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Animal Cell Physiology
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THE PLASMA MEMBRANE
OBJECTIVES
Distinguish between cytoplasm and cytosol.
Explain the concept of selective permeability.
Define the electrochemical gradient and describe its components
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Structure of the Plasma Membrane
The Lipid Bilayer
The basic structural framework of the plasma membrane is the lipid bilayer, two bac
layers made up of three types of lipid moleculesphospholipids, cholesterol, and g
About 75% of the membrane lipids are phospholipids, lipids that contain phosphoru
Present in smaller amounts are cholesterol(about 20%), a steroid with an attached O
group, and various glycolipids(about 5%), lipids with attached carbohydrate groups.
The bilayer arrangement occurs because the lipids are amphipathic molecules, whic
they have both polar and nonpolar parts. In phospholipids, the polar part is the phocontaining head, which is hydrophilic.
The nonpolar parts are the two long fatty acid tails, which are hydrophobic hydroc
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Arrangement of Membrane Proteins
Membrane proteins are classified as integral or peripheral according to whether they are firmthe membrane (Figure 3.2).
Integral proteinsextend into or through the lipid bilayer among the fatty acid tails and are firm
it.
Most integral proteins are transmembrane proteins,which means that they span the entire li
protrude into both the cytosol and extracellular fluid.
A few integral proteins are tightly attached to one side of the bilayer by covalent bonding to f
membrane lipids, integral membrane proteins are amphipathic.
Their hydrophilic regions protrude into either the watery extracllular fluid or the cytosol, and
regions extend among the fatty acid tails.
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As their name implies, peripheral proteins are not as firmly embedded in the membrane.
They are attached to the polar heads of membrane lipids or to integral proteins at the inn
or outer surface of the membrane.
Many integral proteins are glycoproteins,proteins with carbohydrate groups attached to t
that protrude into the extracellular fluid.
The carbohydrates are oligosaccharides (oligo-few; -saccharides sugars), chains of 2 to 60
monosaccharides that may be straight or branched.
The carbohydrate portions of glycolipids and glycoproteins form an extensive sugary coat
called the glycocalyx.
The pattern of carbohydrates in the glycocalyx varies from one cell to another. Therefore,
glycocalyx acts like a molecular signature that enables cells to recognize one another.
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Functions of Membrane Proteins
Generally, the types of lipids in cellular membranes vary only slightly.
In contrast, the membranes of different cells and various
intracellular organelles have remarkably different assortments of
proteins that determine many of the membranes functions
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Some integral proteins form ion channels, pores or holes that specific ions, such
ions (K), can flow through to get into or out of the cell. Most ion channels are seallow only a single type of ion to pass through.
Other integral proteins act as carriers,selectively moving a polar substance or io
side of the membrane to the other. Carriers are also known as transporters.
Integral proteins called receptorsserve as cellular recognition sites. Each type of
recognizes and binds a specific type of molecule. For instance, insulin receptors hormone insulin. A specific molecule that binds to a receptor is called a ligand o
receptor.
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Some integral proteins are enzymes that catalyze specific chemical reactions at the ins
surface of the cell.
Integral proteins may also serve as linkers that anchor proteins in the plasma membran
cells to one another or to protein filaments inside and outside the cell. Peripheral prote
enzymes and linkers.
Membrane glycoproteins and glycolipids often serve as cellidentity markers. They may
(1) recognize other cells of the same kind during tissue formation or (2) recognize and r
potentially dangerous foreign cells.
The ABO blood type markers are one example of cell-identity markers. When you receiv
transfusion, the blood type must be compatible with your own, or red blood cells may c
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Membrane Permeability
The term permeablemeans that a structure permits the passage
of substances through it, while impermeable means that a structure d
not permit the passage of substances through it.
The permeability of the plasma membrane to different substances
varies.
Plasma membranes permit some substances to pass more readily thothers.
This property of membranes is termed selective permeability
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The lipid bilayer portion of the membrane is permeable to nonpolar, un
molecules, such as oxygen, carbon dioxide, and steroids, but is imperme
large, uncharged polar molecules such as glucose.
It is also slightly permeable to small, uncharged polar molecules such as
a waste product from the breakdown of amino acids.
The slight permeability to water and urea is an unexpected property sin
molecules.
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These two small molecules are thought to pass through the lipid
bilayer in the following way.
As the fatty acid tails of membrane phospholipids and glycolipids
randomly move about, small gaps briefly appear in the
hydrophobic environment of the membranes interior.
Water and urea molecules are small enough to move from onegap to another until they have crossed the membrane
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3.3TRANSPORT ACROSS
THE PLASMA MEMBRANE
OBJECTIVE
Describe the processes that transport substances
across the
plasma membrane
Substances generally move across cellular membranes via
transport processes that can be classified as passive or active, depending
they require cellular energy. In passive processes, a substance moves down its concentration or elec
to cross the membrane using only its own kinetic energy (energy of mot
Kinetic energy is intrinsic to the particles that are moving.
There is no input of energy from the cell.
An example is simple diffusion.
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In active processes,cellular energy is used to drive the substance uph
against its concentration or electrical gradient.
The cellular energy used is usually in the form of adenosine triph
(ATP).
An example is active transport.
Another way that some substances may enter and leave cells is a
process in which tiny, spherical membrane sacs referred to as ve
used.
Examples include endocytosis, in which vesicles detach from the
membrane while bringing materials into a cell, and exocytosis, th
merging of vesicles with the plasma membrane to release mater
the cell
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Simple Diffusion
Simple diffusionis a passive process in which substances move f
the lipid bilayer of the plasma membranes of cells without the membrane transport proteins
Facilitated Diffusion
Solutes that are too polar or highly charged to move through
by simple diffusion can cross the plasma membrane by a pascalled facilitated diffusion.
In this process, an integral membrane protein assists a speci
across the membrane.
The integral membrane protein can be either a membrane c
carrier
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Osmosis
Osmosis is a type of diffusion in which there is net movement of a solvent
selectively permeable membrane.
During osmosis, water molecules pass through a plasma membrane in two
(1) by moving between neighboring phospholipid molecules in the lipid bi
simple diffusion, as previously described, and
(2) by moving through aquaporins integral membrane proteins that functi
channels.
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Active Processes
Active Transport
Active transport is considered an active process because energy is requ
carrier proteins to move solutes across the membrane against a concen
gradient.
Two sources of cellular energy can be used to drive active transport:
(1) Energy obtained from hydrolysis of adenosine triphosphate (ATP) is
source in primary active transport;
(2) energy stored in an ionic concentration gradient is the source in secoactive transport.
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PRIMARY ACTIVE TRANSPORT
In primary active transport, energy derived from hydrolysis of ATP ch
the shape of a carrier protein, which pumps a substance across a p
membrane against its concentration gradient.
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