1.3 Membrane Structure

Glycolipid

membrane bound phospholipids that are bound to lipid molecules

Glycoproteins

membrane bound proteins that are bound to polysaccharides

Amphipathic

part of a molecule is hydrophobic and part is hydrophillic

Peripheral Proteins

- hydrophilic on their surface, so are not embedded in the membrane. - Most of them are attached to the surface of integral proteins and this attachment is often reversible. - Some have a single hydrocarbon chain attached to them which is inserted into the membrane, anchoring the protein to the membrane surface

Integral Proteins

- hydrophobic on at least part of their surface and they are therefore embedded in the hydrocarbon chains in the centre of the membrane. - transmembrane; can extend across the membrane, with hydrophilic parts projecting through the regions of phosphate heads on either side.

Describe the evidence from electron microscopy that supported this model.

-A lipid bilayer -Proteins coat outer surface -Proteins don't permeate the lipid bilayer

Why is it important to regulate the degree of fluidity?

-It needs to be fluid enough that the cell can move -It needs to be fluid enough that the required substances can move across the membrane -If too fluid, the membrane could not effectively restrict the movement of substances across itself

What did experiments with the markers find out and what conclusions could be drawn from them?

It showed that membrane proteins are free to move within the membrane rather that be fixed in a peripheral layer.

State the functions of glycoproteins found in the plasma membrane

-Proteins with an oligosaccharide chain attached -Important for cell recognition by the immune system as hormone receptors

Structure of phoshpolipid

1. Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule 2. Consist of two non-polar tails (hydrophobic) composed of two hydrocarbon chains. 3. Because phospholipids contain both hydrophilic (water-loving) and lipophilic (fat-loving) regions, they are classed as amphipathic

Cell membranes are represented according to a fluid-mosaic model because

1. Fluid - the phospholipid bilayer is viscous and individual phospholipids can move position 2. Mosaic - the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components

Davson Danielli Problems

1. It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio 2. It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial) 3. It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores) 4. The temperatures at which membranes solidified did not correlate with those expected under the proposed model

Components of the Plasma Membrane

1. Phospholipids - Form a bilayer with phosphate heads facing outwards and fatty acid tails facing inwards 2. Cholesterol - Found in animal cell membranes and functions to improve stability and reduce fluidity 3. Proteins - May be either integral (transmembrane) or peripheral and serve a variety of roles

Properties of the Phospholipid Bilayer

1. The bilayer is held together by weak hydrophobic interactions between the tails 2. Hydrophilic / hydrophobic layers restrict the passage of many substances 3. Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility 4. This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)

Davson and Danielli

1. The model was described as a 'lipo-protein sandwich', as the lipid layer was sandwiched between two protein layers 2. The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers

Where in the plasma membrane can cholesterol be found?

Attached to the phosphate heads on the periphery of the membrane.

Outline how the electron micrograph image was obtained and why it undermines the Davson-Danielli model of the cell membrane.

Freeze-etched electron micrographs - rapid freezing of cells and then fracturing them. - The fracture occurs along lines of weakness, including the centre of membranes. - Globular structures scattered through freeze-etched images of the centre of membranes shown transmembrane proteins. Structure of membrane proteins. - Improvements in biochemical techniques allowed proteins to be extracted from membranes. - found to be very varied in size and globular in shape so were unlike the type of structural protein that would form continuous layers on the periphery of the membrane. - the proteins were hydrophobic on at least part of their surface so they would be attracted to the hydrocarbon tails of the phospholipids in the centre of the membrane. - "Only peripheral proteins were found associated with a phospholipid bilayer" Fluorescent antibody tagging. - Red or green fluorescent markers were attached to antibodies that bind to membrane proteins. - The membrane proteins of some cells were tagged with red markers and other cells with green markers. - The cells were fused together. Within 40 minutes the red and green markers were mixed throughout the membrane of the fused cell. - showed that membrane proteins are free to move within the membrane rather than being fixed in a peripheral layer. New model needed to explain presence of transmembrane protein

Why was the Davson-Danielli model proposed, what did it help explain?

Helped explain that despite being very thin, membranes are on effective barrier to the movement of certain substances. Explained where proteins were located (though wrongly).

Functions of Membrane Proteins JETRAT

Junctions - Serve to connect and join two cells together Enzymes - Fixing to membranes localises metabolic pathways Transport - Responsible for facilitated diffusion and active transport Recognition - May function as markers for cellular identification Anchorage - Attachment points for cytoskeleton and extracellular matrix Transduction - Function as receptors for peptide hormones

Define the term plasma membrane.

Lipids and proteins that form an external boundary of the cytoplasm of the cell.

Explain how the hydrophobic and hydrophilic properties of phospholipids help to maintain the structure of cell membranes.

Phospholipid molecules make up the cell membrane and are hydrophilic (attracted to water) as well as hydrophobic (not attracted to water but are attracted to other hydrophobic tails). They have a hydrophilic phosphate head and two hydrophobic hydrocarbon tails. Cell membranes are made up of a double layer of these phospholipid molecules. This is because in water the hydrophilic heads will face the water while the hydrophobic tails will be in the center because they face away from the water. The phospholipid bilayer makes the membrane very stable but also allows flexibility. The phospholipid in the membrane are in a fluid state which allows the cell to change it's shape easily.

What type of substances can pass through the phospholipid bilayer?

Small uncharged lipid soluble substances, i.e. oxygen, carbon dioxide

Cholesterol is a type of lipid, but it is not a fat or oil. What group does it belong to?

Steroid

Structure of Membrane Protein

The amino acids of a membrane protein are localised according to polarity: 1. Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer 2. Polar (hydrophilic) amino acids are located internally and face aqueous solutions

Explain how hydrophobic and hydrophilic properties of the phospholipid bilayer allow a membrane to maintain its structure.

They use the emergent property, which allows them to self-organize their heads to stay wet and their tails to stay dry.

Arrangement in Membranes

When phospholipids are mixed with water the phosphate heads are attracted to the water but the hydrocarbon tails are attracted to each other, but not to water. Because of this, the phospholipids become arranged into double layers, with the hydrophobic hydrocarbon tails facing inwards towards each other and the hydrophilic heads facing the water on either side. These double layers are called phospholipid bilayers. They are stable structures and they form the basis of all cell membranes.

Phospholipid

a molecule consisting of 1 phosphate head attached to 2 hydrocarbon tails

Cholesterol

a type of lipid found within mammalian membranes which reduces membrane fluidity and permeability to some solutes

Davson-Danielli model

an early model (1930's) of membrane structure building on Gorter and Grendel's bilayer model suggesting a layer of proteins adjacent to the phospholipid bilayer

Channel proteins

are integral proteins Inside of channel hydrophilic so charged particles can pass them

Cholesterol purpose

reduces the fluidity and permeability of the plasma membrane to hydrophilic particles Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross It increases the flexibility as it prevents the tails from crystallizing and behaving like a solid. It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

Fluid mosaic model

the currently accepted model of membrane structure proposed by Singer and Nicolson in 1966 aka Singer-Nicolson Model - Proteins were embedded within the lipid bilayer rather than existing as separate layers