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Membrane
Transport

How cells control what enters and exits through the plasma membrane: from passive diffusion to active pumps and vesicle trafficking.

AP Biology

The Plasma Membrane

Every living cell is enclosed by a selectively permeable plasma membrane built from a phospholipid bilayer. The hydrophilic heads face outward toward water, while the hydrophobic tails face inward, creating a barrier to most polar and charged molecules.

Channel Hydrophilic heads Hydrophobic tails Extracellular fluid Cytoplasm

Why Transport Matters

Cells must constantly exchange materials with their environment. Nutrients must enter, wastes must exit, and ion gradients must be maintained for signaling and energy production. Without regulated transport, homeostasis collapses.

Nutrient Uptake
Glucose, amino acids, and ions must cross the membrane to fuel metabolism
Waste Removal
\(CO_2\) and metabolic byproducts must exit the cell efficiently
Signal Transduction
Ion channels enable nerve impulses and hormonal responses

Passive Transport

Passive transport moves substances down their concentration gradient: from regions of higher concentration to regions of lower concentration. No cellular energy (ATP) is required: the driving force is the second law of thermodynamics.

1
Simple diffusion: Small, nonpolar molecules (\(O_2\), \(CO_2\), \(N_2\)) pass directly through the lipid bilayer.
2
Facilitated diffusion: Polar molecules and ions move through channel proteins or carrier proteins without energy input.
3
Osmosis: The diffusion of water across a selectively permeable membrane, driven by solute concentration differences.

Simple Diffusion in Action

Membrane High concentration Low concentration Net movement

Osmosis and Tonicity

Osmosis is the net movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration. The direction of water flow depends on tonicity.

Hypotonic
Lower solute outside: water flows into the cell. Cell swells; may lyse.
Isotonic
Equal solute on both sides: no net water movement. Cell maintains shape.
Hypertonic
Higher solute outside: water flows out of the cell. Cell shrivels (crenation).

Predict water direction from total water potential rather than concentration alone.

Turgor raises water potential and can oppose solute effects; these stated initial conditions still favor water entry. Values are an explicitly stated teaching model.

A plant-cell schematic at 295K has total water potential minus 6.854 bar, below the bath at minus 2.451 bar. The arrow points into the cell despite its positive 0.5 bar pressure.
Turgor raises water potential and can oppose solute effects; these stated initial conditions still favor water entry. Values are an explicitly stated teaching model.

A plant cell presses against a wall while an animal cell changes shape

Plant wall is thick and distinct from inner membrane; large central vacuole and peripheral cytoplasm visible. Animal cell has flexible boundary and no wall. Separate exact panel: Paired plant states retain the same wall while the inner membrane is pressed outward or retracted; animal membrane has no surrounding wall.For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.

Wall support changes the response

Wall support changes the response. For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.

Active Transport

Active transport moves substances against their concentration gradient (low → high). This requires energy, typically from ATP hydrolysis. It allows cells to accumulate needed molecules and expel wastes even when the gradient opposes movement.

Pump ATP energy input High concentration Low concentration Membrane
A conceptual membrane has higher sodium outside and lower sodium in the cytosol. A violet pump moves three sodium out and two potassium in per ATP. A separate gold symporter brings one sodium and one glucose inward, using the gradient. Protein forms and their juxtaposition are schematic.
In this conceptual membrane, the Na+/K+ ATPase uses one ATP to move three Na+ out and two K+ in, maintaining a sodium electrochemical gradient. A separate symporter uses that gradient to bring Na+ and glucose inward; the 1:1 ratio is a selected teaching model and varies among transporters. The cutaways do not represent constitutively open pores: both transport cycles and the protein shapes are schematic. The juxtaposition does not assert a shared membrane face in a particular tissue.

Primary vs Secondary Active Transport

Primary
Directly uses ATP hydrolysis to power transport. Example: \(Na^+/K^+\) ATPase breaks \(\text{ATP} \rightarrow \text{ADP} + P_i\) to pump ions.
Secondary (Cotransport)
Uses the gradient created by primary transport. One substance moves down its gradient, providing energy to move another against its gradient.

Bulk Transport: Endo- and Exocytosis

Large molecules and particles that cannot cross the membrane through channels or pumps are moved in membrane-bound vesicles. These processes require ATP and involve remodeling of the plasma membrane itself.

1
Endocytosis brings material into the cell by forming an inward pocket that pinches off as a vesicle. Types: phagocytosis (solids), pinocytosis (fluids), receptor-mediated endocytosis (specific molecules).
2
Exocytosis releases material out of the cell by fusing a vesicle with the plasma membrane, dumping contents into the extracellular space. Example: neurotransmitter release at a synapse.

Passive vs Active: Key Distinctions

Passive Transport Active Transport
Energy Required No Yes (ATP)
Direction High → Low (down gradient) Low → High (against gradient)
Proteins Needed Sometimes (facilitated) Always (pumps or vesicles)
Saturability Facilitated: yes; Simple: no Yes (limited pump number)
Examples \(O_2\) diffusion, aquaporins, glucose via GLUT \(Na^+/K^+\) pump, endocytosis

How to Identify the Transport Type

1
Determine the gradient: Is the substance moving from high to low concentration, or low to high?
2
Check for energy: Does the process require ATP? If yes → active. If no → passive.
3
Check for proteins: Does a channel or carrier assist? If yes → facilitated diffusion. If no protein and nonpolar molecule → simple diffusion.
4
Check for vesicles: Is the membrane engulfing or expelling material? If yes → endocytosis or exocytosis (bulk transport).
5
Is it water? Water moving across a membrane via aquaporins → osmosis (a special case of facilitated diffusion).

Example 1: Oxygen in the Alveoli

In the lungs, \(O_2\) concentration in the alveoli is higher than in the blood of pulmonary capillaries. How does \(O_2\) enter the blood?

Alveolus High \(O_2\) Low \(CO_2\) Capillary Blood Low \(O_2\) High \(CO_2\)
\(O_2\)
\(CO_2\)

Example 2: Glucose Uptake

After a meal, blood glucose concentration is higher outside muscle cells than inside. Glucose is a large, polar molecule. How does it enter the cell?

GLUT Blood (high glucose) Muscle cell Membrane

Example 3: The \(Na^+/K^+\) Pump

A neuron at rest has high \(Na^+\) outside and high \(K^+\) inside. To maintain this gradient after firing, the cell must move \(Na^+\) out and \(K^+\) in, both against their gradients. What type of transport is this?

1
3 \(Na^+\) bind to the pump on the cytoplasmic side.
2
ATP is hydrolyzed to \(\text{ADP} + P_i\). The pump changes shape.
3
3 \(Na^+\) are released outside. 2 \(K^+\) bind from outside.
4
Phosphate detaches → pump returns to original shape → 2 \(K^+\) released inside.

Common Mistakes

✕ Osmosis moves solute
Osmosis is the movement of water, not solute. Water moves toward higher solute concentration.
✕ All diffusion needs proteins
Simple diffusion does NOT require proteins. Only facilitated diffusion uses channels or carriers.
✕ Facilitated = active
Facilitated diffusion is passive: it uses proteins but NO ATP. The molecule still moves down its gradient.

Summary of Transport Types

Type Energy Protein? Direction
Simple diffusion None No Down gradient
Facilitated diffusion None Yes (channel/carrier) Down gradient
Osmosis None Aquaporins (optional) Water → high solute
Primary active ATP (direct) Yes (pump) Against gradient
Secondary active ATP (indirect) Yes (cotransporter) Against gradient
Endocytosis/Exocytosis ATP Vesicle-based Into / out of cell

Question 1 of 5

A red blood cell is placed in a hypertonic solution. What will happen to the cell?

Question 2 of 5

Glucose enters a muscle cell via GLUT transporters down its concentration gradient. This is an example of:

Question 3 of 5

The \(Na^+/K^+\) ATPase pumps 3 \(Na^+\) out and 2 \(K^+\) in per cycle. Why is this considered primary active transport?

Question 4 of 5

A white blood cell engulfs a bacterium by wrapping its membrane around the pathogen and pinching off a vesicle. This process is called:

Question 5 of 5

Which of the following substances can cross the phospholipid bilayer by simple diffusion without any transport protein?