What are some of the main functions of the plasma membrane?
It separates the living cell from its surroundings
It is involved in cell recognition and messaging
It controls cell’s internal environment (selective permeability)
What are functions common to membranes in general?
They define compartments (organelles) inside the cell
They are involved in metabolic processes
What did Langmuir discover ?
That the fatty acids in phospholipids are oriented vertically with polar group in water
What did Gorter and Grendel find in 1925 ?
That red blood cells were covered by a bilayer of phospholipids
What did Davson and Danielli propose in 1935?
The sandwich model: phospholipid bilayer lies between two layers of globular proteins
What model did Singer and Nicolson propose in 1972?
The fluid mosaic model: the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
How do phospholipids move within the bilayer?
They move laterally (most of the time)
Flip-flop (rarely)
What has an influence on membrane fluidity?
Saturation of fatty acids
Presence of cholesterol (in animals)
How do fatty acids influence membrane fluidity?
Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
How does cholesterol influence membrane fluidity?
It reduces phospholipids movement & membrane fluidity at moderately high temperatures
Maintains membrane fluidity at low temperatures by preventing tight packing
What determine most of the membrane’s specific functions ?
Proteins
What is the role of carbohydrates on the plasma membrane ?
They are involved in cellular recognition
Why is the plasma membrane selectively permeable ?
In order to regulate molecular traffic
Do polar or non-polar molecules cross the lipid bilayer the most easily and why ?
Non-polar (hydrophobic) molecules
They can dissolve in the lipid bilayer
What are the three transport mechanisms that allow to control molecular traffic ?
Passive transport (No ATP needed)
Active transport
Bulk transport
What are the two types of passive transport?
Simple diffusion
Facilitated diffusion
What is diffusion ?
The tendency for molecules to spread out evenly into the available space
Molecules diffuse DOWN ther concentration gradient
What is osmosis ?
The diffusion of water across a selectively permeable membrane ( from the region of lower solute concentration to the region of higher solute concentration)
What is tonicity ?
The ability of a solution to cause a cell to gain or lose water
What is an isotonic solution ?
The solute concentration in solution = to that inside the cell (no net water movement accross the plasma membrane)
What is an hypertonic solution ?
The solute concentration in solution > than that inside cell (cell loses water)
What is an hypotonic solution ?
Solute concentration in solution < than that inside cell (cell gains water)
What is osmolarity ?
The total concentration of all solutes
In what units is osmolarity measured ?
In Osmoles (Osm)
What are examples of solutions isosmotic to the humain body ?
Saline solution (0.9% NaCl) 5% glucose solution
How is called the control of water balance in an organism ?
Osmoregulation
What happens to a plant in a hypotonic solution ?
The cell swells until the wall opposes uptake
The cell is now turgid (firm)
What happens to a plant in a isotonic solution ?
No net movement of water into the cell
The cell becomes flaccid (limp), and the plant may wilt
What happens to a plant in a hypertonic solution ?
Plant cells lose water
Membrane pulls away from the wall (plasmolysis)
What happens during facilitated diffusion ?
Transport proteins speed up the passive movement of molecules across the plasma membrane
What to transport proteins might be used ?
Carrier proteins (permease) Channel proteins
What does a channel protein ?
It provides corridors that allow a specific molecule or ion to cross the membrane
What does a carrier protein ?
It undergoes a subtle change in shape that translocates the solute-binding site across the membrane
Why is facilitated diffusion considered passive ?
Because the solute moves down its concentration gradient
What does active transport ?
It moves substances against their concentration gradient
What are characteristics of active transport ?
it is performed by specific integral membrane proteins
It requires energy, usually in the form of ATP
What is an example of active transport ?
The sodium-potassium pump (Na+/K+-ATPase)
What do ion pumps create ?
They create a membrane potential (a voltage difference across a membrane)
How is the mmbrane potential created ?
By differences in the distribution of positive and negative ions
How are called transport proteins that generate voltage across a membrane (such as ion pumps) ?
electrogenic pumps
What is the main electrogenic pump of animal cells ?
The Na - K pump
The main electrogenic pump of plants, fungi, and bacteria ?
The proton pump
How can the membrane potential (voltage across membrane) be used
This energy can be used for cellular work ( ATP synthesis and cotransport )
When does cotransport occur ?
When the active transport of a solute indirectly drives transport of another solute
To what end do plants commonly use the gradient of hydrogen ions generated by proton pumps ?
To drive active transport of nutrients into the cell
How do large molecules, such as polysaccharides and proteins, cross the membrane ?
In bulk via vesicules
What are the two types of bulk transport ?
Endocytosis
Exocytosis
How does endocytosis work ?
THe cell takes in macromolecules by forming vesicles from the plasma membrane
What are the three types of endocytosis ?
Phagocytosis : the cell engulfs particle by forming vacuole
Pinocytosis : cell takes in extracellular fluid in vesicle
Receptor-mediated endocytosis: cell acquires bulk quantities of specific substance (ligands) that may not be highly concentrated in extracellular fluid
How does exocytosis work ?
The transport vesicles migrate to the membrane, fuse with it, and release their contents
How do many secretory cells use to export their products ?
Exocytosis