Cells 2 Flashcards
Plasma Membrane composition:LIpids: What are phospholipids?
They make a lipid bilayer, the hydrophobic tails are pointing inwards and the hydrophilic heads outwards. A hydrophobic interior is created. The entire membrane is connected by bonds and forces and thus, it is difficult to break.
CHOLESTEROL (as part of the animal cell membrane only)
It is a lipid which does not contain phosphate group, only a small polar head. It is therefore hydrophobic and almost entirely embedded in the bilayer. It decreases the fluidity and the permeability of the membrane
Proteins:
They may be integral (totally or partially perforating the membrane) or peripheral (connected to the membrane’s surface). Some integral proteins contain a channel in their center to help large or charged molecules pass through. Some are bonded to carbohydrates(sugars) called glycoproteins.
FUNCTIONS OF MEMBRANE PROTEINS
• Hormone binding sites (i.e. insulin)
• Cell adhesion and cell recognition (through glycoproteins) between cells of
the same tissue (i.e cadherin)
• Channels for passive transport (facilitated diffusion-i.e. acetylcholine receptor
in nervous synapses used both as a neurotransmitter receptor AND a channel
for sodium diffusion)
• Pumps for active transport (i.e. calcium pump)
• Enzymes to break ATP and release energy for active transport
The Singer and Nicolson model of the membrane
The membrane structure is described as the “fluid mosaic model”. This model describes most of the properties of the membrane but not all.
• Fluid: The phospholipids are in a “fluid” state within the bilayer, they do not have a fixed position in the membrane; they can move sideways. This property makes them able to form vesicles, very important during endocytosis and exocytosis.
• Mosaic: The embedded proteins in the bilayer give the impression of a mosaic.
Passive Transport
Passive transport happens when particles (small molecules or ions) move through the membrane down their concentration gradient, that is, from an area of high to an area of low concentration without the use of energy.
Diffusion
Small molecules, such as oxygen and carbon dioxide, diffuse directly through
the bilayer from a region of high to a region of low concentration without the use of energy. This diffusion is a result of random motion of molecules close to the membran
Facilitated Diffusion
Large molecules or charged molecules (i.e ions) cannot move through the
hydrophobic bilayer but need to use a channel protein instead.
Osmosis
Osmosis is a special form of diffusion which involves the movement of
solvent molecules. The solvent is biological systems is water. Water passes from a region of low solute concentration (dilute solution called hypotonic) to a region of high solute concentration (concentrated solution called hypertonic). When two regions contain equal concentrations this solution is called iso
Active Transport
Active transport happens when particles (large molecules or ions) move through the membrane against their concentration gradient, that is, from an area of low to an area of high concentration with the use of energy (in the form of ATP).
Pumps
The process occurs through carrier proteins found on the membrane. These
accept the molecule(s) and by a change in their structure they convey it to the other side of the membrane. Usually these proteins are working as enzymes (ATPase) breaking the ATP →ADP + Pi + release energy. The best example of the pump action is the sodium-potassium pump which transfers Na+ outside the membrane (removes them from the cell) and K+ ions inside (accumulates them in the cell).
Endocytosis
The active uptake of material into the cell in bulk is called endocytosis
Exocytosis
the release of either useful or waste material out of the cell is called exocytosis.
Endocytosis Example 1: Pagocytosis
Phagocytosis is the uptake of large SOLID particles inside the cell (cell
“eating”). This happens through the formation of an invagination of the membrane which encloses the particle. A vesicle is created, which buds off the membrane and transfers the particle inside the cytoplasm AWAY from the membrane.
Example of exocytosis 2:Pinocytosis
inocytosis is the uptake of LIQUID particles inside the cell (cell “drinking”).
This happens similarly through the formation of vesicles which are carried away from the plasma membrane.
