3. 2. 3 Transport Across Cell Membrane Flashcards
Give a list of all of the structures found in the cell surface membrane.
How does movement across membranes occur by?
1 - Simple diffusion (involving limitations imposed by the nature of the phospholipid bilayer)
2 - Facilitated diffusion (involving the roles of carrier proteins & channel proteins)
3 - Osmosis (explained in terms of water potential)
4 - Active transport (involving the role of **carrier proteins & the importance of the hydrolysis of ATP)
5 - Co-transport (illustrated by the absorption of sodium ions & glucose by cells lining the mammalian ileum)
Explain how movement across membranes occurs by simple diffusion?
Simple diffusion is the movement of substances from an area of high concentration to an area of lower concentration.
Simple diffusion does not require a membrane but can occur directly across a phospholipid bilayer if the particles are small & uncharged (like oxygen & carbon dioxide). This occurs because they are small enough to move between the phospholipid molecules & they are neither repelled nor attracted to the fatty acid tails of the phospholipid bilayer. Although water is polar, it is very small so can also pass freely into the cell.
Explain how movement across membranes occurs by facilitated diffusion?
Facilitated diffusion allows large or highly charged particles (i.e glucose & charged ions), unable to pass by simple diffusion by enabling these molecules to enter the cell with the help of channel proteins, which allow substances to pass directly through them. Channel proteins are very specific & only allow one substance to pass through.
Carrier proteins can move one or two proteins at the same time: symporters move one down & another in the same direction but up its concentration gradient. Airporters do the same but the particles are moved in opposite directions. These do not require energy so are passive movements but since they require additional proteins they are known as facilitated diffusion.
Explain how movement across membranes occurs by osmosis?
Osmosis is diffusion of water molecules
Osmosis can only occur across membrane water potential. The likelihood that water will move to/from a location due to the pressure it is exerting. The maximum water potential is 0, the water potential of pure water.
Explain how movement across membranes occurs by active transport?
Active transport is the movement of particles against a concentration gradient (from an area of low to high concentration) at a rate faster than diffusion.
1 - Molecule binds to carrier protein, on one side of the membrane.
2 - ATP is hydrolysed into ADP & P. The phosphate binds to carrier protein on the other side of the membrane, causing the carrier protein to change shape
3 - Molecule travels through the newly opened channel in the cell
4 - Phosphate molecule is released & recombines with ATP
5 - Carrier protein returns to original shape
Explain how movement across membranes occurs by co-transport?
1 - Glucose is absorbed by co-transport into the bloodstream in the small intestine
2 - In the ileum (the final part of the small intestine) the concentration of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen (middle) of the ileum by co-transport.
How does glucose enter the ileum epithelium with sodium ions?
MUST ALSO BE ABLE TO DRAW THIS
1 - Sodium ions are actively transported out of the ileum epithelial cells, into the blood, by the sodium-potassium pump. This creates a concentration gradient - there’s now a higher concentration of sodium ions in the lumen of the ileum than inside the cells.
2 - This causes sodium ions to diffuse from the lumen of the ileum into the epithelial cell down their concentration gradient. they do this via the sodium-glucose co-transporter proteins
3 - The co-transporter carries glucose into the cell with the sodium. As a result the concentration of glucose inside the cell increases
4 - Glucose defuses out of the cell, into the blood, down its concentration gradient through a protein channel, by facilitated diffusion
Illustrate co-transport by the absorption of sodium ions & glucose by cells lining the mammalian ileum.
Link to illustration below -
link
How may cells be adapted for rapid transport?
Cells may be adapted for rapid transport across their internal or external membranes by an increase in surface area of, or by an increase in the number of protein channels and carrier molecules in, their membranes
