Transport Across The Cell Membrane Flashcards
(17 cards)
5 functions of the cell-surface membrane
- Act as a barrier to permanently exclude some substances, permanently retain others, isolating contents of cell from external environment, allow passage of others in and out the cell
- Act as a site for cell recognition
- Act as a site for the binding of hormones/transmitter chemicals via protein receptor molecules
- Allow communication between cells
- Form an impermeable barrier to water soluble substances
4 things the cell-surface membrane is chemically made up of
- Phospholipid bilayer
- Proteins
- Cholesterol
- Carbohydrates
Function of cholesterol
Controls membrane fluidity
The more cholesterol = the less fluid , the less permeable the membrane
Keep membranes stable - without it the cells would burst
Diffusion
The net movement of molecules from a region where they are more highly concentrated to one where their concentration is lower, until evenly distributed
What molecules are involved in diffusion
Small, non-polar molecules e.g. oxygen, carbon dioxide
Why do charged ions and polar molecules do not diffuse easily
Charged ions and polar molecules do not diffuse easily because of the hydrophobic tails of the phospholipids in the membrane
How do charged ions and polar molecules diffuse
Facilitated diffusion through transmembrane carriers and channels
What does facilitated diffusion rely on
The kinetic energy of the diffusing molecules
No ATP is required
How does facilitated diffusion occur
Down a concentration gradient through protein channels and carrier proteins
Protein channels
Form water-filled hydrophilic channels across the membrane
Allow specific water soluble ions to pass through
Channels are selective - if the particular ion is not present, the channel remains closed
Carrier proteins
When a molecule that is specific to the protein is present, it binds with the protein
This causes it to change shape
The molecule is then released to the inside of the membrane
No external energy needed
Active transport
Molecule to be transported binds to carrier protein
ATP binds to carrier protein
Energy released as ATP is hydrolysed to ADP+Pi
Results in carrier protein changing shape and transfers the molecule to the other side of the membrane
Carrier protein reverts back to original shape
Phosphate recombined with ADP to form ATP during respiration
4 adaptations of the small intestine for transport
- Villi which increase SA
- Thin epithelium is 1 cell thick: short diffusion pathway
- Epithelial cells have micro-villi: further increases SA
- Capillaries provide a good blood supply: maintains concentration gradient
Role of diffusion in absorption of glucose & amino acids into the ileum
Usually a greater conc of glucose&amino acids within the ileum than the blood as carbs&proteins are digested continuously
Blood circulates all the time and so the concentration gradient is maintained
Once it moves from the ileum in the blood, it is taken away in the blood and hence the conc lowers again
Role of active transport in absorption of glucose & amino acids into the ileum
Diffusion cannot absorb all the glucose&amino acids by itself as there are times when the concentration of glucose& amino acids is higher in the blood than in the ileum and thus diffusion cannot occur
Glucose has to travel against its concentration gradient
Glucose&amino acids are absorbed via active transport , taken into the blood with sodium ions
Co-transport of a glucose molecule
Sodium potassium pump pumps sodium into blood stream which reduces epithelial cell sodium concentration ( active transport )
Sodium is transported into the epithelial by facilitated diffusion
Sodium is coupled to glucose ( co-transport )
Glucose is transported into blood stream by facilitated diffusion
How is an ATP molecule formed from its component molecules
ATP consists of 3 phosphate groups, adenine and a ribose sugar
Formed though a condensation reaction
Held together by hydrogen bonds
ATP synthase is the enzyme that joins them
