FC303E

Biology
Theme 6: Energy transfer overview
 Energy
Living cells require energy from outside sources.
Some animals obtain energy by eating plants, and
some animals feed on other organisms that eat
plants.
Energy flows into an ecosystem as sunlight and leaves
as heat.
Photosynthesis generates O2 and organic molecules,
which are used in cellular respiration.
Cells use chemical energy stored in organic molecules
to regenerate ATP, which powers work.
 At the end of this class you
should be able to:
• Describe aerobic and anaerobic respiration
• Explain the reactions that occur in glycolysis and
understand that further steps release more ATP
• State the products of anaerobic respiration
• Compare the energy yields for aerobic and
anaerobic respiration
 Respiration
There are two methods of respiration:
• Aerobic respiration requires oxygen and
produces large amounts of ATP, along with
CO2 and H2O
• Anaerobic respiration doesn’t require
oxygen. It produces small amounts of ATP
along with lactate (in animals) or ethanol
and CO2 (in plants)
 Cellular respiration
Cellular respiration includes both aerobic and
anaerobic respiration but is often used to refer to
aerobic respiration

Although carbohydrates, fats, and proteins are all

consumed as fuel, it is helpful to trace cellular
respiration with the sugar glucose:

C6H12O6 + 6 O2  6 CO2 + 6 H2O + Energy (ATP + heat)

 Aerobic respiration
Aerobic respiration can be divided into four steps:
• Glycolysis – glucose is split into two 3-carbon
pyruvate molecules
• Link reaction – in a series of reactions the pyruvate
molecules are converted to acetylcoenzyme A (2-
carbon molecule)
• Krebs cycle (aka Citric Acid Cycle) – A cycle of
redox reactions taking in acetylcoenzyme A and
producing some ATP along with large amounts of
NADH and FADH2
• Oxidative phosphorylation – uses electrons to
synthesise ATP
Stages and Their Locations

Glycolysis Link reaction Krebs cycle

mitochondrial mitochondrial
cytoplasm
matrix matrix
Aerobic respiration

https://www.youtube.com/watch?v=4Eo7JtRA7lg
 Glycolysis
Glycolysis (“splitting of sugar”) breaks down glucose
into two molecules of pyruvate.

Glycolysis occurs in the cytoplasm and has a number

of steps which can be grouped into four stages:
• Phosphorylation of glucose into glucose phosphate
• Splitting the phosphorylated glucose
• Oxidation of triose phosphate
• Production of ATP
 Glycolysis - Phosphorylation of
glucose
In order to split a glucose molecule into pyruvate it
must first be made more reactive. This is done by
the addition of two phosphate molecules, obtained
from the hydrolysis of ATP to ADP.
 Glycolysis - Splitting
phosphorylated glucose
Each phosphorylated glucose molecule is split into
two 3-carbon molecules, known as triose
phosphates.
 Glycolysis - Oxidation and ATP
production
NAD+ removes hydrogen from each of the triose
molecules. The phosphate groups are also removed
and the 3-carbon molecule pyruvate is formed. The
phosphate groups are added to ADP to form ATP.
Glycolysis
Overall 1
molecule of
glucose gives:
• 2 molecules of
ATP
• 2 molecules of
NADH
• 2 molecules of
pyruvate
Glycolysis Summary
• ATP donates a
phosphate group to
glucose.

• Triose phosphate is
oxidised to pyruvate.

• A six-carbon sugar
bisphosphate splits
into two three-carbon
sugar phosphates
 Production of ethanol
In some bacteria and fungi (e.g. yeast) the pyruvate
molecule formed at the end of glycolysis loses a
molecule of CO2 and gains a hydrogen atom from
NADH to produce ethanol.

pyruvate + NADH → ethanol + CO2 + NAD+

This reaction has been used by humans for

thousands of years to make beer and wine.
 Production of lactate
• When muscles are working very hard they
may use up oxygen faster than it can be
supplied
• Pyruvate can take up to two hydrogen atoms
from NADH to form lactate:
pyruvate + NADH → lactate + NAD+
• The lactate is later converted back to
pyruvate when oxygen is again available
• Lactate causes muscle cramps if allowed to
build up so must be removed as soon as
possible
 Energy yields
Energy is obtained from cellular respiration in two ways:
• Substrate-level phosphorylation (glycolysis and the
Krebs cycle)
• Oxidative phosphorylation (the electron transfer chain)
This is the major source of ATP
As the Krebs cycle and oxidative phosphorylation are not
available in anaerobic respiration, due to pyruvate being
converted to lactate or ethanol, the energy yields are much
smaller.
https://www.khanacademy.org/test-
prep/mcat/biomolecules/krebs-citric-acid-cycle-and-
oxidative-phosphorylation/v/oxidative-phosphorylation-and-
chemiosmosis
 The Krebs cycle
In the presence of O2, pyruvate enters the
mitochondrion.

The inner membranes of mitochondria are

impermeable so a specific carrier is needed for
pyruvate to enter.

Before the Kreb’s cycle can begin, pyruvate must be

converted to acetyl CoA, which links the cycle to
glycolysis.
 The Krebs cycle
The Krebs cycle, also called the citric acid cycle, takes
place within the mitochondrial matrix.
There are 8 reactions in the matrix in which the
acetyl group is completely oxidised to CO2.
This is a cycle, so the molecule which is used up in
the 1st step is regenerated in the last step.
The cycle oxidizes organic fuel, generating 1 ATP, 3
NADH, and 1 FADH2 per turn.
 The Krebs cycle

The Krebs cycle has eight steps, each catalyzed by a

specific enzyme.

The acetyl group of acetyl CoA joins the cycle by

combining with oxaloacetate, forming citrate.

The next seven steps decompose the citrate back to

oxaloacetate, making the process a cycle.
The NADH and FADH2 produced by the cycle relay
electrons extracted from food to the electron transport
chain.
 The Kreb’s cycle

Each glucose molecule gives 2 acetyl groups, and therefore

2 turns of the cycle.

Each turn of the Kreb’s cycle produces 1 ATP, 3NADH and 1

FADH2.

The ATP is made by substrate level phosphorylation.

NADH and FADH2 account for most of the energy extracted

from food.
 The electron transport chain
The electron transport chain is a series of electron
carriers in the cristae of the mitochondrion.
Most of the chain’s components are proteins, which
exist in multiprotein complexes.
The first carrier accepts electrons from NADH and
passes them to the next carrier.
The carriers alternate reduced and oxidized states as
they accept and donate electrons.
 The electron transport chain
Electrons are transferred from NADH or FADH2 to the
electron transport chain.

Electrons are passed through a number of proteins

including cytochromes (each with an iron atom) to
O2.

The electron transport chain generates no ATP. The

chain’s function is to break the large free-energy drop
from food to O2 into smaller steps that release energy
in manageable amounts.
 The electron transport chain
These two electron carriers (NADH and FADH2) donate
electrons to the electron transport chain.

The electron transport chain powers ATP synthesis via

oxidative phosphorylation.

More info:
https://bio.libretexts.org/Bookshelves/Introductory_and_G
eneral_Biology/Book%3A_General_Biology_(Boundless)/7
%3A_Cellular_Respiration/7.4%3A_Oxidative_Phosphorylat
ion/7.4B%3A_Chemiosmosis_and_Oxidative_Phosphorylati
on
 The electron
transport chain

Electrons drop in free

energy as they go
down the chain.
The last carrier
donates electrons to
O2, forming H2O. This Photo by Unknown Author is licensed under CC BY
 Electron transport chain
NADH passes the electrons to the electron transport
chain.

Unlike an uncontrolled reaction, the electron

transport chain passes electrons in a series of steps
instead of one explosive reaction.

O2 pulls electrons down the chain in an energy-

yielding tumble and the energy produced is used to
regenerate ATP.
 Chemiosmosis
Electron transfer in the electron transport chain
causes proteins to pump H+ from the mitochondrial
matrix to the intermembrane space.
H+ then moves back across the membrane, passing
through channels in ATP synthase.
ATP synthase uses the exergonic flow of H+ to drive
phosphorylation of ATP.
This is an example of chemiosmosis, the use of
energy in a H+ gradient to drive cellular work.
 Chemiosmosis
The energy stored in a H+ gradient across a
membrane couples the redox reactions of the
electron transport chain to ATP synthesis.

The H+ gradient is referred to as a proton-motive

force, emphasizing its capacity to do work.
 ATP production
The process that generates most of the ATP is called
oxidative phosphorylation because it is powered by
redox reactions.

Oxidative phosphorylation accounts for almost 90%

of the ATP generated by cellular respiration.

A smaller amount of ATP is formed in glycolysis and

the Kreb’s cycle by substrate-level phosphorylation.
 ATP production
During cellular respiration, most energy flows in
this sequence:
glucose  NADH  electron transport chain
 proton-motive force  ATP

About 40% of the energy in a glucose molecule is

transferred to ATP during cellular respiration,
making about 38 ATP.
 Kahoot!
• https://create.kahoot.it/details/cellular-
respiration/5335ddbc-4540-415a-8c9c-
c1dcb092874d
 Summary
Respiration is the process of obtaining energy from
glucose (or other substrates)

Respiration can be aerobic or anaerobic

The steps for respiration are: glycolysis, the link

reaction, the Krebs cycle and oxidative
phosphorylation

Anaerobic respiration produces ethanol or lactate

and has a much lower energy yield