1. Photosynthesis

2. To provide this energy to us!
All energy on earth comes from the sun.
We depend on:
Plants
Algae (underwater plants)
Cyanobacteria (photosynthetic bacteria-unicellular)
To provide this energy to us!
These organisms obtain energy directly from the sun and are called autotrophs
We are heterotrophs-we obtain energy by eating other organisms

3. A little bit about Cyanobacteria
The endosymbiotic theory proposes that an ancestor of cyanobacteria was engulfed by an ancestor of today’s eukaryotic cell and gave rise to plant cells
Algal and plant cells contain chloroplasts (primarily in the mesophyll and guard cells)

5. Leaf Structure and Photosynthesis

6. The leaf is perfectly designed for maximum function
Ground tissue: function = photosynthesis
palisade and spongy mesophyll have many Chloroplasts and air spaces enable efficient gas exchange.
Vascular tissue: function = support and transport
xylem and phloem
Dermal tissue separates the inside of the leaf from the air outside the plant.
Epidermal cells
waxy cuticle
Guard cells for stomata

7. Structure of a leaf

8. Photosynthesis primarily occurs in chloroplasts of leaves
Lilac (Syringa) 8

9. Leaves are thin to maximize surface area exposed to sunlight.
It also limits the distance that gases need to travel to the chloroplasts

10. Plant structure Obtaining raw materials sunlight CO2 H2O nutrients
leaves = solar collectors
CO2
stomates = gas exchange
H2O
uptake from roots
nutrients
N, P, K, S, Mg, Fe…

11. stomate
transpiration
gas exchange

12. Plant Structure: Leaves
Stomata
Stomata = pores in epidermis used for gas exchange
CO2 into cell for photosynthesis
H2O out of leaf by evaporation to facilitate transpiration (process by which water pulled up plant)
Guard cells = epidermal cells that open and close stomata
Stomata open when guard cells swell with water
Stomata close when guard cells collapse together (shrivel)

13. Stomates

14. Stomates Stomata are open in the daytime and closed at night
Light activated proton pumps in guard cell membranes cause K+ to move from neighbouring epidermal cells into guard cells-this results in H2O moving into guard cells (by osmosis) causing them to swell.
Increased turgor pressure causes them to open.
As the concentration of sucrose in guard cells decreases in the evening, water moves out of the cells and stomata close.

15. Stomata typical of dicots Stomata typical of monocots
Potato (Solanum)
Maize (Zea)
Scanning electron microscope images 15

16. chloroplasts in plant cell chloroplasts contain chlorophyll
absorb sunlight & CO2
cross section of leaf
leaves
CO2
chloroplasts in plant cell
chloroplasts contain chlorophyll
chloroplast
make energy & sugar

17. Plant structure Chloroplasts Thylakoid membrane contains
ATP
thylakoid
Chloroplasts
double membrane
stroma
fluid-filled interior
thylakoid sacs
grana stacks (sing. granum)
Thylakoid membrane contains
chlorophyll molecules
electron transport chain
ATP synthase
H+ gradient built up within thylakoid sac
outer membrane
inner membrane
granum
stroma
thylakoid
A typical mesophyll cell has chloroplasts, each about 2-4 microns by 4-7 microns long.
Each chloroplast has two membranes around a central aqueous space, the stroma.
In the stroma are membranous sacs, the thylakoids.
These have an internal aqueous space, the thylakoid lumen or thylakoid space.
Thylakoids may be stacked into columns called grana.

18. Chloroplasts Have 3 membranes
The third membrane is called the thylakoid.
The thylakoid is folded and looks like stacks of coins called granum
The stroma is the space surrounding the grana

19. Chloroplasts
Chlorophyll molecules are embedded in the thylakoid membrane
Act like a light “antenna”
These molecules can absorb sunlight energy.
Image from Biology 11: College Preparation. Pg 73. Nelson, Toronto

20. Chlorophyll There are several types of chlorophyll
Chlorophyll a (blue-green) and chlorophyll b (yellow-green)
Both contain a porphyrin ring attached to a long H-C tail or phytol tail (hydrophobic to anchor into membrane)
Porphyrin rings are also found in the cytochromes of ETC

23. Chlorophyll
The ring has a Mg atom in the centre and alternating single/double bonds
Electrons in the porphyrin ring absorb light energy and begin photosynthesis
Chlorophyll a differs in that it has a methyl group (-CH3) at position –R while chlorophyll b contains an aldehyde group (-CHO)
Chlorophyll a is the primary light-absorbing pigment.

24. Overview

25. Overall Reaction
What is the equation for photosynthesis? Similar to what?
carbon dioxide + water  glucose (simple sugar) + oxygen
CO2 (g) + H2O (l) + light energy [CH2O]+ O2(g)
6CO2 + 12H2O + light energy  C6H12O6 + 6O2 + 6H2O
How are they different?

26. 3.1 Homework
P.145
Q 1-7

27. 3.2 Light energy and photosynthetic pigments

28. Overview There are 3 distinct stages to photosynthesis;
Capturing light energy
Using this energy to make ATP and reduced NADP+ (energy shuttling coenzyme-similar to NAD+)
Using the free energy of ATP and the reducing power of NADPH + H+ to synthesize organic compounds such as glucose, from CO2

29. Light (dependent)Reactions (Stages 1 and 2)
Happen ONLY in sunlight on the thylakoid membrane
Light is absorbed by chlorophyll molecules
The energy generates molecules of ATP

30. Light Independent Reactions-Calvin Cycle-Carbon Fixation (formerly the “dark reactions”)
Happen in sunlight, and in the dark.
Hence “independent of light”
ATP generated by sunlight drives the Calvin Cycle.
Monosaccharides (eg. glucose) are manufactured in the cycle.
Monosaccharides are used to “build” polysaccharides (eg. Starch).

32. Summary
Image from: Biology 11: College Preparation. Pg 74. Nelson, Toronto

33. It’s not the Dark Reactions!
Photosynthesis
Light reactions
light-dependent reactions
energy conversion reactions
convert solar energy to chemical energy
ATP & NADPH
Calvin cycle
light-independent reactions
sugar building reactions
uses chemical energy (ATP & NADPH) to reduce CO2 & synthesize C6H12O6
It’s not the Dark Reactions!

34. Absorption of Light by Chlorophyll

36. Pigments of photosynthesis
Chlorophylls & other pigments
embedded in thylakoid membrane
arranged in a “photosystem”
collection of molecules
structure-function relationship

37. A Look at Light
The spectrum of color V I B G Y O R

38. Light: absorption spectra
Photosynthesis gets energy by absorbing wavelengths of light
chlorophyll a
absorbs best in red & blue wavelengths & least in green
accessory pigments with different structures absorb light of different wavelengths
chlorophyll b, carotenoids, xanthophylls
Why are plants green?

39. What wavelengths of light do you think plants use the least in photosynthesis?

40. Chlorophyll A and B absorb light mostly in the red and blue regions of the spectrum
Carotene and xanthophyll absorb light from other regions and pass the energy to chlorophyll

41. Light absorption by chlorophyll

42. Photosynthetic pigments are arranged as “photosystems”

43. Photosystems of photosynthesis
2 photosystems in thylakoid membrane
collections of chlorophyll molecules
act as light-gathering molecules
Photosystem II
chlorophyll a
P680 = absorbs 680nm wavelength red light
Photosystem I
chlorophyll b
P700 = absorbs 700nm wavelength red light
reaction center
Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane.
When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center.
At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
This starts the light reactions.
Don’t compete with each other, work synergistically using different wavelengths.
antenna pigments

45. Measuring light absorption
A spectrometer is used to measure the amount of absorption at each wavelength
An action spectrum shows the rate of photosynthesis at different light intensities

46. Brown seaweeds subjected to low light intensities
Chlorophyll C instead of chlorophyll B
fucoxanthin
-very wide absorption spectrum
-absorbs light in parts of the spectrum where chlorophyll is less efficient

47. Red seaweeds
Have chlorophyll D
Phycoerythrin

48. Green seaweeds Found in shallow water Do not have fucoxanthin
Does not have phycoerythrin

49. Accessory Pigments
Chlorophyll is not the only light-absorbing molecule in chloroplasts
Accessory pigments like orange carotenoids (beta-carotene) and yellow xanthophylls help absorb other light energy which may damage chlorophyll

50. Changing colours
During spring and summer leaves appear green because of the high concentration of chlorophyll
As temperatures cool, chlorophyll is broken down and we see the oranges and yellows and even reds (anthocyanin)

52. Practice Questions
P.154
Q 1-3, 6-8