Chapter 4Cell Structure

Membranes

3A

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

Acid

s

Un

satu

rate

dS

atu

rad

ed

All C-C bonds are single bonds Straight chain allows maximum

interaction of fatty acid tails Make membrane less fliuid Solid at room temperature

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.

The cell uses two main forms of transporting substances in and out of the cell: passive transport and active transport.

Passive transport requires no energy1. Simple diffusion2. Facilitated diffusion3. Osmosis

Transport of Substances Through the Cell Membrane

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) solutionhttp://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