1.3 Cell Membranes and Transports Flashcards
What is a phospholipid made of?
2 Fatty acids and 1 phosphate attached to a glycerol
Why are phospholipids used to make membranes?
because they have special properties including:
-phosphate head attracted to water (HYDROPHILIC)
-tails repel water (HYDROPHOBIC)
these factors mean that when they are placed in water they will arrange themselves in a specific way so that both parts of the molecule are satisfied.
Hydrophilic
attract water and allow water soluble and non-polar molecules through
Hydrophobic
repel water and allow lipid soluble and polar molecules through
Polar and Non polar
-the phosphate ‘head’ is charged = polar
-the fatty acid ‘tail’ isn’t charger = non-polar
Width of phospholipid bilayer
7-8nm
The other main components of the phospholipid bilayer are:
PROTEINS
What do proteins do?
-They can form channels to allow certain molecules to pass through.
-They can have carbohydrates attached to them to act as signal molecules for cell recognition.
-They can act as receptors for hormones and other signal molecules to bind to.
When was the Fluid Mosaic Model developed?
-was developed by Singer and Nicolson in 1972.
-It built upon the work by Davson and Danielli in 1935 who thought there was a phospholipid bilayer between two layers of globular proteins. They thought it was trilaminar and made of lipids and proteins
Why is it fluid?
because the lipid layer of the membrane isn’t fixed in one place, the phospholipids can move about freely.the membrane actually has the consistency of cooking oil
Why is it mosaic?
because of the patchwork pattern produced by the scattered proteins when membrane is viewed from above
Intrinsic proteins
or Integral proteins are embedded in the bilayer, if they span it from one side to the other they can also be called TRANSMEMBRANE Proteins. They can form channels or act as carriers
Extrinsic proteins
or Peripheral proteins are loosely attached to one side of the membrane. These are often enzymes or receptor molecules
Controlling the position of proteins in membranes:
The non-polar amino acids cause proteins to be embedded in membranes while polar amino acids cause portions of the proteins to protrude from the membrane.
Creating hydrophilic channels through membranes:
Polar amino acids are found inside membrane proteins and create a channel through which hydrophilic molecules can pass through.
Where are hydrophilic parts with polar/ charged R groups?
located around the phospholipid heads or on the outside
Where are hydrophobic portions with non polar R groups?
near/ between the hydrophobic tails
What charge do channel proteins have?
charged/ polar R groups lining the channel
What affects how fluid the phospholipid membrane is?
-There is also some cholesterol which makes the membrane more structured.
-You can also have different proportions of unsaturated and saturated fatty acids in the phospholipids to control how fluid the membrane is. The more unsaturated fatty acids the more fluid.
What are glycoproteins and their function?
-proteins with carbs attached
-Cell to cell recognition
-Cell to cell communication
-Immunity
-Recognition of hormones
-Binding of molecules (adhesion) and crosslinking cells for strength and stability
Cholesterol
-Cholesterol helps prevent extremes…
It both stabilises (makes more rigid) the membrane AND also helps maintain the fluidity.
-It integrates between the phospholipids, and without it the membrane would be too fluid AND it helps to separate the phospholipids so that the fatty chains can’t come together and crystallise.
-It is present in Animal cell membranes but absent in bacteria and most plants – because they have cell walls to provide stability
Glycocalyx
-Is the covering of glycoproteins and glycolipids that surrounds a cell membrane
-The glycocalyx is involved in immunity, compatibility, inflammation regulation, fertilisation, embryonic development etc.
-It helps molecules with cell recognition, cell-to-cell communication and intercellular adhesion.
-The word literally means ’sugar-coat’, and this network of glycoproteins and glycolipids is often very obvious in bacteria or as a layer of slime on fish etc.
What affects permeability of a membrane?
1- Temperature
2- Ethanol
3- pH
Temperature affecting permeability
-the higher the temperature, the more kinetic energy the phospholipid and protein molecules have.
-That creates gaps or pores in the phospholipid bilayer. Over 40°C proteins will lose their tertiary structure because the H-bonds break and they become denatured. These will get larger and larger.
-Smaller molecules will have more energy than larger ones when heated up.
-Diffusion happens more rapidly when the molecules have more energy
Ethanol affecting permeability
-Ethanol will destroy a membrane by dissolving or emuLsifying the lipids in the lipid bilayer.
-By doing so it creates gaps in the membrane and whatever pigment inside the cell will leak out eg Betacyanin or Betalain, the purple pigment in Beetroot.
pH affecting permeability of a membrane
-Extremes of pH will also destroy a membrane (with very low pHs being worse than very high pHs – ie very acidic worse than very alkaline).
-It does this by denaturing the Proteins in the membrane and therefore creating gaps or holes as the tertiary structure of the proteins are destroyed.
-This is again due to the H-bonding being affected by the H+ ions present in the Acid for example.
Fick’s Law
states the rate of diffusion is in direct proportion to:
surface area x concentration / length of diffusion path
Molecules that can pass straight through the lipid bilayer are…
-fat soluble and hydrophobic
-oxygen and CO2
-vitamins D.A.K.E
-steroids and lipid hormones
How does water pass through the bilayer?
Because Water is a very small molecule it can also get through the bilayer BUT the majority of water passes through Channel Proteins called AQUAPORINS
Molecules that need to use carrier or channel proteins to pass through because they are…
-water soluble, polar molecules that are hydrophilic these include:
-sugars
-amino acids
-nucleotides
-ions
-water
4 Main methods by which substances can move across a cell membrane:
1- Lipid diffusion
2- Facilitated diffusion
3- Active transport
4- Osmosis
(simple) lipid diffusion
A few substances can diffuse directly through the lipid bilayer part of the membrane.
The only substances that can do this are hydrophobic (lipid-soluble) molecules such as steroids, and a few extremely small hydrophilic molecules, such as H2O, O2 and CO2.
Since lipid diffusion is a passive process, no energy is involved and substances can only move down their concentration gradient.
Rate of diffusion is affected by:
-Concentration gradient
-Thickness of membrane/distance
-Surface area
-Temperature (higher = more kinetic energy)
-Size of molecule
-Nature of the molecule (lipid-soluble molecules diffuse faster than water-soluble)
-Number of channels
Facilitated diffusion
Facilitated Diffusion is the diffusion of substances across a membrane through a trans-membrane protein molecule. The transport proteins tend to be specific for one molecule, so substances can only cross a membrane that contains an appropriate protein. This is a passive diffusion process, so no energy is involved and substances can only move down their concentration gradient.
Channel proteins
Channel Proteins form a water-filled pore or channel in the membrane. This allows charged substances to diffuse across membranes. Most channels can be gated (opened or closed), allowing the cell to control the entry and exit of ions. In this way cells can change their permeability to certain ions. Ions like Na+, K+, Ca2+ and Cl- diffuse across membranes through specific ion channels.
Carrier proteins
Carrier Proteins have a binding site for a specific solute and constantly flip between two states so that the site is alternately open to opposite sides of the membrane. The substance will bind on the side where it at a high concentration and be released where it is at a low concentration. Important solutes like glucose and amino acids diffuse across membranes through specific carriers.
Co-transport
-Sometimes carrier proteins have two binding sites and so carry two molecules at once.
-This is a coupled movement of two substances via the same carrier protein. It involves a combination of facilitated diffusion and active transport.
-However, this process often combines both passive Facilitated Diffusion for one molecule with Active Transport of the second (energy requiring)
pg 48
