PLANT NUTRITION PART 16: MORE ON THE RATE OF PHOTOSYNTHESIS

As mentioned in the previous post, any factor that directly affects a process if its quantity is changed is called the LIMITING FACTOR.



By looking at graph 1, we can see that the rate of photosynthesis increases as the light intensity increases (0 to A). We say that light intensity is the LIMITING FACTOR. Beyond point A, light intensity is no longer the LIMITING FACTOR since the rate remains constant even though the light intensity increases. In this case we have to consider other factors that could cause the rate to become constant (A to B).

Graph 2 shows that the rate does not increase so much despite the temperature being increased from 20 to 30 degree celsius (with the carbon dioxide being kept constant). This means that temperature is not the actual LIMITING FACTOR. But if the conditions are reversed, the temperature being constant and the carbon dioxide being increased from 0.03% to 0.13%, the rate increases (Look at graph 3). Both of these indicate that carbon dioxide concentration is the LIMITING FACTOR in A to B (Graph 1).

The LIMITING FACTOR in E to F (Graph 3) is the temperature. Increasing the temperature from 20 to 30 degree celsius causes an increase in the rate of photosynthesis (Look at graph 4) though the carbon dioxide concentration remains constant at 0.13%.

END OF CHAPTER 6

PLANT NUTRITION PART 15: FACTORS AFFECTING PHOTOSYNTHESIS

The rate of photosynthesis is affected by the following factors:

• LIGHT INTENSITY

• CONCENTRATION OF CARBON DIOXIDE

• TEMPERATURE

When we consider light intensity, immediately we would think that, the higher the light intensity, the higher would the rate of photosynthesis be, right? That's right actually. But at certain point even if the light intensity is increased, the rate will not increase any further. This is because, the chlorophyll in the chloroplasts can only absorb so much energy from sunlight. It's just like pouring water into a glass. The more you pour in water the higher will the water level be in the glass but if the level is already reaching the brim of the glass, no matter how much water you pour in, the water level will not increase any further. UNDERSTAND?

In the above analogy, when the water level reaches the brim of the glass, we can say that the glass is the LIMITING FACTOR because despite the availability of water, the glass can only take so much. Now, lets consider light intensity which is a factor affecting the rate of photosynthesis. In this case, the LIMITING FACTOR is the chlorophyll right? Right you are, if you consider the absorption of light at a point at which the chlorophyll can no longer absorb more than it should. Because, no matter how much light energy there is available, the chlorophyll can only absorb so much. So at this point, increasing the light intensity will not increase the rate of photosynthesis any further.

But mind you, before reaching that particular point where the graph levels off, light intensity is actually the LIMITING FACTOR because the rate of photosynthesis depends on it. Meaning, before this particular point (where the graph levels off), the rate of photosynthesis will only increase, when the light intensity is increased.


Now, lets consider the effect of carbon dioxide concentration. By looking at the graph below, the rate of photosynthesis increases as the concentration (%) of carbon dioxide is increased. We say that carbon dioxide is the LIMITING FACTOR since the rate depends on it. But at certain point, even after increasing the concentration of carbon dioxide, the rate remain constant. After this point, carbon dioxide concentration is no longer the LIMITING FACTOR.



Temperature may also affect the rate of photosynthesis. Since photosynthesis is an enzyme-controlled reaction, the rate depends on the temperature at which the reaction takes place. As can be seen from the graph, as the temperature is increased, the rate will also increased. We say that the temperature is the LIMITING FACTOR since the rate really depends on it. But at the optimum temperature, the reaction decreased and eventually stops at 45 degree celsius because at high temperature the enzyme catalysing photosynthesis is denatured. So, after the optimum temperature, temperature is no longer the LIMITING FACTOR.

PLANT NUTRITION PART 14: WATER CULTURE EXPERIMENT

The water culture experiment is used to find out whether nitrogen, phosphorus and magnesium are essential for plant growth.


HOW TO MAKE A COMPLETE CULTURE SOLUTION?

• 1000cc distilled water
• 0.25g potassium nitrate
• 0.25g magnesium sulphate
• 0.25g potassium acid phosphate
• 1g calcium nitrate
• 2 drops iron(III) chloride solution

(Important: The chemicals should be added to the water in the order shown above)

To investigate whether Nitrogen is really needed for plant growth, omit the nitrates and use potassium chloride and calcium sulphate.

To investigate whether Magnesium is really needed for plant growth, omit magnesium sulphate and use potassium sulphate.

PRECAUTIONS

• Before setting up the experiment, all apparatus are to be sterilize to ensure that the apparatus is free from micro-organisms which might interfere with the growth of the seedlings.
• The outside of the gas jars are to be covered completely with black papers to prevent light from entering the gas jars. This prevents algae from growing around the roots and hinder their normal functioning.
• The gas jars are to be placed in such a way that they receive enough sunlight but be very careful not to place the gas jars in direct sunlight as it may caused the leaves of the seedlings to scorch and heat up the culture solutions.
• Keep the cotton wool around the seedling dry to prevent the stem from rotting.
• Renew the culture solution every 2 weeks (Remember: Depletion).
• Aerate the solution by blowing air through the delivery tube to supply oxygen for root respiration.
  

PLANT NUTRITION PART 13: MINERAL NUTRITION

And you think photosynthesis is enough for plants to make food right? Nope.... there is more to it actually. After carbohydrate (glucose) is synthesized, some of them need to be converted into amino acids and later from amino acids, proteins are synthesized. For this to occur, plants need minerals. The minerals needed by plants are divided into two groups: Macronutrients and Micronutrients.

MACRONUTRIENTS: Needed in large amount. Examples are Nitrogen, Phosphorus, Sulphur, Magnesium, Potassium and Calcium.

MICRONUTRIENTS: Needed in small amount. Examples are Iron, Manganese, Boron, Cobalt, Zinc, Copper and Molybdenum.

THE ROLE OF MACRONUTRIENTS IN PLANT GROWTH
Do you need to know all these? Hold your horses. In the Biology syllabus, you are required to know the functions and deficiency symptoms of Nitrogen and Magnesium only.


NITROGEN

• Plants generally dependent on nitrogen in the form of nitrate ions or ammonium ions.
• Symptom of deficiency: Chlorosis and stunted growth (Why? Nitrogen is a component of chlorophyll and amino acids and hence proteins are needed for growth. Therefore if plants lack Nitrogen, chlorophyll formation will be affected and there will be insufficient proteins for growth, hence stunted growth)

MAGNESIUM

• Magnesium is also a component of chlorophyll.
• Deficiency symptoms: Chlorosis develops upward from the base of the plant. Unlike in Nitrogen deficiency, yellowing is only in-between the veins (the veins remain green). Chlorosis is accompanied by death of the entire leaf or portions of it.

PLANT NUTRITION PART 12: RATE OF PHOTOSYNTHESIS

The following set up can be used to investigate the effect of different light intensities, different temperatures and different carbon dioxide concentrations on the RATE of photosynthesis.
MODIFICATIONS

• For investigating the effect of different light intensities on the rate of photosynthesis, the distance of the light source should be altered. For example, 50cm, 40cm, 30cm, 20cm and 10cm away from the plant. Air bubbles are given out from the cut end of the plant. Allow some time for the plant to adapt to the conditions provided before taking readings. When they are coming at a regular rate, count the number of air bubbles over a period of time, say five minutes. Repeat this a few times to obtain the average rate (Note: the bubbles produced are oxygen gas released as a result of photosynthesis). The readings may be tabulated like the one shown in the following figure (The heat from the light source may affect the temperature of the water bath, so throughout the investigation, the temperature should be monitored so that it is always constant)

• To investigate the effect of different temperature on the rate of photosynthesis, the temperature of the water bath may be altered by using hot water and ice cubes.
• To investigate the effect of different carbon dioxide concentrations, sodium hydrogen carbonate solutions of different concentrations may be used. For example, 0.01M, 0.02M, 0.03M up to 0.1M.

EXPECTED OBSERVATIONS:

• Effect of different light intensities: The closer the light source to the plant, the higher the light intensity and the more bubbles will be produced and this means that the higher the rate of photosynthesis. Likewise, the further the light source away from the plant, the lower the light intensity and the less bubbles will be produced and this means that the lower the rate of photosynthesis.
• Effect of different temperatures: The lower the temperature the less bubbles will be produced and this means that the lower the rate of photosynthesis. As the temperature is increased, more and more bubbles will be produced and this indicates that the rate of photosynthesis increases with the increase in temperature. WHAT DO YOU THINK HAPPENED IF THE TEMPERATURE IS ABOVE 40 DEGREE CELSIUS? WILL THE RATE INCREASE? OR DECREASE?
• Effect of different carbon dioxide concentrations: The lower the carbon dioxide concentration, the less bubbles will be produced and this means that the lower the rate of photosynthesis. The higher the concentration of carbon dioxide, the more bubbles will be produced and this means that the higher the rate of photosynthesis.
  

PLANT NUTRITION PART 11: ADAPTATION OF LEAVES FOR PHOTOSYNTHESIS

The term ADAPTATION means - the way in which a particular structure suits to perform a particular function.

LEAVES' ADAPTATION FOR MAXIMIZING THE RATE OF PHOTOSYNTHESIS

• Leaves are generally horizontally positioned on the plants so that more sunlight can be absorbed.
• Leaves are generally broad which offers a large surface area which enable the leaves to absorb as much sunlight as possible. Having broad shape also enables more carbon dioxide to diffuse in into the leaves. (Some plants having small leaves compensated this by having numerous of these small leaves)
  
• Most leaves are thin and carbon dioxide has to diffuse across only short distances to reach the inner cells.
• Some leaves have thick waxy cuticle on the upper surface of the leaves to prevent evaporation of water (Remember, water is one of the requirements for photosynthesis. So it must be conserved).
• More chloroplasts containing chlorophyll are located on the upper surface of the leaves (mainly in the palisade mesophyll cells) to maximize absorption of energy from sunlight by the chlorophyll (Remember, the upper surface of the leaves are more exposed to sunlight, so only right that there should be more chloroplasts on the upper surface of the leaves).
• The palisade mesophyll cells are vertically arranged ends-on so that more cells are exposed to sunlight and hence more sunlight can be absorbed by the chlorophyll in these cells.
• Lots of stomata are generally found in the lower surface of the leaves. Being on the lower surface reduces transpiration hence conserving water. Being numerous in number allow more carbon dioxide to diffuse in into the leaves.
• There are lots of intercellular air spaces in the spongy mesophyll layer. This enables carbon dioxide to be stored for use in photosynthesis. Apart from this, it allows gaseous exchange to take place.
• The presence of branching network of veins provides a good water supply to the photosynthesizing cells. The veins also help to remove the products of photosynthesis to other parts of the plants.
  

PLANT NUTRITION PART 10: IMPORTANCE OF PHOTOSYNTHESIS

WHY IS PHOTOSYNTHESIS IMPORTANT TO LIFE ON EARTH?

• Plants play an important role in maintaining the percentage of carbon dioxide at 0.03% in the atmosphere.
• There has been a gradual increase in carbon dioxide output globally.
• This increase is attributed to large scale deforestation, forest fires and the burning of fossil fuels.
• The carbon dioxide in the atmosphere traps heat. This heat-trapping effect of carbon dioxide is called the GREENHOUSE EFFECT which may lead to global warming which would increase the atmospheric temperature of the earth.
• So the importance of photosynthesis is that it helps to remove carbon dioxide from the atmosphere - this helps to greatly reduce the dangers of global warming.
• Apart from maintaining the percentage of carbon dioxide in the atmosphere, photosynthesis also helps to give out oxygen which is useful for other living organisms (for cellular respiration).
  

PLANT NUTRITION PART 9: FATE OF GLUCOSE

WHAT HAPPENS TO THE GLUCOSE PRODUCED DURING PHOTOSYNTHESIS?

• Glucose is used up by actively respiring cells during respiration to release energy.

• In the actively growing root tips and shoot tips, glucose is converted into cellulose to be used for making the cell wall needed for growth of the newly divided cells.

• Together with nitrates, sulphates and phosphates, glucose are used in the synthesis of amino acids which combined to form proteins which are converted into new protoplasm within the cells. Some of these newly synthesized proteins may be used to synthesize enzymes (Some of these proteins are also send to the storage organs for storage).

• Some of the glucose may be converted to sucrose in the storage organs like the seeds and fruits as well as the bulbs and tubers (like onion bulbs and potato tubers). Some of the glucose combined to form starch and this starch is then stored in the storage organs (Note: the potato tuber stores starch)
  

Onion Bulb

Potato Tuber

PLANT NUTRITION PART 8: IS OXYGEN PRODUCED DURING PHOTOSYNTHESIS?

Let us recall back the equation:


From the equation, it can be concluded that oxygen is produced as a by product of photosynthesis. Now, how do we know that oxygen is really produced? OK, here's the procedure:

• Set up the apparatus as shown in the following figure and make sure that the test tube is completely full of water.
• Place the apparatus in bright sunlight for a few days.
• Carefully remove the test tube from the top of the funnel, allowing the water to run out but not allowing the gas to escape.
• Light a wooden splint and then blow it out so that it is just glowing. Carefully put it into the gas in the test tube. If it relights the glowing splint, then the gas is oxygen.
  

CONTROLS
When setting up an experiment and a control, which of the two procedures constitutes the CONTROL depends on the way the prediction is worded. For example, if the prediction is that "in the absence of light, the pondweed will not produce oxygen", then the control is the plant in the light. If the prediction is that "the pondweed in the light will produce oxygen", then the control is the plant in darkness.

PLANT NUTRITION PART 7: IS CHLOROPHYLL NEEDED?

This experiment makes use of plants having variegated leaves (leaves in which in some parts chlorophyll is absent). The following figure shows an example of such plants.



PROCEDURE

• Destarch a potted plant with variegated leaves.
• Choose a leaf and make a simple drawing of the leaf showing clearly the regions which are green in colour as well as the regions which are not green in colour.
• Expose the plant to bright sunlight for 2 hours.
• After 2 hours removed the pre-chosen leaf and test it for the presence of starch using Iodine solution.

OBSERVATION

• Iodine solution changed to blue-black only in the green regions of the leaf whereas the Iodine solution remain unchanged in the non-green regions.
  

CONCLUSION

• Chlorophyll (in the green regions) is needed by green plants to photosynthesize.
  

PLANT NUTRITION PART 6: IS CARBON DIOXIDE NEEDED?

Carbon dioxide is one of the four requirements for photosynthesis to take place. Again, since it is a requirement, photosynthesis will not take place without it.

PROCEDURE:

• Set up two destarched potted plants as shown in the following figure.
• In set up X, a container containing saturated sodium hydrogen carbonate is placed (This is to provide the plant with carbon dioxide) whereas in set up Y, soda lime is used instead (This is to absorb the carbon dioxide inside the polythene bag as well as the carbon dioxide given off by microorganisms in the soil).
• Expose bot set ups to bright sunlight for two hours.
• After 2 hours, remove one leaf from each set up and test each leaf for the presence of starch using Iodine solution.
  

OBSERVATION

• The Iodine solution in the leaf taken from set up X turns blue black whereas the Iodine solution in the leaf taken from set up Y remain unchanged.
  

CONCLUSION

• The outcome of the experiment shows that carbon-dioxide must be present for plants to photosynthesize.
  

PLANT NUTRITION PART 5: IS LIGHT NEEDED?

The objective of this experiment is to investigate if light is one of the requirements for plants to photosynthesize.

PROCEDURE

• Destarch a potted plant.
• Cover one leaf of this destarched plant with a black paper with patterns cut in it (See the following figure).
• Expose the plant to bright sunlight for two hours.
• After 2 hours, test the leaf for the presence of starch using Iodine solution (Remember the procedure outlined in the previous post)
  

OBSERVATION

• The Iodine solution will change colour to Blue-black in the uncovered region whereas in the regions which is covered with black paper, the Iodine solution remain unchanged.

CONCLUSION

• Light is unable to penetrate in the region which is covered with the black paper, so no photosynthesis and therefore no starch is produced.
• Light penetrates in the uncovered region, so photosynthesis takes place and therefore starch is produced and detected when the leaf is tested with Iodine solution.
• The experiment shows that light from the sun is necessary for photosynthesis to take place.
  

PLANT NUTRITION PART 4: DESTARCHING AND TEST FOR STARCH IN LEAF

In the next part (part 5) of this chapter, practical investigations to prove whether chlorophyll, light and carbon dioxide are really needed for green plants to photosynthesize will be discussed. Before these practical investigations can be discussed, it is SOOOOO necessary for you to know HOW and WHY destarching is necessary.

First thing first. In investigating these three requirements, we need to ask ourselves - How do we know whether photosynthesis is really taking place or not? And then ask further - How do we know whether these requirements are responsible or involved in any ways, in photosynthesis? Well, the answer to the first one - If photosynthesis is really taking place, starch must have been produced, right? And the second question - By conducting an investigation where one requirement is deprived from the photosynthesizing plants. OK, enough about this. Lets move on to the real objective of this part of the chapter.

WHAT IS DESTARCHING?

• Removal of starch from the leaves.

WHY DESTARCHING?

• To ensure that at the beginning of the above mentioned investigations, there are no starch in the leaves of the plants to be used in the investigations. We really need to ensure that starch is really produced or not as a result of photosynthesis in the presence or absence of the above mentioned requirements.
  

HOW IS IT CONDUCTED?

• By placing the plants to be used in the investigations in a dark place, for example a dark cupboard for 24 to 48 hours (Darkness will stop photosynthesis in these plants and as a result all starch stored in the leaves will be used as a source of energy)
  

HOW DO WE KNOW IF DESTARCHING IS SUCCESSFUL?

• If we want to know whether the destarching is successful or not, we may take one of the leaf and test it for the presence of starch using Iodine solution.
• SUCCESSFUL??? It means, the outcome of the starch test must be NEGATIVE, the brown Iodine should not change. This means that starch is absent in the tested leaf - meaning, the DESTARCHING IS SUCCESSFUL.

IODINE TEST ON LEAF FOR THE PRESENCE OF STARCH
Starch test using Iodine solution? Hmm... peculiar right? You must be thinking about the same test like the one you did in your food test practical classes? Well....not that easy missy or mister. Here's the procedures:

Procedures:

• Heat some water to boiling point in a beaker and then TURN OUT the Bunsen flame.
• Use forceps to dip a leaf in the hot water for about 30 seconds (This kills the cytoplasm, denatures the enzymes and makes the leaf more permeable to Iodine solution)
• Push the leaf to the bottom of a test tube and pour in some alcohol (ethanol) into the test tube (The alcohol will boil and dissolve out most of the chlorophyll. This makes colour changes with Iodine easier to see)
  

• Pour the green alcohol into a spare beaker.
• Remove the leaf (brittle at this stage) and dip it once more into the hot water to soften it.
• Spread the decolourized leaf flat on a white tile and place a few drops of Iodine solution on to it.
• If the Iodine solution changed colour to Blue black, it means that starch is still present and it means that the destarching is not successful.
• If the Iodine solution does not changed colour, it means that starch is absent in the leaf and it means that the destarching is successful

Once you get the successful outcome, then you can start to investigate!!!

PLANT NUTRITION PART 3: PHOTOSYNTHESIS

Autotrophs such as green plants contain green pigments called chlorophyll (in the chloroplasts). A pigment absorbs certain wavelengths of light and reflects other. A chlorophyll pigment absorbs blue and red light and reflects green light. Therefore the leaves appear to be green.



When light is absorbed by green plants, the light energy is used to synthesize organic compounds such as sugars from inorganic compounds, water (absorbed by the roots of the plants from the soil) and carbon dioxide (obtained from the atmosphere and enters the plants through the stomata of the leaves by diffusion). This process is known as photosynthesis and can be summarized by the following equation.



The GLUCOSE produced during photosynthesis can be converted into other forms of carbohydrates such as starch (for storage) and other complex sugars (sucrose). Notice in the equation, oxygen is also produced as a by product. This oxygen is useful and can be used by other organisms and the plants themselves for respiration.

REQUIREMENTS (CONDITIONS) FOR PHOTOSYNTHESIS:

• Carbon dioxide
• Water
• Chlorophyll
• Light energy

These requirements must be present for plants to photosynthesize. Without one, plants will not be able to photosynthesize.

WHERE IN THE LEAF DOES PHOTOSYNTHESIS TAKING PLACE?
CHLOROPLASTS...... CHLOROPLASTS..... CHLOROPLASTS.... *Hitting the white screen with my purple broom stick*

NOTE: SINCE THE STUDENTS' PRESENTATIONS, I LONGED FOR MY PURPLE BROOM STICK!!!

PLANT NUTRITION PART 2: MODES OF NUTRITION

Living things may be classified according to their modes of nutrition. They may be HETEROTROPHS or AUTOTROPHS.

AUTOTROPHS

• These are organisms that can synthesize their organic materials from inorganic materials in the environment (OR in other words, they are able to make their own food).
• GREEN PLANTS must have come to you mind at this moment right? You are right. Yes green plants are examples of autotrophs but there are other organisms which can be considered as autotrophs like some bacteria.
• BUT for some bacteria, they are using chemical energy instead of light energy so for these bacteria we say that they are chemo-synthesizing instead of photosynthesizing (Remember, since green plants are using light energy from the sun, we say that they are photosynthesizing)
• An autotroph can be a multicellular organism or unicellular organism such as Euglena (Notice the chloroplasts containing chlorophyll?)

HETEROTROPHS

• These are organisms which cannot make their own food.
  
• A heterotroph obtains its food (hence source of energy) from the organic molecules that have already been produced by the autotrophs.
  

PLANT NUTRITION PART 1:THE LEAF

This chapter covers the ways in which plants make use of the carbon dioxide from the atmosphere and water from the soil to make food (carbohydrate) with the help of energy from sunlight which is trapped by chlorophyll in the chloroplasts. This is photosynthesis. Apart from this process, this chapter also covers the way in which plants convert this carbohydrate into proteins and other materials by using minerals absorbed from the soil. Before photosynthesis is to be discussed further, we must first pay tribute to the important organ of the plants in which this process occurs, THE LEAF.

The following figure shows the whole structure of a simple dicotyledonous leaf:


Note: Being able to draw a leaf accurately (based on observing a real specimen) is very important and commonly asked in the practical paper. Students should not take the proportion and the number of veins lightly when drawing a leaf. However drawing the vein network is usually not required but if you are asked specifically in the question to draw the vein network, drawing some (at least covering 10% of the leaf) should suffice. AND mind your labelling. By now, you should be able to identify and label the petiole, midrib, veins, lamina, leaf apex and axil

The following figure shows the internal structure of a dicotyledonous leaf (transverse section):


Can you identify the upper and lower epidermal layers, the palisade mesophyll layer, the spongy mesophyll layer and the intercellular air spaces?

The following figure shows the internal structure of a dicotyledonous leaf (3-Dimensional):


Note: Notice the positions of the xylem and the phloem in the vascular bundle. In a leaf the xylem is always above the phloem (And just for comparison: the phloem is on the outer side while the xylem is in the inner side of the stem). Can you visualise how the vascular bundles in the stem are connected to those in the leaves?