1. Biological Membranes
Sections

2. Learning Objectives
Explain the significance of the selectively permeable membrane to the transport of molecules
Describe the structure of the cell membrane using the fluid-mosaic model and relate it to the semi-permeable membrane
Relate the properties of the lipid bilayer to the properties & functions of cell membranes
Describe ways that membrane proteins associate with the lipid bilayer & their functions & give examples

3. Plasma Membrane Plasma membrane = cell membrane
Forms boundary between inside & outside cell
Controls movement of molecules into & out of the cell
Takes up substances the cell needs
Disposes of the cell’s wastes
Is selectively permeable = allows some substances to cross more easily than others

4. Why Selectively Permeable?
By controlling what crosses the plasma membrane, a cell controls its volume and its internal composition
Inside the cell can be very different from the surrounding environment

5. Phospholipids Bilayers
Phospholipids are the main structural component of the cell membrane
Made up of a glycerol, phosphate group, and two fatty acids
Amphipathic = one side is hydrophobic and one side is hydrophilic
Head is polar & hydrophilic (water loving)
Tails are nonpolar & hydrophobic (water fearing)

7. Phospholipids form a two-layer sheet called a phospholipid bilayer
Hydrophobic tails point inward (are protected from the watery environment)
Hydrophilic heads point outward, toward the environment & cytoplasm

8. Hydrophobic middle of the cell membrane is why it’s selectively permeable
Nonpolar, hydrophobic molecules
Soluble in lipids
Can easily pass through the membrane
Polar, hydrophilic molecules & ions
Not soluble in lipids
Need a transport protein to pass through the membrane
Ex. Aquaporins = channel transport proteins that allow water to cross the cell membrane

10. Fluid Mosaic Model
Fluid mosaic model = states that the cell membrane is made up of many parts that are in constant motion
Mosaic = picture made up of many smaller tiles
The plasma membrane has many different proteins embedded in the phospholipid bilayer
Fluid = not solid, can flow & move
Most proteins & phospholipids can move laterally, & phospholipids can also flip to the opposite layer

13. Membrane Fluidity
The plasma membrane needs to stay fluid & flexible to function properly
Unsaturated fatty acid tails of phospholipids
Kinks stop phospholipids from packing too tightly together
Cholesterol is embedded in the membrane of animal cells
High temps: Holds phospholipids together
Low temps: Holds phospholipids apart to keep the membrane fluid

15. Membrane Composition Mostly made of phospholipids
Different types of proteins as well
Integral membrane proteins = proteins that are embedded (stuck in) the membrane
Transmembrane proteins = integral proteins that extend completely through the membrane (stick out both sides)

16. Peripheral membrane proteins = proteins attached to the surface of the plasma membrane
Not stuck in the bilayer
Can be easily removed without changing the structure of the bilayer

17. Membrane Proteins Proteins perform most of the functions of a membrane
Activities of the membrane are varied
Each type of cell has a different set of proteins
Membranes within a cell also have different sets of proteins

19. Membrane also contains molecules used for cell identification
Glycoprotein = protein with sugars attached
Glycolipid = lipid with sugars attached