chapter 7: cell membrane structure and function
TRANSCRIPT
Chapter 7:
CELL MEMBRANE
STRUCTURE AND FUNCTION
Evelyn I. Milian
Instructor
2012
BIOLOGY I
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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PLASMA MEMBRANE
(Cell Membrane or Cytoplasmic Membrane)
• The plasma membrane is the cell’s flexible outer limiting barrier that separates the cell’s internal environment from the external (extracellular) environment.
• It is present in prokaryotes and eukaryotes.
• Main functions of the plasma membrane:
1. Regulation of exchange with the environment. It is a selective barrier that regulates the flow of nutrients into the cell and discharge of wastes out of the cell.
2. Sensitivity to the Environment. It detects changes in the surroundings and plays a role in communication, transmitting signals both among cells and between cells and their external environment.
3. It is involved in energy transfer and chemical reactions.
4. Structural Support. Specialized connections between plasma membranes, or between plasma membranes and extracellular materials, give tissues stability.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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Fluid Mosaic Model of the Plasma Membrane
• The “Fluid Mosaic Model” describes the structure of the plasma membrane as a mosaic formed by a phospholipid bilayer with proteins and carbohydrates. The proteins can move laterally, giving fluidity to the plasma membrane.
• The phospholipid molecules (made up of two fatty acids joined to glycerol and a phosphate group) are arranged in two layers (a bilayer) or parallel sheets, and are amphipathic molecules—they have a hydrophilic region and a hydrophobic region.
• The hydrophilic (“water-loving”) “heads” (phosphate group and glycerol)
face outward, and the hydrophobic (water-fearing) “tails” (fatty acids)
face inward.
• The eukaryotic cell membrane also has glycolipids (carbohydrate-lipids), glycoproteins (carbohydrate-proteins) and cholesterol molecules (a type of lipid).
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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• The eukaryotic plasma membrane has a greater variety of lipids than the prokaryotic membrane. It contains sterols (such as cholesterol) which adds rigidity to the membrane. Because of their larger size, eukaryotic cells have a much lower surface-to-volume ratio than prokaryotic cells. As the volume of cytoplasm enclosed by a membrane increases, the membrane is placed under greater stress. The sterols in the membrane may help it withstand the stress.
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The Fluidity of the Plasma Membrane
• Membranes are fluid structures (rather like cooking oil) because most of the membrane lipids and proteins easily rotate and move sideways (laterally) in their own half of the bilayer.
• Membrane fluidity is greater when there are more double bonds in the fatty acid tails of the bilayer lipids.
• Cholesterol (a steroid lipid) makes the lipid bilayer stronger but reduces fluidity at moderate temperatures.
• Because of its fluidity, the lipid bilayer self-seals when torn or punctured.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
The Fluidity of Membranes
• Lipids and proteins move laterally
in the membrane, but flip-flopping
across the membrane is rare.
• Unsaturated hydrocarbon tails of
phospholipids have kinks that
keep the molecules from packing
together, enhancing membrane
fluidity (unsaturated fatty acids
have double bonds).
• Cholesterol reduces membrane
fluidity at moderate temperatures
by reducing phospholipid
movement, but at low
temperatures it hinders
solidification by disrupting the
regular packing of phospholipids.
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Selective Permeability of the Plasma Membrane
• Selective permeability (semi-permeability):
The property of the plasma membrane to admit some substances into the cell while excluding others.
• Factors determining plasma membrane permeability:
1) Size of molecules
2) Solubility of molecules into lipids
3) Charge on ions
4) Presence of carrier molecules (transport proteins)
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Arrangement of Membrane Proteins
• Integral proteins:
Extend into or through the lipid bilayer.
Integral transmembrane proteins:
• Span the entire lipid bilayer and protrude
into both the cytosol and extracellular fluid.
• Peripheral proteins:
Associated with the inner or outer surface
of the membrane.
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Functions of Membrane Proteins:
Transport of Ions or Molecules
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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Functions of Membrane Proteins:
Transport of Ions or Molecules
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Functions of Membrane Proteins: Signal Transduction
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Functions of Membrane Proteins: Enzymatic Activity
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Functions of Membrane Proteins: Cell-Cell Recognition
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Functions of Membrane Proteins: Intercellular Joining and Attachment to Cytoskeleton and Extracellular Matrix (Linkers)
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
Functions of Membrane Proteins
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Fig. 03.03
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Gradients Across the Plasma Membrane
• Concentration gradient: *
A difference in the concentration of a chemical
substance from one place to another, such as from the
inside to the outside of the plasma membrane.
• Electrical gradient or potential (membrane potential):*
A difference in electrical charges between two regions
(across the plasma membrane).
• * Both help move substances across the plasma
membrane; the combined influence is termed an
electrochemical gradient.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
CELLULAR TRANSPORT: Gradients
Across the Plasma Membrane
a) Sodium ions and oxygen molecules
are more concentrated in the
extracellular fluid, whereas
potassium ions and carbon dioxide
are more concentrated in cytosol.
b) Because the inner surface of the
plasma membrane of most cells is
negative relative to the outer
surface, an electrical gradient exists
across the membrane.
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TRANSPORT ACROSS THE PLASMA MEMBRANE
• Passive Transport
The movement of a substance across the cell membrane down its concentration gradient, that is, from an area of higher concentration to an area of lower concentration; without expenditure of energy.
• Simple diffusion through the lipid bilayer, diffusion through ion
membrane channels, facilitated diffusion, osmosis.
• Active Transport
The movement of a substance across a cell membrane against its concentration gradient, from lower concentration to higher concentration; requiring the expenditure of cellular energy (from ATP, a high-energy molecule).
• Group translocation, bulk transport (endocytosis, exocytosis)
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TRANSPORT ACROSS THE PLASMA MEMBRANE:
Passive Transport
• Diffusion
Diffusion is a passive process in which there is a net or
greater movement of molecules or ions from a region of
high concentration to a region of low concentration
until equilibrium is reached (they move down their
concentration gradient); no energy is required—it is
spontaneous.
Both the solutes, the dissolved substances, and the
solvent, the liquid that does the dissolving (such as
water in cells), undergo diffusion.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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FIGURE 3.6 – Diffusion.
A crystal of dye placed in a
cylinder of water dissolves
(beginning) and then diffuses
from the region of higher dye
concentration to regions of
lower dye concentration
(intermediate). At equilibrium,
dye concentration is uniform
throughout ,although random
movement continues.
* This is also called simple
diffusion.
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TRANSPORT ACROSS THE PLASMA MEMBRANE:
Passive Transport
• Diffusion Through the Lipid Bilayer
Nonpolar, hydrophobic molecules can diffuse across the lipid bilayer; examples are: respiratory gases, some lipids, small alcohols, and ammonia.
It is important for gas exchange, absorption of some nutrients, and excretion of some wastes.
• Diffusion Through Membrane Channels
Most membrane channels are ion channels, allowing passage of small, inorganic ions which are hydrophilic.
Ion channels are selective and specific and may be gated or open all the time.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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TRANSPORT ACROSS THE PLASMA MEMBRANE:
Passive Transport
• Facilitated Diffusion
The spontaneous movement of a substance across the plasma membrane, from an area of higher concentration to an area of lower concentration (down its concentration gradient), mediated by a transmembrane transport protein (permease), but does not require energy (ATP).
Examples of transport proteins are:
• Channel proteins such as the aquaporins for water transport.
• Carrier proteins such as the glucose transporter.
In facilitated diffusion, a substance diffuses faster than the physical condition indicates it should.
Molecules and ions that move across membranes by facilitated diffusion include glucose, urea, fructose, galactose, and some vitamins.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
TRANSPORT ACROSS THE PLASMA MEMBRANE:
Passive Transport
• Facilitated diffusion of
glucose across a plasma
membrane.
The transporter (GluT)
binds to glucose in the
extracellular fluid, changes
its shape, and releases
glucose into the cytosol.
Facilitated diffusion
requires a transporter
protein but does not use
ATP (energy molecule).
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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TRANSPORT ACROSS THE PLASMA MEMBRANE
• Osmosis
The movement (diffusion) of water through a selectively
permeable membrane from an area of higher water
concentration to an area of lower water concentration,
(down its concentration gradient) until equilibrium is reached.
• Remember that water is the most versatile solvent (dissolving
agent or medium); and a solute is a substance that is dissolved
in another substance.
Osmotic pressure: The force with which a solvent (such as water) moves from a solution of lower solute concentration to a solution of higher solute concentration. In other words, it is the pressure needed to stop or prevent the flow of water across a membrane.
• * The higher the solute concentration, the higher the solution’s
osmotic pressure.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
TRANSPORT ACROSS THE
PLASMA MEMBRANE: Osmosis
• Two sugar solutions of different
concentrations are separated by
a selectively permeable
membrane, which the solvent
(water) can pass through but the
solute (sugar) cannot. Water
molecules move randomly and
may cross through the pores in
either direction, but overall, water
diffuses from the solution with
less concentrated solute to that
with more concentrated solute.
This transport of water, or
osmosis, eventually equalizes
the sugar concentrations on both
sides of the membrane.
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(a) As the experiment starts, water molecules move from the left arm into the right arm,
down the water concentration gradient. (b) After some time, the volume of water in the
left arm has decreased and the volume of solution in the right arm has increased. At
equilibrium, net osmosis has stopped. (c) If pressure is applied to the solution in the
right arm, the starting conditions can be restored.
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TRANSPORT ACROSS THE PLASMA MEMBRANE: Osmosis
• Tonicity is the ability of a solution to change the volume of cells by altering their water concentration. In other words, tonicity is the ability of a solution surrounding a cell to cause that cell to gain or lose water.
• Isotonic solution
A medium or solution in which the overall concentration of solutes equals that found inside a cell (iso = equal). If the solution is isotonic to the cell, there is no net movement of water. The cell is also said to be isotonic in relation to the surrounding solution.
• Hypotonic solution
A medium whose concentration of solutes is lower than that inside the cell (hypo = under, less). If the solution is hypotonic, the cell gains water.
• Hypertonic solution
A medium having a higher concentration of solutes than inside the cell (hyper = above, more). If the solution is hypertonic, the cell loses water.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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Passive Transport:
Osmosis
• Tonicity describes the behavior of cells in a fluid environment.
• Isotonic solution: its concentration of solutes equals that found inside a cell.
• Hypotonic solution: its concentration of solutes is lower than that inside the cell.
• Hypertonic solution: its concentration of solutes is higher than that inside the cell.
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FIGURE 3.8 – Tonicity and its
effects on red blood cells
(RBCs).
Tonicity = A measure of the
solution’s ability to change the
volume of cells by altering their
water content.
One example of an isotonic
solution for RBCs is 0.9% NaCl.
Cells placed in an isotonic
solution maintain their shape
because there is no net water
movement into or out of the cell.
Cells placed in a hypotonic
solution gain water, increase in
size and will burst (hemolysis).
Cells placed in a hypertonic
solution lose water and undergo
crenation (or plasmolysis); the
cytoplasm shrinks.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
Osmosis, Turgor Pressure and Plasmolysis
a) In hypotonic surroundings, the
vacuole of a plant cell fills with water,
but the rigid cell walls prevent the cell
from expanding. The cells of this
healthy begonia plant are turgid.
Turgor pressure is the pressure of
the cell contents against the cell wall;
in plant cells, it is determined by the
water content of the vacuole and
provides internal support.
b) When the begonia plant is exposed to
a hypertonic solution, its cells
become plasmolyzed as they lose
water (contraction of cell contents).
c) The effects of turgor loss are seen
during wilting, when leaves and stems
droop as a result of cells losing water.
The plant eventually dies.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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TRANSPORT ACROSS THE PLASMA MEMBRANE
• Active Transport
The movement of a substance across a cell membrane
against its concentration gradient, from lower
concentration to higher concentration; requiring the use
of cellular energy (from ATP, a high-energy molecule).
Examples of solutes actively transported: ions, amino acids,
monosaccharides.
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TRANSPORT ACROSS THE PLASMA MEMBRANE
• Active Transport
Primary active transport
• Energy derived from ATP changes the shape of a
transporter protein (“pump”), which pumps a substance
across a plasma membrane against its concentration
gradient. Example: sodium-potassium pump (Na+/K+).
Secondary active transport (Cotransport)
• Energy stored in an ionic concentration gradient is
used to drive other substances across the membrane
against their own concentration gradients (this transport
indirectly uses energy obtained from the hydrolysis of ATP).
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PRIMARY ACTIVE TRANSPORT ACROSS THE PLASMA MEMBRANE
FIGURE 3.11 – The sodium-potassium pump (Na+ /K+ ATPase) expels sodium ions
(Na+) and brings potassium ions (K+) into the cell.
Sodium-potassium pumps maintain a low intracellular concentration of sodium ions.
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SECONDARY ACTIVE TRANSPORT ACROSS THE PLASMA MEMBRANE
FIGURE 3.12 – Secondary active transport mechanisms.
(a) Antiporters carry two substances across the membrane in opposite directions.
(b) Symporters carry two substances across the membrane in the same direction.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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TRANSPORT ACROSS THE PLASMA MEMBRANE:
Bulk Transport in Vesicles
• Vesicle: Small, spherical sac that has budded off from an existing membrane.
1) Endocytosis = bringing a substance or particle into cell
The uptake of large biological molecules and particles
into a cell by the formation of a new vesicle from the
plasma membrane; a segment of the plasma membrane
surrounds the substance, encloses it, and brings it in.
2) Exocytosis = releasing a substance or particle from cell
Export of substances from a cell through the fusion of
cytoplasmic vesicles with the plasma membrane (releasing
their contents to the outside of the cell).
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
Endocytosis in Animal Cells
a) Phagocytosis (“cell eating” by phagocytes): A cell (such as a macrophage or white blood cell of the immune system) ingests and destroys solid particles (for example, microbes or cell debris) by packaging it in a vesicle or vacuole, which is digested by hydrolytic enzymes.
b) Pinocytosis (“cell drinking”): The cell “gulps” droplets of extracellular fluid (containing molecules), forming a vesicle around them. Pinocytosis is nonspecific in the substances it transports.
c) Receptor-mediated endocytosis: cells take up specific ligands, molecules that bind to specific cell receptors (membrane proteins).
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FIGURE 3.14 –
Phagocytosis.
Pseudopods (extensions
from the plasma membrane)
surround a particle and the
membranes fuse to form a
vesicle called a phagosome.
* Phagocytosis is a vital
defense mechanism that
helps protect the body from
disease. It is carried out by
defensive cells called
phagocytes.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
Bulk Transport in Vesicles:
Endocytosis: Eating and Drinking by Cells
• Phagocytosis.
A white blood cell ingests and destroys a microbe.
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FIGURE 3.15 – Pinocytosis.
The plasma membrane folds
inward, forming a pinocytic
vesicle.
* Most body cells carry out
pinocytosis, the nonselective
uptake of tiny droplets of
extracellular fluid.
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FIGURE 3.13 – Receptor-mediated
endocytosis of a low-density
lipoprotein (LDL) particle.
• Receptor-mediated endocytosis
imports materials that are needed by
the cells (for example: lipoproteins,
transferrin, some vitamins,
antibodies, certain hormones).
• Receptor proteins recognize the
molecules needed and a vesicle is
formed to take them into the cell.
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TRANSPORT ACROSS THE PLASMA MEMBRANE:
Bulk Transport in Vesicles
2) Exocytosis = releasing a substance or particle from a cell.
Export (secretion) of materials from the cell by fusion
of cytoplasmic vesicles with the plasma membrane
(releasing their contents to the outside of the cell).
Examples of materials released include:
• digestive enzymes, hormones, neurotransmitters,
waste products.
BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
Exocytosis
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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EXOCYTOSIS (Animation)
http://www.biologie.uni-hamburg.de/b-online/library/biology107/bi107vc/fa99/terry/membranes.html
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MOVEMENT OF SUBSTANCES
ACROSS CELL MEMBRANES:
Bulk Transport in Vesicles
Endocytosis and Exocytosis in
Eukaryotic Cells
• Endocytosis: Process in which
substances or particles are taken in by
the invagination of the plasma membrane
forming a vesicle (or vacuole).
• Exocytosis: Process by which
substances or particles are released
from a vesicle inside a cell when it fuses
with the plasma membrane.
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function
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Education, Inc.-Pearson Benjamin Cummings. CA, USA.
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Edition. Pearson Education, Inc. – Pearson Benjamin Cummings. CA, USA.
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Eleventh Edition. John Wiley & Sons, Inc. NJ, USA. www.wiley.com/college/apcentral.
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