cell membranes Flashcards
What are membranes
- Selectively permeable barriers
- Maintain constant internal environment
- Enclose cell contents
- Internal organelles often have different environments to the cytosol
- Very thin (7-10nm
What is a phospholipid bilayer
it is a Dynamic structure
able to move freely around
what is a phospholipid and describe its structure
•Amphiphilic – both hydrophilic and hydrophobic parts
•Polar head; negatively charged phosphate group
•Non polar tail; no charge
•Phospholipids ‘self-assemble’ to create the bilayer
-Tails exist as saturated or unsaturated
•Double bond creates a kink in the fatty acid chain of the tail
what is saturated and unsaturated bonds
- Saturated – single bonds only
* Unsaturated – at least one double bond
what does high conc of saturated and unsaturated acids form
- Higher concentrations of unsaturated acids promote membrane fluidity
- Higher concentration of saturated fatty acids reduce cell membrane fluidity
two main types of carbohydrates
- glycolipids
- glycoproteins
three main roles of carbohydrates
-Maintain membrane stability and cell protection (glycocalyx)
•Aid cell-cell adhesion (cell adhesion molecules)
•Facilitate cell recognition
what is cholesterol and describe its structure
-Is a lipid. Approximately 20% of membrane lipid is cholesterol.
•Also has polar and non polar regions
•Wedges itself between the phospholipid tails
•Can migrate and even ‘flip’ between the membrane layer
what are the two main types of proteins
- integral
- peripheral
what are the different functions that proteins serve
•Serve different functions: enzymes, carrier proteins, channel proteins, receptors, cell adhesion, recognition
what is integral membrane protein
- Firmly inserted into the membrane
- Span the bilayer of the membrane
- Transmembrane portion is hydrophobic
- Extracellular and cytosolic portions are hydrophilic
- Carrier proteins and channels are integral protein
what is peripheral membrane protein
-Loosely attached to membrane
•May be removed easily from the membrane with minimal disruption
what is membrane fluidity affected by
- Extreme temperatures (hot or cold)
- Concentration of unsaturated fatty acids
- Cholesterol levels
what happens to the membrane fluidity at low temps
-Less kinetic energy
•Phospholipids pack together
•Membrane less fluid
what happens to the membrane fluidity at high temps
-More kinetic energy
•Phospholipids tend to move further away
•Membrane more fluid
what happens to the membrane fluidity when there is an addition of cholesterol
-Maintains fluidity by preventing lipids from getting too close or too far apart from each other
are the proportions of the individual components the same in membranes of different cell-types?
- Reb Blood cell – equal amounts of lipid and protein and a small amount of carbohydrate
- Nerve cells – higher amounts of lipid in cell membranes (80% lipid)
what is passive transport
-Moves down a concentration (electrochemical) gradient
•Uses inherent kinetic energy
what are the three types of passive transport
- Three types:
- Simple diffusion
- Facilitated diffusion
- Osmosis
what is active transport
- Moves against a concentration (electrochemical) gradient
- Has directionality
- Requires a specific carrier protein
- Needs an external energy source
what are the two types of active transport
- Two types:
- Primary
- Secondary (Symport, Antiport)
simple diffusion in plasma membrane
- Small, uncharged, non polar molecules
- O2, CO2, NO, urea
- No metabolic energy required
- Uses kinetic energy (natural motion)•No specificity
- Rate of diffusion proportional to concentration gradient
facilitated diffusion in plasma membrane
Small, polar molecules
•H2O, Glucose, Na+
•Uses specific integral proteins (carrier proteins)
•Rate of diffusion proportional to concentration gradient BUT also on one other factor..
rate of diffusion in facilitated diffusion
The rate of diffusion reaches a plateau (transport maximum; Tm) in facilitated diffusion as carrier proteins become fully occupied
what is osmosis
Net movement of water down a concentration gradient
osmosis in plasma membrane
-Water highly polar, but small amounts can ‘sneak’ through the membrane unaided
•Water moves freely and reversibly through specific protein channels called Aquaporins
•Bulk movement of water is facilitated
what is active transfer mechanisms
Utilises external energy sources and specific carrier proteins (pumps)
Allows cells to be very specific about which substances move in and out of the cell
No pump, no transport
what is primary active transport and how does it work
- Movement of Na+ and K+ uses the Na+/K+ ATPase pump
- Maintaining ion concentration differences inside the cell are crucial
- Hydrolysis of ATP results in phosphorylation of the pump
- Results in conformational change of pump
- Causes transport of 3Na+ out and 2K+ in to the cell (against their concentration gradients)•Other examples include calcium and hydrogen pumps
what is secondary active transport and how does it work
Movement of a substance (against its concentration gradient) is coupled to ion movement (down it’s concentration gradient)
•The energy used for secondary active transport comes from the electrochemical gradient across the membrane
•Example: Na+/Glucose transport
what are the two types of transfers called in secondary active transport
- symport
- antiport
what is symport in secondary active transport
-transported substances move in the same direction
•Na+/Glucose transporter
what is antiport in secondary active transport
-transported substances move in opposite direction
•Na+/Ca2+ exchanger – uses the Na electrochemical gradient to transport Ca2+ out of the cell
what two process transfer large molecules
- endocytosis (into the cell)
- exocytosis (out of the cell)
what is endocytosis
Moves large molecules into the cell (ingestion / uptake)
what are the 3 types of endocytosis
-Phagocytosis
•Pinocytosis
•Receptor-mediate
what is exocytosis
Moves large molecules out of the cell (excretion / secretion
what is exocytosis used for
- Hormone secretion
- Neurotransmitter release
- Mucous secretion
what are vesicles
-Bubble-like, membranous sacs
•Made of a phospholipid bilayer containing fluid
•Transport bubble
•Protects substances being transported
what is phagocytosis (endocytosis) and what is it used for
- ‘cell eating’
- Ingestion of large particles by specialised cells (phagocytes; in the immune system)
- Macrophages
- Certain white blood cells
- Used for removing bacteria and debris
- When a particle (e.g. bacteria) binds to a phagocyte, the cell membrane wraps itself around the particle
- Resulting in a phagosome containing the particle
what is pinocytosis (endocytosis)
- ‘cell-drinking’
- Occurs in most cells
- Process used to take in extracellular fluid with dissolved substances
- Droplet enters the cell and fuses with an endosome (sorting vesicle
what is receptor mediated (endocytosis)
- Main mechanism for specific uptake of macromolecules
- Very selective method
- Uptakes substances (hormones, cholesterol) found in small amounts •Receptors for this are specific membrane proteins
- Some viruses can hijack this route and enter our cell
how does endocytosis work and in depth exocytosis
…
how does exocytosis work
- Substances to be moved out of the cell e.g. (Hormones, Waste products, Neurotransmitters)
- Process is stimulated by a cell-surface signal that results in a cascade of events:
- Substance is enclosed in a vesicle
- Transported to plasma membrane
- Vesicle attaches to ‘docking’ sites on the membrane
- Vesicle •Transported to plasma membrane
- Fuses with membrane and ejects substance
