Skills you'll gain in this topic:
- Explain how plasma membrane structure contributes to selective permeability.
- Describe how molecules move across membranes based on size, charge, and polarity.
- Differentiate between passive and active transport processes.
- Analyze how membrane properties affect ion and water movement.
- Predict changes in cell conditions based on permeability and concentration gradients.
Membrane Permeability
Thanks to the structure of the membrane, with the hydrophobic tails and hydrophilic heads, the cellular membrane has selective permeability. This allows some substances to cross easily, while others may not be able to cross or may require a special transport protein to do so.
The membrane acts like a barrier separating the inside of the cell from the external environment of the cell. This separation is super important - it allows the cell to maintain conditions inside that are different from the outside world!
The Fluid Mosaic Model and Selective Permeability
Remember the fluid mosaic model from Topic 2.4? This model explains why membranes are selectively permeable! The phospholipid bilayer creates a hydrophobic barrier in the middle of the membrane, while the embedded proteins create pathways for specific molecules to cross.
Think of the membrane like a bouncer at a club:
- Some molecules (VIPs) can pass right through
- Others need a special escort (transport protein)
- Some aren't getting in no matter what!
Concentration Gradients: The Key to Transport
The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane. A concentration gradient exists when there's a difference in the concentration of a substance on either side of the membrane. These gradients are essential because they:
- Drive passive transport processes
- Store potential energy that cells can use
- Allow cells to maintain different internal conditions than their environment
What Can Cross the Membrane?
The type of molecule determines how it crosses the membrane:
-
Small nonpolar molecules (like O₂, CO₂, and N₂) are the VIPs - they can pass directly through the phospholipid bilayer without any help. These molecules are crucial for respiration and photosynthesis!
-
Small polar uncharged molecules (like water/H₂O) can squeeze through the membrane in small amounts, but usually need help. Water molecules are tiny enough that some can slip between phospholipids, but for larger movements of water, cells use special transport proteins called aquaporins.
-
Large polar molecules and ions (like glucose, amino acids, Na⁺, K⁺) cannot cross the membrane on their own - they need transport proteins to help them across. These molecules are too hydrophilic to pass through the hydrophobic interior of the membrane.
The hydrophobic fatty acid tails are what controls the movement of substances described above. They repel charged and polar molecules and make it very challenging for them to come across.
Quick Reference Chart: How Molecules Cross the Membrane
| Molecule Type | Examples | Can Cross Freely? | Needs Transport Protein? |
|---|---|---|---|
| Small nonpolar | O₂, CO₂, N₂ | ✅ | ❌ |
| Small polar uncharged | H₂O | ✅ (small amounts) | ✅ (large amounts) |
| Large polar | Glucose, amino acids | ❌ | ✅ |
| Ions | Na⁺, K⁺, Cl⁻, Ca²⁺ | ❌ | ✅ |
Diffusion Across the Plasma Membrane
Passive Transport: Going with the Flow
Passive transport is the net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy. Think of it like water flowing downhill—no energy needed!
-
Simple Diffusion
- Molecules move directly through the phospholipid bilayer
- No transport proteins needed
- Only works for small, nonpolar molecules (O₂, CO₂, N₂)
- Example: Oxygen diffusing from your lungs into your bloodstream
-
Facilitated Diffusion
- Still moves down the concentration gradient (high to low)
- Requires transport proteins to help molecules cross
- Used for polar molecules and ions that can't pass through the hydrophobic membrane core
- Example: Glucose entering cells through glucose transporters
-
Osmosis
- Special case of facilitated diffusion for water molecules
- Water moves across membranes through special proteins called aquaporins
- Direction depends on solute concentration on both sides
💡 Quick Tip: Passive transport plays a huge role in importing materials and exporting wastes from the cell. Your body uses passive processes whenever possible because they don't require energy!
Active Transport: Fighting the Current
Active transport requires the direct input of energy to move molecules. In some cases, active transport is utilized to move molecules from regions of low concentration to regions of high concentration (against the concentration gradient). This process requires energy in the form of ATP.
Think of it like pushing a boulder uphill—you need energy to move against the natural direction!
Key Features of Active Transport:
- Uses special carrier proteins called "pumps"
- Directly uses ATP as an energy source
- Creates and maintains concentration gradients
- Essential for nerve cell function, nutrient absorption, and more
Example: The sodium-potassium pump moves sodium ions out of the cell and potassium ions into the cell, even though this goes against their concentration gradients.
| Transport Type | Energy Required? | Direction | Examples |
|---|---|---|---|
| Simple Diffusion | No | High → Low | O₂, CO₂ |
| Facilitated Diffusion | No | High → Low | Glucose, amino acids |
| Active Transport | Yes (ATP) | Low → High | Na⁺/K⁺ pump, calcium pumps |
Maintaining Solute and Water Balance
Organisms use these transport mechanisms to maintain solute and water balance, which is crucial for survival. Here's how:
-
Selective Permeability Creates Control: By controlling what can cross the membrane, cells can maintain different concentrations of substances inside versus outside
-
Active Transport Maintains Gradients: Cells use energy to pump ions and molecules against their concentration gradients, maintaining the internal environment
-
Passive Transport Allows Equilibration: When appropriate, cells allow substances to move down their concentration gradients to balance conditions
-
Coordinated Transport: Cells often use multiple transport mechanisms together to maintain homeostasis
Transport of Large Molecules
For really big molecules that can't fit through membrane proteins, cells use special processes:
Exocytosis: Shipping Packages Out
- Internal vesicles fuse with the plasma membrane
- Contents are released to the outside
- Requires energy (ATP)
- Used for: secreting hormones, neurotransmitters, and waste products
Endocytosis: Bringing Packages In
- The plasma membrane folds inward, forming a vesicle around material
- The vesicle pinches off and enters the cell
- Requires energy (ATP)
- Three main types:
-
Phagocytosis ("Cell Eating")
- Cell engulfs large particles or even whole microorganisms
- Forms food vacuoles
- Common in white blood cells that "eat" bacteria
-
Pinocytosis ("Cell Drinking")
- Cell "gulps" liquid droplets with dissolved substances
- Non-specific—takes in whatever is in the liquid
-
Receptor-Mediated Endocytosis
- Very specific—only brings in molecules that bind to receptors
- Receptors cluster in coated pits that form vesicles
- Examples: cholesterol uptake, insulin signaling
Cell Walls
Cell walls provide two major functions for cells:
- A structural boundary that gives the cell shape and protection
- A permeability barrier that filters what can reach the plasma membrane
Unlike the plasma membrane, the cell wall is NOT selectively permeable in the same way. Water and most dissolved substances can pass through the cell wall, but it blocks larger particles and some pathogens from reaching the cell membrane.
Cell Wall Composition
Different types of organisms have cell walls made of different materials:
🌱 Plant cell walls: Made primarily of cellulose, a complex carbohydrate. Plant cell walls are arranged in layers and contain other polysaccharides like hemicellulose and pectin.
🦠 Bacterial cell walls: Made of peptidoglycan, a mesh-like layer of sugars and amino acids. The thickness of this layer is what determines if a bacterium is Gram-positive or Gram-negative.
🍄 Fungal cell walls: Mainly composed of chitin, a tough polysaccharide similar to the material in insect exoskeletons.
Each of these materials provides structural support while still allowing for the selective movement of substances to the plasma membrane beneath.
Why Cell Walls Matter for Permeability
The cell wall works together with the plasma membrane to control what enters and exits the cell. While the plasma membrane is the primary permeability barrier, the cell wall:
- Prevents the cell from bursting when water enters (especially important in plant cells)
- Filters out large molecules before they reach the plasma membrane
- Provides an additional layer of protection against the environment
Think of it like this: if the plasma membrane is the bouncer deciding who gets into the club, the cell wall is like the metal detector everyone has to go through first!
Membrane transport processes are crucial for maintaining the cell's internal environment. Through passive transport, cells efficiently move materials down concentration gradients without energy cost, while active transport and bulk transport methods allow cells to move substances against gradients and transport large molecules when needed. The formation of concentration gradients through selective permeability, combined with various transport mechanisms, allows organisms to maintain solute and water balance essential for survival. Understanding these mechanisms helps explain how cells maintain homeostasis, acquire nutrients, and remove wastes—fundamental processes that keep all living organisms functioning properly.
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.
| Term | Definition |
|---|---|
| active transport | The movement of ions and molecules across a membrane against their concentration gradient, requiring metabolic energy from ATP. |
| concentration gradient | A difference in the concentration of a substance across a membrane, with higher concentration on one side and lower concentration on the other. |
| endocytosis | A process in which a cell takes in large molecules and particulate matter by folding the plasma membrane inward to form vesicles that engulf external material. |
| exocytosis | A process in which internal vesicles fuse with the plasma membrane to release large molecules and other materials from the cell. |
| metabolic energy | Energy derived from cellular metabolism, typically in the form of ATP, used to power cellular processes. |
| passive transport | The net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy. |
| plasma membrane | The selectively permeable membrane that surrounds the cell, composed of phospholipids, proteins, and other molecules that regulate what enters and exits the cell. |
| selective permeability | The property of a membrane that allows certain substances to pass through while restricting the passage of others. |
| solute | A substance dissolved in a solvent to form a solution; the component present in smaller amount in a solution. |
| vesicle | Small membrane-bound sacs that transport and store materials within or between cells. |
| water balance | The regulation of water movement into and out of cells to maintain proper cellular function and organism homeostasis. |
Frequently Asked Questions
What is passive transport and how does it work?
Passive transport is the movement of molecules down their concentration gradient (high → low) across a selectively permeable membrane without using cellular metabolic energy (EK 2.5.A.2). Types you should know for the AP exam (LO 2.5.A): - Simple diffusion: small nonpolar molecules (O2, CO2) slip through the lipid bilayer. - Osmosis: water diffusion across a membrane; faster via aquaporins (channel proteins). - Facilitated diffusion: polar molecules and ions cross with help from channel proteins or carrier proteins (no ATP). Selective permeability of the membrane creates the gradients that drive passive transport (EK 2.5.A.1). Contrast with active transport (moves substances up a gradient and requires energy). On the exam you may be asked to describe or predict movement based on tonicity, channels/carriers, or concentrations—practice those scenarios (see the Topic 2.5 study guide: https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ). For extra practice, try problems at https://library.fiveable.me/practice/ap-biology.
Why do molecules move from high concentration to low concentration without energy?
Molecules move from high to low concentration without the cell spending metabolic energy because of random molecular motion and the tendency toward increased entropy. At any temperature molecules have kinetic energy and bounce around; when more molecules are on one side, random collisions make it more likely some will move to the emptier side. Over time there’s a net flow down the concentration gradient (diffusion) until equilibrium is reached. This is passive transport (EK 2.5.A.2)—no ATP required. Membrane selectivity can slow or direct diffusion, so some solutes need channel or carrier proteins (facilitated diffusion, aquaporins for water) even though the process is still passive (LO 2.5.A). For AP review, focus on distinguishing passive vs active transport (EK 2.5.A.3) and examples—see the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice questions (https://library.fiveable.me/practice/ap-biology).
What's the difference between passive transport and active transport?
Passive transport: net movement of molecules down their concentration gradient (high → low) without direct metabolic energy (EK 2.5.A.2). Types: simple diffusion (small nonpolar molecules), osmosis (water through bilayer or aquaporins), and facilitated diffusion via channel or carrier proteins. It increases entropy and moves toward equilibrium. Active transport: movement that requires direct input of energy (often ATP) to move solutes—sometimes against their concentration gradient (low → high) (EK 2.5.A.3). Examples: ATP-driven pumps (sodium–potassium pump), proton pumps, and cotransporters (symport/antiport). For large cargos, cells use energy-dependent bulk transport: endocytosis (phagocytosis/receptor-mediated) and exocytosis (EK 2.5.B.1). Why it matters for the AP exam: expect to identify mechanisms, direction relative to gradients, and energy use (LO 2.5.A/B). For a quick topic review, check the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice questions (https://library.fiveable.me/practice/ap-biology).
How does selective permeability actually work in cell membranes?
Selective permeability means the plasma membrane lets some things through but blocks others, so cells can build concentration gradients (EK 2.5.A.1). Mechanically: the phospholipid bilayer is hydrophobic in the middle, so small nonpolar molecules and gases (O2, CO2) diffuse across easily (simple diffusion). Water moves by osmosis—slowly through the bilayer but faster via aquaporins. Polar or charged solutes need protein help: channel proteins (open pores) or carrier proteins (bind-and-shift) allow facilitated diffusion down a gradient (EK 2.5.A.2). Moving solutes uphill requires active transport and ATP (e.g., sodium–potassium pump) or coupled transporters (symport/antiport) (EK 2.5.A.3). Large items use energy-dependent vesicle trafficking: endocytosis (including receptor-mediated, phagocytosis) and exocytosis (EK 2.5.B.1). On the AP exam, expect questions that link these mechanisms to tonicity, concentration gradients, and energy use. Review the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice problems (https://library.fiveable.me/practice/ap-biology).
I'm confused about concentration gradients - can someone explain this simply?
A concentration gradient is just a difference in the amount (concentration) of a substance on one side of a membrane versus the other. Think of it like a hill: molecules naturally "roll" downhill from high concentration to low concentration—that’s diffusion (passive transport, no ATP). If the molecule is water, that downhill movement through a membrane is osmosis (aquaporins help). Because membranes are selectively permeable (EK 2.5.A.1), gradients form and drive passive transport (EK 2.5.A.2). Moving molecules uphill (low → high) needs energy—that’s active transport (EK 2.5.A.3), like the sodium-potassium pump using ATP. On the AP exam, you’ll be asked to explain these ideas and predict movement based on gradients (LO 2.5.A). For a quick review, see the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice problems (https://library.fiveable.me/practice/ap-biology).
What happens during endocytosis step by step?
Endocytosis is an energy-requiring way cells bring in big stuff. Step-by-step: 1. Trigger/recognition—cargo (particles, fluids, or ligands) binds the membrane (often via receptors for receptor-mediated endocytosis). 2. Membrane invaginates—the plasma membrane folds inward around the cargo. 3. Vesicle formation—the membrane pinches off, forming an internal vesicle (phagocytosis for large particles, pinocytosis for fluids, receptor-mediated for specific molecules). 4. Uncoating/maturation—coat proteins (like clathrin if present) are removed and the vesicle matures. 5. Trafficking—the vesicle moves along the cytoskeleton to its destination. 6. Fusion with endosome/lysosome—the vesicle fuses with endosomes or lysosomes where contents are sorted or degraded; useful molecules may be recycled to the membrane. On the AP, this fits LO 2.5.B and EK 2.5.B.1 (big molecules moved by vesicles). Review this topic in the Unit 2 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice problems (https://library.fiveable.me/practice/ap-biology).
Why does active transport need energy if passive transport doesn't?
Passive transport doesn't need cellular energy because it moves substances down their concentration gradient—from high to low—which is energetically favorable (EK 2.5.A.2). Examples: simple diffusion, osmosis, and facilitated diffusion through channel or carrier proteins (aquaporins, ion channels). Active transport needs energy because it moves solutes against their concentration (or electrochemical) gradients—from low to high—which is energetically unfavorable and therefore requires input (EK 2.5.A.3). Cells supply that energy directly (usually ATP hydrolysis) or indirectly (coupled transport). Classic ATP-driven examples are the sodium–potassium pump and proton pumps; symports and antiports use the energy stored in gradients established by ATP pumps. Moving large materials by endocytosis or exocytosis also requires energy to remodel the membrane (EK 2.5.B.1). For AP exam practice and quick review on membrane permeability, check the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and try practice problems at (https://library.fiveable.me/practice/ap-biology).
How do cells transport really big molecules across membranes?
Cells move really big stuff (like proteins, polysaccharides, whole particles) across membranes using energy-dependent vesicle trafficking—endocytosis to bring things in and exocytosis to send things out (EK 2.5.B.1). In endocytosis the plasma membrane folds in and pinches off vesicles: phagocytosis engulfs large particles, pinocytosis “drinks” fluid, and receptor-mediated endocytosis uses specific receptors to concentrate cargo. Exocytosis fuses internal vesicles with the membrane to secrete large molecules (both processes use ATP and cytoskeletal motors). These aren’t diffusion or channel/carrier transport—they’re active, vesicle-based mechanisms required when cargo is too big for protein pores (LO 2.5.B). For AP review, study the CED terms (endocytosis, exocytosis, receptor-mediated, phagocytosis) and check the Topic 2.5 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ). Want extra practice? Try the AP Bio practice set on Fiveable (https://library.fiveable.me/practice/ap-biology).
What's the difference between endocytosis and exocytosis?
Endocytosis and exocytosis are opposite energy-requiring ways cells move large stuff across the plasma membrane (LO 2.5.B). In endocytosis the cell folds its membrane inward to form new vesicles that engulf extracellular material—examples: phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis (specific uptake). In exocytosis internal vesicles fuse with the plasma membrane to release large molecules (like secreted proteins or waste) to the outside. Both processes use ATP-driven membrane remodeling and vesicle trafficking (so they’re not passive diffusion or simple active transport of ions). On the AP exam you may be asked to describe these mechanisms or distinguish them from channel/carrier-mediated transport (EK 2.5.B.1 and EK 2.5.B.2). Review the Topic 2.5 study guide here (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice more at the unit page (https://library.fiveable.me/ap-biology/unit-2) or with 1000+ practice questions (https://library.fiveable.me/practice/ap-biology).
Can you give me examples of when cells use active transport vs passive transport?
Passive transport: things move down their concentration gradient without ATP. Examples: O2 and CO2 diffusing across the membrane; water moving by osmosis through aquaporins; glucose or ions moving by facilitated diffusion through channel or carrier proteins (e.g., glucose uniporter). These match EK 2.5.A.2 keywords: diffusion, osmosis, channel/carrier proteins, aquaporins. Active transport: the cell spends energy (usually ATP) to move solutes against their gradient. Examples: the Na+/K+ pump (ATP-driven) that keeps high K+ inside and high Na+ outside; proton pumps that acidify lysosomes or plant vacuoles; secondary active transport like the Na+/glucose symporter (uses Na+ gradient set up by the pump to bring glucose in) and antiports that exchange ions. For large cargos, endocytosis and exocytosis require energy (EK 2.5.B.1). These are fair-game on AP Bio—know definitions, examples, and when energy’s required. Review the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice problems (https://library.fiveable.me/practice/ap-biology).
How do concentration gradients form across membranes?
Concentration gradients form because membranes are selectively permeable: some solutes cross easily (small nonpolar molecules, or via channels) while others don’t, so concentrations differ on each side (EK 2.5.A.1). Simple diffusion and facilitated diffusion move solutes down their gradient (high → low) without energy (EK 2.5.A.2). Cells create or maintain gradients using active transport—ATP-driven pumps (e.g., Na+/K+ pump, proton pumps) that move ions against their gradient (low → high) and by coupled transport (symport/antiport) that uses one gradient to drive movement of another (EK 2.5.A.3). Water moves by osmosis through membranes or aquaporins, responding to solute gradients and tonicity. Large-scale moves use endocytosis/exocytosis for bulk material (EK 2.5.B.1). On the AP exam you should be ready to describe both passive and active mechanisms and name examples (use LO 2.5.A terms). For a focused review, check the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and extra practice at (https://library.fiveable.me/practice/ap-biology).
I don't understand how exocytosis works - does the cell membrane break?
No—the cell membrane doesn’t “break” during exocytosis. Exocytosis is an active, controlled process (LO 2.5.B; EK 2.5.B.1). A vesicle made inside the cell (from Golgi or endomembrane system) moves to the plasma membrane, the two lipid bilayers fuse, and the vesicle’s contents are released outside. The vesicle membrane becomes part of the plasma membrane, so membrane continuity is preserved rather than ruptured. This lets cells secrete large molecules (like proteins or polysaccharides) and adjust membrane composition without losing barrier function. Exocytosis requires energy and specific fusion machinery to make the membranes merge safely. For AP review, this is directly tied to Topic 2.5 (membrane transport)—check the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and try practice problems (https://library.fiveable.me/practice/ap-biology) to see diagrams and common exam phrasing.
Why can't large molecules just pass through the membrane like small ones do?
Big molecules can’t just slip through the membrane because of selective permeability. The phospholipid bilayer lets small nonpolar molecules (and water through aquaporins) diffuse down their concentration gradients, but large or polar molecules can’t pass the hydrophobic core easily. Cells use proteins instead: channel and carrier proteins do facilitated diffusion for medium-sized polar solutes, while active transport (ATP-driven pumps like the sodium-potassium pump) moves ions against gradients. For really large stuff (proteins, macromolecules, lots of cargo) the cell uses energy-requiring vesicle processes—endocytosis to bring things in and exocytosis to send things out (EKs 2.5.A.1–3 and EK 2.5.B.1). On the AP exam, you should be able to name these mechanisms and explain when energy is required. If you want a clear summary and practice questions on this topic, check the Topic 2 study guide on Fiveable (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and more practice at (https://library.fiveable.me/practice/ap-biology).
What does it mean when they say membranes are selectively permeable?
“Sellectively permeable” means the plasma membrane lets some substances cross more easily than others, so cells can control what goes in and out. Because the membrane is a phospholipid bilayer with embedded proteins, small nonpolar molecules (O2, CO2) diffuse through the lipids, water moves quickly through aquaporins or by osmosis, and charged or large polar molecules (ions, glucose) need channel or carrier proteins—sometimes using facilitated diffusion (no ATP) or active transport (ATP-driven pumps like the sodium–potassium pump) to move against a concentration gradient (EK 2.5.A.1–3). That selective flow creates and maintains concentration gradients and tonicity, which cells use to balance solutes and water (LO 2.5.A/B). On the AP exam you might be asked to describe these mechanisms or predict effects of changing gradients in an FRQ. For a focused review, see the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ) and practice questions (https://library.fiveable.me/practice/ap-biology).
How do I remember all the different types of membrane transport for the AP exam?
Think of membrane transport as three big buckets and a fourth for bulk movement—that keeps it simple for the exam. - Passive (no ATP): diffusion (small nonpolar down a concentration gradient), osmosis (water down its gradient; tonicity matters), facilitated diffusion (channel proteins or carrier proteins; aquaporins for water). - Active (ATP required): primary active (ATP-driven pumps like Na⁺-K⁺ pump, proton pump move solutes against gradients), secondary active (uses gradient energy—symport and antiport). - Bulk transport (energy needed): endocytosis (phagocytosis, receptor-mediated endocytosis) and exocytosis. Quick mnemonic: “DOF-ASP” = Diffusion, Osmosis, Facilitated—Active (pumps/symport/antiport)—bulk (Endo/Exo). On the exam, tie each to CED keywords: selective permeability, concentration gradient, ATP-driven transport, tonicity, and channel/carrier proteins. Practice translating diagrams and scenarios into which mechanism is occurring (this shows up in both MCQs and FRQs). Review the Topic 2.5 study guide (https://library.fiveable.me/ap-biology/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ), the Unit 2 overview (https://library.fiveable.me/ap-biology/unit-2), and drill 1000+ practice questions (https://library.fiveable.me/practice/ap-biology).