1.3 Membranes and Transport Flashcards
What are all cells surrounded by?
A cell membrane
May also be called the cell surface membrane or the plasma membrane
How does the cell membrane appear under the electron microscope?
As a double line
What is the usual distance across the cell membrane under the electron microscope?
7-8nm
What are the principal biochemical constituents of the cell membrane?
Protein and phospholipids
How are the phospholipid molecules arranged in the cell membrane?
As a bilayer
Hydrophilic heads and hydrophobic tails (tails on the inside of the membrane)
What proteins can be found in the membrane?
Channel proteins (like a tube straight through the membrane. The inside of the tube is polar.) Extrinsic proteins (partially crosses bilayer) Intrinsic (fully cross the bilayer) Surface proteins (Some have a carbohydrate chain used for cell recognition = glycoproteins) Carrier proteins (Some require ATP. Could be enzymes, electron carrier/proton pump. 'Pick up' a molecule, turn so they face the inside of the cell and deposit the molecule inside.)
What is the fluid-mosaic model?
The components are free to move with respect to each other
Draw a simple diagram to illustrate the fluid mosaic model.
Label:
phospholipid bilayer
channel, surface, extrinsic, intrinsic & carrier proteins
glycoproteins
What are the major functions of the cell membrane?
Taking up nutrients & other requirements of the cell
Secreting chemicals
Cell recognition
What is the cell membrane selectively permeable to?
Water and some solutes
What affects the permeability of a cell membrane?
Diffusion is affected by:
Steepness of the concentration gradient - If there is a greater difference in the concentrations of a solute inside and outside the cell, then it will travel faster down the concentration gradient.
Distance - shorter distance = shorter time eg alvioli
Warmer temperatures = more kinetic energy for the particles so they move faster.
Size of the molecule - smaller = better because it can diffuse through the phospholipid bilayer (eg O2 and CO2) rather than through a channel protein. Also have more kinetic energy at any given temperature.
How do lipid-soluble substances and water-soluble substances enter and leave a cell?
Lipid-soluble substances can dissolve in the phospholipid bilayer and diffuse through.
Water-soluble substances use temporary protein channels.
What is diffusion?
The movement of molecules or ions from a region of high concentration to one of low concentration.
It is passive (doesn’t require energy).
What is osmosis?
A particular form of diffusion in which water molecules move down a water concentration gradient through a selectively permeable membrane.
What is water potential?
The potential for water to move out of a solution by osmosis.
It has the symbol psi (like a little trident).
Pure water has the highest w/p, given the value 0psi.
All solutions have a lower potential than water bc they have a lower proportion of water molecules. Therefore psi(of a solution) always has a nevative value
In plant cells, what is water potential the sum of?
The solute potential (psi little s) (the effect of solutes lowering the w/p of the cell sap)
and psi little p (opposite pressure provided by the cell wall. Usually positive)
What happens to a cell when it becomes plasmolysed?
The cytoplasm of a plant cell draws away from the cell wall, pulling the cell membrane with it.
Animal cells simply shrink.
This is due to a lack of water inside the cell.
What happens if a cell has an increased volume of water in it?
Animal cells will burst.
Plant cells continue to take in water until the opposing wall pressure prevents it. At this point the cell is referred to as turgid.
What is facilitated diffusion?
It allows rapid exchange due to substances being helped across the membrane by carrier proteins. Does not require ATP.
What is phagocytosis?
Where a large particle may enter the cell, become enclosed by a membrane to form a vesicle and be transported through the cytoplasm.
Draw a diagram to illustrate phagocytosis.
.
What is secretion?
Also called exocytosis.
Substances leaving the cell after being transported through the cytoplasm in a vesicle.
This and phagocytosis mean that the membrane is continually having portions removed or added to it.
What is pinocytosis?
The entry of liquid into a cell by the same mechanism as phagocytosis.
What is active transport?
Requires ATP from respiration
Movement of solutes against a concentration gradient
Will not take place in the presence of a respiratory inhibitor eg cyanide
