Unit 2: Cell Structure and Function

AP Biology — 116 practice questions with detailed explanations.

Unit Study Guide

AP Biology Unit 2: Cell Structure and Function Hub Overview

Executive Summary

Unit 2 serves as the architectural blueprint of biological systems, bridging the macromolecular constituents of Unit 1 with the ultrastructural complexity of the cell—the fundamental unit of life. This unit demands a rigorous, molecularly precise understanding of cellular compartmentalization, the fluid mosaic model of biological membranes, and the highly regulated thermodynamic exchange of materials across these borders. You must master the evolutionary divergence of cellular domains (Bacteria, Archaea, Eukarya), focusing on the emergence of endomembrane systems and the endosymbiotic origins of mitochondria and chloroplasts. Furthermore, a deep understanding of surface-area-to-volume ratios is critical, as it dictates the physical limits of cellular size and metabolic efficiency. Mastery of Unit 2 requires integrating spatial geometry with molecular polarity to explain how cells maintain the strict homeostatic conditions necessary for life.

Molecular Deep-Dive

1. Membrane Architecture and Dynamics

The cell membrane is not a static barrier; it is a dynamic, two-dimensional liquid. The Fluid Mosaic Model describes the plasma membrane as a highly mobile collage of phospholipids, cholesterol, and proteins that move fluidly within the plane of the lipid bilayer.

Phospholipid Amphipathicity: Composed of a hydrophilic phosphate head and hydrophobic fatty acid tails. This amphipathic nature drives the spontaneous formation of bilayers in aqueous environments, creating a selectively permeable barrier.

Cholesterol as a Fluidity Buffer: Embedded within the hydrophobic core, cholesterol acts as a bidirectional temperature buffer. At high temperatures, its rigid ring structure restrains phospholipid movement, reducing fluidity. At low temperatures, it prevents the tight packing of tails, maintaining membrane fluidity and preventing crystallization.

Membrane Proteins: Integral proteins span the hydrophobic core (transmembrane), often serving as channels, carriers, or receptors. Peripheral proteins are loosely associated with the inner or outer surface, frequently participating in cell signaling or tethering the membrane to the extracellular matrix (ECM) or cytoskeleton.

2. Cellular Transport and Thermodynamics

Movement across the membrane is strictly governed by thermodynamics (entropy and free energy).

Passive Transport: Does not require metabolic ATP. Includes simple diffusion (small, nonpolar molecules like

O_2

and

CO_2

moving down their concentration gradient) and facilitated diffusion (polar molecules and ions moving through specific transmembrane proteins down their electrochemical gradient).

Active Transport: Requires the hydrolysis of ATP to move solutes against their concentration gradient. The Sodium-Potassium Pump ( $Na^+/K^+$ ATPase) is the quintessential example, exporting 3

Na^+

ions for every 2

K^+

ions imported, thereby generating a vital electrochemical gradient.

Bulk Transport: Macromolecules are transported via vesicles. Exocytosis secretes cellular products out of the cell, while Endocytosis (phagocytosis, pinocytosis, and receptor-mediated endocytosis) internalizes extracellular material.

3. Cellular Compartmentalization

Eukaryotic cells possess a complex endomembrane system and specialized organelles that localize specific metabolic pathways, increasing chemical efficiency and preventing cross-interference.

Endosymbiotic Theory: Mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by an ancestral eukaryotic cell. Molecular evidence includes their double-membrane structure, circular, naked DNA, and 70S ribosomes.

The Endomembrane System: The Rough ER (studded with ribosomes) synthesizes proteins destined for membranes or secretion; the Smooth ER synthesizes lipids and detoxifies poisons; and the Golgi apparatus modifies, sorts, and packages these macromolecules into lysosomes or secretory vesicles.

AP Exam Trap

Trap 1: Teleological Language and Water Potential

The Trap: Students frequently write that "water wants to move to the hypertonic side" or "the cell is trying to reach equilibrium."

The Fix: The AP exam strictly penalizes teleological (purpose-driven) language. Water does not "want" anything. You must state, "Water spontaneously moves down its concentration gradient from an area of higher water potential to an area of lower water potential due to the random thermal motion of molecules (osmosis)."

Trap 2: Misunderstanding Tonicity Dynamics

The Trap: Assuming a cell placed in a "10% sucrose solution" is automatically hypertonic without knowing the cell's internal solute concentration, or assuming tonicity is a fixed property of a solution alone.

The Fix: Always frame tonicity as a comparative relationship. "The extracellular solution is hypertonic to the cell, meaning the solution has a higher relative solute concentration and lower water potential than the cell's cytoplasm."

Trap 3: Surface Area to Volume Limitations

The Trap: Thinking that a larger cell is more efficient because it can hold more organelles and process more material.

The Fix: As a cell grows, its volume increases geometrically faster than its surface area. The plasma membrane (Surface Area) cannot expand fast enough to service the expanding metabolic demands of the cytoplasm (Volume). You must explicitly state that a higher SA:V ratio is required for efficient nutrient/waste exchange, which limits cell size.

Interactive Glossary

Term	Molecularly Precise Definition
:---	:---
Amphipathic	A molecule containing both a hydrophilic (water-attracting) polar region and a hydrophobic (water-repelling) nonpolar region; fundamental to the spontaneous formation of phospholipid bilayers in aqueous environments.
Aquaporin	A specialized integral channel protein that facilitates the highly selective, rapid osmotic diffusion of water molecules across the selectively permeable membrane.
Concentration Gradient	A thermodynamic difference in the density of a chemical substance across a space or membrane; represents potential energy that drives the passive diffusion of molecules.
Electrochemical Gradient	A dual thermodynamic gradient across a membrane composed of a chemical force (ions moving from high to low concentration) and an electrical force (cations/anions moving toward opposite charges).
Endocytosis	The active, energy-dependent uptake of extracellular material into the cell via the invagination and pinching off of the plasma membrane, forming intracellular vesicles.
Flaccid	A cellular state in plant cells where the protoplast has lost water and pulled away slightly from the cell wall due to an isotonic or hypertonic external environment, lacking turgor pressure.
Osmoregulation	The homeostatic mechanism by which organisms control the internal concentration of water and solutes to maintain cellular turgidity and function.
Plasmolysis	The severe detachment of the cell membrane from the cell wall in plant cells caused by critical water loss when placed in a heavily hypertonic environment.
Selectively Permeable	The property of biological membranes that allows certain molecules to cross with ease while restricting others, strictly based on size, charge, and molecular polarity.
Tonicity	The relative concentration of solutes in the extracellular fluid compared to the intracellular fluid, determining the net direction of osmotic water flow across a semi-permeable membrane.

Quantitative Skill-Set

To conquer AP Biology Unit 2, you must be completely fluent in specific mathematical applications:

Water Potential ( $\Psi$ ) Calculations:

* Master the core equation: $\Psi = \Psi_p + \Psi_s$ (Total Water Potential = Pressure Potential + Solute Potential).

* Calculate Solute Potential using the formula: $\Psi_s = -iCRT$ .

* $i$ = ionization constant (e.g., 1 for sucrose, 2 for $NaCl$ ).

* $C$ = molar concentration (mol/L).

* $R$ = pressure constant ( $0.0831$ L·bars/mol·K).

* $T$ = temperature in Kelvin ( $^{\circ}C + 273$ ).

Application:* Predicting the exact net movement of water into or out of a cell, and calculating the exact pressure potential at dynamic equilibrium.

Surface Area to Volume (SA:V) Geometry:

* Calculate SA and Volume for cubes, cylinders, and spheres.

* *Cube Formulas:* $SA = 6s^2$ ; $V = s^3$ (where $s$ is side length).

Skill:* Demonstrate mathematically why dividing a large cell into multiple smaller cells dramatically increases the total SA:V ratio, thereby optimizing exchange capabilities.

Study Moves

Draw the Fluid Mosaic Model: Do not simply look at textbook diagrams. Physically draw a cross-section of a plasma membrane from scratch. Include cholesterol, integral proteins, peripheral proteins, glycoproteins, and explicitly label the hydrophilic heads and hydrophobic tails.

Water Potential Math Drills: Create a spreadsheet of varied practice problems. Change the temperature, alter the solute (sucrose vs. sodium chloride), and adjust the molarity. Ensure you can calculate the exact point of equilibrium where

\Psi_{inside} = \Psi_{outside}

Trace the Endomembrane Pathway: Pick a complex protein (like a secreted antibody) and trace its physiological pathway from mRNA translation on a ribosome, through the RER lumen, via transport vesicles to the Golgi, and out of the cell via exocytosis. Map this spatially.

Exam Linkage

Unit 2 is a foundational keystone for the entire AP Biology curriculum. It does not exist in a vacuum and will heavily appear in multi-unit FRQs.

Unit 1 (Chemistry of Life): The molecular polarity of water and the emergent amphipathic nature of lipids (Unit 1) directly explain the structural integrity and selective permeability of the cell membrane (Unit 2).

Unit 3 (Cellular Energetics): The chemiosmotic coupling of the Electron Transport Chain relies entirely on the compartmentalization of the mitochondria and chloroplasts, utilizing transmembrane proteins to generate an electrochemical gradient.

Unit 4 (Cell Communication): Cell signaling fundamentally begins at the membrane. Ligands binding to G-Protein Coupled Receptors (GPCRs) or the reception of hydrophobic steroids inside the cell cannot be understood without a deep grasp of membrane permeability.

Unit 6 (Gene Expression): The central dogma of biology relies on the physical compartmentalization of the eukaryotic nucleus (transcription) and the cytoplasm (translation), requiring mature mRNA to cross the nuclear envelope via nuclear pores.

Top 5 Concepts to Master

1The cell membrane is a fluid mosaic of phospholipids and proteins that exhibits selective permeability, allowing only certain molecules to pass through based on size and polarity.
2Cells move materials across their membranes through passive transport, which requires no energy, and active transport, which requires ATP to move substances against their concentration gradient.
3Eukaryotic cells use internal membranes to create specialized compartments, such as the endomembrane system and membrane-bound organelles, which optimize metabolic processes and increase cellular efficiency.
4Cells communicate through complex signal transduction pathways where a signaling ligand binds to a receptor, triggering a cascade of intracellular events that ultimately alter cellular behavior.
5Water potential predicts the movement of water across a semipermeable membrane, driven by differences in solute concentration and physical pressure, ultimately determining cell tonicity and shape.

Key Terms & Definitions

Practice with Flashcards

plasma membrane

The plasma membrane is a thin, flexible barrier that surrounds all cells and regulates the movement of substances in and out of the cell. It consists of a phospholipid bilayer embedded with various proteins and cholesterol molecules.

fluid mosaic model

This model describes the plasma membrane as a dynamic and flexible structure made of a variety of molecules. The phospholipids move fluidly in the plane of the membrane, creating a mosaic pattern of scattered proteins within the lipid bilayer.

phospholipid bilayer

The phospholipid bilayer is a two-layered arrangement of phosphate and lipid molecules that forms the foundation of the cell membrane. The hydrophobic lipid tails face inward, while the hydrophilic phosphate heads face outward toward the watery environments inside and outside the cell.

selective permeability

Selective permeability is a property of the cell membrane that allows only certain molecules to pass through. Small, nonpolar molecules can easily cross the membrane, while large or charged molecules require specific transport proteins.

facilitated diffusion

Facilitated diffusion is the passive transport of molecules across the cell membrane via transmembrane proteins. It does not require energy and is used for molecules that are too large or polar to pass directly through the lipid bilayer.

active transport

Active transport is the movement of molecules across a cell membrane from a region of lower concentration to a region of higher concentration. This process requires energy in the form of ATP to move substances against their concentration gradient.

Na+/K+ pump

The sodium-potassium pump is a protein complex that uses active transport to exchange sodium and potassium ions across the cell membrane. It pumps three sodium ions out of the cell and two potassium ions into the cell, utilizing ATP to maintain cellular ion gradients.

endocytosis

Endocytosis is a cellular process in which substances are brought into the cell by engulfing them with the cell membrane. The membrane folds over the substance, forming a vesicle that pinches off inside the cell to transport the materials.

exocytosis

Exocytosis is the process by which cells expel materials in membrane-bound vesicles to the extracellular environment. The vesicle fuses with the plasma membrane, releasing its contents outside the cell while recycling the membrane lipids and proteins.

tonicity

Tonicity refers to the relative concentration of solutes in the fluid surrounding a cell compared to the concentration inside the cell. It determines the direction of osmotic water flow, which can cause a cell to shrink, swell, or stay the same size.

osmosis

Osmosis is the diffusion of free water molecules across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration. This passive transport process is crucial for maintaining proper cellular hydration and internal pressure.

water potential

Water potential is a measure of the potential energy of water in a system compared to pure water, influencing the direction of water movement. It is determined by the combined effects of solute potential and pressure potential, with water always moving from areas of higher water potential to lower water potential.

surface-area-to-volume ratio

This ratio compares the surface area of a cell to its internal volume, directly affecting the efficiency of cellular transport. As a cell grows larger, its volume increases faster than its surface area, eventually limiting the exchange of nutrients and wastes across the membrane.

prokaryotic cell

A prokaryotic cell is a simple, typically single-celled organism that lacks a membrane-bound nucleus and specialized organelles. Its DNA is located in a nucleoid region, and it generally possesses a cell wall and ribosomes.

eukaryotic cell

A eukaryotic cell is a complex cell that contains a true membrane-bound nucleus and various specialized organelles. Animals, plants, fungi, and protists are composed of these cells, which allow for compartmentalized cellular functions.

nucleus

The nucleus is a highly specialized, membrane-bound organelle that houses the cell's genetic material. It coordinates cellular activities, including growth, metabolism, protein synthesis, and cell division.

endoplasmic reticulum

The endoplasmic reticulum is an extensive network of flattened membranes and sacs that performs multiple critical functions. The rough ER has attached ribosomes and synthesizes proteins, while the smooth ER lacks ribosomes and synthesizes lipids.

Golgi apparatus

The Golgi apparatus is an organelle consisting of stacked membranes that modifies, sorts, and packages proteins. It receives vesicles from the endoplasmic reticulum and prepares their contents for secretion outside the cell or transport to other organelles.

mitochondria

Mitochondria are double-membraned organelles that serve as the powerhouses of the cell by generating ATP through cellular respiration. They have their own independent DNA and can self-replicate to meet the energy demands of the cell.

lysosome

A lysosome is a membrane-bound organelle containing digestive enzymes that break down waste materials and cellular debris. They are essential for recycling cellular components and destroying pathogens that have been taken inside the cell.

⚠️ Common Misconceptions — Exam Traps

❌ Plant cells do not have mitochondria because they have chloroplasts to generate energy.

✅ Correct: Plant cells have both chloroplasts and mitochondria. Chloroplasts perform photosynthesis to produce glucose, but mitochondria are still required to perform cellular respiration and convert that glucose into usable ATP to power cellular processes.

❌ Prokaryotic cells do not have any organelles.

✅ Correct: While prokaryotic cells lack membrane-bound organelles (like a nucleus or endoplasmic reticulum), they do contain non-membrane-bound organelles, such as ribosomes, which are necessary for protein synthesis.

❌ The cell wall is the selectively permeable barrier that controls what enters and exits the cell.

✅ Correct: The cell wall provides structural support and protection but is fully permeable to most small molecules. The plasma membrane (cell membrane) is the selectively permeable structure that regulates the transport of substances in and out of the cell.

❌ Water requires transport proteins to cross the cell membrane during osmosis.

✅ Correct: Because water is a small, polar molecule, it can slowly diffuse directly through the hydrophobic core of the phospholipid bilayer without the aid of transport proteins. Aquaporins (transport proteins) increase the rate of water transport but are not strictly required for osmosis to occur.

❌ Facilitated diffusion requires cellular ATP energy because it uses membrane proteins.

✅ Correct: Facilitated diffusion is a passive transport mechanism. Even though it utilizes channel or carrier proteins to move polar or large substances across the membrane, it relies strictly on the concentration gradient and does not require an input of cellular ATP energy.