Photosynthesis

Question 1

What is photosynthesis, where does it occur and what is the chemical equation?

Answer 1

• Production of carbon compounds in cells using light energy and CO2
• Takes place in chloroplasts
  Equation:
  6CO2 + 6H2O –> (light) C6H12O6 + 6O2

Question 2

Out of what two stages is photosynthesis made of?

Answer 2

1. Light-dependent reaction on the thylakoids of chloroplasts, involves photolysis to produce O2
2. Light-independent reaction in the stroma of chloroplasts, involves carbon fixation to produce carbohydrates

Question 3

What light and wavelength is required for photosynthesis to occur?

Answer 3

• Visible light with wavelength between 400 and 700 nanometers, violet has the shortest wavelength and red the longest

Question 4

What makes chloroplasts look green? Link this to wavelength and the visible light spectrum?

Answer 4

• Chlorophyll a and chlorophyll b
• Chlorophyll absorbs red and blue light most effectively and reflects green light more than other colours.
• Most other wavelengths (photons) are absorbed

Question 5

What is the action spectrum?

Answer 5

• Shows the rate of photosynthesis depending on the various wavelengths of light
• It indicates which wavelengths are most efficient in photosynthesis
• Rate of photosynthesis is lowest at 550nm
• Be able to draw one

Question 6

What is the absorption spectrum?

Answer 6

• Shows which wavelength of visible light is absorbed by chlorophyll a or b. It is measured by a spectrometer
• It shows the percentage of light absorbed at each wavelength
• Be able to draw one

Question 7

What is photolysis?

Answer 7

• The first stage of photosynthesis in which water is split by light
• This is where oxygen is produced
• The energy in photons in used to split water molecules
  Equation:
  2H2O + photons –> 4e- + 4H+ + O2
• Electrons generate ATP, H+ used later on and O2 diffuses out of the plant

Question 8

How are carbohydrates produced in photosynthesis?

Answer 8

• Energy absorbed from light helps turn CO2 and H2O into carbohydrates and other carbon compounds
• This is an anabolic reaction, small molecules are joined to form large ones
• Using ATP and hydrogen, CO2 is converted into organic molecule

Question 9

What is the Calvin cycle?

Answer 9

• The light-independent cycle of chemical reactions where CO2 is assimilated to make sugars
  CO2 + H2O –> carbohydrate + O2

Question 10

What are limiting factors? Which factors exist in photosynthesis?

Answer 10

• A factor that restricts the rate of reaction when present in low amount
• Temperature, light intensity and CO2 concentration
• Water only under extreme conditions

Question 11

How can light intensity be a limiting factor?

Answer 11

• Rate of photo. slows down at a lack of light, rate increases up to a certain point with sufficient supply
• Reaches a maximum at which the rate can no longer increase
• This is because at a certain point all photo systems are occupied by photons
• The effect light intensity has can be measured by the CO2 uptake or O2 output

Question 12

How can CO2 concentration be a limiting factor?

Answer 12

• CO2 concentration (or its uptake) eventually also reaches a limit
• This is because all active sites of the enzymes involved in the production of carbohydrates are occupied, no further increase possible

Question 13

How can temperature be a limiting factor?

Answer 13

• Optimum temperature differs between plants
• Temperatures beyond the optimum level, the enzymes start to denature and rate of photosyn. decreases and eventually stops

Question 14

Explain limiting factors?

Answer 14

• Once the rate remains constant, another factor is limiting and the plant reached its maximum potential
• At optimum temp. the rate is limited by light intensity and CO2 concentration

Question 15

How do you design an experiment?

Answer 15

• Using aquatic plant
• Chose the independent variable (limiting factor) and list control variables
• Decide on the number of repeats for each value of the independent variable
• Measure volume of CO2 produced to measure rate of photosyn.

Question 16

How does photosynthesis change the earth’s atmosphere, oceans and rock deposition?

Answer 16

• The rise of oxygen concentrations were used in the oxidation of minerals such as iron and ended up in sediments and rocks
• Algae and other organisms in the oceans have increased the O2 concentration, called partial pressure
• When the earth was covered with oceans, the atmospheric oxygen started to build up where the multicellular organisms evolved
• Cyanobacteria contributed to the rise in O2 through photosynthesis

Question 17

What is chromatography?

Answer 17

• Technique used to separate mixtures of substances based on the movement of the different substances by capillary action
• The mobile phase is the solvent (alcohol) used to develop the chromatogram
• The paper is the stationary phase

Question 18

What are the different pigments in a green leaf?

Answer 18

• Chlorophylls: beta-carotene and xanthophyll
• The pigments are extracted from the leaves using solvents and then transferred on the chromatography paper.
• The pigments move at different rates on the stationary phase so they separate out to form a chromatogram

Question 19

What is the Rf value?

Answer 19

• Ratio of distance moved by pigment to the distance moved by the solvent
  Rf = distance of sample/distance of solvent
• The Rf values can be compared to the known Rf values of plant pigments to identify them

Question 20

Why are thin layer chromatography better than paper chromatography?

Answer 20

• Gives a better result as well-defined and well-separated spots are obtained