Bio 211 Intro Molecular and Cell Biology
Lecture 13 “Glycolysis”
Reading: Campbell Chap. 9 pp. 147-156
In the previous cell chemistry lecture, we have mostly looked at the different
molecules in cells and some of their chemical properties. In the last half of this unit, and
beginning with today’s lecture, we will focus on some of the most important chemical
processes that take place in cells. Crucial to much of metabolism is how the cell breaks
down glucose to derive energy. The reactions of glycolysis, which will we begin to look
at today is the first phase of this catabolic process.
Outline:
1.
2.
3.
4.
Processes that yield energy
Redox reactions
Electron carriers
Glycolysis
1. Processes that yield energy
Cells derive their energy from the chemical bonds of organic molecules. The
metabolic processes that break down sugars and other food sources to derive energy
consist of two major types:
a. Fermentation: Break down 6C sugars to 3C or 2C compounds to derive some
energy. Occurs without oxygen.
b. Respiration: Break down 6C sugars in presence of oxygen to CO2 and H2O;
most efficient source of energy.
C6H12O6 + 6O2
----> 6 CO2 + 6H2O + energy (ATP + heat)
Most important is the process of respiration, so we will examine these pathways
first, then come back to fermentation. Respiration consists of 3 main phases:
(1) glycolysis
(2) Krebs cycle
(3) electron transport/oxidative phosphorylation
Each of these processes occurs in a specialized location in cells; to fully
understand the importance of these processes, we will briefly identify some of the
structures in cells that carry out these reactions.
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Cells in eukaryotic organisms consist of
a. cell membrane: lipid bilayer that serves as a boundary
b. nucleus: contains the genetic material of the cell
c. cytoplasm: the region surrounding the nucleus, made of cytosol and organelles
d. organelles, such as the mitochondrion: carry out particular specialized
functions in the cell. The mitochondrion carrys out many processes related to energy
metabolism.
Glycolysis occurs in the cytosol.
The Krebs cycle occurs in the mitochondrion (matrix).
The electron transport chain and oxidative phosphorylation occurs in the
mitochondrion (inner membrane).
Summarized in Fig. 9.6
Before we look at these individual processes in more detail, beginning with
glycolysis, let’s take a brief look at some of the kinds of reactions and carriers that are
involved in energy metabolism.
2. Redox reactions
In many chemical reactions, there is a transfer of one or more electrons from one
reactant to another. These are called oxidation-reduction or redox reactions. In an
oxidation-reduction reaction, one substance loses electrons; we say it is oxidized. The
other substance gains electrons; we say it is reduced.
In the reaction:
Xe- + Y ----> X + YeX is the electron donor or the reducing agent; Y is the electron acceptor or the
oxidizing agent.
The overall process of respiration involves oxidation and reduction:
C6H12O6
+
6O2
----> 6 CO2
+
6H2O
Glucose is oxidized and oxygen is reduced in the process.
3. Electron carriers
Many of the individual reactions that comprise the process of respiration are
oxidation-reduction reactions. In this section, we will be introduced to some of the
molecules that are typically involved as electron carriers in redox reactions. When we see
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these molecules involved, we can be fairly certain that oxidation and reduction reactions
are going on.
The name of one essential carrier is NAD+: nicotinamide adenine dinucleotide.
The molecule consists of two nucleotides joined together. Fig. 9.4.
Oxidized NAD+ can carry two electrons and a proton donated from an organic
substrate. We describe the reaction as:
NAD+ + 2e- + 2H+ ----> NADH + H+
oxidized
reduced
The electrons and protons in the reaction are derived from food molecules. The
enzymes that catalyze redox reactions involving NAD carriers are called dehydrogenases.
Chemical energy from reduced NADH is usually released by the processes of
electron transport and oxidative phosphorylation, which we will describe in more detail
shortly. Essentially what happens is the electrons and protons are passed from reduced
NADH through a series of electron carriers and ultimately to oxygen, forming water. The
energy is conserved in these transfers and is used to make ATP.
4. Glycolysis
Glycolysis is the first of the three phases of cellular respiration. In glycolysis,
glucose is broken down in the cytosol into two molecules of pyruvate. The products of
glycolysis include some stored energy as ATP, NADH, which can be a source of
additional energy and the 3 carbon compound pyruvate, which can be oxidized further by
the Krebs cycle.
Glycolysis means “splitting of sugar”. Glucose a 6C sugar is split into two 3C
sugars and the 3C sugars are further oxidized.
The catabolic pathway consists of ten steps each catalyzed by a specific enzyme.
We can divide the steps into two phases:
a. energy investment phase: requires ATP
b. energy pay-off phase: makes ATP by “substrate-level phosphorylation” and
makes NADH by oxidation/reduction.
Today we will work through the major events of glycolysis and where the key end
products are formed. (Figures 2.22 from human biology course)
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In the first phase of glycolysis, 2 ATP are used to activate glucose by
phosphorylation, giving fructose 1,6 bisphosphate. Fig. 2.22. This 6 C sugar is then split
into two 3C molecules in preparation for the pay-off phase.
In the pay-off phase the 3C sugar PGAL or phosphoglyceraldehyde is converted
into a more oxidized 3C sugar pyruvate. During these conversions, one molecule of
NADH and 2 molecules of ATP are made from each PGAL.
The ATP that are made in glycolysis come from transfer of phosphate groups from
the phosphorylated 3C sugars to ADP. Transfer of a phosphate from a substrate to ADP
directly is called “substrate-level phosphorylation”. Other ATP made in the process of
respiration mostly come from “oxidative phosphorylation” in which the energy from
electron transport is conserved as a proton gradient that drives ATP formation.
In the next panel we will summarize the key products made by the process of
glycolysis (Fig. 9.9). In the energy-investment phase, 2 ATP are used up to activate a
molecule of glucose. In the energy-payoff phase, 4 ATP and 2 NADH are produced from
every glucose that is broken down and oxidized into 2 pyruvate (a 3C sugar). The bottom
panel shows the net yield of products:
Glucose
----> 2 Pyruvate + 2 H2O
2 ADP + 2 Pi ----> 2 ATP
2 NAD+
----> 2 NADH + H+
Since 2 ATP are used up in the reaction, the production of 4 ATP in the payoff
phase results in a net yield of 2 ATP.
Summary: In today’s lecture you were introduced to some of the processes important for
producing energy in the cell. The process of cell respiration has 3 distinct pathways
including glycolysis, the Krebs cycle and electron transport/oxidative phosphorylation. In
the next lecture, we will start by taking a more detailed look at glycolysis as a sort of
review, then we will examine the steps used to completely breakdown sugar molecules to
CO2 and H2O via the Krebs cycle. As your homework for the next class period, I would
like you to write out the steps of glycolysis from your book, pages 166-167, and highlight
the steps at which ATP is made or used and where NAD is reduced.
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