3 - PLANT NUTRITION AND TRANSPORT 

Photosynthesis 

• Produces food for plant (glucose) 
• Happens in the leaves of all green plants 
• Happens inside chloroplasts - in leaf cells and other green parts of plant, contain green pigment (chlorophyll) 
• Chlorophyll - green pigment, absorb sunlight, uses its energy to convert carbon dioxide and water into glucose (oxygen also produced - waste product) 
• Converts light energy to chemical energy (stored in the glucose) 
• Chemical energy released when glucose is broken down during respiration 

Leaves - 

•  Broad - large surface area exposed to light 
• Most chloroplasts found in palisade layer - near top of leaf, more exposure to light 
• Upper epidermis - transparent, light can pass through it to palisade layer 
• Network of vascular bundles - xylem and phloem (transport vessels), deliver water and nutrients, take away glucose produced, support leaf structure 

• Waxy cuticle - reduces water loss by evaporation 
• Adaptions of leaves for efficient gas exchange also make photosynthesis more efficient (eg: lower surface full of small holes - stomata, let carbon dioxide diffuse directly into leaf) 

Rate of photosynthesis  

Limiting factor - 

• Depends on environmental conditions (eg: winter - low temp., night - light etc.) 
• Stop photosynthesis from happening any faster 
• Light intensity, CO2, concentration, temperature 

Limiting factor: light 

• Chlorophyll uses light energy to perform photosynthesis 
• Light intensity increased - rate of photosynthesis increases steadily (only up to a certain point - only limiting factors are then either CO2 levels or temp) 
• Not enough light = slows down rate 

Limiting factor: carbon dioxide 

• CO2 - raw material needed for photosynthesis (only about 0.04% of air, fairly scarce for plants) 
• Increases rate of photosynthesis up to a point 
• Graph flattens out - no longer the limiting facto 

Limiting factor: temperature 

• Affects enzymes involved 
• Temp increases - rate of photosynthesis increases up to a point 
• Too high - denatures enzymes 

Experiments 

Testing leaf for starch - 

• Put in boiling water with tweezers/forceps - stops any chemical reactions happening inside leaf 
• Put in boiling tube with ethanol 
• Heat in water bath - gets rid of any chlorophyll, leaf end up pale/white 
• Rinse leaf in cold water 
• Add a few drops of iodine solution
• Present = blue/black 

Showing whether photosynthesis is taking place (starch tests) - 

1 - Chlorophyll 

• Use variegated leaves - only green parts contain chlorophyll (eg: ivy leaves) 
• Take variegated leaf from plant that has been exposed to light 
• Record which bits are green etc. 
• Test leaf for starch (previous experiment) 
• Present = blue/black (will only occur in green parts of leaf) 
• Only parts of the leaf that contained chlorophyll are able to photosynthesise and produce starch 

2 - CO2 

• Set up plant and dish of soda lime in bell jar (sealed) 
• Light shining through bell jar 
• Soda lime will absorb CO2 out of the air in the jar 
• Test leaf for starch (with iodine solution) after set period of time = none present 
• No starch been made in the leaf, CO2 needed for photosynthesis 

3 - Light 

• Plant that has grown without any light 
• Cut leaf from plant - test for starch using iodine solution 
• Light needed for photosynthesis 

Showing rate of photosynthesis (oxygen production) - 

1 - Light 

• Canadian pondweed used to measure the effect of light intensity on the rate of photosynthesis - rate at which the pondweed produces oxygen corresponds to the rate at which photosynthesis is taking place 
• The faster the rate of oxygen production, the faster the rate of photosynthesis 

1. Source of white light placed at a specific distance from pondweed 

2. Leave to photosynthesise for set time 

3. Syringe draws up gas produced 

4. Record results 

5. Repeat with light at different distances 

• Can be altered to measure effect of temperature or CO2 on photosynthesis 
• eg: put in beaker of water, different concentrations of CO2 bubbled through 
• eg: changing temp of water