Cell Membranes

Cells depend on the cell membrane to separate the inside of a cell from its external environment in order to maintain its internal environment, and to regulate the movement of materials from one environment to the other. In order to do that, the cell membrane must;

1) Transport raw materials into the cell

2) Transport waste out of the cell

3) Prevent the entry of unwanted matter

4) Prevent the escape of matter needed inside the cell to perform cell functions

When cell membranes were first viewed using electron microscopy, it was discovered that the cell membrane is in fact a bilayer, which means that it is composed of two layers of molecules. When this bilayer was analyzed chemically, it was discovered that it was composed of phospholipids, which is composed of two fatty acids bonded to a glycerol backbone. These fatty acids form nonpolar, hydrophobic tails, and the phosphate chain forms a polar, hydrophilic head.

Hydrophobic - Not attracted to water

Hydrophilic - Attracted to water

These phospholipids which make up a cell membrane spontaneously arrange themselves into a spherical bilayer because their water-attracting heads face both the inside and the outside of the sphere, whereas their hydrophobic tails face each other in the middle of the bilayer.

Because of the fact that lipids do not dissolve in water, the phospholipid bilayer creates a border to help contain the more fluid lipid centre of the cell. However, the cell does not just contain the phospholipids in its border, it also contains numerous proteins studded in the phospholipid bilayer, as well as cholesterol molecules. Cholesterol allows animal cell membranes to function in a range of temperatures. At high temperatures, cholesterol helps maintain rigidity and at low temperatures, it helps maintain membrane fluid, flexibility and functionality. Cholesterol also helps the membrane become less permeable to most biological molecules. Plants have a different lipid other than cholesterol.

Osmosis

Osmosis is the diffusion of a solvent across a semi-permeable membrane separating two solutions. For cells, the solvent is water, and the concentration from a region of high to low applies to the rule of diffusion.

There are three directions of osmosis when it applies to cell membranes;

1) Isotonic conditions - This is when the water concentration inside the cell equals the water concentration outside the cell. Therefore, equal amounts of water interchange between the external environment and the internal environment of the cell. Ex. Blood plasma and the fluid are usually isotonic.

2) Hypotonic conditions - This occurs when the water concentration outside the cell is greater than inside the cell. Therefore, water moves into the cell. The cell may burst if too much water is moved into it. The destruction of a cell through this process is called lysis.

3) Hypertonic conditions - This occurs when the water concentration inside the cell is greater than outside the cell. Therefore, water moves out of the cell. When water moves out of a cell, the process is called plasmolysis. This results in a higher concentration of solutes in the extracellular fluid.

The Fluid-Mosaic Model

In order for a cell to continue performing its life functions, the conditions inside of it must remain nearly constant. This steady state that results from maintaining constant conditions is called homeostasis. The cell membrane is responsible for maintaining homeostasis. In order to for it to do this, the cell membrane is semi-permeable, allowing some molecules to pass, and others to not. In addition to this, every cell in a multicellular organism is bathed in a thin layer of extracellular fluid, which consists of a mixture of water and other dissolved materials. On both sides of the cell membrane , water is the solvent, which can be thought of as the meeting place for all molecules.

How Cells Obtain Nutrients

Diffusion is the movement of molecules from a region of low concentration to a region of high concentration or vice versa. Some molecules can move easily through the cell membrane, such as small uncharged ones like oxygen. To explain how diffusion works, you have to consider the fact that molecules are in constant motion. They collide with each other and with the walls of their container, changing direction and speed. This random movement of molecules is known as Brownian motion. This drives the process of diffusion.

In diffusion, water acts as the solvent that dissolves other substances, called solutes. Diffusion always results in the net movement from areas with high concentration to low concentration, and this difference of concentrations between these regions is called the concentration gradient. It also results in the equal concentration of water on both sides of a cell membrane.

There is also a different type of membrane protein other than the carrier protein, which is a channel protein. Channel proteins have a tunnel-like shape that allows CHARGED particles (ions) to pass through the lipid bilayer. In order for the ion to pass through a channel protein, it must be small enough and have the right charge. A positively charged channel protein repels positively charged ions and a negatively charged channel protein repels negatively charged ions. In order for channel proteins to function, NO cellular energy is required.

Diffusion may not allow certain substances that are vital to a cell's health to pass the cell membrane. Therefore, a cell membrane is semi-permeable and has specialized transport proteins in the cell membrane to help different kinds of substances move in and out of the cell. Because of the structure of these transport proteins, they are highly selective about what they let in and take out of the cell. The transport protein will only help move one type of dissolved molecule or ion based on the shape, size, and electrical charge. It will accept only a NON-charged molecule with a specific shape. A carrier protein facilitates the movement of molecules from a region of high concentration to a region of low concentration. This is called facilitated diffusion.

Cells do not completely depend on passive transport to obtain nutrients, such as diffusion, channel proteins and carrier proteins. Passive transport would allow some toxic wastes to remain inside a cell and a cell must concentrate nutrients inside in order to maintain a intracellular environment vastly different from outside the cell. In order to do this, cell membranes use active transport, which is a process of moving substances against their concentration gradient.

When a person is resting, his/her cells use up to 40% of their energy just on active transport. But some cells use more than others, such as kidney cells which have to filter your blood and use 90% of their energy on active transport. Some examples of substances needed for active transport for certain cells are;

- Kidney cells pump glucose and amino acids out of the urine and into the blood.

- Intestinal cells pump in nutrients from the gut.

- Root cells pump in nutrients from the soil.

- Gill cells pump out sodium ions from sea water.

The pump that drives active transport in every animal cell is known as a sodium potassium pump which pumps sodium and potassium ions. When 3 positive sodium ions inside the cell and 2 positive potassium ions from outside the cell bind to the transporter's protein complex, the transporter uses a form of energy called ATP and this allows the protein to change shape and move the sodium ions outside of the cell and the potassium ions inside the cell. After this process, the pump returns back to its original shape.

Another protein in the cell membrane pumps unwanted hydrogen ions (H+) out of the cell to keep the cell's interior from become too acidic. It does this by using the energy stored in the sodium-ion concentration gradient to push another positive ion OUT of the cell.

There is a constant tendency for sodium ions to diffuse into the cell and for potassium ions to diffuse out, so the sodium-potassium pump must work constantly. Therefore, it is no surprise that there is a lot of energy used by the cell. Such high energy usage by this pump is caused by rapid, repeated changes of shape in a transporter protein complex.

The sodium potassium pump is also used to bring glucose and amino acids into the cell. How this works is by using a type of carrier membrane protein to help the sodium ion and a molecule, like glucose, enter the cell. A sodium ion and a glucose molecule binds to the carrier protein, and the protein changes shape, allowing the sodium ion to ride down the concentration gradient into the cell along with the glucose molecule with the provided energy. Plant and bacterial cells use hydrogen instead of sodium to do this.

Some materials that they cell must take in are too big or too polar to cross through the cell membrane by passive transport or active transport. Therefore, the cell must use another method that does not require the substance to pass through the lipid bilayer. This method is endocytosis, in which the cell membrane folds in on itself to create a membrane enclosed sac, called a vesicle or vacuole.

There are three main forms of endocytosis;
1) Pinocytosis - Involves the intake of a small droplet of extracellular fluid.
2) Phagocytosis - Involves the intake of a large droplet of extracellular fluid, such as bacteria or bits of organic matter.
3) Receptor-assisted endocytosis - Involves the intake of specific molecules that attach to special proteins in the cell membrane that serve as receptors. Ex. Animal cells bring in cholesterol using this method.

Exocytosis is the reverse of endocytosis,where the vesicle from inside the cell moves to the cell surface. When this happens, the vesicle membrane fuses with the cell membrane and restores the membrane lost during endocytosis.