1. Membrane Structure & Function
Lecture #2
Membrane Structure & Function
Cell Communication

2. Membrane structure, I Selective permeability
Amphipathic~ hydrophobic & hydrophilic regions
Singer-Nicolson: fluid mosaic model

3. Membrane structure, II Phospholipids~ membrane fluidity
Cholesterol~ membrane stabilization
“Mosaic” Structure~
Integral proteins~ transmembrane proteins
Peripheral proteins~ surface of membrane
Membrane carbohydrates ~ cell to cell recognition; oligosaccharides (cell markers); glycolipids; glycoproteins

4. Membrane structure, III
Membrane protein function: •transport •enzymatic activity •signal transduction •intercellular joining •cell-cell recognition •ECM attachment

5. Membrane traffic
Diffusion~ tendency of any molecule to spread out into available space
Concentration gradient
Passive transport~ diffusion of a substance across a biological membrane
Osmosis~ the diffusion of water across a selectively permeable membrane

6. Water balance Osmoregulation~ control of water balance
Hypertonic~ higher concentration of solutes
Hypotonic~ lower concentration of solutes
Isotonic~ equal concentrations of solutes
Cells with Walls:
Turgid (very firm)
Flaccid (limp)
Plasmolysis~ plasma membrane pulls away from cell wall

7. Specialized Transport
Transport proteins
Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient
Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

8. Types of Active Transport
Sodium-potassium pump
Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane
Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane •phagocytosis •pinocytosis •receptor-mediated endocytosis (ligands)

9. Exocytosis
secretes macromolecules by the fusion of vesicles with the membrane
Transport vesicles leave the trans face of the Golgi apparatus
Secretory Cells
Pancreas cells – Insulin
Neurons – Neurotransmitters
Plant Cells – Carbohydrates for cell wall

10. Endocytosis new vesicles from the exterior of the plasma membrane form
Almost the reverse of exocytosis
Three types:
Phagocytosis – “cellular eating”
Pinocytosis – “cellular drinking”
Receptor mediated – acquire specific substances
Human cells:
Used to take in cholesterol for membrane synthesis

11. Protein Synthesis All cell made proteins start in the nucleus as DNA
DNA is copied into an mRNA strand that leaves the nucleus
mRNA, rRNA and tRNA combine to synthesize a protein
Different proteins are synthesize on the ER and Free Ribosomes in Eukaryotes

12. Proteins
Free ribosomes will synthesize proteins that are used within the cell
Ribosomes on the ER synthesize proteins that leave the cell and are used in signaling
Glycoproteins (polysaccharides + amino acids)

13. A side note to signaling
Smooth ER is responsible for the synthesis of lipids and hormones
Hormones are large molecules composed of lipids and modified amino acids
These molecules are used as signal molecules in both short distance and long distance signaling in the endocrine system

14. Signal-transduction pathway
Def: Signal on a cell’s surface is converted into a specific cellular response
Local signaling (short distance): √ Paracrine (growth factors) √ Synaptic (neurotransmitters)
Long distance: hormones

15. Cell Signaling Earl W. Sutherland, Jr.
Vanderbilt University
Nobel Prize, Physiology, 1971
Epinephrine’s ability to convert blood glycogen to glucose in the liver

16. Stages of cell signaling
Glycogen depolymerization by epinephrine
3 steps:
•Reception: target cell detection •Transduction: single-step or series of changes •Response: triggering of a specific cellular response

17. Receptors
Monomers – single receptor molecules that initiate signal with recognized binding; can activate relay proteins or enzymes
Dimers – both monomers must have a recognized signal molecule which causes them to form a dimer, which can activate relay proteins

18. Protein phosphorylation
Protein activity regulation
Adding phosphate from ATP to a protein (activates proteins)
Enzyme: protein kinases (1% of all our genes)
Example: cell reproduction
Reversal enzyme: protein phosphatases

19. Second messengers Non-protein signaling pathway (
Example: cyclic AMP (cAMP)
Ex: Glycogen breakdown with epinephrine
Enzyme: adenylyl cyclase
G-protein-linked receptor in membrane (guanosine di- or tri- phosphate)

20. Cellular responses to signals
Cytoplasmic activity regulation
Cell metabolism regulation
Nuclear transcription regulation