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1.3.U.1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
1.3.U.1 Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.
Be able to:
Draw a simplified diagram of the structure of the phospholipid, including a phosphate-glycerol head and two fatty acid tails.
Define hydrophilic and hydrophobic.
Define amphipathic and outline the amphipathic properties of phospholipids.
Explain why phospholipids form bilayers in water, with reference to hydrophilic phosphate heads and two hydrophobic hydrocarbon tails.
Phospholipids are one of the principal components of cell membranes (in conjunction with membrane proteins)
Phospholipids typically share a common basic structure that includes:
A polar organic molecule (e.g. choline, serine)
A phosphate group
A glycerol molecule (replaced by sphingosine in sphingomyelin)
Two fatty acid tails (may be saturated or unsaturated)
Hydrophilic and Hydrophobic Properties
Cell membranes are composed of phospholipids that consist of a hydrophilic (attracted to water) head and a hydrophobic (repelled by water) tail. This property is described as Amphipathic
phospholipid-web
The phospholipid head contains a negatively charged phosphate group which because of its charge is attracted water because of its polarity
The fatty acid hydrocarbon tail has no charge and is therefore repelled by water
When placed in water, the phospholipids naturally form a double layer with the heads facing outwards towards the water and the tails facing each other inwards
This forms a very stable structure that surrounds the cell because of the attractions and bonds that are formed between the heads to the water and to each other, and the hydrophobic interactions between the tails
Even though it is a very stable structure, it is still fluid, as the phospholipids can move along the horizontal plane
To increase stability, many cells have cholesterol embedded between the phospholipids
Phospholipids may vary in the length and relative saturation of the fatty acid tails
Shorter fatty acid tails will increase fluidity as they are less viscous and more susceptible to changes in kinetic energy
Lipid chains with double bonds (unsaturated fatty acids) have kinked hydrocarbon tails that are harder to pack together
1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.
1.3.U2 Membrane proteins are diverse in terms of structure, position in the membrane and function.
Be able to:
State the primary function of the cell membrane.
Contrast the structure of integral and peripheral proteins.
List at least four functions (with example) of membrane bound proteins.
Contrast the two types of transport proteins: pumps and channels.
Hormone binding sites (receptor proteins)
Proteins embedded in the membrane, which bind to specific hormones.
When the hormone binds, it causes the receptor protein to undergo a conformational change, which signals the cell to perform a function.
For example, insulin receptors.
Pumps for active transport
Proteins that use ATP to move substances from a low concentration to a high concentration across the membrane.
For example, Sodium/Potassium (Na+/K+) pumps and the proton (H+) pumps
Immobilized Enzymes
Integral proteins that catalyze specific chemical reactions.
Many of these enzymes catalyze metabolic reactions or are a part of a metabolic pathway, such as ATP Synthase in aerobic respiration.
Cell-to-cell communication
Receptors for neurotransmitters at synapses between two nerve cells.
Glycoproteins on the surface can also be used for cell identification purposes.
Video to watch to complete the functions of membrane protein worksheet
Cell Adhesion
Proteins that form tight bonds between adjacent cells in tissues and organs.
For example, gap junctions.
Channels for passive transport
Integral proteins that span the membrane and provide a passageway for molecules to move from an area of high concentration to low concentration.
Specific proteins are also used for facilitated diffusion.
The extracellular matrix typically provides structural and biochemical support to surrounding cells, including:
Providing sites for anchorage by cells within a tissue and segregating separate tissues from one another
Sequestering and storing growth factors until receipt of a chemical signal (thereby regulating intercellular communication)
In plant cells the extracellular matrix includes cell wall components (like cellulose) and hence plays an important role in:
Regulating water uptake (maintenance of cell turgor)
Providing mechanical strength and rigidity to the cell (maintains cell shape)
1.3.U.3 Cholesterol is a component of animal cell membranes.
1.3.U.3 Cholesterol is a component of animal cell membranes.
Be able to:
Identify the structure of cholesterol in molecular diagrams.
Describe the structural placement of cholesterol within the cell membrane.
Cholesterol is able to stop the hydrocarbon from crystalizing and behaving as a solid, but the cholesterol also restricts molecular motion, which reduces the fluidity of the membrane. Also reduces the permeability to the hydrophilic particles like sodium ions and hydrogen ions
It is absent in plant cells, as these plasma membranes are surrounded and supported by a rigid cell wall made of cellulose
Most of the cholesterol molecule is hydrophobic and therefore embeds within the tails of the bilayer. A small portion (hydroxyl –OH group) is hydrophilic and is attracted to the phospholipid head.
1.3.A.1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
1.3.A.1 Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.
Be able to:
Describe the function of cholesterol molecules in the cell membrane.
Cholesterol embedded in the membrane will reduce the fluidity making the membrane more stable by the hydrophilic interactions with the phospholipid heads. Cholesterol adds firmness and integrity to the plasma membrane and prevents it from becoming overly fluid, it also helps maintain its fluidity by disrupting the regular packing of the hydrocarbon tails.
At the high concentrations it is found in our cell's plasma membranes (close to 50 percent, molecule for molecule) cholesterol helps separate the phospholipids so that the fatty acid chains can't come together and crystallize.
Therefore, cholesterol helps prevent extremes-- whether too fluid, or too firm-- in the consistency of the cell membrane.
1.3.S.1 Drawing of the fluid mosaic model
1.3.S.1 Drawing of the fluid mosaic model
Be able to:
draw and label the structure of membranes.
When drawing the plasma membrane include:
Phospholipid bilayer
Integral proteins shown spanning the membrane
Peripheral proteins on membrane surface
Protein channels with a pore
Glycoproteins with a carbohydrate side chain
Cholesterol between phospholipids in the hydrophobic region
An indication of thickness (10nm)
Integral proteins are embedded in the phospholipid of the membrane, whereas peripheral proteins are attached to its surface. Glycoproteins are carbohydrates attached to surface proteins.
NOTE: When you draw a peripheral protein for the IB exam, the peripheral protein must not be embedded into the membrane in order to score the point. Also, you should label the whole phospholipid bilayer not just the individual phospholipid on the majority of mark scheme
1.3.S.2 Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.
1.3.S.2 Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.
Be able to:
Describe the observations and conclusions drawn by Davson and Danielli in discovering the structure of cell membranes
The fluid-mosaic model was not the first scientifically accepted paradigm to describe membrane structure. The first model that attempted to describe the position of proteins within the bilayer was proposed by Hugh Davson and James Danielli in 1935. Davson and Danielli proposed that the lipid bilayer was coated on either side with a layer of globular proteins. The hydrophobic tails of the lipids are orientated towards each other, while the hydrophilic heads are oriented to the outside. Although the membrane composition is correct, there are some problems with the proposed model:
Membranes are not identical. They differ in thickness and the ratio of proteins:lipids.
Membranes have distinct inside and outside layers (defined by the membrane proteins which are present on the surface of the membrane)
Other than predicted by the model, the membrane proteins do not have a very good solubility in water - in fact they have hydrophilic and hydrophobic regions. The hydrophobic side is anchored inside the membrane.
When the membrane proteins would cover the lipid bilayer, their hydrophobic regions would be in contact with water, which destabilizes this construct. Even if they would be oriented towards the membrane, they would face towards the hydrophilic heads of the phospholipids causing the same effect. Additionally the proteins would also separate the hydrophilic phospholipid heads from the water. So there is no real stable solution in embedding the membrane with proteins.
The model was described as a 'lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers
The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers
1.3.S.3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
1.3.S.3 Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.
Be able to:
Describe conclusions about cell membrane structure drawn from freeze-etched electron micrograph images of the cell membrane.
Describe conclusions about cell membrane structure drawn from cell fusion experiments.
Describe conclusions about cell membrane structure drawn from improvements in techniques for determining the structure of membrane proteins..
Compare the Davson-Danielli model of membrane structure with the Singer-Nicolson model.
There were a number of problems with the lipo-protein sandwich model proposed by Davson and Danielli:
It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
It did not account for the permeability of certain substances (did not recognize the need for hydrophilic pores)
The temperatures at which membranes solidified did not correlate with those expected under the proposed model
Falsification Evidence:
Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size
Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane
Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place
Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively
When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)
Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane
These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localized to the outside of the membrane structure
New Model:
In light of these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972
According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers
This model, known as the fluid-mosaic model, remains the model preferred by scientists today (with refinements)
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