Cell Structure and the Plasma Membrane

This is Unit Two of Biology, Part Two: the cytoskeleton, the junctions between cells, and the membrane itself. Part One went through the organelles. This part covers what holds a cell in shape, what connects it to its neighbours, and the boundary that decides what gets in. The cytoskeleton is introduced as one thing and then immediately taken apart. There are three fiber types and they resist different stresses — microfilaments take tension, microtubules take compression, and intermediate filaments do neither kind of movement, only structural support. Then three junctions that get confused constantly. A tight junction is a watertight seal. A desmosome is a spot weld that anchors without sealing. A gap junction is a channel that lets molecules through. Sealing, anchoring and passing are three different jobs. The membrane arrives as the fluid mosaic model, and one word in that name needs care. Fluid does not mean loose — a fluid membrane reseals after a fine needle punctures it. The part closes on diffusion, and on a term whose ordinary reading is wrong: equilibrium here is dynamic, so the molecules never stop moving. By the end you will be able to do five things. One: name the three fibers and their stresses. Two: tell the three junctions apart. Three: say what the fluid mosaic model claims. Four: tell integral from peripheral proteins. Five: say what drives diffusion.

Five entries. The first names a network, and its own warning says the network is not one structure. The three that follow are the fiber types, and the useful way to hold them apart is by which stress each one takes. The fifth entry covers two structures built from the widest of them.

The network.

Bio 0297 The Cytoskeleton, Introduced

The cytoskeleton is a network of protein fibers that maintains a cell's shape, anchors organelles and moves cytoplasm and vesicles. It is not one structure but three distinct fiber types, covered in the entries that follow.

The narrowest.

Bio 0299 Microfilaments

Microfilaments are the cytoskeleton's narrowest fibers, built from two intertwined actin strands, and they function in cellular movement and shape. Actin is not confined to muscle — every eukaryotic cell relies on microfilaments.

The tension bearers.

Bio 0301 Intermediate Filaments

Intermediate filaments are cytoskeletal fibers wound from several strands of fibrous protein, bearing tension and anchoring the nucleus. Unlike microfilaments they play no role in cell movement — their function is purely structural support.

The widest.

Bio 0303 Microtubules

Microtubules are the cytoskeleton's widest, hollow fibers, built from tubulin dimers, resisting compression and tracking vesicle movement. Microtubules resist compression the way microfilaments resist tension — different stresses, different fibers.

Built from microtubules.

Bio 0305 Flagella and Cilia

Flagella are long and few, moving an entire cell; cilia are short and numerous, moving the cell or material past it. Despite differing in length and number, both share the same internal nine-plus-two microtubule arrangement.

Two entries on connection. The first is material the cell secretes and then sits in, which makes it easy to mistake for part of the cell. The second is a plant channel that has to cross a wall as well as a membrane to do its job.

Secreted, and outside.

Bio 0307 The Extracellular Matrix

The extracellular matrix is secreted protein and carbohydrate, mainly collagen, that holds animal cells into tissue and lets them communicate. It sits outside the plasma membrane — secreted material, not part of the cell itself.

Through the wall.

Bio 0309 Plasmodesmata

Plasmodesmata are channels through adjoining plant cell walls that connect neighbouring cells' cytoplasm, letting material pass directly between them. They cross both the wall and the membrane — a wall alone would keep neighbouring cells apart.

Four entries. The three junctions are routinely confused with one another, and the distinction is functional rather than structural: one seals, one anchors, one lets things through. The capstone then uses the whole unit so far to identify an unlabelled cell.

A watertight seal.

Bio 0311 Tight Junctions

A tight junction is a watertight seal, built mainly from claudin and occludin proteins, holding two adjacent animal cells tightly together. It seals the space between cells — students often mistake it for a channel like the gap junction, which does the opposite.

A spot weld.

Bio 0312 Desmosomes

A desmosome is a spot-weld junction, built from cadherin proteins linked to intermediate filaments, holding adjacent epithelial cells together. It anchors cells mechanically and does not seal the space between them as a tight junction does.

A channel.

Bio 0313 Gap Junctions

A gap junction is a protein-lined channel, built from six connexin proteins forming a connexon, letting ions and small molecules pass between adjacent cells. It is the animal counterpart to a plasmodesma, though structurally distinct.

The capstone. An unlabelled sample, identified.

Bio 0316 Identifying a Cell from Its Organelles

Identifying whether an unlabelled microscope sample is prokaryotic, an animal cell or a plant cell, using only the organelles visible in it. The reasoning is eliminative rather than confirmatory: no single organelle decides a classification alone, and a wall, a nucleus and a chloroplast each rule out different possibilities. What identifies the sample is the combination that survives.

Three entries introducing the membrane. The model's name contains a word worth pausing on, the molecule that makes the structure inevitable, and a pair of protein classes separated by where they sit rather than by what they do.

The model.

Bio 0321 The Fluid Mosaic Model

The fluid mosaic model describes the plasma membrane as a shifting mosaic of phospholipids, cholesterol, proteins and carbohydrates. A fluid membrane is not a loose one — it reseals after a fine needle punctures it.

Why it orients.

Bio 0323 Membrane Phospholipids, Oriented

A membrane phospholipid is amphiphilic — a hydrophilic head and hydrophobic tails — and that dual character orients every phospholipid the same way. Amphiphilic names a molecule with both a water-loving and a water-fearing region, neither purely polar nor nonpolar.

Two protein classes.

Bio 0325 Integral and Peripheral Proteins

An integral protein spans or embeds within the bilayer; a peripheral protein attaches to the surface without entering it. Location — embedded against surface-attached — is what distinguishes them, not function.

Two entries. A carbohydrate coating that exists on one side of the membrane only, and then an entry showing what happens when a recognition system is used by something it was not built to recognise.

The outer coating.

Bio 0327 The Glycocalyx

The glycocalyx is the carbohydrate coating on a cell's exterior surface, attached to membrane proteins as glycoproteins or to membrane lipids. It exists only on the outward-facing surface — no equivalent coating exists on the cytoplasmic side.

A door already open.

Bio 0328 How a Virus Exploits Membrane Receptors

A virus can enter a cell by binding a cell-surface receptor the cell normally uses for another purpose. The receptor's ordinary job is recognition, and the virus exploits an existing doorway rather than making one.

Two entries on fluidity. The first says it is a property of how the whole membrane packs rather than of any single molecule. The second covers a molecule that does something unusual — it moderates fluidity in both directions at once.

How tightly it packs.

Bio 0329 Membrane Fluidity

A membrane's fluidity depends on how tightly its phospholipid tails pack, and unsaturated tails, kinked by double bonds, pack more loosely. Fluidity is a property of the whole membrane's packing, not of one phospholipid molecule alone.

The buffer.

Bio 0330 Cholesterol's Role in the Membrane

Cholesterol sits alongside membrane phospholipids and buffers the effect of temperature, keeping the membrane fluid at low temperatures and stable at high ones. It moderates fluidity in both directions at once — a buffer, not a fluidizer or a stiffener alone.

Four entries to close. Selective permeability is a matter of degree rather than a gate. Then diffusion, which the cell spends no energy on at all, and the state it arrives at — where the molecules keep moving and only the net direction stops.

Some things, not others.

Bio 0333 Selective Permeability

A plasma membrane is selectively permeable, letting some substances cross freely while blocking others largely by size and polarity. It is not an all-or-nothing gate — the same membrane can pass one substance freely while blocking another almost entirely.

Passive movement.

Bio 0335 Diffusion

Diffusion is the passive movement of a substance from higher to lower concentration until the concentration is equal throughout a space. It needs no cellular energy — the concentration gradient itself, not the cell, drives the movement.

When the gradient goes.

Bio 0337 Dynamic Equilibrium

Dynamic equilibrium is the state a diffusing substance reaches once its gradient disappears — molecules still moving, but with no net movement in any direction. The equilibrium is dynamic, not static: molecular motion never stops, only the net direction does.

What changes the rate.

Bio 0339 Factors Affecting Diffusion Rate

Diffusion rate rises with a steeper gradient, higher temperature, smaller mass and a thinner, more permeable membrane. Solvent density is one factor among several — a denser medium slows diffusion without changing the gradient.

Five things to carry out of Part Two. The cytoskeleton is three fiber types, not one structure. Microfilaments are built from actin and take tension; microtubules are hollow, built from tubulin, and take compression; intermediate filaments give structural support and play no part in movement at all. Flagella and cilia differ in length and number and share the same nine-plus-two core. The three junctions do three different jobs. A tight junction seals the space between cells. A desmosome spot-welds them together without sealing. A gap junction is a channel that lets ions and small molecules through — the animal counterpart to a plant's plasmodesma. The fluid mosaic model describes a shifting arrangement of phospholipids, cholesterol, proteins and carbohydrates. Fluid does not mean loose: the membrane reseals after a puncture. Fluidity comes from how tightly the tails pack, and cholesterol buffers it in both directions rather than pushing it one way. Integral proteins embed in or span the bilayer; peripheral proteins attach to its surface. What separates them is position, not what they do — and the carbohydrate glycocalyx exists only on the outward face, which is the surface a virus binds when it enters through a receptor the cell already uses. And diffusion is driven by the concentration gradient rather than by the cell, which spends no energy on it. Dynamic equilibrium is where it ends: the gradient is gone, the net movement is zero, and the molecules are still moving. Part Three turns to the transport the cell does spend energy on.