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Lecture 3a.
Membranes and Transport
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The Cell Membrane
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Overview: Membranes
Structure of cell membranes Functions of cell membranes How things get in and out of cells
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What is a membrane?
Basically, a covering
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Structure of the cell membrane
Separates intracellular fluids from extracellular fluids
Contains lipids, proteins and carbohydrates Lipids
Phospholipids Cholesterol
Proteins Integral Peripheral
Carbohydrates Form the glycocalyx
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Fluid Mosaic Model
Double bilayer of lipids with imbedded, dispersed proteins
Bilayer consists of phospholipids, cholesterol, and glycolipids Glycolipids are lipids with bound carbohydrate Phospholipids have hydrophobic and hydrophilic
bipoles
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Fluid Mosaic Model
Figure 3.3
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Membrane Junctions – link cells together
Tight junction – impermeable junction that encircles the cell
Desmosome – anchoring junction scattered along the sides of cells
Gap junction – a nexus that allows chemical substances to pass between cells
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Membrane Junctions: Tight Junction
Figure 3.5a
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Figure 3.5b
Membrane Junctions: Desmosome
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Figure 3.5c
Membrane Junctions: Gap Junction
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Phopholipid bilayer
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Cell Membrane Structure
Phospholipids: Hydrophilic head
(likes water) Hydrophobic tails
(cannot be in contact with water)
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Cell Membrane Structure
Ions and water soluble compounds cannot cross the membrane without help. Why not?
This separates the extracellular fluid from the cytosol which is important for homeostasis
Nonpolar molecules, fat soluble organic molecules (e.g. steroids), and water can cross
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What else is in membranes?
1. Cholesterol
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What else is in membranes?
2. Proteins
- Integral proteins (many are transmembrane)
- Peripheral proteins
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Membrane Proteins
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Membrane protein functions
1. Anchoring - attach to cytoskeleton
2. Enzymes - catalyze reactions
3. Recognition (identifiers) usu. glycoproteins (e.g. MHC, ABO)
4. Receptors - signaling (ligand)
5. Transport
Carriers - transport things in/out
Channels – pore allowing ions in/out
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What else is in membranes?
3. Membrane carbohydrates
Glycocalyx: Glycoproteins Glycolipids
Used for cell-cell recognition Often, doctors can tell if certain cells are
normal or abnormal by their glycoproteins and glycolipids
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Why have a cell membrane anyway?
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Cell MembraneAKA “plasma membrane”
General functions1. Physical isolation
Important to maintain a different environment inside the cell relative to outside. --May not sound that interesting, but this is key to life: need to separate ion concentrations to create potential energy, preserve homeostasis. Note: this is also important for internal organelles (e.g lysosome, mitochondrion). Cf. bacteri
2. Regulation of exchange in the environment Controlling this exchange is critical
3. Sensitivity
4. Structural support
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Overcoming the Cell Barrier
The cell membrane is a barrier, but: Things must get in and out (like what?)
Nutrients, oxygen must get in products and wastes must get out
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Cell membranes are selectively permeable.
What does this mean?
Only certain things can cross How are compounds transported across the
cell membrane? Passive transport (no ATP needed) Active transport (ATP required)
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Types of Transport
Diffusion (passive) Carrier-mediated transport (passive or active) Vesicular transport (active)
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Terms
Before we get into detail about the types of transport, we need to discuss some key terms… Solutions
Molecules (solutes) dissolved in a liquid (solvent) All molecules are constantly in random motion, which
causes mixing Brownian motion
Random tendency of ALL molecules to move due to their inherent kinetic energy: energy of motion
Concentration The amount of solute in a solvent
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Concentration Gradient
More solute in one region of a solvent than in another
Examples?
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Concentration gradients
Concentrations of some key ions are very different on the inside versus the outside of cells creating a gradient
IN:[Na+] = low[K+] = high[Ca2+] = very
low[Cl-] = low
OUT:[Na+] = high[K+] = low[Ca2+] = low[Cl-] = high
(blood, interstitial fluid)
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Types of Transport
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Passive transport: Diffusion
Due to: random motion and collision of molecules (NOT a pulling or pushing force)
Movement “down” a concentration gradient (from area of high concentration to area of low concentration)
Simple diffusion – nonpolar and lipid-soluble substances Diffuse directly through the lipid bilayer
Facilitated diffusion – larger, polar molecules Transported substances bind carrier proteins or pass
through protein channels (e.g. glucose, amino acids, and ions)
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Diffusion – solid in water
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What factors Affect Diffusion Rates?
Distance Molecule size Temperature Gradient Electrical force
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What can/can’t diffuse through the cell membrane?
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Simple Diffusion
Materials which diffuse directly through cell membrane: lipid-soluble compounds (alcohols, fatty acids,
and steroids) dissolved gases (oxygen and carbon dioxide)
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Channel-Mediated Facilitated Diffusion
Materials which pass through transmembrane proteins (channels): water soluble compounds ions
Note: even though a channel is required, this is still a passive process. Diffusion, by definition, is always passive.
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Carrier-Mediated Facilitated Diffusion
Carrier-mediated transport of ions and organic substrates into or out of the cell down their concentration gradient. Still passive
Can also be called passive carrier-mediated transport
Note: if energy is required to move something in or out of the cell using a carrier protein, it is called active transport (ATP required)
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Facilitated Diffusion is passive transport with a carrier
Animation
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How Facilitated Diffusion Works
Carrier proteins transport molecules too large to fit through channel proteins (glucose, amino acids): molecule binds to receptor site on carrier protein protein changes shape, molecules pass through receptor site is specific to certain molecules
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Diffusion Through the Plasma Membrane
Figure 3.7
Extracellular fluid
Cytoplasm
Lipid-solublesolutes
Lipidbilayer
Lipid-insolublesolutes
Watermolecules
Small lipid-insolublesolutes
(a) Simple diffusion directly through the phospholipid bilayer
(c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge
(b) Carrier-mediated facilitated diffusion via protein carrier specific for one chemical; binding of substrate causes shape change in transport protein
(d) Osmosis, diffusion through a specific channel protein (aquaporin) or through the lipid bilayer
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Osmosis is the net diffusion of water across a membrane.
How can water move across a cell membrane?
- aquaporins
- directly?
Osmosis occurs when solutes cannot cross the membrane (no diffusion) so the solvent, water, crosses instead
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Osmosis = Water Movement
Water molecules diffuse across membrane toward solution with more solutes
Volume increases on the side with more solutes
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Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8a
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Effect of Membrane Permeability on Diffusion and Osmosis
Figure 3.8b
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Osmosis in cells
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Osmosis
Isotonic cell ok Hypotonic Swelling or hemolysis (burst).
Like in the bathtub Hypertonic crenation (shrinkage)
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How Osmosis Works
More solute molecules, lower concentration of water molecules
Key to osmosis: membrane must be freely permeable to water, selectively permeable to solutes.
(i.e. some solutes must be impermeable. Otherwise, diffusion would occur)
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KEY CONCEPT Concentration gradients tend to even out due to
random motion of particles In the absence of a membrane, diffusion
eliminates concentration gradients When different solute concentrations exist on
either side of a selectively permeable membrane, either:
1. diffusion of permeable molecules equalizes concentrations OR
2. osmosis moves water through the membrane to equalize the concentration gradients
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Active Transport
Active transport proteins: move substrates against concentration gradient Uses carrier proteins (just like carrier-mediated
facilitated diffusion) requires energy, such as ATP Examples
ion pumps move ions (Na+, K+, Ca+, Mg2+) exchange pump countertransports 2 ions at the same
time
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Active Transport
Primary active transport requires ATP at the same time that substrate is moved
Secondary active transport requires energy later use of an exchange pump (such as the Na+-K+
pump) indirectly to drive the transport of other solutes
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Primary Active Transport: Sodium-Potassium Exchange Pump
Primary Active transport (carrier mediated): sodium ions (Na+) out, potassium ions (K+) in 1 ATP moves 3 Na+ out, 2 potassium ions (K+) in
Maintains concentrations of Na+ and K+ Also an example of countertransport
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Figure 3.10
Cytoplasm
Extracellular fluidK+ is released andNa+ sites are ready tobind Na+ again; thecycle repeats.
Cell ADP
Phosphorylationcauses theprotein tochange its shape.
Concentration gradientsof K+ and Na+
The shape change expels Na+ to the outside, and extracellular K+ binds.
Loss of phosphaterestores the originalconformation of thepump protein.
K+ binding triggersrelease of thephosphate group.
Binding of cytoplasmic Na+ to the pump proteinstimulates phosphorylationby ATP.Na+
Na+
Na+
Na+Na+
K+K+
K+
K+
Na+
Na+
Na+
ATPP
P
Na+
Na+Na+
K+
K+
P
Pi
K+
K+
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Secondary Active Transport Example: Na+/glucose symporter
Figure 3–20
Na+ concentration gradient drives glucose transport
ATP energy pumps Na+ back out
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Types of Active Transport
Figure 3.11
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Vesicular Transport
Materials move into or out of the cell by means of vesicles, also called bulk transport Endocytosis (Clathrin-mediated)
Receptor mediated endocytosis Pinocytosis Phagocytosis
ExocytosisALL are active processes (require ATP) though they are not usually referred to as “active transport”
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Receptor-Mediated Endocytosis
Endosome
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Figure 3–22a
Pinocytosis
Pinocytosis (cell drinking) Endosomes “drink” extracellular fluid
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Phagocytosis
Phagocytosis (cell eating) pseudopodia (psuedo = false,
podia = feet) engulf large objects in
phagosomes
Figure 3–22b
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Figure 3–7b
Exocytosis
Is the reverse of endocytosis
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Summary
Table 3–3
The 7 methods of transport
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Summary
Structure of cell membranes Phospholipid bilayer, proteins, carbohydrates,
cholesterol Functions of the cell membrane Transport Mechanisms of the cell membrane:
diffusion and osmosis active transport proteins vesicles in endocytosis and exocytosis