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Questions and Answers
What is the primary function of scramblases in cellular membranes?
Which type of membrane protein is embedded in and anchored to the cell membrane?
How do large polar molecules and ions typically permeate lipid bilayers?
What distinguishes channel proteins from carrier proteins?
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What describes the overall structure of the cell membrane?
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What role does the hydrophobic core of lipid bilayers play in membrane permeability?
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Which factor increases the fluidity of the cell membrane?
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Which statement correctly describes the distribution of ions inside and outside the cell?
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What is a function of flippases in the cell membrane?
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What two processes do carrier proteins utilize to transport substances?
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Which of the following is true about lipid-soluble substances during membrane transport?
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Which statement is true about the lipid bilayer of membranes?
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What effect does higher temperature have on membrane fluidity?
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Which of the following lipids is predominantly found in the outer leaflet of the membrane?
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Which statement about cholesterol in membranes is accurate?
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What role do floppases play in cellular membranes?
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What is the primary function of cellular membranes?
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How does cholesterol affect membrane fluidity?
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Which statement correctly describes the asymmetrical distribution of phospholipids in membranes?
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What mechanisms are employed for the transport of substances across membranes?
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Which best describes primary active transport?
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What vital role do membrane junctions play?
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How do transporters maintain the balance between intracellular and extracellular environments?
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What effect does temperature have on membrane fluidity?
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What characterizes aquaporins in cell membranes?
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How do gated ion channels control ion permeability?
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What limits the rate of facilitated diffusion?
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What is the primary function of the sodium-potassium pump?
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Which of the following describes secondary active transport?
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What type of channels are highly selective for potassium ions?
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What is a key characteristic of non-gated ion channels?
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During the action of the sodium-glucose co-transporter, what happens as Na+ ions move down their concentration gradient?
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Which of the following is not a type of gated ion channel?
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What defines primary active transport?
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What is the primary role of the Na+-Ca2+ counter-transport system?
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How does ouabain affect the Na+/K+ ATPase pump?
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Which type of transport does not require metabolic energy?
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What characterizes secondary active counter-transport?
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Which statement best describes facilitated diffusion?
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What is a key difference between primary active transport and secondary active transport?
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Which of the following transport mechanisms depends on a sodium gradient?
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What is the significance of sodium's role in cellular transport mechanisms?
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Study Notes
Cellular Membranes
- Defines the cell boundaries
- Serves as a barrier between cytoplasm and external environment
- Regulates the movement of substances into and out of the cell
- Contributes to intra- and inter-cellular communication
- Forms membrane junctions linking adjacent cells together
Structure & composition
- Membrane is a 5nm-thick lipid bilayer with associated proteins
- Proteins can be integrated into the bilayer or attached to the inner or outer surface of the membrane
- It follows the fluid-mosaic model
Membrane Fluidity
- Molecules within the membrane can move and change places
- It depends on lipid composition, temperature, and cholesterol
- Factors that increase fluidity include: short tails of phospholipids, more unsaturated bonds, higher temperature, and low cholesterol levels
Membrane Asymmetry
- The membrane is asymmetric
- The outer leaflet is rich in sphingomyelin and phosphatidylcholine
- The inner leaflet is rich in phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol
Role of enzymes in lipid asymmetry
- Flippases: Utilize ATP to move PS and PE from the outer leaflet to the inner leaflet
- Floppases: Utilize ATP to move PC and SM from the inner leaflet to the outer leaflet
- Scramblases: Non-specific ATP-independent enzymes that facilitate the movement of lipids down their concentration gradients
Membrane Proteins
- Integral membrane proteins: Embedded in, and anchored to the cell membrane
- Peripheral membrane proteins: Adhere to integral proteins on either side of the membrane
- Serve as receptors, adhesion molecules, transporters, and enzymes
Permeation of Lipid Bilayers
- Gasses and small molecules permeate lipid bilayers
- Hydrophobic core of lipid bilayers acts as a barrier to the diffusion of large and/or charged molecules
- Membrane permeation of large polar molecules and ions is mediated by transport proteins
Principles of Membrane Transport
- Ion concentrations differ inside and outside the cell
- Distribution of ions inside and outside the cell is controlled by:
- activity of transport proteins
- permeability characteristics of the lipid bilayer
Transport Pathways
- Channel proteins: have open spaces and allow movement of molecules based on charge or size
- Carrier proteins: bind with the substance and carry it to the other side
- Function via two processes:
- Diffusion
- Active transport
Simple Diffusion
- Lipid-soluble substances dissolve in the lipid bilayer and diffuse through, like oxygen
- Lipid-insoluble molecules pass through channels, like water
- Transport is selectively permeable
- Flow is down the concentration gradient
Ion channels
- Integral, membrane-spanning proteins
- Can be gated or non-gated
- Selectivity is based on:
- Size of the channel
- Charge of the lining
- Negatively lined channels allow cations, not anions
- Positively lined channels allow anions, not cations
Gated Ion Channels
- Controls ion permeability
- When the gate is open, ions flow through by passive diffusion
- Voltage gated respond to changes in electrical potentials
- Chemical gated respond to signaling molecules, like cAMP or IP3
- Ligand-gated respond to hormones or neurotransmitters, like ACh
Facilitated Diffusion
- “Carrier-mediated diffusion”
- Occurs down the concentration gradient
- Requires membrane carrier
- Molecule enters the pore within the protein, binds to the receptor
- Protein undergoes a conformational change
- Pore opens to the opposite end, molecule is released
- Examples include: glucose (GLUT) and amino acids
Limiting Rate of Facilitated Diffusion
- Rate of transport is limited by the time of the conformational changes
- Rate of facilitated diffusion reaches a maximum, Vmax
Active Transport
- Maintains ion concentration
- Goes "uphill" against the concentration gradient
- Two types:
- Primary Active Transport: Energy derived from ATP
- Secondary Active Transport: Energy is derived from energy created originally by primary active transport
- Depends on carrier proteins
Sodium-Potassium Pump (Primary)
- 3 Na+ ions are exported out of the cell
- 2 K+ ions are imported into the cell
- 1 ATP molecule is consumed
- Binding of ions stimulates phosphorylation of the pump
- Works against the concentration gradient
- Maintains low Na+ and high K+ inside the cell
- Critical for cell function
Secondary Active Transport
- When ions are transported by primary active transport, a large concentration gradient forms
- This gradient is a storehouse of energy
- Diffusion energy of the ion can pull other substances along with it
- Requires carrier proteins
Sodium-Glucose Co-Transporter
- As Na+ moves down its concentration gradient, it carries along glucose
- Sodium-dependent glucose transporters (SGLT) are found in the epithelial cells lining the small intestine and the proximal tubule of the nephron
Sodium Counter-Transporter (Antiport)
- Counter-transport: transport in the opposite direction of the primary ion
- Na+-Ca2+ counter-transport occurs through almost all cell membranes
- Na+-H+ counter-transport occurs in several tissues, like the kidneys
- Na+ into the tubular cell
- H+ into the tubular lumen
- Can transport a large number of H+ at once
- Key to maintaining body fluids
Transporters Comparison
| Type of Transport | Active or Passive | Carrier-Mediated | Uses Metabolic Energy | Dependent on Na+ Gradient |
|---|---|---|---|---|
| Simple diffusion | Passive | No | No | No |
| Facilitated diffusion | Passive | Yes | No | No |
| Primary Active transport | Active | Yes | Yes, direct | No |
| Secondary Active Transport (Cotransport) | Active | Yes | Yes, indirect | Yes |
| Secondary Active Transport (Counter-transport) | Active | Yes | Yes, indirect | Yes |
Review: Na+/K+ ATPase
- Pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell, using ATP
Review: Na+/K+ ATPase
- If an ATPase-inhibitor (ouabain, glycosides) is used, the pump will be inhibited and the ion gradients will be disrupted
Clinical Relevance
- Ouabain and Digitalis are given to patients with heart disease to make the heart pump more strongly
- These drugs work by partially inhibiting the Na+-K+ pump in the heart
- This inhibition leads to an increase in intracellular Na+
- The increase in Na triggers the Na+-Ca2+ antiporter to import Ca2+, increasing intracellular Ca2+
- The increase in Ca2+ leads to a stronger heart contraction
Review Questions (1st order)
- Membrane fluidity depends on: lipid composition, temperature, and cholesterol
- Cholesterol maintains fluidity of the membrane
- Flippases move phospholipids from the outer to the inner leaflet, Floppases move from the inner to the outer leaflet, and Scramblases move lipids in both directions
- Facilitated diffusion uses carrier molecules
- Ion channels can be gated by voltage, chemicals, or ligands
- The time required for conformational changes limits the rate of diffusion
- Primary active transport uses energy derived from ATP, while secondary active transport uses the energy from a pre-established gradient
2nd Order statements/questions
- Cholesterol plays a regulatory function in antibiotic resistance
- Consider how this is possible by considering the function of cholesterol in membrane fluidity and its interaction with antibiotics
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Description
Explore the key concepts and structures of cellular membranes. This quiz covers membrane composition, fluidity, and asymmetry, providing insights into how membranes serve as barriers and facilitate communication between cells. Test your understanding of the fluid-mosaic model and the factors influencing membrane dynamics.
