Cell Structure and Function Part 2

• Cell surface membrane
• Intracellular membranes

• Phospholipids and proteins
• Glycoproteins and glycolipids
• Cholesterol

Amphipathic because phosphate group is polar, hydrocarbon chains non-polar

Phospholipids are held together by hydrophobic interaction not covalent bonds

• For it to be selective permeable
• For it to fold or fuse during exo/endocytosis
• For membrane proteins to be embedded and to function normally

• Phospholipid settle into a closely packed arrangement [membrane solidified]
• Solidified membrane permeability changes, membrane proteins may not function normally

• Type of phospholipids (saturated or unsaturated) :
• Saturated hydrocarbon tails - can pack together closely
• High M.P solidifies at lower temp
• Unsaturated hydrocarbon tails - have kinks (double bond)
• cannot pack together as closely
• Low M.P remains fluid at low temp

• Presence of Cholesterol :
• stabilise fluidity of lipid bilayer
• at high temp, cholesterol makes membrane less fluid by restraining phospholipid movement
• at low temp, cholesterol hinders close packing and so prevents membrane from solidifying

• non polar molecules can dissolve in lipid bilayer
• it impedes passage of ions and polar molecules (hydrophilic)
• Hence require transport proteins
• Selectively permeable

• Glycoproteins embedded in membrane
• Held in place by interactions btwn nonpolar grps of protein and hydrophobic hydrocarbon chains of phospholipids

Extrinsic (peripheral) and intrinsic (integral)

are loosely bound on either side of the lipid bilayer

are partly embedded or (transmembrane proteins) penetrate all the way into the lipid bilayer

glycoproteins

• form the glycocalyx :
• Cell cell recognition
• Cell cell adhesion
• Receptor sites

• transport of substances across the membrane
• enzymatic activity
• cell cell recognition
• cell cell adhesion
• signal transduction
• attachment to cytoskeleton and extracellular matrix

• Channel proteins provides hydrophilic channel that is selective for a particular solute.
• Carrier proteins shuttle substance from one side to the other by changing shape i.e some hydrolyse ATP to actively pump substances across membrane (sodium potassium pump)

• Fluid - both phospholipids and proteins being free to move within the membrane
• (phospholipids held together by hydrophobic interactions)
• Mosaic - scattered manner in which membrane proteins are embedded in the membrane

• Separates the cytoplasm from the external environment
• Selective barriers to regulate passage of substances in and out of cell and organelles
• Increase SA for exchange of substances and attachment of proteins for specific functions
• Cell cell recognition (presence of foreign proteins on membrane will trigger off production of antibodies for removal of foreign cells)
• Receptor sites (membrane proteins that form receptors will have a binding site that only the specific hormone can recognise and bind to)
• cell cell adhesion

• Tight junction:
• region where membranes of 2 adjacent cells are very tightly pressed together and bound by specific proteins
• serves as continuous barrier preventing any leakage of substances via intercellular spaces
• Desmosomes: dense regions on 2 adjacent cell membranes (made of proteins that hold the 2 membranes tightly to one another)
• holds cells together in tissues that are subjected to stretching (muscles cells)
• Gap Junction:
• consists of channels or pores through cell membrane of 2 adjacent cells connecting the cytoplasm of these cells (tunnel like that)
• permits passage of ions and small molecules to diffuse freely between neighbouring cells (prevent passage of proteins)

• Passive
• Diffusion
• Facilitated Diffusion
• Osmosis

• Active (requires ATP)
• Active Transport
• Endocytosis/Exocytosis

is the net movement of particles from a region where they are at a relatively high concentration to a region where they are at a lower concentration.

• refers to the diffusion of substances with the aid of transport proteins in the membrane in response to a diffusion gradient
• These substances are usually polar hence cannot easily diffuse across hydrophobic barrier of cell membrane.
• Speed up rate of diffusion
• Passive (NO ATP)
• Rate depends on number of transport proteins available & diffusion gradient

• Protein Carriers :
• binding sites where they bind to specific solute to be transported and undergo a series of conformational changes to transfer bound solute across membrane
• (can be inihibited by molecules that resemble normal solute)
• Protein Channels :
• channels form aqueous pores that extend across the lipid bilayer and allow specific solutes to pass through them

• is defined as the passage of solvent molecules from a region of high water potential to a region of low water potential, across a partially permeable membrane.
• -200kPa and -400kPa, the solution with more -ve value has the lower water potential
• Water is polar

• Soln with high water potential :
• cells lyse
• plant cell turgid

• Soln with same water potential :
• no net movement of water
• incipient plasmolysis (plant)

• Soln with lower water potential :
• Crenated water move out shrivel
• Plant cells flaccid (plasmolysis)

• transport of substances in and out of cells regardless of presence of concentration gradient
• Active (requires ATP) thus anything that inhibits formation of ATP stops active transport (i.e CYANIDE)
• Protein carriers needed (ATPase)

• Protein carrier changes conformation when it is phosphorylated by ATP.
• 3 Na+ pumped OUT
• 2 K+ pumped IN
• ions move agnst conc gradient as the conc. of Na+ outside the cell is ALWAYS much higher than inside

• Proteins pump H⁺ from a region of lower concentration to region of higher concentration with expenditure of ATP
• i.e. in lysosomes, create acidic enviroment within lumen

• Endocytosis - uptake of material into cell by invagination of plasma membrane and its internalisation in a membrane bounded vesicle
• Exocytosis - cell secretes moelcules by fusion of vesicles with the cell surface membrane

• Pinocytosis - taking fluids and solutes dissolved in that fluid into cell by endocytosis
• Phagocytosis - ingesting large particles by endocytosis (phagocytes)
• Receptor-mediated endocytosis - selectively taking up specific molecules by endocytosis (receptor proteins embedded on cell surface membrane, binding of ligand to its receptor triggers invagination of cell membrane, forming endocytic vesicle)

• Common fate :
• Vesicle fuses with lysosome, products absorbed.

• process where the cell secretes certain biomolecules by fusion of vesicles with cell surface membrane
• (also adds membrane and membrane proteins to cell surface membrane)