Chapter 4 Cell Structure

3A Membranes Objectives: Describes the structure and composition of a cellular membrane Differentiate between active and passive transport Describe the processes of diffusion and osmosis

Membrane Structure Membrane: a thin covering that forms the outer boundary of the cell Membranes sense and respond the changes in the surrounding environment

Membrane Structure Composed of molecules of lipids (phospholipids and cholesterol) and proteins. The proteins perform the activities in the membrane Fluid mosaic model: the lipid molecules form a flexible two-layered film in which proteins are embedded

Membrane Structure Lipid Bi-layer: phospholipids organized in two layers The phospholipids are in a shape like a head and a tail Hydrophobic tails inside Hydrophilic heads outside

Fluidity of the Membrane Membrane is Fluid Lipids have rapid lateral movement Fluidity depends on lipid composition

Fatty Acids Unsaturated Saturaded All C-C bonds are single bonds Straight chain allows maximum interaction of fatty acid tails Make membrane less fliuid Solid at room temperature Saturaded Some C=C bond (double bonds) Bent chain keeping tails apart Make membrane more fluid Liquid at room temperature

Membrane Properties

Selective Permeability Selective Permeability means that the plasma membrane allows some substances to cross through it while also keeping other substances out.

Transport of Substances Through the Cell Membrane The cell uses two main forms of transporting substances in and out of the cell: passive transport and active transport. Passive transport requires no energy Simple diffusion Facilitated diffusion Osmosis

Simple Diffusion Diffusion is the passive movement of particles (atoms, ions or molecules) from a region in which they are in higher concentration to regions of lower concentration. It continues until the concentration of substances is uniform throughout. High temperatures increase diffusion; large molecules slow diffusion

Facilitated Diffusion Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special transport proteins. Channel proteins Carrier proteins

Osmosis Osmosis is a special case of diffusion that involves water molecules. In osmosis water diffuses from a hypotonic (low solute concentrated) solution to a hypertonic (high solute concentrated) solution http://www.youtube.com/watch?v=w3_8FSrqc-I Example: paramecium http://www.youtube.com/watch?v=fh_yjLppNAg

Active Transport Active transport is the movement of molecules across a cell membrane from an area of lower concentration to one of higher concentration Assisted by enzymes Requires energy