Flashcards in 6 - Cell membranes Deck (22):
Structure of the Phospolipid bilayer?
Forms bilayer, heads out tails in, when in the water of the cytoplasm
Dosent allow in water soluble substances
What does the membrane contain?
Why is it called a fluid-mosaic model?
The phospholipid and protein molecules fit together to form a continuous pattern - mosaic
Fluid, the components are in constant motion and move laterally - fluid
Role of phospholipids?
Head is hydrophilic - attracts water
Tail is hydrophobic - repels water
Centre of bilayer is hydrophobic - doesn't allow water-soluble substances (ions) through it
Role of cholesterol?
Provides stability by preventing too much movement in the cell
Net movement of particles from an area of high concentration to an area of low concentration
It is a passive process
Factors affecting the rate of diffusion?
Length of diffusion pathway
Size of molecules or ions
Temperature - affects kinetic energy
What can diffuse through the phospholipid bilayer?
Non polar molecules - lipid soluble
O2, CO2 , Ethanol
Why can't ions diffuse through the phospholipid bilayer?
Water soluble (polar) and thus can't go through the hydrophobic phospholipid bilayer
How do ions get diffuse through the phospholipid bilayer?
Sodium ions are the main example
cell specific sized
Cell can change the number of channels present, the more channel proteins the more ions diffuse through
Can use gated channels to control the diffusion of ions
'Voltage-gated channels' use an electrical charge to open their channels
How does glucose pass through the phospholipid bilayer?
Large and have small charges on their OH groups means they can't pass through the phospholipid tails
glucose binds temporarily to carrier proteins which changes shape and the glucose moves through
Structure of channel and carrier proteins
Ion specific due to their specific territory structure
How does water pass through the phospholipid bilayer?
Do have dipoles but are very small
Can diffuse through by osmosis using aquaporins - water channel proteins
Cell membranes are selectively permeable so water needs the aquaporins
What is osmosis?
The net movement of water molecules from an area of high water potential to an area of lower water potential through a partially permeable membrane - moves down a water potential gradient
The unit for water potential?
psi Ψ, Pure water has a Ψ of 0, water potential units are therefore negative
What is active transport?
The movement of particles from an area of low concentration to an area of high concentration using energy produced from the hydrolysis of ATP
In the phospholipid bilayer, the active transport of ions uses carrier proteins
Why do sodium ions help runners?
Stimulates the rate of glucose uptake in the blood as specialised carrier proteins in the cell surface membranes of the epithelial cells lining of the small intestine which co-transport glucose and sodium into the cell at the same time.
Co-transport of Na+ and glucose
- Sodium ions actively transported out of the ileum epithelial cells into the blood, by sodium potassium pump
- Sodium ions diffuse from lumen of the ileum into the epithelial cell, down the concentration gradient
- Co-transporter carries glucose in the cell with sodium
- Concentration of glucose increases
- Glucose diffuses out of the cell into the blood through a protein channel by facilitated diffusion
- Sped up by large surface area of microvilli
- Sped up by having large number of carrier proteins
- Sped up by having a lot of mitochondria
Test for permeability of cell membrane?
Use scape and cut five equally sized pieces of beetroot. Rinse to remove and pigment
Add five pieces to the test tube, each containing 5cm3 of water. Use a measuring cylinder or pipette to measure water.
Place each test tube in a water bath at a different temperature for the same time
Remove the pieces of beetroot from the tube leaving just the coloured liquid
Now use a colorimeter - passes light through and see how much is absorbed, higher the absorbance the more pigment released so the higher the permeability
Connect colorimeter to a computer and use software to collect data and create a graph
Effect of temperature on the permeability of the membrane?
1. Below 0C - membrane is rigid as the molecules don't have much energy - channel proteins and carrier proteins deform, increasing the permeability - ice crystals may form and pierce the membrane making it highly permeable
2. Temperatures between 0 and 45C - Partially permeable - molecules not packed together and not rigid - as it increases becomes more permeable as the molecules move more
3. Above 45C - Bilayer starts to melt (break down) - more permeable - Water expands and pressures the membrane - Channel proteins and carrier proteins deform - increases the permeability
What is facilitated diffusion?
Down a concentration gradient - higher to lower concentration
Carrier proteins move large molecules
Channel proteins for ions - ion specific