Chapter 4: Transport Across Cell Membranes Flashcards
Define plasma membrane:
The membrane which surrounds all cells and organelles (as they all have the same basic structure).
When surrounding the cell it is known as the cell surface membrane (boundary between cell cytoplasm and environment).
What 5 molecules form the cell membrane structure?
Phospholipids
Proteins
Cholesterol
Glycolipids
Glycoproteins
What is a phospholipid?
A water soluble lipid with a phosphate group which forms a bilayer due to the hyrophilic, polar ‘head’ and hydrophobic non polar tail.
Important component of the cell-surface membrane.
Define amphipathic molecule:
The compination of hydrophobic and hydrophilic components in a molecule.
What are the 3 main functions of the phospholipid bilayer?
Allow lipid-soluble substances in and out.
Prevent water-soluble substances entering or leaving.
Make the membrane flexible and self-sealing.
What are the 2 ways which proteins can be found in the CSM?
- Transmembrane protein
- Embedded protein
Define transmembrane protein:
A protein spanning the phospholipid bilayer wither found as a protein channel or carrier protein.
Define embedded protein:
A protein found on the surface of the bilayer which acts as support or a receptor.
What are the 6 main functions of membrane proteins?
- Stuctural support
- Channels for water soluble substances
- Active transport across membrane (carrier proteins)
- Cell-surface receptors to identify cells
- Adherance between cells
- Receptors
Where is cholesterol found in the CSM?
Embedded in the hydrophobic area (as they are also hydrophobic).
What are the 5 main functions of cholesterol in the CSM?
- Strength
- Prevent water loss
- Prevent soluble ion loss
- Limit movement of fatty acid tails by pulling them together
- Control fluidity
What is the structure of a glycolipid?
A carbohydrate covalently bonded with a lipid. They are extensions of phospholipids.
What are the main functions of glycolipids?
Cell-surface receptor (e.g. blood groups)
Stability of the membrane
Attatchment to other cells
What is the structure of a glycoprotein?
Carbohydrate chains attached to extrinsic proteins on the outer surface of the cell membrane.
What are the main functions of glycoproteins?
Cell surface receptors (mainly hormones and neurotransmitters)
Recognition sites for other cells
Attachment sites to form tissues
Why don’t most molecules diffuse across the CSM freely?
- Not lipid soluble
- Too large to pass through channels
- Same charge as protein channels
- Polar so cant pass through the non-polar tails
Why is the cell-surface membrane described as having a ‘fluid-mosaic model’?
Fluid: molecules can move around each other. Membrane has a flexible structure so is constantly changing shape.
Mosaic: variety of different molecules like the pattern of a mosaic.
Define diffusion:
The net movement of ions or molecules from a regin of high concentration to one of lower concentration until evenly distributed.
What type of transport can be used to describe diffusion? Why?
Passive
There is no external source of energy, only the inate energy of the system.
Fill in the gaps:
All particles are in _____ _____ and this is completely ______ due to the kinetic energy they posess.
As a result, the particles are constantly _______ ___ each other and other objects.
constant motion
random
bouncing off
Name 3 factors which affect the rate of diffusion:
Distance of concentration gradient
Surface area
Difference in concentration
Give the equation for Ficks Law:
Rate of diffusion ∝ (Surface area x concentration gradient) / Length of diffusion pathway
Define facilitated diffusion:
The movement of molecules through transmembrane channels and carriers that span the membrane. The two proteins involved are protein channels and carrier proteins.
How does facilitated diffusion occur in protein channels?
Water filled hydrophilic channels are formed across the membrane, allowing specific water soluble ions to pass through. The channels are selective so only change shape for specific ions and molecules. Looser binds are formed than with carrier proteins.
How does facilitated diffusion occur in carrier proteins?
Carrier proteins span the plasma membrane, allowing molecules to bind to it, changing its’ shape and relearing the molecule to the other side of the membrane. These bonds are tighter than those in protein channels
Define osmosis:
The movement of water from a region of high water potential to one of lower water potential through a selectively permeable membrane.
Why is it only water that moves via osmosis?
The selectively permeable membrane only allows water across it, not the solute.
Define hypertonic in terms of concentration and water potential:
High concentration outside cell.
Water potential is higher inside the cell.
Define isotonic in terms of concentration and water potential:
Concentration and water potential are equal both inside and outside the cell.
Define hypotonic in terms of concentration and water potential.
Low concentration outside cell
Water potential is higher outside cell
Define water potential:
The pressure exerted by water
What is te highest possible water potential value?
0 kPa
How are red blood cells impacted by osmosis?
HINT: haemolysis
Red blood cells contain a variety of solutes dissolved in their cytoplasm so they absorb water by osmosis if placed in pure water causing them to burst. This is why red blood cells live in blood plasma.
Also they become shrivelled if placed in a solution with a lower water potential than its own.
Define 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.
What are the 2 main uses of active transport?
To directly move molecules
To move molecules individually using a pre-established concentration gradient (co-transport).
How is active transport different from passive transport?
Metabolic energy is needed in the form of ATP
Substances move against (up) a concentration gradient
Carrier protein molecules are used, acting as ‘pumps’. E.g. the sodium-potassium pump
The process is very selective
Describe direct active transport:
1) Carrier mpolecules span the plasma membrane and contain binding sites for the molecules or ions being transported.
2) The molecule/ion binds to the receptor site on the protein, chaning the protein shape.
3) ATP binds to the carrier protein from the inside of the cell due to the shape change. This initiates the hydrolysis of ATP which causes the protein molecule to change shape again and open.
4) The molecule/ion is released into the cell due to the shape change.
5) The phosphate molecule is finally released from the carrier protein causing it to resume its initial shape.
Name an example of more than one molecule/ion being moved via active transport:
Sodium-potassium pump
What are microvilli?
Finger-like projections of the CSM on epithelial cells lining the ileum (part of the small intestine).
What is the function of microvilli?
Increase SA for the insertion of protein channels and carrier proteins to increase transport across membranes.
How does diffusion aid the absorption of carbohydrates and amino acids?
Generally more of them in the ileum than the bloodstream (due to constant flow of blood to maintain conc. gradient) so the can move through protein channels and carrier proteins via facilitated diffusion.
Why is active transport necessary for the absorption of glucose and amino acids?
Diffusion can only equal the concentrations of gluscose and amino acids in the ileum and bloodstream so active transport is neccessary so that everything is absorbed.
Describe the absorption of glucose and amino acids via co-transport:
This is an example of the sodium-potassium pump in use:
1) Sodium ions are actively transported out of epithelial cells by a carrier protein molecule, maintaining the concentration gradient of sodium in the epithelial cell. This is where potassium ions enter the cell.
2) There is a higher concentration of sodium ions in the lumen of the intestine than inside the cell so they diffuse down their concentration gradient into the cell through a different type of carrier protein (co-transport protein).
3) As htey diffuse in through the carrier protein, they carry the glucose/AA molecule with them.
4) The glucose/AA molecules pass into the bloodstream via facilitated diffusion.
How does each molecule in the co-transport move in realtion to its concentration gradient?
Sodium: down concentration gradient
Glucose/ AA: against concentration gradient (with sodium conc. grad.)
What form of transport does glucose move by in c-transport?
Indirect
