Photosynthesis is the primary energy source for life on Earth, utilizing sunlight to convert carbon dioxide and water into oxygen and glucose. It takes place through two key sets of reactions - the light-dependent reactions that use sunlight to produce ATP and NADPH, and the light-independent Calvin cycle that uses these products to fix carbon into sugars. The rate of photosynthesis is influenced by several environmental factors, including light intensity, temperature, carbon dioxide concentration, and water availability, with the lowest level determining the overall rate.

1. Photosynthesis

2. Our Primary Energy
Source
• What is the primary energy source for life on
this planet?
The Sun

3. The Nature of Light
• Visible light is the component
of the Electromagnetic
spectrum of energy that a
plant uses for photosynthesis.
Spectrum – range of wavelengths
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5. What Colors?
• Spectrum of light for
photosynthesis:
• 400nm – 700 nm
Broken down by wavelength:
–400-525 nm – violet-blue
–525-625 nm - green-yellow
–625-700 nm – orange-red

6. Action Spectrum
• Compares the
wavelength of light and
the efficiency of
photosynthesis.
• Most graphs will show
the:
• % use of light in that
wavelength OR
• Rate of photsynthesis

7. Absorption Spectrum
• Since light can
perform work only
if it absorbed…
– Chl a.
– Chl b.
– Carotenoid
• Wavelengths of
light best absorbed
by the main
photosynthetic
pigments

8. Absorption Spectrum

9. Absorption Spectrum
• Greatest light absorbed in violet to blue
and red range
• Least absorption in green - yellow
range. This light is reflected.

10. Absorption vs. Action

11. Pigments absorb the light
for photosynthesis
• When light strikes a pigment, it moves an
electron to an excited state.
• Chlorophyll a (chl a) —main pigment in
photosynthesis—absorbs violet-blue and red.
Converts light to chemical energy
• Chlorophyll b(chl b) —accessory pigment in
photosynthesis—absorbs blue/green to
orange/red
• Carotenoids and xanthophylls —accessory
pigment—absorbs violet to green

12. The Chloroplast
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13. DRAW

14. THE CHLOROPLAST

15. Chloroplast
• Thylakoid –
– photosynthetic membranes.
– Arranged in stacks called GRANA.
– Contain chlorophyll: photosystems
– Light DEPENDENT reactions occur
• Stroma –
– Outside the thylakoid
– Light INDEPENDENT reactions occur.

16. Photosynthesis – an overview
(the absolute minimum you must know!!!)
• 6CO2 + 12H2O light C6H12O6 + 6O2 + 6H2O
• Light energy needed (photon)
• ATP used and generated
• NADP+ :the electron carrier, that combines with 2 electrons and one
hydrogen ion.
• NADP+ becomes reduced into NADPH, when it receives the electrons.
• Glucose and oxygen are made.
NADP : nicotinamide adenine dinucleotide phosphate

17. Photosynthesis – an overview

18. Photosynthesis – an overview

19. Photosynthesis – an overview

20. The
Light
Reactions

21. The Light Dependent Reactions

22. The Light Dependent Reactions
• Thylakoid
• PHOTOLYSIS: Sunlight (photon energy) SPLITS
WATER.
» Creates O2
» Creates a H+ gradient
» Replaces electrons in chlorphyll
• SunLight energizes electrons in chlorophyll. Electrons
move down ET chain:
» Pumps H+ ions thru membrane
» Remaining Electron energy is reduces NADP >>>>>
NADPH
» H+ ions diffuse through ATP Synthase: Chemiosmotic
Photophosphorylation …. Creates ATP

23. Summary of the Light
Reactions
Input:
• Light
• Water
• ADP
• NADP+
Output:
• ATP
• NADPH
• O2 ( waste)

24. From Dependent to
Independent

25. The
Light
Independent
Reactions

26. 26

27. Light Independent Reactions(overview)
• Take place in the stroma
• Uses CO2 from atmosphere
• Uses ATP and NADPH.
• NADPH IS OXIDIZED!
• THE CARBON MOLECULE IS REDUCED!
• Forms glucose (hexose) and RuBP (continues in the
cycle)

28. Light Independents Rxns.
Note the ATP and NADPH numbers for 1 glucose

30. Light Independent Reactions
• STROMA
• 3CO2 enter system (2 “turns” for 1 glucose)
• Bond Energy stored in ATP and NADPH used to build
carbon molecule. Reduces 6C3P molecule.
• 3C5PP molecules continue in the cycle
• Carbohydrate produced (C3P) (C3P X 2 – 2P = Glucose)
• ADP and NADP reusable.
• Water made (waste product)

31. Photosynthesis- Tracking the Atoms
6CO2 + 12H2O18 C6H1206 + 6O2
18 + 6H20

32. External Factors
affecting
Photosynthesis

33. Overview
• Environmental Factors:
–Influences Rate of
photosynthesis
–Rate of enzyme action
affected

34. Limiting Factors
• Law of Limiting Factors:
When a chemical process depends on
more that one essential condition being
favorable, the rate of reaction will be
limited by the factor nearest its minimum
value

35. Limiting Factors of
Photosynthesis
•Light intensity
•CO2 concentration
•Temperature

36. The Effect of Light Intensity on
Photosynthetic Rate
• Low light – shortage of NADPH
and ATP.
• As intensity increase – the
point where all chlorophyll is
saturated by light is reached.
•Rate Limiting Step - reduction
of C3P
• NO NADPH = NO REDUCTION
• Consider overall rate –
accounts for shading of leaves.

37. The Effect of Temperature on
Photosynthetic Rate
Low Temperatures:
Rate limiting step:
• Enzymes that catalyze
Calvin cycle work more
slowly!
• NADPH accumulates.
•High Temperature: Best at
35°C
Rate limiting step:
• C5PP carboxylase will not
work effectively- denature
•Carbon Fixation is
limited.
•NADPH accumulates!

38. The Effect of Light Intensity and Temperature on
Photosynthetic Rate
Which is
the
limiting
factor
here;
light
intensity
or
temp.?

39. The Effect of CO2
Concentration
CO2 – substrate for Calvin
Cycle Rxns.
• if all active sites are used –
reaction cannot go faster.
RATE- LIMITING STEPS:
•Low/Medium CO2
concentrations:
• Carbon Fixation – C5PP
and NADPH accumulate
• High Concentrations:
• not a factor

40. Measuring the Rates of
Photosynthesis

41. Measuring Rates
1. Production of O2 :
• Allows for measurement of volume of gas
• Measures presence of O2 - burning splint

42. Measuring Rates
2. Uptake of CO2 –
• Difficult to measure directly
–If CO2 is absorbed from water – pH
increases
–Use Bromothymol Blue – Qualitative
• Turns yellow in the presence of CO2

43. Measuring Rates
3. Biomass
– Equal sized samples of plants taken at
specific time intervals
– Percentage increase in dry mass
measured
– Growing conditions must be the same
– Indirect

44. Oxygen Concentration
What would a graph for increasing levels of CO2 look like?

45. The End
(ignore any slides after this
one)

46. Water and Minerals
• Water – shortage causes a decrease in
rate
– Stomates close – prevent gas exchange –
decreases rate
– Rate limiting: Photolysis, Carbon Fixation
• Minerals –
– Mg and N – needed to form chlorophyll
– Zn, Mn, Fe, Cu – needed for ETC, enzyme
reactions.

47. Transpiration

48. Transpiration
• Gas exchange in plants: CO2 in and
O2 out
• Occurs in the presence of water
• Occurs in the spongy mesophyll
• Water evaporates from the surface of
the leaves and stems of plants.

49. Factors Affecting Transpiration
• Depends on internal:
– Stomate close – internal
• Produce hormone – Abscisic Acid

50. Factors Affecting Transpiration
External – Abiotic Conditions:
• Light – guard cells close stomate in dark
• Temperature – higher temperature –
higher transpiration rate.
• Humidity – lower humidity outside leaf
increase rate of transpiration
• Wind – higher wind speeds – increased
rate of transpiration!

51. C3, C4 and CAM Plants
• Differences in the Calvin Cycle:
– C3 –
• plant that makes a three carbon compound as its first
stable compound in the Calvin Cycle
• Use Calvin Cycle exclusively: Stomate open during day.
• 95% of plants.
– C4 –
• plant that makes a four carbon compound as its first
stable compound in the Calvin Cycle.
• In Hot climates: stomate partially close during the day.
• Corn, Sugar Cane.
– CAM
• Plants that grow in hot, dry climates
• Stomate closed during the day.
• Dark Reactions actually take place at night!
• Cactus, Pineapple

52. CAM =
Crassulacean Acid
Metabolism

53. C3 and C4 Leaf