1. plasma Membrane
Chapter 4

2. Plasma membrane
At only 8 nm thick, it would take over 8,000 membranes to equal the thickness of a paper.
The plasma membrane is the edge of life.
The plasma membrane controls what enters and what leaves the cell.
It is selectively permeable.

3. Plasma membrane
Cellular membranes are fluid mosaics of lipids and proteins.
Most abundant lipids in membranes are phospholipids.
Phospholipids are amphipathic.
Hydrophobic
Hydrophilic

4. The phospholipid molecule
A phospholipid molecule consists of 5 parts.
Glycerol
Fatty acid
Phosphate group
Organic molecule (choline)

5. The phospholipid molecule
The polar head is hydrophilic (attracted to water)
The non-polar tail are hydrophobic (repel water)
Polar head (Choline + phosphate group)
Non-polar tails (Glycerol + 2 fatty acids)

6. The phospholipid molecule

7. The phospholipid molecule

8. Plasma membranes are phospholipid bilayers
Hydrophilic heads face water outside and inside cell.
Hydrophobic tails face inward towards each other.
Sandwich model

9. Plasma membranes are phospholipid bilayers

10. Fluidity of membranes
Membranes are not static sheets of molecules locked rigidly.
Membranes are held together primarily by hydrophobic interactions weaker than covalent bonds.
Most of the lipids and some proteins can shift around.

11. Fluidity of membranes

12. Fluid mosaic model Plasma membranes have several parts: Phospholipids
Glycolipids
Glycoprotein
Cholesterol
Transport proteins
Channel protein
Carrier protein
Integral proteins
Peripheral proteins

13. Fluid mosaic model

14. cholesterol
The steroid cholesterol which is wedged between phospholipid molecules has different effects on membrane fluidity at different temperatures.
At 37 degrees Celsius (98.6 F) cholesterol makes the membrane less fluid by restraining phospholipid movement.
At lower temperatures it hinders solidification by disrupting the regular packing of phospholipids

15. cholesterol

16. cholesterol
The membrane is fluid, and the molecules are always moving. It has about the same consistency as olive oil.
Cholesterol is an amphipathic molecule, meaning, like phospholipids, it contains a hydrophilic and a hydrophobic portion.
Cholesterol's hydroxyl (OH) group aligns with the phosphate heads of the phospholipids.
The remaining portion of it tucks into the fatty acid portion of the membrane

17. cholesterol
Because of the way cholesterol is shaped, part of the steroid ring (the four hydrocarbon rings in between the hydroxyl group and the hydrocarbon "tail") is closely attracted to part of the fatty acid chain on the nearest phospholipid.
This helps slightly immobilize the outer surface of the membrane and make it less soluble to very small water-soluble molecules that could otherwise pass through more easily

18. cholesterol
Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules.
In other words, it keeps the membrane from turning to mush.
Cholesterol also helps keep some proteins in place.

19. Membrane proteins and their function
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to cytoskeleton and extracellular matrix

21. G protein coupled receptors
G-protein-coupled receptors (GPCRs) are the largest and most diverse group of membrane receptors in eukaryotes.
These cell surface receptors act like an inbox for messages in the form of light energy, peptides, lipids, sugars, and proteins.
Such messages inform cells about the presence or absence of life-sustaining light or nutrients in their environment, or they convey information sent by other cells.

22. G protein coupled receptors
G proteins are specialized proteins with the ability to bind the nucleotides guanosine triphosphate (GTP) and guanosine diphosphate (GDP).
GPCRs are a large family of cell surface receptors that respond to a variety of external signals.
Binding of a signaling molecule to a GPCR results in G protein activation, which in turn triggers the production of any number of second messengers.
Through this sequence of events, GPCRs help regulate an incredible range of bodily functions, from sensation to growth to hormone responses.

23. G protein coupled receptors

24. Movement of substances into & out of cells
Phospholipid bilayer prevents the movement of water soluble molecules and ions.
But exchange between cytoplasm and environment is essential.
There are five mechanisms by which this exchange is achieved:
Diffusion
Facilitated diffusion
Osmosis
Active transport
Bulk transport

25. Movement of substances into & out of cells
Diffusion: O2, CO2, glycerol, alcohol, steroid hormones, H2O
Non- polar or very small. (concentration gradient needed)
Facilitated diffusion: large polar molecules like glucose, amino acids & sodium and chloride ions need channel and carrier proteins.
Osmosis: A special type of diffusion involving water only.
Active transport: ATP required because ions move up concentration gradient. (sodium-potassium pump)
Bulk transport: proteins, polysaccharides, part or whole cell by endocytosis (in) and exocytosis (out).

26. Bulk transport Endocytosis
Engulfing of material by membrane to form small sac or endocydic vavuole.
Phagocytosis
Cell eating by phagocytes – bulk uptake of solid material
Pinocytosis
Cell drinking vesicles – bulk uptake of liquid

27. endocytosis

28. Bulk transport Exocytosis
Process by which material is removed from cell
Digestive enzymes by the pancreas
Golgi body carrying enzymes to cell surface

29. exocytosis