How cells Acquire energy Chapter 5 ,6
Topics to be covered
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ATP and cellular work
Enzymes
Cellular respiration
Fermentation
ATP and cellular work
 Carbohydrates, fats and other fuel molecules that we obtain from food cannot be directly used as
fuel by our cells
 Chemical energy released by the breakdown of these fuel molecules are stored in the form of ATP
 ATP then powers cellular work
Structure of ATP
The tail of 3 phosphate groups in the ATP is the key part that provides energy for cellular work.
Each phosphate is negatively charged and these -ve charges repel each other.
The crowding of the -ve charge in the tail contributes to the Potential Energy of ATP.
Structure of ATP
• The transfer of chemical energy within the living cells is managed by a nucleotide,
Adenosine Tri Phosphate (ATP)
• The chemical energy is stored when ATP is made and it is broken when released
• ATP has 3 phosphate groups attached to it. The bonds holding the last 2P (phosphates)
are the high-energy phosphate bonds, which are easily broken.
• When these are broken, the energy released is transferred to a lower energy
molecule or released to the environment.
The ATP Cycle
Phosphate Transfer
This energy helps cells perform
• Transport work
• Mechanical work,
• Chemical work.
ENZYMES
Any of several complex proteins that are produced by cells and act as catalysts in specific biochemical reactions
• An enzyme is a protein molecule that acts as a catalyst to speed the
rate of the reaction and are Reusable
• The formation, breakdown and rearrangement of molecules to provide organisms with
essential energy and building blocks are known as Biochemical reactions
• The input of energy required to get these reactions started is called Activation energy.
• Raising the Temperature routinely helps in supplying the activation energy, however, this
rise in temperature results in denaturation of the proteins.
• The use of catalyst helps in increasing the rate of the reaction, without affecting the cells
proteins.
• Enzymes can be used over and over again until they are worn out or damaged
• The production of these proteins is directly under the control of an organism’s genetic
material (DNA).
• The instructions for manufacture of these enzymes are found on the genes of the cell.
• Organisms make their own enzymes.
How does the enzyme work?
When enzyme binds with substrate (reactant), it provides
o Physical stress
o Chemical stress
When chemical bonds get Stress (strain)– they break effortlessly /easily
Enzyme
Turnover number /Sec
Catalase
Carbonic Anhydrase
3-Ketosteroid isomerase
Acetylcholinesterase
2,800,000
600,000
280,000
25,000
Enzyme-Substrate-Complex
Enzyme Activity
Enzymes Bind selectively to Substrates:
•The 3-D shape, Size and Charge are responsible for allowing the enzyme to combine with a reactant* and lower
the activation energy.
*(chemical substance that is present at the start of a chemical reaction)
•The molecule to which the enzyme attaches (binds) itself is called the substrate and the temporary molecule
formed is called the Enzyme-Substrate-Complex.
• The active site is the place on the enzyme which has the shape & chemistry that fits the substrate molecule.
• It is a pocket or a grove on the surface of the enzyme into which the substrate slips.
• This induces in a change in the shape of the enzyme at the active site to embrace the substrate and catalyze
the reaction…Induced fit hypothesis.
• Active site is a place where chemical bonds are formed or broken.
• This is the site where activation energy is lowered and the electrons are shifted to
change the bonds.
• Though the active site molds itself to the substrate, enzymes cannot fit all substrates
and are specific to certain substrates or group of very similar substrate molecules.
How enzyme works
EA activation energy
Enzyme inhibitors
An inhibitor is a molecule that attaches itself to an enzyme and interferes with the enzyme’s ability to form
an enzyme-substrate complex.
Competitive inhibition:
Some inhibitors have a shape that closely resembles the normal substrate (substrate imposters) of the
enzyme and hence the enzyme cannot differentiate between the two. The inhibitor competes with the
substrate for the active site of the enzyme.
As long as the inhibitor is bound to the enzyme, the active site of the enzyme is not available for the
substrate and hence the product is not formed.
The reaction [of the enzyme catalyzes] doesn't occur atall and hence the product is not formed. This is termed
as Competitive inhibition.
Non Competitive inhibition:
Other inhibitors bind to the enzyme
at a site remote from active site, but
the binding changes the enzyme’s
shape, hence not allowing the
substrate to bind at the active site.
Feedback Inhibition
•
•
Reversible inhibition
Prevents the cell from wasting resources
Cellular Respiration
• Requires a cell to exchange two gases
with its surroundings
• Aerobic process
• Cellular respiration is defined as a
“process of aerobic harvesting of
chemical energy from organic fuel
molecules”
Aerobic cellular respiration is a series of
enzyme controlled chemical reactions
in which O2 is involved in the breakdown
of glucose to Co2 and water and the
chemical-bond energy from glucose is
released to the cell in the form of ATP.
The following equation summarizes the net result of the reaction
between sugar and oxygen to form carbon dioxide and water :
Glucose + Oxygen
C6H12O6 + 6O2
Carbon dioxide + water + energy
6CO2 + 6H2O + energy (ATP + heat)
Of all the covalent bonds in glucose, the ones that are
easiest to break are the C-H and O-H bonds which are
present on the outside of the molecule.
When these bonds are broken, two things happen:
1.The energy of the electrons can be used to phosphorylate
ADP molecules to produce higher-energy ATP molecules
and
2.Hydrogen ions (protons) are released.
 The ATP is used to power the metabolic activities of the cell. The chemical
activities that remove electrons from glucose result in the glucose being oxidized.
 These high energy electrons must be controlled. Electron transfer molecules like
NADH and FADH2 temporarily hold the electrons and transfer them to other
electron carriers.
 ATP is formed when these transfers take place.
 In aerobic cellular respiration oxygen serves as the terminal electron acceptor.
When the electrons are added to oxygen it becomes a negatively charged ion (O--)
and hence becomes reduced.
 The positively charged hydrogen ions that are released from glucose molecule
combine with the negatively charged oxygen ions to form water.
 Once all the hydrogen are removed from the glucose molecule, the remaining
carbon and oxygen atoms are rearranged to form individual molecules of CO2. The
redox reactions are complete.
 All the hydrogen removed from glucose combines with oxygen to form water.
The energy released is used to generate ATP. The process can produce 32 ATP for
each glucose molecule consumed.
 In eukaryotic cells, the process of releasing energy from food begins in the
cytoplasm and is completed in the mitochondria.
 There are three distinct enzymatic pathways or stages involved:
 Glycolysis, Krebs cycle and Electron transport chain.
Glycolysis:
• Glycolysis (glycos = sugar; lysis = split) takes place in the cytoplasm of the cells
and results in the breakdown of glucose with the release of electrons and the
formation of ATP.
• Glucose has energy added to it from 2 ATP molecules. This extra energy makes
some of the bonds in glucose unstable and glucose is more readily broken
down.
• After passing through four enzymatic reactions, 6-C is cleaved into 2
molecules of 3-C molecules.
• These undergo 5 more reactions to form Pyruvic acid or pyruvate.
Pyruvate
Pyruvate
•
Electrons released from the bond splitting are picked by NAD+ to form NADH.
•
In addition to NADH, glycolysis also makes 4 ATP molecules directly when enzymes
transfer phosphate groups from fuel molecules to ADP.
•
Since 2 ATP molecules are consumed in starting the reaction, the net gain of ATP is
2 ATP per molecule of glucose in glycolytic pathway.
•
2 NADH2 are formed.
•
These have large potential energy that can be used to form ATP through ETC.
Thus the generalized reaction that summarizes the events of glycolysis is:
Glucose + 2 ATP + 2 NAD+
4 ATP + 2 NADH + 2 Pyruvic acid
Summary:
1. Requires the use of 2 ATPs
2. Ultimately results in the formation of 4 ATPs
3. Results in the formation of 2 NADHs, - These have large potential energy that can be
used to form ATP through ETC
4. Results in the formation of 2 molecules of pyruvic acid
Citric Acid Cycle or Krebs Cycle
The 2 molecules of pyruvic acid are groomed inside mitochondria for citric acid synthesis.
They first lose a carbon as CO2, resulting in formation of acetate;
Electrons are stripped from the molecules and transferred to NAD+ forming NADH;
Each acetate molecule s attached to Coenzyme A (CoA)
The acetyl-coenzyme A proceeds through the Krebs cycle and is completely
oxidized.
The acetyl portion of the molecule is transferred to 4-carbon compound,
oxaloacetate and a new 6-C compound, citrate is formed.
The CoA is released to participate in another reaction with pyruvic acid.
For every acetic acid molecule that enters the cycle, two CO2 molecules are
released as waste.
Some energy is used to produce ATP directly.
However most energy is captures in the form of NADH and FADH2.
All carbon atoms that enter the cell as fuel are accounted for as CO2 exhausted
and the 4 C oxaloacetate is recycles.
The series of compounds formed here are called
keto acids.
Tricarboxylic acid cycle (TCA)
These reactions take place in the mitochondria.
The Krebs cycle is also known as citric acid cycle and ).
2 Molecules of Acetyl CoA
from each Glucose
molecule; and multiply
number of CO2, GTP, and
reducing equivalents
produced accordingly
 Acetic acid joins a 4C acceptor compound, Oxalo acetate to form 6C citric acid.
 When every acetic acid molecule that enters the cycle, two CO2 molecules exit
as waste
Some of the energy is used to produce ATP directly
However much more energy is captured by NADH and FADH2 in the form of
electrons.
All carbon atoms that entered the cycle are lost as CO2.
This is the account from one molecule of acetic acid, however from a glucose
molecule, 2 acetic acid molecules are formed, hence all the above products will
be doubled.
Thus the overall reaction in the Krebs cycle is:
2 Pyruvic acid+ 8NAD+ + 2FAD + 2ADP + 2H3PO4
6CO2 + 8 NADH + 8H+ + 2FADH2 + 2ATP
Overview of Citric Acid Cycle
Electron transport system
This is the final stage of aerobic cellular respiration and is dedicated to generating energy, ATP.
These are a series of redox reactions with oxygen as the final electron acceptor.
The negatively charged oxygen combines with hydrogen ions to form water.
This step makes the process aerobic.
Why does electron transfer to oxygen release energy?
• When electrons move from glucose to oxygen, it is as
though the electrons were falling.
• This “fall” of electrons releases energy during cellular
respiration.
• The path that electrons take on their way down from
glucose to oxygen involves many steps.
Structure of mitochondria
The mitochondria consists of two membranes; an outer enclosing membrane and an inner
folded membrane.
The reactions of the ETC are associated with this inner membrane. The inner membrane
being highly folded offers a large surface area and hence can accommodate thousands of
copies of the ETC.
Each chain uses the energy released by the ‘fall’ of electrons to move hydrogen ions (H+)
across the inner mitochondrial membrane.
This pumping causes ions to become more concentrated on one side of the membrane
than on the other. This difference in the proton concentration stores potential energy.
There is a tendency for the hydrogen ions to gush back to the low concentration region,
which occurs through a regulator enzyme called ATP synthase.
How electron transport drives ATP synthesis
Summary of ATP yield during Cellular Respiration
Food
Polysaccharides
Sugars
Fats
Glycerol Fatty acids
Glycolysis
Acetyl
CoA
Proteins
Energy from food. The
monomers from
carbohydrates
(polysaccharides
and sugars), fats, and proteins
can all serve as fuel for cellular
respiration.
Amino acids
Citric
Acid
Cycle
Electron
Transport
ATP
Figure 6.12
FERMENTATION: Anaerobic Harvest Of Food Energy
–Some of your cells can actually work for short periods without oxygen.
–Fermentation is the anaerobic (without oxygen) harvest of food energy.
Fermentation in Human Muscle Cells
–After functioning anaerobically for about 15 seconds:
• Muscle cells will begin to generate ATP by the process of fermentation
–Fermentation relies on glycolysis to produce ATP.
• Glycolysis:
– Does not require oxygen
– Produces two ATP molecules for each glucose broken down to pyruvic acid
Pyruvic acid, produced by glycolysis, is
 Reduced by NADH, producing NAD+, which keeps
glycolysis going.
• In human muscle cells, lactic acid is a byproduct.
Fermentation in Microorganisms
– Fermentation alone is able to sustain many types of
microorganisms.
– The lactic acid produced by microbes using fermentation is
used to produce:
• Cheese, sour cream, and yogurt dairy products
• Soy sauce, pickles, olives
• Sausage meat products
• Yeast are a type of microscopic fungus that:
• Use a different type of fermentation
• Produce CO2 and ethyl alcohol instead of lactic acid
• This type of fermentation, called alcoholic fermentation, is used to
produce:
• Beer
• Wine
• Breads
Fermentation is the incomplete oxidation of glucose:
C6H12O6 + (H+ and e- acceptor)
smaller hydrogen containing
molecules + energy (ATP + heat)
Typically glucose proceeds through glycolysis producing pyruvic acid.
The pyruvic acid then undergoes several alternative changes depending on the kind of
organism and the specific enzymes it possesses.
In some organisms pyruvic acid is reduced to lactic acid, while in some organisms, it is
reduced to alcohol and carbondioxide.
NADH
NAD+
Pyruvic acid
NADH
Lactic acid
NAD+
ethanol + carbon di oxide
Alcoholic fermentation is the anaerobic respiration that the yeast cells follow when
oxygen is lacking………………….the cell profits 2 ATPs in this fermentation.
INPUT
2 ADP
2 P
OUTPUT
2 ATP
2 CO2 released
Glycolysis
2 NAD 2 NADH
Glucose
2 NADH 2 NAD
2 Pyruvic
 2 H
acid
2 Ethyl alcohol
Bread with air
bubbles produced
by fermenting yeast
Beer
fermentation
Figure 6.16