Edexcel CB6



During photosynthesis, photosynthetic organisms (green plant & algae) use energy from sun to make glucose.

Some glucose is to make larger, complex molecules that plants or algae needs to grow, make up organisms biomass- mass of living organisms.

Energy sstored in organisms' biomass works its way through area te main producers of food for nearly all life on Earth.

Photosynthesis happens inside chloroplasts- contains chlorophyll which absorbs light, energy transferred to chloplasts by light. 

Equation: Carbon dioxide + Water ----> Glucose + Oxygen

Photosynthesis is an endothermic reaction- energy is taken in during reaction.

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PRACTICAL- Investigating Light Intensity

Pondweed/algal balls can be used to measure effect on rate of photosynthesis, rate at which pondweed produces oxygen corresponds to rate at which it's photosynthesising- faster rate of oxygen produced, faster rate of photosynthesis.


  • Apparatus is set up, gas syringe should be empty to start with sodium hydrogencarbonate may be added to water to make sure plant has enough carbon dioxide. 
  • Source of white light is place at a specific distance from pondweed/algal balls.
  • Pondweed/algal balls is left to photosynthesise for set amount of time.
  • As it photosynthesises, oxygen released will collect in gas syringe. Allows you to accurately measure volume of oxygen produced. 
  • Whole experiment is repeated with light source at different distances from pondweed/algal balls. Rate of oxygen produced at each distance can be calculated (Volume produced/ time taken)
  • For this experiment any variables that could affect results should be controlled e.g. temperature- can be controlled by putting conical flask in water bath & Carbon dioxide concentration- can be controlled by adding set amount of sodium hydrogen to a set volume of water.
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Limiting factors

Limiting Factors

Light Intensity: Distance from lamp and light intensity are inversely proportional to eachother- as distance increases, light  intensity decreases. However light intensity decreases in proportion to square of distance called Inverse square law: Light intensity is directly proportional to 1/distance squared

Carbon Dioxide Levels

Increasing carbon dioxide concentration increases photosynthesis up to a point, after the graph flattens out showing carbon dioxide isn't the limiting factor anymore, as long as light and carbon dioxide are in supply the limiting factor is temperature.


Limiting factor because if it's too low- enzymes needed for photosynthesis work more slowly at low temp.- if plant gets too hot enzymed needed for photosynthesis will denature happens at about 45 degrees celcius.

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Transport in Plants

Root Hair Cells

  • Gives plant a large surface area for absorbing water and mineral ions from soil.
  • Concentation of mineral ions is usually higher in root hair cells than in soil around them so mineral ions are absorbed by active transport, water is absorbed by osmosis.


  • Made out of colums of elongated living cells with small pores in end walls, allows stuff to flow through
  • transports food substances (mainly sucrose) made in leaves to rest of plant for immediate use (growing regions) or storage- translocation requires energy from respiration, transport goes in both directions.

Xylem Tubes

  • Made of dead cells joined end to end with no end walls between them & a hole down middle, strengthened with material called lignin. 
  • Carry water & mineral ions from roots to stem & leaves. 
  • Movement of water from roots through xylem & out of leaves- transpiration stream
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Caused by evaporation & diffusion of water from plant's surface- most transpiration happens in leaves.

Loss of water creates slight shortage of water in leaf & so more water is drawn up from rest of plants through xylem vessels to replace it.

Means more water is drawn up from roots & there's a constant transpiration stream of water through the plant. 

Transpiration stream carries mineral ions- dissolved in water along with it. 

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  • Has tiny pores on surface of plant
  • Mostly found on lower surface of leaves.
  • Allows carbon dioxide & oxygen to diffuse directly in & out of a leaf.
  • Also allows water vapour to escape during transpiration,
  • Transpiration is the side effect of the way, leaves are adapted for photosynthesis.
  • Have to have stomata so gases can be exchanged easily- more water inside plant than in air outside- water escapes from leaves through stomara by diffusion.
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Transpiration Rate

Environmental Factors:

  • Light Intensity- brighter the light, greater the transpiration rate. Stomata begin to close as it gets darker- photosynthesis can't happen in the dark, so they don't need to be open to let carbon dioxide in- when stomata is close very little water can escape.
  • Temperature- warmer it is, faster transpiration happens. When it's warm water particles have more energy to evaporate and diffuse out of stomata.
  • Air Flow- better the air flow around leaf (stronger wind, greater transpiration rate) If air flow around leaf is poor, water vapour just surrounds leaf & doesn't move away, this means there's a high concentration of water in air outside leaf, diffusion then happens quickly from an area of higher concentration to an area of lower concentration.


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Estimation of Transpiration Rate

  • You can use potometer to estimate transpiration rate- measures water uptake by plant but it's assumed that water uptake by plant is directly related to water loss from leaves (transpiration)
  • Set up apparatus and then record starting position of air bubble. 
  • Start a stopwatch and record distance moved by bubble per unit time (per hour) calculating speed of air bubble movement gives an estimate of transpiration rate.
  • You can use potometer to estimate how light intensity, temperature or air flow around plant affect transpiration rate. Remember to only change one variable at a time and control the rest. 
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