Chapter 5
The Working Cell:
Chemical Energetics and Enzymes
I. Energy: The capacity to do work. The
ability to change matter
Can exist in two forms:
1. Kinetic energy: Energy of motion. Energy
that is actively performing work. Examples:
Heat: Energy of particles in motion.
 Light: Energy of photons of light

2. Potential energy: Stored energy due to
position or arrangement of matter. Examples:

Chemical energy: Potential energy of molecules due to
the arrangement of atoms. The most important type of
energy for living organisms.

Position: Bicycle at the top of a hill.
Kinds of Energy
Chemical
Nuclear
Electromagnetic
Light
Electrical
Mechanical
Heat
Sound
II. Energy Transformation
Energy can be converted from one kind to another.
Transformations are inefficient, generating heat.
Examples:
Light energy --------
--> Chemical energy (sugar) + Heat
Chemical energy --
---> Mechanical energy + Heat
Electrical energy ---
--> Light energy + Heat
Chemical energy ---
--> Biological work + Heat
Heat is easily measured energy, because all other
forms of energy can be converted to heat.
From a biological standpoint, heat is a poor kind of
energy which is not very useful to do work.
Why? Because heat is lost to the environment.
III. All energy transformations are subject to
the First and Second Laws of
Thermodynamics
1. First Law of Thermodynamics: Energy can
be transformed (e.g.: chemical to mechanical),
but cannot be created nor destroyed. The total
amount of energy in the universe is constant.
Biological Consequence: Living organisms
cannot create the energy they need to live.
They must capture it from their environment.
Sources of energy used by living organisms:
Sun and chemical energy.
2. Second Law of Thermodynamics
ENERGY CONVERSIONS ARE INEFFICIENT
In any energy transformation, a certain amount of
energy is lost as heat.
By comparison, living organisms are relatively
efficient.
Electrical Energy ------->
Chemical Energy -->
(Gasoline)
Chemical Energy -------->
(Glucose)
--------->
---------------->
5% Light + 95% Heat
25% Mechanical
75% Heat
-----------> 40% ATP + 60% Heat
III. Laws of Thermodynamics (Cont.)
2. Second Law of Thermodynamics: The universe
inevitably tends toward a state of increased
disorder or chaos (entropy). For this reason,
energy transformations are inefficient.
Entropy (S): Measure of disorder.
Disorganized, less usable energy (heat).
Only way to overcome entropy, is to put energy into
the system.
Biological Consequences: Living organisms must
constantly take in energy to avoid entropy
(disintegration, death and decay).
High quality energy is a limited resource, because
usable energy, decreases over time.
IV. Chemical Reactions Either Store or Release
Energy:
I. Exergonic Reactions:
 Release free energy.
 Also exothermic (release heat).
 Products have less energy than the reactants.
 Example:
Cellular respiration is an exergonic process:
C6H12O6 + 6 O2 ----> 6 CO2 + 6 H2O + Energy
Sugar
Oxygen
High Energy Reactants
Carbon
Water
Dioxide
Low Energy Products
IV. Chemical Reactions Store or Release Energy:
II. Endergonic Reactions:
 Require net input of free energy.
 Also endothermic (absorb heat).
 Products have more energy than the reactants.
 Create products that are rich in potential energy.
 Example:
Photosynthesis is an endergonic process:
6 CO2 + 6 H2O + Sunlight ----> C6H12O6 + 6 O2
Carbon
Water Energy
Dioxide
Low Energy Reactants
Sugar
Oxygen
High Energy Products
Chemical Reactions Either Store or Release Energy
Endergonic Reactions
Exergonic Reactions
Require Energy
Higher Energy Products
Release Energy
Lower Energy Products
Metabolism: All chemical processes that occur within a
living organism. Either catabolic or anabolic reactions.
I. Catabolic Reactions:
 Release
energy (exergonic).
 Break
down large molecules (proteins, polysaccharides)
into their building blocks (amino acids, simple sugars).
 Often
coupled to the endergonic synthesis of ATP.
Examples:
1. Cellular respiration is a catabolic process:
C6H12O6 + 6 O2 -------> 6 CO2 + 6 H2O + Energy
Sugar
Oxygen
Carbon dioxide Water
2. The digestion of sucrose is a catabolic process:
Sucrose + Water -------> Glucose + Fructose + Energy
Disaccharide
Monosaccharides
Metabolism: Catabolism + Anabolism
II. Anabolic Reactions:
 Require
energy (endergonic).
 Build
large molecules (proteins, polysaccharides) from
their building blocks (amino acids, simple sugars).
 Often
coupled to the exergonic breakdown or hydrolysis
of ATP.
Examples:
1. Photosynthesis is an anabolic process:
6 CO2 + 6 H2O + Sunlight ----> C6H12O6 + 6 O2
Carbon
Dioxide
Water
Sugar
Oxygen
2. Synthesis of sucrose is an anabolic process:
Glucose + Fructose + Energy -------> Sucrose + H2O
Monosaccharides
Disaccharide
V. ATP: Shuttles Chemical Energy in the Cell
 Coupled
Reactions:

Endergonic and exergonic reactions are often coupled to each
other in living organisms.

The energy released by exergonic reactions is used to fuel
endergonic reactions.
 ATP “shuttles”
energy around the cell from exergonic
reactions to endergonic reactions.

One cell makes and hydrolyzes about 10 million ATPs/second.

Cells contain a small supply of ATP molecules (1-5 seconds).
 ATP powers
nearly all forms of cellular work:
1. Mechanical work: Muscle contraction, beating of flagella and
cilia, cell movement, movement of organelles, cell division.
2. Transport work: Moving things in & out of cells.
3. Chemical work: All endergonic reactions.
A. Structure of ATP (Adenosine triphosphate)

Adenine: Nitrogenous base.

Ribose: Pentose sugar, same ribose of RNA.

Three Phosphate groups: High energy bonds.
B. ATP Releases Energy When Phosphates Are
Removed:
Phosphate bonds are rich in chemical energy and
easily broken by hydrolysis:
ATP + H2O ----> ADP + Energy + Pi
ADP + H2O ----> AMP + Energy + Pi
Structure and Hydrolysis of ATP
C. Regeneration of ATP:
ATP can be regenerated through
dehydration synthesis:
ADP + Energy + Pi ----> ATP + H2O
Phosphorylation: Transfer of a phosphate
group to a molecule. Requires energy.
The energy required for this endergonic
reaction is obtained by trapping energy
released by other exergonic reactions
(E.g.: Cellular respiration).
ATP Shuttles Energy From Exergonic
Reactions to Endergonic Reactions
VI. Enzymes:

Protein molecules that catalyze the reactions of living
organisms.

Enzymes increase the rate of a chemical reaction without
being consumed in the process.

Name: Substrate (or activity) + ase suffix
Examples:

Sucrase

Lipase

Proteinase

Dehydrogenase (Removes H atoms)

Enzymes are specific: Catalyze one or a few related
reactions.

Enzymes are efficient. Can increase the rate of a reaction
10 to billions of times!!!!
VI. Enzymes:
 Enzymes
increase the rate of a chemical reaction by
lowering the activation energy required to initiate the
reaction.
 Activation energy of a reaction: Energetic barrier that
reactant molecules must overcome for reaction to
proceed.
Creation of new bonds requires breaking of old bonds.

Both exergonic and endergonic reactions

Transition state :“Intermediate” state of reactants
Enzymes Lower the Energy of Activation of
a Chemical Reaction
Enzyme Mechanism of Action:
1. Binding: Enzyme binds to the reactant(s),
forming an enzyme-substrate complex.

Substrate: The reactant the enzyme acts upon to
lower the activation energy of the reaction.

Active site: Region on enzyme where binding to
substrate occurs.
• Active site dependent upon proper 3-D
conformation.
Enzyme Mechanism of Action:
2. Induced fit model: After enzyme binds to
substrate, it changes shape and lowers activation
energy of the reaction by one of several
mechanisms:

Straining chemical bonds of the substrate

Bringing two or more reactants close together

Providing “micro-environment” conducive to
reaction
3. Release: Once product is made, it is released
from active site of enzyme.
Enzyme is ready to bind to another substrate
molecule.
Mechanism of Enzyme Action
VII. Environmental Factors Affect Enzyme Function
A. Temperature: Optimal temperature.
 Most
reactions are too slow at low temperatures.
 Most
enzymes are denatured over 50-60oC.
B. Salt concentration: Optimal concentrations vary for each
enzyme.
C. pH: Optimal pH varies, but most enzymes work best
close to pH 7.

Pepsin optimal pH is 2

Amylase optimal pH is 8.5
D. Other essential molecules are required by some enzymes:

Cofactors: Inorganic atoms that must bind (Cu, Fe, Zn)

Coenzymes: Organic compounds that must bind (vitamins)
VII. Regulation of enzyme activity
A. Concentration of substrate (enzyme constant)
B. Concentration of enzyme (substrate constant)
C. Enzyme Inhibitors

Competitive inhibitors: Bind to active site of enzyme.
Inhibitor resembles normal substrate.
Example:
• AZT: An AIDS drug. Inhibits an HIV enzyme
responsible for viral replication. Resembles
nucleotide.

Noncompetitive inhibitors: Bind to another site (allosteric
site) of enzyme.
Active site is altered and unable to bind substrate.
Inhibitor usually does not resemble substrate.
Competitive versus Noncompetitive Enzyme Inhibition
VII. Regulation of enzyme activity
D. Feedback Inhibition: Final product of a metabolic
pathway can inhibit enzymes that catalyze its synthesis.
Enzyme 1
Enzyme 2
Enzyme 3
Enzyme 4
A ---------> B ---------> C ----------> D --------> E (Product)
Product (E) inhibits enzyme 1, shutting down pathway.
Example:
ATP regulates its own production this way.
VII. Regulation of enzyme activity
E. Inhibition can be temporary or permanent:
 Reversible inhibition:
Inhibitor binds through weak bonds (H-bonds).
 Inhibitor is released when more substrate is added.
 Once inhibitor is released, enzyme is functional.
 Example: Many reactions of cellular metabolism.

• ATP feedback inhibition is temporary
 Irreversible
inhibition:
Covalent bonds form between enzyme and inhibitor.
 Enzyme is permanently inactivated.
 Example: Many drugs, pesticides, and poisons.

• Penicillin inhibits bacterial cell wall synthesis.
• Malathion inhibits nervous system enzyme
(acetylcholinesterase).
The Cell Membrane and Cell Transport
Functions of Cell Membranes
1. Separate cell from nonliving environment. Form
most organelles and partition cell into discrete
compartments.
2. Regulate passage of materials in and out of the
cell and organelles. Membrane is selectively
permeable.
3. Receive information that permits cell to sense
and respond to environmental changes.



Hormones
Growth factors
Neurotransmitters
4. Communication with other cells and the
organism as a whole. Surface proteins allow cells
to recognize each other, adhere, and exchange
materials.
I. Fluid Mosaic Model of the Membrane
1. Phospholipid bilayer: Major component is a
phospholipid bilayer.

Hydrophobic tails face inward

Hydrophilic heads face water
2. Mosaic of proteins: Proteins “float” in the
phospholipid bilayer.
3. Cholesterol: Maintains proper membrane
fluidity.
The outer and inner membrane surfaces are
different.
Membrane
Phospholipids
Form a
Bilayer
The Membrane is a Fluid Mosaic of
Phospholipids and Proteins
Notice that inner and outer surfaces are different
A. Fluid Quality of Plasma Membranes
 In
a living cell, membrane has same fluidity as
salad oil.

Unsaturated hydrocarbon tails INCREASE membrane
fluidity
 Phospholipids
and proteins drift laterally.
Phospholipids move very rapidly
 Proteins drift in membrane more slowly

 Cholesterol: Alters
fluidity of the membrane
Decreases fluidity at warmer temperatures (> 37oC)
 Increases fluidity at lower temperatures (< 37oC)

B. Membranes Contain Two Types of Proteins
1. Integral membrane proteins:
Inserted into the membrane.
Hydrophobic region is adjacent to hydrocarbon tails.
2. Peripheral membrane proteins:
Attached to either the inner or outer membrane surface.
Functions of Membrane Proteins:
1. Transport of materials across membrane
2. Enzymes
3. Receptors of chemical messengers
4. Identification: Cell-cell recognition
5. Attachment:


Membrane to cytoskeleton
Intercellular junctions
Membrane Proteins Have Diverse Functions
C. Membrane Carbohydrates and Cell-Cell
Recognition
 Found
on outside surface of membrane.
 Important
for Cell-cell recognition: Ability of one cell
to “recognize” other cells.
Allows immune system to recognize self/non-self
 Include:

• Glycolipids: Lipids with sugars
• Glycoproteins: Proteins with sugars
• Major histocompatibility proteins (MHC or
transplantation antigens).
Vary greatly among individuals and species.
 Organ transplants require matching of cell
markers and/or immune suppression.

The cell plasma membrane is Selectively Permeable
A. Permeability of the Lipid Bilayer
1. Non-polar (Hydrophobic) Molecules
• Dissolve into the membrane and cross with ease
• The smaller the molecule, the easier it can cross
• Examples: O2 , hydrocarbons, steroids
2. Polar (Hydrophilic) Molecules
• Small polar uncharged molecules can pass through
easily (e.g.: H2O , CO2)
• Large polar uncharged molecules pass with
difficulty (e.g.: glucose)
3. Ionic (Hydrophilic) Molecules
• Charged ions or particles cannot get through
(e.g.: ions such as Na+ , K+ , Cl- )
Transport Proteins in the membrane:
Integral membrane proteins that allow for
the transport of specific molecules across
the phospholipid bilayer of the plasma
membrane.
How do they work?
May provide a “hydrophilic tunnel” (channel)
 May bind to molecule and physically move it
 Are specific for the atom/molecule transported

III. Passive transport: Diffusion of molecules
across the plasma membrane
A. Diffusion: The net movement of a substance
from an area of high concentration to area of
low concentration.
Does not require energy.
B. Passive transport: The diffusion of substance
across a biological membrane.
Only substances which can cross bilayer by
themselves or with the aid of a protein
 Does not require the cell’s energy

Passive Transport: Diffusion Across a
Membrane Does Not Require Energy
IV. Osmosis:
The diffusion of water across a semi-permeable
membrane.
Through osmosis water will move from an area
with higher water concentration to an area with
lower water concentration.
Solutes can’t move across the semi-permeable
membrane.
Osmotic Pressure: Ability of a solution to take up water
through osmosis.
Example: The cytoplasm of a cell has a certain osmotic
pressure caused by the solutes it contains.
There are three different types of solution when compared to
the interior (cytoplasm) of a cell:
1. Hypertonic solution: Higher osmotic pressure than cell due to:
Higher solute concentration than cell or
Lower water concentration than cell.
2. Hypotonic solution: Lower osmotic pressure than cell due to:
Lower solute concentration than cell or
Higher water concentration than cell.
3. Isotonic solution: Same osmotic pressure than cell.
Equal concentration of solute(s) and water than cell.
V. Cells depend on proper water balance
Animal Cells:
Do best in isotonic solutions.
Examples:

0.9% NaCl (Saline)

5% Glucose
If solution is not isotonic, cell will be affected:
 Hypertonic
solution: Cell undergoes crenation.
Cell “shrivels” or shrinks.

Example: 5% NaCl or 10% glucose
 Hypotonic
solution: Cell undegoes lysis. Cell
swells and eventually bursts.

Example: Pure water.
V. Cells depend on proper water balance
Plant Cells: Do best in hypotonic solutions, because
the cell wall protects from excessive uptake of
water.
 Hypertonic
solution: Cell undergoes plasmolysis.
Cell membrane shrivels inside cell wall.
 Isotonic
solution: Cell becomes flaccid or wilts.
 Hypotonic
solution: Turgor. Increased firmness of
cells due to osmotic pressure.

This is the reason why supermarkets spray fruits and
vegetables with pure water, making them look firm and
fresh.
VI. Facilitated Diffusion:
Some substances cannot cross the membrane by
themselves due to their size or charge.
Membrane proteins facilitate the transport of solutes
down their concentration gradient.
No cell energy is required.
Transport Proteins
 Specific
: Only transport very specific molecules
(binding site)
Glucose
 Specific ions (Na+, K+, Cl- )

Facilitated Diffusion Uses a Membrane
Transport Protein
VI. Active Transport:
 Proteins use energy from ATP to actively “pump”
solutes across the membrane
 Solutes are moved against a concentration gradient.
 Energy is required.
Example:
The Na+-K+ ATPase pump:
Energy of ATP hydrolysis is used to move
Na+ out of the cell and K+ into the cell
Endocytosis:
Moving materials into cell with vesicles.
Requires use of cell energy.
1. Pinocytosis (“Cell drinking”): Small droplets of liquid are
taken into the cell through tiny vesicles.
Not a specific process, all solutes in droplets are taken in.
2. Phagocytosis (“Cell eating”): Large solid particles are
taken in by cell.
Example: Amoebas take in food particles by surrounding
them with cytoplasmic extensions called pseudopods.
Particles are surrounded by a vacuole.
Vacuole later fuses with the lysosome and contents are
digested.
Endocytosis Uses Vesicles to Move
Substances into the Cell
Endocytosis:
3. Receptor mediated endocytosis: Highly specific.
Materials moved into cell must bind to specific
receptors first.
Example: Low density lipoproteins (LDL):
 Main form of cholesterol in blood.
 Globule of cholesterol surrounded by single layer of
phospholipids with embedded proteins.
 Liver cell receptors bind to LDL proteins and remove
LDLs from blood through receptor mediated
endocytosis.
 Familial hypercholesterolemia: Genetic disorder in
which gene for the LDL receptor is mutated.
Disorder found in 1 in 500 human babies worldwide.
Results in unusually high levels of blood cholesterol.
Blood Cholesterol is Taken Up by Liver Cells
through Receptor Mediated Endocytosis
Exocytosis:
Used to export materials out of cell.
Materials in vesicles fuse with cell
membrane and are released to outside.
Tear glands export salty solution.
 Pancreas uses exocytosis to secrete insulin.
