5 Cellular Membranes

5 Membrane Composition and Structure Cell membranes are bilayered, dynamic structures that:  Perform vital physiological roles  Form boundaries between cells and their environments  Regulate movement of molecules into and out of cells

5 Membrane Composition and Structure Lipids are like the water of a lake in which proteins “float.” This general design is called the fluid mosaic model.

Figure 5.1 The Fluid Mosaic Model

Figure 5.2 A Phospholipid Bilayer Separates Two Aqueous Regions

5 Membrane Composition and Structure Although all biological membranes are structurally similar, some have quite different compositions of lipids and proteins.

5 Membrane Composition and Structure All biological membranes contain proteins. The ratio of protein to phospholipid molecules varies depending on membrane function. The association of protein molecules with lipid molecules is not covalent.

5 Membrane Composition and Structure Integral membrane proteins have hydrophobic regions of amino acids that penetrate or entirely cross the phospholipid bilayer.  Transmembrane proteins show different “faces” on the two sides of the membrane. Peripheral membrane proteins lack hydrophobic regions and are not embedded in the bilayer.

5 Membrane Composition and Structure Some of the proteins and lipids can move around in the membrane. Some proteins are restricted in movement.

5 Membrane Composition and Structure Carbohydrate-bound lipid is called glycolipid. Most of the carbohydrate in the membrane is covalently bonded to proteins, forming glycoproteins. glycolipid glycoprotein

5 Cell Recognition and Adhesion Cells are able to arrange themselves into groups by two processes:  Cell recognition  Cell adhesion

5 Cell Recognition and Adhesion  Homotypic binding occurs when both cells possess the same type of cell surface receptor and their interaction causes them to stick together.  Heterotypic binding occurs between two different but complementary proteins and resembles a plug and socket.

5 Cell Recognition and Adhesion Specialized cell junctions form between cells in a tissue. Animals have three types of cell junctions: tight junctions, desmosomes, and gap junctions.

5 Cell Recognition and Adhesion Tight junctions link adjacent epithelial cells to function to:  Restrict the migration of membrane proteins and phospholipids from one region of the cell to another  Prevent substances from moving through the intercellular space

5 Cell Recognition and Adhesion Desmosomes act like spot welds on adjacent cells, holding them together.

5 Cell Recognition and Adhesion Gap junctions are connections that facilitate communication between cells. Gap junctions are made up of specialized protein channels called connexons.

5 Passive Processes of Membrane Transport Biological membranes are selectively permeable: allow some substances to pass, while others are restricted. Some substances can move by  Simple Diffusion: movement from high concentration to low concentration  Facilitated Diffusion: passive movement of substances via a protein

5 Passive Processes of Membrane Transport Diffusion is the process of random movement toward the state of equilibrium.

5 Passive Processes of Membrane Transport Diffusion rates are determined by:  temperature  size of the molecule  electrical charge of the molecule  concentration gradient.

5 Passive Processes of Membrane Transport Small molecules can move across the lipid bilayer by simple diffusion. The more lipid-soluble the molecule, the more rapidly it diffuses. Polar and charged molecules such as amino acids, sugars, and ions do not pass readily across the lipid bilayer.

5 Passive Processes of Membrane Transport Osmosis is the diffusion of water across membranes. 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).

5 Passive Processes of Membrane Transport Isotonic solutions have equal solute concentrations. Hypertonic solutions have a greater solute concentration than the solution to which it is compared. Hypotonic solutions have a lower solute concentration than the solution to which it is compared.

5 Passive Processes of Membrane Transport One way for polar and charged substances to enter cells is through the process of facilitated diffusion.

Figure 5.9 A Gate Channel Protein Opens in Response to a Stimulus

Figure 5.11 A Carrier Protein Facilitates Diffusion (Part 1)

5 Active Transport In contrast to diffusion, active transport requires the expenditure of energy. Ions or molecules are moved across the membrane against the concentration gradient. ATP is the energy currency used either directly or indirectly to achieve active transport.

5 Active Transport Three different protein-driven systems are involved in active transport:  Uniport  Symport  Antiport

Figure 5.12 Three Types of Proteins for Active Transport

5 Active Transport If ATP is used directly for the pumping system, as in the sodium–potassium pump, the system is a primary active transport system.

5 Active Transport Secondary active transport systems use established gradients to move substances. This form of transport uses ATP indirectly.

Figure 5.14 Secondary Active Transport

5 Endocytosis and Exocytosis Endocytosis  Brings macromolecules, large particles, small molecules, and even other cells into the eukaryotic cell. Exocytosis  Process by which materials packaged in vesicles are secreted from the cell.

Figure 5.15 Endocytosis and Exocytosis

5 Endocytosis and Exocytosis Three types of endocytosis: Phagocytosis = cell eating: involves the largest vesicles (entire cells can be engulfed). Pinocytosis = cell drinking: involves smaller vesicles; dissolved substances and fluids are brought into the cell. Receptor-mediated endocytosis: similar to pinocytosis, but highly specific; receptor proteins are exposed on the outside of the cell in regions called coated pits.

Figure 5.16 Formation of a Coated Vesicle (Part 1) Clathrin molecules form the “coat” of the pits.

Figure 5.16 Formation of a Coated Vesicle (Part 2)

Figure 5.17 More Membrane Functions (Part 1) Membranes have many functions

Figure 5.17 More Membrane Functions (Part 2) Membranes have many functions