Chapter 5 – The Working Cell
5.1 Membranes are fluid mosaics of lipids and proteins with many functions
• The plasma membrane is the edge or boundary that encloses the living cell. Membranes are composed of
a phospholipid bilayer that has embedded and attached proteins. Biologists termed it as a fluid mosaic.
Cell proteins have 6 major functions:
1. Maintain cell shape and coordinate changes inside and outside the cell by their attachment to the
cytoskeleton and extracellular matrix
2. Some serve as receptors for chemical messengers from other cells
3. Some are enzymes and serve to carry out the steps of metabolic reactions
4. Some are involved in cell to cell recognition. They serve as tags that other cells can recognize.
5. May participate in intercellular junctions that attach adjacent cells.
6. May also function in the transport of substances across the membrane.
*the membrane is selectively permeable, so some substances move across more easily than others*
5.2 Membranes form spontaneously, a critical step in the origin of life
When a mixture of phospholipids and water is shake, the phospholipids will organize into bilayers. This
requires no genetic information and occurs simply because of the properties of the phospholipids themselves.
A plasma membrane that allows cells to regulate their chemical exchanges with the environment is a basic
requirement of life.
5.3 Passive transport is diffusion across a membrane with no energy investment
• Molecules move randomly as a result of thermal motion (heat energy). Diffusion results from this motion.
This is the tendency of particles to spread out evenly in the available space.
• The molecules are moving down the concentration gradient until all particles are equally distributed.
Each molecule moves independently; however, there is a net movement from the side that is more
concentrated to the side that is less concentrated.
• Passive transport occurs when no energy has to be expended to do the work. Most traffic in the cells is
accomplished by diffusion. Small, nonpolar molecules diffuse easily. Ions and polar molecules can diffuse,
if they are moving down the gradient and if they have the help of transport proteins.
5.4 Osmosis is the diffusion of water across a membrane
• Water is one of the most important substance moving across the membrane by passive transport.
Osmosis is the diffusion of water. Terms:
Solute – substance that dissolves a solvent
Solvent – the substance that is dissolved
Solution – the combination of solvent and solute
• Water will move across a membrane until the solute concentrations are equal on both sides. Polar water
molecules cluster around hydrophilic (water loving) solutes causing a net movement of water down its
own concentration gradient.
5.5 Water balance between cells and their surroundings is crucial to organisms
• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. This is dependent on
the concentration of solutes that cannot cross the plasma membrane relative to the concentration of
solutes inside the cell.
• In isotonic solutions, cells gain water at the same rate they lose water.
• In hypotonic solutions, the cell gains water. The solution has higher water concentration than the cell.
Water moves in by osmosis.
• In hypertonic solutions, water leaves the cell causing is to shrink. The solution has lower water
concentration than the cell. Water exits by osmosis.
5.6 Transport proteins facilitate diffusion across membranes
Nonpolar, hydrophobic molecules can dissolve in the lipid bilayer and cross easily. Polar or charged
substances can move across the membrane with the help of specific transport proteins in the process of
facilitated transport. Without the transport protein, the substance cannot cross the membrane –or- it crosses
so slowly that it is not useful to the cell. Facilitated diffusion is a type of passive transport because it does not
use energy. Substances such as sugars, amino acids, ions, and water can cross using facilitated diffusion.
5.7 Research on another membrane protein led to the discovery of aquaporins
An aquaporin is a protein channel that allows for the transport of water. A single aquaporin can allow the
entry or exit of up to 3 billion water molecules per second. Dr. Peter Agre received the 2003 Nobel Prize in
Chemistry for their discovery.
5.8 Cells expend energy in the active transport of a solute
In active transport, the cell uses energy to move a solute against the concentration gradient. The energy is
ATP. This occurs by:
1. Solutes on the cytoplasm side attach to specific binding sites on the transport protein
2. ATP transfers a phosphate to the transport protein
3. The protein changes shape and releases the solute on the other side of the membrane
4. The phosphate detaches and the transport protein returns to its original shape
5.9 Exocytosis and endocytosis transport large molecules across membranes
• Exocytosis is used to export large molecules, such as proteins or polysaccharides. In this transport, a
vesicle filled with macromolecules buds from the Golgi body and moves in to the plasma membrane. Once
it reaches the membrane, the two fuse and vesicle’s contents are released outside of the cell.
• Endocytosis is used to transport large materials in to the cell. A vesicle forms as the plasma membrane
folds around materials outside of the cell to bring it in.
3 types of endocytosis:
• Phagocytosis or “cell eating” brings solid particles in to the cell
• Pinocytosis or “cell drinking” brings liquids in to the cell
• Receptor mediated endocytosis is highly selective. Receptor proteins for certain molecules are embedded
on places in the membrane. When they collect a particular substance, they membrane indents to form a
coated pit which will create a vesicle to bring in the material.
5.10 Cells transform energy as they perform work
• A cell is a chemical factory in which thousands of reactions occur within a microscopic space. Some of
these reactions build compounds needed by the cell, while others break down compounds to release
energy. To understand how living cells work, you must first understand energy.
• Energy is the capacity to perform work. Work is performed when an object is moved against an opposing
force. In other words, it is the ability to rearrange matter. There are two types of energy: kinetic and
potential.
• Kinetic is the energy of motion. Moving objects transfer motion to other matter. While
transferring this motion, they release heat, which is a form of kinetic energy associated with the
movement of atoms.
• Potential energy is the energy an object possesses as a result of its location. Chemical energy is
the potential energy of molecules and is the most important energy type to living organisms. Life
depends on the ability to convert energy from one form to another
Two laws govern energy transformations:
Thermodynamics is the study of energy transformations that occur in matter.
• First Law of Thermodynamics – (law of energy conservation) the total amount of energy in the
universe is constant. Energy can be transferred or transformed but it cannot be created or
destroyed.
• Second Law of Thermodynamics – During every energy conversion, some of the energy becomes
unusable (unable to do work). In most transformations, some energy is converted to heat.
•
Cells cannot use heat to perform work, so it is “lost” to the surroundings. This “loss” of heat is actually a
disordered form of energy, and its release into the environment makes the universe more disorganized or
in a state of more entropy. So the second law states that energy conversions reduce the order of the
universe and increase its entropy.
5.11 Chemical reactions either store or release energy
 Chemical reactions are of two types: endergonic and exergonic.
 Endergonic (“energy in”) reactions require an input of energy in order to begin. They will yield products
full of potential energy (chemical potential). The reaction starts out with reactants that have little
potential energy. Energy is taken in from the surroundings as the reactions occur, so that the products
will actually contain more energy than the reactants. The energy is in the covalent bonds of the products.
o Example: photosynthesis – starts with carbon dioxide and water, but needs light energy to
proceed.
 Exergonic (“energy out”) reactions release energy. An exergonic reaction begins with reactants whose
covalent bonds contain energy.
o Example: Krebs Cycle and Electron Transport of cellular respiration
5.12 ATP drives cellular work by coupling exergonic and endergonic reactions
 ATP (adenosine triphosphate) powers nearly all forms of cellular work. It is made of an adenine (a
nitrogenous base), ribose (a five carbon sugar), and three phosphate groups attached. Due to a slight
negative charge on each phosphate, the bonds between them are easily broken by hydrolysis. This
hydrolysis releases the energy in the bond, a phosphate detaches and leaves, and the ATP is converted to
ADP (adenosine diphosphate).
 The hydrolysis reaction is exergonic and is accomplished by transferring a phosphate from the ATP to
another molecule. This is called phosphorylation and most cellular work depends on this.
5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers
 Energy must be absorbed to weaken bonds so that they can break and new bonds can form. This is called
energy of activation (EA) and is the amount of energy the reactants must absorb to be activated and start
a chemical reaction. However, most of the essential reactions must occur quickly and precisely for a cell
to survive. The cells must be able to overcome this energy barrier in order to proceed.
 Enzymes have special capabilities and can increase the rate of reaction without being changed by the
reaction. An enzyme does not add energy to the reaction; it speeds up a reaction by lowering the amount
of activation energy. In catalyzing metabolic reactions, enzymes are critical to life.
5.14 A specific enzyme catalyzes each cellular reaction
 Enzymes are proteins and the unique three dimensional shapes shape determines what reaction it can
catalyze. The substrate is a specific reactant that an enzyme works on. The active site is a location where
a substrate attaches to the enzyme.
 An active site is a pocket or groove on the surface of the enzyme. An active site only fits one kind of
substrate. When the substrate binds to the enzyme, the active site changes slightly to embrace the
substrate. This induced fit allows the active site to be in a position to catalyze the reaction.
 For every enzyme, there are conditions under which it is most effective.
o Temperature affects molecular motion, but too high temperatures can denature an enzyme and
alter its specific shape.
o Salt concentration and pH also influence enzyme activity. Few enzymes can tolerate salt because
the salt ions interfere with the chemical bonds holding the enzyme together.
o Low pH can cause a similar problem due to hydrogen ions.
 Most enzymes will not function without a nonprotein helper called a cofactor. A cofactor can be inorganic
(ex: ions of zinc, iron or copper) or it can be an organic cofactor called a coenzyme (ex: vitamin).
5.15 Enzyme inhibitors can regulate enzyme activity in a cell
 A chemical that interferes with an enzyme’s activity is called an inhibitor. If it attaches to an enzyme by
covalent bonds, the inhibition is usually irreversible. If it attaches by weak bonds, like hydrogen bonds,
then it is usually reversible.
 There are two types of inhibition:
o Competitive – where the inhibitor resembles a normal substrate and competes with the
substrate for the active site. This will block the substrate from bonding
o Noncompetitive – the inhibitor does not enter the active site and binds to the enzyme
somewhere else causing the substrate to no longer fit.
 Inhibitors are not always harmful. Usually, they are important mechanisms in metabolic control.
 One type of inhibition is called feedback inhibition. If a cell is producing more product than it needs, then
the product can act as an inhibitor of that pathway.
5.16 Many drugs, pesticides, and poisons are enzyme inhibitors
Many beneficial drugs act as enzyme inhibitors. Ibuprofen inhibits an enzyme that helps in the production of
prostoglandins (molecules that increase pain sensations). Penicillin blocks an enzyme needed by bacteria to
make cell walls. Cancer drugs block enzymes needed to promote cell division. HIV drugs target key viral
enzymes.