1. 5.10 MEMBRANE STRUCT. AND FUNCTION
Membranes organize the chemical activities of cells
Membranes provide structural order for metabolism
Form most of the cell's organelles
Compartmentalize chemical reactions
The plasma membrane forms a boundary between a living cell and its surroundings
Exhibits selective permeability
Controls traffic of molecules in and out

2. 5.11: PHOSPHOLIPID BILAYER
Membrane phospholipids form a bilayer
Phospholipids are the main structural components of membranes
Two nonpolar hydrophobic fatty acid "tails"
One phosphate group attached to the hydrophilic glycerol "head"

3. LE 5-11a
Hydrophilic head
Phosphate
group
Symbol
Hydrophobic tails

4. In membranes, phospholipids form a bilayer Two-layer sheet
Phospholipid heads facing outward and tails facing inward
Selectively permeable
Polar lipid-soluble molecules pass through
Nonpolar molecules not soluble in lipids do not pass through

5. LE 5-11b
Water
Hydrophilic
heads
Hydrophobic
tails
Water

6. 5.12: FLUID MOSAIC MODEL (CELL MEMB)
The membrane is a fluid mosaic of phospholipids and proteins
A membrane is a mosaic
Proteins and other molecules are embedded in a framework of phospholipids
A membrane is fluid
Most protein and phospholipid molecules can move laterally (more fluid than solid)
Membrane glycoproteins and glycolipids function in cell identification

7. LE 5-12 Extracellular matrix Glycoprotein Carbohydrate Glycolipid
Plasma
membrane
Phospholipid
Proteins
Microfilaments
of cytoskeleton
Cholesterol
Cytoplasm

8. Proteins make the membrane a mosaic of function
Proteins perform most membrane functions
Identification tags (glycoproteins)
Junctions between adjacent cells
Enzymes
Receptors of chemical messages from other cells (signal transduction)
Transporters of substances across the membrane

9. 5.14: PASSIVE TRANSPORT (NO ATP!)
Passive transport is diffusion across a membrane
Diffusion is the tendency for particles to spread out evenly in an available space
From an area of high concentration to an area of low concentration
Passive transport across membranes occurs when a molecule diffuses down a concentration gradient
Small nonpolar molecules such as O2 and CO2 diffuse easily across the phospholipid bilayer of a membrane

10. LE 5-14a
Molecules of dye
Membrane
Equilibrium

11. LE 5-14b
Equilibrium

12. 5.15: FACILITATED DIFFUSION
Transport proteins may facilitate diffusion across membranes
In facilitated diffusion
Transport proteins that span the membrane bilayer help substances diffuse down a concentration gradient
To transport the substance, a transport protein may
Provide a pore for passage
Bind the substance, change shape, and then release the substance

13. LE 5-15
Solute
molecule
Transport
protein

14. Osmosis is the diffusion of water across a membrane
In osmosis water, molecules diffuse across a selectively permeable membrane
From an area of low solute concentration
To an area of high solute concentration
Until the solution is equally concentrated on both sides of the membrane
The direction of movement is determined by the difference in total solute concentration
Not by the nature of the solutes
Animation: Osmosis

15. cluster of water molecules
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

16. SECTION 5.17
Water balance between cells and their surroundings is crucial to organisms
Osmoregulation is the control of water balance
Tonicity is the tendency of a cell to lose or gain water in solution
Isotonic solution: solute concentration is the same in the cell and in the solution
No osmosis occurs
Animal cell volume remains constant; plant cell becomes flaccid

17. Hypotonic solution: solute concentration is greater in the cell than in the solution
Cell gains water through osmosis
Animal cell lyses; plant cell becomes turgid
Hypertonic solution: solute concentration is lower in the cell than in the solution
Cell loses water through osmosis
Animal cell shrivels; plant cell plasmolyzes

18. LE 5-17 Isotonic solution Hypotonic solution Hypertonic solution H2O
Animal
cell
(1) Normal
(2) Lysed
(3) Shriveled
Plasma
membrane
H2O
H2O
H2O
H2O
Plant
cell
(4) Flaccid
(5) Turgid
(6) Shriveled
(plasmolyzed)

19. 5.18: ACTIVE TRANSPORT USING PROTEINS
Cells expend energy for active transport
Active transport requires energy to move solutes against a concentration gradient
ATP supplies the energy
Transport proteins move solute molecules across the membrane
Animation: Active Transport

20. LE 5-18 Transport protein P P Protein changes shape Phosphate detaches
ATP
Solute
ADP
Solute binding
Phosphorylation
Transport
Protein reversion

21. 5.19: ACTIVE TRANSPORT USING VESICLES
Exocytosis and endocytosis transport large molecules
To move large molecules or particles through a cell membrane
A vesicle may fuse with the membrane and expel its contents outside the cell (exocytosis)
Membranes may fold inward, enclosing material from the outside (endocytosis)

22. LE 5-19a
Fluid outside cell
Vesicle
Protein
Cytoplasm

23. Endocytosis can occur in three ways
Phagocytosis ("cell eating")
Pinocytosis ("cell drinking")
Receptor-mediated endocytosis

24. Faulty membranes can overload the blood with cholesterol
SECTION 5.20: CONNECTION
Faulty membranes can overload the blood with cholesterol
Cholesterol is carried in the blood by low-density lipoprotein (LDL) particles
Normally, body cells take up LDLs by receptor-mediated endocytosis
Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors
People with hypercholesterolemia have more than twice the normal level of blood cholesterol