Membranes 1 Ch. 5 Outline - Membranes Membrane Models  Fluid-Mosaic Plasma Membrane Structure and Function  Phospholipids  Proteins Plasma Membrane Permeability  Diffusion  Osmosis  Transport Via Carrier Proteins Cell Surface Modifications

Membranes 2 Structure and Function: The Phospholipid Bilayer The plasma membrane is common to all cells Separates:  Internal living cytoplasmic from  External environment of cell Phospholipid bilayer:  External surface lined with hydrophilic polar phosphate heads  Cytoplasmic surface lined with hydrophilic polar phosphate heads  Nonpolar, hydrophobic, fatty-acid tails sandwiched in between

3 The Fluid Mosaic Model

Membranes 4 Fluid-Mosaic Model Four components:  Basic membrane referred to as phospholipid bilayer  Protein molecules ­Float around like icebergs on a sea ­Membrane proteins may be peripheral or integral  Peripheral proteins are found on the membrane surface  Integral proteins are partially or wholly embedded (transmembrane) in the membrane ­Some have carbohydrate chains attached  Cholesterol – helps stiffen membrane

5 Transmembrane Proteins

6 Phospholipid & Cholesterol Molecules

Membranes 7 Fluid-Mosaic Model Carbohydrate chains: ­Give the cell a “sugar coat”, glycocalyx. ­Protects cell ­Facilitates adhesion between cells ­Facilitates reception of signal molecules ­Functions in cell-to-cell recognition

8 The Fluid Mosaic Model

Membranes 9 Fluidity of Plasma Membrane Fluidity of membrane is dependent on its lipid components and is critical to proper functioning. ­Body temperature – consistency of olive oil. ­More unsaturated fats = more fluid membrane ­Fluidity means that cells are pliable. ­Proteins free to “drift” laterally in phospholipid bilayer.

10 Lateral Migration of Membrane Proteins

Membranes 11 Functions of Membrane Proteins Channel Proteins:  Tubular  Allow passage of molecules through membrane Carrier Proteins:  Combine with substance to be transported  Assist passage of molecules through membrane Cell Recognition Proteins:  Provides unique chemical ID for cells  Help body recognize foreign substances Receptor Proteins:  Binds with messenger molecule  Causes cell to respond to message Enzymatic Proteins:  Carry out metabolic reactions directly

12 Membrane Protein Diversity

Membranes 13 Permeability of Membrane Plasma membrane is differentially (selectively) permeable  Allows some material to pass  Inhibits passage of other materials Passive Transport:  No ATP requirement  Molecules follow concentration gradient Active Transport  Requires carrier protein or help of vesicles  Requires energy in form of ATP

14 Types of Membrane Transport: Overview

Membranes 15 Types of Transport: Diffusion A solution consists of:  A solvent (liquid), and  A solute (dissolved solid) Diffusion  Net movement of solute molecules down a concentration gradient  Molecules move both ways along gradient  More move from high to low concentration than vice versa  Equilibrium: ­When NET change stops ­Solute concentration uniform – no gradient

16 Types of Membrane Transport: Diffusion

17 Types of Membrane Transport: Diffusion Across Lung

Membranes 18 Types of Transport: Osmosis – Passive transport Osmosis:  Special case of diffusion  Focuses on solvent (water) movement rather than solute  Diffusion of water across a differentially (selectively) permeable membrane ­Solute concentration on one side high, but water concentration low ­Solute concentration on other side low, but water concentration high  Water diffuses both ways across membrane but solute can’t  Net movement of water is toward low water (high solute) concentration Osmotic pressure is the pressure that develops due to osmosis

19 Types of Membrane Transport: Osmosis

Membranes 20 Types of Transport: Osmosis Isotonic Solution – normal cell environment  Solute and water concentrations equal on both sides of membrane Hypotonic Solution  Concentration of solute lower than on other side  Cells placed in a hypotonic solution will swell  May cause cells to break – Lysis  Creates turgor pressure in plant cells Hypertonic Solution  Concentration of solute higher than on other side  Cells placed in a hypertonic solution will shrink – Plasmolysis (plants) & crenation (animals) Plasmolysis (plants) & crenation (animals)

21 Osmotic Effects on Cells

Membranes 22 Types of Transport: Carrier Proteins  Facilitated Transport – no cell energy needed ­Explains passage of small molecules like glucose and amino acids (even though polar) ­Can’t get through membrane lipids ­Combine with carrier proteins ­Follow concentration gradient  Active Transport – requires extra cell energy ­Moves molecules against concentration gradient ­Molecules combine with carrier protein ­Example: sodium-potassium pump

23 Types of Membrane Transport: Facilitated Transport

24 Active Transport: The Sodium-Potassium Pump

Membranes 25 Types of Transport: Membrane-Assisted Transport Macromolecules transported into or out of the cell inside vesicles = active transport  Exocytosis – Vesicles fuse with plasma membrane and secrete contents outside  Endocytosis – Cells engulf substances into pouch which becomes a vesicle inside cell ­Phagocytosis – Large, solid material taken into vesicle ­Pinocytosis – Liquid or small, solid particles go into vesicle ­Receptor-Mediated –form of pinocytosis using a receptor protein in a coated pit

26 Membrane-Assisted Transport: Exocytosis

27 Membrane-Assisted Transport: Three Types of Endocytosis

Membranes 28 Cell Surface Modifications: Junctions Cell Surfaces in Animals  Junctions Between Cells  Anchoring junctions attach adjacent cells ­Adhesion Junctions  Many intercellular filaments between cells (heart, bladder, stomach = tissues that get stretched) ­Desmosomes  Single point of attachment between cells (skin)

Membranes 29 Cell Surface Modifications: Junctions Cell Surfaces in Animals ­Tight Junctions  Form impermeable barriers (like a zipper)  Serve as barriers (blood-brain barrier, intestines, kidney tubules) ­Gap Junctions  Two plasma membrane channels are joined. Allows communication when small molecules and ions pass between two cells (muscle cells)

30 Cell-Surface Modifications: Junctions

Membranes 31 Cell Surface Modifications Extracellular Matrix  External meshwork of polysaccharides and proteins (collagen, elastin, cartilage, bone)  Found in close association with the cell that produced them Plant Cell Walls  Plants have freely permeable cell wall, with cellulose as the main component ­Plasmodesmata channels penetrate cell wall ­Each contains strands of cytoplasm that allow passage of small molecules between cells

32 Cell-Surface Modifications: Extracellular Matrix

33 Cell-Surface Modifications: Plasmodesmata