What are the four main functions of the plasma membrane?
- Regulates the passage of substances into and out of cells and organelles.
2.Detects chemical messengers at the cell surface.
3.Links adjacent cells through membrane junctions.
4.Anchors cells to the extracellular matrix.
Describe the structure of the phospholipid bilayer.
- Composed of amphipathic phospholipids.
- Hydrophobic fatty acid tails face inward, forming a nonpolar core.
- Hydrophilic polar heads face outward, interacting with the extracellular fluid and cytosol.
What is the role of cholesterol in the cell membrane?
- Limits the ordered packing of fatty acids to maintain membrane fluidity.
- Provides structural integrity and flexibility.
What are the two types of membrane proteins?
- Integral Proteins: Amphipathic and embedded in the lipid bilayer. Includes transmembrane proteins for ion channels and signalling.
- Peripheral Proteins: Non-amphipathic and located on the membrane surface, bound to integral proteins or polar lipid regions.
What is the function of transmembrane proteins?
They facilitate ion/water transport, transmit chemical signals, and anchor extracellular/intracellular protein filaments.
What are the three types of membrane junctions?
Desmosomes, Tight junctions, and Gap junctions.
What does desmosomes do as a membrane junction.
Provide mechanical strength by anchoring adjacent cells, especially in stretch-prone tissues like skin
What do tight junctions do?
Seal adjacent cells to limit molecule movement between cells and support trans-epithelial transport.
What do gap junctions do?
Allow direct communication between cells via connexon protein channels.
How do desmosomes anchor cells?
- Dense plaques on the cytoplasmic surface anchor cadherins (proteins) that extend into the extracellular space and bind with cadherins from adjacent cells.
- Provide mechanical strength in stretch-prone tissues.
What is the function of tight junctions?
- Form barriers to limit the movement of ions and molecules between cells.
- Ensure substances pass through cells via transcellular pathways.
- Maintain specialized membrane surface functions by blocking protein movement between apical and basolateral surfaces.
Describe the structure of gap junctions.
- Formed by connexin proteins that assemble into connexons, creating a channel for direct molecule passage between cells.
- Allow communication of ions and small molecules, critical in cardiac muscle and electric synapses.
What are the main types of transport across the membrane?
Simple Diffusion, Facilitated Transport, and Active Transport
What is simple diffusion?
Movement of molecules from high to low concentration without energy use.
What is Facilitated Transport?
Utilizes specific protein channels or carriers for selective molecule transport.
What is Active Transport ?
Moves molecules against the gradient using ATP.
Why is diffusion across a membrane slower than in water?
The hydrophobic interior of the lipid bilayer limits the movement of polar and charged molecules.
What factors influence ion diffusion across a membrane?
Ion diffusion depends on:
1. Concentration Gradient: Difference in ion concentration across the membrane. 2. Electrical Gradient: Charge difference influencing ion movement.
What are the two main pathways for epithelial transport?
- Transcellular Pathway: Molecules pass through cells via membrane transporters.
- Paracellular Pathway: Molecules pass between cells through junctional gaps.
What is simple diffusion?
The net movement of molecules from a region of higher concentration to a region of lower concentration until equilibrium is reached.
How does distance affect diffusion time?
Diffusion time increases with the square of the distance, making diffusion inefficient over long distances.
How do tight junctions affect molecule passage in epithelial tissues?
Tight junctions force substances to move transcellularly (through cells) rather than between them, allowing for selective and controlled transport.
Why are membranes critical for maintaining homeostasis?
Membranes regulate the internal environment by controlling molecule movement, facilitating communication, and providing structural integrity.
What does it mean for phospholipids to be amphipathic?
Amphipathic molecules have both hydrophobic (nonpolar fatty acid tails) and hydrophilic (polar phosphate head) regions.
