Lesson 4: Introduction to Cellular Metabolism Flashcards
Cellular Respiration
Energy is required by living things to power all life functions.
The energy sources of the cell are the sugars and other nutrient molecules that
are the end products of digestion.
Cellular respiration is the gradual breakdown of nutrient molecules such as glucose and fatty acids in a series of reactions that release energy in the form of ATP.
The most common source of energy is glucose. If there is not enough glucose, fatty acids or amino acids can be used instead.
Oxidation and Reduction (Redox)
Cellular respiration
involves a series of redox
reactions.
Redox reactions are
common in metabolic
pathways.
When two molecules
react, one starts
oxidised and is reduced
while the other starts in
a reduced state and
becomes oxidised.
There are three different
ways in which a molecule
can be oxidised or reduced.
In this simple equation for respiration, glucose is oxidised as hydrogen atoms are
gradually removed from it and added to the oxygen which becomes reduced.
C6H12O6 + 6O2 🡪 6CO2 + 6H2O + energy
In redox reactions, the reduced molecule always has more potential energy
since electrons passing from one molecule to the other carry energy with them.
Glycolysis
The first stage of cellular respiration is glycolysis.
Glucose is broken down in the cytoplasm by a series of enzymes to produce
pyruvate and a small amount of ATP according to the reaction below.
Glucose 🡪 2 pyruvate + 2 ATP
Anaerobic vs Aerobic Respiration
The rest of the stages of cellular respiration are dependent upon the presence or
absence of oxygen.
Aerobic respiration is the most efficient way of producing ATP an occurs in the
mitochondria.
Anaerobic respiration occurs in the cytoplasm.
In animals, the pyruvate produced by glycolysis is converted to lactate.
In organisms such as yeast and bacteria, anaerobic respiration is known as
fermentation and produces ethanol and carbon dioxide.
Anaerobic Respiration in Humans
In humans anaerobic respiration occurs during exercise if the rate of oxygen
delivery cannot keep up with the needs of aerobic respiration.
This produces far less ATP than aerobic respiration but can allow the person to
continue exercising for a short period of time.
This produces a build up of lactate in the muscles which leads to cramping.
Lactate is taken by the blood to the liver where it is converted back to pyruvate.
The liver will either use it for aerobic respiration or convert the pyruvate
back to glucose using energy.
Fermentation
Organisms such as yeast and bacteria have lower energy needs
and perform anaerobic respiration as their main form of energy
production.
Humans use yeast and bacteria in many industries such as
brewing and baking
Aerobic Respiration
Aerobic respiration can be broken into four parts:
Glycolysis
Link Reaction
Krebs Cycle
Electron Transport Chain and Chemiosmosis
Glycolysis
Means “breaking of
glucose”
Occurs in the cytoplasm of
the cell
Is anaerobic
Produces pyruvate, 2 ATP
and 2 NADH + H+
One molecule of glucose is
converted to two
molecules of pyruvate.
Steps of
Glycolysis
Phosphorylation
Lysis
Oxidation
ATP Formation
Phosphorylation
The addition of two phosphate groups to glucose to form hexose
bisphosphate
The phosphates come from 2 ATP molecules which are used up in
this step of the reaction.
Phosphorylation causes the normally stable glucose to become a
more unstable phosphorylated compound which can by split into
two three carbon sugars in the next step of glycolysis.
Lysis
The unstable hexose bisphosphate is split into two 3
carbon sugar phosphates.
Oxidation
Another phosphorylation takes place in which an inorganic phosphate
(Pi) is added to each of the three carbon sugar phosphates.
The energy to add the Pi comes from an oxidation reaction not ATP.
The triose bisphosphate is oxidised at the same time as NAD+ is
reduced to NADH + H+.
ATP Formation
A series of reactions in which two phosphate groups from each triose
bisphosphate are transferred onto two ADP molecules to from two
molecules of ATP and one molecule of pyruvate.
Four molecules of ATP are formed in total but since two were used in
the phosphorylation step we only see a net gain of 2 ATP molecules.
The Link
Reaction
If oxygen is present the pyruvate produced in glycolysis will move into the
mitochondrial matrix by facilitated diffusion.
In the link reaction, pyruvate is converted to acetyl CoA.
Each of the two pyruvates are decarboxylated producing a 2 CO2, and 2
acetyl groups.
Hydrogen is removed from each acetyl group and added to NAD in a
redox reaction.
CoA is added to each acetyl group.
Acetyl CoA now enters the Krebs Cycle.
The Krebs Cycle
The oxidation of acetyl groups is coupled with the reduction of hydrogen
carriers.
As soon as acetyl CoA enters the cycle the CoA is removed to be recycled.
The acetyl groups (two carbons) combines with oxaloacetate (a four carbon
acid) to form citrate (a six carbon compound)One of the acetyl groups is dehydrogenated (oxidization) to release a pair of
hydrogen atoms which reduce NAD+ to NADH + H+ and decarboxylated to form
a molecule of CO2. This produces the 5 carbon acid called α- Ketoglutarate.
The α-ketoglutarate is also dehydrogenated to release a pair of hydrogen
atoms which reduces NAD+ to NADH + H+ and decarboxylated to form a
molecule of CO2. This produces a four carbon compound called succinyl-CoA.Succinyl-CoA is converted into Fumerate through two reactions that release the
co-enzyme A group, produce a molecule of ATP and release two hydrogen atoms
which reduce FAD to FADH2.
Fumerate gets converted into oxaloacetate through a dehydrogenation reaction
that releases two hydrogen atoms which reduce NAD+ to NADH + H+.
