14 October 2009
(P. J. Hollenbeck)
478-86; DVD 14.6, 147
Probs: exam II ‘05, #1; ECB Q14-16 &17
Living cells require a continuous input of energy.
As you know, this is because living
systems continually consume energy that is "spent" in constructing, maintaining and organizing the
cell (overcoming entropy).
The input of energy comes from our star, the sun, and is converted into
useful chemical energy by photosynthesis.
Although we will focus on photosynthesis in plant cells,
much of what we know about the detailed structure of the machinery comes from studies of
Recall that the breakdown of sugar through glycolysis, the TCA cycle, electron transport and ATP
glucose + O ==> CO + H O + ATP
Photosynthesis is essentially the opposite:
CO + H O + solar energy ==> glucose + O
As cell biologists, we want to understand photosynthesis at two levels:
(i) be able to compare and contrast the organization of the mitochondrion and the chloroplast,
recognizing where key reactions occur and how compartment boundaries are important in
(ii) be able to understand the overall schemes of glucose metabolism and glucose synthesis.
Structure and function of the chloroplast
[see fig. below and
14-25 thru -27 in
(A) Origin & organization of compartments
(1) Although we will concentrate on the organization of photosynthesis in plant cells,
remember that simpler and more “ingenious” systems of photosynthesis exist in
bacteria, and these pre-date the evolution of plants, as we discussed in the last lecture.
(2) The chloroplast, like the mitochondrion, seems likely to have its origin in
endosymbiosis between an early eukaryotic cell and a procaryote: in this case, a
photosynthetic bacterium similar to modern cyanobacteria.
(3) Chloroplasts have an outer envelope comprised of two separate membranes with
very little space in between: the
outer envelope membrane
, which has porins and is
leaky like the outer mitochondrial membrane; and the
inner envelope membrane
which is impermeable and contains a variety of transporters, similar to the inner
(4) Within the envelope is a third, internal, membrane system called the
that encloses an elaborately shaped compartment.
The compartment has the
form of many membranous sacs called thylakoids that are organized in many places into
Although the geometry looks complex, the lumens of these sacs
and their stacks are all continuous with each other, forming a single innermost
compartment called the
The space between the thylakoid membrane
and the chloroplast envelope is called the