MEMBRANE STRUCTURE AND FUNCTION

5.1 Membranes are a fluid mosaic of phospholipids and proteins Membranes are composed of phospholipids and proteins –Membranes are commonly described as a fluid mosaic –This means that the surface appears mosaic because of the variety of proteins embedded in the phospholipids and fluid because most of the proteins can drift about in the phospholipids

Hydrophilic head WATER Hydrophobic tail WATER

Phospholipid bilayer Hydrophobic regions of protein Hydrophilic regions of protein

5.1 Membranes are a fluid mosaic of phospholipids and proteins Many phospholipids are made from unsaturated fatty acids that have kinks in their tails –This prevents them from packing tightly together, which keeps them liquid (more like oil!) –This is aided by cholesterol wedged into the bilayer to help –keep it liquid at lower temperatures –keep it from turning to mush at higher temperatures

5.1 Membranes are a fluid mosaic of phospholipids and proteins Membrane proteins: –Integral proteins: those proteins that enter the lipid bilayer. –Transmembrane proteins: those proteins that go across the entire membrane span and stick out both sides –Peripheral proteins: those proteins that are stuck like “post-it notes” onto the outside or inside of the plasma membrane

5.1 Membranes are a fluid mosaic of phospholipids and proteins

Membranes contain integrins, which give the membrane a stronger framework –Integrins attach to the extracellular matrix on the outside of the cell as well as span the membrane to attach to the cytoskeleton. –These are, of necessity, transmembrane proteins.

Cholesterol Glycoprotein Glycolipid Carbohydrate of glycoprotein Phospholipid Microfilaments of cytoskeleton Integrin

5.1 Membranes are a fluid mosaic of phospholipids and proteins Some glycoproteins in the membrane serve as identification tags that are specifically recognized by membrane proteins of other cells –For example, cell-cell recognition enables cells of the immune system to recognize and reject foreign cells, such as infectious bacteria –Carbohydrates that are part of the extracellular matrix are significantly involved in cell-cell recognition

5.1 Membranes are a fluid mosaic of phospholipids and proteins Many membrane proteins function in – Structure (Integrins) – Cell-cell recognition (Glycoproteins) – Enzymes – Signal transduction – Transport

Enzymes

Messenger molecule Activated molecule Receptor Signal Transduction The “message” gets across, but not the original molecule. This is a “second messenger” system.

Transport Molecules actually make it from one side to another.

Cell membrane exhibit “Selective Permeability” Selective permeability: Cell membranes are designed to allow some substances to cross more easily than others. Nonpolar molecules (carbon dioxide and oxygen) cross easily Polar molecules (glucose and other sugars) do not cross easily

Water How Does Water Get Across the Membrane? 2 Ways… but let’s hold off for now

5.3 Passive transport: No energy is spent Diffusion is a process in which particles spread out evenly in an available space –Particles move from an area of more concentrated particles to an area where they are less concentrated –This means that particles diffuse down their concentration gradient –Eventually, the particles reach equilibrium where the concentration of particles is the same throughout

5.3 Passive transport is diffusion across a membrane with no energy investment Diffusion across a cell membrane does not require energy, so it is called passive transport –The concentration gradient itself represents potential energy for diffusion Molecules of dye MembraneEquilibrium

Two different substances MembraneEquilibrium 2 Substances Diffuse Independently of One Another

The Rate of Diffusion depends on… 1. The steepness of the concentration gradient 2. The size of the solute 3. The temperature of the solvent 4. The presence of charged groups

Osmosis: A Special Type of Diffusion Diffusion of water down its concentration gradient Water will always move to dilute. http: Osmosis problems http

Real-Life Osmosis: Tonicity

Using a Protein to Get Material Across the Cell Membrane

Facilitated Diffusion Diffusion proceeds down a concentration gradient, but through a protein NO ATP REQUIRED (the concentration gradient runs the show)

Active Transport Moves molecules against their concentration gradient (uphill) Going uphill requires energy…in the form of ATP

Types of Active Transport

Na+/K+ ATPase: Primary Active Transport httphttp:ATPase

Secondary Active Transport Active Transport You’ve pushed Na+ uphill against its concentration gradient…now use it!

Phagocytosis (Cell Eating)

Pinocytosis (Cell Drinking)

Receptor-Mediated Endocytosis httphttp:receptor mediated endocytosis

Exocytosis (Cell “Spitting”)