Plant Nutrition: Photosynthesis Factors

Plant Nutrition: Factors Affecting Photosynthesis

• Controls & Experimental Groups:
  • Control groups receive all necessary substances, the substance being tested is NOT included.
  • Experimental groups receive everything except the substance being tested
  • Experimental control is designed to test a certain variable.

Plant Nutrition: Testing Leaves for Starch

• Destarching Plants: Plants are left in a dark cupboard for 24 hours. This causes them to use up their stored starch reserves as they cannot photosynthesize in the absence of light.
• Importance of Destarching: Ensures any starch detected is generated during the experiment, not stored before.

Experimental Skills 6.1: Testing a Leaf for Starch

• Purpose: To identify if a leaf has been photosynthesizing
• Materials: 250 cm³ beaker, burner, tripod and gauze, boiling tube, some iodine solution, forceps, alcohol (methylated spirits), fresh green leaf, white tile.
• Safety Precautions: Alcohol is flammable, do not collect the alcohol until you have turned out the burner, put a damp cloth over the alcohol if it burns.
• Method:
  1. Boil water in a beaker.
  2. Place a leaf in boiling water to break down cell membranes and denature enzymes.
  3. Turn out the flame.
  4. Place leaf in alcohol to dissolve the chlorophyll.
  5. Remove leaf from alcohol and place in hot water.
  6. Place leaf on white tile and add iodine solution
  7. Blue-black color indicates presence of starch.

Experimental skills 6.1/Questions and Answers

• Chlorophyll Dissolving: Alcohol dissolves chlorophyll, a green pigment in leaves
• Boiling Water: Boiling water breaks down cell membranes, allowing chlorophyll to leave.
• Observation/Conclusion: Blue-black color indicates starch. This suggests photosynthesis occurred.

Experimental Skills 6.2: Investigating the Necessity for Chlorophyll for Photosynthesis

• Variegated Leaves: Leaves with both green and white patches, green parts contain chlorophyll, while white parts do not.
• Method:
  1. Draw the variegated leaf, label green and white parts.
  2. Test the leaf for starch, following the procedure described earlier.
  3. Record observations by drawing and labeling the leaf after adding iodine solution.

Experimental Skills 6.2/Questions and answers

• Starch Location: Green parts of the leaf contained starch, indicating chlorophyll presence.
• Conclusion: Chlorophyll is essential for photosynthesis, as starch is only present in the parts of the leaf where chlorophyll was present.

Experimental Skills 6.3: Investigating the Necessity for Light for Photosynthesis

• Method: Cover part of a leaf with black paper to block light.
• Method: 1. Create a shape or initials in black paper to attach to leaf on plant. 2. Attach to leaf on plant. 3. Place plant in a sunny area for 24 hours. 4. Remove leaf and paper. 5. Test leaf for starch following the procedure. 6. Record results by drawing and labeling the leaf after adding iodine solution

Experimental skills 6.3/Questions and answers

• Importance of Destarching: Destarching ensures that any observed starch is a product of the experiment's new light exposure and not from previous, non-experimental, photosynthesis.
• Duration of Experiment: The leaf must remain attached for at least 24 hours

Experimental Skills 6.4: Investigating the Necessity for Carbon Dioxide for Photosynthesis

• Materials: Healthy potted plant, 2 transparent containers (e.g., conical flasks), 2 rubber stoppers with slits to fit, potassium hydroxide solution, distilled water, petroleum jelly,
• Method: Design and set up apparatus, place plant in either one of two containers labeled one carbon dioxide and the other without.
  1. Set up the apparatus as in Figure 6.20, taking note of precautions to prevent air entering.
  2. Fill one flask with potassium hydroxide (to remove carbon dioxide) and other flask with distilled water.
  3. Place the leaf stalk in one rubber bung carefully positioning into one of the flasks and securing with the bung.
  4. Repeat the process with another leaf.
  5. Place setup in a light place for 1 day.
  6. Remove the apparatus from the light.
  7. Test leaves for starch.

Experimental Skills 6.4/Questions and answers

• Control: A leaf in distilled water serves as a control, allowing the researcher to observe any discrepancies due to carbon dioxide’s presence.
• Purpose of Potassium Hydroxide: The potassium hydroxide absorbs carbon dioxide, creating a comparative testing space for the effect of carbon dioxide absorption.

Experimental Skills 6.5: Investigating the Effect of Varying Light Intensity on the Rate of Photosynthesis

• Materials: Healthy aquatic plant (with stems), Beaker, Water, Lamp, Ruler, and timer
• Method: Measure the distance between the lamp and the plant.
  1. Collect a piece of aquatic plant. Cut off a piece about 7 to 9 cm long to be able to use in the experiment.
  2. Set up the apparatus and attach the paper clip to the top of the plant. Ensure the end of the cut stem floats upwards.
  3. Place lamp as close to beaker as possible and measure distance.
  4. Record the number of bubbles produced by the cut stem of the aquatic plant for 1 minute of light intensity using a timer for time measurement.
  5. Repeat steps 2-3 five different times (increasing the light intensity/ distance from the plant from the lamp) to get reliable data sets.
  6. Create a line graph with distance on the x-axis and mean number of bubbles (over 3 trials) produced in 1 minute on the y-axis.

Plant Nutrition: Limiting Factors in Photosynthesis

• Limiting Factors: These are factors that reduce the rate of photosynthesis. These include Light intensity, Carbon dioxide concentration, and Temperature.
• Light Intensity: Photosynthesis increases with light intensity, until the maximum rate is reached and another limiting factor takes over.
• Carbon Dioxide Concentration: Photosynthesis increases as carbon dioxide concentration increases, only up to a certain point where a different limiting factor takes over.
• Temperature: Enzymtic reactions limit the rate of photosynthesis at lower temperatures. As the temperature increases, the reactions also increase until an optimal temperature is reached.

Two Limiting Factors at Once

• When there is more than one limiting factors in an experiment, it is necessary to create different values/ conditions for the chosen limiting factor to observe the rate of photosynthesis and how different levels of the selected variable affect the rate.

Stomata

• Stomata are openings in plant leaves; carbon dioxide enters through stomata, allowing for photosynthesis.
• Stomata may close in hot or bright conditions, due tothe loss of water, reducing photosynthetic rates.

Glasshouses: Optimizing Conditions

• Temperature: Glasshouses can regulate temperature, either heating or allowing for ventilation to maintain optimal conditions. This is essential for some plant types.
• Water: Automatic watering systems are often used in glasshouses to ensure optimal water availability. Water needs can be altered according to plant type.
• Light Intensity: For insufficient light, artificial light or reflectors help to increase the available light. If the light intensity is too high, blinds/shade can be used to reduce the intensity.
• Carbon dioxide: Carbon dioxide can be increased via addition of carbon dioxide gas, which can increase photosynthesis rates .