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CELL MEMBRANES AND
TRANSPORT
CH. 5
STRUCTURE OF MEMBRANES
• Bilayer membrane ~7 nm wide
• Contains embedded proteins
FLUID MOSAIC MODEL
• Membrane described as
fluid b/c both
phospholipids and proteins
can move about by
diffusion
• Bilayer has fluidity we
associate with olive oil
• Move sideways in their
own layers
• Proteins move like icebergs
in sea
FLUID MOSAIC MODEL
• Some phospholipid tails are saturated,
some are unsaturated
• More unsaturated = more fluid
WHY???
FACTORS AFFECTING FLUIDITY
• Longer tail = less fluid membrane
• Lower temp. = less fluid membrane
PHOSPHOLIPIDS
• Tails of phospholipids
are nonpolar, so it is
difficult for polar
molecules (water
soluble) to pass
through the
membrane
• Ex: sugars, amino acids,
and proteins cannot
leak out of the cell
CHOLESTEROL
• hydrophilic heads + hydrophobic tails
• Fits in between phospholipids
• Animal cells: amount cholesterol= amount
phospholipids in cell membrane
• Plant cells: little to no cholesterol; Prokaryotes =
none!
• Regulates fluidity, stabilize membranes
• Hydrophobic regions: prevent ions/polar molecules
from passing through membrane
• Important in myelin sheath around nerve cells: ion leaks
would slow signals
CHOLESTEROL
TWO TYPES OF PROTEINS IN
CELL MEMBRANES:
1. Intrinsic (aka integral):
found in inner layer,
outer layer, or most
commonly spanning
the entire membrane
(transmembrane proteinscross both sides of
membrane)
2. Extrinsic (aka
peripheral): found on
either the inner or
outer surface of the
membrane
INTRINSIC PROTEINS
• Stay in membrane due to its hydrophobic and
hydrophilic regions
• Most float like mobile icebergs although some are
fixed like islands to structures inside or outside the
cell and do not move about
EXTRINSIC PROTEINS
• Many are bound to intrinsic proteins
• Some are held in place by binding to molecules
inside or outside the cell
PROTEINS’ ROLES IN MEMBRANES
• Transport proteins: hydrophilic channels
• Moving Ions, polar molecules
• Enzymes: on plasma membrane on small intestine surface
hydrolyze disaccharides
• Proteins for photosynthesis + cell respiration: E- transport
chains in mitochondria membrane and thylakoid
membrane of chloroplast
• Receptors: bond to antigen and begin communication
pathway
GLYCOLIPIDS AND GLYCOPROTEINS
• Many proteins and lipids in membrane have
carbohydrate chains attached that face
the outside of the membrane
• Glycolipids = carb. (polysaccharide) attached to
lipid
• Glycoproteins = carb. attached to protein
GLYCOLIPIDS AND GLYCOPROTEINS
• Form H-bonds with water to stabilize the membrane
• Form sugary coating on membrane called
glycocalyx
• Act as receptor molecules
for cell-cell recognition
(immune cells)
SIGNALING RECEPTORS
• Coordinate activities
of animal cells
• Recognize messenger
molecules like
hormones and
neurotransmitters
• When molecule binds
with receptor, it
triggers a series of
chemical reactions in
the cell
EXAMPLE: CELL INSULIN RECEPTORS
ENDOCYTOSIS RECEPTORS
• Bind to
molecules
that are
parts of the
structures
that are to
be engulfed
by cell
CELL MARKER RECEPTORS
• Aka antigens; Allow cell-cell recognition
• Each cell type has its own specific antigen (similar
to how different countries have different flags)
• Ex: ABO blood types
TRANSPORT PROTEINS
• Provide channels or passageways for ions and polar
molecules to pass through cell membrane
• Two main types:
1. Channel proteins – Facilitated diffusion (NO ATP!)
2. Carrier proteins – Active transport (ATP REQUIRED!)
REVIEW AND PRACTICE!
• http://www.wisconline.com/objects/ViewObject.aspx?ID=ap1101
• KAHOOT!
TRANSPORT ACROSS THE
CELL MEMBRANE
CH. 5
CELL SURFACE MEMBRANE (CSM)
TRANSPORT
• Phospholipid bilayer
creates effective
barrier against water
soluble molecules and
ions
• Prevents aqueous
contents from escaping
• Some essential transport
is achieved through:
1. Diffusion
2. Facilitated diffusion
3. Osmosis
4. Active transport and
bulk transport
DIFFUSION (SIMPLE DIFFUSION)
• Diffusion: net movement, as a result of random
motion of its molecules or ions, of a substance from
a region of high concentration to a region of low
concentration
• Particles move down a concentration gradient
FACTORS THAT AFFECT
RATES OF DIFFUSION
• ‘steepness’ of concentration gradient: greater
difference = higher rate
• Temperature: higher temperature = higher
rate
• Surface area of diffusion: greater surface area
= higher rate (more contact space)
• Nature of molecules/ions: small molecules =
greater rate
DIFFUSION
• Respiratory
gases cross
membrane by
diffusion
(uncharged
and nonpolar)
FACILITATED DIFFUSION
• Diffusion that takes place with the help
“assistance” of a certain protein
molecule
• Channel proteins
• Carrier proteins
CHANNEL PROTEINS
• Water-filled pores
• Fixed shape
• Allow charged substances
(usually ions) to diffuse
through membrane
• Gated to allow for selectivity
and control of movement
CARRIER PROTEINS
• Flips between shapes to alternately open binding
sites on interior/exterior of membrane
• Direction of movement depends on concentration
gradient inside and outside of cell
OSMOSIS
• Type of diffusion involving water molecules only
WATER POTENTIAL
• Water potential: tendency of water molecules to
move from one place to another
• Measured by Greek symbol psi Ψ
WATER POTENTIAL PROBLEMS
1) In beaker A, which has a higher water potential, distilled
water or the beet core?
1) Where will water flow in the diagram B? Explain why.
WATER POTENTIAL
• Water potential for pure
water (distilled) is 0
• Solute potential - amount
that solutes lower the
water potential
• Solutes make water
potential less than 0,
the more solute, the
more negative the
water potential!
Pressure potential
(symbol Ψp) Increasing
pressure increases
water potential
Pressure potential
makes water potential
less negative (more
positive)
WATER POTENTIAL
WATER POTENTIAL IN ANIMAL CELLS
OSMOSIS IN PLANT CELLS
ACTIVE TRANSPORT (ATP NEEDED)
• Active transport moves molecules/ions against
concentration gradient
• Achieved by carrier proteins (specific for structure of
particular ion/molecule)
• ATP supplied by cellular respiration
• Can occur into or out of the cell
SODIUM-POTASSIUM PUMP
• Found in CM of all animal cells
• Use ~30% cell energy and ~70% neuron cell energy
• Pump 3 Na+ ions out of the cell and 2 K+ ions into
the cell
• Net result: inside of cell become more negative than the
outside
ACTIVE TRANSPORT
• Important in re-absorption in the kidneys, where
certain useful molecules and ions have to be reabsorbed into the blood after filtration into the
kidney tubules
• Involved in
absorption of
products of
digestion in the
gut
ACTIVE TRANSPORT
• Used to load sugar from photosynthesizing cells of
leaves into phloem tissue for transport around the
plant
BULK TRANSPORT (ATP REQUIRED!)
• Bulk transport involves the mechanism of moving
large quantities of molecules into the cell
(endocytosis) or out of the cell (exocytosis)
• Large molecules such as proteins or
polysaccharides, part of cells, or even whole cells
may be transported across the membrane
PHAGOCYTOSIS
• ‘cell eating’
• Bulk up take of solid material
• Cells specializing in this are called phagocytes and
the vacuoles are called phagocytic vacuoles
• Ex: engulfing of bacteria by immune cells
PINOCYTOSIS
• ‘cell drinking’ – bulk uptake
of liquid into cell
• Vacuoles ( vesicles) formed
are often extremely small, in
which the process is called
micro-pinocytosis
• Human egg cells take up
nutrients from cells that
surround it by pinocytosis
EXOCYTOSIS
• Ex: how plants transport materials out of CM to
make cell wall
• Ex: secretion of digestive enzymes of the pancreas
• Secretory vesicle from the Golgi carry the enzymes to the
CSM and release their contents