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Asignatura: biologia, Profesor: INDIFERENTE INDIFERENTE, Carrera: Ingeniero Químico, Universidad: UPV-EHU
Tipo: Apuntes
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5 The Dynamic Cell Membrane
5.1 What Is the Structure of a Biological Membrane?
The general structure of membranes is kknow as the th flfl uid mosaic modelid i d l.
The phospholipid bilayer is like a “lake” in which a variety of proteins “float.”
Figure 5.1 The Fluid Mosaic Model
5.1 What Is the Structure of a Biological Membrane?
Artificial bilayers can be made in the laboratory.
Lipids maintain a bilayer organization spontaneously—helps membranes fuse during phagocytosis, vesicle formation, etc.
5.1 What Is the Structure of a Biological Membrane?
Membranes may vary in lipid composition
PhPhospholipids vary—fatty acid chain h li id f tt id h i length, degree of saturation, phosphate groups
Membranes may be up to 25 percent cholesterol
5.1 What Is the Structure of a Biological Membrane?
Phospholipid bilayer is flexible, and the interior is fluid, allowing lateral movement of molecules.
Fluidity depends on temperature and lipid composition.
5.1 What Is the Structure of a Biological Membrane?
Membranes contain proteins, the number of proteins varies with cell function
Some membrane proteins extend across the lipid bilayer—with hydrophobic and hydrophilic regions or domains.
5.1 What Is the Structure of a Biological Membrane?
Two types of membrane proteins:
Figure 5.4 Interactions of Integral Membrane Proteins
5.1 What Is the Structure of a Biological Membrane?
Transmembrane proteins may have different domains on either side of the membrane.
The two sides of the membrane can have very different properties.
5.1 What Is the Structure of a Biological Membrane?
Some membrane proteins can move freely within the bilayer, while some are anchored to a specific region.
Some can be anchored by cytoskeleton elements, or lipid rafts —lipids in semisolid state.
5.1 What Is the Structure of a Biological Membrane?
Membranes have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules.
Glycolipids
Glycoproteins
Figure 5.1 The Fluid Mosaic Model
5.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cells arrange themselves in groups by cell recognition and cell adhesion.
These processes can be studied in sponge cells—the cells are easily separated and will come back together again.g
Figure 5.6 Cell Recognition and Adhesion (A)
5.2 How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cell junctions are specialized structures that hold cells together:
Figure 5.7 Junctions Link Animal Cells Together (A)
Tight junctions help ensure directional movement of materials.
Figure 5.7 Junctions Link Animal Cells Together (B)
Desmosomes are like “spot welds”
Figure 5.7 Junctions Link Animal Cells Together (C)
Gap junctions allow communication
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion : the process of random movement toward equilibrium
Equilibrium —particles continue to move, but there is no net change in distribution
Figure 5.8 Diffusion Leads to Uniform Distribution of Solutes
5.3 What Are the Passive Processes of Membrane Transport?
Net movement is directional until equilibrium is reached.
Diffusion is net movement from regions of greater concentration to regions of lesser concentration.
5.3 What Are the Passive Processes of Membrane Transport?
Diffusion rate depends on:
5.3 What Are the Passive Processes of Membrane Transport?
Osmosis : the diffusion of water
Osmosis depends on the number of solute particles present, not the type of particles.
Figure 5.9 Osmosis Can Modify the Shapes of Cells
5.3 What Are the Passive Processes of Membrane Transport?
If two solutions are separated by a membrane that allows water, but not solutes to pass through, water will diffuse from the region of higher water concentration (lower solute concentration) to the region of lower water concentration (higher solutewater concentration (higher solute concentration).
5.3 What Are the Passive Processes of Membrane Transport?
Isotonic solution: equal solute concentration (and equal water concentration)
Hypertonic solution: higher solute concentration
Hypotonic solution: lower solute concentration
