in to the xylem.

In the diagram, the part labelled ‘Cells inside root’ is the root cortex.

  1. Identify and label the cuticle, cellular and tissue structures of a dicotyledonous leaf, as seen in cross-section under the light microscope and describe the significance of the features of a leaf in terms of functions, to include:
  • Distribution of chloroplasts – photosynthesis
  • Stomata, palisade and mesophyll cells – gas exchange
  • Vascular bundles (xylem and phloem) – transport and support.

So first, the structure of a dicot leaf:

Note: The cuticle is a waxy layer covering the top and bottom of the leaf.

It is largely the mesophyll cells that contain chloroplast, and of these mesophyll cells, the palisade mesophyll cells contain the most chloroplast. Therefore, these are the cells that perform photosynthesis. Also, note that the palisade cells are closer to the top of the cells, and so receive more sunlight than the spongy mesophyll.

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.

When plant cells respire, they mostly use the oxygen given off from photosynthesis, however, a small portion that diffused in through the stomata from the air is also used.

When plants photosynthesise, they require CO2. This diffuses in through the stomata, through the intercellular airspaces present between the spongy mesophyll, and in to the photosynthesising cells.

Now, as for the vascular bundles: These contain the xylem and the phloem, usually encased in an endodermis which can be one to several cells thick. In a dicot leaf, the xylem is usually 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. Xylem vessels are very strong as they have a woody material called lignin deposited in their cell walls. Therefore, they also act as a 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).

  1. Describe the importance of:
  • Nitrate ions for protein synthesis

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 important for plant growth.

  • Magnesium ions for chlorophyll synthesis.

Magnesium forms the central ion in a chlorophyll molecule.

  1. 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 general increase in cell size. This means more proteins! So a deficiency of nitrate ions will result in stunted plant growth, and 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.

  1. Describe the uses, and the dangers of overuse, of nitrogen-containing fertilisers.

Nitrogen-containing fertilisers are added to crop soil, so that the plants may have a sufficient supply of nitrate ions for growth. This improves crop yield.

One essential property of fertilisers, in order for plants to be able to take up the nutrients in them, is that they are water soluble. This means, that if too much fertiliser is added to the soil, or if it is added at the wrong time (e.g. just before rain), then the excess fertiliser may be washed out of the soil (this process is called leaching) and into nearby waterbodies.

This may result in excessive growth of algae, which is known as eutrophication. Eutrophication will cause the algae to block the sunlight entering the water, which will affect the water plants present in the body. It may also use up the oxygen in the lake, causing marine life to die.

 

Notes Submitted by Sarah.

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