1. Lipid Bilayer

2. History of Lipid Bi-Layer
Researchers noted lipid soluble molecules entered cells more rapidly than water soluble molecules, suggesting lipids are component of plasma membrane.
Later analysis showed it was phospholipids formed into a bilayer, with proteins embedded in outer membrane. (RBC soaked in acetone)
Plasma membrane is phospholipid bilayer in which protein molecules are partially or wholly embedded. Embedded proteins are scattered throughout membrane in irregular patterns, varies among membranes.
Proteins have the ability to move throughout exterior of membrane, along with phospholipids. They do not flip-flop. This supports the fluid mosaic model.

5. Plasma Membrane
Membrane structure has two components, lipids and proteins.
Lipids are arranged into a bilayer.
Most plasma membranes are phospholipids, which spontaneously arrange themselves into a bilayer.
Nonpolar tails (Fatty acid tails) are hydrophobic and are directed inward; polar heads (phosphate head) are hydrophilic and are directed outward to face extracellular and intracellular fluids.

6. Membrane Fluidity
At body temperature, membrane has consistency of olive oil.
In each monolayer, the hydrocarbon tails wiggle, and entire phospholipid molecule can move sideways at a rate of about 2 micrometers/second.
Phospholipid molecules rarely flip-flop from one layer to the other.
Fluidity of the phospholipid bilayer allows cells to be pliable.
Some proteins are held in place by cytoskeletal filaments; most drift in fluid bilayer.

7. Membrane Structures
Glycolipids: have a structure similar to phospholipids except the hydrophilic head is a variety of sugar, they are protective and assist in various functions.
Cholesterol: lipid found in animal plasma membranes; reduces the permeability of membrane.
Glycoproteins: have an attached carbohydrate chain of sugar that projects externally.
The plasma membrane is asymmetrical; glycolipids and proteins occur only on outside and cytoskeletal filaments attach to proteins only on the inside surface.

8. Membrane Proteins
Plasma membrane and organelle membranes have unique proteins; RBC plasma membrane contains 50+ types of proteins.
Membrane proteins determine most of the membranes functions.
Channel Proteins: allow a particular molecule to cross membrane freely.
Carrier Proteins: selectively interact with a specific molecule so it can cross the plasma membrane
Receptor Proteins: shaped so a specific molecule can bind to it. (Hormones)
Enzymatic Proteins: catalyze specific metabolic reactions.

9. Phospholipid Bilayer

10. Cell to Cell Recognition
Carbohydrate chains of glycolipids and glycoproteins identify cell; diversity of the chain is enormous.
Chains vary by number of sugars(15 to several hundred)
Chains vary in branching.
Sequence of sugars in chains vary.
Glycolipids and glycoproteins vary from species to species, from individual to individual of same species, and even from cell to cell of same ind.
Immune system rejection of transplanted tissues is due to recognition of unique glycolipids and glycoproteins. Blood types are due to unique glycoproteins on the membranes of RBC.

11. Membrane Permeability
The plasma membrane is differentially permeable, only certain molecules can pass through freely.
A permeable membrane allows all molecules to pass through; an impermeable membrane allows no molecules to pass through; a semipermeable membrane allows some molecules to pass through.
Small non-charged lipid molecules (alcohol, oxygen) pass through the membrane freely.
Small polar molecules (carbon dioxide, water) easily pass following their concentration gradient.
Macromolecules cannot freely cross a plasma membrane.
Ions and charged molecules have difficulty crossing the hydorphobic phase of the bilayer.

12. Passive and Active Transport
Both passive and active mechanisms move molecules across membrane.
Passive transport moves molecules across membrane without expenditure of energy by cell; includes diffusion and facilitated transport.
Active Transport uses energy (ATP) to move molecules across a plasma membrane; includes active transport, exocytosis, endocytosis, and pinocytosis.

14. Diffusion and Osmosis
In diffusion, molecules move from higher to lower concentration (ex. down their gradient)
Lipid soluble molecules (Alcohol) diffuse.
Gases readily diffuse through lipid bilayer.
Osmosis is the diffusion of water across a differentially permeable membrane.
Tonicity is strength of a solution in relationship to osmosis; determines movement of water into or out of cells.

17. Phospholipid Bi-Layer

19. Receptor Mediated Endocytosis

20. Endocytosis

21. Gap Junction

22. Gap Junctions
Gap junctions are intercellular channels some 1.5–2 nm in diameter. These permit the free passage between the cells of ions and small molecules (up to a molecular weight of about 1000 daltons).
They are constructed from 4 (sometimes 6) copies of one of a family of a transmembrane proteins called connexins.
Because ions can flow through them, gap junctions permit changes in membrane potential to pass from cell to cell.
Examples:
The action potential in heart (cardiac) muscle flows from cell to cell through the heart providing the rhythmic contraction of the heartbeat.
At some synapses in the brain, gap junctions permit the arrival of an action potential at the synaptic terminals to be transmitted across to the postsynaptic cell without the delay needed for release of a neurotransmitter.
As the time of birth approaches, gap junctions between the smooth muscle cells of the uterus enable coordinated, powerful contractions to begin.
Several inherited disorders of humans such as
certain congenital heart defects and
certain cases of congenital deafness
have been found to be caused by mutant genes encoding connexins.

23. Desmosomes Desmosomes
Desmosomes are localized patches that hold two cells tightly together. They are common in epithelia (e.g., the skin). Desmosomes are attached to intermediate filaments of keratin in the cytoplasm.
Pemphigus is an autoimmune disease in which the patient has developed antibodies against proteins (cadherins) in desmosomes. The loosening of the adhesion between adjacent epithelial cells causes blistering.
Carcinomas are cancers of epithelia. However, the cells of carcinomas no longer have desmosomes. This may account for their ability to metastasize.
Hemidesmosomes
These are similar to desmosomes but attach epithelial cells to the basal lamina ("basement membrane" – View) instead of to each other.
Pemphigoid is an autoimmune disease in which the patient develops antibodies against proteins (integrins) in hemidesmosomes. This, too, causes severe blistering of epithelia.

24. Plasmodesmata Plasmodesmata
Although each plant cell is encased in a boxlike cell wall, it turns out that communication between cells is just as easy, if not easier, than between animal cells. Fine strands of cytoplasm, called plasmodesmata, extend through pores in the cell wall connecting the cytoplasm of each cell with that of its neighbors.
Plasmodesmata provide an easy route for the movement of ions, small molecules like sugars and amino acids, and even macromolecules like RNA and proteins, between cells. The larger molecules pass through with the aid of actin filaments.
Plasmodesmata are sheathed by a plasma membrane that is simply an extension of the plasma membrane of the adjoining cells. This raises the intriguing question of whether a plant tissue is really made up of separate cells or is, instead, a syncytium: a single, multinucleated cell distributed throughout hundreds of tiny compartments!

25. Plasmodesmata

26. Tight Junction

27. Adhesion Junction