Transport across cell membranes Flashcards
Why are phospholipids important
HydrophIlic HEADS = attracted to water
HydrophObic TAILS = repelled by water
Functions of phospholipids
Allow lipid-soluble substances to enter and leave the cell
Prevent water-soluble substances entering and leaving
Make membrane flexible and self-sealing
Proteins in bilayer use
Never extend completely across surface of bilayer
Give mechanical support
Cell receptors
Functions of proteins in membrane
Structural support
Channels transporting water-soluble substances
Allow active transport
Form cell-surface receptors for identifying cells
Help cells adhere together
Receptors
Cholesterol molecules function
Add strength
Hydrophobic to prevent water loss and dissolved ions
Pull together fatty acid tails of phospholipid molecules, limiting movement
What are glycolipids made up of
Carbohydrate + lipid
Functions of glycolipids
Recognition sites
Maintain stability
Help cells attach to one another
What are Glycoproteins made up of
Carbohydrate chains on outer surface of of cell membrane
Function of glycoproteins
Recognition sites
Help cells attach to form tissues
Allow cells to recognise one another
Why molecules do not freely diffuse across permeable cell surface membrane
Not soluble in lipids
Too large
Same charge as charge on protein channels -> so repel
Polar so have difficultly passing through non-polar hydrophobic tails
Why is a fluid mosaic model of cell-surface membrane called fluid and mosaic
Fluid = phospholipid molecules can move relative to one another
Mosaic = Proteins vary in shape, size pattern (same way mosaics do)
What form of transport is diffusion
Passive
What occurs in all passive forms of transport
Particles constantly in motion due to kinetic energy they possess
Motion is random
Bounce off one another
What does passive transport mean
The energy comes from the natural, inbuilt motion of particles, rather than some external source
Diffusion definition
The net movement of molecules or ions
From an area of high concentration to an area of low concentration
Facilitated diffusion definition
Diffusion involving presence of protein carrier molecules to allow the passive movement of substances across plasma membranes
Two types of proteins involved in facilitated diffusion
Protein channels
Carrier proteins
Where does facilitated diffusion occur
Down a concentration gradient
Protein channels form
Water-filled hydrophilic channels across the membrane
What do protein channels allow
Specific water-soluble ions to pass through
How do protein channels work
Channels are selective, open in presence of specific ion and otherwise are closed
What happens as ions pass through protein channels
Ions bind to protein and change shape in way that closes it to one side of the membrane and opens is to the other side
What to carrier proteins do
Span plasma membrane
Specific molecules bind with the protein
Change shape so molecule is released to inside of membrane
Go from area of high concentration to low so no external energy is needed
Osmosis definition
Passage of water from high water potential to lower through a selectively permeable membrane
Water potential unit
Trident thing
Units of pressure
kPa
Osmosis rules
More solute added lower water potential
Water + solute = water potential will always less than 0
Water moves from area of high -> low water potential
Example of osmosis when left side of membrane has lower solute concentration
Solute and water move randomly by kinetic energy
Selectively permeable plasma membrane only allows water molecules to move across
Water molecules diffuse to right, where there is more solute and less water
Dynamic equilibrium established
What happens if the water potential value is very low
More negative the value the lower the water potenial
Osmosis in animal cells
Animal cells in liquid, blood plasma
So water potential in cells and outside is equal
Water potential outside cell higher = bursts, net movement enters cell
Water potential outside is lower = shrinks
Active transport defintion
Movement of molecules into or out of a cell from low to high concentration using ATP and carrier proteins
What is ATP used for in active transport
Directly move molecules
Co-transport (moving molecules using concentration gradient) thats been set up by direct active transport
Why is active transport different from passive forms
Metabolic energy in form of ATP
Moved against concentration gradient
Carrier protein molecules
Very selective
How does direct active transport occur
1) Carrier proteins span plasma membrane and bind to molecule to be transported to one side
2) Molecule binds to receptor sites
3) Inside of cell ATP binds to protein
4) ATP split into ADP + phosphate molecule
5) Protein molecules change shape and open to opposite side of membrane
6) Molecule released to other side of membrane
7) Phosphate molecule released from protein
8) Protein revert to original shape for repetition
9) Phosphate molecule recombines with ADP to form ATP
Difference between facilitated diffusion and active transport
Facilitated = DOWN concentration gradient
= DOESN’T need metabolic energy
Active transport = AGAINST concentration gradient
=NEEDS metabolic energy
What is Sodium-potassium pump an example of
More than one molecule moved in cell as one is removed
Explain sodium-potassium pump
Sodium ions removed actively as potassium ions are actively taken in
How to increase the rate of movement across membranes
Epithelial cells possess microvilli
- more SA for insertion of carrier proteins through diffusion and facilitated diffusion
Increase number of protein channels and carrier proteins
Role of diffusion in absorption
Carbs and proteins digested
Greater concentration of glucose and amino acids in ileum than blood
Glucose moves by facilitated diffusion in ileum to blood
Continuously removed by cells
Maintains concentration gradient in ileum and blood
Difference between carrier and channel proteins
Channel proteins = open and close, only charged substances diffuse
Carrier = fixed specific shape (allowing one particular charged substance), diffuse down conc gradient
Two functions of membranes in cells
Regulate movement and exchange of substances
Separate internal and external environment
Why do phospholipids form in an aqueous environment
Have a hydrophilic head and hydrophobic tail
Molecules arranges themselves between tails and surrounding water is minimised
How do molecules move to the other side of a cell surface membrane if there are no carrier proteins
Move from outside of cell to inside directly in between phospholipids by simple diffusion
Because there is a higher conc of the molecules outside the cell than inside and diffusion takes place from high to low DOWN the conc gradient. Molecules are small and nonpolar so can move between the phospholipids
(Remember this can only happen if the molecule is non polar)
If the molecule is not lipid soluble how would it travel across the cell membrane
Facilitated diffusion
Specific channel protein or carrier protein
When does facilitated vs simple diffusion occur
Simple = lipid soluble non polar doesnt need carrier
Facilitated = not lipid soluble, polar needs carrier
Explain the appearance of cytoplsam under a microscope (not touching cell wall)
Cell has been placed in solution with lower water potential
Water moves out by osmosis
Causing cytoplasm/ vacuole. Protoplast to shrink and decrease in volume
What would happen if you put a cheek cell in a test tube of pure water
Cheek cell has lower water potential than water so take water into cell by osmosis and swell and eventually burst
Onion cell placed in pure water waht would happen
Cell wall provides additional strength that can withstand pressure so does not burst
What is simple diffusion
Movement DOWN conc gradient
Movement of O2 from plasma into red blood cell
What is facilitated diffusion
Movement of glucose from high to low conc
Movement down conc gradient
What is cotransport vs facilitated diffusion
Simple diffusion = no carrier proteins, go through phospholipid bilayer
Co transport = with carrier protein involving facilitated diffusion and active transprot. ATP
Facilitated diffusion = use channel and carrier proteins, transport polar and ions NO ATP
What is the purpose of using carrier proteins recycling atp
Maintains concentration gradient for sodium
Why is beetroot used in the experiment
Pigment is red
Amount of pigment thtat leaves indicated permeability
