2.3 Transport Across Cell Membrane Flashcards
Describe the cell membrane as a fluid-mosaic model
Bilayer of phospholipids with embedded proteins, cholesterol and carbohydrates.
What is the structure of a phospholipid?
A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group.
How do phospholipids arrange themselves in the cell membrane and why?
Are in a bilayer with the hydrophilic phospholipid head facing outwards towards solutions on either side and the hydrophobic fatty acid tails facing inwards.
What benefit does cholesterol bring to the cell membrane and is it found in bacterial cell membranes?
Cholesterol regulates membrane fluidity by making the membrane more impenetrable and thicker. It is not found in bacterial cell membranes.
What factors affect membrane permeability and how?
Temperature - high temperatures make membranes more fluid.
Solvent concentration - The more easily the phospholipid bilayer is dissolved, the more permeable the membrane is.
pH- pH affects the protein structure in the membrane
How can cell membrane permeability be investigated using beetroot?
Use a cork borer to collect samples of uniform diameter.
Cut discs of a uniform depth using a sharp scalpel on a white tile and rinse in cold water. This removes excess pigment that has leaked through physically broken cell membranes.
Prepare at least five concentrations of ethanol (e.g. 0%, 10%, 20%, 30%, 40%) in beakers.
Place the discs into the corresponding solution for 10 minutes.
Make sure the samples are completely covered by the ethanol solutions and mixed frequently throughout the 10 minutes.
Remove the discs from the solutions to prevent further changes and allow a fair comparison between the experiments.
Calibrate a colorimeter by using a cuvette of distilled water at an absorbance of 520nm.
The cuvettes must be dry and the clear sides must not be touched to prevent potential errors in the readings.
Measure the absorbance of each solution.
Plot the results in a graph with concentration on the x-axis and absorbance on the y-axis.
The darker the solution, the more pigment has been released. This is reflected in a higher reading for absorbance.
What is diffusion?
Net movement of particles from a high to low concentration
Which molecules can travel across the cell membrane by simple diffusion?
Small and non-polar
What is facilitated diffusion?
Facilitated diffusion describes the net movement of particles down a concentration gradient through channel or carrier proteins.
Which molecules cross the cell membrane by facilitated diffusion?
Large and polar (repelled by hydrophobic tails of cell membrane)
How do carrier proteins work?
When a substance binds to carrier proteins, there is a change in the shape of the carrier protein.
This change in shape moves the substance from the outside of the cell to the inside.
What is osmosis?
Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (high water potential) to a concentrated solution (low water potential).
What is active transport?
Movement from a low to a high concentration which requires energy
How does active transport take place across the membrane
Carrier proteins
What is the mechanism of transport of glucose and amino acid into the epithelial cells in the ileum?
Co-transport
How is glucose co-transported into epithelial cells?
Sodium ions are actively transported into the blood by the sodium-potassium pump, from the inside of epithelial cells lining the ileum.
The concentration of Na+ is now lower inside the cell than outside.
Two Na+ ions from the lumen of the ileum bind to a transmembrane protein known as the sodium-glucose cotransporter.
At the same time, the cotransporter binds with a molecule of glucose from the ileum and moves it into the cell.
The sodium ions diffuse into the cell, moving the glucose into the cell at the same time.
