III' Biological Membranes

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III. Biological Membranes

• Brenda LeadyFall 2005

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• A cell can be divided into
• Plasma membrane
• A.k.a. cytoplasmic membrane/ cell membrane
• Cytoplasm
• nucleus

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Plasma membrane

• Separates inside from outside
• Intracellular fluid
• ICF
• Inside cells
• Extracellular fluid
• ECF
• Outside cells

ECF
ICF
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Fig 3.2
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Fluid Mosaic Model

• Thin lipid bilayer with proteins
• Constantly changing mosaic
• Phospholipids (most), cholesterol and glycolipids

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Fig 3.3
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Phosopholipid

• Polar head is hydrophilic
• Nonpolar tail is hydrophobic
• Self orienting and self assembling
• Assemble with heads out and tails in

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Surfaces differ

• Differ in kinds and amounts of lipids
• Integral membrane proteins
• Embedded in membrane
• Most transmembrane
• Transport (channels, carriers), receptors
• Peripheral Membrane Protein
• Attached only loosely
• Easily dislodged
• May function in cell structure, enzyme or movement

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Fig 3.4
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Membrane Transport

• Interstitial fluid
• ECF derived from blood
• Bathes cells
• Constant traffic between cell and interstitial
  fluid
• Membrane is selectively/ differentially permeable
• Lets some things in or out but not others

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Active/ Passive

• Passive processes
• Substances cross membrane without energy input
  from the cell
• Active processes
• Require cell to use ATP to move substances

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Passive Processes

• Diffusion
• Filtration

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Diffusion

• Important in every cell
• Tendency for molecules or ions to scatter evenly
  throughout the environment
• All molecules are in constant motion bumping into
  each other
• Move from areas of higher concentration to areas
  of lower concentration
• Along or down a concentration gradient

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Fig 3.6
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Why do substances move?

• They are being hit by solvent molecules
• Like balls on a pool table

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Speed influenced by

• Concentration difference
• Greater difference faster diffusion
• Size of molecule
• Smaller molecules diffuse faster
• Temperature
• Warmer temperatures make diffusion go faster

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Eventually

• Concentrations equal out and NET diffusion stops
• Molecules are in constant motion and so movement
  NEVER stops

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Plasma membranes allow molecules to pass if

• Lipid soluble
• Small enough to pass through membrane channels
• Assisted by carrier molecules

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Types of diffusion

• Simple
• Facilitated
• Osmosis

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Fig 3.7
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Simple Diffusion

• Unassisted diffusion of very small or lipid
  soluble substances
• Oxygen, carbon dioxide, fat soluble vitamins,
  alcohol

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Fig 3.7
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Facilitated Diffusion

• Passive transport process using carriers or
  water-filled channels
• Glucose, other sugars, amino acids, and ions

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Carrier

• Transmembrane intergral protein with specificity
  for certain large substances (sugars and amino
  acids for example)
• Envelopes and releases substances to other side
• Limited by number of carriers available

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Fig 3.7
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Channel

• Transmembrane protein that transports water or
  ions
• Some always open, other not
• Also limited by number available

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Fig 3.7
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Osmosis

• Diffusion of water through a semipermeable
  membrane
• Crosses membrane on its own or through channels
• Aquaporins
• Occurs when water concentration differs on 2
  sides of a membrane

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• If water concentration is equal, no net movement

membrane
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• If solute concentration different, water
  concentration is different
• If one side has more water, it moves to the other
  side with less
• Solute follows its concentration gradient

Direction of salt movement
membrane
80 salt 20 water
10 salt 90 water
Direction of water movement
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Fig 3.8
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However

• If membrane is not permeable to solute,
• Water moves but NOT the solute
• Volume changes on one side

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osmosis
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Tonicity

• Ability of a solution to change the shape or tone
  of cells by altering internal water volume

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• Isotonic- same tonicity
• Cell does not shrink or swell
• 0.9 saline
• Hypertonic
• Solution more concentrated
• Cell shrinks due to water loss
• Crenate
• Hypotonic
• Solution less concentrated than cell
• Cells swell due to water gain
• Swell and pop (lyse)
• Hyper- and hypo- can also refer to the
  relationship of the solution to the cell
• You have to be told if you are referring to the
  cell or the solution

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Fig 3.9
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Filtration

• Generally only across capillary walls
• Forces water and solutes through a capillary wall
  using pressure
• Passive process based on a PRESSURE gradient
• Higher pressure to lower pressure
• Well see this again later

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Active Processes

• Active transport
• Vesicular transport

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Active processes

• Must use ATP to move the substances
• Unable to pass using passive means
• Too large, not lipid soluble, against gradient

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Active transport

• Requires carrier proteins to move against/ up a
  concentration gradient
• From where there is less to where there is more
• Transports only specific substances

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Fig 3.10
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Vesicular Transport

• Large particles, macromolecules and fluids
• Exocytosis- moving stuff out of cell
• Endocytosis- moving stuff into cell
• Phagocytosis- cell eats debris or invaders
• Forms phagosome, fuses with lysososme for
  digestion
• Transcytosis- in one side and out the other

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Fig 3.12
Exocytosis
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Exocytosis
Endocytosis
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Phagocytosis
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Absorption of Digested Food
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Figure 23.21
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Absorption of Digested Food

• Carbohydrates
• Broken down into monosaccharides
• Facilitated diffusion or active transport
• Proteins
• Broken down into amino acids
• Facilitated diffusion or active transport

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• Lipids
• Triglycerides broken down into monoglycerides and
  fatty acids
• Associate with bile slats to form micelles
• Move to epithelial surface and use simple
  diffusion into cell
• Without micelles, float on watery chyme and never
  contact cells
• Inside cells, remade into triglycerides and
  coated with proteins to form chylomicron
• Processed by Golgi for export outside cell
• A few free fatty acids enter blood but
  chylomicrons enter lacteal (lymph capillary)
• Eventually lymph dumps into blood
• Chylomicrons broken down to free fatty acids and
  glycerol that can enter body cells

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• Nucleic acids
• Broken down into pentose sugars, nitrogenous
  bases and phosphate ions
• Active transport into cells and into blood

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MembraneReviewQuestions
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What type of transport does require the cell to
spend ATP?

• Diffusion
• Osmosis
• active transport
• facilitated diffusion

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I add sodium chloride to a beaker of water
SALT

• What is the solute?
• The solvent?

Water
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Diffusion is

• Substances travel down a concentration gradient
• Solvent travels down a concentration gradient
• Substances travel up a concentration gradient
• Solvent travels up a concentration gradient

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Osmosis is movement of the solvent across a
membrane(2 are correct)

• From an area with more solvent to an area with
  less solvent
• From an area with less solvent to an area with
  more solvent
• From an area with more solute to an area with
  less solute
• From an area with less solute to an area with
  more solute

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A
C
E
B
D
F
Figure 1
Figure 2
Figure 3

• In figures 1, 2 and 3, is the cell hypertonic,
  hypotonic or isotonic to the solution?
• Figure 1
• Figure 2
• Figure 3

hypertonic
hypotonic
isotonic
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A
C
E
B
D
F
Figure 1
Figure 2
Figure 3

• In figures 1, 2 and 3, is the solution
  hypertonic, hypotonic or isotonic to the cell?
• Figure 1
• Figure 2
• Figure 3

hypotonic
hypertonic
isotonic
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A
C
E
B
D
F
Figure 1
Figure 2
Figure 3

• In each figure, which arrow depicts how the
  solute will move (the dots)?
• Figure 1
• Figure 2
• Figure 3

A
D
both
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A
C
E
B
D
F
Figure 1
Figure 2
Figure 3

• In each figure, which arrow depicts how the
  solvent will move?
• Figure 1
• Figure 2
• Figure 3

B
C
both
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A
C
E
B
D
F
Figure 1
Figure 2
Figure 3

• In each figure, which arrow depicts the direction
  of osmosis?
• Figure 1
• Figure 2
• Figure 3

B
C
both
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A
C
E
B
D
F
Figure 1
Figure 2
Figure 3

• In each figure, which arrow depicts the direction
  of diffusion?
• Figure 1
• Figure 2
• Figure 3

A
D
both
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A
C
E
B
D
F
10 salt
7 glucose
6 albumin
15 glucose
5 salt
6 albumin
Figure 1
Figure 2
Figure 3

• In each figure, which arrow depicts the direction
  of diffusion?
• Figure 1
• Figure 2
• Figure 3

A
D
both
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A
B
10 salt
5 salt
Figure 1

• What would make diffusion go FASTER?
• Temperature- warmer or colder?
• Concentration- 8(lower) or 20(higher)
• Molecule size- larger or smaller