BIO1PS 2012
Plant Science
Lecture 9
Leaves and Photosynthesis Pt. I
Dr. Michael Emmerling
Department of Botany
Room 410
[email protected]
Learning Objectives
Describe the cell types in a typical dicot leaf
Describe the structure of a typical C3 plant
leaf
Describe Joseph Priestley’s landmark
experiment
Leaves
•Function
•Modifications
•Structure - internal and external
•Stomata and water conservation (regulation)
•Photosynthetic cells
•Transport
Leaf Structure and Function
Adaxial
• cuticle
• epidermis
• palisade mesophyll layer
• spongy mesophyll layer
• vascular tissue (xylem, phloem)
• (sclerenchyma) support
• epidermis
• cuticle
Abaxial
outer protective layer
photosynthesis
transport - water, sugars
outer protective layer
Leaf Structure
adaxial
outer protective layer
photosynthesis
transport - water, sugars
outer protective layer
abaxial
Ladiges et al. (2010), 4th ed., Fig. 6.14
Photosynthetic Cells
Palisade mesophyll
• tightly packed, regularly shaped cells close to the
adaxial surface
Spongy mesophyll
• loosely packed, irregularlyshaped cells, with more air
spaces - thus “spongy”
A Typical Plant Cell
Ladiges et al. (2010), 4th ed., Fig. 4.2
Photosynthetic Cells
• photosynthesis is carried out in chloroplasts
• chloroplasts
• are concentrated in the tightly packed palisade mesophyll
•
•
and also present in the spongy mesophyll
are pressed up against the cell walls near the air spaces
tend not to occur where adjacent cells touch
• diffusion of CO2 to chloroplasts is facilitated
• there may be 20-30 (or more) chloroplasts per cell
Chloroplasts in Cells
Knox et al. (2005), 3rd ed., Fig. 3.15
Ladiges et al. (2010), 4th ed., Fig. 4.17
Leaf Structure and Function
Adaxial
• cuticle
• epidermis
• palisade mesophyll layer
• spongy mesophyll layer
• vascular tissue (xylem, phloem) transport - water, sugars
• (sclerenchyma) support
• epidermis
• cuticle
Abaxial
Transport - Vasculature
Water
• transported into the leaf by the xylem
Products of photosynthesis
• transported out of (and into) the leaf by the
phloem
Vascular bundle
• xylem (transporting water)
• phloem (transporting sugars)
Bundle sheath cells
• surround the vascular bundle
"Normal" Leaf Anatomy
In contact with
bundle sheath cells
Leaf Structure and Function
Adaxial
• cuticle
• epidermis
• palisade mesophyll layer
• spongy mesophyll layer
• vascular tissue (xylem, phloem)
• (sclerenchyma) support
• epidermis
• cuticle
Abaxial
Other Leaf Cells
• sclereid (a type of
sclerenchyma). Also
known as “stone cells”
• strong cells, often large
and irregularly shaped,
providing support to
leaves, seed coats and
fruits (pear)
• have thickened cell wall
Ladiges et al. (2010), 4th ed., Fig. 16.7
Hakea
Ladiges et al. (2010), 4th ed., Fig. 41.17
Bulliform Cells
Involved in leaf movement –
especially rolling
Ladiges et al. (2010), 4th ed., Fig. 16.37
Leaf Structure and Function
Adaxial
• cuticle
• epidermis
• palisade mesophyll layer
• spongy mesophyll layer
• vascular tissue (xylem, phloem)
• (sclerenchyma) support
• epidermis
• cuticle
Abaxial
outer protective layer
photosynthesis
transport - water, sugars
outer protective layer
Function of Leaves
Highly modified leaves
• can serve a variety of other functions
• are important adaptations to different habitats or
environmental conditions
Generate energy from photosynthesis
• this is the primary function
• sugars (primarily sucrose) from carbon dioxide
(CO2) and water (H2O)
Timeline of Organisms
~3.5 billion years ago
Fossil bacteria
Anaerobic CO2 fixation (modern sulfur bacteria)
CO2 + 2H2S
->
CH2O + 2S + H2O
~3.5 - 2.5 billion years ago
“Cyanobacterial” stromatolites start producing oxygen by
photosynthesis
CO2 + 2H2O
->
CH2O + O2 + H2O
~2.3 - 1.8 billion years ago
Oxygen-rich atmosphere, aerobic respiration
CH2O +O2
->
CO2 + H20
J.B. Priestley’s Experiments
Candle quickly goes out
“Dephlogisticated air” or
“vital air” was required
Introduce a live
mouse…..
Mouse "goes out" (quickly)
J.B. Priestley’s Experiments
Introduce a live
mint sprig into the
“contaminated air”
THEN a live mouse
Mouse is
happy
"The injury which is continually done to the
atmosphere by the respiration of such a large
number of animals ... is, in part at least,
repaired by the vegetable creation."
J.B. Priestley
"History" of Photosynthesis
Stephen Hales, 1727
plants obtain nutrition from the air
Joseph Priestley, 1776
"Experiments and Observations on Different Kinds
of Air"
1772: Contaminated air (by burning a candle)
could be restored by a sprig of mint
Jan Ingen-Housz, 1773
green parts of plants exposed to sunlight were
required to “purify” air
"History" of Photosynthesis
Jan Ingen-Housz, 1779
Court Physician to Empress Maria Theresa of Austria
• green parts of plants exposed to sunlight were
required to "purify" air
• plants in the dark did not purify the air
"Experiments Upon Vegetables, Discovering Their great
Power of Purifying the Common Air In the Sun-shine, and
of Injuring it in the Shade and at Night." (1779)
"History" of Photosynthesis
Jean Senebier, 1782
purification of air required "fixed air"
Antoine Lavoisier, 1785
fixed air was CO2
Jan Ingen-Housz, 1796
CO2 was the source of carbon for plants
N. T. de Saussure, 1804
CO2 + H2O = O2 + organic matter
"History" of Photosynthesis
Julius Mayer, 1845
Energy used by plants and animals is
derived from the sun
Photosynthesis transforms the energy of
light into chemical energy
Theodor W. Engelmann, 1882
Action spectrum of photosynthesis
Photosynthesis
The synthesis of high energy
chemical compounds (food) from
low energy compounds (H2O and
CO2 ) using the energy present in
photons of light
Photosynthesis
6 CO2 + 6 H2O
light
C6H12O6 + 6 O2
e.g. glucose
fructose
6 mol + 6 mol
1 mol + 6 mol
•occurs (primarily) in the mesophyll cells
•dependent on both photochemical and
enzyme-mediated reactions
Light-Dependent Reactions
• photons are absorbed by pigments in
photosystems
• photosystems are large protein complexes
embedded in thylakoid membranes
• chlorophyll absorbs light of specific wavelengths
• energy is converted into movement of electrons
and protons
• proton gradient is used to synthesise ATP