Membranes
Lecture Cellular Series 4
Reading Assignments
Read Chapter 11 Membrane Structure Review Chapter 21 pages 709-717 (Animal Cell Adhesion) 709(Animal Review Chapter 12 Membrane Transport Review Chapter 15 regarding Endocytosis and Exocytosis
Selective and Semi-permeable Barriers
1
A. Membrane Composition and Structure
Biological membranes consist of lipids, proteins, and carbohydrates. The fluid mosaic model describes a phospholipid bilayer in which membrane proteins move laterally within the membrane. Phospholipids are the most abundant lipid in the plasma membrane and amphipathic, amphipathic, containing both hydrophobic and hydrophilic regions.
The Fluid Mosaic Model
A. Membrane Composition and Structure
Cell membranes are bilayered, dynamic bilayered, structures that:
Perform vital physiological roles Form boundaries between cells and their environments Regulate movement of molecules into and out of cells
The plasma membrane exhibits selective permeability.
It allows some substances to cross it more easily than others
2
A Phospholipid Bilayer Separates Two Aqueous Regions
A. Membrane Composition and Structure
The lipid portion of a cellular membrane provides a barrier for water-soluble watermolecules. Membrane proteins are embedded in the lipid bilayer. bilayer. Carbohydrates attach to lipid or protein molecules on the membrane, generally on the outer surface, and function as recognition signals between cells.
A. Membrane Composition and Structure
All biological membranes contain proteins. The ratio of protein to phospholipid molecules varies depending on membrane function, which can very greatly. Many membrane proteins have hydrophilic and hydrophobic regions and are therefore also amphipathic. amphipathic.
3
Davson-Daniellis Sandwich Model of membrane Davson- Danielli structure (1935): Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers. Was supported by electron microscope pictures of membranes.
Singer and Nicolsons Fluid Mosaic Model Nicolson (1972): Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer. bilayer.
Hydrophobic region of protein
Phospholipid bilayer
Hydrophobic region of protein
Freeze-fracture experimentation provided evidence for the SingerNicolson model of membrane structure (embedded proteins than spanned membrane). Additional evidence when different cells are fused and the migration of membrane proteins are observed.
4
Phospholipids are free to move laterally but flip-flop (transmembrane rotation) only rarely. Unsaturation (double bonds) kink tails of fatty acids and prevent orderly stacking. Thus saturated phospholipids are less fluid than unsaturated phospholipids. Cholesterol distorts the tails and generally stiffens cell membranes.
ER is where phospholipids
get synthesized and added to the endomembrane system. Flippases play a needed role. Transport vesicles resupply cellular membrane.
A. Membrane Composition and Structure
Integral membrane proteins are partially inserted into the phospholipid bilayer. Peripheral proteins attach to its surface by ionic bonds. The association of protein molecules with lipid molecules is not covalent; both are free to move around laterally, according to the fluid mosaic model.
5
Interactions of Integral Membrane Proteins
EXTRACELLULAR SIDE N-terminus
C-terminus CYTOPLASMIC SIDE
Helix
A. Membrane Composition and Structure
Integral membrane proteins have hydrophobic regions of amino acids that penetrate or entirely cross the phospholipid bilayer. bilayer. Transmembrane proteins have a specific orientation, showing different faces on the faces two sides of the membrane. Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer. bilayer.
Integral or transmembrane proteins play several different roles in a cell. Each of these distinctive proteins is encoded by a particular gene and thus has a very specific amino acid sequence.
6
Membrane proteins can associate with the lipid bilayer in several different ways.
B. Animal Cell Adhesion
Tight junctions prevent passage of molecules through space around cells, and define functional regions of the plasma membrane by restricting migration of membrane proteins over the cell surface. Desmosomes allow cells to adhere strongly to one another. Gap junctions provide channels for chemical and electrical communication between cells.
Exploring Intercellular Junctions in Animal Tissues
TIGHT JUNCTIONS Tight junctions prevent fluid from moving across a layer of cells Tight junction At tight junctions, the membranes of neighboring cells are very tightly pressed against each other, bound together by specific proteins. Forming continuous seals around the cells, tight junctions prevent leakage of extracellular fluid across a layer of epithelial cells. 0.25 m DESMOSOMES Tight junctions Intermediate filaments Desmosome Gap junctions Desmosomes (also called anchoring junctions) function like rivets, fastening cells together into strong sheets. Intermediate filaments made of sturdy keratin proteins anchor desmosomes in the cytoplasm.
1 m
GAP JUNCTIONS Gap junctions (also called communicating junctions) provide cytoplasmic channels from one cell to an adjacent cell. Gap junctions consist of special membrane proteins that surround a pore through which ions, sugars, amino acids, and other small molecules may pass. Gap junctions are necessary for communication between cells in many types of tissues, including heart muscle and animal embryos.
Space between Plasma membranes cells of adjacent cells
Extracellular matrix Gap junction
0.1 m
7
C. Passive Processes of Membrane Substances Transport
can diffuse passively across a membrane by: unaided diffusion through the phospholipid bilayer, facilitated bilayer, diffusion through protein channels, or by means of a carrier protein.
Table 5.1
C. Passive Processes of Membrane Transport
Solutes diffuse across a membrane from a region with a greater solute concentration to a region of lesser. Equilibrium is reached when the concentrations are identical on both sides.
8
C. Passive Processes of Membrane Transport
The rate of simple diffusion of a solute across a membrane is directly proportional to the concentration gradient across the membrane. A related important factor is the lipid solubility of the solute. In osmosis, water will diffuse from a region of its higher concentration (low concentration of solutes) to a region of its lower concentration (higher concentration of solutes).
Osmosis is the movement of water across a semipermeable membrane
9
C. Passive Processes of Membrane Transport
Small molecules can move across the lipid bilayer by simple diffusion. The more lipid-soluble the molecule, the lipidmore rapidly it diffuses. An exception to this is water, which can pass through the lipid bilayer more readily than its lipid solubility would predict. Polar and charged molecules such as amino acids, sugars, and ions do not pass readily across the lipid bilayer. bilayer.
Semi-permeable Even with respect to diffusion
C. Passive Processes of Membrane Transport
In hypotonic solutions, cells tend to take up water while in hypertonic solutions, they tend to lose it. Animal cells must remain isotonic to the environment to prevent destructive loss or gain of water.
10
Osmosis Modifies the Shapes of Cells
Shriveled
Normal
Lysed
Plasmolyzed
Flaccid
Turgid (Normal)
C. Passive Processes of Membrane Transport
The cell walls of plants and some other organisms prevent cells from bursting under hypotonic conditions. Turgor pressure develops under these conditions and keeps plants upright and stretches the cell wall during cell growth.
A Paramecium (or any organism living in a hypotonic solution) has a special problem. Water tends to move into the cells and swell and burst them. Paramecium has a particular structure, called a contractile vacuole, which constantly pumps water outside of the cell, and thus reduces pressure upon the membrane.
11
C. Passive Processes of Membrane Transport
Channel proteins and carrier proteins function in facilitated diffusion. Rem: Polar and charged molecules such as Rem: amino acids, sugars, and ions do not pass readily across the lipid bilayer. bilayer.
A Gate Channel Protein Opens in Response to a Stimulus
A Carrier Protein Facilitates Diffusion
12
C. Passive Processes of Membrane Transport
The rate of carrier-mediated facilitated diffusion is at maximum when solute concentration saturates the carrier proteins so that no rate increase is observed with further solute concentration increase.
D. Active Transport
Active transport requires energy to move substances across a membrane AND against a concentration gradient.
D. Active Transport
Three different protein-driven systems proteinare involved in active transport:
Uniport transporters move a single type of solute, such as calcium ions, in one direction. Symport transporters move two solutes in the same direction. Antiport transporters move two solutes in opposite directions, one into the cell, and the othe...