Lecture Cell Chapters 5 and 6 Biological Membranes and Cell Communication

Membrane structure, I Selective permeability Amphipathic~ hydrophobic & hydrophilic regions Singer-Nicolson: fluid mosaic model

Carbohydrate chains Glycoprotein Carbohydrate chain Extracellular fluid Hydrophobic Hydrophilic Glycolipid Figure 5.6: Detailed structure of the plasma membrane. Although the lipid bilayer consists mainly of phospholipids, other lipids, such as cholesterol and glycolipids, are present. Peripheral proteins are loosely associated with the bilayer, whereas integral proteins are tightly bound. The integral proteins shown here are transmembrane proteins that extend through the bilayer. They have hydrophilic regions on both sides of the bilayer, connected by a membrane-spanning α-helix. Glycolipids (carbohydrates attached to lipids) and glycoproteins (carbohydrates attached to proteins) are exposed on the extracellular surface; they play roles in cell recognition and adhesion to other cells. Cholesterol Hydrophilic α-helix Integral proteins Peripheral protein Cytosol Fig. 5-6, p. 111

Membrane structure, II Phospholipids~ membrane fluidity Cholesterol~ membrane stabilization “Mosaic” Structure~ Integral proteins~ transmembrane proteins Peripheral proteins~ surface of membrane Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

(b) Fluid mosaic model. According to this model, a cell Hydrophilic region of protein Hydrophobic region of protein Phospholipid bilayer Figure 5.2: Two models of membrane structure. Integral (transmembrane) protein Peripheral protein (b) Fluid mosaic model. According to this model, a cell membrane is a fluid lipid bilayer with a constantly changing “mosaic pattern“ of associated proteins. Fig. 5-2b, p. 108

The Lipid Bilayer Lipids of the bilayer fluid or liquid-crystalline state Proteins move within the membrane

Membrane Proteins

Functions of Membrane Proteins

Membrane traffic Diffusion~ tendency of any molecule to spread out into available space Concentration gradient Passive transport~ diffusion of a substance across a biological membrane without using energy Osmosis~ the diffusion of water across a selectively permeable membrane

Diffusion

Osmosis

Pressure applied to piston to resist upward movement Water plus solute Pure water Selectively permeable membrane Figure 5.12: Osmosis. The U-tube contains pure water on the right and water plus a solute on the left, separated by a selectively permeable membrane. Water molecules cross the membrane in both directions (red arrows). Solute molecules cannot cross (green arrows). The fluid level would normally rise on the left and fall on the right because net movement of water would be to the left. However, the piston prevents the water from rising. The force that must be exerted by the piston to prevent the rise in fluid level is equal to the osmotic pressure of the solution. Molecule of solute Water molecule Fig. 5-12, p. 117

Water balance Osmoregulation~ control of water balance Hypertonic~ higher concentration of solutes Hypotonic~ lower concentration of solutes Isotonic~ equal concentrations of solutes Cells with Walls: Turgid (very firm) Flaccid (limp) Plasmolysis~ plasma membrane pulls away from cell wall

Osmotic Pressure

When a cell is placed in an isotonic solution, water Outside cell Inside cell H20 molecules Solute molecules No net water movement Figure 5.13: The responses of animal cells to osmotic pressure differences. (a) Isotonic solution. When a cell is placed in an isotonic solution, water molecules pass in and out of the cell, but the net movement of water molecules is zero. Fig. 5-13a, p. 118

(b) Hypertonic solution. When a cell is placed in a Outside cell Inside cell Net water movement out of the cell Figure 5.13: The responses of animal cells to osmotic pressure differences. (b) Hypertonic solution. When a cell is placed in a hypertonic solution, there is a net movement of water molecules out of the cell (blue arrow). The cell becomes dehydrated and shrunken. Fig. 5-13b, p. 118

(c) Hypotonic solution. When a cell is placed in a Outside cell Inside cell Net water movement into the cell Figure 5.13: The responses of animal cells to osmotic pressure differences. (c) Hypotonic solution. When a cell is placed in a hypotonic solution, the net movement of water molecules into the cell (blue arrow) causes the cell to swell or even burst. 10 μm Fig. 5-13c, p. 118

Turgor and Plasmolysis

Plasma membrane Nucleus Vacuole Vacuole Vacuolar membrane (tonoplast) Figure 5.14: Turgor pressure and plasmolysis. Vacuolar membrane (tonoplast) Plasma membrane Cytoplasm Fig. 5-14, p. 119

Specialized Transport Transport proteins Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

Facilitated Diffusion

Outside cell Glucose High concentration of glucose Low concentration Figure 5.16: Facilitated diffusion of glucose molecules. Glucose transporter (GLUT 1) Cytosol 1 Glucose binds to GLUT 1. Fig. 5-16a, p. 120

glucose is released inside cell. Figure 5.16: Facilitated diffusion of glucose molecules. 2 GLUT 1 changes shape and glucose is released inside cell. Fig. 5-16b, p. 120

GLUT 1 returns to its original shape. Figure 5.16: Facilitated diffusion of glucose molecules. 3 GLUT 1 returns to its original shape. Fig. 5-16c, p. 120

Types of Active Transport Sodium-potassium pump Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane •phagocytosis •pinocytosis •receptor-mediated endocytosis (ligands)

concentration gradient concentration gradient Higher Lower Outside cell Active transport channel concentration gradient Potassium concentration gradient Sodium Figure 5.17: A model for the pumping cycle of the sodium–potassium pump. Lower Cytosol Higher (a) The sodium–potassium pump is a carrier protein that requires energy from ATP. In each complete pumping cycle, the energy of one molecule of ATP is used to export three sodium ions (Na+) and import two potassium ions (K+). Fig. 5-17a, p. 121

causes carrier protein to change shape, releasing 3 Na+ outside cell. 2. Phosphate group is transferred from ATP to transport protein. 3. Phosphorylation causes carrier protein to change shape, releasing 3 Na+ outside cell. 1. Three Na+ bind to transport protein. 4. Two K+ bind to transport protein. Figure 5.17: A model for the pumping cycle of the sodium–potassium pump. 6. Phosphate release causes carrier protein to return to its original shape. Two K+ ions are released inside cell. 5. Phosphate is released. Fig. 5-17b, p. 121

Phagocytosis Large particles enter cell

Receptor-Mediated Endocytosis

Intracellular junctions PLANTS: Plasmodesmata: cell wall perforations; water and solute passage in plants ANIMALS: Tight junctions~ fusion of neighboring cells; prevents leakage between cells Desmosomes~ riveted, anchoring junction; strong sheets of cells Gap junctions~ cytoplasmic channels; allows passage of materials or current between cells

Tight Junctions

Gap Junctions

Cell Signaling Synthesis, release, transport of signaling molecules neurotransmitters, hormones, etc ligand binds to a specific receptor 2. Reception of information by target cells

Cell Signaling 3. Signal transduction 4. Response by the cell receptor converts extracellular signal into intracellular signal causes change in the cell 4. Response by the cell

Cell Signaling

1 Cell sends signal Signaling molecules Receptor 2 Reception Signaling protein 3 Signal transduction Figure 6.2: Overview of cell signaling. Protein Enzyme Protein that regulates a gene 4 Response Altered membrane permeability Altered metabolism Altered gene activity Fig. 6-2, p. 136

Local Regulators Paracrine regulation Local regulators diffuse through interstitial fluid act on nearby cells Local regulators histamine growth factors prostaglandins nitric oxide

Local Signals

Neurotransmitters Chemical signals released by neurons (nerve cells)

Hormones Chemical messengers Secreted by endocrine glands in plants and animals Secreted by endocrine glands in animals Transported by blood to target cells

Hormones