How a Thin, Oily Film Decides What Enters Every Cell

Every leaf, muscle and drop of blood is built from cells, and nothing smaller can carry out all the activities of life on its own. Each cell is wrapped in a membrane only a few molecules thick that decides what goes in and what stays out.

This lesson covers the cell theory and the eukaryotic cell, the cell wall and plasma membrane, and how substances cross the membrane.

The cell theory states that all living organisms are made of cells and products of cells, and that new cells arise only from pre-existing cells; a eukaryotic cell has a plasma membrane, cytoplasm with membrane-bound organelles, and a nucleus enclosed by a nuclear envelope.

The cell theory:

- All living organisms are composed of cells and products of cells
- All cells arise from pre-existing cells
- Schleiden and Schwann together proposed that plants and animals are made of cells, and Rudolf Virchow explained that new cells form by the division of pre-existing cells

General structure of a eukaryotic cell:

- Plasma membrane — the outer boundary of every cell
- Cell wall — outside the membrane in plants and fungi
- Cytoplasm — the fluid matrix in which organelles are suspended
- Organelles — endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, mitochondria, plastids, ribosomes, microbodies and cytoskeleton
- Nucleus — a double-membraned envelope with pores, holding chromatin and a nucleolus

An everyday example. An onion peel seen under a school microscope shows rows of brick-shaped cells, each with a cell wall and, once stained, a dark nucleus.

The substance. Viruses are the boundary case of the cell theory — they are not made of cells and cannot multiply outside one, which is why they are not considered true living cells.

The plant cell wall is a rigid, non-living layer of cellulose, hemicellulose, pectins and proteins, cemented by a middle lamella and crossed by plasmodesmata, while the plasma membrane is a fluid lipid bilayer in which proteins float like tiles in a mosaic.

Cell wall:

- Composition — cellulose, hemicellulose, pectins and proteins in plants; chitin in fungi; cellulose, galactans, mannans and calcium carbonate in algae
- Primary wall — thin and able to grow, formed in young cells
- Secondary wall — laid down inside the primary wall as the cell matures
- Middle lamella — a layer mainly of calcium pectate that cements neighbouring cells together
- Plasmodesmata — fine cytoplasmic strands that pass through the walls and middle lamella, connecting the cytoplasm of neighbouring cells
- Functions — shape, mechanical support, protection from bursting, and a barrier against pathogens

Plasma membrane — the fluid mosaic model:

- A lipid bilayer of phospholipids, with polar heads facing outwards and non-polar tails facing inwards
- Integral proteins are buried in the bilayer, and peripheral proteins lie on its surface
- The quasi-fluid lipid layer lets proteins move sideways within it
- The model was proposed by Singer and Nicolson
- This fluidity allows cell growth, the formation of junctions, secretion, endocytosis and cell division

An everyday example. A crisp cucumber slice going limp in salted water loses water through its membranes by osmosis, while its cell walls keep the cells from collapsing completely.

The substance. The cell wall is freely permeable, while the plasma membrane is selectively permeable — the wall gives support, but only the membrane controls what enters the cell.

Passive transport moves substances down their concentration gradient without energy, facilitated diffusion is passive transport that needs a carrier or channel protein, and active transport moves substances against their gradient using energy from ATP.

Passive transport:

- Movement from higher to lower concentration, needing no energy
- Simple diffusion — small, non-polar molecules such as oxygen and carbon dioxide pass straight through the lipid bilayer
- Osmosis — water moves across a selectively permeable membrane from a dilute solution to a more concentrated one

Facilitated diffusion:

- Still down the gradient, and still without energy
- Polar molecules and ions, which cannot cross the non-polar lipid layer, pass through carrier or channel proteins
- Carriers are specific and can become saturated when all of them are in use

Active transport:

- Movement against the concentration gradient, from lower to higher concentration
- Needs energy from ATP and carrier proteins that act as pumps
- Example: the sodium-potassium pump, which moves sodium ions out of the cell and potassium ions in

Comparing the three:

- Energy — none, none, ATP
- Carrier protein — not needed, needed, needed
- Direction — down, down, against the gradient

An everyday example. The scent of an incense stick spreading through a room is diffusion, while root cells pulling mineral ions out of dilute soil water rely on active transport.

The substance. Facilitated diffusion is easily confused with active transport because both use proteins — the deciding feature is energy, not the presence of a carrier.