Membrane structure Flashcards
What are the characteristics of the phospholipid membrane?
It is fluid, flexible and dynamic
Why does a phospholipid bilayer form in water?
Triglycerides have one 1 glycerol and 3 fatty acids. The phospholipids have one fatty acid replaced by a phosphate group.
The phosphate heads are hydrophilic (water-loving, polar) and the fatty acids tails are hydrophobic (water-hating, non-polar).
The phospholipid bilayer forms in water due to the amphipathic properties of the molecules.
What differs the membrane proteins from one another?
They have diverse structures, carry out different functions and have different positions in the plasma membrane.
Their job is to assist the membrane in carrying out its function.
What does amphipathic mean?
Having hydrophobic and hydrophilic properties
What are integral and peripheral membrane proteins?
It depends on their position in the plasma membrane. Integrals are embedded in the plasma membrane.
Peripheral proteins are attached to the outside of the membrane.
What are some examples of the functions of the membrane proteins?
Channels: these proteins have a channel that allows the passive transport of substances
Carriers: these proteins bind to substances on one side, change shape and transport them to the other side
Recognition: these proteins help differentiate between self and non-self
Enzymes: speed up the rate of reaction
Glycolipids: a phospholipid and carbohydrate attached together. Helps maintain structure
A pump on the other hand uses energy and is therefore involved in active transport
What does Cholesterol do in the cell membrane?
- It is a steroid that is found in animal cell membranes and maintains the structure of the membrane.
- It is an amphipathic molecule and is an integral structure.
- Its presence restricts the movement of phospholipids, which reduces the membrane fluidity.
- At lower temperatures it disrupts the regular structure of the hydrocarbon tails which prevents the solidification of the membrane (more fluid)
- Cholesterol also reduces membrane permeability (lets fewer substances through) to some substances
- The fluidity of membranes allows the mixing of dyes
- It anchors the peripheral proteins (which are not amphipathic) into the cell membrane
- Cholesterol is composed of a non-polar part (a four-ring structure, which is hydrophobic), a hydrocarbon tail and a polar hydroxyl group
- The majority of cholesterol is hydrophobic (non-polar) therefore it is attracted to the hydrophobic hydrocarbon tails in the centre and disrupts their regular packing
- The hydroxyl group of cholesterol is hydrophilic (attracted to phospholipid heads)
- The fluidity of the membranes must be closely monitored so that an exchange of substances can still take place
What structures must be included when drawing a fluid mosaic model?
- Glycoprotein
- Glycolipid
- Carbohydrate
- Integral protein (included different membrane proteins)
- Peripheral protein
- Cholesterol
- Phospholipid bilayer (head and tail included)
Be able to draw the structure 2D
What evidence from electron microscopy has led to the proposal of the Davson-Danielli model?
- The total lipid SA to the cell membrane was 2:1
- The cells are covered by a fatty molecule layer that is two molecules thick
- The Davson-Danielli model suggests the membrane is made up of a lipid bilayer while the heads are arranged outwards
- It also suggested that two protein layers coat each side of the membrane
- In the electron microscopy three layers were visible (one of which were dark)
- Proteins appear dark, the middle part was lighter-coloured (phospholipids are lighter)
What was the evidence for the falsification of the Davson-Danielli model?
- Proteins are amphipathic however, the majority of the protein is non-polar (hydrophobic) which makes it unlikely that they are found in contact with an aqueous environment.
- Also this model assumed that all cell membranes had identical structures, but different membranes carry out different functions
- Using the freeze-fracture techniques, the embedded proteins were confirmed
The Singer and Nicholsen model proposed 1972
- proteins are individually embedded, so that the hydrophilic proteins and phospholipid heads are maximally exposed (instead of covered) to water
- It also showed that hydrophobic proteins and phospholipid tails were in non-aqueous areas inside the bilayer
How are amphipathic properties linked to polar and non-polar?
Charged —> polar —> likes water
The hydrophobic tails repel the polar ions because the ions like water (hydrophilic)
Polar: hydrophilic (water-loving)
Non-polar: hydrophobic (water-hating)
