Why this unit matters
Understanding cell architecture explains how organisms maintain internal stability and carry out essential processes.
What you will learn
- Identify major organelles and connect their structures to core cellular functions.
- Explain membrane structure and selective permeability using phospholipids and embedded proteins.
- Distinguish passive and active transport and predict movement across concentration gradients.
Understand the core ideas
Cell structure reflects physical and chemical constraints. Prokaryotic cells lack membrane bound organelles, so many processes occur in a shared cytoplasmic space, while eukaryotic cells compartmentalize reactions in organelles such as mitochondria, chloroplasts, and endomembrane compartments. Compartmentalization can increase efficiency by concentrating enzymes and substrates and by separating incompatible reactions. Organelles are not isolated from one another conceptually, because AP Biology reasoning often tracks material flow between nucleus, ribosomes, endoplasmic reticulum, Golgi, membranes, and extracellular space.
The plasma membrane is a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates in many eukaryotes. Amphipathic phospholipids create a semipermeable barrier: small nonpolar molecules diffuse readily, while charged or large polar solutes generally require transport proteins. Movement direction depends on gradients in concentration or electrochemical potential. Passive transport moves down a gradient, while active transport moves against a gradient using energy, often through ATP hydrolysis or coupled transport. Osmosis is water movement across a selectively permeable membrane and can change cell volume depending on external solute concentration.
Cell size links directly to exchange efficiency through surface area to volume ratio. As a cell grows, volume increases faster than surface area, reducing membrane area available per unit cytoplasm for exchange. Cells address this through division, elongated or flattened shapes, and membrane specializations such as microvilli. Data interpretation questions often compare transport rates or survival across cells in different tonicities, so the key is to connect membrane permeability and geometry to measurable outcomes rather than memorizing isolated definitions.
Key terms
- phospholipid bilayer
- A double layer of amphipathic phospholipids with hydrophilic heads outward and hydrophobic tails inward, forming the core membrane structure.
- selective permeability
- The property of a membrane that allows some substances to cross more easily than others based on size, charge, polarity, and transport proteins.
- osmosis
- The net diffusion of water across a selectively permeable membrane from higher free water potential to lower free water potential.
- surface area to volume ratio
- A geometric relationship that influences how efficiently a cell can exchange materials with its environment.
Data reasoning: membrane transport in different solutions
Equal cells are placed in three solutions. Relative volume after 20 minutes is 1.20 in solution A, 1.00 in solution B, and 0.82 in solution C.
- Interpret increased volume in solution A as net water influx, indicating an external hypotonic condition.
- Interpret unchanged volume in solution B as near isotonic conditions with no strong net water movement.
- Interpret decreased volume in solution C as net water efflux, indicating an external hypertonic condition.
- Conclude that water movement follows osmotic gradients and that membrane selectivity, not cell choice, drives the direction.
A common misconception
Claim: Active transport means particles move quickly, while passive transport means they move slowly.
Correction: Active versus passive refers to energy use and gradient direction, not to transport speed.
Lessons in this unit
- Prokaryotic and eukaryotic cell organizationCompare internal organization and infer functional consequences of compartmentalization.
- Membrane structure and fluid mosaic modelUse membrane composition to explain transport and signaling capacity.
- Transport across membranesModel diffusion, osmosis, facilitated diffusion, and active transport in realistic contexts.
- Cell size and surface area constraintsRelate geometry to exchange rates and why cells remain small or specialized.
Study task
Unit checkpoint
How is facilitated diffusion different from active transport?
Facilitated diffusion moves substances down their gradient through membrane proteins without energy input, while active transport moves substances against a gradient using energy, often from ATP.