AS Module 1
10.3 Plasma membranes and processes of passage through them.
After this tutorial, you should be able to:-
| Explain the arrangement of phospholipids, proteins and carbohydrates in the fluid-mosaic model of membrane structure. |
| Describe which factors effect diffusion and link to Fick’s law. |
| Relate water potential to osmosis, and describe what factors are required for it to take place. |
| Define the role of protein molecules and energy in active transport and facilitated diffusion. |
Plasma membranes
The plasma membrane separates the cell from its surroundings and is important in controlling what enters and leaves the cell. For this reason much attention has been focused on its structure.
The plasma membrane contains lipids. Lipids are fat molecules of which there are many kinds. Cell membranes consist of 2 layers of a type of lipid called phospholipids. These consist of a hydrophobic (water hating) hydocarbon "tail"and a hydrophilic (water loving) glycerol "head". Although in the body, the cell wall is surrounded on both sides by water, the non-polar ends (hydrophobic) are attracted to each other. These lipids are held together by a protein framework randomly dotted on both sides of the membrane in a mosaic pattern. These proteins are attached to the surface of the membrane while others penetrate into it to varying extents; some of them extend right through it and stick out on the other side. This membrane is a fluid structure which has led to this model of a cell membrane to be called a fluid-mosaic model (see diagram 1).
Diffusion
Diffusion is simply a name for the fact that, in a liquid or gaseous state, molecules will move from an area of
higher concentration to an area of lower concentration. Since materials must get in and out of a cell somehow, they are usually forced in and out by this process.Unfortunately, there is the membrane in the way. For some molecules this isn't much of a problem: they can slip right through, (When a membrane allows a substance to slip through it, it is called
permeable). When a membrane blocks a certain substance, it is called impermeable. When a membrane blocks some substances and not others, it is called semi-permeable.) Others, however, are blocked, and must come in using a different route. There are three main ways for molecules to come in and out of a cell: osmosis, facilitated diffusion, and active transport.
The above demonstrates the principal of diffusion through a membrane. The red dots, which the membrane is
permeable to, spread out into equal numbers on each side. The blue dots, which the membrane is not permeable to, remain the same.Click
here to see an animation of diffusion
Factors effecting Diffusion
The rate at which diffusion
occurs is determined by :- Surface area of membrane.
Thickness of membrane.
The difference in concentration being separated by the membrane.
Fick’s law shows these factors as:-
Osmosis
Although the plasma membrane is fully permeable to respiratory gases, it is by no means permeable to all substances. The
porous nature of the membrane means that only those molecules that are small enough will diffuse through. Larger molecules either penetrate slowly or not at all. Thus, the plasma membrane is said to be selectively permeable or semipermeable, permitting the passage of some substances but not other.Osmosis is really a special case of diffusion : it involves
the passage of water molecules from a region of high water concentration to a region of lower water concentration. The concentration of the water depends on the amount of other molecules within it e.g. sucrose. If a container of water is divided by a semipermeable membrane and one side contains a strong sucrose solution (hypertonic) i.e. less water molecules, whilst the other side contains a weak sucrose solution (more water molecules), then there will be a net movement of water molecules into the hypertonic solution where there are less water molecules.To see an animation of osmosis, click
here.
The water molecules perform this net movement because the water molecules in the weak solution have a greater potential energy than those in the strong sucrose solution. Therefor:
Water will diffuse from a region of high water potential to a region of lower water potential.
Or, put another way:
From a solution of less negative water potential to a solution of more negative water potential.
The larger the gap (gradient) in water potential, the greater the rate at which the water will diffuse.
Active transport
As has been stated, water molecules are small enough to pass through the cell membrane passively (without the use of energy or help), whereas larger molecules such as sugars can not. These larger molecules can enter the cell by using a process called active transport. The proteins which run through the plasma membrane lipid bilayer have the ability to "swallow" a molecule on one side of the membrane and "spit it out" on the other.
If this process of transport transfers molecules from a low concentration outside of the cell to a high concentration inside the cell i.e. going against the concentration gradient, then it is called active transport. This is because the protein in the cell membrane has to actively move the molecule against the concentration gradient.
Click here for a diagram of active transport.
Facilitated diffusion
If however, the proteins in the plasma membrane are transferring molecules from a high concentration to a low concentration, then they are simply helping the natural process of diffusion. For this reason this process is called facilitated diffusion.
Click here for an animation of facilitated diffusion.
The above processes require energy to perform their operation. These energy needs are met by the molecule ATP or adenosine triphosphate. This is broken down by the enzyme ATPase to ADP of adenosine diphosphate. This breakdown releases energy which is used by the proteins to transport material.
Endo and exocytosis
We have considered how the membrane allows ions and molecules to pass into and out of the cell. But there are times when larger particles need to enter or leave the cell. White blood cells ingest bacteria, and amoeba engulf their prey. Membrane transport systems cannot do this job, but the membrane itself can.
Endocytosis is the term used when materials are surrounded by and taken up into membrane-lined vesicles. This can occur at a relatively large scale, for example when bacteria are ingested (phagocytosis).
To see an animation of endocytosis, click here and here.
Exocytosis is the term for the emptying of a membrane-lined vesicle at the surface of the cell. For example, in cells producing hormones, vesicles containing the hormone fuse with the cell surface membrane to release their contents.
Now click on
this to test your knowledge!Cell Biology Links