Cell Communication and Cell Reproduction

This is Unit Four of Biology, Part One: signals, and what receives them. The unit opens on how cells talk to each other, and the first thing to fix is that a signal is not broadcast to whoever is nearby. A ligand affects only cells carrying a receptor for it. Release does not equal effect. Four signalling modes then sort by distance and duration. Paracrine signals act locally and briefly — and their short range comes from being destroyed quickly, not from being weak. Endocrine signals travel in blood, arrive dilute, and last longer; that dilution is a consequence of the bloodstream, not evidence of a feeble signal. Receptors then divide by location, and one type behaves quite differently from the rest. An internal receptor binds a ligand that crossed the membrane on its own, and the resulting complex acts on D N A directly — with no cascade at all. The three surface receptor types are told apart by what binding triggers: a channel opening, a separate protein being switched on, or an enzyme activating. The G-protein-linked receptor does none of the work itself — it only throws a switch. By the end you will be able to do five things. One: tell the signalling modes apart. Two: say why a ligand is selective. Three: say what an internal receptor does. Four: tell the three surface receptors apart. Five: say what keeps a ligand out of the membrane.

Two entries. The first distinguishes signalling between cells from signalling within one, and its warning says the two are stages rather than rivals. The second names the pair every signal in this unit depends on.

Between, and within.

Bio 0761 Intercellular and Intracellular Signaling

Intercellular signalling is communication between cells; intracellular signalling carries a message within one cell, from receptor to response. The two levels connect rather than compete — most intercellular signals end by triggering an intracellular one.

The pair.

Bio 0763 Ligands and Receptors

A ligand is a signalling molecule released by one cell that binds a specific receptor protein on or in a target cell. A ligand affects only receptor-bearing cells — releasing it does not affect every cell it reaches.

Four entries sorted by how far a signal travels. Two of the warnings correct the same instinct — that a short-range or dilute signal must be a weak one. Neither is true, and both have specific causes.

Nearby cells.

Bio 0765 Paracrine Signaling

Paracrine signalling releases a ligand that diffuses to nearby cells, producing a fast, short-lived, locally concentrated response. The short range comes from rapid degradation of the ligand, not from the ligand being weak or slow-acting.

A paracrine case.

Bio 0767 Synaptic Signaling, a Paracrine Case

A synapse is the small gap between two nerve cells, where the presynaptic cell releases neurotransmitters — a paracrine ligand. A nerve impulse travels electrically inside one cell; only the crossing between cells is chemical.

Through the blood.

Bio 0769 Endocrine Signaling

Endocrine signalling releases hormones into the bloodstream, producing a slower but longer-lasting response over long distances. A hormone's low circulating concentration is a consequence of dilution in blood, not a sign the signal is weak.

Back to itself.

Bio 0771 Autocrine Signaling

Autocrine signalling releases a ligand that binds receptors on the same cell that released it, or on nearby cells of the same type. It is self-directed by definition — not merely paracrine signalling renamed for a shorter distance.

One entry, and it is the exception to everything in the previous topic. Some signals never enter the space between cells at all — they pass through a channel joining two cytoplasms directly, which imposes a size limit.

Cytoplasm to cytoplasm.

Bio 0773 Direct Signaling via Gap Junctions and Plasmodesmata

Gap junctions in animal cells and plasmodesmata in plant cells are direct channels letting small signalling molecules pass between adjacent cells. They admit only small molecules and ions — proteins and D N A are far too large to pass.

Two entries dividing receptors by where they sit. The internal one is the outlier in this whole unit: it produces a response without any of the transduction machinery Part Two is about.

In the cytoplasm.

Bio 0775 Internal Receptors

An internal receptor sits in the cytoplasm and binds a hydrophobic ligand small enough to cross the membrane unaided. No second-messenger cascade is needed — the resulting complex acts on D N A directly, without transduction.

Spanning the membrane.

Bio 0777 Cell-Surface Receptors: Three Domains

A cell-surface receptor spans the membrane with an extracellular domain that binds ligand, a transmembrane domain, and an intracellular domain. The receptor converts the signal at the membrane — the ligand's job ends at binding, and it is never delivered inward.

Four entries. Three receptor types, told apart by what binding actually triggers, plus the procedure entry that walks the switching cycle of the second one — including how it turns itself back off.

Opens a channel.

Bio 0779 Ion Channel-Linked Receptors

An ion channel-linked receptor opens a membrane channel when its ligand binds, letting specific ions flow down their gradient. The channel is closed by default and opens only while the ligand remains bound — not permanently once triggered.

Throws a switch.

Bio 0781 G-Protein-Linked Receptors

A G-protein-linked receptor crosses the membrane seven times and, once bound by ligand, activates a separate G-protein. The receptor itself does not carry out the response — it only switches the G-protein on.

On, then off again.

Bio 0783 The G-Protein Activation Cycle

A heterotrimeric G-protein cycles between an inactive G D P-bound state and an active G T P-bound state, resetting itself once the signal has passed. The reset is its own G T P hydrolysis — and a signal will not terminate on schedule if that hydrolysis step is impaired.

Activates an enzyme.

Bio 0785 Enzyme-Linked Receptors and Receptor Tyrosine Kinases

An enzyme-linked receptor has an intracellular domain that is itself an enzyme or binds one directly; a receptor tyrosine kinase is the common case. Phosphorylation follows dimerisation — an unpaired receptor tyrosine kinase does not activate on its own.

Three entries. A ligand's chemistry decides where its receptor has to be: one crosses the membrane freely, one cannot cross at all, and one is a gas whose reach is limited by how briefly it exists.

Crosses unaided.

Bio 0787 Small Hydrophobic Ligands: Steroid Hormones

A steroid hormone is a small, hydrophobic ligand built on a four-ring skeleton, able to diffuse across the plasma membrane. It is not water-soluble — a carrier protein, rather than its own polarity, is what keeps it circulating in blood.

Cannot cross.

Bio 0789 Water-Soluble Ligands

A water-soluble ligand is polar, and either cannot cross the plasma membrane or cannot cross it unaided, so it binds a surface receptor instead. Polarity, not size alone, is what keeps it out of the membrane's hydrophobic core.

A gas.

Bio 0791 Nitric Oxide as a Gaseous Ligand

Nitric oxide is a short-lived gaseous ligand that diffuses directly across the plasma membrane and acts only over short distances. Its very short half-life limits its reach — it cannot function as a long-distance, endocrine-style ligand.

Five things to carry out of Part One. The signalling modes sort by range and duration. Paracrine acts locally and briefly because the ligand is destroyed quickly. Endocrine travels in blood, arrives dilute and lasts longer. Autocrine returns to the cell that sent it — self-directed by definition, not paracrine over a shorter distance. A ligand affects only cells that carry a receptor for it. Releasing a signal is not the same as producing an effect, and most of the cells a hormone physically reaches will do nothing at all. An internal receptor is the outlier. Its ligand crosses the membrane unaided, and the resulting complex acts on D N A directly — no second messenger, no cascade, none of the machinery the next part is about. The three surface receptors differ by what binding triggers. An ion channel opens while the ligand stays bound. A G-protein-linked receptor throws a switch and does nothing else itself. An enzyme-linked receptor activates an enzyme, and a receptor tyrosine kinase must pair up before it can. And a ligand's chemistry decides where its receptor sits. Polarity keeps a water-soluble ligand out of the membrane's core, so it binds at the surface. A steroid crosses freely and rides a carrier protein in blood. Nitric oxide crosses too, and its half-life keeps it local. Part Two follows what happens after binding.