Sarah Huggens LIMITING FACTORS OF PHOTOSYNTHESIS “THE FACTOR THAT IS PRESENT AT THE LOWEST OR LEAST FAVOURABLE VALUE.”

Light Intensity Constant temperature, limiting factor is light intensity. No light intensity, no photosynthesis. As light intensity increases, so will rate of photosynthesis until it plateaus at higher temperatures. Light intensity is no longer the limiting factor. Light intensity is directly proportional to R.O.P 3 effects of light:  Stomata open – CO 2 enters leaves  Trapped by chlorophyll – excites electrons  Splits water molecules to produce protons – use in chemiosmosis and to produce reduce NADP for the light independent reaction.

How does this link to the Calvin Cycle? More light energy available to excite more electrons for the light dependent reaction. Electrons – photophosphorylation, production of ATP and reduced NADP. These are used as sources of hydrogen and energy in the Calvin Cycle to reduce GP to TP. If there is no light – GP cannot be reduced to TP, so GP accumulates and levels of TP will fall. This will also mean that less TP can be regenerated to RuBP and this will also fall.

Carbon Dioxide Increasing CO 2 will increase the rate of photosynthesis until it plateaus. Temperature is now the limiting factor. 500 million years ago – 20x more CO 2 than there is now, and this fell during Carboniferous period. Gradually decreased until industrialisation began. Burning fossil fuels now increases CO 2 levels and releases CO 2 that was fixed during CF period. Oceans – CO 2 sinks. Greenhouses – CO 2 levels drop to around 0.02%. Introduce more CO 2 by burning methane or oil-fired heaters. Enhanced CO 2 = increase R.O.P as long as there is no other limiting factor

How does this link to the Calvin Cycle? More carbon dioxide – more GP > more TP and more RuBP However, lots of stomata open – increased transpiration and may lead to wilting. This sometimes triggers the stress response of closing the stomata (less CO 2 ) If CO 2 falls below 0.01% = RuBP will accumulate. Levels of TP and GP fall.

Temperature Increasing temperature can increase R.O.P but will reach a plateau. Temperature too high (above 25 o C), enzymes involved in Calvin Cycle (i.e rubisco) become less efficient – oxygen more successfully competes for active site than CO 2. Between 0 o C and 25 o C, R.O.P doubles for each 10 o C rise. Light dependent stage not very affected by temperature. High temperatures cause more water loss from stomata. Stress response – close stomata limiting CO 2 availability.

How does this link to the Calvin Cycle? As temperatures rise above 25 o C the oxygenase activity of rubisco increases more than carboxylase activity. More photorespiration than photosynthesis. ATP and NADPH from light dependent are wasted. Damage of proteins. Increase in temperature = increase in water loss via transpiration. Stress response.

Measuring rate of photosynthesis LIGHT INTENSITY R.O.P usually found by measuring the oxygen production – photosynthometer. Oxygen produced collects in capillary tube. Gas bubble in the capillary tube can be manipulated to be against the scale, and it’s length can be measured. The length the bubble travels can be converted into volume if the radius of the capillary is known. (πr 2 ) Can compare rates if same apparatus is used throughout, and temperature is kept constant. Darkened room – only light available is from the light source which can be manipulated. (However this light may produce a significant amount of heat.) Need to allow plant to acclimatise to conditions Sodium hydrogencarbonate provides CO 2 for efficient photosynthesis.

Measuring rate of photosynthesis TEMPERATURE Keep all other factors constant. Light intensity that produces high rate of photosynthesis. Alter temperature of water bath and measure the volume of gas produced. Accuracy – solubility of oxygen. CARBON DIOXIDE All factors constant except for number of drops of sodium hydrogencarbonate. Measure volume of gas produced.

Leaf Discs Cut leaf discs using a borer or straw. 5 discs in syringe (10cm 3 ) Half fill syringe with NaHCO 3 Place finger over end of syringe and pull the plunger. This removes air from the air spaces of spongy mesophyll. Air replaced by NaHCO 3 – density increases and discs sink. Illuminate syringe from above – time it takes to float. 1/t = rate of photosynthesis.