plasma membrane content Flashcards
what is the plasma membrane comprised of?
- phospholipid bilayer comprised of phospholipid molecules which have a hydrophilic phosphate head and hydrophobic tail (fatty acid)
why is the plasma membrane flexible? (3)
-
individual phospholipid moleculesare capable of sideways movement- highly mobile within the membrane
- a single lipid molecule can travel rapidly from one place to another - cholesterol (phytosterol in plants) is interspersed among phospholipid molecules- prevents fatty acid tails from packing too closely together and acts as a buffer against temp fluctuations
- low temp: phos. molecules cluster closely together (little energy to move): reduced fluidity THEREFORE cholesterol is inserted& fluidity increases due to greater distance between molecules/ increased freedom of movement
- high temp: more space between molecules (energy++): high fluidity THEREFORE cholesterol is inserted & motion of tails decreases: reduced fluidity
describe the fluid mosaic model
*the membrane is a dynamic and flexible mosaic of various proteins, lipids, and carbohydrates which form ‘mosaics’
- the phospholipid bilayer acts like a fluid: the phospholipids can move laterally within their layer, allowing the membrane to be flexible
main functions of plasma membrane
-
SELECTIVELY PERMEABLE MEMBRANE
-allows essential molecules (O2, H2O, nutrients) to enter - removes waste products/prevents harmful substances from entering
- keeps internal contents confined in one area
- proteins attached/embedded in membrane gather info about environment
- receptors enable cell to respond to external stimuli/facilitates communication between cells thru signalling pathways
- facilitates transport of molecules via:
1. passive (diffusion, osmosis, facilitated diffusion)
2. active (energy moves molecules against gradient)
3. endo/exocytosis (bulk transport of materials)
describe the hydrophobic fatty acid tail
- tail is repelled from the aqueous intracellular (INTERNAL) and extracellular (EXTERNAL) solutions
- forced to face inwards towards each other, forming the phospholipid bilayer
describe the role of surface proteins
- enable cell-cell interaction/communication
- facilitate exchange of substances with the external environment
- help to keep cell anchored
role of transmembrane proteins? provide an example
- regulate the movement of substances across the membrane
e.g some transmembrane proteins relay messages from one side of the membrane to the other (when molecule with msg can’t pass through the membrane)
e.g maintains differences of ion concentration through transmembrane ion pumps/channels
describe the interaction between receptors and ligands
- each receptor protein is specific for a single molecule/small no. of molecules with a related STRUCTURE to which the receptor binds
- molecules (i.e lipids, proteins, ions, carbs) that bind to receptors are called ligands
- once bound, the receptor protein is activated/inactivated and undergoes a conformational change
-> this change can lead to enzyme activation, ion channel opening, etc (eventually control the transmission of msg within/between cells/change the behaviour of cells
what are glycoproteins?
- membrane receptor proteins that carry a carbohydrate molecule
- addition of carbohydrate group can protect protein core to increase longevity of the protein in the rough extracellular environment
since receptors are proteins, they are…
genetically encoded
- the specific set of receptor proteins that a cell carries is determined by the genes that the cell express
what are MHC proteins and their role?
-
membrane recognition protein molecules act as MARKERS that identify the cell as a normal body cell
-> in contrast to cancerous cells, invading micro-organisms, transplant from unrelated individual - immune system can recognise proteins on ‘self’ cells but attack non-self molecules
- each organism has its own kind of recognition proteins (same species can be differentiated by the proteins on the surfaces of their cell membranes)
what do adhesion proteins do?
- link cells together to maintain the 3D structure and normal functioning of tissues
-
cytosine facing end is connected to parts of the internal cytoskeleton
-> cytoskeleton is an internal skeleton of microtubles that extend throughout eukaryotic cells - this gives cells their shape/ability to move and arrange organelles *gives cell stability/anchors cell in its environment
why are transport proteins important?
- the lipid bilayer is impermeable to many substances, including ions and water-soluble molecules
- transport proteins allow specific substances to move across a membrane
-> transport proteins can be open channels through which a substance moves on its own
-> others use energy to actively pump a substance across
name 4 types of membrane proteins
- receptor
- recognition
- adhesion
- transport
what is extracellular fluid
the liquid medium outside of the plasma membrane through which nutrients are supplied and wastes removed
what is the plasma membrane described as and why is this important?
selectively/differentially permeable- allows some substances through and not others
- small molecules can pass through pores between phospholipids
- some molecules can be helped through/held back
- permeability ensures that essential molecules (eg glucose, amino acids, lipids) enter
- concentration of substances inside cells is sustained, allowing cell to maintain a constant internal environment
explain why water is polar and why this is important
- O is negatively charged, H2 is positively charged
- O is attracted to hydrogen atoms of a nearby water molecule- forming hydrogen bonds with it
*water molecules can interact with other charged particles (e.g. polar molecules/ions)
define hydrogen bonds
- a week intermolecular chem. bond between a hydrogen atom on one molecule and a second, more electronegative atom on another molecule/different part of the same molecule
describe the relationship between non-polar/polar molecules and their ability to form hydrogen bonds with water
- polar molecules form hydrogen bonds w water molecules (hydrophilic)
- non-polar molecules cannot dissolve (bc they can’t form hydrogen bonds with water molecules) thus they are hydrophobic
what are the 2 equations involving polar/non-polar solvents and solutes? (hint. ‘like dissolves in like’)
- polar solvent (eg water) + polar solute -> solution
- non-polar solvent + non-polar solute -> solution
*lipids are non-polar- do not dissolve in water but do dissolve in each other.
why do most gases dissolve in water?
- water molecules are held together by hydrogen bonds, forming a dynamic, 3D network.
- this structure creates tiny gaps or spaces between the water molecules, which can accommodate small gas molecules
explain the process of simple diffusion and the types of molecules that can pass through
- occurs when substances move from a region of higher concentration to a region of lower concentration
- the structure of the bilayer makes it essentially impermeable to most water-soluble substances (e.g. glucose/amino acids) and charged molecules (ions)
- small non-charged molecules (e.g. CO2/O2 are lipid soluble and can diffuse thru membrane rapidly (can move from hydrophilic heads-> hydrophobic tails
*water molecules can pass through the membrane via simple diffusion because their small size allows them to slip through temporary gaps in the dynamic lipid bilayer (inefficient)
transport of hydrophilic molecules/ions across cellular membranes is assisted by…
transport proteins
*these molecules/ions move through channels in the membrane assisted by the proteins
**macromolecules cannot pass through membrane (too large)
what the difference between endocytosis and exocytosis?
-exocytosis involves fusion of the plasma membrane with small membrane-bound containers/sacs called vesicles that form inside the cytoplasm, expelling the contents of the vesicle into the extracellular environment
- endocytosis involves inward formation of a cavity/pouch of plasma membrane that encompasses some extracellular contents, then seals back on itself to form a vesicle in the cytoplasm containing the extracellular contents
