1. Plasma Membrane Structure and Function
The plasma membrane separates the internal environment of the cell from its surroundings.
The plasma membrane is a phospholipid bilayer with embedded proteins.
The plasma membrane has a fluid consistency and a mosaic pattern of embedded proteins.
Embedded proteins may be partially or completely embedded within the phospholipid bylayer. The phospholipid bilayer has a fluid consistency, similar to liquid cooking oil. The fluidity of the phospholipid bilayer and the mosaic pattern of proteins embedded within this bilayer are two features of the fluid-mosaic model of membrane structure.

2. Fluid-mosaic model of membrane structure
The membrane is composed of a phospholipid bilayer in which proteins are embedded. The hydrophilic heads of phospholipids are part of both the outside surface and the inside surface of the membrane. The hydrophobic tails make up the interior of the membrane. Note the plasma membrane’s asymmetry – carbohydrate chains are attached to the outside surface, and cytoskeleton filaments are attached to the inside surface.

3. Cells live in fluid environments, with water inside and outside the cell.
Hydrophilic (water-loving) polar heads of the phospholipid molecules lie on the outward-facing surfaces of the plasma membrane.
Hydrophobic (water-fearing) nonpolar tails extend to the interior of the plasma membrane.
The hydrophobic interior of the plasma membrane repels many water-soluble molecules that would otherwise cross the membrane.

4. Cholesterol strengthens the plasma membrane.
Plasma membrane proteins may be peripheral proteins or integral proteins.
Aside from phospholipid, cholesterol is another lipid in animal plasma membranes; related steroids are found in plants.
Cholesterol strengthens the plasma membrane.
By strengthening the plasma membrane, cholesterol (animal cells) and related lipids (plant cells) may help to regulate the fluidity of the plasma membrane.
Peripheral proteins on the inside surface of the membrane may be held in place by cytoskeleton filaments. Integral proteins are embedded in the membrane but can move laterally within the membrane. Some integral proteins may protrude from only one surface of the membrane, but most span the membrane with hydrophilic and hydrophobic regions corresponding to those of the membrane.

5. When phospholipids have carbohydrate chains attached, they are called glycolipids.
When proteins have carbohydrate chains attached, they are called glycoproteins.
Carbohydrate chains occur only on the exterior surface of the plasma membrane.
The outside and inside surfaces of the plasma membrane are not identical.

6. Channel protein
A channel protein allows a particular molecule or ion to cross the plasma membrane freely. Cystic fibrosis, an inherited disorder, is caused by a faulty chloride (Cl-) channel; a thick mucus collects in airways and in pancreatic and liver ducts.

7. Carrier protein
A carrier protein selectively interacts with a specific molecule or ion so that it may cross the plasma membrane. A faulty carrier for glucose may be the cause of diabetes mellitus in some persons. The cells starve in the midst of plenty, and glucose spills over into the urine.

8. Cell recognition protein
The major histocompatibility complex (MHC) glycoproteins are diverse and vary for each person, making organ transplants difficult. Foreign MHC proteins are detected by the body’s immune system.

9. In animal cells, the carbohydrate chains of cell recognition proteins are collectively called the glycocalyx.
The glycocalyx can function in cell-to-cell recognition, adhesion between cells, and reception of signal molecules.
The diversity of carbohydrate chains is enormous, providing each individual with a unique cellular “fingerprint”.

10. Receptor protein
A receptor protein has a shape that corresponds to a specific molecule so the specific molecule can bind to it. An example can be seen in pygmies. Pygmies are shorter than average because their plasma membrane growth hormone receptors do not interact correctly with growth hormone.

11. Enzymatic protein
An enzymatic protein can catalyze a metabolic reaction within the cell. As an example, Cholera bacteria release a toxin that interferes with the functioning of an enzyme that helps regulate the sodium content of cells. Sodium ions and water leave cells of the intestine, and the affected individual can die from severe diarrhea.

12. The Permeability of the Plasma Membrane
The plasma membrane is differentially permeable.
Macromolecules cannot pass through because of size, and tiny charged molecules do not pass through the nonpolar interior of the membrane.
Small, uncharged molecules pass through the membrane, following their concentration gradient.
Molecules move passively from and area of high concentration to an area of low concentration, following their concentration gradient.

13. How molecules cross the plasma membrane
The small, curved arrows indicate that these structures cannot cross the plasma membrane, and the large arrows indicate that these substances can cross the plasma membrane.

14. Movement of materials across a membrane may be passive or active.
Passive transport does not use chemical energy; diffusion and facilitated transport are both passive.
Active transport requires chemical energy and usually a carrier protein.
Exocytosis and endocytosis transport macromolecules across plasma membranes using vesicle formation, which requires energy.
Exocytosis and endocytosis are included as methods of active transport.

15. Diffusion
Diffusion is the passive movement of molecules from a higher to a lower concentration until equilibrium is reached.
Gases move through plasma membranes by diffusion.

16. Process of diffusion
When crystals of dye are placed in water, they are concentrated in one area.

17. The dye dissolves in the water, and there is a net movement of dye molecules from higher to lower concentration within the container.

18. At equilibrium, the water and the dye molecules are equally distributed throughout the solution.

19. Gas exchange in lungs occurs by diffusion
Oxygen diffuses from the alveoli into the capillaries because there is a higher concentration of oxygen in the alveoli than in the blood of the capillaries.

20. Osmosis
The diffusion of water across a differentially permeable membrane due to concentration differences is called osmosis.
Diffusion always occurs from higher to lower concentration.
Water enters cells due to osmotic pressure within cells.

21. Osmosis demonstration
A thistle tube, covered at the base with differentially permeable membrane, contains a 10% sugar solution.

22. The solute (green particles) cannot pass through the membrane, but the water (blue particles) passes through in both directions. There is a net movement of water into the thistle tube, where there is a lower percentage of water molecules.

23. Due to the incoming water molecules, the level of solution rises in the thistle tube.

24. Osmosis in cells
A solution contains a solute (solid) and a solvent (liquid).
Cells are normally isotonic to their surroundings, and the solute concentration is the same inside and out of the cell.
“Iso” means the same as, and “tonocity” refers to the strength of the solution.
A 0.9% solution of sodium chloride is isotonic to red blood cells, therefore, intravenous solutions have this tonicity.

25. Osmosis in plant and animal cells
Arrows indicate the direction of movement of water. In an isotonic solution, there is no net movement of water, and the cell neither gains nor loses water.

26. Hypotonic solutions cause cells to swell and possibly burst.
“Hypo” means less than.
Animal cells undergo lysis in hypotonic solution.
Increased turgor pressure occurs in plant cells in hypotonic solutions.
Plant cells do not burst because they have a cell wall.

27. In hypotonic solution, a cell gains water
In hypotonic solution, a cell gains water. The animal cell may undergo lysis (burst). In the plant cell, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.

28. Hypertonic solutions cause cells to lose water.
“Hyper” means more than; hypertonic solutions contain more solute.
Animal cells undergo crenation (shrivel) in hypertonic solutions.
Plant cells undergo plasmolysis, the shrinking of the cytoplasm.

29. In a hypertonic solution, a cell loses water
In a hypertonic solution, a cell loses water. Animal cells shrivel (undergo crenation). Plant cell vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts can be seen in the center of the cell.

30. Transport by Carrier Proteins
Some biologically useful molecules pass through the plasma membrane because of channel proteins and carrier proteins that span the membrane.
Carrier proteins are specific and combine with only a certain type of molecule.
Facilitated transport and active transport both require carrier proteins.

31. Facilitated transport
During facilitated transport, substances pass through a carrier protein following their concentration gradients.
Facilitated transport does not require energy.
The carrier protein for glucose has two conformations and switches back and forth between the two, carrying glucose across the membrane.
After glucose binds to the open end of the carrier, the carrier closes behind the glucose molecule. As glucose moves along, the constricted end of the carrier opens in front of the glucose molecule. After glucose is released into the cytoplasm, the carrier returns to its former conformation. This process can occur as often as 100 times per second.

32. Facilitated diffusion of glucose
A carrier protein speeds the rate at which a solute crosses a membrane from higher solute concentration to lower solute concentration. At (1), the molecule enters the carrier protein. At (2), the carrier undergoes a change in shape (conformation) that releases the molecule to the other side of the membrane. At (3), the carrier returns to its former shape.
This model of facilitated transport suggests that after a carrier has assisted the movement of a molecule to the other side of the membrane, it is free to help other similar molecules pass through the membrane. Because the glucose carrier is designed mainly for glucose, glucose can cross the membrane many times faster than the other sugars. This is a good example of differential permeability of a plasma membrane.

33. Active transport
During active transport, ions or molecules are moved across the membrane against the concentration gradient – from an area of lower to higher concentration.
Energy in the form of ATP is required for the carrier protein to combine with the transported molecule.

34. Active transport
Active transport allows a solute (ion or molecule) to cross the membrane against its concentration gradient – from lower solute concentration to higher solute concentration. At (1), the molecule enters the carrier. During (2), the breakdown of ATP induces a change in shape that drives the molecule across the membrane. At (3), the carrier protein returns to its former shape or state.

35. Carrier proteins involved in active transport are called pumps.
The sodium-potassium pump is active in all animal cells, and moves sodium ions to the outside of the cell and potassium ions to the inside.
The sodium-potassium pump carrier protein exists in two conformations; one that moves sodium to the inside, and the other that moves potassium out of the cell.
The term “pump” is used because the carrier protein is using energy to pump a substance across a membrane against its concentration gradient, much like a water pump moves water against the force of gravity.

36. The sodium-potassium pump
A carrier protein actively moves three sodium ions (Na+) to the outside of the cell for every two potassium ions (K+) pumped to the inside of the cell. Note that chemical energy of ATP is required.
In this view, the protein carrier has a shape that allows it to take up to three sodium ions (Na+).

37. In this view, ATP is split, and the terminal phosphate group is transferred to the carrier protein.

38. The attached phosphate group from ATP allows the carrier protein to change in shape, and the three sodium (Na+) ions are released to the outside of the cell. The new shape (conformation) of the carrier protein allows the carrier to take up two potassium (K+) ions.

39. When potassium ions are inside the carrier protein, the phosphate group is released from the carrier protein.

40. Finally, a change in shape that occurs when the phosphate group is released, causing the carrier protein to release the potassium (K+) ions to the inside out the cell. The new (original) shape of the carrier enables it to take up three sodium (Na+) ions once again.

41. Exocytosis and Endocytosis
During exocytosis, vesicles fuse with the plasma membrane for secretion.
Some cells are specialized to produce and release specific molecules.
Examples include release of digestive enzymes from cells of the pancreas, or secretion of the hormone insulin in response to rising blood glucose levels.
Release of the hormone insulin from the pancreas is called regulated secretion, because vesicles fuse with the plasma membrane only when insulin is needed to reduce blood glucose.

42. Exocytosis
Exocytosis deposits substances on the outside of the cell and secretion occurs.

43. Endocytosis
During endocytosis, cells take in substances by invaginating a portion of the plasma membrane, and forming a vesicle around the substance.
Endocytosis occurs as:
Phagocytosis – large particles
Pinocytosis – small particles
Receptor-mediated endocytosis – specific particles

44. Phagocytosis
Phagocytosis occurs when the substance to be transported into the cell is large; amoebas ingest food by phagocytosis. Certain types of human white blood cells are amoeboid and engulf worn-out cellular debris or bacteria using phagocytosis. When an endocytic vesicle fuses with a lysosome, digestion of the vesicle contents occurs.

45. Pinocytosis
Pinocytosis occurs when a macromolecule, such as a polypeptide, is to be transported into the cell. The resulting vesicle or vacuole is small. Pinocytosis occurs continuously, but the loss of plasma membrane due to vesicle formation is offset by exocytosis.

46. Receptor-mediated endocytosis
Receptor-mediated endocytosis is a form of pinocytosis. The substance to be taken in binds with a specific receptor protein, which migrates to a pit or is already in a coated pit. The resulting vesicle contains the substance and the receptor. Receptor-mediated endocytosis is responsible for cells taking uplow-density lipoprotein (LDL) when LDL receptors gather in a coated pit. In individuals with a genetic disorder called familial hypercholesterolemia, the LDL receptor is unable to properly bind to the coated pit, and cells are unable to take up cholesterol. Cholesterol accumulates in the walls of arterial blood vessels, causing severe health problems.

47. Summary
The structure of the plasma membrane allows it to be differentially permeable.
The fluid phospholipid bilayer, its mosaic of proteins, and its glycocalyx make possible many unique functions of the plasma membrane.
Passive and active methods of transport regulate materials entering and exiting cells.