MLB 111 UNIT 3
STUDY UNIT 3: Photosynthesis
NUMBER OF LECTURES: 5
LECTURER: Dr N Olivier
REFERENCE: Hillis et al., Chapter 10
Core Concepts:
3.1 Energy in light
3.2 Energy transfer to chemical energy
3.3 Carbohydrate synthesis
STUDY OBJECTIVES:
You have mastered this study theme when you can:
1. Distinguish among autotrophic and heterotrophic organisms by giving definitions and
examples. (p197)
Autotrophs
• able to use only water and CO2 to make their own carbohydrates which can be used
directly or converted into other types of molecules (e.g. cellulose). Uses sunlight
energy
• for anabolic and catabolic processes
• e.g. plants, algae, cyanobacteria
Heterotrophs
• cannot make own carbohydrates
• must consume other organisms (autotrophs or heterotrophs) to obtain molecules for
their activities
• e.g. animals, fungi, most bacteria
2. Write down the chemical equation that summarizes the overall reaction of
photosynthesis and relating it to the chemical equation that summarizes the overall
reaction of aerobic respiration. (p197)
Photosynthesis:
Sunlight + 6CO2 + 6H2O -> C6H12O6 + 6O2
*C6H12O6 is glucose
Process cannot happen without sunlight
Respiration: (opposite reaction to photosynthesis)
C6H12O6 + 6O2 -> 6CO2 + 6H2O + free energy
1
,3. Describe the pioneering experiment of Ruben et al. that proved that oxygen released
during photosynthesis was released from H2O and not CO2. (p197-198)
Experiment done in 1941. Using isotopes of oxygen to identify source of O2 produced
during photosynthesis.
O2 comes from H2O, not CO2:
Water is seen as a reactant - shown on both sides of the equation as it is also released as a
product. This is a redox reaction.
So revised equation: 6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O
(*either version of equation is accepted)
How it was proved:
• Carbon dioxide was made (labelled) with the 18O2 isotope ("heavy" oxygen)
• Other isotope is 16O2
• Experiment was repeated where H2O was labelled with 18O2 isotope.
• In that version, the resultant O2 was the heavy version - showed that it came from the
water not the carbon dioxide.
4. Name the two stages of photosynthesis and argue how these provide evidence for the
endergonic / anabolic nature of photosynthesis. (p199)
1. Light reactions
• light-dependent reactions
• convert light energy into chemical energy
• products: ATP and NADPH
• NADPH plays role in many reactions - reducing agent in photosynthesis and other
anabolic reactions.
2. Carbon-fixation reactions
• light-independent reactions
2
, • uses ATP and NADPH (from light reactions) and CO2 (gas!)
• product: carbohydrates
• anabolic/building
5. Identify the elements of stroma, thylakoid disks, grana, inner and outer membranes of
chloroplasts in a plant cell and apply this knowledge to discuss where the light-dependent
and light-independent reactions of photosynthesis take place. (p199)
Remember diagram/structure of chloroplast from study unit 1. Stroma is inside inner
membrane. Chloroplast has three membranes. Light reaction is in thylakoid membrane.
Note light reactions vs carbon-fixation reactions.
Light independent reactions - stroma
Light dependent reactions - thylakoid
6. Describe the relationship between wavelength and energy in the electromagnetic
spectrum. (p199-200)
Energy in light
• the amount of energy in electromagnetic radiation is inversely proportional to its
wavelength
• longer wavelength = lower energy
3
STUDY UNIT 3: Photosynthesis
NUMBER OF LECTURES: 5
LECTURER: Dr N Olivier
REFERENCE: Hillis et al., Chapter 10
Core Concepts:
3.1 Energy in light
3.2 Energy transfer to chemical energy
3.3 Carbohydrate synthesis
STUDY OBJECTIVES:
You have mastered this study theme when you can:
1. Distinguish among autotrophic and heterotrophic organisms by giving definitions and
examples. (p197)
Autotrophs
• able to use only water and CO2 to make their own carbohydrates which can be used
directly or converted into other types of molecules (e.g. cellulose). Uses sunlight
energy
• for anabolic and catabolic processes
• e.g. plants, algae, cyanobacteria
Heterotrophs
• cannot make own carbohydrates
• must consume other organisms (autotrophs or heterotrophs) to obtain molecules for
their activities
• e.g. animals, fungi, most bacteria
2. Write down the chemical equation that summarizes the overall reaction of
photosynthesis and relating it to the chemical equation that summarizes the overall
reaction of aerobic respiration. (p197)
Photosynthesis:
Sunlight + 6CO2 + 6H2O -> C6H12O6 + 6O2
*C6H12O6 is glucose
Process cannot happen without sunlight
Respiration: (opposite reaction to photosynthesis)
C6H12O6 + 6O2 -> 6CO2 + 6H2O + free energy
1
,3. Describe the pioneering experiment of Ruben et al. that proved that oxygen released
during photosynthesis was released from H2O and not CO2. (p197-198)
Experiment done in 1941. Using isotopes of oxygen to identify source of O2 produced
during photosynthesis.
O2 comes from H2O, not CO2:
Water is seen as a reactant - shown on both sides of the equation as it is also released as a
product. This is a redox reaction.
So revised equation: 6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O
(*either version of equation is accepted)
How it was proved:
• Carbon dioxide was made (labelled) with the 18O2 isotope ("heavy" oxygen)
• Other isotope is 16O2
• Experiment was repeated where H2O was labelled with 18O2 isotope.
• In that version, the resultant O2 was the heavy version - showed that it came from the
water not the carbon dioxide.
4. Name the two stages of photosynthesis and argue how these provide evidence for the
endergonic / anabolic nature of photosynthesis. (p199)
1. Light reactions
• light-dependent reactions
• convert light energy into chemical energy
• products: ATP and NADPH
• NADPH plays role in many reactions - reducing agent in photosynthesis and other
anabolic reactions.
2. Carbon-fixation reactions
• light-independent reactions
2
, • uses ATP and NADPH (from light reactions) and CO2 (gas!)
• product: carbohydrates
• anabolic/building
5. Identify the elements of stroma, thylakoid disks, grana, inner and outer membranes of
chloroplasts in a plant cell and apply this knowledge to discuss where the light-dependent
and light-independent reactions of photosynthesis take place. (p199)
Remember diagram/structure of chloroplast from study unit 1. Stroma is inside inner
membrane. Chloroplast has three membranes. Light reaction is in thylakoid membrane.
Note light reactions vs carbon-fixation reactions.
Light independent reactions - stroma
Light dependent reactions - thylakoid
6. Describe the relationship between wavelength and energy in the electromagnetic
spectrum. (p199-200)
Energy in light
• the amount of energy in electromagnetic radiation is inversely proportional to its
wavelength
• longer wavelength = lower energy
3
