AP Biology Unit 2 Notes: How Cells Control Exchange Across Membranes

Membrane Transport: Big Picture and Selective Permeability

Cells survive by controlling what enters and leaves. That sounds simple, but it’s a constant balancing act: you need nutrients, water, and ions coming in; you need wastes going out; and you must maintain internal conditions that allow enzymes and other cellular processes to work. Membrane transport is the set of processes that move substances across a cell membrane (or across internal membranes like the ER, Golgi, lysosomes, and mitochondria).

Why membranes are “selectively permeable”

A cell membrane is primarily a phospholipid bilayer. Each phospholipid is amphipathic—it has a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails. In water, phospholipids spontaneously form a bilayer with tails tucked inward. This structure creates a hydrophobic interior that acts like a barrier.

That barrier matters because not everything can pass through equally. Selective permeability means the membrane allows some substances to cross more easily than others. In general:

  • Small nonpolar molecules (like oxygen and carbon dioxide) cross relatively easily because they dissolve in the hydrophobic interior.
  • Small polar molecules (like water) can cross, but often need help to cross quickly.
  • Ions and large polar molecules (like glucose, amino acids, and charged ions such as sodium) do not cross the hydrophobic core easily; they typically require transport proteins.

A common misconception is that the membrane is a rigid wall with “holes.” In reality, it’s a fluid structure where the hydrophobic interior is a major gatekeeper; proteins provide most of the controlled pathways.

Gradients: the driving force behind transport

Transport is usually driven by differences across the membrane. Two gradients appear constantly in cell transport problems:

  • Concentration gradient: difference in solute concentration across a membrane.
  • Electrochemical gradient: for ions, the “push” depends on both concentration and charge differences (the electrical part comes from membrane potential).

Substances tend to move in the direction that reduces these gradients. When movement occurs “down” a gradient (from high to low concentration for solutes), it can happen without added energy input.

Passive vs. active transport

Understanding whether energy is required is central in AP Biology.

  • Passive transport: movement across a membrane that does not require cellular energy input (ATP). This includes simple diffusion, osmosis (diffusion of water), and facilitated diffusion.
  • Active transport: movement that requires energy (directly or indirectly) to move substances against their gradients (from low to high concentration for solutes, or against an electrochemical gradient for ions). This includes membrane pumps, cotransport, and bulk (vesicular) transport.

It’s tempting to think “active = faster” and “passive = slower,” but that’s not reliable. Some passive processes (like ion flow through an open channel) are extremely fast. The key distinction is energy requirement and direction relative to the gradient, not speed.

What affects how fast substances cross?

Several factors influence diffusion and transport rates:

  • Steepness of the gradient: bigger difference usually means faster net movement.
  • Temperature: higher temperature increases molecular motion.
  • Membrane surface area: more area allows more exchange.
  • Distance/thickness: thicker barrier slows diffusion.
  • Size and polarity/charge of the molecule: large or charged substances cross slowly without proteins.
  • Number and activity of transport proteins: especially important for facilitated diffusion and active transport.

“Net movement” and dynamic equilibrium

Diffusion doesn’t mean molecules stop moving once concentrations equalize. At dynamic equilibrium, molecules still move randomly in both directions, but there is no net movement because the rates balance.

Exam Focus
  • Typical question patterns:
    • Predict which molecules can cross the membrane directly versus needing a protein (based on size, polarity, and charge).
    • Interpret diagrams showing gradients and ask for the direction of net movement.
    • Explain why maintaining gradients is essential for cell function (linking transport to homeostasis).
  • Common mistakes:
    • Saying molecules “want” to move—use cause-and-effect language (random motion + gradients).
    • Confusing “selectively permeable” with “only lets good things in.” It’s about chemical properties and protein control.
    • Treating equilibrium as “no movement” instead of “no net movement.”

Facilitated Diffusion

Facilitated diffusion is passive transport that uses membrane proteins to help substances cross the membrane down their concentration or electrochemical gradient. It matters because many biologically important molecules (glucose, amino acids, ions) are too polar or too charged to cross the lipid bilayer efficiently on their own.

Why proteins are needed

The interior of the bilayer is hydrophobic. Polar molecules and ions are energetically “uncomfortable” entering it, so crossing is slow without assistance. Transport proteins solve this by providing a hydrophilic pathway or binding site.

There are two major types of proteins involved in facilitated diffusion:

Channel proteins: pathways through the membrane

Channel proteins form a hydrophilic tunnel. They are especially important for ions and water.

  • Ion channels: selective for particular ions (often based on size and charge). Many are gated, meaning they open and close in response to a stimulus:
    • Voltage-gated (respond to membrane potential)
    • Ligand-gated (respond to a chemical signal)
    • Mechanically gated (respond to stretch)
  • Aquaporins: channels that greatly increase water permeability. This is crucial because water is polar; while it can cross slowly on its own, cells often need rapid water movement.

A useful analogy: channels are like a hallway with a door—when open, many molecules can pass quickly.

Carrier proteins: shape-changing shuttles

Carrier proteins bind a specific solute and undergo a conformational change that moves the solute from one side of the membrane to the other.

Carrier-mediated facilitated diffusion is:

  • Specific: a carrier typically transports only one solute (or a small family of related solutes).
  • Saturable: there are limited carrier proteins. At high solute concentrations, all carriers may be occupied, producing a maximum transport rate.

Analogy: carriers are like a revolving door—only so many people can pass per minute, no matter how crowded it gets.

Facilitated diffusion vs. simple diffusion

Both are passive (no ATP required), but they behave differently as concentration changes.

  • Simple diffusion through the lipid bilayer tends to increase approximately proportionally with concentration gradient (more molecules available to cross).
  • Facilitated diffusion increases with gradient at first, but can plateau when transport proteins are saturated.

This is a common AP-style graph interpretation task: if transport rate levels off at high concentration, you’re likely looking at a protein-mediated process.

“Downhill” can mean electrically downhill too

For ions, “down the electrochemical gradient” means considering both:

  • movement toward lower concentration, and
  • movement toward the opposite charge (e.g., a positive ion is attracted to a more negative side).

Students sometimes incorrectly predict ion movement using concentration only. In many cells, the electrical gradient can be strong enough to oppose or reinforce the concentration gradient.

Examples (concept first, then application)

1) Glucose uptake in many cells: Glucose is large and polar, so it commonly enters via carrier proteins (often called GLUT transporters in many organisms). It moves into cells when extracellular glucose concentration is higher than inside.

2) Nerve signaling: Ion channels (especially gated channels) allow rapid ion movement that changes membrane potential, enabling electrical signaling.

What can go wrong: misconceptions to avoid

  • Facilitated diffusion is sometimes mistaken for active transport because it uses proteins. The key is direction: in facilitated diffusion, the solute moves down its gradient and does not require ATP input.
  • Don’t assume channels are always open. Many are gated, and opening/closing is a major control point.
Exam Focus
  • Typical question patterns:
    • Interpret a graph of transport rate vs. solute concentration to distinguish simple diffusion from facilitated diffusion (look for saturation).
    • Identify whether a depicted protein is a channel or a carrier based on mechanism (tunnel vs. binding + conformational change).
    • Predict movement direction based on gradients (including ion movement and membrane charge).
  • Common mistakes:
    • Calling facilitated diffusion “active transport” just because a protein is involved.
    • Ignoring saturation—assuming transport rate can increase without limit.
    • Predicting ion flow from concentration gradient only, ignoring the electrical component.

Tonicity and Osmoregulation

Tonicity describes how a surrounding solution affects the movement of water into or out of a cell by osmosis. It’s one of the most testable parts of membrane transport because it connects membrane permeability, diffusion, and cell structure (especially plant vs. animal cells).

Osmosis: diffusion of water

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from regions where water is more “available” to regions where it is less available.

A very common way to phrase this (and it’s useful for AP):

  • Water tends to move toward the side with higher solute concentration (because solutes “tie up” water molecules, reducing the amount of free water).

Be careful: water does not move because it “wants to dilute” the solute. It moves due to random molecular motion and differences in effective water concentration across the membrane.

Tonicity vs. concentration

Tonicity depends on nonpenetrating solutes—solutes that cannot cross the membrane. If a solute can cross, it may not cause a lasting osmotic effect because it will diffuse until balanced.

So, tonicity is not just “which side has more solute total,” but “which side has more solute that can’t cross.”

The three tonicities (and what happens to cells)

When comparing the solution outside the cell to the cytoplasm:

  • Isotonic: same effective concentration of nonpenetrating solutes. No net water movement; cell volume stays stable.
  • Hypotonic: outside has lower nonpenetrating solute concentration than inside. Water enters the cell; the cell swells.
  • Hypertonic: outside has higher nonpenetrating solute concentration than inside. Water leaves the cell; the cell shrinks.
Animal cells vs. plant cells

The outcome depends strongly on whether there is a cell wall.

  • Animal cells (no cell wall):

    • In hypotonic solution, they may swell and potentially lyse (burst) because the membrane can’t withstand unlimited stretching.
    • In hypertonic solution, they shrink (often described as crenation in red blood cells).

  • Plant cells (cell wall present):

    • In hypotonic solution, water enters and the cell becomes turgid. The cell wall resists expansion, creating turgor pressure, which is actually beneficial—turgor helps support the plant.
    • In hypertonic solution, water leaves, the plasma membrane pulls away from the cell wall, and the cell undergoes plasmolysis, which can harm the plant.

A frequent misconception is that plant cells “prefer isotonic.” In reality, many plant cells function best slightly hypotonic to maintain turgor.

Worked tonicity examples

Example 1 (animal cell): A red blood cell is placed in a solution that is hypertonic relative to the cytoplasm.

  • Step 1: Identify the gradient. Outside has higher nonpenetrating solute.
  • Step 2: Predict water movement. Water moves out of the cell by osmosis.
  • Step 3: Predict result. The cell shrinks.

Example 2 (plant cell): A plant cell is placed in a hypotonic solution.

  • Step 1: Outside has lower nonpenetrating solute.
  • Step 2: Water moves into the cell.
  • Step 3: The membrane pushes against the cell wall; turgor pressure rises.
  • Step 4: The cell becomes turgid, typically a healthy state.

Osmoregulation: maintaining water and solute balance

Osmoregulation is how organisms control internal water balance and solute concentration. It matters because cells must keep enzymes and membranes functioning within narrow conditions; too much swelling or shrinking disrupts cellular processes.

Organisms face different osmotic challenges depending on environment:

  • Freshwater organisms often live in hypotonic surroundings (water tends to enter their bodies). They must prevent excess water accumulation and avoid losing too many salts.
  • Marine organisms may live in hypertonic surroundings (water tends to leave their bodies). They must prevent dehydration and manage salt gain.

Mechanisms vary across organisms, but common strategies include:

  • Contractile vacuoles in some freshwater protists: collect and expel excess water.
  • Kidneys and excretory systems in animals: regulate water and ion excretion.
  • Plant stomata and vacuoles: help manage water retention and turgor.

Don’t overgeneralize: “marine fish drink seawater” is often true for many bony fish, but different groups have different strategies. What AP typically wants is the core idea: organisms regulate internal conditions using physiological mechanisms that move water and solutes.

Exam Focus
  • Typical question patterns:
    • Given solute concentrations (or a diagram), identify whether a solution is hypertonic/hypotonic/isotonic and predict water movement.
    • Compare effects of tonicity on animal vs. plant cells (lysis vs. turgor vs. plasmolysis).
    • Explain why tonicity depends on nonpenetrating solutes (often embedded in scenarios with permeable solutes).
  • Common mistakes:
    • Confusing tonicity with total solute concentration without considering solute permeability.
    • Saying “water moves to where there is more water” without connecting it to solute and membrane selectivity.
    • Assuming plant cells burst in hypotonic solution (cell wall prevents lysis under typical conditions).

Mechanisms of Transport

In AP Biology, “mechanisms of transport” typically means you can explain and distinguish the major ways substances cross membranes, including passive transport (reviewed above), active transport, and bulk transport. The unifying theme is that cells maintain homeostasis by controlling movement using proteins and vesicles.

Primary active transport: pumps that use ATP directly

Primary active transport uses energy from ATP hydrolysis to move solutes against their gradients. The key idea: pumps create and maintain gradients that the cell then uses for many other processes.

A classic example is the sodium-potassium pump (common in animal cells). Its cycle (conceptually) involves:

1) Binding specific ions on one side of the membrane.
2) Using ATP to phosphorylate the pump protein, changing its shape.
3) Releasing ions on the other side, against their gradients.
4) Dephosphorylation returns the pump to its original shape.

Why this matters: maintaining ion gradients supports nerve impulses, helps regulate cell volume, and powers secondary active transport.

Another important pump type is the proton pump, which moves hydrogen ions across membranes. Proton gradients are widely used—most famously in energy-related membranes (like mitochondria and chloroplasts) and also in some plasma membranes where they drive cotransport.

A misconception to avoid: ATP is not used to “push” a solute through an open channel. Instead, ATP changes the protein’s shape, enabling movement against the gradient.

Secondary active transport: using a gradient as “stored energy”

Secondary active transport (often called cotransport) uses the energy stored in an ion gradient (created by primary active transport) to move another substance against its gradient.

Two common cotransport arrangements:

  • Symport: both substances move in the same direction.
  • Antiport: substances move in opposite directions.

Mechanistically, you can think of it like this:

1) A pump (primary active transport) builds a steep ion gradient.
2) The ion “wants” to move back down its gradient.
3) A cotransporter harnesses that downhill movement to drive another solute uphill.

Example: glucose absorption in the intestine (conceptual)
In many epithelial cells, sodium concentration is kept low inside the cell by a sodium-potassium pump. Sodium then moves down its gradient from the intestinal lumen into the cell through a symporter that carries glucose along with it—allowing glucose uptake even when glucose inside the cell is already high.

Students often confuse this with facilitated diffusion because glucose may be moving into the cell through a protein. The difference is that glucose is being moved against its gradient, using energy indirectly from the sodium gradient.

Bulk transport: moving large materials with vesicles

Some cargo is too large (or too complex) to cross via transport proteins. Cells solve this with vesicular transport, which uses membrane-bound vesicles.

Endocytosis: bringing materials into the cell

Endocytosis is when the membrane invaginates (folds inward) to form a vesicle that brings material into the cell.

Types commonly emphasized:

  • Phagocytosis: “cell eating”—engulfing large particles (like bacteria). Often performed by immune cells.
  • Pinocytosis: “cell drinking”—uptake of extracellular fluid and dissolved solutes.
  • Receptor-mediated endocytosis: highly specific uptake where ligands bind to membrane receptors, triggering vesicle formation. This increases efficiency and selectivity.

Why receptor-mediated endocytosis matters: it shows how cells can import scarce molecules even when they are at low concentrations.

Exocytosis: exporting materials out of the cell

Exocytosis occurs when a vesicle fuses with the plasma membrane and releases its contents outside. This is central to:

  • secretion of hormones and enzymes,
  • neurotransmitter release at synapses,
  • delivery of membrane proteins and lipids to the plasma membrane.

A common mistake is to treat exocytosis as only “waste removal.” It is also a major pathway for secretion and membrane remodeling.

Putting it together: how cells coordinate multiple transport mechanisms

Real cells usually use several mechanisms simultaneously:

  • Pumps build ion gradients.
  • Those gradients power cotransport.
  • Channels allow rapid, regulated changes.
  • Vesicles move large cargo and add/remove membrane components.

This coordination is a frequent AP theme: structure enables function. The membrane’s proteins and vesicle trafficking systems are the structures that make controlled exchange possible.

Worked mechanism identification (AP-style reasoning)

Scenario: A cell takes up a polar nutrient from a region of low nutrient concentration outside the cell to a higher concentration inside the cell, and the uptake stops when ATP production is inhibited.

  • Step 1: Direction relative to gradient: low outside to high inside means against the gradient.
  • Step 2: Energy dependence: uptake stops when ATP is inhibited, so energy is required.
  • Step 3: Conclusion: this is active transport (primary or secondary). If the scenario also mentions coupling to an ion moving down its gradient, that would point to secondary active transport.
Exam Focus
  • Typical question patterns:
    • Given a scenario (direction of movement, ATP dependence, coupling to another ion), classify transport as primary active, secondary active, facilitated diffusion, or simple diffusion.
    • Explain how an ion pump indirectly powers uptake of another solute (cotransport reasoning).
    • Interpret diagrams of endocytosis/exocytosis and identify the type (phagocytosis vs receptor-mediated, etc.).
  • Common mistakes:
    • Assuming any protein-mediated transport must require ATP (facilitated diffusion does not).
    • Mixing up primary vs secondary active transport (primary uses ATP directly; secondary uses a gradient built by primary).
    • Describing endocytosis/exocytosis as substances “going through” the membrane rather than being packaged in vesicles that bud or fuse.