1. MEMBRANE STRUCTURE AND FUNCTION
5.10 Membranes organize the chemical activities of cells
Membranes
Provide structural order for metabolism

2. The plasma membrane of the cell is selectively permeable
Controlling the flow of substances into or out of the cell
Cytoplasm
Outside of cell
TEM 200,000 
Figure 5.10

3. 5.11 Membrane phospholipids form a bilayer
Have a hydrophilic head and two hydrophobic tails
Are the main structural components of membranes
CH2
CH3 CH N + O O– P C
Phosphate group
Symbol
Hydrophilic head
Hydrophobic tails
Figure 5.11A

4. Phospholipids form a two-layer sheet
Called a phospholipid bilayer, with the heads facing outward and the tails facing inward
Water
Hydrophilic heads
Hydrophobic tails
Figure 5.11B

5. 5.12 The membrane is a fluid mosaic of phospholipids and proteins
A membrane is a fluid mosaic
With proteins and other molecules embedded in a phospholipid bilayer
Fibers of the extracellular matrix
Carbohydrate (of glycoprotein)
Glycoprotein
Microfilaments of cytoskeleton
Phospholipid
Cholesterol
Proteins
Plasma membrane
Glycolipid
Cytoplasm
Figure 5.12

6. 5.13 Proteins make the membrane a mosaic of function
Many membrane proteins
Function as enzymes
Figure 5.13A

7. Other membrane proteins
Function as receptors for chemical messages from other cells
Messenger molecule
Receptor
Activated molecule
Figure 5.13B

8. Membrane proteins also function in transport
Moving substances across the membrane
ATP
Figure 5.13C

9. 5.14 Passive transport is diffusion across a membrane
In passive transport, substances diffuse through membranes without work by the cell
Spreading from areas of high concentration to areas of low concentration
Equilibrium
Membrane
Molecules of dye
Figure 5.14A
Figure 5.14B

10. Small nonpolar molecules such as O2 and CO2
Diffuse easily across the phospholipid bilayer of a membrane

11. 5.15 Transport proteins may facilitate diffusion across membranes
Many kinds of molecules
Do not diffuse freely across membranes
For these molecules, transport proteins
Provide passage across membranes through a process called facilitated diffusion
Solute molecule
Transport protein
Figure 5.15

12. 5.16 Osmosis is the diffusion of water across a membrane
In osmosis
Water travels from a solution of lower solute concentration to one of higher solute concentration
Lower concentration of solute
Higher concentration of solute
Equal concentration of solute
H2O
Solute molecule
Selectively permeable membrane
Water molecule
Solute molecule with
cluster of water molecules
Net flow of water
Figure 5.16

13. 5.17 Water balance between cells and their surroundings is crucial to organisms
Osmosis causes cells to shrink in hypertonic solutions
And swell in hypotonic solutions
In isotonic solutions
Animal cells are normal, but plant cells are limp
Plant cell
H2O
Plasma membrane
(1) Normal
(2) Lysed
(3) Shriveled
(4) Flaccid
(5) Turgid
(6) Shriveled (plasmolyzed)
Isotonic solution
Hypotonic solution
Hypertonic solution
Animal cell
Figure 5.17

14. The control of water balance
Is called osmoregulation

15. 5.18 Cells expend energy for active transport
Transport proteins can move solutes against a concentration gradient
Through active transport, which requires ATP P
Protein changes shape
Phosphate detaches
ATP
ADP
Solute
Transport protein
Solute binding 1
Phosphorylation 2
Transport 3
Protein reversion 4
Figure 5.18

16. 5.19 Exocytosis and endocytosis transport large molecules
To move large molecules or particles through a membrane
A vesicle may fuse with the membrane and expel its contents (exocytosis)
Fluid outside cell
Cytoplasm
Protein
Vesicle
Figure 5.19A

17. Membranes may fold inward
Enclosing material from the outside (endocytosis)
Vesicle forming
Figure 5.19B

18. Endocytosis can occur in three ways Phagocytosis Pinocytosis
Receptor-mediated endocytosis
Pseudopodium of amoeba
Food being ingested
Phagocytosis
Pinocytosis
Receptor-mediated endocytosis
Material bound to receptor proteins
PIT
Cytoplasm
Plasma membrane
TEM 54,000
TEM 96,500 
LM 230
Figure 5.19C