M1 Ch5: Membranes Flashcards
General functions of membranes
General functions of membranes:
-Keeping all cellular compononents inside the cell
-Selective permeability
-Isolates organelles from rest of cytoplasm so cellular processes occur seperately
-A site of biochemical reactions
-Allows cell to change shape
Model for membranes
Membranes can be viewed with the fluid mosaic model
What does the fluid mosaic model suggest?
The fluid mosaic model suggests that proteins are found within the phospholipid bilayer
Phospholipid structure
Phospholipid structure :
-Hydrophilic phosphate head
-Hydrophobic fatty acid tails
Two structures formed from phospholipids in water
Structures formed from phospholipid in water:
-Micelle (circular)
-Bilayer (linear)
Glycoproteins
Glycoproteins - branching carbohydrate portion of a protein which acts as a recognition site for chemicals eg hormones
Glycolipids
Glycolipids: acts as a recognition site eg for toxins
Cholesterol inside the membrane
Cholesterol = stability and flexibility
Extrinsic protein location
Extrinsic proteins are confined to the inner or outer surface of the membrane
Intrinsic protein location
Intrinsic proteins span the whole phospholipid bilayer
Intrinsic protein
Intrinsic proteins are carrier molecules/channels - they help transport substances that cannot diffuse across the membrane. They also act as receptors or enzymes for catalysing reactions
Channel proteins (intrinsic proteins)
Channel proteins provide a hydrophillic channel that allows for passive movement of polar molecules and ions, and are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic r groups on the outside of the protein
Carrier proteins (intrinsic proteins)
Carrier proteins allow for passive trsnsport and active transport
Why was the fluid mosaic model given that name?
Meaning of the fluid mosaic model for membranes:
-Fluid = because the phospholipid molecules can move around within each layer > means membrane is flexible and can change shape
-Mosaic = because membrane is studded with protein molecules that vary in arrangement
Diffusion
Diffusion is particles moving from high to low concentration
Energy not needed
What is facilitated diffusion
Facilliated diffusion occurs when large molecules diffuse through carrier proteins or channel proteins, as it is faster than moving through the phospholipid bilayer
Carrier proteins
Carrisr proteins move large molecules across membranes
How carrier proteins function
Carrier proteins function:
1. Molecule attaches to carrier protein on membrane
2. Protein changes shape
3. Releases molecules on opposite side of membrane
Channel proteins
Channel proteins form pores in membranes for charged particles to diffuse through
Factors affecting rate of diffusion
Factors that affect the rate of diffusion:
-Concentration gradient = high conc = fast diffusion (as diffusion happens, difference in conc between two sides of membrane decreases until equilibrium is reached and diffusion slows
-Thickness of exchange surface = thinner = shorter distance = more diffusion
-Surface area = large sa = more diffusion
-Microvilli = increase sa = increase diffusion
How does a larger surface area increase rate of diffusion?
Large surface area means that more particles can be exchanged in the same amount of time
What does facilliated diffusion depend on?
Facilitated diffusion depends on:
-Concentration gradient = higher = faster but levels off when equilibrium reached
-More channel proteins increase facilliated diffusion
Aquaporins
Aquaporins are a type of channel protein that allow for facilliated diffusion of water
Osmosis
Osmosis is the movement of water particles from high to low concentration across a semi permeable membrane
What is water potential?
Water potential is the likelihood of water molecules diffusing in and out of a solution
Isotonic
Isotonic is when two solutions have equal water potential
Factors affecting osmosis
Factors that affect osmosis:
-Water potential gradient (higher=faster osmosis) (difference in water potential decreases overtime with osmosis)
-Exchange surface thickness = thinner = faster osmosis
-Surface area of exchange surface = larger sa = faster osmosis
Active transport
Active transport uses energy to move molecules and ions across membranes against a concentration gradient
Active transport and carrier proteins
In active transport, molecule attaches to carrier proteins, protein changes shape, moves molecule across membrane, releases it on other side
Differences between active transport and facilliated diffusion
Differences between active transport and facilliated diffusion:
-Active tranport moves solutes from low to high conc, facilliated diffusion moves from high to low conc
-Active transport requires energy (ATP from respiration undergoes hydrolysis to release ADP and inorganic phosphate ((energy))) wheras diffusion doesnt
Co-transporters
Co-transporters are types of carrier proteins
1- they bind two molecules at a time
2- conc gradient of one molecule is used to move other molecule against its own conc gradient
Factors affecting rate of active transport
Factors that affect active transport:
-Speed of individual carrier proteins = faster = faster AT rate
-Number of carrier proteins = more = faster AT rate
-rate of respiration + ATP availability = respiratiom inhibited means AT cant take place
Roles of cell surface membranes
Roles of cell surface membranes:
-Separates the inside of the cell from the outside
-Acts as a barrier to large or polar molecules
-Controls the entry and exit of substances
-Has antigens on the surface for cell recognition / recognition
Roles of membranes within cells
Roles of memebranes with cells:
-Compartmentalisation - separating out certain organelles to allow for optimum
conditions for certain processes e.g. respiration
-Some membranes such as the cristae in mitochondria will have enzymes embedded in
them for reactions such as respiration to happen on.
-The nuclear membrane has pores on it to allow molecules such as mRNA out to allow
for transcription of proteins to occur.
What is meant by compartmentalisation for membranes?
Compartmentalisation seperates out certain organelles to allow for optimum conditions for certain processes
Why must membranes have fluidity?
Membranes must have fluidity in order for:
-Diffusion of substances across the membrane
-For membranes to fuse
-For cells to move and change shape
How does heat affect membrane fluidity?
As temperature increases, kinetic energy of phospholipids increases, which creates gaps between the bilayer. Molecules pass through the gaps, and the permeability of membrane increases
How do solvents affect membrane fluidity?
Solvents that are less or not polar can disrupt the structure of the bilayer
Fick’s law
Fick’s law:
rate of diffusion is proportional to: (surface area x difference in concentration) divided by (length of diffusion path((membrane thickness)) )
Affect of the polarity of molecules and membranes
Small non-polar molecules diffuse rapidly across membranes, wheras small polar molecules diffuse slowly across the membranes. Charged ions, however, cannot diffuse across the membrane.
Advantages of the membrane folding over
Advantages of a folded membrane:
-Process occurs within/across membranes > process is enzyme controlled
-Large surface area > more enzymes > increases rate of reaction > increased rate of ATP production
Why do solvents like water not disrupt phospholipid membrane structure?
Solvents like water are polar > phospholipids do not dissolve in water
Affect of excess alcohol consumption on the liver membrane
-Alcohol is less/not polar > lipid soluble > disrupts bilayer structure of liver > diffuses through bilayer > reduces transport of materials > prevents normal functioning > potential cell death > liver cannot filter blood > fatal
Source of energy in diffusion
Energy in diffusion comes from the random movement of particles
Water potential (Ψ) unit
Water potential unit (Ψ) is kilopascals - kPa
What has the highest water potential?
Pure water has the highest water potential (0kPa). All solutions have a lower water potential than pure water, and has negative water potential.
Results of increasing amount of solute
Increasing amount of solute lowers the water potential
Why does increasing the amount of solute decrease the water potential?
Increasing the amount of solute decreases the water potential because water molecules bind to solute molecules, reducing the number of water molecules that are free to diffuse
What is solute potential? (Ψs)
The contribution that solutes make to the water potential of a solution is the solute potential (Ψs) and is a negative value
Osmotic concentration
Osmotic concentration is the amount of dissolved solutes in a solution
Name for solution with high osmotic concentration
A solution with high osmotic concentration is concentrated
Name for solution with low osmotic concentration
A solution with low osmotic concentration is dilute
Isotonic
An isotonic solution has the same osmotic concentration as the cytoplasm of the cell, meaning the cell does not change
Hypertonic
A hypertonic solution has higher osmotic concentration than the cytoplasm of the cell, meaning the cell will shrink (Crenated)
Hypotonic
A hypotonic solution has lower osmotic concentration than the inside of the cell, meaning the cell expands (Lysis)
Crenated cells
Crenated cells shrink
Lysis of cells
Lysis of cells cause cells to expand
Endocytosis process
Endocytosis (substances entering cells):
-Membrane folds inwards
-Fuses with itself
-Vesicles form
-Vesicles move through cytoplasm
-Into membranes > via active transport > requires energy (ATP)
-Binds to molecule
-Substance released into cell
How are carrier proteins able to move molecules across the membrane?
Carrier proteins have binding sits for specific chemicals -> when the chemical binds, tertiary structure of protein changes -> this allows chemical across the membrane where chemical is released
Channel proteins
Channel proteins -> central pore -> which is lined with hydrophillic amino acids and contains water -> so hydrophillic channels move across channel across the membrane
-Channel proteins are selective with the molecules
-Some only open in response to a trigger eg change in voltage across membrane
Factors that affect membrane permeability
Factors that affect membrane permeability:
-Temperature
-Solvent concentration
How can you use a beetroot to investigate how different factors affect membrane structure and permeability?
-Beetroot cells contain a dark purple-red pigment
-The higher the permeability of the beetroot cell membrane, the more of this pigment leaks out of the cell
Apparatus used when measuring volume
Volume is recorded using a measuring cylinder
What is a colorimeter?
-A colorimeter is a machine that passes light through a coloured liquid sample and measures how much of that light is absorbed (and therefore gives an indication of how much of the colour is present in the solution)
-A colour filter is used in the light path to ensure that the correct wavelength of light is used to measure the optical density of the specific pigment in the solution (e.g. the beetroot pigment called betalain)
-The colorimeter must be zeroed before each colorimeter tube (called a cuvette) is inserted. This can be done using distilled water in a cuvette
Affect of temperature on membrane permeability
-As tempreature increases, membrane permeability also increases
-This is because as temp increases, the phospholipids within the cell membrane move more because they have more energy
-Increased movement means the phospholipids are not as tightly packed together, increasing the permeability of fthe membrane
-In addition, the volume of water inside the cells expands, putting pressure on the membrane, causing channel and carrier proteins to deform so they can no longer control what enters and leaves the cell
-Temperature also affects the conformation (3D shape) of proteins as at high temperatures the intermolecular forces between amino acids are broken which affects the protein’s specificity and function
Limitations of the beetroot practical in measuring membrane permeability + their solutions
Limitations of the beetroot practical in measuring membrane permeability:
-Cuvettes may differ in thickness (very slightly). A thicker (or scratched) cuvette will absorb slightly more light than a thinner unscratched cuvette
->Solution: use the same cuvette for every reading, or repeat the investigation many times and find a mean
-The beetroot pieces may not be identical in size and shape, meaning some test tubes could contain slightly more beetroot tissue than others
->Solution: cut the discs as accurately as possible using a scalpel and ruler, and repeat each investigation several times to find a mean
-Some parts of beetroot tissue have more pigment in their cells than others
->Solution: conduct several repeats, using different parts of the beetroot and find a mean
