CHAPTER 4:
Outcomes:
4.1
THE ENERGY OF LIFE
-Understand the laws of thermodynamics and energy conversions.
-Know the forces that cause water and solutes to move across membranes
passively.
-Understand how enzymes function in chemical reactions.
-Understand which type of solutes move by simple diffusion and which by
Facilitated diffusion. Understand the importance of osmosis to all cells.
-Know the mechanisms by which substances are moved across
membranes against a concentration gradient.
-Understand how material can be imported into or exported from a cell
by being wrapped in membranes.
All Cells Capture and Use Energy
Energy is the ability to do work. Energy is fundamental to life, as many cellular processes
require energy. The total amount of energy in any object is the sum of energy’s two forms,
namely Potential Energy, which is stored energy available to do work, and Kinetic Energy,
which is energy being used to do work, any moving object possesses kinetic energy. Calories
are units used to measure energy. One calorie (cal) is the amount of energy required to raise
the temperature of 1 gram of water by 1 C. The most common unit for measuring the energy
content of food is Kilocalories (kcal) which equals 1000 calories.
Thermodynamics is the study of energy transformations. The first and second laws of
thermodynamics describe the energy conversions vital for like, as well as those that occur in
the nonliving world.
The First Law of Thermodynamics is the Law of Energy Conservation. It states that energy
cannot be created or destroyed, although energy can be converted to other forms. This means
that the total amount of energy in the universe is constant.
There are different forms of energy, e.g.:
-Solar/Light Energy: energy received from the sun that is absorbed to produce carbohydrates
during photosynthesis.
-Chemical Energy: this is energy stored in the bonds of molecules, which can be released to
perform work, e.g. the energy available in ATP, fats and carbohydrates.
-Kinetic Energy: energy of movement.
-Heat Energy: energy that is given off in the form of heat during a chemical reaction.
The Second Law of Thermodynamics states that all energy transformations are inefficient
because every reaction loses some energy to the surroundings as heat. This process is
irreversible and the lost heat energy will not return to a useful form. Heat energy results from
random molecular movements. Because heat is disordered, and all energy eventually becomes
heat energy it follows that all energy transformations must head toward increasing disorder.
Entropy is a measure of this randomness. In general the more disordered a system is, the higher
its entropy.
4.2
Networks of Chemical Reactions Sustain Life
The word Metabolism encompasses all the chemical reactions in cells. These reactions
rearrange atoms into new compounds and each reaction either releases or absorbs energy.
Two categories of chemical reactions are found:
An Endergonic Reaction requires an input of energy to proceed. The products therefore
contain more energy than the reactants
E
n
e
r
g
y
Activation energy
Energy level of products
Energy level of reactants
Reaction
An Exergonic Reaction releases energy. The products therefore contain less energy than the
reactants
E
n
e
r
g
y
Activation energy
Energy level of reactants
Energy level of products
Reaction
Most chemical reactions can proceed in both directions, i.e. when enough product forms, some
of it converts back to reactants. Arrows going in both directions between reactants and products
indicate reversible reactions. At chemical equilibrium the reaction goes in both directions at
the same rate. This does not necessarily mean that the amounts of products and reactants are
equal but rather their rates of formation are equal.
Electrons can carry energy. Most energy transformations in organisms occur in OxidationReduction (Redox) reactions, which transfer energized electrons from one molecule to
another. Oxidation means the loss of electrons from a molecule, atom or ion. Oxidation
reactions are exergonic and they release energy as they degrade complex molecules into
simpler products. Reduction means a gain of electrons (plus any energy contained in the
electrons). Reduction reactions are therefore endergonic and they require a net input of energy.
Oxidation and reduction occur simultaneously because electrons removed from one molecule
during oxidation must join another molecule and reduce it, i.e. if one molecule is reduced (gains
electrons), then another must be oxidized (loses electrons).
Some proteins are electron-shuttling specialists. Groups of electron carriers often align in
membranes. In an Electron Transport Chain, each protein accepts an electron from the
molecule before it and passes it to the next. Small amounts of energy are released at each step
of an electron transport chain and the cell uses this energy in other reactions.
4.3
ATP Is Cellular Energy Currency
The covalent bonds of Adenosine Triphosphate or ATP temporarily store energy. This energy
is released during exergonic reactions in the cells. ATP is a nucleotide with an adenine base, a
five-carbon sugar and three phosphate groups (PO4). The phosphate groups place three negative
charges very close to one another. This arrangement makes the molecule unstable so it releases
energy when the covalent bonds between the phosphates break. In eukaryotic cells
mitochondria produce most of the cell’s ATP during cellular respiration.
Energy is released from ATP in exergonic reaction where the endmost phosphate group is
removed through hydrolysis to yield Adenosine diphosphate, a free phosphate group and a
burst of energy.
ATP + H2O  ADP + PO4 + energy
Coupled Reactions are simultaneous reactions in which one provides the energy that drives
the other. Cells couple the hydrolysis of ATP to endergonic reactions that occur at the same
time and drive the endergonic reactions, which lead to work or synthesis of new molecules. In
this coupling a cell uses ATP as an energy source by Phosphorylating (transferring its
phosphate group to) another molecule. This transfer of phosphate may either energize the target
molecule or change the shape of the target molecule.
Organisms require huge amounts of ATP and they recycle ATP at a fast rate, adding phosphate
groups to ADP to reconstitute ATP. Because the molecule is unstable, it cannot be stored for
long periods of time. Cells rather store other molecules such as lipids of carbohydrates, which
are then diverted to the metabolic pathway of respiration when ATP supplies are running low.
This produces additional ATP.
4.4
Enzymes Speed Biochemical Reactions
Enzymes are organic molecules that Catalyse (speed up) chemical reactions without being
consumed. They are among the most important of all biological molecules. Most enzymes are
Proteins but some may be made up of RNA. Their functions are varied and many organelles
such as the mitochondria, chloroplasts, lysosomes etc. are specialized sacs of enzymes.
Enzymes speed up reactions by lowering the Activation Energy, the amount of energy
required to start a reaction. Enzymes bring the reactants (or Substrates) into contact with one
another so that less energy is required for the reaction to proceed. Most enzymes can only
catalyse on or a few reactions. The key to this specificity lies in the shape of the enzyme’s
Active Site, the region to which the substrates bind. The substrates fit like puzzle pieces into
the active site (lock-and-key model). Once the reaction occurs, the enzyme releases the
products. Its active site is then empty and ready to pick up more substrate.
Activation energy: not
catalysed
E
N
E
R
G
Y
Activation energy: catalysed
Energy level: reactants
Energy level: products
TIME
+
Enzyme
+
Substrate
[E-S]
Enzyme
Products
The six major enzyme categories are:
-Oxidoreductases: these are enzymes that are involved in the removal or addition of protons
(and electrons) from/to compounds, e.g. the dehydrogenase enzymes.
-Transferases: enzymes that are required when parts of a compound/molecule is transferred
to another one, e.g. in transamination.
-Hydrolases: enzymes involved in hydrolysis reactions.
-Isomerases: enzymes involved in the conversion of a compound to its isomers, e.g. glucose
converted to fructose.
-Ligases: enzymes involved in the formation of bonds with the breakdown of ATP (as energy
source).
Non-protein helpers called Cofactors are substances that must be present for an enzyme to
catalyse a chemical reaction. Cofactors are often oxidized or reduced during the reaction, but
they are also not consumed. Instead, they return to their original state once the reaction is
complete.
Cells regulate the rates of their chemical reactions. One way of regulation is through Negative
Feedback (also called Feedback Inhibition) in which the product of a reaction inhibits the
enzyme that controls its formation. Negative feedback works in two general ways to prevent
too much of a substance from accumulating:
a) Non-Competitive Inhibition: Product molecules bind to the enzyme at a location other than
the active site in a way that alters the enzyme’s shape so that it can no longer bind substrate.
+
Enzyme
Substrate
Active site cannot
accommodate the
substrate
+
Enzyme
[Enzyme-substrate]
Inhibitor
b) Competitive Inhibition: The product of a reaction binds to the enzyme’s active site
preventing it from binding substrate.
E+P
Enzyme
Substrate
Enzyme
Inhibitor
[Enzyme-substrate]
The opposite of negative feedback is Positive Feedback in which a product activates the
pathway leading to its own production
Enzymes are very sensitive to conditions in the cell.
●An enzyme can become denatured and stop working if the pH changes or salt
concentrations become too high or too low.
●Enzymes are sensitive to Substrate Concentrations. When we start with a low substrate
concentration and a high enzyme concentration all the active sites on the enzyme molecules
will not be fully occupied. By increasing the substrate concentration, but keeping the enzyme
concentration constant, more active sites are filled at any given time and more product will be
formed. This increase will continue up until the point where there are enough substrate
molecules to fill all the active sites at any given time. Increasing the substrate concentration
beyond this point will not increase the rate of product formation and the reaction rate will
remain constant.
●Temperature also greatly influences enzyme activity. Each enzyme has an Optimum
Temperature at which it functions best. Enzyme action generally speeds up as the temperature
rises because reactants have more kinetic energy, but if it gets too hot the enzymes denature
and can no longer function.
●Pharmaceutical Drugs can also inhibit enzyme function.
4.5
Membrane Transport May Release Energy or Cost Energy
Membranes control movement in and out of the cell and are Selectively Permeable. This
movement can be Active (energy required) or Passive (no energy inputs needed). Some
substances pass freely though membranes while others require help from proteins. Due to
regulation of membrane transport, the interior of a cell is chemically different from the outside.
Concentrations of some dissolved substances (solutes) are higher inside the cell than outside
and others are lower. Likewise, the inside of each organelle may be chemically different from
the solution in the rest of the cell.
The term Gradient describes any such differences between two neighbouring regions. In a
Concentration Gradient, a solute is more concentrated in one region than in a neighbouring
region. Over time, a concentration gradient dissipates, unless energy is expended to maintain
it.
In Passive Transport, a substance moves across a membrane without direct expenditure of
energy. All forms of passive transport involve Diffusion, the spontaneous movement of a
substance from a region where it is concentrated to a region where it is less concentrated.
Simple Diffusion is a form of passive transport in which a substance moves down its
concentration gradient without the use of a carrier molecule or protein. Substances may enter
or leave cells by simple diffusion only if they can pass freely through the membrane, e.g. lipids,
oxygen, carbon dioxide.
Osmosis is the simple diffusion of water across a selectively permeable membrane. This is
important when water and not solutes can move through a membrane.
.
An Isotonic solution will have the same solute concentration as the cell. When a cell is placed
in an isotonic solution no nett movement of water is found, i.e. the amount of water moving
into the cell is equal to the amount flowing out. The cell size will remain constant.
A Hypotonic solution will have less solutes and thus be less concentrated resulting in a higher
water potential. When a cell is thus placed in a hypotonic solution water will move into the cell
resulting in the cell swelling.
A Hypertonic solution will have more solutes and thus be more concentrated resulting in a
lower water potential. When a cell is placed in a hypertonic solution water will move out of the
cell resulting in shrinking of the cell.
When epidermal cells of the onion are placed in a concentrated salt solution the plasma
membrane will be drawn away from the cell wall as the cell contents contract. This process is
referred to as Plasmolysis.
Facilitated Diffusion is a form of passive transport in which membrane proteins assist the
movement of a polar solute along its concentration gradient. This process does not require
energy.
In Active Transport a cell uses a transport protein to move a substance against its
concentration gradient. Energy for active transport comes from ATP. An example is the
Sodium-Potassium Pump. Cells must contain high concentrations of potassium and low
concentrations of sodium to perform many functions. The Na+- K+ pump is responsible for
example for the flow of impulses along nerve and muscle cells. During the resting stage of the
cell, there will be a lower Na+- ion concentration on the inside than the outside, and a lower
K+-ion concentration on the outside than the inside. These concentration differences are
maintained by the active pumping of the specific ions against their concentration gradients.
ATP provides the energy for this movement via a carrier protein.
During the resting stage the carrier protein has a shape that can accommodate 3 Na+ (A, B).
When the ATP splits the inorganic phosphate (P) attaches to the carrier protein causing a
Conformational change (change in physical structure) of the carrier protein (C, D). The 3
Na+ are then released on the outside. The carrier protein can then take up 2 K+ from the outside
(E). With the release of the P, the carrier protein again changes shape resulting in the release
of the K+ on the inside (F), and the whole cycle is repeated again.
NB. For every 3 Na+ moved out of the cell, 2K+ are moved into the cell.
Outside: high [K+]
; low [Na+]
ATP
P
A. Inside: high [Na+]; low [K+]
B.
F.
ADP
C.
E.
P
P released
D.
P
P
Large particles must enter or leave the cell with the help of Vesicles. Endocytosis allows a
cell to engulf fluids and large molecules and bring them into the cell. The cytoskeleton deforms
in a way that forms a small indentation in the cell membrane. The indentation becomes a bubble
of membrane that closes in on itself forming a vesicle that traps whatever was outside the
membrane.
There are two main forms of endocytosis – Large solid particles are taken in through the
process of Phagocytosis, whereas the cell engulfs fluids and dissolved substances in small
amounts through Pinocytosis.
Phagocytosis:
Outside
Inside
Pinocytosis:
This follows the same pattern as phagocytosis, but liquids with solutes are taken in.
Exocytosis, the opposite of endocytosis, uses vesicles to transport fluids and large particles out
of cells. Vesicles within the cells move to the cell membrane and join with it, releasing the
substance outside the membrane.
Outside
Inside
Summary
Metabolism
Consists of
Enzymes
Are Proteins that Catalyse
Chemical Reactions
Are
Endergonic
If they Require
Net Input of
ATP
If they
Release
Energy
Is a Molecule that stores
Requires
Exergonic
Exists in two forms
Active transport
Creates
Potential Energy
Kinetic Energy
Stores
Facilitated Diffusion
Dissipates
Simple Diffusion
Concentration Gradient