Cells: The Living Units: Part A 3 Cells: The Living Units: Part A

(f) Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Glycoprotein Figure 3.4f

Membrane Junctions Three types: Tight junction Desmosome Gap junction

Membrane Junctions: Tight Junctions Prevent fluids and most molecules from moving between cells Where might these be useful in the body?

(a) Tight junctions: Impermeable junctions prevent molecules Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Interlocking junctional proteins Intercellular space (a) Tight junctions: Impermeable junctions prevent molecules from passing through the intercellular space. Figure 3.5a

Membrane Junctions: Desmosomes “Rivets” or “spot-welds” that anchor cells together Where might these be useful in the body?

(b) Desmosomes: Anchoring junctions bind adjacent cells together Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Plaque Intermediate filament (keratin) Linker glycoproteins (cadherins) (b) Desmosomes: Anchoring junctions bind adjacent cells together and help form an internal tension-reducing network of fibers. Figure 3.5b

Membrane Junctions: Gap Junctions Transmembrane proteins form pores that allow small molecules to pass from cell to cell For spread of ions between cardiac or smooth muscle cells

(c) Gap junctions: Communicating junctions allow ions and small mole- Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Channel between cells (connexon) (c) Gap junctions: Communicating junctions allow ions and small mole- cules to pass from one cell to the next for intercellular communication. Figure 3.5c

Membrane Transport Plasma membranes are selectively permeable Some molecules easily pass through the membrane; others do not

Types of Membrane Transport Passive processes No cellular energy (ATP) required Substance moves down its concentration gradient Active processes Energy (ATP) required Occurs only in living cell membranes

Passive Processes What determines whether or not a substance can passively permeate a membrane? Lipid solubility of substance Channels of appropriate size Carrier proteins PLAY Animation: Membrane Permeability

Passive Processes Simple diffusion Carrier-mediated facilitated diffusion Channel-mediated facilitated diffusion Osmosis

Passive Processes: Simple Diffusion Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer PLAY Animation: Diffusion

(a) Simple diffusion of fat-soluble molecules Extracellular fluid Lipid- soluble solutes Cytoplasm (a) Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer Figure 3.7a

Passive Processes: Facilitated Diffusion Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which: Exhibit specificity (selectivity) Are saturable; rate is determined by number of carriers or channels Can be regulated in terms of activity and quantity

Facilitated Diffusion Using Carrier Proteins Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids) Binding of substrate causes shape change in carrier

(b) Carrier-mediated facilitated diffusion via a protein Lipid-insoluble solutes (such as sugars or amino acids) (b) Carrier-mediated facilitated diffusion via a protein carrier specific for one chemical; binding of substrate causes shape change in transport protein Figure 3.7b

Facilitated Diffusion Using Channel Proteins Aqueous channels formed by transmembrane proteins selectively transport ions or water Two types: Leakage channels Always open Gated channels Controlled by chemical or electrical signals

(c) Channel-mediated facilitated diffusion Small lipid- insoluble solutes (c) Channel-mediated facilitated diffusion through a channel protein; mostly ions selected on basis of size and charge Figure 3.7c

Passive Processes: Osmosis Movement of solvent (water) across a selectively permeable membrane Water diffuses through plasma membranes: Through the lipid bilayer Through water channels called aquaporins (AQPs)

(d) Osmosis, diffusion of a solvent such as Water molecules Lipid billayer Aquaporin (d) Osmosis, diffusion of a solvent such as water through a specific channel protein (aquaporin) or through the lipid bilayer Figure 3.7d

Passive Processes: Osmosis Water concentration is determined by solute concentration because solute particles displace water molecules Osmolarity: The measure of total concentration of solute particles When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

Solute and water molecules move down their concentration gradients (a) Membrane permeable to both solutes and water Solute and water molecules move down their concentration gradients in opposite directions. Fluid volume remains the same in both compartments. Left compartment: Solution with lower osmolarity Right compartment: Solution with greater osmolarity Both solutions have the same osmolarity: volume unchanged H2O Solute Solute molecules (sugar) Membrane Figure 3.8a

(b) Membrane permeable to water, impermeable to solutes Solute molecules are prevented from moving but water moves by osmosis. Volume increases in the compartment with the higher osmolarity. Both solutions have identical osmolarity, but volume of the solution on the right is greater because only water is free to move Left compartment Right compartment H2O Solute molecules (sugar) Membrane Figure 3.8b

When osmosis occurs, water enters or leaves a cell Importance of Osmosis When osmosis occurs, water enters or leaves a cell Change in cell volume disrupts cell function PLAY Animation: Osmosis

Tonicity Tonicity: The ability of a solution to cause a cell to shrink or swell Isotonic: A solution with the same solute concentration as that of the cytosol Hypertonic: A solution having greater solute concentration than that of the cytosol Hypotonic: A solution having lesser solute concentration than that of the cytosol

Figure 3.9 (a) Isotonic solutions (b) Hypertonic solutions (c) Hypotonic solutions Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out). Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of solutes than are present inside the cells). Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of solutes than are present in cells). Figure 3.9

Summary of Passive Processes Energy Source Example Simple diffusion Kinetic energy Movement of O2 through phospholipid bilayer Facilitated diffusion Movement of glucose into cells Osmosis Movement of H2O through phospholipid bilayer or AQPs Also see Table 3.1