Transport across membranes Flashcards
Phospholipids
Made up of phosphate head and fatty acid tail
Cholesterol
provide strength and are hydrophobic so help prevent water loss by limiting the movement of phosoplipids and fatty acids
2 types of proteins
carrier and channel
Why is it called a fluid- mosaic model
phospholipids can move in membranes, and it’s made from different molecules
hydrophobic
water hating
hydrophillic
water loving
glycoprotein
Protein in the membrane with a carbohydrate chain attached that act as mechanical support and cell receptors
glycolipid
carbohydrate covalently bonded to a lipid which acts as a cell surface receptor for specific chemicals
What do channel proteins allow through the membrane?
water soluble Ions
What do carrier proteins allow through the membrane?
Larger molecules/ ions
How do phospholipids arrange themselves in a membrane
In a bilayer
Diffusion
The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until they are evenly distributed.
Extracellular
outside the cell
What cells can move through the bilayer ?
small, uncharged, lipid soluble molecules
Facilitated diffusion
Diffusion involving the presence of protein carrier and channel molecules to allow the passive movement of charged and larger molecules across plasma membranes
Channel proteins
water filled hydrophilic channels across the membrane which only open to allow specific ions across.
Carrier proteins
when a specific molecule eg glucose binds and conformational change occurs which releases the molecule to the other side of the membrane
water potential
the tendency for water molecules to move (kilopascals)
Osmosis
Passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane
Equilibrium
Osmosis stops when both sides of the membrane have equal water potential
What sort of process is diffusion and osmosis
Passive
hypotonic
When the water potential is higher, and the net movement of molecules enters the cell causing it to swell and burst
isotonic
When the water potential is equal and the net movement of molecules neither enters nor exits the cell so there is no change.
hypertonic
the water potential is lower so the net movement of molecules leaves the cell so the cell shrivels
Active transport
the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.
explain how an ion is transported into a cell by active transport
- The ion binds to a complimentary receptor in the carrier protein
- Inside the cell ATP binds to protein
- The ATP splits into ADP and a phosphate molecule
- This results in the carrier protein changing shape and opening to allow the ion to be released.
- The phosphate is released from the protein causing the protein to revert to it original shape
Explain how co-transport occurs using absorption of sodium ions and glucose by cells lining the mammalian ileum as an example
Sodium ions actively transported out of epithelial cells by sodium potassium pump (carrier protein) into the blood
This maintains a higher concentration of sodium ions in the lumen of the illeum compared with the inside of the epithelial cells
Sodium ions diffuse into the epithelial cell through a co-transport protein (carrier protein)
The movement of sodium ions carries glucose/amino acids throught the same protein into the epithelial cell
Glucose/amino acids move into the blood by facilitated diffusion (through another carrier protein)
Factors affecting the rate of diffusion
- concentration gradient
- particle charge/ size
- temperature
- surface area
- distance
- channels and carrier proteins
What is the function of phospholipids in the phospholipid bilayer
- 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 do most molecules not freely diffuse across the cell surface membrane
-not soluble in lipids
-too large
-of the same charge as the protein channels
-polar
What does the addition of solute do to pure water
lower the water potential
How does water potential affect water movement
-The more solute that is added the lower its water potential
-Water will move by osmosis from a region of higher (less negative) water potential to one of lower water potential (more negative)
What is the water potential of pure water
0
