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Cellular Membranes

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Title: Cellular Membranes


1
Cellular Membranes
  • The outer boundary of the cell as well as the
    boundary of many of the internal organelles is
    made of a cellular membrane
  • Composed primarily of phospholipids that are
    arranged in a bilayer (2 layers) with proteins,
    carbohydrates and cholesterol molecules are
    integrated within
  • Fluid mosaic model mosaic of proteins floats in
    or on the fluid lipid bilayer like boats on a pond

2
4 Components of Cellular Membranes
  • Phospholipid bilayer
  • Flexible matrix, barrier to permeability
  • Transmembrane proteins
  • Integral membrane proteins
  • Interior protein network
  • Peripheral or Intracellular membrane proteins
  • Cell surface markers
  • Glycoproteins and glycolipids

3
Phospholipid Models
4
Phospholipids
  • Structure consists of
  • Glycerol a 3-carbon polyalcohol
  • 2 fatty acids attached to the glycerol
  • Nonpolar and hydrophobic (water-fearing)
  • Phosphate group attached to the glycerol
  • Polar and hydrophilic (water-loving)
  • Spontaneously forms a bilayer
  • Fatty acids are on the inside
  • Phosphate groups are on both surfaces

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6
Plasma Membrane Structure
7
  • Bilayers are fluid
  • Hydrogen bonding of water holds the 2 layers
    together
  • Individual phospholipids and unanchored proteins
    can move through the membrane

8
  • Environmental influences on fluidity
  • Saturated fatty acids make the membrane less
    fluid than unsaturated fatty acids
  • Kinks introduced by the double bonds keep them
    from packing tightly
  • Most membranes also contain sterols such as
    cholesterol, which can either increase or
    decrease membrane fluidity, depending on the
    temperature
  • Warm temperatures make the membrane more fluid
    than cold temperatures

9
Plasma Membrane Function
  • At the surface of a cell, the plasma membrane
    separates the intracellular fluid (ICF or
    cytosol) from the extracellular fluid (ECF) of a
    cell
  • Provides a means to communicate with other cells
  • Provides a gateway for exchange between the ECF
    and ICF
  • the arrangement of phospholipids in a bilayer
    makes most of the thickness of the membrane
    NON-POLAR and thus creates an extremely effective
    barrier against the movement of polar substances
    into or out of the cell
  • membrane proteins determine what gets or stays
    in/out of a cell which allows the composition of
    the ICF to be optimal for cellular functions
    control the movement of substances into/out of
    the cell

10
Cell Membrane Proteins
  • Integral or transmembrane proteins
  • completely pass through the bilayer
  • extracellular portion is exposed to the ECF
  • composed of polar amino acids
  • intracellular portion is exposed to the ICF
  • composed of polar amino acids
  • connect the ICF and ECF
  • composed of nonpolar amino acids
  • different classes of integral proteins are based
    on function
  • Peripheral membrane protein
  • associated only with the intracellular surface of
    the cell membrane (located in the ICF)
  • capable of detaching and moving into the cytosol
    to interact with other molecules within the cell

11
Membrane Proteins
  • Various functions
  • Transporters
  • Enzymes
  • Cell-surface receptors
  • Cell-surface identity markers
  • Cell-to-cell adhesion proteins
  • Attachments to the cytoskeleton

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13
Integral Membrane Protein Structure
14
Membrane Carbohydrates
  • The small polysaccharides that are part of the
    plasma membrane are always immersed in the ECF
  • covalently bound to an integral membrane protein
    or a phospholipid head
  • 2 varieties
  • Glycolipids
  • polysaccharides are covalently bound to the polar
    head of a phospholipid
  • Glycoproteins
  • polysaccharides are covalently bound the
    extracellular portion of an integral membrane
    protein

15
Modes of Membrane Transport
  • Transmembrane Transport
  • movement of small substances through a cellular
    membrane (plasma, ER, mitochondrial..)
  • ions, fatty acids, H2O, monosaccharides,
    steroids, amino acids
  • polar substances use integral membrane proteins
  • nonpolar substances pass directly through the
    phospholipid bilayer
  • Vesicular Transport
  • transport of substances that are TOO LARGE to
    move through a membrane
  • proteins, cellular debris, bacteria, viruses
  • use vesicles to move substances into/out of the
    cell

16
Vesicles
  • Bubbles of phospholipid bilayer membrane with
    substances inside
  • Created by the pinching or budding of the
    phospholipid bilayer membrane
  • reduces the amount of membrane at that location
  • Can fuse (merge) with another phospholipid
    bilayer membrane within the cell
  • adds to the amount of membrane at that location

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18
Vesicular Transport
  • Exocytosis
  • moves substance out of the cell
  • fusion of a vesicle with substances with the cell
    membrane
  • Endocytosis
  • moves substances into the cell
  • cell membrane creates pseudopods (false feet)
    which traps substances in the ECF within a vesicle

19
Types of Endocytosis
  • Phagocytosis
  • cell eating
  • endocytosis of few very large substances
    (bacteria, viruses, cell fragments)
  • vesicles containing cells fuse with lysosomes
    which digest the cells
  • Pinocytosis
  • cell sipping
  • endocytosis of extracellular fluid
  • Receptor-mediated endocytosis
  • endocytosis of a specific substance within the
    ECF
  • requires that the substance attaches to the
    extracellular portion of an integral membrane
    receptor protein

20
Endocytosis
21
Solutions
  • The ICF and the ECF are homogeneous mixtures of
    substances including water, ions, amino acids,
    disaccharides, triglycerides called solutions
  • Solutions are divided into 2 parts
  • Solvent
  • substance present in greatest amount
  • the solvent of the body is water
  • Solute(s)
  • substance(s) present in lesser amounts
  • every other substance in the body is a solute
  • ions, carbohydrates, proteins, fats, nucleotides
  • Solutions are described in terms of their
    concentration
  • the amount of solutes in a given volume of
    solution
  • Units include molarity, , weight per volume

22
ECF vs. ICF
  • The total solute concentration of the ECF ICF
  • The concentration of each solute in the ECF is
    different from its concentration in the ICF
  • the cell membrane transports each solute between
    the ICF and ECF creating gradients for each
    solute to maintain optimal conditions within the
    ICF

EXTRACELLULAR
INTRACELLULAR
Major cations
Na


K

Mg
Major anions
Cl
-
-
HCO
3
Protein
Phosphates
--
SO
4
Org. acids
150
100
50
0
0
50
100
150
Concentration (mM)
Concentration (mM)
23
Concentration Gradients
  • The difference in the amount of a substance
    between 2 locations
  • area of greater amount vs. area of lesser amount
  • the difference may be LARGE or small
  • may exist across a physical barrier (membrane) or
    across a distance without a barrier
  • Substances in an area of high concentration
    naturally move toward regions of lower
    concentrations
  • Form of mechanical energy where location of
    greater amount provides more collisions
  • collisions cause substances to spread out

24
Concentration Gradients
25
Movement of Substances Relative to Gradients
Active Transport
  • The movement of a substance from a region of
    lesser amount to a region of greater amount is
    called active transport
  • moves a substance UP or AGAINST a concentration
    gradient and requires an energy source
  • since this movement subtracts from the area of
    lower concentration and adds to the area of
    higher concentration it creates a larger
    difference between the 2 areas causing the
    gradient to increase

26
Movement of Substances Relative to Gradients
Passive Transport
  • The movement of a substance from a region of
    greater amount to a region of lesser amount is
    called passive transport
  • moves a substance DOWN or WITH a concentration
    gradient and does NOT require an energy source
  • actually releases the (same amount of) energy
    required to create the gradient
  • causes the gradient to decrease since this
    movement subtracts from the area of higher
    concentration and adds to the area of lower
    concentration creating a smaller difference
    between the 2 areas

27
Transmembrane Concentration Gradients Equilibrium
  • Equilibrium
  • a condition that is met when substances move
    passively down a gradient until there is equal
    concentrations of a substance between 2 locations
    (NO gradient)
  • no net movement of substances from one location
    to another
  • substances continue to move due to heat energy,
    but movement occurs equally in both directions
  • Equilibrium of substances across the various
    membranes in the cells of the body DEATH
  • your body is in a constant battle to ensure
    equilibrium of solutes across the membranes is
    never reached

28
Transmembrane Concentration Gradients Steady
State
  • Steady State
  • a condition that is met when substances move
    passively down a gradient, but then are moved
    actively back up the gradient
  • the gradient of a substance is MAINTAINED by the
    constant expenditure of energy by the cell
    fighting against equilibrium
  • no net movement of substances from one location
    to another
  • one substance moving passively down the gradient,
    is exactly balanced by one substance moving
    actively up the gradient
  • Life depends upon the ability of the organism to
    exist in a steady state

29
Transmembrane Transport of Nonpolar vs. Polar
Substances
  • Nonpolar substances cross a membrane through the
    phospholipid bilayer
  • ineffective barrier against the movement of
    nonpolar molecules across a membrane
  • it is impossible to control the movement of
    nonpolar molecules through a membrane
  • Polar substances cross a membrane by moving
    through integral membrane transporting proteins
  • Carriers or Channels
  • Each carrier and channel has a unique tertiary
    shape and therefore is designed to transport a
    different substance across a membrane
  • the cell can control the movement of polar
    molecules through a membrane by controlling the
    activity of the transporting proteins

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31
Channels
  • Integral membrane proteins containing a water
    filled hole (pore) which allows the movement of
    very small polar substances (ions and water)
  • passively transport polar substances very quickly
    across membrane down their respective
    concentration gradient
  • some channels have gates which can be
  • open
  • allows substance to cross
  • closed
  • does not allow substance to cross
  • channels that do not have gates are always open
    and are considered to be leaky channels

32
Carriers
  • Integral membrane proteins that carry 1 or more
    small polar substances (monosaccharides, amino
    acids, nucleotides)
  • The movement of only one substance across a
    membrane is called uniport
  • The movement of more than one substance across a
    membrane is called cotransport
  • change their shape between 2 conformations
  • flip-flop between being open to the ECF and ICF
  • transport substances much more slowly across a
    membrane compared to channels
  • the maximum rate at which these proteins can
    transport substances across a membrane is limited
    by how fast they can change shapes

33
Patterns of Transport by Carriers
  • Symport
  • 2 substances are moved in the same direction
  • Antiport
  • 2 substances are moved in opposite directions
  • Some carriers passively transport substances down
    their respective concentration gradient
  • Other carriers actively transport substances up
    their respective concentration gradient
  • carriers called pumps hydrolyze a molecule of ATP
    and use the energy to pump substances across the
    membrane against the gradient
  • Still other carriers simultaneously transport at
    least one substance up its concentration gradient
    and one substance down its concentration gradient

34
Passive Transmembrane Transport
  • Movement a substance to move DOWN its gradient
  • Releases energy as a result of the movement
  • Nonpolar substances diffuse through the nonpolar
    phospholipid bilayer in a process called simple
    diffusion
  • Polar molecules require the help (facilitation)
    of integral membrane proteins (channels or
    carriers) to cross the bilayer in a process
    called facilitated diffusion
  • Diffusion does not occur if there is no gradient
  • equilibrium

35
Facilitated Diffusion
36
Factors Affecting the Rate of Diffusion
  • The rate at which a substance moves by way of
    diffusion is influenced by 3 main factors
  • Concentration gradient
  • a large concentration gradient, results in a high
    rate of diffusion
  • Temperature
  • a high temperature, results in a high rate of
    diffusion
  • heat causes motion
  • Size of the substance
  • a large substance, has a low rate of diffusion
  • larger objects move more slowly

37
Osmosis
  • The diffusion of water (solvent) across a
    selectively permeable membrane is called osmosis
  • requires water channels called aquaporins
  • Solutes generate a force (osmotic pressure) that
    pulls water molecules toward the solutes
  • the greater the osmotic pressure, the greater the
    amount of water movement

38
Tonicity
  • The difference in the total solute concentration
    of the ECF vs. the ICF determines the direction
    and extent of osmosis across the plasma membrane
  • If the total solute concentration of the ECF
    ICF, then the ECF is considered an isotonic
    (same) solution
  • water does not move into or out of cell causing
    no change in the cell volume
  • If the total solute concentration of the ECF gt
    ICF, then the ECF is considered a hypertonic
    (more) solution
  • water diffuses out of the cell causing the cell
    to shrink (crenate)
  • If the total solute concentration of the ECF lt
    ICF, then the ECF is considered a hypotonic
    (less) solution
  • water diffuses into the cell causing the cell to
    swell

39
Osmosis and Cell Volume Changes
40
Primary Active Transport
  • Integral membrane pumps use the energy stored in
    a molecule of ATP to transport substances across
    a membrane UP a concentration gradient
  • The pump
  • hydrolyzes the high energy bond in a molecule of
    ATP (releasing energy)
  • uses the energy of ATP hydrolysis to flip-flop
    between conformations while moving substances UP
    a concentration gradient
  • This process converts chemical energy (ATP) to
    mechanical energy (gradient across the membrane)
  • the gradient can be used if necessary by the cell
    as a form of energy to do work

41
Sodium, Potassium ATPase
  • Na,K-ATPase (Na,K pump)
  • located in the plasma membrane
  • actively cotransports
  • 3 Na from the ICF (lesser amount) to the ECF
    (greater amount)
  • 2 K from the ECF (lesser amount) to the ICF
    (greater amount)

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43
Secondary Active Transport
  • Integral membrane carriers move at least 2
    different substances across a membrane
  • One substance moves across a membrane UP a
    concentration gradient
  • One substance moves across a membrane DOWN a
    concentration gradient
  • The movements of the substance DOWN a
    concentration gradient releases energy which the
    carrier uses to flip-flop between conformations
    and move a second substance UP a concentration
    gradient
  • piggyback transport
  • This type of transport is called secondary
    because this process is driven by a gradient that
    is created by a previous occurring primary active
    transport process

44
Secondary Active Transport Na, Glucose
Cotransporter
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