1. The plasma membrane functions
The functions of the plasma membrane include:
Isolation
Regulation of exchange with the environment
Sensitivity to the environment
Structural support

2. Plasma Membrane
Physical barrier - separates intracellular fluids from extracellular fluids
Helps in maintaining homeostasis
Plays a dynamic role in cellular activity – selectively permeable

3. Fluid Mosaic Model Double bilayer of phospholipids
Phospholipids have hydrophobic tails and hydrophilic heads
CH2
CH3 CH N + O O– P C
Phosphate group
Hydrophilic head
Hydrophobic tails

4. The plasma membrane includes proteins
Integral proteins
Within the membrane
Peripheral proteins
Bound to inner or outer surface of the membrane

5. The plasma membrane includes proteins
Anchoring proteins (stabilizers)
Attach to inside or outside structures
Recognition proteins (identifiers)
Label cells as normal or abnormal
Enzymes
Catalyze reactions
Receptor proteins
Bind and respond to ligands (ions, hormones)
Carrier proteins
Transport specific solutes through membrane
Channels
Regulate water flow and solutes through membrane

8. Specialized junctions
Structures on the plasma membrane surfaces
Microvilli, Cilia,
Stereocilia
Apical surface
Lateral
surface
Specialized junctions

9. Features of Apical Surface of Epithelium - Microvilli
Projections that increase surface area
Folding of the plasma membrane

10. Features of Apical Surface of Epithelium - Cilia
These structures are designed for motility.
Epithelia that need to move substances across their surface (like mucous in the air passages) have cilia.
Each cilium or flagellum has a basal body located at its base.
Basal bodies anchor the cilia or flagella and are thought to be responsible for their formation.
They look like centrioles and are believed to be derived from them

11. Flagella: (ex) spermatoza
Extra long cilia
Moves cell

12. Cell junctions – 3 groups
Tight junction
designed to restrict the movement of material between the cells they link
Gap junction
create cytoplasmatic communication bridges between cells
Anchoring junction
attach cells to one another or to extracellular matrix

13. Membrane Junctions Tight junction Gap junction Anchoring junction

14. Tight Junctions
An intercellular junction between cells in which the outer layers of the cell membranes fuse,
reducing the ability of larger molecules and water to pass between the cells.
Tight junctions prevent the free movement of molecules between cells in the intestine and allow the intestinal cell to control absorption

15. Gap junctions
Example – intercalated discs in the heart, electrical synapses

16. Cell transport mechanisms - How things enter and leave the cell

17. 2 groups of movement Passive transport – no energy is needed Diffusion
Carrier-mediated
Active transport – requires ATTP
Pumps
Vesicular transport

18. Characteristics of selectively permeable membranes
EXTRACELLULAR
FLUID
Materials may cross
the plasma membrane
through active or
passive mechanisms.
Plasma
membrane
Passive mechanisms
do not require ATP.
Active mechanisms
require ATP.
Diffusion is
movement driven
by concentration
differences.
Carrier-mediated
transport involves
carrier proteins, and
the movement may
be passive or active.
Vesicular transport
involves the
formation of
intracellular
vesicles; this is an
active process.
CYTOPLASM

20. Passive transport
All molecules in the body are in constant motion regardless of the presence of a membrane (kinetic energy)
Motion stops only at absolute zero
By international agreement, it is defined as 0K on the Kelvin scale, −273.15°C on the Celsius scale and −459.67°F on the Fahrenheit scale
When a membrane is present the movement in a certain direction can be limited or changed
A molecule will move in a certain direction until collide with another molecule. When this happens, the direction of the movement will change

21. Diffusion
Diffusion is the tendency for molecules to spread out evenly into the available space
The driving force is kinetic energy
Slow in air and water but important over small distances
Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction
Depends on a concentration gradient. (What is a concentration? A concentration gradient?)
At dynamic equilibrium, as many molecules cross one way as cross in the other direction

22. Factors Affecting Diffusion
Distance (inversely related)
Molecule size (inversely related)
Temperature (directly related)
Gradient size (directly related)
Electrical forces
Attraction of opposite charges (+,–)
Repulsion of like charges (+,+ or –,–)

23. Diffusion The movement of molecules will happen in ALL directions
What is usually important is the net rate of diffusion in a certain direction
The net movement will be from high to low concentration until equilibrium is reached
At equilibrium, the net movement is equal in all directions

24. When a membrane is present
Membrane can be:
Freely permeable (this does not apply to plasma membrane) – allows passage of all substances
Selectively permeable – permits passage of some materials and prevents passage of others
Impermeable – cells can be impermeable to specific substances, but no living cell has a completely impermeable membrane

25. Permeability characteristics of membranes
Freely permeable membranes
Selectively permeable membranes
Impermeable membranes
Ions
Carbohydrates
Ions
Carbohydrates
Ions
Carbohydrates
Protein —
Protein —
Protein —
Water
Water
Water
Lipids
Lipids
Lipids
Freely permeable membranes
allow any substance to pass without
difficulty.
Selectively permeable membranes,
such as plasma membranes, permit the
passage of some materials and prevent
the passage of others.
Nothing can pass through impermeable
membranes. Cells may be impermeable
to specific substances, but no living cell
has an impermeable membrane.

26. Selectively permeable membranes
Selective based on:
Characteristics of material to pass
Size
Electrical charge
Molecular shape
Lipid solubility
Characteristics of membrane
What lipids and proteins present
How components are arranged

28. Diffusion through cell membrane
Diffusion is divided into 2 types:
1. Simple diffusion – the movement of particles through the membrane with no assistance
Nonpolar / lipid-soluble substances that diffuse directly through the lipid bilayer
Gases readily diffuse through lipid bilayer. (Ex. movement of oxygen inside cells and CO2 outside)
Diffusion of water and other lipid-insoluble molecules happens via protein channels
The channels are highly selective as a result of the diameter, shape, charge and chemical bonds

29. Diffusion of lipid-soluble materials

30. Diffusion of lipid-insoluble materials

31. Diffusion through cell membrane
2. facilitated diffusion - Assisted by carrier protein
Materials are bound to specific proteins and move through water-filled protein channels (big polar molecules; ex. – glucose)
The facilitated diffusion rate depends on the rate in which the carrier protein molecule can undergo changes that allow passage
Carrier Proteins
Are integral transmembrane proteins
Show specificity for certain polar molecules
Their number will influence the amount that can be transferred through the membrane

33. Osmosis
Osmosis is a simple diffusion of water.
It occurs through a selectively permeable membrane
Occurs when the concentration of a water is different on opposite sides of a membrane
Membrane must be freely permeable to water, selectively permeable to solutes

34. Osmosis – osmolality, osmolarity and osmotic pressure
Osmolality (molecular weight) - One osmole is 1 gram molecular weight
Osmolarity (concentration) - One osmole in one liter
Osmotic pressure – defined by the concentration of solute particles in a solution
Is defined by the number of particles, not their size or nature
Each particle in a solution, regardless of its mass, exerts the same pressure against the membrane

35. Effects of Solutions of Varying Tonicity
Tonicity – description of how the solution affects a cell
Isotonic – solutions with the same solute concentration as that of the cytosol
Hypertonic – solutions having greater solute concentration than that of the cytosol
Hypotonic – solutions having lesser solute concentration than that of the cytosol

36. Passive Membrane Transport: Filtration
The passage of water and solutes through a membrane by hydrostatic pressure
Pressure gradient pushes solute-containing fluid from a higher-pressure area to a lower-pressure area
Depending on the size of the membrane pores
only solutes of a certain size may pass through it.

37. Transport that uses ATP
A movement that can be against concentration gradient
Uses ATP to move solutes across a membrane
Two types:
Active transport - use of carrier proteins
Vesicular transport

38. Types of Active Transport
2 types according to the source of energy used for the transport
Primary active transport
The energy for the transport derived directly from a high energy molecule – ATP
The hydrolysis of ATP causes phosphorylation of a transport protein that in turn changes its shape.
That change “promotes” the passage of materials (ex. Sodium-potassium pump)

39. 1 2 3 6 5 4 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+
Fig
EXTRACELLULAR
FLUID
[Na+] high
Na+
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
[Na+] low
ATP P
Na+ P
CYTOPLASM
[K+] high
ADP 1 2 3 K+
Figure 7.16, 1–6 The sodium-potassium pump: a specific case of active transport K+ K+ K+ K+ P K+ P 6 5 4

40. Types of Active Transport
Secondary active transports – one ATP-powered pump can drive secondary transport of other solutes.
The energy is derived from the energy stored in creating the concentration gradient
This concentration difference was created by the primary active transport that used ATP
Secondary transport, like the primary, depends on carrier proteins, but without the need of energy

41. – + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+
Fig. 7-19 – + H+
ATP H+ – +
Proton pump H+ H+ – + H+ H+ – + H+
Diffusion
of H+
Sucrose-H+
cotransporter
Figure 7.19 Cotransport: active transport driven by a concentration gradient H+ –
Sucrose + – +
Sucrose

42. Active transport
Symport system – two substances are moved across a membrane in the same direction
Antiport system – two substances are moved across a membrane in opposite directions (Na/K)

44. Vesicular Transport
Transport of large particles and macromolecules across plasma membrane using vesicles and ATP
Endocytosis – enables large particles and macromolecules to enter the cell. Few types:
Receptor-mediated endocytosis – selective process that depends on the binding of extracellular material to a specific receptor
This binding initiates the endocytosis
Phagocytosis – “cell eating”; endocytosis of solid objects
pseudopods engulf solids and bring them into the cell’s interior
Happens in specialized cells
Pinocytosis – “cell drinking”; endocytosis of liquids.
This is not a selective process and does not involve receptor

45. Vesicular Transport
Exocytosis – moves substance from the cell interior to the extracellular space
Transcytosis – moving substances into, across, and then out of a cell
Vesicular trafficking – moving substances from one area in the cell to another

47. Passive Membrane Transport – Review
Process
Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through membrane
Facilitated diffusion
Movement of glucose into cells
Osmosis
Movement of H2O in & out of cells
Filtration
Hydrostatic pressure
Formation of kidney filtrate

48. Active Membrane Transport – Review
Process
Energy Source
Example
Active transport of solutes
ATP
Movement of ions across membranes
Exocytosis
Neurotransmitter secretion
Endocytosis
White blood cell phagocytosis