Cell Membrane & Cell Theory

All living things share seven characteristics, remembered by the acronym MRS GREN. An organism is classified as living only if it displays all seven — and every living organism is built from cells.

1. Movement
2. Respiration
3. Sensitivity
4. Growth
5. Reproduction
6. Excretion
7. Nutrition

CELL THEORY

Cell theory is the unifying statement that connects all living things. It was proposed by Matthias Schleiden and Theodor Schwann (1838–39) and completed by Rudolph Virchow (1855), who added the principle that every cell arises from a pre-existing cell. The theory has three parts:

1. All living organisms are composed of one or more cells (and the products of cells).
2. The cell is the structural and functional unit of life — the chemical reactions of life take place inside cells.
3. New cells arise only from the division of pre-existing cells, inheriting the genes that direct their growth and function.

MEMBRANE STRUCTURE

The cell membrane separates the cytoplasm from the extracellular environment and controls what enters and leaves the cell. It is semi-permeable: some molecules pass through freely, while others are excluded because they are too large or because they carry a positive or negative charge.

The membrane contains two layers of phospholipids. A phospholipid is a lipid in which one of the fatty acids has been replaced by a phosphate group. It is represented as a round hydrophilic (water-attracting) head — the phosphate group — with two hydrophobic (water-repelling) fatty acid tails attached by a glycerol backbone. Because the heads attract water and the tails repel it, the phospholipids arrange themselves into a bilayer: heads facing the watery solutions on the outer and inner surfaces of the membrane, tails pointing inward, away from the water.

CELL MEMBRANE STRUCTURE — a phospholipid bilayer with proteins interspersed. Hydrophilic heads face the watery exterior and interior; hydrophobic tails point inward. Transport proteins span the bilayer and provide a channel through it.

The accepted model of the membrane is the fluid mosaic model (S.J. Singer and Garth Nicolson, 1972). The membrane is a "mosaic" because proteins are scattered through the bilayer like tiles, and "fluid" because the lipids and proteins are free to move around within it. The model has been revised repeatedly over the past 75 years as new technologies allowed each version to be tested against better evidence — an example of science as an ongoing process rather than a finished answer.

• Cholesterol and sterols - lipids positioned between the phospholipids that keep the membrane fluid and flexible; cholesterol in animal cells, sterols in plant cells.
• Membrane proteins - proteins interspersed throughout the bilayer (the membrane is therefore sometimes described as a lipoprotein membrane); they carry out transport, recognition and communication.
• Glycoproteins - proteins with a carbohydrate (sugar) chain attached; they act as receptor molecules and allow cells to recognise one another.
• Membrane enzymes - enzymes positioned in or on the membrane catalyse reactions at its surface; this is especially important in prokaryotic cells, which have no internal organelles to house them.

In summary: phospholipid bilayer → lipids and proteins interspersed → all components free to move → fluid mosaic structure.

INTEGRAL AND PERIPHERAL PROTEINS

Membrane proteins are classified by the way they attach to the bilayer:

• Integral (transmembrane) protein - permanently attached and completely penetrates the bilayer.
• Peripheral protein - temporarily attached, rests on the membrane surface, and never enters the hydrophobic core.

Integral proteins span the bilayer; a glycoprotein is an integral protein carrying a carbohydrate chain; peripheral proteins rest on the surface; cholesterol sits between the fatty acid tails.

Functions of integral proteins:

1. Form transport channels — movement through them can be passive or active.
2. Act as receptor molecules — specifically shaped binding sites for chemicals such as hormones.
3. Act as enzymes, catalysing specific metabolic pathways.
4. Act as adhesion points, connecting neighbouring cells together.
5. Carry attached carbohydrate chains as glycoproteins, allowing cells to recognise each other.

Functions of peripheral proteins:

1. Provide support — anchoring the internal cytoskeleton and the extracellular matrix to the membrane.
2. Enable communication — starting a chain reaction that stimulates a response from DNA or other organelles.
3. Act as enzymes on specific substrates.
4. Transfer electrons — as in the electron transport chain of respiration.

MEMBRANE FUNCTION

The fluid structure of the membrane is critical to its function. Transport processes such as endocytosis and exocytosis fail if fluidity is lost, and the free movement of lipids and proteins is what allows vesicles to fuse with the membrane. Acting as a selectively permeable barrier, the membrane and its channel, carrier and pump proteins control all exchange between the cell and its environment — the subject of the Transport Across Membranes note.