Unit 2: Cell Structure and Function

Cell Structure and Organelles

Living things are made of cells, and the cell is life’s basic unit of structure and function. A central theme for AP Biology is structure–function: the way a cell (or any cell part) is built helps explain what it can do. Eukaryotic cells, in particular, act like coordinated systems of compartments, membranes, and molecular machines rather than a list of isolated “facts.”

Prokaryotic vs. eukaryotic cells (the first major split)

All cells share a few core features: a plasma membrane, cytoplasm, DNA as genetic material, and ribosomes to make proteins.

Prokaryotic cells (Bacteria and Archaea) are generally smaller and lack membrane-bound organelles. Their genetic material is typically one continuous, circular DNA molecule found in the nucleoid (not enclosed by a nucleus). Many prokaryotes have:

• A cell wall for structural support; in most bacteria it is composed of peptidoglycan.
• A capsule (in some species) for extra protection and adhesion.
• Pili for attachment and sometimes DNA transfer.
• Flagella for motility (built differently than eukaryotic flagella).
• Small ribosomes in the cytoplasm.

A common misconception is that “prokaryotes are simple.” They are less compartmentalized, but they can be biochemically sophisticated and carry out the same core tasks (DNA replication, transcription, translation, metabolism) using highly evolved molecular machinery.

Eukaryotic cells (animals, plants, fungi, protists) contain many internal membranes that form membrane-bound organelles. The major advantage is compartmentalization, which allows different processes to occur simultaneously in different microenvironments.

Why compartmentalization matters

Membranes partition the cell into regions with distinct conditions (pH, enzymes, ion concentrations). This separation improves:

• Efficiency by concentrating enzymes and substrates.
• Protection by isolating potentially harmful chemistry (like macromolecule breakdown).
• Control by regulating what enters and exits each compartment.

A useful analogy is a factory with specialized rooms and controlled access rather than a single open warehouse.

The nucleus: information storage and control

The nucleus is usually the largest organelle in a eukaryotic cell. It stores hereditary information (DNA) organized as chromatin and, during cell division, as chromosomes. The nucleus helps direct cell activities and is central to the cell’s ability to reproduce.

It is enclosed by a nuclear envelope (double membrane) containing nuclear pores that regulate traffic (RNA out, proteins in). The nucleolus is the most visible structure inside and is where rRNA is produced and ribosome subunits begin assembly.

Separating DNA from cytoplasm allows extra layers of gene regulation (RNA processing, splicing, export control), supporting complex multicellular life.

Ribosomes: protein synthesis machines (not membrane-bound)

Ribosomes are the sites of protein synthesis. They are round structures made of rRNA and proteins, organized into a large and a small subunit.

• Free ribosomes in the cytosol often make proteins used in the cytosol.
• Bound ribosomes on the rough ER often make proteins destined for secretion, membranes, or certain organelles.

Ribosomes are not membrane-bound organelles, but AP questions often test whether you connect the location of translation to the final destination of the protein.

Endomembrane system: making, modifying, and shipping

The endomembrane system produces and transports many cell products.

Endoplasmic reticulum (ER)

The ER is a continuous channel extending throughout the cytoplasm that provides mechanical support and helps with transport.

• Rough ER (RER) is studded with ribosomes. It is a major site for synthesizing proteins for secretion, membranes, and lysosomes. As proteins enter the RER, they fold and can be chemically modified (for example, carbohydrate additions). The RER environment supports quality control; misfolded proteins may be retained or targeted for breakdown. Rough ER is a major way eukaryotic cells compartmentalize their work.
• Smooth ER (SER) lacks ribosomes. It synthesizes lipids (including phospholipids), and also produces lipid-based molecules such as certain hormones and steroids. It participates in carbohydrate metabolism, detoxifies drugs/poisons (notably in liver cells), and stores calcium ions (important in muscle cells).

A helpful memory cue is “smooth = synthesis” (especially lipids) and storage, not digestion.

Golgi apparatus (Golgi complex)

The Golgi apparatus modifies, processes, and sorts products received from the ER. It acts as the cell’s packaging and distribution center, placing finished products into vesicles that carry materials to specific destinations, including the plasma membrane for secretion.

Cells that secrete lots of protein (like gland cells) typically have abundant RER and Golgi.

Lysosomes and digestive compartments

Lysosomes contain digestive enzymes that break down old organelles, debris, or ingested particles. Their enzymes work best in an acidic interior, which is kept separate from the cytosol by the lysosomal membrane.

Lysosomes commonly form when enzyme-containing vesicles from the trans Golgi fuse with vesicles made during endocytosis. Lysosomes are also essential in apoptosis (programmed cell death).

Plants often rely heavily on large central vacuoles for storage and digestion-like roles, and many protists use food vacuoles.

Vacuoles (especially important in plants)

Vacuoles are fluid-filled sacs that can store water, food, wastes, salts, or pigments. Plant cells often have a large central vacuole that stores water/solutes and helps generate turgor pressure against the cell wall, supporting the plant and aiding cell growth by expansion.

Energy-related organelles: mitochondria and chloroplasts

Mitochondria

Mitochondria convert energy from organic molecules into ATP (adenosine triphosphate). They have an outer membrane and an inner membrane; the inner membrane folds into cristae. These membranes create regions including the intermembrane space and the internal matrix.

Mitochondria have their own DNA and ribosomes, supporting endosymbiotic theory.

Chloroplasts (plants and algae)

Chloroplasts perform photosynthesis and have a double outer membrane. They contain chlorophyll, the pigment that gives plants their green color. Internally, membranes form thylakoids (stacked as grana) surrounded by stroma.

Chloroplasts also have their own DNA and ribosomes, another line of evidence for endosymbiosis.

A key structural idea is that mitochondria and chloroplast membranes support electron transport chains and chemiosmosis (the reaction details are emphasized later, but structure–function appears here).

Peroxisomes (specialized oxidation compartments)

Peroxisomes break down fatty acids and detoxify harmful substances. Many reactions produce hydrogen peroxide (H2O2) as a byproduct; peroxisomes contain enzymes that convert H2O2 into water and oxygen, illustrating how compartmentalization isolates potentially damaging chemistry.

Cytoskeleton: support, movement, and organization

The cytoskeleton is a dynamic network of protein fibers that determines cell shape, helps move materials, and enables movement.

• Microtubules are hollow tubes made of tubulin. They are important in cell division (spindles) and intracellular transport, and form the core of cilia/flagella.
• Microfilaments are thin rods made of actin. Actin monomers assemble and disassemble as needed, allowing microfilaments to grow/shrink for movement, cell shape changes, and muscle contraction.
• Intermediate filaments provide tensile strength, helping cells resist pulling forces.

Cytoskeletal “tracks” interact with motor proteins to move vesicles and organelles.

Cilia and flagella (eukaryotic cell movement)

Eukaryotic cilia and flagella are microtubule-based structures with locomotive properties. Their coordinated beating motion can move single-celled organisms (or cells like sperm) and can also move fluid across tissues (like the respiratory tract). Cilia are usually short and numerous; flagella are longer and fewer.

Do not confuse eukaryotic flagella with bacterial flagella; they differ in structure and how movement is generated.

Cell walls, extracellular matrix, and cell junctions

Cells interact with their surroundings, and those interactions influence function.

Cell walls

A cell wall provides support and protection.

• Plants: cell walls contain cellulose.
• Fungi: cell walls contain chitin.
• Bacteria: many have peptidoglycan-based walls.

Animal cells do not have cell walls.

Extracellular matrix (ECM) in animals

Animal cells often secrete an extracellular matrix of proteins/carbohydrates that supports tissues and helps cells anchor and communicate.

Cell junctions and adhesion

Membrane adhesion proteins help form junctions between adjacent animal cells:

• Tight junctions seal gaps, preventing leakage.
• Desmosomes fasten cells together like rivets.
• Gap junctions form channels that allow small molecules/ions to pass between cells.

In plants, plasmodesmata connect neighboring cells through cell walls.

Plant cells versus animal cells (key differences)

Plant cells differ from animal cells in several predictable structure–function ways:

• Plant cells have a cell wall (cellulose) outside the plasma membrane.
• Plant cells have chloroplasts (photosynthesis) containing chlorophyll.
• Plant cytoplasm is often largely displaced by a large central vacuole.
• A commonly tested distinction is that plant cells typically do not contain centrioles.

Exam Focus

• Typical question patterns:
  • Predict which organelles are abundant given a cell’s function (secretion, detoxification, movement).
  • Compare prokaryotic vs. eukaryotic structures and infer consequences (for example, drug specificity).
  • Interpret microscope images/diagrams to identify organelles by structure.
• Common mistakes:
  • Calling ribosomes membrane-bound organelles or placing them “inside” the Golgi.
  • Confusing smooth vs. rough ER roles (lipids/detox/steroids vs. protein synthesis/processing).
  • Treating organelles as independent rather than a coordinated ER → Golgi → vesicle trafficking system.

Microscopy, Cell Size, and the Surface Area-to-Volume Problem

Cells are microscopic, but their size is constrained by diffusion and exchange. As cells increase in volume, the surface area-to-volume ratio decreases, making exchange of materials less efficient. Surface areas and volumes can be computed with standard geometry formulas, and the same idea scales up to whole organisms.

How we see cells: microscopy basics

• Light microscopes can be used to study living or stained cells and can magnify up to about 1,000 times, but resolution is limited.
• Electron microscopes reveal much finer detail that cannot be easily observed with light microscopy, but require extensive preparation and are not used to observe living cells in a normal way.

AP questions often ask you to interpret micrographs and connect visible structure to function.

Why cells (and organisms) are size-limited: exchange constraints

Much exchange (nutrients, gases, wastes) happens at membranes. As size increases:

• Volume (material demand) grows faster than surface area (exchange capacity).
• Diffusion becomes increasingly ineffective over longer distances.

This creates selection for small cells, cell division, or shapes that increase surface area.

The SA:V concept also applies to organisms. As organisms get larger, SA:V decreases, which affects properties such as heat exchange. Small organisms lose heat at much higher rates than larger organisms because their higher SA:V allows more efficient heat exchange with the environment.

Surface area-to-volume ratio (SA:V)

For a sphere:

SA = 4\pi r^2

V = \frac{4}{3}\pi r^3

As radius increases, volume scales with r^3 while surface area scales with r^2, so SA:V decreases.

Cells often increase surface area using folds or projections (like microvilli in intestinal cells).

Strategies to deal with SA:V constraints

Cells and organisms increase effective exchange by:

• Using many small cells rather than one huge cell
• Adopting flattened or elongated shapes
• Adding membrane folds or surface projections

“Bigger” is not automatically “better”: bigger cells can store more, but are often less efficient at exchange unless they compensate structurally.

Example: comparing SA:V for two cubes

For a cube with side length a:

SA = 6a^2

V = a^3

• If a = 1: SA = 6, V = 1, so SA:V = 6:1.
• If a = 2: SA = 24, V = 8, so SA:V = 3:1.

Doubling side length halves SA:V.

Internal transport and cytoplasmic streaming

Even within a cell, materials must move from entry points to where they are used. Diffusion alone can be slow across long distances, so larger cells rely more on cytoskeleton-based transport and cytoplasmic streaming to distribute materials.

Exam Focus

• Typical question patterns:
  • Use SA:V reasoning to explain why cells divide or why microvilli increase absorption.
  • Interpret diffusion experiments (like agar cubes) relating diffusion distance to size.
  • Predict how changing cell shape affects exchange efficiency.
• Common mistakes:
  • Mentioning surface area and volume without linking to the power relationship (SA scales with squared dimensions; V with cubed dimensions).
  • Assuming diffusion time scales linearly with distance; diffusion becomes dramatically less effective over longer distances.
  • Forgetting that increased surface area only helps if the membrane also has the transport capacity (channels, carriers, pumps).

Plasma Membrane Structure: The Fluid Mosaic Model

The plasma membrane is the selectively permeable boundary between the cell and its environment. It regulates transport, communication, recognition, and the maintenance of gradients.

What the membrane is made of

The core is a phospholipid bilayer. Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails. In water, they self-assemble into a bilayer (heads outward, tails inward) due to the hydrophobic effect.

The membrane is composed mostly of phospholipids and proteins and is semipermeable.

The fluid mosaic model

The membrane is a fluid mosaic:

• Fluid: many lipids/proteins move laterally.
• Mosaic: a patchwork of lipids, proteins, and carbohydrates.

Fluidity supports bending (vesicles), clustering, and dynamic responses.

Membrane proteins: structure types and major functions

Membrane proteins carry out most specific membrane functions.

• Peripheral proteins are loosely associated with the bilayer surface (inner or outer surface).
• Integral proteins are embedded in the bilayer and are often amphipathic.
• Transmembrane proteins span the bilayer.

Major roles include:

• Transport proteins (channels and carriers); some are pumps powered by ATP.
• Channel proteins that selectively allow passage of certain ions or molecules.
• Receptor proteins that act as docking sites for signaling molecules (for example, hormones).
• Enzymes catalyzing reactions at the membrane.
• Anchors connecting cytoskeleton and ECM.
• Adhesion proteins forming junctions between adjacent cells.

A key reasoning point is that the lipid bilayer provides the basic barrier, but proteins provide specificity.

Carbohydrates on the membrane: identity and communication

Carbohydrates attached to proteins (glycoproteins) or lipids (glycolipids) function in cell recognition, adhesion, and communication. Their carbohydrate side chains are found only on the outer (extracellular) surface of the plasma membrane, which makes membrane orientation biologically important.

Cholesterol and membrane fluidity

In animal cells, cholesterol buffers membrane fluidity, helping prevent membranes from becoming too rigid at low temperatures or too fluid at high temperatures.

Selective permeability: what crosses easily and what doesn’t

The hydrophobic interior strongly affects permeability. A useful idea is “like dissolves like”: hydrophobic (nonpolar) substances are more compatible with the membrane’s hydrophobic core.

In general:

• Small nonpolar molecules (O2, CO2) cross relatively easily.
• Small uncharged polar molecules cross slowly.
• Ions and large polar molecules do not cross without proteins.

Example: why salt doesn’t diffuse freely through the membrane

Salt dissolves into ions (Na+ and Cl−). Charged ions interact strongly with water and are energetically disfavored from entering the membrane’s hydrophobic interior, so cells require channels or pumps to move ions across.

Exam Focus

• Typical question patterns:
  • Explain how membrane composition affects permeability and transport capacity.
  • Predict effects of changing lipid composition or cholesterol on membrane fluidity.
  • Identify membrane components (receptors, glycoproteins, channels) and relate them to function.
• Common mistakes:
  • Saying “proteins make the membrane” rather than recognizing a lipid bilayer with embedded proteins.
  • Assuming all small molecules cross easily; polarity and charge matter more than size alone.
  • Confusing receptor proteins (signaling) with transport proteins (movement across the membrane).

Passive Transport: Diffusion, Osmosis, Facilitated Diffusion, and Dialysis

Transport across membranes is driven by gradients. Passive transport does not require outside energy because net movement is down a gradient.

What affects movement across membranes

Whether a molecule can cross depends strongly on:

1. The semipermeability of the membrane
2. The size and charge/polarity of the molecule

Hydrophobic molecules can often pass through the bilayer directly; hydrophilic substances usually require assistance from proteins.

Diffusion and concentration gradients

Diffusion is the net movement of particles from higher concentration to lower concentration due to random molecular motion. Individual molecules still move randomly in all directions; at equilibrium, movement continues but net change is zero.

Simple diffusion across the bilayer

Simple diffusion occurs when molecules pass directly through the lipid bilayer, typically small nonpolar (hydrophobic) molecules.

Example: If O2 concentration is higher outside a cell than inside, O2 diffuses inward until concentrations balance.

Facilitated diffusion

Facilitated diffusion is passive transport using membrane proteins to move substances down their concentration gradient.

• Channel proteins create a hydrophilic passage; often selective and sometimes gated.
• Carrier proteins bind a solute and change shape to move it across.

A common AP trap is to assume that any protein-mediated transport uses ATP. Facilitated diffusion is still passive because it goes down the gradient.

Example (glucose): If glucose is higher outside than inside, a glucose transporter can move glucose into the cell without ATP. If the gradient reverses, transport can reverse depending on transporter type and regulation.

Osmosis and aquaporins

Osmosis is diffusion of water across a selectively permeable membrane. Water moves from higher “free water” (lower solute concentration) toward lower “free water” (higher solute concentration), assuming the solute cannot cross freely.

Water often travels through aquaporins, water-specific channels that increase membrane water permeability.

A useful comparison: in diffusion, the membrane is often permeable to the solute; in osmosis, it typically is not.

Tonicity (hypotonic, hypertonic, isotonic)

Tonicity describes how a solution affects water movement and cell volume based on nonpenetrating solutes.

• Hypotonic: lower solute outside than inside → water enters.
• Hypertonic: higher solute outside than inside → water leaves.
• Isotonic: equal solute concentrations → no net water movement.

Plant cells are protected by their cell wall during osmotic changes. If a plant cell loses water, the membrane can pull away from the wall; this is plasmolysis. If it takes in water, the membrane can press tightly against the wall, contributing to turgor.

Osmoregulation strategies

Water balance can be regulated by structures and systems such as:

• Contractile vacuoles in some freshwater protists (pump out excess water)
• Kidneys in animals
• Guard cells in plants

Dialysis

Dialysis is the diffusion of solutes across a selectively permeable membrane.

Kidney dialysis is a specialized medical process that filters blood using machines and concentration gradients.

Membranes, ions, and polarization

Ions such as Na+ and K+ often require membrane proteins to cross membranes. As ions move, membranes can become polarized, laying groundwork for electrical effects that matter later in transport and signaling.

Exam Focus

• Typical question patterns:
  • Predict water movement and cell volume changes in hypertonic/hypotonic/isotonic solutions.
  • Distinguish simple diffusion vs. facilitated diffusion using molecule properties and protein involvement.
  • Interpret graphs of transport rate vs. concentration (facilitated diffusion can saturate when proteins are fully occupied).
  • Apply dialysis ideas to selectively permeable membranes and concentration gradients.
• Common mistakes:
  • Defining tonicity by total solute rather than nonpenetrating solute.
  • Saying water moves from “hypotonic to hypertonic” without tying it to solute concentration and membrane permeability.
  • Assuming diffusion stops at equilibrium.
  • Assuming any protein-mediated transport uses ATP.

Water Potential: A Quantitative Way to Predict Osmosis (Especially in Plants)

Tonicity is qualitative; water potential provides a quantitative way to predict water movement, especially in plant contexts.

What water potential means

Water potential measures the potential energy of water compared with pure water. Water moves from higher water potential to lower water potential.

Water potential is influenced by:

• Solute concentration (reduces water potential)
• Pressure (can raise water potential)

The water potential equation

\Psi = \Psi_s + \Psi_p

Pure water is often treated as \Psi = 0 under standard conditions; adding solute makes \Psi more negative.

Solute potential

\Psi_s = -iCRT

Where:

• i = ionization constant
• C = molar concentration
• R = pressure constant (provided when needed)
• T = temperature in Kelvin

Convert temperature using (°C + 273). Adding solute lowers water potential; the more solute particles, the more negative \Psi_s becomes.

Pressure potential

Pressure potential \Psi_p reflects physical pressure on water. In plant cells, the central vacuole can generate **turgor pressure** against the cell wall, making \Psi_p positive and increasing overall \Psi.

Worked example: predicting water movement with water potential

Given:

• Cell interior: \Psi_s = -0.7 MPa, \Psi_p = 0.3 MPa
• Outside solution: \Psi = -0.2 MPa

Compute inside water potential:

\Psi_{inside} = -0.7 + 0.3

\Psi_{inside} = -0.4

Compare potentials:

• Inside: -0.4 MPa
• Outside: -0.2 MPa

Water moves from higher to lower water potential, so it moves from -0.2 to -0.4, meaning water moves into the cell.

A common confusion is thinking “more negative means more water.” More negative means lower potential energy, so water moves toward it.

Exam Focus

• Typical question patterns:
  • Calculate \Psi for a cell and solution and predict direction of water movement.
  • Use water potential to explain turgor pressure changes in plant cells.
  • Compare tonicity reasoning with water potential reasoning in complex scenarios.
• Common mistakes:
  • Reversing the direction rule (moving water from lower \Psi to higher \Psi).
  • Forgetting \Psi_p can be positive in walled cells.
  • Forgetting the negative sign in \Psi_s = -iCRT or failing to convert Celsius to Kelvin.

Active Transport and Bulk Transport: Moving Against Gradients

Passive transport cannot build or maintain gradients. Active transport uses energy to move substances against gradients, and vesicle-based processes move large materials.

What “active” really means

Active transport moves substances against a concentration gradient (low to high) or against an ion’s electrochemical gradient, requiring energy (often ATP). If a gradient is maintained despite diffusion’s tendency to eliminate it, the cell must be spending energy somewhere.

Primary active transport: ATP-powered pumps

In primary active transport, ATP directly powers a pump.

Sodium-potassium pump example: It uses ATP to move three Na+ out of the cell and two K+ in, maintaining gradients vital for many processes.

Secondary active transport: using stored gradient energy (cotransport)

In secondary active transport, the cell uses energy stored in an ion gradient (usually created by primary active transport) to move another substance.

• Symport: both move in the same direction.
• Antiport: move in opposite directions.

Secondary active transport is still “active” because at least one substance is moved against its gradient, even if ATP is not used directly at that transporter.

Membrane potential and electrochemical gradients

For ions, both concentration differences and electrical forces matter. As ions move, membranes can become polarized, and the combined influence is the electrochemical gradient. Cells often maintain a negative interior, affecting ion flow through channels and the work pumps must do.

Bulk transport: endocytosis and exocytosis

Large particles and macromolecules require vesicles.

• Endocytosis brings material in by engulfing it with the plasma membrane, forming a vesicle/vacuole.
  • Pinocytosis: ingestion of liquids.
  • Phagocytosis: ingestion of solids.
  • Receptor-mediated endocytosis: specific uptake using receptors; often involves clathrin-lined pits. When a ligand binds its receptor, the membrane invaginates and a vesicle forms around the ligand.
• Exocytosis exports material by vesicle fusion with the plasma membrane. It can eject wastes or secretions such as hormones; it is essentially the reverse of endocytosis.

Conceptual example (receptor-mediated uptake): If a receptor is defective, uptake can decrease even when the ligand is abundant outside because vesicle formation is not triggered properly.

Bulk flow (pressure-driven transport)

Bulk flow is the one-way movement of fluids driven by pressure rather than diffusion.

Examples include the movement of blood through vessels and fluid transport in xylem and phloem in plants.

Exam Focus

• Typical question patterns:
  • Distinguish primary vs. secondary active transport and identify the true energy source.
  • Predict effects of inhibiting ATP production on gradients and transport.
  • Analyze vesicle-based transport (endocytosis/exocytosis), including receptor function and clathrin involvement.
  • Distinguish diffusion/osmosis from pressure-driven bulk flow in biological systems.
• Common mistakes:
  • Labeling any movement through a membrane protein as “active”; direction relative to the gradient matters.
  • Saying secondary active transport uses no energy; it uses gradient energy ultimately created using ATP.
  • Confusing endocytosis with exocytosis or assuming bulk transport happens through channels.

Organelles as an Integrated System: Information Flow, Trafficking, and Coordination

Cells function as coordinated systems. The nucleus, ribosomes, ER, Golgi, vesicles, cytoskeleton, and membrane cooperate to make proteins and deliver them to correct locations.

From DNA to functional protein: location matters

A simplified eukaryotic flow:

1. DNA in the nucleus is transcribed to RNA.
2. RNA is processed and exported through nuclear pores.
3. Ribosomes translate RNA to protein.
4. Proteins destined for secretion/membranes are usually synthesized at ribosomes on rough ER.
5. Vesicles carry proteins from ER to Golgi.
6. Golgi modifies, sorts, and ships products to final destinations (membrane, secretion, lysosome, etc.).

This explains many structure–function prompts, especially those involving secretion.

Vesicle trafficking depends on membranes and the cytoskeleton

Vesicles do not simply drift; cytoskeletal tracks and motor proteins guide them. This enables targeted delivery of membrane proteins, localized secretion, and rapid reorganization—especially important in large polarized cells like neurons.

Maintaining distinct internal environments

Compartments maintain distinct conditions that match their functions:

• Lysosomes maintain an acidic lumen.
• Mitochondria maintain gradients across the inner membrane.
• The ER maintains conditions suited for protein folding/processing.

These conditions are actively maintained by membrane properties and transport proteins.

Endosymbiotic theory (why mitochondria and chloroplasts are special)

The endosymbiotic theory proposes that mitochondria and chloroplasts originated as prokaryotes that entered a host cell and formed a mutualism.

Evidence includes:

• Their own DNA
• Their own ribosomes
• Double membranes
• Replication resembling prokaryotic cell division

AP questions often ask you to connect a specific observation to why it supports endosymbiosis.

Real-world connections

• Some antibiotics target prokaryotic structures (like ribosomes) more than eukaryotic ones, reflecting structure–function differences.
• Genetic diseases or toxins can disrupt ER folding, Golgi sorting, or membrane trafficking, causing widespread dysfunction because so many pathways depend on correct protein location.
• Cancer and immune disorders often involve altered membrane receptors, signaling, and transport systems.

Example: predicting organelle changes after a functional shift

If a cell evolves to produce large amounts of a secreted protein (like digestive enzymes), you would predict:

• Increased rough ER (protein synthesis/initial processing)
• Increased Golgi (modification/packaging)
• Increased vesicle activity near the plasma membrane (exocytosis)

AP rewards the causal explanation rather than a memorized list.

Exam Focus

• Typical question patterns:
  • Trace a protein’s path from synthesis to destination and name the organelles involved.
  • Use evidence to support endosymbiotic theory.
  • Predict system-level consequences of disrupting trafficking (ER stress, secretion defects).
• Common mistakes:
  • Treating organelles as isolated stations instead of a connected network.
  • Confusing destinations of proteins made on free vs. bound ribosomes.
  • Listing endosymbiotic evidence without explaining why it supports the theory.