6.0 Introduction

1. Cells Interact with Their Environment
1. Certain Materials Pass Through Membrane Passageways
2. Plasma Membrane Is Lipid Sheet with Embedded Proteins fig 6.1

1. The Phospholipid Bilayer
1. Membrane's Lipid Layer Is Composed of Phospholipids fig 6.2
2. Phospholipids
1. Form the foundation of cell membranes
1. Backbone is a three-carbon glycerol molecule
2. Attach to three fatty acid chains in a fat molecule
3. Attach to two fatty acid chains in a phospholipid molecule
4. Phosphate group attaches a polar organic alcohol to the third carbon

2. One end of the molecule is strongly nonpolar and water insoluble
3. The other end is strongly polar and water soluble
4. Phospholipids are diagrammed as a polar head with two nonpolar tails fig 6.2

3. Phospholipids Form Bilayer Sheets
1. Interactions between phospholipids and water
1. Nonpolar tails are pushed away from water molecules
2. Nonpolar tails cannot form hydrogen bonds with water
3. Water molecules form bonds with each other excluding nonpolar tails

2. Spontaneously form a lipid bilayer fig 6.3
1. Polar heads face water on either side
2. Nonpolar tails face inward toward each other

3. Lipid bilayer sheets are the foundation of biological membranes
1. Nonpolar interior repels water-soluble molecules
2. Proteins in the lipid bilayer allow passage of polar molecules

2. The Lipid Bilayer Is Fluid
1. Lipid Molecules Move within the Stable Bilayer
1. Hydrogen bonding of water holds lipid layer together
2. Phospholipids and unanchored proteins move freely within membrane fig 6.4

2. Fluidity of Membrane Depends on Alignment of Phospholipid Tails
1. Closely aligned tails create less fluid membranes
2. Less closely aligned tails create more fluid membranes
1. Associated with double-bonded carbons in the tail chain
2. May contain short lipids that prevent contact between tails

1. The Fluid Mosaic Model
1. Plasma Membrane Composed of Lipids and Globular Proteins fig 6.5
1. First thought a protein layer coated both surfaces of the lipid bilayer
2. Was the widely accepted Davson-Danielli model of 1935
3. Model not workable due to insolubility of membrane proteins
1. Nonpolar proteins would separate polar portions of lipid from water
2. Bilayer would then dissolve

2. Next Model Inserted Proteins within Lipid Bilayer
1. Singer and Nicolson revision of 1972
2. Nonpolar segments of proteins in contact with nonpolar interior of bilayer
3. Polar portions protrude from membrane surface
4. Called the fluid mosaic model fig 6.6

2. Components of the Cell Membrane
1. Cell Membranes Are Assembled From Four Components tbl 6.1
1. Lipid bilayer foundation
1. Other components distributed within foundation
2. Provides a flexible matrix which is a barrier to permeability

2. Transmembrane proteins
1. Move within the lipid bilayer, not located in fixed positions
2. Provide passageways through which molecules and information pass

3. Network of supporting fibers
1. Structurally supported by proteins like spectrin
2. Connects membrane proteins to cell's actin filament cytoskeleton
3. Control lateral motion of key membrane proteins

4. Exterior proteins and glycolipids
1. Membranes assembled in the ER, transferred to the Golgi
2. Golgi adds glycocalyx, chains of sugars, to membrane proteins and lipids
3. Sugar molecules function as cell identity markers

3. Examining Cell Membranes
1. Transmission Electron Microscopy
1. Tissue placed in hard matrix like epoxy fig 6.7
2. Cut into thin sections with microtome
3. Shavings of epoxy and tissue placed on grid
4. Beam of electrons directed through sample on grid
5. Resolution good enough to show double layers of membrane

2. Scanning Electron Microscopy
1. Bounces electrons off surface of sample
2. Tissue is often freeze-fractured, produces crack between layers
3. Structures associated with membrane stick to one side or other
4. Membrane halves coated with electron-attracting metal forming a cast
5. Cast examined under SEM

4. IKinds of Membrane Proteins
1. Advantages of Flexible Design of the Mosaic Model
1. Broad range of interactions between membrane and environment
2. Mostly associated with presence of proteins fig 6.8

2. Six Key Classes of Membrane Proteins
1. Transport channels
1. Membrane selects what substances will enter
2. Take up molecules present in high concentration

2. Enzymes
1. Chemical reactions carried out on interior surface of membrane
2. Enzymes attached directly to membrane

3. Cell surface receptors
1. Membranes sensitive to chemical messages
2. Receptor proteins on surface act as antennae

4. Cell surface identity markers
1. Markers on membrane identify cells to other cells
2. Acts as specific ID tags

5. Cell adhesion proteins
1. Cells use certain proteins to glue themselves to one another
2. Some are detachable, others are permanent

6. Attachments to the cytoplasm
1. Surface proteins may interact with other cells
2. Often linked to cytoskeleton by proteins

5. Structure of Membrane Proteins
1. Anchoring Proteins is the Bilayer
1. Membrane proteins attached to external surface by anchoring substance
1. Phospholipid molecule called phosphatidylinositol
2. Proteins move about tethered to phospholipid

2. Other proteins completely traverse bilayer
1. Part of protein extends through bilayer
2. May be nonpolar helix beta-pleated sheets of nonpolar amino acids fig 6.9
3. Nonpolar portion held within interior of bilayer
4. Polar ends protrude from both sides of membrane

2. Extending Proteins across the Bilayer
1. Anchors
1. Example: Proteins that attach spectrin of cytoplasm to membrane interior fig 6.10
2. Receptors for extracellular signals are also single-pass anchors
3. Portion of receptor that sticks outward binds with molecules
4. Binding induces changes in part of protein on the inside

2. Channels
1. Proteins that wind back and forth through the membrane
2. Locked into shape by several helical segments
3. Create a hole in the membrane like that in a donut
4. Example: Photosynthesis transmembrane protein
5. Forms crescent-shaped channel through membrane fig 6.11
6. Water-soluble molecules pass through these channels
7. Each channel admits only certain substances

3. Pores
1. Non-polar beta-pleated sheet transmembrane proteins
2. Characteristic motif where sheets fold back over themselves
3. Form a pore called a beta-barrel
4. Examples include porins of bacterial outer membranes fig 6.12

1. Diffusion
1. Molecules Dissolved in Water Are in Constant Random Motion
1. Process called diffusion fig 6.13
2. Causes net movement from higher to lower concentration
3. Equilibrium when there is uniform concentration

2. Selective Permeability
1. Cell membranes studded with proteins that act as channels
2. Each channels only allows passage of certain material
3. Results in selective permeability
4. Allows cell to concentrate specific substances or combinations

3. Diffusion of Ions Through Channels
1. Ions move across membrane through channels
1. Ions are solutes with unequal number of protons and electrons
2. Cations are positive due to excess of protons
3. Anions are negative with excess of electrons

2. Ions interact with polar molecules of water
3. Due to charge, ions are repelled by non-polar lipid bilayer interior
4. Ion channels have water-filled pore across membrane
1. No interaction between channel and ion
2. Net movement dependent on concentration and voltage

2. Facilitated Diffusion fig 6.14
1. Carriers Transport Solutes Across the Membrane
1. Selective carriers allow passage of specific molecules in both directions
2. Facilitate movement with physical binding
3. Direction of net movement dependent on concentration gradient
4. Net movement from high concentration to low concentration

2. Facilitated Diffusion in Red Blood Cells
1. RBC carrier proteins transports Cl– one way, HCO₃– the other way
2. RBC glucose transporter
1. Add phosphate to newly entering glucose
2. As a charged molecule it cannot pass back out
3. Not a channel, transport due to molecule shape change

3. Carrier-Mediated Processes Saturate
1. Rate of movement can become saturated
1. Increasing concentration affects movement only to a certain point
2. When all carriers are occupied diffusion reaches its limit
3. Capacity of the transport system is at maximum
4. Example: Transport of Cl– and HCO₃– in red blood cells

2. Prevents buildup of unwanted materials
3. Essential characteristics
1. Specific to certain molecules with a given carrier
2. Passive process driven by internal and external concentrations
3. System may become saturated when all carriers are in use

3. Osmosis
1. Cytoplasm of Cell is an Aqueous Solution
1. A mixture of water and molecules
2. Solvent: Water, most common molecules in the solution
3. Solute: Other molecules dissolved in the water

2. Molecules Diffuse Down a Concentration Gradient
1. Both water and molecules diffuse from regions of high to low concentration
2. Membrane prevents equal motion of solvent and solute
1. Many solutes cannot pass through biological membranes
2. Water can freely pass through membrane

3. Osmosis: Diffusion with net movement of water across a membrane fig 6.15
4. Concentration of all solutes establishes osmotic concentration
1. Solution with higher concentration is hyperosmotic
2. Solution with lower concentration is hypoosmotic
3. Solutions with equal concentrations are isosmotic

5. Cellular changes fig 6.16
1. Shrinks (looses water) when hypoosmotic to environment
2. Swells (gains water) when hyperosmotic to environment

3. Osmotic Pressure
1. Hydrostatic pressure of cytoplasm pushes against cell membrane
2. Osmotic pressure: Force required to stop osmosis across membrane fig 6.17
3. Equilibrium between osmotic concentration difference and pressure
1. When pressure is too high most unsupported cells burst
2. Cells with cell walls can withstand pressure and will not burst

4. Maintaining Osmotic Balance
1. Many cells adjust to being hyperosmotic to environment
2. Extrusion
1. Example: Contractile vacuole of Paramecium
2. Vacuole collects water, transports to point near cell surface
3. Pore opens to outside
4. Vacuole contracts and pumps water out of cell

3. Isoosmotic solutions
1. Many cells adjust internal solute concentration to match environment
2. Cells are isoosmotic with environment
3. Multicellular organisms similarly regulate composition of body fluids
4. Terrestrial animals bathe cells in isoosmotic solution

4. Turgor
1. Plant cells are hyperosmotic with respect to their immediate environment
2. Possess a high solute concentration within the central vacuole
3. Turgor pressure: Internal pressure of plant cells
4. Pressure pushes cytoplasm against cell wall, causes rigidity

1. Bulk Passage Into and Out of the Cell
1. Endocytosis fig 6.18
1. Fuel for cells are large molecules that cannot cross the membrane
2. Employ process that envelopes food particles with part of membrane
3. Three types: Phagocytosis, pinocytosis, receptor-mediated endocytosis
4. Phagocytosis and pinocytosis
1. Phagocytosis: Material brought in is particulate
2. Pinocytosis: Material is liquid, contains dissolved molecules

5. Receptor-mediated endocytosis
1. Associated with transport of specific macromolecules
2. Cytoplasmic side of plasma membrane is covered with clathrin
3. Indentations in plasma membrane called clathrin-coated pits fig 6.19
4. Pit closes over when proper molecule enters
5. Process is highly specific, very fast but transient

6. Fluid-phase endocytosis is same process with fluids

2. Exocytosis
1. Reverse of endocytosis
2. Materials extruded from cell by discharge from surface vesicles fig 6.20
3. Utilized by plants to construct cell wall
4. Includes protist contractile vacuole discharge
5. Used by animal cells to secrete chemical materials

1. Active Transport
1. Diffusion, Facilitated Diffusion and Osmosis Are Passive Processes
2. Transport of Molecules Also Occurs Against Concentration Gradient
1. Expends energy
2. Involves highly selective protein carriers
3. Molecules moved may be ions, sugars, amino acids or nucleotides fig 6.21
4. Enables cell to concentrate materials inside itself
5. Allows cell to export materials even if concentrated on outside

3. The Sodium-Potassium Pump
1. Cells maintain low internal concentration of sodium: Pump it out fig 6.22
2. Cells maintain high internal concentration of potassium: Pump it in
3. Energy provided by adenosine triphosphate (ATP)
4. Associated with conformational changes in transmembrane protein
1. Step 1: Three molecules of Na⁺ bind to cytoplasmic subunits
2. Step 2: Complex binds, cleaves one ATP; ADP released, Pi remains bound
3. Step 3: Three Na⁺ molecules move across channel are released on outside
4. Step 4: Complex binds two K⁺ molecules
5. Step 5: Pi released, complex disassociates K⁺, released to the inside
6. Step 6: Without phosphate group protein reverts to original conformation

5. Process removes three Na⁺ and brings in two K⁺

2. Coupled Channels
1. Some Channels Do Not Use ATP
1. Cotransport
1. Accumulate molecules against concentration gradient
2. Moves molecules and Na⁺ together

2. Composed of two components
1. Establishing the down gradient uses ATP
2. Traversing the up gradient through cotransport/coupled channels

2. Establishing the Down Gradient
1. Sodium potassium pump
1. Actively pumps sodium ions out of cell, uses ATP
2. Establishes gradient where sodium is lower inside cell

2. The proton pump
1. Pumps protons (H⁺) across membrane, expends energy
2. Creates proton gradient with more H+ on outside of membrane
3. Diffusion drives protons back down concentration gradient
4. Protons return by other channel

3. Traversing the Up Gradient
1. Accumulate amino acids and sugars against concentration gradient
2. Cotransport moves molecules and Na⁺ together fig 6.23
1. Na⁺ moves down its concentration gradient
2. Molecule moves up its concentration gradient

3. Countertransport couples Na⁺ movement with Ca⁺⁺ or H⁺
1. Na⁺ and molecule bind to same transport protein
2. Bind on opposite sides of membrane
3. Na⁺ moves down its gradient
4. Molecule extruded against its concentration gradient

4. Down gradient from proton pump used in ATP production fig 6.24
1. Cell expends energy to produce ATP, energy-storing molecule
2. Process called chemiosmosis

4. Summarization of Mechanisms for Transport Across Membranes tbl 6.2

3. Chloride Channels and Cystic Fibrosis
1. Cystic Fibrosis is a Fatal Human Disease
1. Affected individual has thicken mucus
1. Mucus causes tissue inflammation, clogs airways and organ ducts
2. Death attributes to organ damage

2. Individuals surviving longer with medical treatment

2. A Hereditary Disorder
1. Results from a defect in a single gene
1. One in 25 individuals carries at least one copy of defective gene
2. Must inherit gene from both parents to develop disease

3. Difficult Disease to Characterize
1. Many organs and systems affected
2. Finally determined to be a defect in membrane's ability to export Cl^-
1. Buildup of Cl^- in cells attracts water, thickens surrounding mucus
2. Gene codes for CFTR protein, transmembrane conductance regulator

3. New treatment includes inserting working copy of gene into cells