Membrane Structure and Function

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Membrane Structure and Function
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Plasma Membrane

• Is the boundary that separates the living cell
  from its nonliving surroundings
• Selectively Permeable (chooses what may cross the
  membrane)
• Fluid mosaic of lipids and proteins
• Lipid bilayer
• Contains embedded proteins

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Phospholipids

• Are the most abundant lipid in the plasma
  membrane
• Are amphipathic, containing both hydrophilic
  (head) and hydrophobic regions (tails)
• Head composed of phosphate group attached to one
  carbon of glycerol is hydrophilic
• Two fatty acid tails are hydrophobic

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Phospholipid Bilayer
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Singer and Nicolson

• In 1972, Singer and Nicolson, Proposed that
  membrane proteins are dispersed and individually
  inserted into the phospholipid bilayer of the
  plasma membrane

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Fluid Mosaic Model

• A membrane is a fluid structure with a mosaic
  of various proteins embedded in it when viewed
  from the top
• Phospholipids can move laterally a small amount
  and can flex their tails
• Membrane proteins also move side to side or
  laterally making the membrane fluid

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• Freeze-fracture studies of the plasma membrane
  support the fluid mosaic model of membrane
  structure

A cell is frozen and fractured with a knife. The
fracture plane often follows the hydrophobic
interior of a membrane, splitting the
phospholipid bilayer into two separated layers.
The membrane proteins go wholly with one of the
layers.
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The Fluidity of Membranes

• Phospholipids in the plasma membrane Can move
  within the bilayer two ways

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The Fluidity of Membranes

• The type of hydrocarbon tails in phospholipids
  affects the fluidity of the plasma membrane

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The Fluidity of Membranes

• The steroid cholesterol has different effects on
  membrane fluidity at different temperatures

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Membrane Proteins Their Functions

• A membrane is a collage of different proteins
  embedded in the fluid matrix of the lipid bilayer

Fibers of extracellular matrix (ECM)
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Types of Membrane Proteins

• Integral proteins
• Penetrate the hydrophobic core of the lipid
  bilayer
• Are often transmembrane
• proteins, completely
• spanning the membrane

EXTRACELLULAR SIDE
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Types of Membrane Proteins

• Peripheral proteins
• Are appendages loosely bound to the surface of
  the membrane

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• Six Major Functions of Membrane Proteins

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Six Major Functions of Membrane Proteins
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The Role of Membrane Carbohydrates in Cell-Cell
Recognition

• Cell-cell recognition
• Is a cells ability to distinguish one type of
  neighboring cell from another
• Membrane carbohydrates
• Interact with the surface molecules of other
  cells, facilitating cell-cell recognition

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Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces
• This affects the movement of proteins synthesized
  in the endomembrane system (Golgi and ER)

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Synthesis and Sidedness of Membranes

• Membrane proteins and lipids are made in the ER
  and Golgi apparatus

ER
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Membrane Permeability

• Membrane structure results in selective
  permeability
• A cell must exchange materials with its
  surroundings, a process controlled by the plasma
  membrane

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Permeability of the Lipid Bilayer

• Hydrophobic molecules
• Are lipid soluble and can pass through the
  membrane rapidly
• Polar molecules
• Do NOT cross the membrane rapidly

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Transport Proteins

• Transport proteins
• Allow passage of hydrophilic substances across
  the membrane

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Passive Transport

• Passive transport is diffusion of a substance
  across a membrane with no energy investment
• CO2, H2O, and O2 easily diffuse across plasma
  membranes
• Diffusion of water is known as Osmosis

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Simple Diffusion

• Diffusion
• Is the tendency for molecules of any substance to
  spread out evenly into the available space
• Move from high to low concentration
• Down the concentration gradient

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Effects of Osmosis on Water Balance

• Osmosis
• Is the movement of water across a semipermeable
  membrane
• Is affected by the concentration gradient of
  dissolved substances called the solutions
  tonicity

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Water Balance of Cells Without Walls

• Tonicity
• Is the ability of a solution to cause a cell to
  gain or lose water
• Has a great impact on cells without walls

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Three States of Tonicity
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Isotonic Solutions

• If a solution is isotonic
• The concentration of solutes is the same as it is
  inside the cell
• There will be
• NO NET
• movement of
• WATER

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Hypertonic Solution

• If a solution is hypertonic
• The concentration of solutes is greater than it
  is inside the cell
• The cell will
• lose water
• (PLASMOLYSIS)

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Hypotonic Solutions

• If a solution is hypotonic
• The concentration of solutes is less than it is
  inside the cell
• The cell will gain
• water

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Water Balance in Cells Without Walls
Animal cell. An animal cell fares best in an
isotonic environment unless it has special
adaptations to offset the osmotic uptake or loss
of water.
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Water Balance of Cells with Walls

• Cell Walls
• Help maintain water balance
• Turgor pressure
• Is the pressure of water inside a plant cell
  pushing outward against the cell membrane
• If a plant cell is turgid
• It is in a hypotonic environment
• It is very firm, a healthy state in most plants
• If a plant cell is flaccid
• It is in an isotonic or hypertonic environment

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Water Balance in Cells with Walls
Plant cell. Plant cells are turgid (firm) and
generally healthiest in a hypotonic environment,
where the uptake of water is eventually balanced
by the elastic wall pushing back on the cell.
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How Will Water Move Across Semi-Permeable
Membrane?

• Solution A has 100 molecules of glucose per ml
• Solution B has 100 molecules of fructose per ml
• How will the water molecules move?

There will be no net movement of water since the
concentration of solute in each solution is equal
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How Will Water Move Across Semi-Permeable
Membrane?

• Solution A has 100 molecules of glucose per ml
• Solution B has 75 molecules of fructose per ml
• How will the water molecules move?

There will be a net movement of water from
Solution B to Solution A until both solutions
have equal concentrations of solute
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How Will Water Move Across Semi-Permeable
Membrane?

• Solution A has 100 molecules of glucose per ml
• Solution B has 100 molecules of NaCl per ml
• How will the water molecules move?

Each molecule of NaCl will dissociate to form a
Na ion and a Cl- ion, making the final
concentration of solutes 200 molecules per mil.
Therefore, there will be a net movement of water
from Solution A to Solution B until both
solutions have equal concentrations of solute
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Facilitated Diffusion

• Facilitated diffusion
• Is a type of Passive Transport Aided by Proteins
• In facilitated diffusion
• Transport proteins speed the movement of
  molecules across the plasma membrane

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Facilitated Diffusion Proteins

• Channel proteins
• Provide corridors that allow a specific molecule
  or ion to cross the membrane

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Facilitated Diffusion Proteins

• Carrier proteins
• Undergo a subtle change in shape that
  translocates the solute-binding site across the
  membrane

A carrier protein alternates between two
conformations, moving a solute across the
membrane as the shape of the protein changes.
The protein can transport the solute in either
direction, with the net movement being down the
concentration gradient of the solute.
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Active Transport

• Active transport
• Uses energy to move solutes against their
  concentration gradients
• Requires energy, usually in the form of ATP

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Active Transport

• The sodium-potassium pump
• Is one type of active transport system

EXTRACELLULAR FLUID
P
P i
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Comparison of Passive Active Transport
Passive transport. Substances diffuse
spontaneously down their concentration
gradients, crossing a membrane with no
expenditure of energy by the cell. The rate of
diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins act as
pumps, moving substances across a membrane
against their concentration gradients. Energy for
this work is usually supplied by ATP.
ATP
Diffusion. Hydrophobic molecules and (at a slow
rate) very small uncharged polar molecules can
diffuse through the lipid bilayer.
Facilitated diffusion. Many hydrophilic
substances diffuse through membranes with the
assistance of transport proteins, either channel
or carrier proteins.
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Maintenance of Membrane Potential by Ion Pumps

• Membrane potential
• Is the voltage difference across a membrane
• An electrochemical gradient
• Is caused by the concentration electrical
  gradient of ions across a membrane
• An electrogenic pump
• Is a transport protein that generates the voltage
  across a membrane

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Proton Pump
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Cotransport

• Cotransport
• Occurs when active transport of a specific solute
  indirectly drives the active transport of another
  solute
• Involves transport by a membrane protein
• Driven by a concentration gradient

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Example of Cotransport

• Cotransport active transport driven by a
  concentration gradient

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Bulk Transport

• Bulk transport across the plasma membrane occurs
  by exocytosis and endocytosis
• Large proteins
• Cross the membrane by different mechanisms

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Exocytosis Endocytosis

• In exocytosis
• Transport vesicles migrate to the plasma
  membrane, fuse with it, and release their
  contents
• In endocytosis
• The cell takes in macromolecules by forming new
  vesicles from the plasma membrane

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Endocytosis
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Exocytosis
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• Three Types of Endocytosis

PHAGOCYTOSIS
PHAGOCYTOSIS
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RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated vesicle
Third Type of Endocytosis
Coated pit
Ligand
A coated pit and a coated vesicle
formed during receptor- mediated endocytosis (TEMs
).
Coat protein
Plasma membrane
0.25 µm
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