Atlas MCAT Course

Systems & Physiology → Transport & Flux

Transport & Flux

Why physiology is the control of movement.

01The idea

Homeostasis is impossible without movement. Every regulated variable in the body — Na⁺, glucose, O₂, CO₂, water — must be moved to be controlled.

If you can predict the direction of movement, the rate of movement, and the coupling between movements, you can reason through renal, gastrointestinal, pulmonary, and cardiovascular systems using the same playbook.

THREAD · Gradients and flow

02Build the model

AFrom physics to physiology: the unifying principle

That driving force may be chemical (concentration gradients), electrical (charge separation), or pressure (bulk flow). This lesson applies the electrochemical, equilibrium, and energy logic you’ve already built — now in biological form.

BDriving forces: why anything moves

Movement occurs because of differences across space.

Gradient	Difference in	Drives
Chemical	Concentration	Diffusion
Electrical	Charge	Ion movement
Pressure	Pressure	Bulk flow

For ions, chemical and electrical forces combine into a net electrochemical driving force.

This is exactly the same logic used in membrane potentials and the Nernst framework.

CDiffusion: random motion, predictable outcome

C.1Simple diffusion

Moves down the concentration gradient with no energy input. The rate depends on gradient and permeability. Examples: O₂, CO₂, small nonpolar molecules. Limitations: inefficient over long distances, and not saturable.

C.2Facilitated diffusion

Uses membrane proteins (channels or carriers) but still moves down the gradient. Saturable. Examples: glucose transporters, ion channels.

DActive transport: going uphill requires energy

D.1Primary active transport

Direct use of ATP to move solutes against their gradients. The canonical example is the Na⁺/K⁺ ATPase: 3 Na⁺ out, 2 K⁺ in.

This directly links transport to metabolism.

D.2Secondary active transport: gradient coupling

Uses existing gradients to move other solutes uphill, and does not use ATP directly. Symport moves both in the same direction; antiport moves them in opposite directions. Example: the Na⁺–glucose symporter.

Two questions sort every mechanism: is it downhill, and if not, where does the energy come from?

This is a systems-level dependency.

EOsmosis and tonicity: water follows rules, not intent

In osmosis, water moves toward higher effective osmolarity, through aquaporins or membranes. Tonicity takes a cell-centered view:

Tonicity describes what the solution does to the cell.

This explains why some solutes “don’t count” long-term.

FChannels vs. carriers: speed vs. control

Channels	Carriers
High throughput	Slower
Gated (voltage, ligand, mechanical)	Undergo conformational cycling
Not governed by binding affinity	Saturable

GEpithelial transport: direction across a layer

Epithelia move substances from one compartment to another. They are polarized tissues, not single membranes.

The transcellular route goes through cells; it requires coordinated apical and basolateral transporters and is highly regulated. The paracellular route goes between cells, through tight junctions, and depends on junction permeability.

This logic underlies renal, intestinal, and pulmonary physiology.

HBulk flow and convection: moving with the fluid

In capillaries and filtration systems, solutes move with fluid flow, driven by pressure gradients. Examples: filtration and reabsorption.

03See the connections

The lesson names these links itself. Read the map left to right.

Driving forces

Chemical
Electrical
Pressure

Mechanisms

Diffusion (simple, facilitated)
Primary active (ATP)
Secondary active (gradient)
Bulk flow

Physiology uses it in

Epithelia (polarized layers)
Cell volume (tonicity)
Kidney, gut, lung, circulation

Next: cardiovascular physiology.

04Reason with it

Four questions. Answer in your head first, then choose. You’ll see the reasoning either way.

Think first.

Glucose enters a cell down its gradient through a GLUT transporter protein. What kind of transport is this?

Think first.

The Na⁺/K⁺ ATPase is poisoned. What happens to Na⁺–glucose symport?

Think first.

A solution has the same total osmolarity as the cell, but its main solute crosses the membrane freely. Over time, what happens to a cell placed in it?

Think first.

What mainly determines how strongly an epithelium can enforce one-way transport?

05MCAT lens

The MCAT emphasizes

Direction from the net driving force, not concentration alone

Can you reconstruct this concept without looking at the page? You should now be able to reason through:

Why Na⁺ gradients power many systems

Check against 02 · D: secondary transport rides the Na⁺ gradient.

Why cells swell or shrink in different solutions

Check against 02 · E: water follows non-penetrating solutes.

Why blocking Na⁺/K⁺ ATPase has global effects

Check against 02 · D: every gradient-coupled transporter fails with it.

Why epithelial transport is directional

Check against 02 · G: polarized transporters and tight junctions.

How to predict ion movement using net driving force

Check against 02 · A and B: sum the chemical and electrical forces.

If something feels unclear, the missing idea is usually thisFlux is driven by gradients and constrained by membranes.

08Remember

REMEMBER · if you keep only a few things

Physiology regulates flux, not static concentrations.
Particles move down their net driving force unless energy is supplied.
Downhill = diffusion, even through a protein.
Primary active uses ATP; secondary active rides a gradient.
Tonicity counts non-penetrating solutes only.
Channels are gated; carriers saturate.
Convection is non-selective.

09Forward hook

Transport explains local movement. The next challenge in physiology is distance.

How does the body move substances over long distances?

That leads directly to circulation, pressure generation, and flow distribution.

Next lesson
Cardiovascular Physiology→