Cells: Membranes And Membrane Transport Flashcards
What type of bonds are non-polar hydrophobic molecules unable to form?
Hydrogen
What are the three components that membranes are composed of?
- Lipids: phospholipids, glycolipids, and sterols
- Proteins
- Carbohydrates in the form of glycoproteins and glycolipids
Describe the structure of a phospholipid molecule
A backbone (glycerol) + 3-C alcohol
Attached to backbone:
- negatively charged phosphate head= polar and easily form H bonds with water
- 2 non-polar fatty acid tails forming hydrophilic tails
What in a result of the amphipathic nature of phospholipids?
In aqueous environment spontaneously organise themselves so that hydrophobic tails point in wards and hydrophilic heads point outward, creating a shield.
This results in the formation of 2 layers.
What is the overarching function of lipid bilayers?
Separate the internal environment of a cell (or organelle) from the external
Regulate the movement of substances in and out
Describe the permeability of a lipid bilayer
Depends one size and hydrophilic/hydrophobic nature:
- non-polar, lipid soluble easily pass through (e.g. steroids)== no ions (Na+)
- large uncharged polar (e.g. glucose) hydrophilic so are repelled
- small polar (water, ethanol) and non polar (O2 and CO2) mostly permeablep
What molecules simply diffuse across a membrane?
Non-polar molecules—> O2 and CO2 in gas exchange
Very small polar (water, ethanol)
What are the two types of membrane proteins?
Integral
Peripheral
What are integral proteins?
- embedded in membrane layer
- difficult to isolate
- amphipathic—> interact w/ phobic exterior and philic interior== embedded
What are peripheral proteins?
- hydrophilic
- found on surface and only interact w phili regions of integral proteins
- easier to remove
What are the 5 functions of membrane proteins?
- Transport proteins movement in and out cell (channel and carrier proteins)
- Recognition cell recognition, immune system: self from non-self cells
- Receptors for chemical signals &binding sites for hormones and neurotransmitters—> trigger intracellular reactions
- Enzymes show enzymatic activity & catalyse reactions
- Adhesion help in cell adhesion to other cells/environment & help cell motility
What are aquaporins?
- Integral protein w/ 4 identical water channels
- Tetrameric protein of 4 monomeric subunit—> each subunit a has a water channel
- lined w/ specific hydrophilic side chains== passage of water but not ions
- Bidirectional depending on conc. gradient
What is the function of an aquaporin?
Rapid movement of water in and out of cell by forming hydrophilic channels
Volume of water that needs to be transported determines amount of aquaporins ( e.g. in kidney there are many)
What are the key characteristics of facilitated diffusion?
- Movement of molecules down a conc. gradient
- Movement is assisted by transport proteins (carrier&channel)
What are channel proteins?
Transmembrane proteins that assemble to form channels for the passage of polar molecules
What is an example of a type of channel protein?
Ion channels:
Tiny pores that act as pathways only for ions—> highly selective so different channels for different ions
What is the selectivity of ion channels due to?
Size of pore acting as a filter
Binding sites of hydrophilic amino acid side chains highly ion specific
Why are ion channels in facilitated diffusion highly specific?
Open and close in response to specific stimuli:
- changes in voltage
- binding of small molecules to channel proteins or ligand-gated channels
-pressure (other mechanical forces)
When gates open, ions pass through pore & down con. Gradient
How does facilitated diffusion work with carrier proteins?
- Binds to solute molecules
- Undergoes conformational change, transferring molecule to other side of membrane
- Sites are highly specific for transported solutes
Example: GLUT (glucose transporter carrier protein) transports glucose into red blood cell down conc. gradient
Why is active transport exergonic?
Releases energy requiring breakdown of ATP
What are the 3 things active transport helps with?
- Take up of essential nutrience (glucose from intestine lumen to epithelial cells lining small intestine)
- Remove secretory waste materials from inside cell to extracellular fluid
- Maintain right ion conc. in cells (e.g. sodium and potassium levels in RBCs)
What are the two main types of active transport?
Direct: energy released by breakdown of ATP used directly to transport molecules across membrane
Indirect: movement of one solute down conc. gradient drives movement of second solute against con. Gradient
What results in selective permeability?
Facilitated diffusion: gated ion channels, carrier proteins
and active transport
Describe the structure of a glycolipid
Covalent bonding of carbohydrates to lipids
Amphipathic (restricted to external surface)
Carb group polar, lipid non-polar (embedded in layer)
What is the general use of a glycolipid?
Membrane stability as form H bonds w/ water molecules surrounding cell
What are glycoproteins?
Covalent bonding of short carb chains (oligosaccherides) to protein molecules
Carb groups of molecule protrude into extracellular environment
What are the 4 functions of glycolipids and glycoproteins?
- Cell recognition act as ‘markers’ to help cells recognise each other (help immune system recognise itself)
- Cell adhesion help attach and bind to other cells to form tissues
- Cell signallingact as receptors for enzymes and other molecules for cell signalling (e.g. receiving/transmitting chemical signals)
- Glycocalyx
What is the glycocalyx
Sticky layer formed by carb groups of the glycolipids/proteins that protrude from cell surface.
Helps protect cell surface in addition to signalling, adhesion, etc)
What are the 2 key statements of the fluid mosaic model?
- Lipid bilayer is fluid: depends on nature of fatty acids in phospholipid molecule & amount of cholesterol
- The proteins (integral &peripheral) are embedded in the fluid layer which resembled]s a mosaic
How do fatty acid chains vary?
- Number of carbon atoms (16-20) (increase number=higher melting point)
- Saturation
How does membrane fluidity change depending on fatty acid saturation?
Saturated: higher melting points=provide stability
Unsaturated: lower melting points =ensure fluidity
How to saturated fatty acids provide stability?
No kinks so chains lay parallel to each other.
Fit together snugly to make membrane dense and rigid.
What happens to phospholipid molecules when temperature drops? Why are the kinks in unsaturated fatty acids important then?
Low temperatures=molecules come closer together.
Makes membrane more gel-like, decreasing fluidity.
Kinks important as prevent molecules from joining too closely & so maintain membrane fluidity.
*this is why saturated matte acids solidify more easily
cold blooded organisms adapt to cold by increasing proportion of unsaturated fatty acids (hibernation) thus regulating fluidity
Describe cholesterol
Amphipathic steroid.
Hydrophilic and hydrophilic sides interact correspondingly to phospholipid regions, holding phospholipid molecules together.
How are cholesterol molecules inserted into the lipid bilayer?
- At low temperatures ‘inserted cholesterol prevents fatty acid chains from fitting closely together—>prevent freezing
- At high temperatures cholesterol stabilises membrane—>reduces fluidity
- Decreases permeability of membrane to ions and molecules
How are large amounts of materials transported through cell?
Endocytosis and Exocytosis
Both require energy==active transport
What are the 3 stages of phagocytosis?
Pagocytosis: ingestion of large solid particles:
[seen in WBC&uncellular organisms e.g. amoeba]
- WBC: Projections of cell called pseudopodia surrounds foreign particles
- They meet and engulf resulting in a vesicle called phagosome
- Phagosome fuses with lysosome to digest particle and release nutrience
What is pinocytosis?
A form on endocytosis where fluid is ingested by cell to form small vesicles
What is exocytosis?
Material enclosed in vesicles
Vesicles move to plasma membrane and fine with it
They then discharge its contents into exterior
lipids and proteins are added
What is an example of exocytosis?
Glycolipids produced by endoplasmic reticulum and modified by Golgi apparatus.
Vesicles released by Golgi apparatus fuse w plasma membrane discharging its contents outside.
Waste is also excreted through exocytosis.
What are the 3 classifications of ion channels?
Voltage-gated
Ligand-gated
Mechanically-gated (respond to mechanical cues e.g. vibrations)
