The structure of biological membranes makes them fluid and dynamic.Membranes control the composition of cells by active and passive transport.
- Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
- Membrane proteins are diverse in terms of structure, position in the membrane and function.
- Cholesterol is a component of animal cell membranes.
- Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.
- The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.
Membrane Parts
Phospholipid Bilayer
-It is in movement, and flexible
-Have kicks that keep the molecules form packing together
-Hydrophilic head (likes water) with a hydrophobic tail (dislikes water)
*Hydrophobic tail repels water from the center layer
*Hydrophilic head attracts water and form outer layer
-It is in movement, and flexible
-Have kicks that keep the molecules form packing together
-Hydrophilic head (likes water) with a hydrophobic tail (dislikes water)
*Hydrophobic tail repels water from the center layer
*Hydrophilic head attracts water and form outer layer
Importance of the Membrane
-Membranes define compartments
-Membrane proteins control what goes in and out of the cell
-Membranes are fluid
Davson Danielli Membrane Model (Protein Sandwich)
-He proposed layers of protein were attacked to the outer skirts of the phospholipid layer
-His evidence was that in he electron microscopes phospholipid layers appeared light and the protein layer appeared dark surrounding it.
Singer Nicolson (Fluid Mosaic Model)
-He froze images of the center of the membranes and due to cracks in the image he concluded that membranes were fluid and had movement
-Membranes were shown to be vary in shape and size due to its movement
-He concluded that proteins weren’t in the outside of the phospholipid layer, but indented within the membrane
-Membranes define compartments
-Membrane proteins control what goes in and out of the cell
-Membranes are fluid
Davson Danielli Membrane Model (Protein Sandwich)
-He proposed layers of protein were attacked to the outer skirts of the phospholipid layer
-His evidence was that in he electron microscopes phospholipid layers appeared light and the protein layer appeared dark surrounding it.
Singer Nicolson (Fluid Mosaic Model)
-He froze images of the center of the membranes and due to cracks in the image he concluded that membranes were fluid and had movement
-Membranes were shown to be vary in shape and size due to its movement
-He concluded that proteins weren’t in the outside of the phospholipid layer, but indented within the membrane
Cholesterol: reduces membrane fluidity by reducing phospholipid movement at moderate temperatures but it also hinders solidification at low temperatures.
Function of Membrane Proteins
-Hormone binding sites: site exposed on the outside of the membrane, which allows hormones to bind, then a signal is transmitted to the center of the cell
-Enzymes: catalyze reactions inside or outside of the cell depending on the active site
-Electron carries: they are arranged in chains inside the membrane so the electrons can pass form one carrier to another
-Channels for passive transport: passages inside the center of membrane proteins, which allow a specific substance to pass through
-Pumps for active transport: the pumps release ATP in order for specific substances to pass through the membrane
Functions of:
-Receptor proteins: act as receptors and recognize other cells
-Integral proteins: carry molecules through the plasma membrane
-Cholesterol: affects membrane fluidity at different temperatures
Active Transport
-Uses energy, in the form of ATP, to move molecules against a concentration gradient, by using membrane protein pumps
- Substances that are transported are sodium and potassium.
Function of Membrane Proteins
-Hormone binding sites: site exposed on the outside of the membrane, which allows hormones to bind, then a signal is transmitted to the center of the cell
-Enzymes: catalyze reactions inside or outside of the cell depending on the active site
-Electron carries: they are arranged in chains inside the membrane so the electrons can pass form one carrier to another
-Channels for passive transport: passages inside the center of membrane proteins, which allow a specific substance to pass through
-Pumps for active transport: the pumps release ATP in order for specific substances to pass through the membrane
Functions of:
-Receptor proteins: act as receptors and recognize other cells
-Integral proteins: carry molecules through the plasma membrane
-Cholesterol: affects membrane fluidity at different temperatures
Active Transport
-Uses energy, in the form of ATP, to move molecules against a concentration gradient, by using membrane protein pumps
- Substances that are transported are sodium and potassium.
Diffusion and Osmosis: Passive movement of particles from a region of high concentration to a region of low concentration (down a concentration gradient)
Endocytosis and Exocytosis
Exocytosis: when the vesicle fuses with the plasma membrane, the contents are then inside of the membrane taken the material to the outer side of the membrane.
Endocytosis: A vesicle is formed by pinching off a small piece of membrane, the material that it contains is from the outside of the process so in this method the material is taken inside the cell.
Endocytosis: A vesicle is formed by pinching off a small piece of membrane, the material that it contains is from the outside of the process so in this method the material is taken inside the cell.
The explanation of the structure of the plasma membrane has changed over the years as new evidence and ways of analysis have come to light. Under what circumstances is it important to learn about theories that were later discredited?
Although the study of membranes have experienced evolution through the past years, it is important for students to understand all previous concepts in order to understand the development within the idea. For example, although the Davson-Danielli Model was wrong about the fact that protein surrounded the membrane, his statements about the Phospholipid Bilayer were correct. Students also need to understand the mistakes made in the past in order to understand the true components of the membrane.
Although the study of membranes have experienced evolution through the past years, it is important for students to understand all previous concepts in order to understand the development within the idea. For example, although the Davson-Danielli Model was wrong about the fact that protein surrounded the membrane, his statements about the Phospholipid Bilayer were correct. Students also need to understand the mistakes made in the past in order to understand the true components of the membrane.