Chloroplasts & Photosynthesis
I. The Plastid Family Tree
A. Plastids exist in a number of different forms which are, to some extent, interconvertible
II. Chloroplast Structure
A. The chloroplast has
three components:
III. Isolation and Fractionation
A. Isolation
B. Fractionation
IV. Chloroplast Composition
A. Proteins
B. Lipids
C. Pigments
D. Chlorophyll structure:
E. Absorption/action spectra
F. Most (75%) of chlorophyll in the chloroplast is complexed with proteins
1. Protein complexes
(along with their functions) isolated from the thylakoid membranes
Careful studies revealed that the photosystems and other particles were
preferentially associated with either the granal or stromal thylakoids:
| Particle | Granal thylakoid | Stromal thylakoid |
| PS II | 85% | 15% |
| PS I | less than 15% | greater than 85% |
| ATPase | 0% | 100% |
| Cytochrome | 50% | 50% |
If a plant produced chloroplasts that lacked grana, what part of
photosynthesis would they be deficient in?
V. Photosynthesis
A. Introduction
1. The summary equation (taking place in the presence of light and a green
plant):
CO2 + H2O ----> (CH2O)n +
O2
2. The global impact of photosynthesis is substantial:
B. Review of the component parts of photosynthesis:
1. Photochemical or "light" reactions
Example:
1/2O2 + 2e- + 2H+ ----> H2O
most negative "V" values indicate a weak affinity for electrons
so these carriers tend to donate electrons and not accept them; for example:
NADH/ NAD+ (50:50 mixture): E0 = -0.32 V (NADH has a
strong tendency to donate electrons).
calculation of light energy:
E(lambda) = hv
where:
(lambda)= wavelength of light;
h = Planck's constant
v= frequency of oscillation
What did Einstein say?:
orbital - electrons are delocalized in a bond joining two or more atoms; of
prime importance in light absorption;
Once an electron has absorbed energy and is in an excited state, the ground
state may be re-established
three ways: fluorescence
in algae
Photosystems I and II and the Light Harvesting Complex
Example: PS I
Chlorophyll molecules absorb photons of light and then pass this energy to the P700 center
The photosystems, LHC's and other complexes are both physically and
biochemically (according to redox potential) arranged in the membrane;
Photons absorbed by both
reactive centers are used to drive electron transfers through the carriers; these
redox reactions provide energy to accomplish two specific tasks:
Empirical studies had shown that chloroplasts in the light synthesized ATP
from ADP plus Pi, but the mechanism for this strongly endothermic reaction was unknown
until Mitchell's hypothesis.
The key was the proton gradient
The gradient creates an electropotential difference
This process which synthesizes ATP using the energy in a photon of light is called photophosphorylation.
2. Synthetic Reactions: The Calvin Cycle
6 CO2 + 18 ATP + 12 NADPH2 ----> 6 carbon sugar + 18
ADP + 18 Pi + 12 NADP+
CO2 + ribulose bisphosphate (RUBP) ----> 2 phosphoglyceric acid
(PGA)
rubisco
(ribulose bisphosphate carboxylase/oxygenase).
If the reaction runs six times, you would fix enough carbon to produce
________ sugar molecules EXCEPT that:
Note that the rubisco reaction does not require ATP or NADPH2.
Why do you need these high energy molecules?
VI. Environmental Effects on Thylakoid Membranes
EM and biochemical studies were done on thylakoid membranes from plants
growing in full sun and plants growing shaded by a canopy of leaves (the leaf canopy
filters out some of the shorter wavelengths, thus enhancing the amount of longer
wavelength light reaching the leaves).
EM analysis revealed that PS I and PS II particle sizes were different in
each habitat;
biochemical studies were done to compare relative PS I and PS II activity:
below the canopy - PS I / PS II ratio = 5.56
above the canopy - PS I / PS II ratio = 0.91
What did this mean?
