Cell Membranes Flashcards
Cell surface membranes are ____ ________
semi permeable
What is the role of membranes within cells?
divide the cell into different compartments and act as a barrier between the organelle and the cytoplasm - also semi permeable
Describe fluid mosaic model
Phospholipid molecules form a continuous, double layer called a bilayer. This bilayer is ‘fluid’ because the phospholipids are constantly moving. Proteins are scattered through the bilayer, like tiles in a mosaic. These include channel proteins and carrier proteins which allow large molecules and ion to pass through the membrane.
Some proteins have a carbohydrate attached these are called ______
glycoproteins
Some lipids also have a carbohydrate attached these are called _______
glycolipids
Describe the role of cholesterol in the membrane
Gives the membrane stability. It fits between the phospholipids. Cholesterol binds to the hydrophobic tails of the phospholipids causing them to pack closely together. This restricts movement making the membrane less fluid and more rigid.
How does temperatures below zero degrees effect the membrane?
Phospholipids don’t have much energy.
Packed closely together and membrane is rigid.
Channel proteins and carrier proteins in the membrane denature, increasing the permeability of the membrane.
Ice crystals may form a pierce the membrane making it highly permeable when it thaws.
How does temperatures between 0 and 45 degrees effect the membrane?
Phospholipids can move around and aren’t packed as closely together
Membrane is partially permeable
Phospholipids can move more as they have more energy - increasing the permeability of the membrane
How does temperatures above 45 degrees effect the membrane?
The phospholipid bilayer starts to melt and the membrane becomes more permeable.
Water inside the cell expands putting pressure on the membrane
Channel and carrier proteins in the membrane denature so they cant control what enters or leaves - increases the permeability of the membrane
LOOK AT PRACTICAL ON PAGE 100
Define diffusion
net movement of particles from an area of a higher concentration to an area of lower concentration - particles diffuse down the concentration gradient
Diffusion is a _______ process
passive = no energy needed
When molecules diffuse directly through a cell membrane, its also known as _______ __________
simple diffusion
Explain the factors effecting the rate of diffusion
- The conc gradient = the higher it is the faster the rate of diffusion. As diffusion takes place, the difference in concentration between the two sides of the membrane decreases until it reaches equilibrium. This means that the diffusion slows down over time
- The thickness of the exchange surface = the thinner the exchange surface the faster the rate of diffusion
- The surface area = the larger the surface area the faster the rate of diffusion
What things are diffused via facilitated diffusion?
Larger molecules
Ions
Polar molecules
How do particles diffuse if they are large or charged?
carrier or channel proteins
passive process
Describe how carrier proteins work
- First a large molecule attaches to a carrier protein in the membrane
- Then the protein changes shape
- This releases the molecule on the opposite side of the membrane
Describe how a channel protein works
Channel proteins form pores in the membrane for charged particles to diffuse through down their concentration gradient.
Factors affecting the rate of facilitated diffusion :
- The concentration gradient = the higher the conc gradient, the faster the rate of facilitated diffusion. As equilibrium is reached the rate of facilitated diffusion will level off .
- The number of channel or carrier proteins = once all the proteins in a membrane are in use, facilitated diffusion cant happen any faster even if you increase the conc gradient. The greater the number of transport proteins in the cell membrane, the faster the rate of facilitated diffusion
Define osmosis
diffusion of water molecules across a partially permeable membrane, from an area of high water potential to an area of lower water potential
Water potential = the potential of water molecules to diffuse out of or into a solution
Pure water has a water potential of _______
How do you lower water potential?
Zero
Adding solutes lowers water potential so the water potential of any solution is negative
The more negative the water potential the stronger the concentration of the solutes in the solution
What happens to a cell in an isotonic solution?
If two solution have the same water potential they’re said to be isotonic so there is no net movement of water molecules as there is no difference in water potential.
What happens to a cell in a hypertonic solution?
If a cell is placed in a solution with lower water potential it may shrink as water moves out by osmosis. Solutions with a lower water potential compared with the inside of the cell are called hypertonic.
What happens to a cell in a hypotonic solution?
If a cell is placed in a solution that has a higher water potential it will swell as water moves into it by osmosis. Solutions with a higher water potential compared with the inside of the cell are called hypotonic.
