OSSF Membranes & Cytoskeletal Proteins
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What is the basic structure of all cellular membranes?
A lipid bilayer containing proteins and carbohydrates.
What are the main functions of cellular membranes?
Separate the cell from its surroundings, compartmentalize the cell, allow communication, change shape, selectively import/export molecules, and recognize the outside world and other cells.
What are cellular membranes mostly made of?
Phospholipids.
What part of a phospholipid is hydrophilic?
The polar phosphate-containing head.
What part of a phospholipid is hydrophobic?
The fatty acid hydrocarbon tails.
Why do phospholipids form a bilayer?
Hydrophilic heads interact with water while hydrophobic tails avoid water and aggregate together.
What is the default shape of a membrane?
A sphere.
What happens when a membrane is torn?
It is self-sealing.
What maintains membrane shapes other than a sphere?
The cellular cytoskeleton.
How do membranes behave?
Like a two-dimensional fluid.
What movements can phospholipids make within the membrane?
They can spin, flex, and move laterally.
Can phospholipids normally flip between membrane leaflets?
Almost never.
Why is lateral movement of membrane lipids and proteins important?
It allows them to diffuse, interact, and become distributed between daughter cells after mitosis.
What happens to membrane fluidity when unsaturated hydrocarbon tails increase?
Membrane fluidity increases.
Why do unsaturated hydrocarbon tails increase fluidity?
Double bonds create kinks that prevent the tails from packing tightly.
What happens when hydrocarbon tails are more saturated and straight?
The membrane becomes denser, less fluid, and less permeable.
What does cholesterol do to the membrane?
It stiffens the bilayer, making it less flexible and less permeable to water-soluble molecules.
Why does cholesterol stiffen the membrane?
It is short and rigid and fills spaces between neighboring phospholipids.
What does cholesterol do at low temperatures?
It protects against lipid crystallization.
What are lipid rafts?
Cholesterol-rich regions containing assemblies of proteins and lipids that float within the membrane.
What are functions of lipid rafts?
Protein trafficking and signal transduction.
How can cells limit diffusion of membrane proteins and lipids?
By lipid rafts, occluding junctions, cytoskeletal anchors, and connections to extracellular matrix.
Where are new phospholipids produced?
In the smooth endoplasmic reticulum.
Where are phospholipid-synthesizing enzymes located?
On the cytosolic surface of the smooth ER.
What does scramblase do?
It randomly moves phospholipids from one membrane leaflet to the other.
What is the result of scramblase activity in the ER?
The two membrane leaflets become the same.
How does membrane move from the ER to the Golgi?
In vesicles.
What does the Golgi apparatus do to membrane composition?
It creates distinct inside and outside faces and produces membrane asymmetry.
What do flippases and floppases do?
They use ATP to move phospholipids between membrane leaflets.
Why is membrane asymmetry important?
It creates functional differences between the two membrane leaflets.
Where is phosphatidylserine normally located?
On the cytosolic side of the membrane.
What is the significance of negatively charged phosphatidylserine?
It contributes to the membrane potential.
Where do glycolipids and glycoproteins face in the plasma membrane?
The cell exterior.
What do glycolipids and glycoproteins form at the cell surface?
The protective glycocalyx.
What is phosphatidylinositol used for?
Signaling pathways.
What are important functions of the Golgi apparatus?
Creating heterogeneous proteins, cell signaling, cell-cell communication, cell protection, membrane asymmetry, and directing vesicles to final destinations.
How does the ER differ from the Golgi?
The ER produces symmetrical membranes, while the Golgi creates distinct membrane composition and asymmetry.
Which molecules rapidly cross membranes by simple diffusion?
Small nonpolar molecules.
Do steroid metabolites cross membranes easily?
Yes.
How do uncharged polar molecules cross membranes?
They slowly diffuse across membranes.
Do small or large uncharged polar molecules cross more rapidly?
Small molecules.
What protein facilitates rapid water movement across membranes?
Aquaporin.
How does water move through aquaporins?
By osmosis.
Can ions freely cross the lipid bilayer?
No. Membranes are impermeable to charged molecules.
What are peripheral membrane proteins?
Proteins associated with the membrane without being embedded through the lipid bilayer.
What are integral membrane proteins?
Proteins associated with or embedded in the membrane.
What are functions of membrane proteins?
Transport, receptors, enzymes, linking the inside and outside of the cell, and anchoring the membrane to cytoskeleton or extracellular matrix.
What are the two general types of membrane transport proteins?
Transporters and channels.
How does a transporter move a solute?
It binds the molecule and undergoes a conformational change to transfer it across the membrane.
How does a channel move solutes?
It forms an aqueous pore through which solutes rapidly cross.
What is passive transport?
Movement down an electrochemical gradient without energy expenditure.
What is active transport?
Movement against an analyte's electrochemical gradient requiring energy directly or indirectly.
What is simple diffusion?
Passive movement of a solute across the membrane without a transporter or channel.
What is facilitated diffusion?
Passive movement through a channel or transporter down an electrochemical gradient.
What limits facilitated diffusion?
The availability of carriers.
What is diffusion?
Movement of solute from high concentration to low concentration.
What is osmosis?
Movement of water from low solute concentration toward high solute concentration.
What does “salt sucks” describe?
Osmosis.
What is an electrochemical gradient?
The combination of a concentration gradient and an electrical gradient.
What two gradients affect ion movement?
Concentration and electrical gradients.
What is membrane potential?
The voltage difference across a membrane.
What is the charge of the cell interior relative to the outside?
The cell interior is more negative.
Why is the cell interior more negative?
Because of negatively charged proteins, negatively charged phospholipids, and slow K+ leakage out of the cell.
Where is K+ concentrated?
Inside the cell.
Where is Na+ concentrated?
Outside the cell.
Which direction does the Na+ electrochemical gradient favor?
Into the cell.
What is primary active transport?
Transport in which ATP directly drives movement of a solute against its electrochemical gradient.
What are examples of primary active transport pumps?
Na+/K+ pump, H+ pump, and Ca2+ pump.
Why is active transport important?
It allows the cell to regulate cytoplasmic solute concentrations and can create electrical differences used to do work.
What is secondary active transport?
Movement of one analyte using the electrochemical gradient of another analyte.
What is a symporter?
A transporter that moves two or more solutes in the same direction.
What is an antiporter?
A transporter that moves two or more solutes in opposite directions.
What is a uniporter?
A transporter that moves one solute in one direction and is not coupled.
What type of transporter is the Na+/K+ pump?
A primary active transporter and an antiporter.
What provides energy for secondary active transport?
The electrochemical gradient of another solute.
What is an example of secondary active transport?
Using the Na+ electrochemical gradient to drive glucose transport.
What happens when a membrane transporter is defective in cystinuria?
Cystine is not properly reabsorbed by the kidney and accumulates in urine.
What can cystine accumulation cause?
Cystine crystals and bladder stones.
Why can channels transport solutes rapidly?
Multiple water molecules or ions can move through simultaneously.
What are gap junctions?
Channels involved in communication between cells.
What are aquaporins?
Channels that allow water movement.
What are gated channels?
Channels that open in response to specific signals.
What is a K+ leak channel?
A generally open channel that is important for generating the normal resting membrane electric potential.
What is a voltage-gated ion channel?
A channel that opens when a specific membrane potential is reached.
What is a ligand-gated ion channel?
A channel that opens when a specific ligand binds.
What is a mechanically gated ion channel?
A channel that opens when mechanical force is applied.
What happens in endocytosis?
The membrane folds to bring material into the cell.
What happens in exocytosis?
Membrane folding/fusion is involved in moving material out of the cell.
What are the three primary cytoskeletal filament types?
Actin microfilaments, intermediate filaments, and microtubules.
What is the cytoskeleton?
A dynamic internal skeleton made of protein filaments that supports and organizes the cell.
Which cytoskeletal component is thinnest?
Actin microfilaments.
What are actin filaments made of?
Globular actin monomers.
What does each actin monomer contain?
An ATP-binding site.
What is the diameter of actin microfilaments?
6–8 nm.
What is the structure of actin filaments?
Two strings of beads twisted together.
Are actin filaments polar?
Yes.
What are the two ends of an actin filament?
Plus and minus ends.
Which actin end grows faster?
The plus end.