Download Notes

• Plasma Membrane Structure and
Function
• Separates the internal environment of the
cell from its surroundings.
• Regulates what materials enter and leave a
cell; maintains homeostasis
• Made of 2 layers of phospholipids
(phospholipid bilayer) with proteins
embedded throughout.
• Fluid consistency (like a light oil) and a
mosaic pattern of proteins.
•Because of consistency and pattern of
components, referred to as fluid-mosaic model
of membrane structure
http://www.stolaf.edu/people/giannini/flashanimat/lipids/membrane fluidity.swf
The Fluid Mosaic Model
A. Nonpolar (hydrophobic) tails
B. Polar (hydrophilic) heads
C. Protein
D. Protein
E.Carbohydrates
F.Ion/small polar molecule
G. Diffusion
H. Glycoprotein (carbohydrate attached to a protein)
I. Glycolipid (carbohydrate attached to a lipid)
J. Phospholipid
K. Phospholipid bilayer
Note – this diagram does not show the following:
•Proteins on inner or outer surface of membrane
•Cholesterol (would be scattered amongst nonpolar tails of
phospholipid)
• Cells live in fluid environments, with
water inside and outside the cell.
• Components of plasma membrane:
–2 layers of phosphlipids
• Glycerol backbone, 2 fatty acid
chains, and a phosphate group
• Polar head and nonpolar tail
–Integral proteins
–Peripheral proteins
–Cholesterol
–Carbohydrates
•Phospholipid bilayer:
•Hydrophilic (water-loving) polar heads
•Face outside and inside of cell.
•Hydrophobic (water-fearing) nonpolar
tails
•Extend to the interior of the plasma
membrane.
•Cholesterol – helps prevent fatty acid tails of
phospholipid bilayer from sticking together;
makes membrane fluid; reduces permeability of
membrane to H+ and Na+
•Carbohydrates:
•Define the cell’s characteristics and help cells
identify chemical signals; might help diseasefighting cells recognize & attack a potentially
harmful cell
•Glycoproteins – proteins with carbohydrates
attached
•Glycolipids – phospholipids with
carbohydrates attached
• Proteins:
• Proteins on inside surface
• Anchor the plasma membrane to
the cell’s internal support structure;
give the cell its shape
• Proteins on outside surface
• Called receptors; transmit signals to
the inside of the cell
• Proteins that span the entire
membrane:
• Called transport proteins; create
tunnels through which certain
substances can enter and leave the
cell
• Remove needed substances or
waste materials through the plasma
membrane
• Contributes to the selective
permeability of the membrane
Functions of membrane proteins
• Some help to transport materials across
the membrane.
•Channel Protein –
allows certain
molecule or ion to
cross membrane freely
•Carrier Protein – interacts
with certain molecule or
ion to help move it across
membrane
•Others receive specific molecules, such as
hormones.
•Receptor Protein – has certain shape so
that specific molecule, such as a hormone or
some other signaling molecule, can attach
•Attachment can cause protein to change
shape & cause cell to perform certain action
•Still other membrane proteins function as
enzymes.
•Enzymatic Protein - Can carry out
metabolic reactions
The Permeability of the Plasma
Membrane
• Selectively permeable – meaning only
certain materials can cross the
membrane.
• Two mechanisms of transport:
–Active – requires ATP (chemical
energy)
–Passive – does not require chemical
energy
• Examples of passive transport:
– Diffusion (no carrier protein needed)
– Facilitated transport (which usually
requires a carrier protein)
• Active transport:
– Requires a carrier protein
– May sometimes involve the formation of
vesicles to take in or get rid of materials
(for example endocytosis and
exocytosis).
• **More on these types of transport later in
chapter**
•How do materials get across the plasma
membrane?
•Small, uncharged molecules pass through
the membrane, following their
concentration gradient (gradual change in
chemical concentration from one area to
another – molecules tend to move from
area of high to low concentration).
•Larger macromolecules or tiny charged
molecules rely on proteins to help them to
get across.
How molecules cross the plasma
membrane
Diffusion and Osmosis
• Diffusion is the passive movement of
molecules from a higher to a lower
concentration until equilibrium is reached.
– Equilibrium – state in which all materials
are evenly concentrated (sometimes
called dynamic equilibrium)
• Movement of molecules still occurs, but
there is no NET movement of
molecules
• Gases move through plasma membrane by
diffusion.
Osmosis
• Defined as the diffusion of water across a
differentially permeable membrane due to
concentration differences.
• Molecules always move from higher to
lower concentration.
• Water enters cells due to osmotic pressure
(pressure that develops in a system due to
osmosis) within cells.
Osmosis demonstration
Rate of diffusion affected by:
•Concentration – higher the concentration,
more collisions between particles, higher the
rate of diffusion
•Temperature – at higher temperatures,
particles move faster, so more collisions occur
between particles; rate of diffusion increases
•Pressure – higher the pressure, the closer the
particles are to one another; they collide more
often; rate of diffusion increases
Osmosis in cells
• A solution contains a solute (solid) and a
solvent (liquid).
• Cells are normally isotonic to their
surroundings, and the solute concentration
is the same inside and out of the cell.
– Cell is in equilibrium; there is no net
movement of water across the cell
membrane.
Video clip
Isotonic = same
No net
movement
of water
X = Solute
X = water
Cells in isotonic solutions
Elodea in fresh water
• Hypotonic solutions cause cells to swell &
possibly burst; “hypo” means less than.
– Lower concentration of solute, higher
concentration of water outside than inside
cell.
– Net movement of water from outside to
inside the cell.
– Animal cells - may undergo lysis (cell is
disrupted – may even burst)
– Plant cells - increased turgor pressure
(makes plant cell rigid); plant cells do not
burst because they have a cell wall.
Hypotonic = less than
Water moves
in
(hypo/hippo –
swells like a
hippo)
X = Solute
X = water
Cells in a hypotonic solution
Elodea in distilled water
• Hypertonic solutions cause cells to lose
water; “hyper” means more than
– Higher concentration of solute, lower
concentration of water outside than inside
cell.
– Net movement of water from inside to
outside of cell.
– Animal cells - undergo crenation (shrivel)
– Plant cells - undergo plasmolysis, the
shrinking of the cytoplasm.
– Turgor pressure is lost as plant cells
shrink.
Hypertonic = more than
Water
moves out
X = Solute
X = water
Cells in a hypertonic solution
Elodea in salt water
Transport by Carrier Proteins
• Some materials cannot enter or leave cell
due to their size and/or nature.
• Channel proteins – open and close to allow
substances to diffuse across plasma
membrane
• Carrier proteins are specific and combine
with only a certain type of molecule; change
shape as they function to move substances
across membrane.
• Facilitated transport and active transport
both require carrier proteins.
Facilitated transport
• Substances pass through a carrier protein
following their concentration gradients
(high  low concentration).
• Does not require energy.
• Brings in materials such as glucose &
amino acids; proteins are specific to
molecules taken in.
• Some molecules can be taken in faster
than others – differential permeability.
Animation
Active transport
• Ions or molecules are moved across the
membrane against the concentration
gradient – from an area of lower to higher
concentration.
• Energy in the form of ATP is required for the
carrier protein to combine with the
transported molecule.
• Occurs in cells such as kidney cells (taking
sodium from urine), thyroid gland cells
(taking in iodine), and cells in digestive tract
(absorbing nutrients).
Active transport
• Carrier proteins involved in active transport
are called pumps.
• The sodium-potassium pump is active in all
animal cells (particularly nerve & muscle
cells), and moves sodium ions to the
outside of the cell and potassium ions to the
inside.
• The sodium-potassium pump carrier protein
exists in two conformations; one that moves
sodium to the outside, and the other that
moves potassium into the cell.
The sodium-potassium pump
http://www.brookscole.com/chemistry_d/templates/student_resources/shared_re
sources/animations/ion_pump/ionpump.html
Exocytosis and Endocytosis
• For molecules that are too large to be
transported with carrier proteins – transported
using vesicle formation – requires energy.
• Exocytosis - vesicles fuse with the plasma
membrane for secretion.
• Causes cell membrane to enlarge –
process occurs during growth.
• Molecules released become part of cell
membrane, part of matrix surrounding cell,
or nourish nearby cells.
Exocytosis
•Some cells are specialized to produce
and release specific molecules.
•In some cases, release of molecules may
occur only in the presence of signals
received by the plasma membrane.
•Examples include release of digestive
enzymes from cells of the pancreas, or
secretion of the hormone insulin in
response to rising blood glucose levels.
• Endocytosis - cells take in substances when a
portion of the plasma membrane folds in, and
forms a vesicle around the substance.
• When vesicle fuses with lysosome, digestion
occurs.
• Endocytosis occurs as:
• Phagocytosis – large particles (food or other
cells)
• Pinocytosis – small particles or liquids
• Receptor-mediated endocytosis – form of
pinocytosis for specific particles such as
vitamins, hormones, or lipoproteins
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter6/animations.html
Phagocytosis
•Seen in unicellular organisms like amoebas.
•White blood cells can use this process to take
in bacteria & worn out red blood cells
Pinocytosis
•Used by root cells of plants.
•Seen in blood cells, cells that line kidneys
and intestines.
Receptor-mediated endocytosis
•Seen during exchange of maternal & fetal
blood in placenta; & intake of cholesterol in
body cells (failure to do so results in high blood
pressure, blocked arteries, and heart attacks).