This comprehensive and detailed review contains Description of important topics and detailed processes that you need to know for exam 3.
*Essential Study Material!!
Glycolysis
Glycolysis is one of the oldest metabolic pathways in cells, having evolved very early in
prokaryotic cells. It is an anaerobic process that does not require oxygen. Glycolysis occurs in
the cytoplasm in eukaryotes, where it breaks down the 6-carbon sugar glucose into two 3-carbon
pyruvate molecules in order to make ATP.
The glycolysis pathway has ten enzyme-driven steps, but functionally we think of glycolysis in
two phases:
- The investment phase during which 2 ATP are USED to phosphorylate glucose to make
it less stable and turn the pathway as a whole into one big -G reaction. This is an
investment because you are supposed + be making ATP not using it!
- The payoff phase is when the pathway MAKES 4 ATP for a net gain of 2 ATP overall
(4 -2 = 2). The is a step in the payoff phase that uses an oxidation reaction, so
glycolysis also produces 2 NADH that will be useful later on in the mitochondria.
By the end of glycolysis, we have accomplished the following reaction.
Glucose »»»-> 2 pyruvate + 2ATP + 2 NADH
Photosynthesis
Photosynthesis is a "coupled process" with two parts that function together to transform light
energy into chemical energy AND make stable storage molecules for that chemical energy. The
light reactions are responsible for capturing light and using that energy to make temporary
energy coupling molecules, ATP, and NADPH, which will power the Calvin cycle to make stable
organic molecules from CO2 in the atmosphere. The Calvin cycle is responsible for making the
first carbon backbone, G3P, from which literally all other organic molecules are made!
Chloroplast Structure
Photosynthesis in eukaryotes occurs entirely within the chloroplast, a membrane bound organelle
derived from photosynthetic prokaryotes called cyanobacteria (aka "pond scum"), In order to
understand how the molecular machinery for photosynthesis works
it is critical to understand the structure of the chloroplast (back to those Big Ideas, the light
reactions are a great example of structure-function relationships).
Unlike the mitochondria which ply evolved once, chloroplasts have evolved many times in green
algae, golden algae, red algae, brown algae, etc., so there are actually different kinds of
chloroplast, but they all share certain fundamental features in common. We
will focus on the chloroplast of green algae and plants, as illustrated below:
, Chloroplasts have a double membrane as you would expect, but they have additional membranes
inside that form hollow disc-shaped structures called thylakoids. Many of the components we
will discuss for the light reactions are integral proteins in the thylakoid membrane. The thylakoid
is important because it creates a new compartment, the thylakoid space of lumen, that is separate
from the general “cytosol” of the chloroplast which is called the stroma. The lumen will be used
to generate high concentrations of hydrogen ions making it very acidic which is not good for
proteins; all of the enzymes with be in the stroma at a more neutral pH.
The Light Reactions
The light reactions occur at the thylakoid membrane, producing ATP and NADPH to power the
Calvin cycle. The light reactions occur in the following steps"
1. Photosystem II captures light to excite electrons in a pair of chlorophyll molecules and
gives those high-energy electrons to the electron carrier plastoquinone (PQ).
2.b The high-energy electron carrier PQ gains to two electrons making it PQ2• which
attracts two H+ from the stroma to make PQH2.
2a. The electrons in the chlorophylls are replaced through the process of photolysis which
splits water to take two electrons and produces oxygen and hydrogen ions as waste
products. The function of photolysis is to resupply electrons to PSII.
3. PQH2 delivers the two high-energy electrons to cytochrome complex, dropping two H+
on the lumen side and becoming PQ again. The negative charged electrons move
through cytochrome complex and H+ from the stroma side move with them until
ending up in the lumen.
4. The low-energy electrons from the cytochrome complex are used to reduce the low-
energy electron carrier PC in the lumen. (Steps 1-4 use the energy from the electrons to
power on H+ pump, which does the work of building an H+ gradient, making PQ +
Cytochrome complex + PC an example of an electron transport chain).
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