Cells · Topic 9 of 17

Cell Membrane & Transport

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Biology · the cell

Cell Membrane & Transport

The membrane is a fluid phospholipid bilayer. Passive transport runs high to low without ATP; active transport uses ATP; water follows solute by osmosis.

01 · Passive vs activePassive = no ATP · active = ATP

02 · OsmosisOsmosis: water moves toward more solute

03 · TonicityHypo → swells · hyper → shrinks

What it is

Diffusion: high → low concentration until evenly spread must know

Osmosis = water crosses a membrane toward the side with more solute must know

Facilitated diffusion: down the gradient through a protein, no ATP

Step by step

Na⁺/K⁺ pump: 3 Na⁺ out, 2 K⁺ in, per ATP

Bulk transport (uses energy)

Step by step

Endocytosis brings material in: phagocytosis (eating), pinocytosis (drinking)

Exocytosis: a vesicle fuses with the membrane and releases contents out

Parts & jobs

Membrane = phospholipid bilayer + proteins (fluid mosaic), selectively permeable must know

Heads hydrophilic (face water); tails hydrophobic (face inward)

Proteins transport and receive signals; outer carbohydrates = cell ID tags

The rules

Hypotonic → cell swells; hypertonic → shrinks; isotonic → no change must know

O₂, CO₂ and fat-soluble molecules cross freely; ions and glucose need proteins

Red blood cell: hypotonic → bursts (lysis); hypertonic → shrivels (crenation)

Plant cell: hypotonic → firm (turgor); hypertonic → plasmolysis, wilting

Worked examples

Cell (1% solute) in 4% salt → hypertonic → water leaves, cell shrinks

Don't confuse

Passive: high → low, no ATP; active: low → high, uses ATP must know

Passive or active transport?

Commonly confused

	Passive	Active
Direction	High → low concentration (down the gradient)	Low → high (against the gradient), or bulk
Cell energy (ATP)	None	Required
Protein help	Channels or carriers in facilitated diffusion	Pump proteins; vesicles in bulk transport
Examples	Diffusion, osmosis, facilitated diffusion	Na⁺/K⁺ pump, endocytosis, exocytosis

Misconceptions

Trap: water moves toward the hypertonic (more solute) side

More detail

Show 5 more facts

Diffusion speeds up with a steep gradient, heat and small molecules

Aquaporins = water channels (red blood cells, kidney tubules)

Receptor-mediated endocytosis: specific cargo binds receptors (e.g. LDL)

Secondary active transport rides a pump-made gradient (glucose, amino acids)

Cholesterol regulates membrane fluidity (animal cells)

Pearl

Water follows solute: whichever side holds more dissolved particles pulls the water toward it.

Sources · OpenStax Concepts of Biology §3.4 The Cell Membrane (fluid mosaic model) · OpenStax Concepts of Biology §3.5 Passive Transport (diffusion, osmosis, tonicity) · OpenStax Concepts of Biology §3.6 Active Transport (pumps, endocytosis, exocytosis) · OpenStax Biology 2e §5.2 Passive Transport (aquaporins, plasmolysis) · OpenStax Biology 2e §5.3 Active Transport (sodium–potassium pump) · OpenStax Biology 2e §5.4 Bulk Transport (phagocytosis, pinocytosis) · Union Test Prep — HESI A2 Biology study guide, p. 1: cells, organelles, membrane, respiration, photosynthesis · Union Test Prep — HESI A2 Biology study guide, p. 2: biological molecules, enzymes, metabolism · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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