1. Chapter 4
Cell Structure

2. 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

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

4. 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

5. 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

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

8. 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

9. Membrane Properties

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

11. 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

12. 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

13. 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

14. 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
Example: paramecium

15. 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