Chapter 5 Plasma Membranes Flashcards
Compartmentalisation
The formation of separate membrane-bound areas in a cell
It is vital because if you contain reactions in specific parts of cells then it allows the specific conditions for reactions to be achieved.
Required for cellular reactions and protects vital cell components
Membrane structure
The cell surface membrane is known as the plasma membrane.
Formed from a phospholipid bilayer. The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane.
Has the fatty acid tails of the phospholipids in between forming a hydrophobic core.
The outer surface of the hydrophilic phosphate heads can interact with water.
Cell membrane theory
Membranes were first seen in 1950s when electron microscopes were made. It showed that they were two black parrallel lines - supporting theorys of lipid bilayer.
Then the fluid-mosaic model was formed as the phospholipids are free to move within the layer, giving it flexibility.
A closer look at cell membrane components
Plasma membranes contain various proteins and lipids. The quanitity and type are dependant on the type of cell.
Glycoprotein - branchin carbohydrate portion of a protein which acts as a recognition site for chemicals. Used for cell signallinge.g. hormones
Glycolipid - acts as a recognition site, e.g. for cholera toxins. Used for immune system
Cholesterol - for stability/flexibility
extrinsic proteins - protein molecule partly embedded. or lying on the surface.
hydrophobic tails of phospholipid molecules.
intrinsic protein - protein molecul spanning the phospholipid bilayer
hydrophilic heads of phospholipds molecules point outwards.`
Intrinsic Proteins
are embedded through both layers of a membrane. Thye have amino acids with hydrophobic r-groups on their external surfaces, which interact withe hydrophobic core of the membrane keeping them in place.
Channel Proteins - provide a hydrophilic channel that allows passive movement of polar molecules and ions down a conc. gradient through membranes. They are held in place by interactions between the hydrophobic core of membrane and the hydrophic R - groups on the outside.
Carrier proteins - important role in passive transport and active transport
Glycoproteins
Are intrinsic proteins - embeedded in cell-surface membrane with attached chains of varying length
Play a role in cell adhesion and act as receptors for chemical signals.
When a chemical binds to the receptor it will cause cell signalling.
Some drugs act by binding to cell receptors
Glycolipids
similar to glycoproteins.
They are lipids with attached carbohydrate sugar chains.
These molecules are cell molecules or antigens and can be recognised by the cells of the immune system for themselves or others.
Extrinsic proteins
Present on only one side of the bilayer.
Hydrophilic R-groups on their outer surface and interact with the polar heads of the phospholipids or intrinsic proteins.
Some move between layers.
Cholesterol
is a lipid with a hydrophilic end and hydrophobic end, like a phospholipid.
regulates fluidity of membranes.
Are positioned between phospholipids in a membrane layer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails
Adds stability to the membranes without allowing them to become to solid by preventing them from grouping together and crystalising.
Factors affecting membrane structure: Temperature
When temperature is increased the phospholipid will have more kinetic energy and will move more. This makes a membrane more fluid and it begins to lose its structure. If the temperature continues to increase cells will eventually break down completely.
This loss in structure increases permeability allowing particles to cross it.
Carrier and channel proteins will denature in the membrane at higher temperatures. So membrane permeability will be affected.
Factors affecting membrane structure: Solvents
If a solvent is polar it makes the hydrophobic tails face away from the water and create a hydrophobic core with hydrophilic heads. However if there was an organic solvent which are less polar than water or a non-polar solvent then it will create disorientation in the membrane creating more gaps etc. and increasing the permeability for it. This is why alcohol is used to tried cuts since it breaks down the membrane of the bacteria and makes it more fluid and permeable allowing the bacteria to me destroyed.
Diffusion
is the net, or overall, movement of particles from an area of high concentration to an area of low concentration.
Passive transport since it doesn’t require energy.
Factors affecting rate of diffusion
Temperature - the higher the temp the higher rate of diffusion since particles have more kinetic energy.
Concentration difference - the greater the difference in concentration between two regions the faster the rate of diffusion because the overall movement from the higher concentration to lower concentration will be larger.
With a barrier :
Surface area - the larger the area of exchange surface the higher the rate of diffusion.
thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion.
Diffusion accross membranes
involves particles crossing the phospholipid bilaye. It can only happen if the membrane is permeable to the particles - non-polar molecules such as oxygen (O2) diffuse through freely down conc. gradient.
The hydrophobic interior repels ions, polar molecule , water.
Small polar molecules move through at a very slow rate so they are partially permeable.
Facilitated diffusion
ions and polar molecules that are large can pass through channel proteins.
Protein channels are selectively permeable.
Can also involve carrier proteins which change shape when a specific molecule binds.
In facilitated diffusion the movement of the molecules is down the conc. gradient and doesn’t require energy.
Rate of facilitated diffusion is dependant on - temp, conc grad, SA, thickness, number of channel protein.
Active Transport
is the movement of moleucules or ions into or out of a cell from a region of lower concentration to a region of higher concentration. The process requires energy and carrier proteins. Energy is needed. and is supplied as ATP.
How carrier proteins do Active transport
They act as pumps
The molecule or ion binds to receptor in the channel or the carrier protein on the outside and ATP binds on the inside and hydrolysed into a phosphate group and ADP. phosphate group causes the carrier protein to change shape and open up and release the molecule and ion.
Phosphate molecule is released and joins with the ADP to form ATP. and the carrier protein returns to its original shape.
This is a selective process with different carrier proteins.
Bulk Transport general
Large molecules like bacterial cells, enzymes and hormones are too large to move through channel proteins so tehy are moved into and out of cells by bulk transport.
Endocytosis
phagocytosis - for solids
pinocytosis - for liquids
process is the same for both.
The cell surface membrane first invaginates when it comes into contact with the material. The membrane enfolds the material until the membrane fuses forming a vesicle. The vesicle pinches off and moves into the cytoplasm to transfer the material for further processing with the cell. For example, vesicles containing bacteria are moved towards lysosomes, where bacteria are digested by enzymes.
Exocytosis
is the reverse of endocytosis. Vesicles usually formed by the golgi apparatus, move towards and fuse with the cell surface membrane. The contents of the vesicle are then release outside the cell.
How is it possible for vesicles to move around
Energy is required for vesicle to move along the cytoskeleton in the form of ATP.
Osmosis
the diffusion of water across a partially permeable membrane. As with all diffusion it is a passive process with no energy.
Water potential
is the pressure exerted by water molecules as they colide witha a membrane or container. They are measured in pascals(pa) or kilopascals (kPa).
Symbols is the greek letter psi.
Water has a water potential of 0
The more concentrated a solution is the more negative its water potential is.
When there is a conc gradient across a membrane the water will diffuse to balance the water potentials.
hydrostatic pressure
the pressure that is created when water diffuses into cells. At cellular level this pressure is relatively large and potentially damaging.
Osmosis in Animal cells
If an animal cell is placed in a solution with higher water potential than that of the cytoplasm, water will move into the cell by osmosis, increasing the hydrostatic pressure inside the cell. The cells surface membrane cannot withstand the increased pressure. It will break and the cell will buurst - cytolysis.
If an animal cell is placed in a solution with lower water potential then it will lose water to the solution by osmosis. This will cause reduction in the volume of the cell-surface membrane to crenation.
To prevent cytolysis and crenation, multicellular organisms have control mechanism to keep cells in isotnic environments.
Osmosis in plant cells
plants are unable to control the water potential of the fluid around them, for example, the roots are usually surrounded by almost pure water.
Plant cells have a cellulose cell wall surrounding the cell-surface membrane. When water enters by osmosis the increased hydrostatic pressure pushes the membrane against the rigid walls. this pressure is called turgor. As the pressure increases it resist against the entry of further water and the cell is said to be turgid.
When plants are in a solution with a lower water potential than their own, water is lost from the cells by osmosi. This causes volume to decrease causign the membrane to pull away from the cell wall. - here the cell is plasmolysed.
