1. CP BIO: Ch. 7 The Cell Membrane
and transport across the membrane

2. Boundary between the cell and its environment
The Plasma membrane
Boundary between the cell and its environment
FLUID MOSAIC MODEL
- Describes structure of cell membranes
“mosaic” – sea of lipids with scattered proteins
“fluid” – molecules float and move around within the layer

3. Phospholipid Phospholipid = lipid molecule with phosphate on one end
glycerol + two fatty acids + phosphate group
phosphate group = polar end
(hydrophilic “head”)
Fatty acids = nonpolar end
(hydrophobic “tails”)

4. Phospholipids form a double layer
Nonpolar tails are on the inside (away from water)
Polar heads are on the outside ( touch water)

5. Features of the Cell Membrane
Semipermeable = some substances can pass through
- some cannot
- depends on: molecule size, charge, polar or nonpolar
Cholesterol – molecules scattered among the phospholipids
- in animal cells
- keep membrane flexible in changing temperatures
Carbohydrates – glucose chains on outside of cell
- Identification “tags”
- Receptor sites for messenger molecules

6. Fluid Mosaic Model

7. Membrane Proteins have many functions
Transport Enzyme Receptor
Allows a specific Catalyzes a Site for
molecule to reaction inside messenger
pass through the cell molecule to
the membrane attach

8. Membrane proteins (2) Identification Junctions Structure
“SELF” – cell belongs Cells join to Keeps internal
in this organism, form tissues, parts organized
immunity communicate

9. How do membranes keep homeostasis?
Cell membranes are selectively permeable
The lipid layer blocks most substances
Some molecules can cross the membrane
By passive or active transport
Some are too big to cross at all

10. PASSIVE TRANSPORT USES NO CELL ENERGY
Molecules move randomly – spread out until evenly distributed
From an area of higher concentration to an area of lower concentration

11. DIFFUSION
Diffusion: movement of particles from an area of high concentration to an area of lower concentration
Two areas with different concentrations = concentration gradient
Particles move in all directions (random)
NET movement is from high concentration to low
“Down the concentration gradient”

12. Diffusion Particles spread out until evenly distributed
 equilibrium (homogeneous)
AT equilibrium, particles continue moving, but in all directions equally
 NO further change in concentration

13. Cell Membranes allow some particles to cross
Particles can diffuse across the lipid bilayer if they are:
Small
Nonpolar (lipid-soluble)
Examples: CO2 , O2, fatty acids can diffuse easily

14. FACILITATED DIFFUSION
Transport proteins “help” particles move across the membrane – DOWN their gradient 
PASSIVE transport – no cell energy used
Proteins are SPECIFIC
each allows only a certain substance to pass
Particles can cross by facilitated diffusion if they are:
Small
POLAR (water soluble) or CHARGED (ions)
Examples: H2O, glucose, Ca+2, Cl-

15. Transport Proteins Pores and Channels – open path through membrane
Carrier proteins – take particle on one side of membrane and release on the other

16. OSMOSIS Diffusion of water across a membrane
Important process in cell homeostasis 
Water crosses the cell membrane easily
- Small enough to pass between lipid molecules
Also pass through special proteins, aquaporins

17. Why osmosis matters
Water crosses membrane easily, faster than many solutes
Will try to reach equilibrium
If NET water moves into or out of cell  changes homeostasis
Unequal water on one side of cell membrane = osmotic pressure

18. TONICITY = osmotic pressure in cells
ISOTONIC
Equal concentrations of solutes inside and outside cell
Equal concentrations of water
Water goes in and out of cell at equal rates

19. Isotonic pressure in cells
No NET movement of water into or out of cell
Normal water pressure in animal cells
Wilted (“flaccid”) water pressure in plant cells

20. When solute concentration is different on two sides of a membrane
lower solute concentration = hypotonic
Higher solute concentration = hypertonic
Solutes will move down their gradient IF THEY CAN CROSS THE MEMBRANE

21. Water concentration is OPPOSITE of solutes
Low solutes  HIGH WATER concentration
High solutes  LOW WATER concentration
Water WILL diffuse down its gradient and crosses the cell membrane easily
Goes TO whichever side has more solutes

22. Cells in hypotonic solutions
Solutes are lower outside cell, water is higher
Water enters cell by osmosis
Cytoplasm swells
- animal cells: swell, may burst (“lyse”)
- plant cells: high osmotic pressure “turgor”
- won’t burst - cell wall
- “Turgid” – stiff and firm, upright stem

23. Cells in hypertonic solutions
Solutes are higher outside cell, water is lower
Water leaves cell by osmosis
Cytoplasm shrinks - “plasmolysis”
- animal cells: shrivel
- plant cells: low turgor pressure
- cytoplasm pulls away from cell wall
- but cell wall does not shrink

24. Osmosis in Animal Cells
Animal cells like ISOTONIC conditions best

25. Plant cells like HYPOTONIC conditions best
Osmosis in Plant Cells
Plant cells like HYPOTONIC conditions best

26. Contractile Vacuoles
Fresh-water protists (like Paramecia or Amoeba) must constantly remove water that comes into the cell by osmosis

27. ACTIVE TRANSPORT – uses cell energy
Two kinds: 1. Molecule transport against the gradient a. from low concentration to high b. pushed across by membrane proteins (pumps) c. uses energy ATP d. small molecules and ions

28. Why would cells use active transport?
1) To concentrate substances:
Examples: kidneys concentrate wastes in urine,
- intestine concentrates nutrients in blood
2) To maintain an ion concentration
- sodium-potassium pump – allows nerve impulses

29. 2) Bulk Transport – uses energy
To bring larger particles into or out of cell
Endocytosis = brings material into cell
- fold cell membrane around it form a vacuole
a. Phagocytosis = “cell eating”
- large particles or whole cells
- examples: amoeba, white blood cells
b. Pinocytosis = “cell drinking”
- small folds of membrane take in liquids

30. Pinocytosis – “cell drinking”
Example: small intestine absorbs some water this way

31. Exocytosis – send OUT of cell
Vacuole containing substance fuses with cell membrane  opens to outside of cell
- for cell secretions
- Examples: hormones from endocrine glands
digestive juices from pancreas or intestine