Membrane Structure and Function Flashcards
Membranes grow through
enlargement of preexistant membranes
The ER incorporates membranes:
- lipid from the cytosol
- proteins through addition or synthesis
- carbohydrate addition occurs in Golgi
Double line at high magnifications
Cell membrane (7.5 nm thick)
The two sides (leaflets) of all membranes
C = cytoplasmic/nucleoplasmic side
E= exoplasmic side
term to describe a membrane meaning that they have a hydrophobic part and hydrophilic part
amiphillic
Aliphatic, nonpolar chains of fatty acids
hydrophobic
Carboxyl groups that make up the head group of the fatty acids
hydrophillic part that faces externally
structure with fatty acid tail and contains carboxyl groups studded on the head group
glycolipid
contains a phosphate with a type of polar group attached
phospholipid
Some PLs and all GLs are _______
sphingolipids
GLs which contain sialic acids
gangliosides
In cellular membranes, the bilayer is __________
asymmetric
Mixture of PLs and GLs spontaneously form a bilayer membrane in an aqueos solution
Membrane asymmetry
C leaflet:
E leaflet:
- C leaflet: most phoshatidyl serine (PS) and phosphatidyl ethanolamine (PE)
- lipid-linked protein
- E leaflet: most phosphatidyl choline (PC) and sphingomyelin (SM) and ALL glycolipids (GL)
- CHO groups on membrane proteins
- GPI-linked proteins
enzyme that causes the lost of asymmetry in some cell membranes
scramblases
The presence of PS on the outer surface (E leaflet) triggers _________ of aged cells by _________
phagocytosis, macrophages
Makes up about 20% of plasma membrane lipid
cholesterol
it can move in the plane of the membrane and between leaflets (“flip-flop”)
Cholesterol decreases the membrane ____________ to small molecules
permeability
Membrane proteins and glycoproteins help with:
- signaling
- transport across the membrane
- cell adhesion
In single pass transmembrane proteins, CHO is always on the _____ side of the cell membrane
Functions:
E side
function: receptors
cell recognition
cell attachment
Epidermal growth factor (EGF) receptor
-tyrosine kinase signaling steps:
- Ligands (EGF) bind to receptors
- Receptors form dimers
- Dimers cross-phosphorylate their cytoplasmic tails
- Tyrosine kinase domains activated
- Other proteins phosphorylated
- Cell proliferation stimulated
Large multipass transmembrane proteins functions:
- receptors(G protein coupled receptor)
- transport proteins ion channels (connexin)
- ion pumps
- glucose transporters
etc.
Acetylcholine receptors (nicotinic) in the membranes of skeletal muscle cells are _________ membrane proteins.
multipass
A group of five receptors make one ion channel in the membrane.
- Ion channel is blocked
- Two AChs bind to receptors
- Channel opens
Anchored proteins (glycosyl-phosphatidylinositol (GPI)) do not _____
functions:
cross the bilayer
- receptors
- enzymes
located on the E side
Alkaline phosphate is a
GPI-linked protein
–In osteoblast membranes (bone forming cells)
–Increases the local concentration of free phosphate
→ calcium phosphate in bone
What is the function of proteins covalently bound to lipid
receptor-related signaling
ex. , G-coupled receptor protein and its GTP coupled binding protein receptor is always on the C side
____ and ____ are lipid-linked signalling proteins that stimulate cell proliferation
Src and Ras
–Ras is a GTP-binding protein
–Src is a tyrosine kinase
Function of proteins bound to the membrane by ionic bonds (peripheral membrane proteins)
bind other structures to the cell membrane on both E and C sides
anchors actin filaments in skeletal muscle cells to a complex of proteins in the muscle cell membrane
Dystrophin
This protein is defective in Duchenne’s Muscular Dystrophy
prevent movement between adjacent cells forcing apical to basal membrane diffusion
tight junctions
the CHO layer on the outer surface of the plasma membrane
the glycocalyx
Attached to lipids and proteins
Always on the E side
Glycocalyx is on the outer surface gives cells a fuzzy appearance
Membrane carbohydrate occurs as glycolipid or glycoprotein and functions include:
–protects proteins from proteolysis
–cell adhesion
–cell recognition (ABO blood groups are GLs)
Cholera toxin and HIV -1 both recognize and bind to specific glycolipids on intestinal epithelial cells. This facilitates their entry into the cells
Plasma membranes (are/are not) homogeneous
not homogeneous
Some rafts are invaginated to form ______
caveolae
The protein caveolin help them form their characteristic flask-like shape
Many signaling molecules and receptors are concentrated in
Selective affinity for:
lipid rafts and caveolae
Selective affinity for:
- Ras and Src proteins
- GPI anchored proteins
Insulin receptors are localized to lipid rafts and caveolae
Membrane Transport: Passive Processes
- Simple diffusion
- Carrier-mediated facilitated diffusion
- Channel-mediated facilitated diffusion
- Osmosis
movement of solute particles “down” its concentration gradient (High to Low)
simple diffusion
movement of H2O “down” its concentration gradient (from high water concentration to low water concentration area).
Water diffuses through plasma membranes two ways:
Osmosis
- Through the lipid bilayer
- Through water channels called aquaporins (AQPs)
Fick’s Law of Diffusion
Rate of diffusion ~ surface area x concentration gradient x membrane permeability/ membrane thickness
membrane permeability~ lipid solubility/molecular size
Facilitated Diffusion Using Channel Proteins
Two types:
Aqueous channels formed by transmembrane proteins selectively transport ions or water
- Leakage channels
- Always open
- Gated channels
- Controlled by chemical or electrical signals
selected on basis of size and charge
The measure of total concentration of solute particles
osmolarity
•When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached
The ability of a solution to cause a cell to shrink or swell
tonicity
A solution with the same solute concentration as that of the cytosol
Isotonic
A solution having greater solute concentration than that of the cytosol
hypertonic
A solution having lesser solute concentration than that of the cytosol
hypotonic
Examples of Isotonic Solutions:
0.9% Saline (0.9 g NaCl/100mL H20 solution) and 5% Glucose (5g glucose/100mL H20 solution)
Requires carrier proteins (solute pumps) and moves solutes against concentration gradient
Active transport
•Types of active transport:
–Primary active transport
–Secondary active transport
Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across the membrane
Primary Active Transport
- Depends on an ion gradient created by primary active transport
- Energy stored in ionic gradients is used indirectly to drive transport of other solutes
Secondary Active Transport
cotransport: uses symport and antiport systems
Mitochondria Membrane Structure
Membrane
E-face:
P-face:
E-face: backed by extracellular space
P-face: backed by protoplasm
Mobility of membrane lipids
PLs and GLs can move in the plane of the membrane, but “flip-flops” are rare
(unless scramblases are present)
Notch
- Notch and its ligand both are single pass transmembrane proteins
- Notch is on cell 1—external part is cleaved and bound to notch ligand
- Ligand is on cell 2– Bound Notch is internalized
Glycocalyx Functions:
- Cell recognition
- Enzymatic activity
- Contain receptors for antibodies, hormones, bacteria, and viruses
are microdomains within the plasma membrane enriched (2X) in cholesterol and sphingolipids. These may be flat or form pockets (caveolae)
Lipid rafts
Membrane proteins involved in signal transduction into the cell concentrate in these rafts
Methods of Membrane Transport
- Simple Diffusion
- Through Lipid Bilayer
- Osmosis
- Carrier-Mediated Transport
- Facilitated Diffusion
- Primany Active Transport
- Seconday Active Transport
- Vesicular Transport
- Endocytosis
- Exocytosis
Membrane Transport
Acts as a selectivly permeable barrier. Substances move across it by passive processes, which depend on the kinetic energy of molecules, and by active processes, which depend on the use of cellular energy (ATP)
The plasma membrane
Factors affecting rate of diffusion through a cell membrane:
- Lipid solubility
- Molecular size
- Cell membrane thickness
- Concentration gradient
- Membrane surface area
- Composition of lipid layer
Passive Membrane Transport: Facilitated Diffusion
Certain lipophobic molecules (e.g., glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which:
- exhibit specificity (selectivity)
- are saturable, i.e., rate is determined by number of carriers or channels
- can be regulated in terms of activity and quantity
Facilitated Diffusion Using Carrier Proteins:
- Transmembrane integral proteins transport specific polar molecules (e.g., sugars and amino acids)
- Binding of substrate causes shape change in carrier
Glucose diffusion
Tonicity
Hypertonic solutions are sometimes infused intravenously into the bloodstream of _________ patients to draw excess water our of the extracellular space and move it into the bloodstream so that it can be eliminated by the kidneys
Hypotonic solutions may be used to _________tissues of patients.
edematous; rehydrate
Active Transport
Operational Model of Ca+ ATPase
Sodium-potassium pump (Na+ -K+ ATPase)
Primary Active Transport
- Located in all plasma membranes
- Involved in primary and secondary active transport of nutrients and ions
- Maintains electrochemical gradients essential for functions of muscle and nerve tissues
Operatinal Model of the Na/K ATPase in the Plasma Membrane
Examples of Secondary Active Transporters
Operational Model of the Na<span>+</span> / Glucose Symporter
Epithelial Transport
A. A cotransport carrier at the luminal border simultaneously transfers glucose against a concentration gradient and Na down a concentration gradient from the lumen into the cell.
B. No energy is directly used by the cotransport carrier to move glucose uphill. Instead, operation of the cotransport carrier is driven by the Na concentration gradient (low Na in ICF compared to lumen) established by the energy-using Na-K pump.
C. The Na-K pump actively transports Na out of the cell at the basolateral border, keeping the ICF Na concentration lower than the luminal concentration.
D. After entering the cell by secondary active transport, glucose is transported down its concentration gradient from the cell into the blood by facilitated diffusion, mediated by a passive glucose carrier at the basal border.
E. The Na-K pump also actively transports K into the cell, maintaining a high intracellular K concentration, but this action has no influence on secondary active transport.
