3.2.3 Transport across cell membranes Flashcards
What are the functions of the phospholipid bilayer in the cell membrane?
allow lipid-soluble substances to enter and leave the cell
prevent water-soluble substances entering and leaving the cell
make the membrane flexible and self-sealing.
Why are phospholipids important components
The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.
The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, repelled by the water on both sides.
What causes substances to dissolve?
If they’re polar/charged- attracted to water molecules and vice versa
Phosphate head- neg charge FA tails- no charge
Where are the ester bonds in a phospholipid?
Glycerol and fatty acid
Glycerol and phosphate (phospho ester bond)
What do ester bonds form between?
Acid and Alcohol
Who came up with the fluid mosaic model, and what are the five points?
Singer and Nicolson in 1972
Phospholipid bilayer
Hydrophilic heads and hydrophobic tails.
Mosaic like pattern of embedded proteins
Fluid movement
Intrinsic (inside- runs through mid of layer and forms channel) and extrinsic (outside, on surface) proteins
What are lipids/proteins held together by and what does this cause?
weak hydrophilic/hydrophobic interactions. Therefore molecules are relatively “free” to move about.
What causes the fluidity of the membrane to increase?
Fluidity of the membrane will increase as:
1) the temperature increases.
2) number of double bonds in fatty acids
increase.
3) decreasing amount of cholesterol (are lipid soluble, hphobic organic molecules that can stick together and go between the phospholipid layers, pulling them together)
Function of proteins in membrane
Transport (e.g intrinsic proteins being hollow)
Intercellular joining
Enzymatic activity (sequential enzymes in a metabolic pathway)
Cell-cell recognition
Signal transduction (e.g. hormones)
Attachment to cytoskeleton + extracellular
Cholesterol function and description
They add strength to the membranes. Cholesterol molecules are very hydrophobic and therefore play an
important role in preventing loss of water and dissolved ions from the cell. They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules but without making the membrane as a whole too rigid.
The functions of cholesterol in the membrane are to:
• reduce lateral movement or other molecules including
phospholipids
• make the membrane less fluid at high temperatures
• prevent leakage of water and dissolved ions from the cell.
Permeability of the cell-surface membrane
most molecules do not freely diffuse
across it because many are:
• not soluble in lipids and therefore cannot pass through the
phospholipid layer
• too large to pass through the channels in the membrane
• of the same charge as the charge on the protein channels and so.
even if they are small enough to pass through, they arc repelled
• electrically charged (in other words are polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.
What is the phospholipid bilayer permeable/ impermeable to?
The phospholipid bilayer is permeable to:
1) small uncharged molecules 2)Lipid soluble molecules (lipids)
Phospholipid bilayers are impermeable to:
1) Large molecules 2) All charged molecules and ions.
Give examples of things the bilayer is permeable/impermeable to
Permeable: SMALL NON-POLAR MOLECULES- O2 N2
(less so than small non-polar molecs but still) SMALL UNCHARGED POLAR MOLECULES CO2 Urea Glycerol
Impermeable: LARGE UNCHARGED POLAR MOLECULES Glucose Sucrose
CHARGED MOLCULES AND IONS Na+ K+ Cl- Ca2+ Mg2+ HCO3-
What is required to transport impermeable substances?
They require transport proteins.
Different molecules require different transport proteins.
Therefore, a cell can “select” which molecule to transport by putting particular transport proteins in its membrane
What needs to be transported?
Nutrients e.g. oxygen, glucose, amino acids
Wastes e.g. carbon dioxide, urea
Secretion of cell products e.g. hormones, enzymes
Transport of ions to maintain concentration and electrical gradients
Simple Diffusion
“The net movement of ions or molecules from a region of high concentration to a region of low concentration.”
Net= overall change
Particles move due to kinetic energy
Movement is random
Particles randomly collide with each other and their surroundings.
It is a passive process – requires no energy.
Only small non-polar molecules will diffuse through the lipid
bilayer (i.e. the substances that the membrane is permeable to).
Facilitated Diffusion: Channel Proteins
- To be held in bilayer, has PHOBIC R-groups on outside (like PHOBIC tails)
- To allow water soluble ions/water to pass, has PHILIC (in tertiary structure so folded inside) channel
Facilitated Diffusion: Carrier Proteins
- Shape is specific to molecules it will transport- (usually larger ones while channel is for small ions)
- When a molecule such as glucose that is specific to the protein is present, it binds with the protein. This causes it to change shape in such a way that the molecule is released to the inside of the membrane
Summarise Channel and Carrier Proteins
Channel Protein Hydrophilic channel Transport of ions Specific to the solute Movement in both directions Carrier Protein Complementary receptor site Transport of larger molecules Specific to the solute Movement in both directions
Active Transport- what it is, if energy is required and why, what proteins it affects etc
“Is the transport of molecules or ions across a membrane against their concentration gradient.”
Energy is required to move the substance against its concentration gradient. Its natural tendency is to diffuse in the opposite direction.
In cells with Specific AT carrier proteins, they use energy from ATP to transport substances across a membrane, but energy not used by a carrier protein directly
How does ATP work?
P stands for phosphate -ve charge
-ve phosphate sticks to protein
changes shape
Factors that affect rate of transport (including facilitated diffusion)
Concentration gradient- increase in it increases rate of diffusion- is steeper
Temperature (kinetic energy)- increase in it increases rate- more KE, more collisions FD-BUT up to a point- proteins denature at too high temp
Distance to diffuse (e.g thickness of walls of alveoli/intestines etc) - increase in it decreases rate, further away, longer it’ll take
Surface area of exchange surface- increase in it increases rate
FD- Number of carrier proteins- increase in it increases rate
Fick’s law
(surface area x conc. grad)/ distance to travel
Rate increases as:
surface area increases
concentration gradient increases
distance decreases.
Draw and interpret graphs for time (x) conc. of substance inside cell (y) and conc. of substance outside (x) rate (y)
On Anki and notes
Bulk Transport Across Membrane
Transport of large macromolecules and solids.
Endocytosis = into the cell 1) Phagocytosis = absorbing solid material. e.g. white blood cell, amoeba.
2) Pinocytosis = absorbing dissolved material (e.g. An amoeba “eating”)
Exocytosis (vesicles formed by golgi apparatus to excrete from cell) = out of the cell e.g. releasing a neurotransmitter for cellular communication, secretion of proteins e.g enzymes
Osmosis real def
the diffusion of water from a hypotonic solution to a hypertonic solution across a selectively permeable membrane.”
1) a net movement
2) equal movement in both directions between isotonic solutions.
(Think of osmosis as the diffusion of water.)
Define: Hypotonic, hypertonic and isotonic
hypertonic = greater concentration of solute
hypotonic = lower concentration of solute
isotonic = the same concentration
Why is a selectively permeable membrane needed?
The semipermeable membrane limits the diffusion of solutes in the water, w/o it, would just cause diff. of solutes- water wouldn’t have a conc. grad
What is water potential? What’s the water pot. of water and solutions? What is it measured in? What water pot. does water move to?
Water potential is the measure of the tendency of water molecules to move. It’s given the symbol ψ (Psi)
It’s measured in MPa or KPa.
For pure water ψ = 0 KPa.
A solution will have a negative water potential.
The higher the concentration the more negative the water potential.
Water moves from high to low Ψ (high means lots of water, low means lots of solute)
(Helps in required practicals) Water pot. equation and explain it
Ψ= − [isotonic concentration x P x T(ºK)] = kPa
(P = pressure constant = 8.31) (T = temperature measured in Kelvin = 273 + ºC)
Isotonic conc.- where graph meets x axis, (for potato experiment, conc outside is equal to conc. inside)
REAL osmosis def.
Diffusion of water from a high water potential to a low water potential across a selectively permeable membrane
What are the different effects of water potential on animal and plant cells? Give an example of what causes lysis in animal cells, and what is it called when cell memb moves away from wall
Ideal
Animal cells
Inside and outside same, Volume stays same
Inside Ψ more +ve, Outside -ve Volume= decreases, shrinks
Inside Ψ -ve, Outside +ve Volume: increases (phospholipid bilayer breaks), cell LYSES- e.g ecstasy, causes thirst, drink too much water
Plant Cells
Inside Ψ= Outside Ψ- ideal Volume = same, FLACCID
Inside Ψ more +ve, Outside -ve Volume= decreases, shrinks PLASMOLYSIS (cell memb moves away from wall)
Inside Ψ -ve, Outside +ve Volume: increases, TURGID
Pressure impact on water potential, diff btw plant and animal cells
An increase in pressure will increase the water potential.
It increases the tendency for the molecules to move.
Animal cells burst when internal pressure is increased so it isn’t applicable- phoshpolipid bilayer breaks
But Plant cells have cell walls.
Pressure can increase inside the cell without it bursting.
Explain, in own words, water potential in plant cells formula and state formula (simple, dont overthink)
Water potential = solute potential + pressure potential
ψ = ψ s + ψ p (is 0 in animal cells as they can’t handle pressure)
This can result in positive water potentials.
As water moves in, ↑ψ p causes ↓ψ s bc more water in cell, meaning more pressure, meaning higher water potential.
Co transport and absorption of glucose in the ileum
1 Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in che cell-surface membrane of the epithelial cells.
2 This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.
3 Sodium ions diffuse into the epithe lial cells down thisconcentration gradient through a different type of protein carrier (co-transport protein) in the cell-surface membrane. As the sodium ions diffuse in through this second carrier protein, they carry either amino acid molecules or glucose molecules into the ceII with them.
4 The glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.
1) Na+s AT out of cell by NaK pump- makes conc grad, higher outside cell
2) Na+ diff into cell through co transport protein- take aa or gl with them
3) Gl + aas conc grad, now into blood through facil
