1. Cellular Respiration
Dr. Mark A. McGinley
Honors College and Department of
Biological Sciences
Texas Tech University

2. Biological Work
• Most of the energy used to do biological work
comes from ATP
• ATP breaks down and releases energy that is
used to do biological work

3. Energetics in a Nutshell
• Photosynthesis converts light energy to
potential energy stored in chemical bonds of
glucose
• Cellular Respiration converts potential energy
in glucose to potential energy stored in ATP
– ATP releases energy used to do work
• Glucose links the two processes

4. Breaking Down Glucose to Release
Potential Energy
• Starts with the process of glycolysis
• Followed by either
– Fermentation (in anaerobic environments)
– Citric Acid Cycle (Krebs Cycle) + electron transport
(in aerobic environments)

5. Glycolysis
• Glucose broken down into two molecules of
pyruvate
– Occurs in the cytosol
• Breaking down ATP requires the input of
energy from 2 molecules of ATP but releases
energy in 4 molecules of ATP
• Thus, net gain of energy of 2 ATPs in glycolysis

6. Glycolysis
• Glucose + 2ATP => 2 pyruvate + 4 ATP + H+

7. Glycolysis
• Glycolysis breaks down glucose to release
energy in two ATPs
– ATPs can release energy to do biological work

8. Problem Facing the Cell
• Glucose <= => 2 pyruvate + H+
• This reaction will continue to break down
glucose to release ATP until the reaction
reaches an equilibrium
• Once equilibrium is reached, glycolysis will
stop, so no more ATP is released

9. Solution
• In order to allow glycolysis to continue cells
must maintain the concentration gradient by
removing pyruvate and H+ from the cell.
• H+ picked up by NAD+ => NADH
• Eventually NAD+ gets saturated

10. Ultimate Solution
• H+ must be removed from NADH in order to
allow glycolysis to continue
• Key Point- How this happens depends on
whether or not there is oxygen in the
environment

11. Anaerobic Environment
• When there is no oxygen in the environment
then pyruvate and H+ are removed from the
cell by fermentation
• Several patterns of fermentation including
– Alcohol fermentation
– Lactic acid fermentation

12. Alcohol Fermentation
• Pyruvate and H+ => acetaldehyde => ethanol
• Ethanol becomes the ultimate “hydrogen
acceptor”

13. Advantages and Disadvantages of
Alcohol Fermentation
• Benefit
– End products of glycolysis are removed from the
cell so glycolysis can continue
• Disadvantage
– Alcohol can be poisonous to cells
– Pyruvate used to help remove H+ from the cell
• Still lots of potential energy stored in pyruvate
• Can’t break down pyruvate to release energy

14. Lactic Acid Fermentation
• Pyruvate + H+ => lactate
• Lactate becomes the ultimate hydrogen
acceptor

15. Advantages and Disadvantages of
Lactic Acid Fermentation
• Benefit
– End products of glycolysis are removed from the
cell so glycolysis can continue
• Disadvantage
– lactate can be poisonous to cells
– Pyruvate used to help remove H+ from the cell
• Still lots of potential energy stored in pyruvate
• Can’t break down pyruvate to release energy

16. Review in Anaerobic Environments
• Glucose broken down by glycolysis and
fermentation
• For each glucose molecule broken down there
is a net gain of two ATPs

17. Aerobic Environments
• When oxygen is present
– O2 + H+ => H20
• Water becomes the ultimate hydrogen
acceptor
– Benefit
• Water is non-toxic and in fact is beneficial
• Pyruvate can be broken down to release more stored
energy

18. Energy From Pyruvate
• Glycolysis occurs in the cytosol
– NADH and pyruvate move into the mitochondria
• In the mitochondria pyruvate is broken down
to release ATP in two processes
– Citric acid cycle (Krebs Cycle)
– Electron transport

19. Pyruvate Links Glycolysis and Citric
Acid Cycle

20. Citric Acid Cycle
• The details of the Citric Acid Cycle are well know
– Not super important for this course
• Key Points
– Inside of the mitochondrion pyruvate breaks down to
produce CO2 + Acetyl CoA
– Acetyl CoA enters Citric Acid Cycle
• Acetyl CoA + oxaloacetate = > citrate
– CO2 released
– 1 ATP produced for each Acetyl CoA that enters the
cycle
• Thus, 2 ATPs per glucose

21. Electron Transport
• In a process very similar to what we talked
about in cyclic electron flow in photosynthesis
– An excited electron moves down an electron
transport chain (located in inner membranes of
mitochondria)
• Energy released used to actively transport H+
• H+ concentration gradient powers Chemiosmosis
– Releases lots of ATP
– 26 or 28 ATP/glucose

22. Electron Transport

23. Review in Aerobic Environments
• Glucose broken down by glycolysis, citric acid
cycle, and electron transport
• For each glucose molecule broken down there
is a net gain of 30 - 32 ATPs
– 2 per glucose from glycolysis
– 2 per glucose from citric acid cycle
– 26 – 28 per glucose from electron transport

24. Advantages of Breaking Down Glucose
in Aerobic Environments
• Benefit
– End products of glycolysis are removed from the
cell so glycolysis can continue
– Ultimate hydrogen acceptor (water) is beneficial
to cells
– Pyruvate can be broken down to release much
more energy