Cell Membranes Flashcards
Explain why the cell surface membrane is called a ‘fluid mosaic model’
‘Fluid’ because the cell membrane comprises of phospholipids and proteins which are free to move laterally within a layer
‘Mosaic’ because the random arrangement of the proteins embedded in the phospholipid molecules resemble a mosaic pattern
What are the functions of the phospholipid bilayer?
As a major component of cell membrane** it…
1) regulates the movement of substances** moving in and out of the cell by acting as a barrier to ions and polar* or large* molecules
2) acts as a boundary *between the intracellular and extracellular aqueous environment
3) allows compartmentalisation **within a cell
4) allows formation of transient pores for simple diffusion of small, non-polar molecules across the membrane
What is the structure and function of cholesterol?
Structure:
- Has a characteristic 4 ring **structure
- Slightly amphipathic**** as it has a hydrophilic,polar, hydroxyl group & a hydrophobic *4 ring *structure
- The hydroxyl group of cholesterol aligns with the charged phosphate heads of the phospholipids while the rest of it is tucked into the hydrophobic core of the membrane.
Function:
-Cholesterol regulates membrane fluidity* i.e. it stabilises the membrane.
➔The membrane is prevented from being overly fluid** at warmer temperatures** as cholesterol restricts phospholipid movement** through its interactions with the phospholipids.
➔The membrane is prevented from being overly firm at lower temperatures as cholesterol prevents the close packing* of phospholipids and hence prevents solidification/crystallisation.
-Cholesterol stabilises the lipid bilayer due to van der Waals interactions*** between the rigid fused ring structure and the lipid bilayer
How are proteins held in the membrane?
- The non-polar, hydrocarbon *chains of the phospholipid bilayer can form hydrophobic interactions with the non-polar R groups of amino acids found on the exterior surface of the channel protein (facing the hydrophobic core).
- The charged phosphate head*** of the phospholipid bilayer can interact with the charged / polar R groups **of amino acids found on the exterior surface of the channel protein
What is the function of a carrier protein?
- bind the solute on one side** of the membrane and as a result, the protein undergoes a conformational change** that allows access of the solute to the opposite side **of the membrane (e.g. glucose transporter)
- some are pumps that usually use ATP* to move solute against a concentration gradient** (from a low solute concentration to a high solute concentration) (e.g Na+ / K+ pump)
What kind of proteins are embedded in the phospholipid bilayer
- channel proteins
- carrier proteins
- enzymes (e.g. ATP synthase on the inner mitochondrial membrane which catalyzes the synthesis of ATP)
- receptor proteins
(e. g. receptor protein at channels on the RER surface which
(1) holds the ribosomes in position and
(2) allow the entry of polypeptides synthesized on the ribosomes on the RER surface into the cisternal space) - functions to stabilize membrane structure as they are non-covalently bound to the cytoskeleton and extracellular matrix
What is the role of glycoproteins and glycolipids on the CSM?
Carbohydrate chains function as
- markers in cell-cell recognition and adhesion which allow cells to be attached to one another to form tissues and organs
- function as receptors
(e. g. RTK for insulin and GPCR for glucagon)
What are the functions of membranes at the surface of cells and membranes within the cell?
(x5 functions)
1) Regulate movement of substances ** as membranes are a SELECTIVELY PERMEABLE MEMBRANE which act as a BOUNDARY
a) between inside & outside of cell, (b) between organelle &cytoplasm (e.g. Golgi apparatus& cytoplasm) & (c) between compartments within an organelle (e.g. mitochondrial matrix & intermembrane space)
2) Membranes allows for COMPARTMENTALISATION** which allow
(i) unique environments ** to be formed for highly specialized activities (e.g acidic environment in lysosomes for hydrolytic enzymes to work)
(ii) spatial separation** of biochemical processes & thus their sequential operation** within a cell
(e. g.protein modification in RER and further protein modification, sorting, and packaging in the GA)
(iii) accumulation of ions** to high concentrations (e.g.accumulation of a high concentration of H+ in the intermembrane space of the mitochondria enable a proton gradient to be established for chemiosmosis)
3) Membranes act as a SURFACE FOR CHEMICAL REACTIONS** to occur in sequential manner ➔ membranes may have functionally related proteins grouped together so that sequential biochemical processes can occur
(e. g. the thylakoid membranes of the chloroplast have electron carriers & ATP synthetase for chemiosmosis **to occur.)
4) Membranes INCREASE SURFACE AREA** for chemical reactions
(e. g. inner mitochondrial membrane is highly folded to hold more electron transport chains and ATP synthetase)
5) Membranes surface topography enable COMMUNICATION of cell with surroundings
➔the unique combination of proteins/glycoproteins/glycolipids on surface of different cells enable
(a)cell-cell recognition** and adhesion** so that tissue formation is possible,
(b)ligands to recognize specific receptors so that signal transduction** can occur.
What is the definition of simple diffusion?
Definition: Net movement of molecules/ions from a region of high concentration to a region of low concentration, down a concentration gradient.
e.g. O2 diffuses from the lungs to the blood
What is the definition of facilitated diffusion?
Definition: Net movement of polar charged molecules/ions from a region of high concentration to a region of low concentration, down a concentration gradient** through a transport protein**.
Transport proteins facilitate diffusion **of substances that are insoluble in phospholipids bilayer
e. g. 1) transmembrane hydrophilic channel proteins with hydrophilic pores(e.g. aquaporins)
2) carrier proteins (e.g. glucose transporters)
Explain what is meant by “active transport” [2m]
- Active transport is the transport of substances across the membrane against** a concentration gradient using carrier proteins* or pumps*
- and energy in the form of ATP **is required
e. g. Na+-K+pump
(e. g. in the maintenance of polarised state of nerve cells) Movementof each molecule or ion is in one direction (unlike diffusion which is reversible)
What is the definition of bulk transport?
What are the 2 types of bulk transport?
Bulk transport is an active process as it requires ATP. However it is NOT active transport as it does not transport molecules across a membrane through a transmembrane carrier protein.
1) Exocytosis: Secretion of macromolecules (e.g.waste materials) to the exterior of the cell by fusion of vesicle membrane with the plasma membrane
2) Endocytosis: Infolding or extension of cell surface membrane to form a vesicle or vacuole, thus allowing cell to acquire macromolecules and particulate matter respectively.
a) Phagocytosis: solids taken into cell via a vacuole (e.g. white blood cells engulf bacteria)
b) Pinocytosis: liquids taken into the cell via vesicles (e.g. human egg cell takes up nutrients from surrounding follicles)
What is the definition of osmosis?
Definition: Net movement of water from a region of high water potential to a region of low water potential down a water potential gradient through a selectively permeable membrane.
Explain why proteins are required for transport of glucose across the cell surface membrane. [3m]
- Glucose is polar and thus hydrophilic
- The hydrophobic core of the phospholipid bilayer would repel the glucose
- Glucose is too large to pass through the transient pores of the pplipid bilayer
- Transport proteins provides a hydrophilic channel through the membrane for the passage of the solutes
Describe two named examples of movement in cells that use ATP. [2]
- active transport* of (named substance e.g. Na+, K+) across membrane against concentration gradient by protein pumps;
- movement of vesicles / organelles, through cytoplasm by motor proteins e.g. kinesin, dynein, myosin;
- exocytosis* of named substance e.g. insulin, glucagon, antibodies, enzymes (vesicles move along cytoskeleton to plasma membrane and fuses with it);
- endocytosis / phagocytosis* of name substance e.g. food particles, pathogen (cell membrane invaginates / pseudopodia extends and surrounds the substances and eventually pinches off the membrane to form a vesicle);
- spindle fibre / chromosome / chromatid, (movement) during, mitosis / meiosis ;
- AVP, e.g. cilia / flagella beating;
