Chapter 5 – The Working Cell
• Energy
– The capacity to do work
• What is work?
 Potential Energy
 Stored energy, eg. in chemical bonds
 Kinetic Energy
 Energy of motion
• What is the Law of Conservation of
Energy?
ATP and Cellular Work
• The chemical energy of organic molecules is released in
cellular respiration to make ATP in the mitochondria
– Consists of adenosine plus a tail of three phosphate groups
– Is broken down to ADP, accompanied by the release of energy
Energy
Adenosine
Adenosine
Phosphate
transferred
to other
molecules
• ATP functions in what is called energy coupling, or the
ATP cycle
Cellular respiration:
chemical energy
harvested from
fuel molecules
Energy for
cellular work
• Most enzymes are proteins with diverse
structure.
• Enzymes are chemical catalysts that:
– Increase the rate of a reaction.
– Are not permanently altered or used up by the
reaction.
– Do not change the nature of the reaction.
enzyme animation..
– Reduce the required amount of activation
energy required to initiate the reaction.
• Many enzymes work by orienting molecules so that
they can better contact each other.
• Each type of enzyme has has a highly-ordered,
characteristic 3-dimensional shape (conformation).
• Ridges, grooves, and pockets lined with specific
amino acids.
• Pockets active in catalyzing a reaction are called the
active sites of the enzyme..
Enzyme
Activation energy
barrier
Reactants
Products
(a) Without enzyme
(b) With enzyme
• The molecule affected by the enzyme is the
substrate.
• Substrates have specific shapes to fit into
the active sites (lock-and-key model):
• Better fit may be induced (induced-fit
model):
• Enzyme undergoes structural change when
substrate binds..
• Enzyme names end with suffix “-ase.”
– Enzymes may be named according to their
mode of action, e.g. kinase.
• May specify both the substrate of the enzyme and
job category.
– Enzymes may be named for the substrate, e.g.
lipase, maltase.
• Different organs may make different
enzymes (isoenzymes) that have the same
activity.
– Differences in structure do not affect the active
sites..
• Enzyme activity can be affected by several
factors:
– Changes in pH and/or temperature may
denature the enzyme. These changes may break
the H bonds stabilizing the molecule.
– Small molecules or ions called cofactors alter
the shape of the active site and/or is needed for
the substrate to bind with the active site, e.g.
Ca++.
– Certain organic molecules called coenzymes
are needed to transport small molecules or ions
(especially H+) from one enzyme to another,
e.g. NAD..
– Metabolic pathways are frequently regulated by
inhibition. End-product inhibition occurs when
one product binds with the enzyme and prevents
it from binding with the substrate. The product
may bind with the enzyme at the allosteric site.
This is negative feedback.
inhibition animation..
Substrate
Active site
Inhibitor
Enzyme
(c) Enzyme inhibition by a molecule that causes the
active site to change shape
• Competitive inhibition occurs when some
other imposter molecule (similar to the
substrate) binds with the enzyme. This
prevents the substrate from participating in
any metabolic pathway. This is typically
how poisons work..
Inhibitor
Substrate
Substrate
Active site
Active site
Enzyme
(a) Normal enzyme action
Enzyme
(b) Enzyme inhibition by a substrate imposter
Membrane Transport
• Working cells must control the flow of materials
– This is the primary function of the plasma membrane
– Transport proteins also help with this task
• Transport process may be categorized by the
energy requirements:
– Passive transport:
• Net movement down a concentration gradient.
– Does not require metabolic energy (ATP).
– Active transport:
• Net movement against a concentration gradient.
– Requires ATP..
Molecules of dye
Membrane
Equilibrium
(a) Passive transport of one type of molecule
Equilibrium
(b) Passive transport of two types of molecules
Diffusion
• Molecules/ions are in constant state of
random motion due to their thermal energy.
• Diffusion is the movement of particles from
an area of high concentration to and area of
low concentration. Net diffusion indicates
the final concentration changes.
Diffusion Animation
• Diffusion of some substances through the
plasma membrane requires carriers or
channels (discussed later)..
• Osmosis is the special term for the net
diffusion of H20 across a selectively permeable
membrane.
• Osmosis is the movement of H20 from a high
[H20] to lower [H20] until equilibrium is
reached.
• Two requirements for osmosis:
– Must be difference in [solute] on the 2 sides of the
membrane.
– Membrane must be impermeable to the solute.
Hypotonic
solution
Hypertonic
solution
Isotonic solutions
Sugar molecule
(solute)
Selectively permeable
membrane
Osmosis
(net movement of water)
•Hypertonic solution = has a higher concentration of solute
•Hypotonic solution = has a lower concentration of solute
•Isotonic solution = has an equal concentration of solute
Osmosis Animation..
Water Balance in Cells
• The survival of a cell depends on its ability to
balance water uptake and loss = Osmoregulation
Animal
cell
Normal
Lysing
Shriveled
Plasma
membrane
Plant
cell
Flaccid (wilts)
(a) Isotonic
Turgid
Shriveled
(b) Hypotonic
(c) Hypertonic..
Carrier-Mediated Transport
• Molecules that are too large and/or polar to
pass through the plasma membrane require
protein carriers.
– Facilitated diffusion is passive transport utilizing
carriers. Molecules move from high
concentration to low concentration.
e.g. GLUT carriers transport glucose into human cells..
– Active transport required the expenditure
of ATP. Molecules are moved from low
concentration to high concentration.
• The most common primary active
transport mechanism is the Na+/K+
pump. In this case, the protein carrier is
an ATPase that converts ATP to ADP +
Pi. The carrier transports 3 Na+ out of the
cell and takes in 2 K+.
Na/K Pump Animation..
Bulk Transport
• Movement of many large molecules, that cannot be
transported by carriers, at the same time.
• Exocytosis:
– Fusion of the membrane-bound vesicles that contains cellular
products with the plasma membrane.
Outside cell
Plasma
membrane
Cytoplasm
(a) Exocytosis
•Endocytosis:
–Exocytosis in reverse.
–Specific molecules can be taken into the cell because
of the interaction of the molecule and protein receptor
(receptor-mediated endocytosis).
Endo/Exocytosis Animation
(b) Endocytosis
•Phagocytosis
•Pinocytosis
Pseudopod
of amoeba
Food being ingested