HL 1 Biology

3.8b Photosynthesis


the production of organic substance/food (including carbohydrates, proteins and lipids), using light energy and simple inorganic substances (such as carbon dioxide and water).” 

  • an example of energy conversion (since light energy converts to chemical energy)
  • prokaryotes = first organisms to perform photosynthesis
      • photosynthesis increase; due to rise in oxygen concentration of atmosphere

<<Pigments and Light Absorption>>

First stage: Absorption of Sunlight

    • visible light: form of electromagnetic radiation
    • Sunlight: a mixture of different wavelengths of visible light (shown as red, green, blue)

Light absorption involves pigments: chemical substances which absorbs light.

    • pigments that absorbs all colors = appear black
        • since no light is emitted
    • pigments that absorbs most colors = the color not absorbed is shown to us
        • only that part of the sunlight is not absorbed
        • and is reflected through our eyes
    • Chlorophyll: the main pigment used to absorb light in photosynthesis


“the main photosynthetic pigment”

    • its structure: helps absorb some colors/wavelengths of light better than others
        • red and blue light are absorbed more than green
    • green light that chlorophyll can’t absorb is reflected
        • thus, makes  chlorophyll. chloroplasts,  plant leaves look green

Some of the light energy absorbed by chlorophyll is used to:

    • produce ATP from ADP + phosphate
    • split water molecules and release hydrogen (photolysis of water)
        • hydrogen later used to make carbohydrate
        • splitting water releases oxygen = a toxic waste product
    • ATP and hydrogen are used to fix carbon dioxide (to produce organic molecules)
    • *Carbon fixation: conversion of carbon in a gas to carbon in solid compound

<<Factors that Affect the Rate of Photosynthesis>>

1. Light Intensity


    • at low light intensity = rate increases as light intensity is increased
    • at high light intensity = further increase has no effect; rate remains plateau (unless factor preventing increase is altered)

2. Carbon Dioxide Concentration


    • at low CO2 concentration = rate increases as concentration is increased
    • at high CO2 concentration = further increase has no effect

Carbon dioxide is essential; needed to make organic molecules

  • Carbon fixation: conversion of CO2 into solid or liquid carbon compounds
  • ATP needed to provide energy and hydrogen from photolysis
  • in darkness: carbon fixation stops and in low light intensitites slows down due to shortage of ATP and hydrogen

3. Temperature

Screen shot 2013-01-06 at 11.01.27 PM

    • at low temp. = rate is very low or zero
    • as temp. increases = rate increases until it reaches optimum temp. (max)
    • above optimum temp. = rate decreases and eventually becomes zero


Measuring the Effect of Temperature by Data Logging (p. 105)

(1) pH is monitored because the pH is raised during photosynthesis, when carbon dioxide is used. The pH is lowered when the carbon dioxide made by respiration is dissolved in the water. By monitoring the high and low pH, a distinct observation can be made when photosynthesis is occurring.

(2) The independent variable: temperature/time and dependent variable: pH.

(3) *Each line could be labeled for their respective temperatures. So, on the x-axis: time and on the y-axis: pH

(4) The optimum temperature is hard to conclude, because of the uncertainty of the pH sensor used in the experiment.Yet, the optimum temperature could range from 22.5 to 27.5˚C according to table 2. Also, although the temperature was varied, the pH at the end of the 4 hours does not show much difference which also leads to inaccuracy in finding the optimum temperature.

(5) There will be differences in the results because a different alga will have a different optimum temperature and optimum rates of photosynthesis.

(6) Since the temperature intervals are too small and the time was measured for a short span, the pH changes resulted to be limited. Also, the pH sensor is not precise enough to clearly show a difference. Thus, by measuring different variables such as the CO2 production or oxygen production could be suggested. As each respective sensor for dissolved oxygen or carbon dioxide in the water, it may show a more precise data due to a greater sensitivity.


Exam Question: Study Guide (pg. 22)

(2a) (i) Light = decrease in CO2 concentration,  Dark = increase in CO2 concentration

(ii)  Warmer = decrease in CO2 concentration, Cooler = increase in Co2 concentration

(b) Light intensity

(c) i) Rises in CO2 production as respiration produces CO2 (ii) Falls in Co2 concentration as photosynthesis uses up  CO2



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