4 - Membranes

Functions of Membrane Proteins

1. Transport

2. Receptors for signal transduction

3. Attachment to cytoskeleton and extracellular matrix

4. Enzymatic activity

5. Intercellular joining

6. Cell-cell recognition

Types of Membrane Transport

• Plasma membranes selectively permeable

  • Some molecules pass through easily; some do not

• Passive processes

  • No cellular energy (ATP) required

  • Substance moves down its concentration gradient

• Active processes

  • Energy (ATP) required

  • Occurs only in living cell membranes

Types of Passive Transport

1. Diffusion

2. Facilitated transport

3. Osmosis

Diffusion

• Passive process

• Describes the spread of particles (which can also be atoms, molecules) through random motion from regions of higher concentration to regions of lower concentration

  • Collisions cause molecules to move down or with their concentration gradient

    • Difference in concentration between two areas

• Diffusion can still occur when there is no concentration gradient (but there will be no net flux)

• Driven by decrease in Gibbs free energy

• Speed influenced by molecule size and temperature

Macroscopic theory of diffusion

• Fick's first law of diffusion

• Fick’s second law of diffusion

Fick's first law of diffusion

• Net flux is proportional to the spatial gradient of the concentration function

• Proposed in an analogy to Fourier’s law of heat transfer

J = -D(dC/dx)

• J = Diffusion flux, amt of substance per unit area per unit time

• D = Diffusion coefficient, length/time

• C = Concentration, amount of substance per unit volume

• x = position, length

What is this equation

Fick’s first law of diffusion

• J = Diffusion flux, amt of substance per unit area per unit time

• D = Diffusion coefficient, length/time

• C = Concentration, amount of substance per unit volume

• x = position, length

Note: I do not think we need the equations memorized just be familiar

Fick’s second law of diffusion

• The time rate of change in concentration is proportional to the curvature of the concentration function

  • Follows from continuity equation and Fick’s first law

  • Can be derived from the one-dimensional random walk

  • Predicts how diffusion causes the concentration to change with time

dc/dt = D * d²C/dx²

What is this equation

Fick’s second law of diffusion

• shows time dependency

Note: I do not think we need the equations memorized just be familiar

What is this equation?

• The relationship between distance and time for diffusion

  • The time for one-dimensional diffusion increases as the square of the distance

• Derived from the variance of the Gaussian distribution

• 2D diffusion: Variance is 4DΔt

• 3D diffusion (molecule across a cell): Variance is 6DΔt

• For distances smaller than a cell (0-10 µm), diffusion takes less than a ms to a few ms

• For larger distances (diameter of a muscle cell, 40-100 µm), diffusion can take several seconds

Liquid diffusion coefficient

• Stokes showed that for a spherical particle, the drag force is related to size and solvent viscosity

• Liquid diffusion coefficients from the Stokes Einstein equation

What is this equation:

Liquid diffusion coefficients from the Stokes

• Where:

  • f = Friction coefficient of the solute

  • kb = Boltzmann’s constant

  • µ = Solvent viscosity

  • r = Solute radius

Permeability

• aka: filtration

• the rate of flow of a liquid or gas through a porous material

Diffusion

• the passive movement of molecules or particles along a concentration gradient

Permeability depends on

1. diffusion coefficient (D)

   • P increases when D increases

2. Membrane thickness (Δx)

   • P decreases when Δx increase

3. Partition coefficient (K)

   • P increases when K increases

Diffusion across the lipid bilayer

• Any molecule will eventually diffuse across a protein-free lipid bilayer down a concentration gradient (might just take a while)

  • The rate of diffusion depends on the size of the molecule and its hydrophobicity

  • Small nonpolar molecules (O2, CO2) diffuse rapidly

  • Small polar molecule (water, urea) diffuse slowly

  • Lipid bilayer is highly impermeable to charged ions

Transport across the lipid bilayer

• Membrane transport proteins are needed to allow essential molecules to pass through the lipid bilayer

  • Ions, sugars, amino acids, nucleotides, cell metabolites

• Transporters

• Channels

Transporters

• Bind a specific solute and undergo conformational changes to transfer the solute across the membrane

• embedded in lipid layer

Channels

• Form aqueous pores that extend across the lipid bilayer, example: aquaporins

  • Much faster transport than transport via proteins

Types of diffusion

1. simple diffusion

2. facilitated diffusion

Simple diffusion

• Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid bilayer

• E.g., oxygen, carbon dioxide, fat-soluble vitamins

Facilitated Diffusion

• Certain lipophobic molecules (e.g., glucose, amino acids, and ions) transported passively by

  • Binding to protein carriers

  • Moving through water-filled channels

Carrier-mediated facilitated diffusion

• passive therefore no energy

• Transmembrane integral proteins are carriers

• Transport specific polar molecules (e.g., sugars and amino acids) too large for channels

• Binding of substrate causes shape change in carrier then passage across membrane

• Limited by number of carriers present

  • Carriers saturated when all engaged

Channel-mediated facilitated diffusion

• Aqueous channels formed by transmembrane proteins

• Selectively transport ions or water

• types

  1. leakage channels

  2. gated channels

Leakage channels

• types of channel in Channel-mediated facilitated diffusion

• always open

Gated channels

• types of channel in Channel-mediated facilitated diffusion

• Controlled by chemical or electrical signals

• required change

Osmosis

Movement of water into or out of cells down a concentration gradient

• passive process

• Movement of solvent (e.g., water) across selectively permeable membrane

  • Water diffuses through plasma membranes

  • Through lipid bilayer

  • Through specific water channels called aquaporins (AQPs)

• Occurs when water concentration different on the two sides of a membrane

  • Water concentration varies with number of solute particles because solute particles displace water molecules

Osmolarity

Measure of total concentration of solute particles

• Measure of solute concentration

Water moves by osmosis until

hydrostatic pressure and osmotic pressure equalize

hydrostatic pressure

back pressure of water on membrane

osmotic pressure

tendency of water to move into cell by osmosis

When solutions of different osmolarity are separated by membrane permeable to all molecules

both solutes and water cross membrane until equilibrium reached

When solutions of different osmolarity are separated by membrane impermeable to solutes

osmosis occurs until equilibrium reached

Osmosis causes cells to

swell and shrink

• Change in cell volume disrupts cell function, especially in neurons

Tonicity

Ability of solution to alter cell's water volume

• Types

  1. isotonic

  2. hypertonic

  3. hypotonic

Isotonic

Solution with same non-penetrating solute concentration as cytosol

Hypertonic

Solution with higher non-penetrating solute concentration than cytosol

Hypotonic

Solution with lower non-penetrating solute concentration than cytosol

Sources of intracellular osmolarity:

1. Macromolecules: Contribute very little to the osmolarity of the cell (few of them compared to small molecules)

   • Charged, which attracts oppositely charged inorganic ions

   • Counterions make a major contribution to osmolarity

2. Small organic molecules (sugars, amino acids, nucleotides):

   • Both charged small molecules and their counterions contribute to osmolarity

3. Osmolarity is mainly due to small organic ions

Cell must control osmolarity or water will _

continuously move into the cell by osmosis

Why are Red blood cells a special case of osmolarity

• No nucleus

• Plasma membrane with a high permeability to water

• High number of Na+ - K+ pumps for controlling cell volume

If placed in a hypotonic solution, what will happen to a red blood cell?

• Water rushes into cell

• Cells burst

(low solute, high water concentration)

If placed in a hypertonic solution, what will happen to a red blood cell?

• Water leaves cell

• Cells shrink

(high solute, low water concentration)

If placed in a isotonic solution, what will happen to a red blood cell?

stay normal

Passive transport allow _

• aka: (facilitated diffusion)

• allows solutes to pass the membrane inactively

  • Uncharged molecule: Transport must be driven by concentration gradient

  • Charged molecule: Transport drive by concentration gradient and electrical gradient

    • Most plasma membranes have an electrical potential difference, inside is negative with respect to the outside

    • Entry of positively charged ions favored

Active transport

Transporter-mediated solute transport against an electrochemical gradient

• Requires energy input

Membrane channels allow _

passive transmembrane movement

Membrane transporters can create _

large differences in intra- and extracellular concentrations

Transporter-mediated solute movement types

1. Uniporters

2. Coupled transports

   1. symporters

   2. antiporters

Uniporters

Mediate the movement of a single solute from one side of the membrane to the other at a rate determined by their Vmax and Km

Coupled transporters

Transfer of one solute depends on the transport of a second

Types:

1. symporters

2. antiporters

Symporters

• type of coupled transporter

• aka: co-transporters

• Simultaneously transports a second solute in the same direction

Antiporters

• type of coupled transporter

• aka: exchangers

• Transfers a second solute in the opposite direction

Types of Membrane Transport: Active Processes

1. Active transport

2. Vesicular transport

Active Processes require

• Both type require ATP to move solutes across a living plasma membrane because

  • Solute too large for channels

  • Solute not lipid soluble

  • Solute not able to move down concentration gradient

Active membrane transport

• Process by which a transporter transfers a solute across the lipid bilayer resembles an enzyme-substrate reaction

  • No modification of solute

• Each transporter has one or more binding sites for its solute

  • Transfers solute by undergoing a conformational change

  • Vmax of transport = rate at which transporter can flip between its two states

  • Km = concentration of solute at which transport is half of the maximum rate

  • Solute binding can be inhibited competitively (other solutes bind) or noncompetitively (inhibitors change the structure of the transporter)

Ways that cells link transporters to an energy source:

1. Coupled transporters couple the uphill transport of one solute to the downhill transport of another

2. ATP-driven pumps couple uphill transport to the hydrolysis of ATP

3. Light driven pumps (bacteria and archaea) couple uphill transport to an input on energy from light

Active transport requires

• Requires carrier proteins (solute pumps)

  • Bind specifically and reversibly with substance

• Requires energy

  • Moves solutes against concentration gradient

Types of active transport:

1. Primary active transport

   • Required energy directly from ATP hydrolysis

2. Secondary active transport

   • Required energy indirectly from ionic gradients created by primary active transport

Primary active transport

• Energy from hydrolysis of ATP causes shape change in transport protein that "pumps" solutes (ions) across membrane

• E.g., calcium, hydrogen, Na⁺-K⁺ pumps

Sodium-potassium pump

• Most well-studied

• Carrier (pump) called Na+-K+ ATPase

• Located in all plasma membranes

• Involved in primary and secondary active transport of nutrients and ions

• Na+ and K+ channels allow slow leakage down concentration gradients

• Na+-K+ pump works as antiporter

Na+-K+ pump work as antiporter

• Pumps against Na+ and K+ gradients to maintain high intracellular K+ concentration and high extracellular Na+ concentration

  • Maintains electrochemical gradients essential for functions of muscle and nerve tissues

  • Allows all cells to maintain fluid volume

Generation of a resting membrane potential

• Produced by separation of oppositely charged particles (voltage) across membrane in all cells

  • Cells described as polarized

• Voltage (electrical potential energy) only at membrane

  • Ranges from –50 to –100 mV in different cells

    • Negative sign indicates inside negative relative to outside

Secondary Active Transport

• Depends on ion gradient created by primary active transport

• Energy stored in ionic gradients used indirectly to drive transport of other solutes

• Cotransport—always transports more than one substance at a time

Types of cotransports

1. Symport system: Substances transported in same direction

2. Antiport system: Substances transported in opposite directions

Ion gradient active transport

• The transfer of two solutes allows the coupled transporters to harvest energy stored in the electrochemical gradient

  • The free energy released during the movement of an organic ion down an electrochemical gradient is used to pump other solutes uphill against their gradient

ABC Transporters

• (ATP-binding cassettes): Contains two ATP-binding sites

• ATP hydrolysis following binding results in conformational changes that alternately expose substrate binding sites to one side of the membrane and then the other

• Transport small molecules across the bilayer

• Each ABC transporter is thought to be specific for a particular molecule or class of molecules (inorganic ions, amino acids, sugars, peptides, proteins)

ABC transporter clinical importance

• Cancer:

  • Multidrug resistance (MDR) protein:

  • Able to pump hydrophobic drugs out of the cytosol

  • Overexpressed in cancer cells

• Malaria:

  • P. falciparum (protist responsible for disease) overexpress the ABC transporter that pumps out chloroquine

• Cystic fibrosis:

  • Caused by a mutation in the gene encoding CFTR

  • CFTR functions as a Cl- channel in epithelial cells

  • Irregular ion concentrations in the extracellular fluid

    • Thick, sticky mucus

Vesicular Transport

• Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles

• Requires cellular energy (e.g., ATP)

Vesicular Transport Functions

• Exocytosis—transport out of cell

• Endocytosis—transport into cell

  • Phagocytosis, pinocytosis, receptor-mediated endocytosis

• Transcytosis—transport into, across, and then out of cell

• Vesicular trafficking—transport from one area or organelle in cell to another

Endocytosis and transcytosis similarities

• Involve formation of protein-coated vesicles

• Often receptor mediated, therefore very selective

• Some pathogens also hijack for transport into cell

• Once vesicle is inside cell it may

  • Fuse with lysosome

  • Undergo transcytosis

Endocytosis Types

1. Phagocytosis (cell eating)

2. Pinocytosis (cell drinking)

3. Receptor-mediated endocytosis

Phagocytosis

• type of endocytosis

• cell eating

• Pseudopods engulf solids and bring them into cell's interior

• Form vesicle called phagosome

• Used by macrophages and some white blood cells

  • Move by amoeboid motion

    • Cytoplasm flows into temporary extensions

    • Allows creeping

Pinocytosis

• fluid-phase endocytosis

• cell drinking

• Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell

  • Fuses with endosome

• Most cells utilize to "sample" environment

• Nutrient absorption in the small intestine

• Membrane components recycled back to membrane

Receptor-mediated endocytosis

• Allows specific endocytosis and transcytosis

  • Cells use to concentrate materials in limited supply

• Clathrin-coated pits provide main route for endocytosis and transcytosis

  • Uptake of enzymes, low-density lipoproteins, iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins

• Different coat proteins

  • Caveolae

  • Coatomer

Caveolae

• coat protein involved in receptor-mediated endocytosis

• Capture specific molecules (folic acid, tetanus toxin) and use transcytosis

  • Involved in cell signaling but exact function unknown

Coatomer

• coat protein involved in receptor-mediated endocytosis

• Function in vesicular trafficking

Exocytosis

• Very targeted

• Cell barding

• Usually activated by cell-surface signal or change in membrane voltage

• Substance enclosed in secretory vesicle

• v-SNAREs ("v" = vesicle) on vesicle find

• t-SNAREs ("t" = target) on membrane and bind

• Functions

  • Hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes

glycocalyx

• "Sugar coating" at cell surface

  • Lipids and proteins with attached carbohydrates (sugar groups)

• Every cell type has different pattern of sugars

  • Specific biological markers for cell to cell recognition

  • Allows immune system to recognize "self" and "non self"

Cell binding/communities

1. Some cells "free"

   • e.g., blood cells, sperm cells

2. Some bound into communities

Cell junction types

• ways cells are bound

  1. Tight junctions

  2. Desmosomes

  3. Gap junctions

Tight Junctions

• Adjacent integral proteins fuse → form impermeable junction encircling cell

  • Prevent fluids and most molecules from moving between cells

  • chemical barrier

• Where might these be useful in body?

  • digestive system

  • placenta (fine control)

  • blood vessels

  • epithelial

  • endothelial

Desmosomes

• "Rivets" or "spot-welds" that anchor cells together at plaques (thickenings on plasma

  membrane)

  • Linker proteins between cells connect plaques

  • Keratin filaments extend through cytosol to opposite plaque giving stability to cell

  • Reduces possibility of tearing (mechanical)

• Where might these be useful in body?

  • muscles

  • skin

  • ligaments

  • tendons

  • tension re-enforcement

Gap Junctions

• Transmembrane proteins form pores (connexons) that allow small molecules to pass from cell to cell

  • For spread of ions, simple sugars, and other small molecules between cardiac or smooth muscle cells

• Where might these be useful in body?

  • cardiac

  • muscle cells

Cell Environment Interactions

• Cells interact directly or indirectly by responding to extracellular signals

• Always involves glycocalyx

  • Cell adhesion molecules (CAMs)

  • Plasma membrane receptors

  • Voltage-gated channel proteins

Roles of Cell Adhesion Molecules

• Thousands on approximately every cell in body

• Anchor to extracellular matrix or each other

• Assist in movement of cells past one another

• Attract WBCs to injured or infected areas

• Stimulate synthesis or degradation of adhesive membrane junctions

• Transmit intracellular signals to direct cell migration, proliferation, and specialization

Roles of Plasma Membrane Receptors

• contact signaling

• chemical signaling

Contact signaling (Plasma Membrane Receptors)

• touching and recognition of cells

• e.g., in normal development and immunity

Chemical signaling (Plasma Membrane Receptors)

• interaction between receptors and ligands (neurotransmitters, hormones, and paracrines) to alter activity of cell proteins (e.g., enzymes or chemically gated ion channels)

• Same ligand can cause different cell responses

• Response determined by what receptor linked to inside cell

Chemical signaling (Plasma Membrane Receptors) Process

• Catalytic receptor proteins become activated enzymes

• Chemically gated channel-linked receptors open and close ion gates → changes in excitability

• G protein–linked receptors activate G protein, affecting an ion channel or enzyme, or causing release of internal second messenger, such as cyclic AMP

Integral proteins

• Firmly inserted into membrane (most are transmembrane)

• Have hydrophobic and hydrophilic regions

  • Can interact with lipid tails and water

• Function as transport proteins (channels and carriers), enzymes, or receptors

Function of integral proteins

1. Transport

2. Signal Transduction

3. Attachment

4. Enzymatic activity

5. Cell-cell junctions

6. Recognition

Transport (integral proteins)

• A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute.

• Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane

integral proteins as receptors for signal transduction

• A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as a hormone.

• When bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell.

Attachment (integral proteins)

• Attachment to the cytoskeleton and extracellular matrix

• Elements of the cytoskeleton (cell's internal supports) and the extracellular matrix (fibers and other substances outside the cell) may anchor to membrane proteins, which helps maintain cell shape and fix the location of certain membrane proteins.

• Others play a role in cell movement or bind adjacent cells together.

Intercellular joining (integral proteins)

• Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions.

• Some membrane proteins (cell adhesion molecules or CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions.

Cell-cell recognition (integral proteins)

• Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells.

Enzymatic activity (integral proteins)

• A membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution.

• A team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway as indicated (left to right) here.