The Cell Membrane

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CHAPTER 7 MEMBRANE STUCTURE AND FUNCTION
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Study guide 20-27
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Diffusion

• 2nd Law of Thermodynamics
• Governs biological systems!
• Universe tends to disorder!

Diffusion movement from highgtlow concentration
of THAT SUBSTANCE!
diffusion
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Diffusion of 2 Solutes
Each substance diffuses down its own
concentration gradient, independent of
concentration gradient of any other substance.
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• Diffusion
• PASSIVE TRANSPORT
• No energy required
• So How does that happen?

Brownian Motion
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Osmosis is the diffusion of water across a
membrane

• Water is very important to life, so we talk
  about water separately
• Diffusion of water from high concentration of
  water to low concentration of water
• across a semi-permeable membrane

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Concentration of water

• Direction of osmosis is determined by comparing
  total solute concentrations
• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water

water
net movement of water
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Osmosis

• 250 times the volume of cell/second!
• Osmosis NET movement of water across a
  selectively permeable membrane driven by a
  difference in solute concentration on either side
  of the memrane.
• Free water moves
• Less solute more free water
• Water flows from low solute to high solute
  
• Until equilibrium

Slide 2
osmosis
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Osmosis
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What determines when and what direction water
will move?

• 3 molecules of albumin 15 molecules of
  glucose
• 66,000 mw 180 mw

Water?
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Isotonic? Hypertonic? Hypotonic?
A B are isotonic A B are hypertonic to C C
is hypotonic to A and B
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Osmotic Pressure

• Pressure generated by diffusion of water across a
  membrane
• When pressure is equal water flow will stop
• Called hydrostatic pressure water-stopping
  pressure
• Osmolarity- in terms of of particles in a
  volume of liquid
• 1 osmolar soln 1 M of osmoltically active
  particles per liter.

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Red Blood Cells in NaCl solutions

• 100mOs 500 mOs

hypotonic
isotonic
hypertonic
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Osmosis problems
Hydrostatic generator
Osmosis problems
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• End
• Diffusion
• Osmosis

Cell membrane
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The Cell Membrane
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Overview

• Cell membrane separates living cell from
  nonliving surroundings
• thin barrier 8nm thick
• Controls traffic in out of the cell
• selectively permeable
• allows some substances to cross more easily than
  others
• hydrophobic vs hydrophilic
• Made of phospholipids, proteins other
  macromolecules

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Phospholipids
Phosphate

• Fatty acid tails
• hydrophobic
• Phosphate group head
• hydrophilic
• Arranged as a bilayer

Fatty acid
Aaaah, one of thosestructurefunction examples
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Phospholipid bilayer
polar hydrophilic heads
nonpolar hydrophobic tails
polar hydrophilic heads
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(No Transcript)
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Two generations of membrane models
How do we know?
1972 Singer, Nicholson Dispersion
model Hydrophyllic regions in aqueous Hydrophobic
in hydrophobic PL FLUID MOSAIC MODEL! Freeze
Fracture evidence Permanent model????
phospholipid bilayer -1920s 1930-60s models
from EM views ??-Not as hydrophyllic as pure
PLs? D D - Hydrophyllic Protein sandwich ??-
all membranes identical??? ??-amphipathic
proteins? Solubile in H2O hydrophobic region in
aqueous?
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More than lipids

• In 1972, S.J. Singer G. Nicolson proposed that
  membrane proteins are inserted into the
  phospholipid bilayer

Its like a fluidIts like a mosaic Its the
Fluid Mosaic Model!
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Evidence for the drifting of membrane proteins
Other Evidence microsurgery on cells
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• Membranes are fluid- like salad oil!
• held in place weak hydrophobic interactions
• PLS drift laterally
• rarely flip-flop between layers
• larger proteins move slower
• some proteins guided by cytoskeleton motors
• some proteins anchored by cytoskeleton

Fluid Membrane
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Membrane is a collage of proteins other
molecules embedded in the fluid matrix of the
lipid bilayer
Extracellular fluid
Phospholipids
Cholesterol
Cytoplasm
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The fluidity of membranes
Maintains and increases/decreases fluidity
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• Fluidity influenced by temperature
• cool - more solid- PLs closely packed
• if rich in unsaturated fatty acids - more fluid
  than those rich in saturated fatty acids -
  kinks prevent tight packing
• cholesterol steroid- wedged between PLs of
  animal cells
• warm -limits mvmnt. of PLs, reduces fluidity
• cool - maintains fluidity, prevents tight
  packing-EX salmon

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• Must be fluid to work w/ enzymes be permeable
• Cells alter lipid makeup to adjust for temp.
  changes
• EX cold organisms ( winter wheat, salmon, bears)
  increase of unsat PLs in autumn
• - prevents solidifying membranes

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Membrane fat composition varies

• Fat composition affects flexibility
• membrane must be fluid flexible
• about as fluid as thick salad oil
• unsaturated fatty acids in phospholipids
• keep membrane less viscous
• cold-adapted organisms, like winter wheat
• increase in autumn
• cholesterol in membrane

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Why areproteins the perfect molecule to build
structures in the cell membrane?
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Classes of amino acids
What do these amino acids have in common?
nonpolar hydrophobic
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Classes of amino acids
What do these amino acids have in common?
I like thepolar onesthe best!
polar hydrophilic
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Membrane Proteins

• Proteins determine membranes specific functions
• cell membrane organelle membranes each have
  unique collections of proteins
• Membrane proteins
• peripheral proteins
• loosely bound to surface of membrane
• cell surface identity marker (antigens)
• integral proteins
• penetrate lipid bilayer, usually across whole
  membrane
• transmembrane protein
• transport proteins
• channels, permeases (pumps)

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• 2 major types of membrane proteins.
• Peripheral proteins - not embedded bound to
  inner/outer surface may be connected to
  integral proteins
• Integral proteins - penetrate bilayer
• - Transmembrane protein
• hydrophobic regions of nonpolar aas in contact
  with bilayer core often alpha helices
• hydrophilic regions of aas in contact w/
  environment beta pleated
• - Unilateral partially through coupled

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Proteins domain anchor molecule
Polar areas of protein

• Within membrane
• nonpolar amino acids
• hydrophobic
• anchors protein into membrane
• On outer surfaces of membrane
• polar amino acids
• hydrophilic
• extend into extracellular fluid into cytosol

Nonpolar areas of protein
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Examples
water channel in bacteria
proton pump channel in photosynthetic bacteria
function through conformational change shape
change
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Many Functions of Membrane Proteins
Outside
Plasma membrane
Inside
Transporter
Enzymeactivity
Cell surfacereceptor
Uniport Symport Antiport
Cell adhesion
Cell surface identity marker
Attachment to thecytoskeleton
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Membrane carbohydrates

• Play a key role in cell-cell recognition
• ability of a cell to distinguish one cell from
  another
• antigens
• important in organ tissue development
• basis for rejection of foreign cells by immune
  system

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• Some proteins reinforce shape of cell
• cytoplasmic side,
• some connect to cytoskeleton
• exterior side,
• some attach to fibers of ECM
• ECM extracellular matrix

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Sidedness of the plasma membrane
What makes the endomembrane system work?
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• Membranes are bifacial and sided
• may differ in lipid composition
• proteins have a direction orientation
• outer surface has carbos
• asymmetry begins w/ synthesis in ER
• proteins in plasma membrane provide a variety
  of major cell functions

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The detailed structure of an animal cells plasma
membrane, in cross section
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• Cell-cell recognition distinguish one type of
  neighboring cell from another
• important in cell sorting
• organization into tissues and organs during
  development
• basis for rejection of foreign cells by
  immune system
• key in on surface molecules, often carbs

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• Carbos - usually branched oligosaccharides
  lt 15 monomers
• may be covalently bonded to
• lipids glycolipids
• proteins glycoproteins
• external OSs vary from species to species,
  individual to individual, and even from cell type
  to cell type w/in same individual -
  identification
• marks each cell type as distinct
• blood types (A, B, AB, O)-RBCs
• Glycocalyx of animal cells fuzzy!

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End membranes
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Movement across the Cell Membrane
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Diffusion

• 2nd Law of Thermodynamics governs biological
  systems
• universe tends towards disorder (entropy)

• Diffusion
• movement from high ? low concentration

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Diffusion

• Move from HIGH to LOW concentration
• passive transport
• no energy needed

movement of water
diffusion
osmosis
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Diffusion across cell membrane

• Cell membrane is the boundary between inside
  outside
• separates cell from its environment

NO!
Can it be an impenetrable boundary?
OUT waste ammonia salts CO2 H2O products
IN food carbohydrates sugars, proteins amino
acids lipids salts, O2, H2O
OUT
IN
cell needs materials in products or waste out
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Diffusion through phospholipid bilayer

• What molecules can get through directly?
• fats other lipids

• What molecules can NOT get through directly?
• polar molecules
• H2O
• ions
• salts, ammonia
• large molecules
• starches, proteins

lipid
salt
NH3
aa
H2O
sugar
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Channels through cell membrane

• Membrane becomes semi-permeable with protein
  channels
• specific channels allow specific material across
  cell membrane

inside cell
sugar
aa
H2O
salt
outside cell
NH3
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Facilitated Diffusion

• Diffusion through protein channels
• channels move specific molecules across cell
  membrane
• no energy needed

facilitated with help
open channel fast transport
The Bouncer
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Active Transport

• Cells may need to move molecules against
  concentration gradient
• shape change transports solute from one side of
  membrane to other
• protein pump
• costs energy ATP

conformational change
ATP
The Doorman
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Active transport

• Many models mechanisms

ATP
ATP
symport
antiport
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Getting through cell membrane

• Passive Transport
• Simple diffusion
• diffusion of nonpolar, hydrophobic molecules
• lipids
• high ? low concentration gradient
• Facilitated transport
• diffusion of polar, hydrophilic molecules
• through a protein channel
• high ? low concentration gradient
• Active transport
• diffusion against concentration gradient
• low ? high
• uses a protein pump
• requires ATP

ATP
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Transport summary
simplediffusion
facilitateddiffusion
ATP
activetransport
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How about large molecules?

• Moving large molecules into out of cell
• through vesicles vacuoles
• endocytosis
• phagocytosis cellular eating
• pinocytosis cellular drinking
• exocytosis

exocytosis
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Endocytosis
fuse with lysosome for digestion
phagocytosis
non-specificprocess
pinocytosis
triggered bymolecular signal
receptor-mediated endocytosis
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The Special Case of WaterMovement of water
across the cell membrane
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Osmosis is diffusion of water

• Water is very important to life, so we talk
  about water separately
• Diffusion of water from high concentration of
  water to low concentration of water
• across a semi-permeable membrane

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Concentration of water

• Direction of osmosis is determined by comparing
  total solute concentrations
• Hypertonic - more solute, less water
• Hypotonic - less solute, more water
• Isotonic - equal solute, equal water

water
net movement of water
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Managing water balance

• Cell survival depends on balancing water uptake
  loss

freshwater
balanced
saltwater
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Managing water balance

• Isotonic
• animal cell immersed in mild salt solution
• example blood cells in blood plasma
• problem none
• no net movement of water
• flows across membrane equally, in both directions
• volume of cell is stable

balanced
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Managing water balance

• Hypotonic
• a cell in fresh water
• example Paramecium
• problem gains water, swells can burst
• water continually enters Paramecium cell
• solution contractile vacuole
• pumps water out of cell
• ATP
• plant cells
• turgid

ATP
freshwater
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Water regulation

• Contractile vacuole in Paramecium

ATP
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Managing water balance

• Hypertonic
• a cell in salt water
• example shellfish
• problem lose water die
• solution take up water or pump out salt
• plant cells
• plasmolysis wilt

saltwater
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Aquaporins
1991 2003

• Water moves rapidly into out of cells
• evidence that there were water channels

Peter Agre John Hopkins
Roderick MacKinnon Rockefeller
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Osmosis
.05 M
.03 M
Cell (compared to beaker) ? hypertonic or
hypotonic Beaker (compared to cell) ? hypertonic
or hypotonic Which way does the water flow? ? in
or out of cell
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Any Questions??