Cellular Respiration: Obtaining Energy
from Food
Cellular Respiration
The glucose produced in photosynthesis can be broken down to release energy for
carrying on various cellular processes
For many endurance athletes, the rate at which oxygen is provided to working
muscles is the limiting factor in their performance (VO2 max)
Your muscles need a continuous supply of energy to perform work (contract
and relax)
Muscle cells obtain this energy from the sugar glucose through a series of
chemical reactions that depend upon a constant input of oxygen (O2)
When there is enough oxygen reaching your cells to support their energy
needs, metabolism is said to be aerobic
Your aerobic capacity is
the maximum rate at which O2 can be taken in and used by your muscle cells
and
therefore the most strenuous exercise that your body can maintain aerobically
If you work even harder and exceed your aerobic capacity, the demand for oxygen in
your muscles will outpace your body’s ability to deliver it
Metabolism then becomes anaerobic, and your muscle cells switch to an
“emergency mode”
Break down glucose very inefficiently and produce lactic acid as a by-product
So exergonic reactions can provide the energy to drive the formation of ATP from
ADP
This is a redox reaction
The exergonic reactions would be from the breakdown of food - essentially
breaking chemical bonds and releasing the stored (potential) energy
Producers and Consumers
Plants and other autotrophs (“self-feeders”) are organisms that make all their own
organic matter, including carbohydrates, lipids, proteins, and nucleic acids, from
nutrients that are entirely inorganic - producers
Heterotrophs (other-feeders) cannot make organic molecules from inorganic ones consumers
Chemical Cycling Between Photosynthesis and Cellular Respiration
The products of photosynthesis are the substances required for cellular respiration
and
the products of cellular respiration are the substances required for photosynthesis
Cellular Respiration: Aerobic Harvest of Food Energy
Cellular respiration is
the aerobic harvesting of chemical energy from organic fuel molecules
and an aerobic process that requires oxygen
Cellular respiration requires that a cell exchange two gases with its surroundings
The cell takes in oxygen in the form of the gas O2
It gets rid of waste in the form of the gas carbon dioxide CO2
Most often, the fuel molecule used by cells is glucose, a simple sugar
(monosaccharide) with the formula C6H12O6
An Overview of Cellular Respiration
Cellular respiration can be divided into three stages
Glycolysis - cytosol
Citric acid cycle - mitochondria
Electron transport - mitochondria
Together, these serve to convert food molecules (e.g., glucose) to CO2, H2O, and ATP
Electron Carriers
Many of the reactions that release energy also release electrons
Since free electrons will damage the cell, these reactions can only occur if there
is a place for these extra electrons to go
Many of the released electrons have high potential energy
This potential energy would go to waste unless there was some means to
harness that energy
How can that potential energy be used to make ATP?
Most of the high-energy electrons are picked up by electron carriers
The main electron carrier in cellular respiration is NAD+ (nicotinamide
adenine dinucleotide)
A secondary electron carrier in cellular respiration is FAD
So NADH and NAD+ constantly cycle back and forth
So do FAD and FADH2
Glycolysis
Glycolysis starts with 1 glucose (6C molecule) and ends with 2 pyruvic acids (3C
molecules)
We also get a net gain of 2 ATP and 2 NADH molecules
A Transition Step
Each pyruvic acid is then converted to an acetyl-CoA molecule
This also leads to the production of one NADH molecule/pyruvic acid
So everything from glycolysis onward is occurring in the mitochondria
The Citric Acid Cycle
The acetyl group (2C) then enters the citric acid cycle
The carbons eventually leave the cycle as CO2
Some ATP is produced during the citric acid cycle
NADH and FADH2 are also produced
The Electron Transport Chain
Embedded in the inner membrane is a series of electron carriers comprising the
electron transport chain
As high energy electrons move down the chain, they become less energetic
Some of the energy from the electrons is used to pump protons across the membrane
setting up a concentration gradient - potential energy
As the protons move down their concentration gradient through the ATP synthase
complexes, they drive the formation of ATP molecules
On average, cellular respiration can produce a maximum of 32 ATP/glucose
Different fuels (foods) can enter cellular respiration at different stages to provide energy
The citric acid cycle requires oxygen
Under anaerobic conditions, only glycolysis occurs and ...
fermentation
Some organisms produce ethanol through fermentation
When animals have to go anaerobic, they produce lactic acid as a byproduct
So fermentation occurs to replenish the NAD+ so that glycolysis can continue to occur
to supply ATP
When we need it, we have three sources of ATP available to us
The relative contribution of each source can vary depending upon the activity, the
intensity, and the duration
Phosphocreatine = creatine phosphate
Muscle fibers
Fast-twitch fibers
Rely on phosphocreatine and fermentation
Designed for strength
Few mitochondria
Fewer blood vessels
Slow-twitch fibers
Rely on aerobic respiration
Designed for endurance
Many mitochondria
Lots of blood vessels