Cell Structure and Function — AP Biology Bio Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Cell organelle structure and function, the fluid mosaic model of the cell membrane, passive and active membrane transport mechanisms, and the link between cell compartmentalization and metabolic efficiency.

You should already know: High-school biology, basic chemistry.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is Cell Structure and Function?

Cell Structure and Function is the study of how the molecular, subcellular, and cellular architecture of living organisms directly enables all life-sustaining biological processes, from nutrient uptake to energy production to cell signaling. This is Unit 2 in the AP Biology CED, worth 10-13% of your total exam score, and forms the foundation for all later topics including cellular energetics, cell communication, and heredity. Common synonyms you will see in exam questions include "subcellular organization", "membrane dynamics", and "cellular compartmentalization". Examiners never test isolated factual recall here: every question will require you to connect a structural feature to its biological role to earn full marks.

2. Cell organelles — function and structure

All cells share four core features: a cell membrane, cytosol, ribosomes, and genetic material, but eukaryotic cells (plant, animal, fungi, protist) have specialized membrane-bound organelles that prokaryotes (bacteria, archaea) lack. Below are the high-yield organelles tested most frequently on the AP exam:

• Nucleus: Structure: Double lipid bilayer (nuclear envelope) with selective nuclear pores, internal nucleolus, and chromatin (DNA wrapped around histone proteins). Function: Stores genomic DNA, regulates gene expression, and assembles ribosomal subunits in the nucleolus. Exam tip: Nuclear pores only allow passage of molecules like mRNA and transcription factors, protecting DNA from damage by cytoplasmic enzymes.
• Ribosomes: Structure: Two subunits made of rRNA and protein, either free-floating in the cytosol or bound to the rough endoplasmic reticulum (RER). Function: Site of protein synthesis. Tip: Free ribosomes make proteins for internal cellular use, while bound ribosomes make proteins for secretion, membrane insertion, or transport to lysosomes.
• Rough Endoplasmic Reticulum (RER): Structure: Folded membrane network contiguous with the nuclear envelope, studded with bound ribosomes. Function: Folds, modifies, and tags proteins made by bound ribosomes for transport to the Golgi apparatus.
• Smooth Endoplasmic Reticulum (SER): Structure: Tubular membrane network with no ribosomes. Function: Synthesizes lipids (phospholipids, steroids), detoxifies drugs and toxins, and stores calcium ions for muscle cell contraction.
• Golgi Apparatus: Structure: Stacked, flattened membrane sacs called cisternae, with a cis (receiving) face that accepts vesicles from the RER and a trans (shipping) face that sends vesicles to target locations. Function: Modifies, sorts, and packages proteins and lipids into vesicles for transport to the cell membrane, lysosomes, or extracellular space.
• Lysosomes: Structure: Membrane-bound sacs filled with hydrolytic enzymes, maintained at an acidic pH of ~4.5. Function: Breaks down damaged organelles, ingested pathogens, and macromolecules via autophagy or phagocytosis. Tip: The low pH is a critical structural adaptation: hydrolytic enzymes are inactive at the neutral pH of the cytosol, so accidental lysosome leakage does not damage the cell.
• Mitochondria: Structure: Double membrane, with a smooth outer membrane and a highly folded inner membrane (cristae), internal matrix with its own circular DNA and ribosomes. Function: Site of aerobic cellular respiration, produces ATP for cellular work.
• Chloroplasts (plant/algae only): Structure: Double membrane, internal stacks of thylakoids called grana, and a gel-like stroma with its own circular DNA and ribosomes. Function: Site of photosynthesis, converts light energy to chemical energy stored in glucose.

Worked Example: A pancreatic cell that secretes large amounts of digestive enzymes will have unusually high volumes of RER and Golgi apparatus. This is because digestive enzymes are secreted proteins, synthesized by bound ribosomes on the RER, processed and packaged by the Golgi, and released via exocytosis. Examiners frequently ask to predict organelle abundance in specialized cell types, so always link organelle function to cell role for full marks.

3. Cell membrane — fluid mosaic

Proposed by Singer and Nicolson in 1972, the fluid mosaic model describes the cell membrane as a dynamic, flexible structure with a mosaic of embedded proteins floating in a phospholipid bilayer.

Core Structure

Phospholipids are amphipathic: their hydrophilic phosphate heads face the aqueous extracellular and intracellular environments, while their hydrophobic fatty acid tails face inward, away from water, forming a stable bilayer.

Fluidity Regulators

Three factors control membrane fluidity, a common FRQ test topic:

1. Temperature: Higher temperatures increase phospholipid movement, raising fluidity; lower temperatures reduce movement, decreasing fluidity.
2. Fatty acid saturation: Unsaturated fatty acid tails have double bonds that create kinks, preventing tight packing and increasing fluidity. Saturated tails pack tightly, reducing fluidity.
3. Cholesterol (animal cells only): Acts as a fluidity buffer: at high temperatures, it restricts phospholipid movement to reduce fluidity; at low temperatures, it prevents tight packing to maintain fluidity.

Mosaic Components

• Integral proteins: Embedded in the bilayer, often transmembrane (span the full width of the membrane), with hydrophobic regions that interact with fatty acid tails and hydrophilic regions exposed to aqueous environments. Functions include transport channels, cell surface receptors, enzymes, and adhesion molecules.
• Peripheral proteins: Loosely attached to the membrane surface or integral proteins, involved in cell signaling and structural support.
• Carbohydrate chains: Attached to lipids (glycolipids) or proteins (glycoproteins) only on the extracellular surface, responsible for cell-cell recognition and immune system identification.

Worked Example: A mutation reduces unsaturated fatty acid content in the cell membranes of a cold-water fish. The membrane will become rigid at low temperatures, reducing the function of embedded transport proteins and slowing nutrient uptake, decreasing survival. This is a common FRQ question, so always explicitly link fatty acid saturation to fluidity and cell function.

4. Membrane transport — passive and active

Membrane transport describes the movement of molecules across the selectively permeable cell membrane, divided into two categories based on energy requirements:

Passive Transport (no ATP input, moves down concentration gradient)

1. Simple diffusion: Movement of small, nonpolar, uncharged molecules (O₂, CO₂, steroid hormones) directly through the phospholipid bilayer, no transport protein required.
2. Facilitated diffusion: Movement of large, polar, or charged molecules (glucose, ions, water) down their gradient via specific integral transport proteins. Channel proteins form hydrophilic pores for fast transport (e.g., aquaporins for water, ion channels for Na⁺/K⁺), while carrier proteins bind to target molecules and change shape to transport them across the membrane.
3. Osmosis: Diffusion of water across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to lower water potential (higher solute concentration). The AP Biology water potential formula is: $$\Psi ={\Psi }_S+{\Psi }_P$$ Where ${\Psi }_S$ = solute potential (always negative, more solute = more negative) and ${\Psi }_P$ = pressure potential (positive in turgid plant cells, 0 in open containers/animal cells). Worked Example: A plant cell with $\Psi =-0.7$ MPa is placed in a beaker of solution with $\Psi =-0.3$ MPa. Water moves from the higher (less negative) water potential of the beaker into the lower (more negative) water potential of the cell, making the cell turgid.

Active Transport (requires ATP input, moves against concentration gradient)

1. Protein pumps: Transmembrane carrier proteins that use ATP to move molecules against their gradient. The most commonly tested example is the Na⁺/K⁺ pump, found in all animal cells: it pumps 3 Na⁺ out of the cell and 2 K⁺ into the cell per ATP molecule, creating an electrochemical gradient used for nerve signaling and nutrient uptake.
2. Bulk transport: Active transport of large molecules or large quantities of material using vesicles:

• Endocytosis: Cell takes in material by forming vesicles from the cell membrane, including phagocytosis (cell eating, ingesting large particles like bacteria), pinocytosis (cell drinking, ingesting extracellular fluid), and receptor-mediated endocytosis (specific molecules bind to surface receptors to trigger vesicle formation).
• Exocytosis: Vesicles from the Golgi fuse with the cell membrane, releasing their contents outside the cell, used for secretion of hormones, enzymes, and neurotransmitters.

Exam tip: For full marks on transport questions, always specify if transport is passive/active, what subtype it is, and if a transport protein is required. Most water transport occurs via aquaporins (facilitated diffusion), not simple diffusion, so explicitly mention this to avoid losing marks.

5. Cell compartmentalisation and metabolic efficiency

Compartmentalization refers to the separation of the eukaryotic cell into distinct, membrane-bound organelles, each with an internal environment optimized for a specific function. This adaptation drastically improves metabolic efficiency, a core AP exam concept, via four key mechanisms:

1. Isolation of incompatible reactions: The acidic environment of lysosomes is kept separate from the neutral cytosol, so hydrolytic enzymes do not damage other cellular components. Similarly, the light-dependent reactions of photosynthesis occur in the thylakoid membrane, while the Calvin cycle occurs in the stroma, preventing cross-interference between reaction pathways.
2. Increased surface area for enzymatic reactions: The folded cristae of mitochondria and stacked thylakoid grana of chloroplasts provide far more surface area for embedded enzyme complexes involved in respiration and photosynthesis, respectively. More surface area = more reaction sites = higher metabolic rate.
3. Concentration of reactants and enzymes: Organelles can accumulate high concentrations of specific enzymes and substrates needed for a particular pathway, increasing reaction rate without raising the concentration of those molecules across the entire cell (which would be toxic or wasteful). For example, enzymes for fatty acid oxidation are concentrated in peroxisomes, so reactions proceed quickly and toxic intermediates are broken down inside the organelle before they can damage the rest of the cell.
4. Prokaryote vs eukaryote efficiency: Prokaryotes have no membrane-bound organelles, so all reactions occur in the cytosol or on the cell membrane. This limits their maximum size and metabolic rate, as they cannot concentrate reactants or isolate incompatible reactions. Eukaryotes are larger and have higher metabolic rates directly because of compartmentalization.

Worked Example: A mutation causes the inner mitochondrial membrane to be smooth instead of folded into cristae. The cellular respiration rate will decrease drastically, as the smooth membrane has far less surface area for electron transport chain proteins and ATP synthase, reducing ATP production per unit time. Always link surface area to reaction rate for FRQ full marks.

6. Common Pitfalls (and how to avoid them)

• Wrong move: Listing organelle functions without linking to structural features. Why students do it: They memorize fact lists instead of the structure-function relationship the AP exam exclusively tests. Correct move: For every organelle question, explicitly connect structure to function, e.g., "Lysosomes have a lipid membrane to maintain an acidic internal pH that activates their hydrolytic enzymes, allowing them to break down waste without damaging the cytosol."
• Wrong move: Confusing passive and active transport, or forgetting to mention aquaporins for osmosis. Why students do it: They assume water moves only via simple diffusion, and mix up concentration gradient direction. Correct move: First identify if transport moves down (passive) or against (active) the gradient, and specify that most water transport occurs via facilitated diffusion through aquaporin channels.
• Wrong move: Mixing up solute potential and water potential calculations. Why students do it: They forget solute potential is always negative, and higher solute concentration means lower (more negative) water potential. Correct move: Write the water potential formula $\Psi ={\Psi }_S+{\Psi }_P$ for every transport problem, and remember water always moves from higher (less negative) to lower (more negative) water potential.
• Wrong move: Stating all eukaryotic cells have the same organelle abundance. Why students do it: They learn the "standard" animal/plant cell model and ignore specialized cell functions. Correct move: Link organelle abundance to cell role, e.g., muscle cells have more mitochondria to produce ATP for contraction, secretory cells have more RER and Golgi to release proteins.
• Wrong move: Mixing up cholesterol's fluidity buffer role. Why students do it: They only remember cholesterol affects fluidity, not its opposing effects at high and low temperatures. Correct move: Memorize that cholesterol reduces fluidity at high temperatures (restricts phospholipid movement) and increases fluidity at low temperatures (prevents tight packing of fatty acid tails).

7. Practice Questions (AP Biology Style)

Question 1 (MCQ)

A researcher observes that a specialized mammalian cell produces and secretes large amounts of a protein hormone. Which of the following organelle combinations would be most abundant in this cell? A) Smooth ER and lysosomes B) Rough ER and Golgi apparatus C) Mitochondria and vacuoles D) Ribosomes and chloroplasts

Worked Solution: Correct answer = B. Secreted proteins are synthesized by bound ribosomes on the rough ER, then transported to the Golgi apparatus for modification and packaging into secretory vesicles that release the hormone outside the cell. A is incorrect: Smooth ER makes lipids, lysosomes break down waste. C is incorrect: Mitochondria produce ATP, vacuoles store materials, and neither is directly involved in protein secretion. D is incorrect: Chloroplasts are only found in photosynthetic organisms, not mammalian cells.

Question 2 (FRQ, 3 points)

A cold-water fish species has a mutation that reduces the percentage of unsaturated fatty acids in its cell membrane phospholipids. (a) Predict the effect of this mutation on membrane fluidity at cold temperatures (1 point). (b) Justify your prediction (2 points).

Worked Solution: (a) Prediction: The membrane will have significantly reduced fluidity (become more rigid) at cold temperatures. (b) Justification: Unsaturated fatty acid tails have double bonds that create kinks in their structure, which prevents phospholipids from packing tightly together at low temperatures, maintaining membrane fluidity. Reduced unsaturated fatty acid content allows phospholipids to pack more tightly, decreasing fluidity, which reduces the function of embedded transport proteins and slows cellular processes, reducing survival in cold water.

Question 3 (FRQ, 4 points)

The Na⁺/K⁺ pump is an active transport protein found in all animal cells. (a) Define active transport, and explain why the Na⁺/K⁺ pump is classified as active (2 points). (b) Describe one biological function of the electrochemical gradient created by the Na⁺/K⁺ pump (2 points).

Worked Solution: (a) Active transport is the movement of molecules across a cell membrane against their concentration gradient, requiring an input of cellular energy (usually ATP). The Na⁺/K⁺ pump pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, both against their respective concentration gradients, and uses one ATP molecule per cycle to power this movement, so it is classified as active transport. (b) One function of the electrochemical gradient is to enable nerve cell signaling: the gradient of positive charge across the membrane creates a resting membrane potential, which allows neurons to fire action potentials when ion channels open, transmitting electrical signals across the body. Alternatively, the gradient can power secondary active transport of nutrients like glucose into intestinal cells, using the movement of Na⁺ down its concentration gradient to bring glucose into the cell against its gradient.

8. Quick Reference Cheatsheet

9. What's Next

This Cell Structure and Function unit forms the core foundation for 60% of the remaining AP Biology syllabus. Next, you will build on your knowledge of mitochondria and chloroplasts to study cellular energetics (Unit 3), including cellular respiration and photosynthesis. You will use your understanding of membrane proteins and transport to learn about cell communication and signal transduction (Unit 4), and your knowledge of cell compartmentalization will help you understand cell division and heredity (Units 5 and 6), as well as the endosymbiotic theory of eukaryotic evolution.

If you struggle with any of the concepts in this guide, or want to practice more AP-style questions on cell structure and function, you can ask Ollie, our AI tutor, for personalized explanations, flashcards, and extra practice problems tailored to your weak areas. You can also find more study guides for other AP Biology units on the homepage, aligned to the latest College Board CED.