2.4.1 Structure of the membrane.
2.4.2 Properties of the membrane phospholipids
2.4.3 Functions of membrane proteins .
2.4.4 Definitions of diffusion and osmosis.
2.4.5 Passive transport across membranes.
2.4.6 Active transport across the membrane.
2.4.7 Vesicle transport within the cell.
2.4.8 Membrane fluidity and transport across membrane by endocytosis and exocytosis.
This model of the bilayer's has the proteins removed for clarity.
The 'head's have large phosphate groups, thus they are hydrophilic (attract water) or polar. These section are suited to the large water content of the tissue fluid and cytoplasm on opposite sides of the membrane.
The fatty acid tails are non-charged, hydrophobic and repel water. This creates a barrier between the internal and external 'water' environments of the cell. The 'tails' effectively create a barrier to the movement of charged molecules
The individual phospholipids are attracted through their charges and this gives some stability. They can however move around in this plane
The stability of the phospholipid can be increased by the presence of cholesterol molecules.
Diffusion: passive movement of particles from a region of high concentration to a region of low concentration.
This model illustrates the main features of the process of diffusion.
The movement of particles is caused by the kinetic energy possessed by the particle.
The direction of movement is random.
Observing groups of particles they appear to move from regions of high concentration to regions of low concentration.
Alternatively the statement can be in terms of pressure. Movement from a region of high pressure to a region of low pressure.
However, most biological diffusion takes place through membranes and involves sources, sinks and diffusion gradient.
This model shows diffusion through a membrane:
Places that supplies and maintains the high concentration of particles are called the source.
The place where the substances is continually removed (or changed) is called the sink.
Maintaining the concentration gradient between the two areas is a feature of biological systems.
e.g. Source = blood oxygen. Sink= respiring cell
Osmosis:
the passive movement of water molecules, across a partially permeable membrane
from a region of lower solute concentration to a region of higher solute concentration
water moves(kinetic energy) through plasma membranes pores called aquaporin.
In this image of osmosis the solute is represented by the green molecules. The water is represented by the blue molecules.
The water molecules have kinetic energy like other molecules.
The water molecules move randomly and will if they come into contact with the membrane pass straight through.
The tendency is for water to pass from lower solute (left) to higher solute (right) concentrations.
The passive movement implies that there is no expenditure of energy in moving the molecules from one side of the membrane to the other:
Some molecules are so small that they pass through the membrane with little resistance
The include Oxygen and Carbon Dioxide
Lipid molecules ( even though very large) pass through membranes with very little resistance also.
Larger molecules move passively through the membrane via channel proteins
These proteins have large globular structures and complex 3d-shapes
The shapes provide a channel through the middle of the protein, the 'pore'
The channel 'shields' the diffusing molecule from the non-charged regions of the membrane.
Molecules are moved against the concentration gradient form a low to high concentration
Active mean that the membrane protein 'pump' requires energy (ATP) to function
The source of energy is ATP is produced in cell respiration
Transported molecules enter the carrier protein in the membrane.
The energy causes a shape change in the protein that allows it to move the molecule to the other side of the membrane.
The sodium-potassium, pump that creates electro-chemical gradient across the cell membrane of all cells.
Cells are -ve charged on the inside relative to the outside.
This pump is modified in the nerve cell to create some of the electrochemical phenomena seen in nerve cells.
Cells will manufacture molecules for secretion outside of the cell. Some of these secretion molecules are complex combinations of proteins, carbohydrates and lipids. The base protein is coded for by a gene whose expression begins the process. The animation below picks up the process with the protein already synthesised in the rough endoplasmic reticulum. The animation plays very slowly so that the sequence can be followed.
1. Protein is already synthesised and present in the rER.
2. The protein is moved through the rER and modified.
3. A spherical vesicle is formed form the end of the rER with the protein inside.
4. The vesicle migrates to the golgi apparatus.
5. Vesicle and golgi membranes fuse. The protein is released into the lumen of the golgi apparatus.
6. The golgi modifies the protein further.
7. A new vesicle is formed from golgi membrane which then breaks away.
8. The vesicle migrates to the plasma membrane fuses secretes content. A process called exocytosis.
a) Exocytosis: vesicle membrane fuses with the plasma membrane.
b) Endocytosis:a vesicle is formed by the infolding of the plasma membrane
In each of the cases above the membranes are able to form and break without loss of the continuity of the plasma membranes. The process is very similar to the childhood game of playing with bubbles of detergent. Bubbles are produced then they can be watched readily joining together or splitting apart. The process is readily observed in cells but proof of the mechanism was not produced until 2002 by 2002 by Lin Yang and Huey Huang.
Membrane fluidity:
(a) The phospholipid molecules can change places in the horizontal plane. This creates the so called fluid property of the membrane.
(b) Molecule exchange in the vertical plane does not occur. This maintains the integrity of the membrane.
(c) Cholesterol embedded in the membrane reduces its fluidity.
Mechanism for the making and breaking of the cell membrane.
The Li Yang and Huey Huang model has shown that it is the proportion of other molecules in the plasma membrane that will determine whether is opens or closes.
It is suggested that the molecules that initiate membrane fusion or breakage will be:lipids;proteins and cholesterol.
They derived their model by a serendipitous observations whilst performing other experiments.
With x-ray diffraction patterns they showed how the phospholipids will form an hourglass shape at the point of contact. (click image for diffraction x-rays)
The sequence of diagrams shows how a membrane might fuse or split and yet self-seal during a wide variety of biological situations. It has not been lost on the researchers the potential uses of this knowledge and mechanism in medical therapies.
In the model:
(a) Membranes approach.
(b) Touching membranes note how the phospholipid heads flow together starting the process of fusion. As noted above this requires the presence of additional molecules.
(c) At the point of contact there is a single lipid bilayer.
(d) The pore is open and the membranes are now continuous.
Mouse-over the image for evidence x-ray diffraction images.
There are biological issues concerning membranes breaking and forming throughout this course. It is astonishing that such a common concept had to wait until 2002 for Lin Yang and Huey Huang to produce the crucial evidence. This outstanding research will still take some time to reach school textbooks.
