1.3 Cell Membranes & Transport Flashcards
Explain the arrangement of the phospholipid bilayer
- hydrophilic heads interact w water in cytoplasm and with water around the cell
- hydrophobic tails point inward
- phospholipid component allows lipid soluble molecules to move across but not water soluble molecules
Extrinsic proteins
- on the surface of the bilayer
- provide structural support and do cell-cell recognition as well as hormone recognition
Intrinsic protein
- extend both layers of phospholipid
- include transport proteins (channels and carriers) for non lipid soluble molecules
Permeability of membrane for small molecules, lipid soluble and water soluble
- small: move between phospholipid molecules through diffusion
- lipid soluble: dissolve and diffuse across easily
- water soluble: cannot ready diffuse so use intrinsic protein molecules
- non-polar dissolve in hydrophobic layer and easily diffuse
Why is it called the fluid mosaic model?
- individual phospholipid molecules can move within a layer to one another (fluid)
-proteins embedded in the bilayer vary in shape and size and in their distribution among phospholipids (mosaic)
What’s the carbohydrate layer around an animal cell called?
- glycocalyx formed from glycoproteins and glycolipids (carbs attach to proteins and phospholipids)
- acts as an antigen in cell to cell recognition
How does the distribution of charges and uncharged parts determine the position of a protein in a membrane?
- charged groups go to hydrophilic heads (extrinsic)
- non charged groups go to hydrophobic tails (instrinsic)
Define diffusion
The passive movement of a molecule or ion down a concentration gradient from a region of high concentration to a region of low concentration
- limited to small non-polar or lipid soluble molecules
What 3 main factors influence the rate of diffusion?
- concentration gradient: increased conc difference = increased rate
- thickness of exchange surface: thinner surface = higher rate
- surface area of membrane: larger area = more room for molecules to diffuse
What are some other factors impacting rate of diffusion?
- size of diffusing molecule
- number of diffusing molecules
- temperature
—> number of carrier proteins available (Fd)
Rate of diffusion equation
Surface area x conc difference
——————————————
Length of diffusion path
Define facilitated diffusion
The passive transfer of molecules or ions down a concentration gradient across a membrane by carrier protein molecules in a membrane
Describe channel proteins
- molecules with pores lined with polar groups
- hydrophilic and therefore ions can pass through
- able to open or close based on cells need
- specific to the molecule/ion that they transport
Describe carrier proteins
- diffusion of large polar molecules
- molecule attaches to binding site of the carrier protein
- able to change shape to release molecule, then revert
Define active transport
The movement of molecules or ions across a membrane against a concentration gradient using energy from the hydrolysis of ATP made in the cell by respiration
Outline some of the features of active transport
- low to high conc
- anything impacting respiration impacts active transport
- intrinsic carrier proteins
- rate is limited by number and availability of carrier proteins
Outline the process of active transport
- molecule or ion combines with specific carrier protein on outside of membrane
- ATP transfers phosphate group to carrier protein on inside of membrane
- carrier protein changes shape and carries the molecule/ion across the membrane and into the cell
- molecule/ion released into cytoplasm
- phosphate ion is released from carrier molecule back to cytoplasm and combines with ADP to form ATP
- carrier protein regains original shape
Width of phospholipid bilayer
8nm
Why does pigment leakage occur when beetroot discs submerged in ethanol?
- ethanol dissolves/emulsifies lipids and denatures proteins creating holes in the membrane
How does solubility in lipid affect the rate of diffusion?
- increased solubility = increased rate of diffusion
- lipid soluble substances can diffuse much easier than non-lipid soluble as they dissolve
Describe how molecular size affects the rate of diffusion
- increased size = lower rate of diffusion
- easier to pass through phospholipid molecules
Suggest two reasons why transport across the membrane is vital to the cell
- obtain nutrients
- secrete molecules ie enzymes
Compare the Davison-Danielli model to the fluid mosaic model
S: phospholipid bilayer w same arrangement of hydrophobic/hydrophilic parts. Both have protein
D: proteins not embedded in bilayer in DD - no intrinsic proteins. Absence of glycoalyx in DD
Why would the presence of unsaturated fatty acids in the phospholipid increase its fluidity?
- unsaturated have double carbon bond
- kink in chain
- less packing/compact so less energy to break intermolecular forces
Position of cholesterol molecules in bilayer
By fatty acid tails
Role of cholesterol molecules in the phospholipid bilayer
- regulate fluidity of membrane, located by fatty acids
- not enough makes it fluid
- too much makes it rigid
Where would hydrophilic parts with charged/polar R groups be located?
Around the phospholipid heads
Where would the hydrophobic parts with non-polar R groups be found?
Arranged between hydrophobic tails
Beetroot prac - what happens when temp is below 40°C?
- increased KE of phospholipid and proteins will cause them to move more which creates gaps in the phospholipids
- pigment will diffuse at greater rate as it has more KE
Beetroot prac - what happens above 40°C?
- protein components of membrane begin to denature forming pores that the pigment can diffuse out of more easily
—> graph levels off as membrane becomes fully permeable
Define osmosis
Net movement of water molecules from an area of high water potential to an area of low water potential across a selectively permeable membrane
Define water potential
A measure of the free energy of water molecules and the tendency of water to move
- the higher the concentration of solute in a solution or cell, the lower the water potential.
Explain hypotonic, isotonic and hypertonic solutions
- hypotonic solution has a lower solute concentration outside cell than inside
- isotonic solution has the same concentration of solute and the same water potential.
- hypertonic solution has a higher concentration of solute outside the cell than inside
Animals cells in hypotonic solutions
- net movement of water is from the higher water potential of the solution to the lower water potential in the cells, by osmosis.
- the cells will expand and as they lack a cell wall, they will burst because of the increased pressure in the cells.
- called haemolysis.
- red pigment is released into the surrounding solution.
Animal cells in hypertonic solutions
- net movement of water is from the higher water potential in the cells to the lower water potential in the solution by osmosis.
- this makes the cells shrivel, and is termed crenating.
Animal cells in isotonic solutions
- no change as no net movement
Plant cells in solution with a lower water potential
- water leaves the cells by osmosis, from a higher water potential inside the cell to a lower water potential in the solution.
- the vacuole and cytoplasm shrink because of the loss of water and the cytoplasm pulls away from the cell wall.
- plasmylosis (plasmolysed cells)
Plant cells in solution with a higher water potential
- become turgid
- water enters by osmosis and the cytoplasm and vacuole expand.
- when it is not physically possible for any more water to enter the cell, the water potential of the cell is 0
What is the water potential of pure water?
0kPa
Water potential of a cell =
Solute potential + pressure potential
What is pressure potential?
- pressure exerted in the cell contents by its cell membrane
- usually +ve, cell trying to push H2O out
What is incipient plasmolysis?
The point at which the cell membrane just begins to move away from the cell wall
—> pressure potential is a,ways 0
What is the water potential during incipient plasmolysis?
= to solute potential
How can you identify incipient plasmolysis in a microscope?
When half of the visible cells are plasmolysed
2 factors that affect the rate of FD
- temp
- amount of carrier proteins
Function of channel protein
To allow FD
What is meant by the term lipid bilayer?
Two layers of phospholipids
Rate of diffusion is proportional to…
Concentration
Describe enodcytosis
The cell membrane engulfs particle to from a vesicle around it
Use your knowledge of osmosis to explain why contractile vacuoles are necessary for the survival of amoeba
- cytoplasm of amoeba has lower water potential than surrounding solution
- water move into cell by osmosis
- water move from cytoplasm to vacuole by osmosis
- water then expelled to prevent bursting and therefore keep amoeba alive
How does the cell wall result in pressure potential?
Generated as a result of cell walls resistance to expansion
Explain arrangements of phospholipids in water
- hydrophilic heads in water as attracted
- hydrophobic tails away as repel
Why would seed growth be stunted by no O2 (active transport link)
- oxygen needed for atp synthesis to enable active transport
- no atp means no active transport means stunted growth because plant can’t get enough phosphate/calcium/magnesium
