Cell Structure and the Plasma Membrane

This is Unit Two of Biology, Part Three: membrane transport. Part Two ended with diffusion, which the cell spends nothing on. This part covers everything else that crosses a membrane, and the organising question is simple: does the cell pay? The first answer is that using a protein does not mean paying. Facilitated transport moves substances through channels and carriers and is still passive, because the gradient supplies the movement and the protein only removes the obstacle. What defines active transport is direction — against the gradient — not machinery. Then osmosis, and one term that cannot be used alone. Tonicity describes a relation between a solution and a particular cell's cytoplasm. Calling a solution isotonic without naming the cell drops the reference point the word depends on, and the book gives that its own misconception entry. The consequences are physical. A cell in the wrong solution bursts or shrivels — and a plant cell, because it has a wall, does neither in the same way. The part closes on bulk transport, where material is wrapped in membrane rather than passed through a protein, and a capstone diagnosing three cells that have each failed differently. By the end you will be able to do five things. One: say why facilitated transport is passive. Two: say what tonicity is relative to. Three: tell active from facilitated transport. Four: say what makes the pump electrogenic. Five: tell the endocytosis variants apart.

Three entries. The first makes the point the whole part turns on — a protein in the path does not make transport active. The next two are the protein types, and the second has a limit the first does not: it can run out.

Helped, not driven.

Bio 0341 Facilitated Transport

Facilitated transport moves a substance down its existing concentration gradient with help from a membrane protein, using no cellular energy. It stays passive — the gradient supplies the movement, and the protein only removes the membrane's obstacle.

A pore.

Bio 0343 Channel Proteins

A channel protein forms a hydrophilic pore for a specific substance, staying open at all times or gated to open on a condition. A channel is specific to one substance or a narrow set — not to whatever happens to fit.

Binds, and changes shape.

Bio 0345 Carrier Proteins

A carrier protein binds a specific substance and changes shape to move it across the membrane, saturating once every carrier is occupied. Past saturation, raising the gradient further does not raise the transport rate — the carriers become the limit.

Two entries. Osmosis moves one substance and one only, and the substance driving that movement is not the one that moves. The second entry supplies the two measures that describe the situation, and warns that one of them counts something narrower than it appears.

Water, specifically.

Bio 0347 Osmosis

Osmosis is the diffusion of water across a selectively permeable membrane, toward the side with lower water concentration. It moves water specifically — the solute driving that movement never crosses the membrane at all.

Two measures.

Bio 0349 Osmolarity and Tonicity

Osmolarity measures a solution's total solute concentration; tonicity describes how that osmolarity, compared to a cell's, moves water. Osmolarity counts dissolved particles, not suspended cells — a cell-crowded solution can still read lower.

Four entries. The three tonicity terms, then a misconception entry devoted entirely to the fact that none of them means anything stated alone, and then the physical consequences — which differ for a cell with a wall and a cell without one.

The three terms.

Bio 0351 Hypotonic, Isotonic, and Hypertonic Solutions

A hypotonic solution draws water in; a hypertonic one draws water out; an isotonic one, matching the cell, moves no net water. These terms are relative to the cell's own cytoplasm and are never a property of the solution alone.

The standing error.

Bio 0352 "Isotonic Means the Same for Every Cell"

Calling a solution isotonic without naming which cell it compares to drops the reference point the term depends on. Tonicity has no meaning stated alone — it describes a relation between one solution and one cell's cytoplasm.

What goes wrong.

Bio 0354 Osmotic Lysis and Crenation

A cell swelling past its membrane's capacity will lyse, or burst; one losing too much water will crenate, shrinking and shrivelling. A mistakenly hypotonic or hypertonic intravenous solution can lyse or crenate blood cells by exactly this mechanism.

With a wall.

Bio 0356 Turgor Pressure and Plasmolysis

Turgor pressure is cytoplasm pushing against a cell wall; plasmolysis is the membrane pulling away from that wall as water is lost. A walled cell cannot lyse the way an unwalled one does — the wall limits how far the membrane can expand.

Two entries. The first defines active transport by direction rather than by mechanism, which is the distinction Topic 1 set up. The second complicates the picture for charged particles, which answer to two gradients that do not always agree.

Low to high.

Bio 0358 Active Transport

Active transport moves a substance against its concentration gradient, low to high, requiring the cell to spend energy. It is defined by direction relative to the gradient — facilitated transport also uses proteins but spends nothing.

Charge, as well as concentration.

Bio 0360 Electrochemical Gradient

An electrochemical gradient combines a charged particle's concentration gradient with the membrane's electrical charge difference. For some ions both gradients pull the same way; for others, such as potassium, they pull in opposite directions.

Three entries. The best-known pump, whose unequal ratio has a consequence worth stating; then a distinction between spending ATP directly and living off a gradient someone else paid for; then a naming scheme that describes direction only.

Three out, two in.

Bio 0362 The Sodium-Potassium Pump

The sodium-potassium pump uses A T P to move three sodium ions out for every two potassium ions in, both against their gradients. The unequal ratio makes the pump electrogenic, shifting the cell interior slightly more negative each cycle.

Direct, or borrowed.

Bio 0364 Primary and Secondary Active Transport

Primary active transport uses A T P directly against a gradient; secondary active transport spends no A T P, drawing on a gradient a primary pump established. Secondary transport still moves a substance against its gradient — it borrows energy another pump paid for.

Direction and count.

Bio 0366 Uniporters, Symporters, and Antiporters

A uniporter carries one substance; a symporter carries two in the same direction; an antiporter carries two in opposite directions. These terms describe direction and count only, independent of whether the transport is active or facilitated.

Five entries. Material too large for any protein gets wrapped in membrane instead. Three variants differ by what they take in — particles, fluid, or one specific substance a receptor recognises — and the last entry runs the whole mechanism backwards.

Wrapped in membrane.

Bio 0368 Endocytosis

Endocytosis is active transport bringing large particles into a cell by folding the plasma membrane around the particle and pinching off a vesicle. It moves material by enclosing it in membrane, not by passing it through existing proteins.

Particles.

Bio 0370 Phagocytosis

Phagocytosis is the endocytosis variant in which a cell engulfs a large particle, extending its membrane around the target before enclosing it. It engulfs particulate matter specifically — taking in surrounding fluid is a separate process.

Fluid.

Bio 0372 Pinocytosis

Pinocytosis is the endocytosis variant taking in extracellular fluid and its dissolved small molecules, producing a much smaller vesicle. Despite its name it is not purposeful drinking — it takes in whatever small molecules happen to be nearby.

One substance.

Bio 0374 Receptor-Mediated Endocytosis

Receptor-mediated endocytosis uses a specific receptor to bind one substance before the membrane folds inward. A missing or defective receptor blocks the pathway entirely — the substance stays in circulation rather than entering the cell.

Outward.

Bio 0376 Exocytosis

Exocytosis is active transport expelling material from a cell by fusing a membrane-bound vesicle with the plasma membrane. Exocytosis and endocytosis move membrane material in opposite directions, and both spend cellular energy.

One entry to close the unit. Three cells are each failing at something, and the diagnosis works by elimination: what a cell can still do narrows which mechanism is broken. The finding it turns on is that bulk transport can fail while everything else works.

The capstone. Three cells, three failures.

Bio 0473 Diagnosing a Membrane Transport Failure

Diagnosing which membrane transport mechanism has failed in each of three malfunctioning cells, using only the transport concepts this part established. The reasoning is eliminative: what a cell can still do rules out mechanisms that must therefore be intact. The key finding is that a cell can transport water, ions and solutes normally while still failing at endocytosis, because bulk transport depends on a distinct mechanism.

Five things to carry out of Part Three. A protein in the path does not make transport active. Facilitated transport moves substances through channels and carriers while the gradient supplies all the movement — the protein only removes the membrane as an obstacle. Carriers saturate, and past that point a steeper gradient buys nothing. Osmosis moves water and only water; the solute driving it never crosses. Tonicity describes a relation between a solution and one particular cell's cytoplasm, so calling a solution isotonic without naming the cell states nothing at all. Active transport is defined by direction — against the gradient — rather than by machinery. Primary active transport spends A T P directly; secondary transport spends none of its own and lives off a gradient a primary pump established. The sodium-potassium pump moves three sodium ions out for every two potassium ions in. That unequal ratio is what makes it electrogenic: each cycle leaves the interior slightly more negative than before. And bulk transport wraps material in membrane rather than passing it through a protein. Phagocytosis takes particles, pinocytosis takes fluid, and receptor-mediated endocytosis takes one substance a receptor recognises — which is why a defective receptor leaves that substance in circulation. Unit Three turns to metabolism, respiration and photosynthesis.