APBIO: Unit 2: Cell Structure and Function
Cell Structure and Organelles
Living things are made of cells, with the cell as life’s basic unit of structure and function.
A central theme for AP Biology is structure–function, which explains how a cell's build aids its functions.
Eukaryotic cells function as coordinated systems composed of compartments, membranes, and molecular machines rather than isolated entities.
Prokaryotic vs. Eukaryotic Cells
All cells share core features:
Plasma membrane
Cytoplasm
DNA as genetic material
Ribosomes for protein synthesis
Prokaryotic Cells:
Found in Bacteria and Archaea.
Generally smaller and lack membrane-bound organelles.
Genetic material: a continuous, circular DNA molecule found in the nucleoid (not within a nucleus).
Often possess:
A cell wall for structural support (most bacteria have peptidoglycan).
A capsule for extra protection and adhesion (in some species).
Pili for attachment and possible DNA transfer.
Flagella for motility (different from eukaryotic flagella).
Small ribosomes in the cytoplasm.
Common misconception: Prokaryotes are simple; they can perform complex biochemical tasks using evolved molecular machinery despite being less compartmentalized.
Eukaryotic Cells:
Include animals, plants, fungi, and protists.
Contain numerous internal membranes forming membrane-bound organelles, allowing compartmentalization.
Compartmentalization:
Membranes divide the cell into regions with distinct conditions (pH, enzymes, ion concentrations).
Benefits include:
Efficiency through concentrated enzymes and substrates.
Protection via isolation of harmful chemical processes.
Control over the entry and exit of materials in each compartment.
Analogy: A factory with specialized rooms versus a single large warehouse.
The Nucleus: Information Storage and Control
The nucleus is typically the largest organelle in a eukaryotic cell, storing hereditary information (DNA) as chromatin and chromosomes during cell division.
Directs cell activities and is essential for reproduction.
Enclosed by a nuclear envelope (double membrane) with nuclear pores regulating RNA and protein traffic.
Nucleolus:
Most visible structure within the nucleus.
Site where rRNA is produced and ribosome subunits are assembled.
Separation of DNA from cytoplasm adds layers of gene regulation, crucial for complex multicellular life.
Ribosomes: Protein Synthesis Machines
Sites of protein synthesis made of rRNA and proteins, organized as large and small subunits.
Free Ribosomes:
Located in the cytosol, producing proteins used within the cytosol.
Bound Ribosomes:
Associated with the rough Endoplasmic Reticulum (ER), synthesizing proteins destined for secretion, membranes, or organelles.
Important for AP Biology to connect translation location to protein destination.
Endomembrane System: Making, Modifying, and Shipping
Produces and transports many cell products.
Endoplasmic Reticulum (ER)
Continuous channel throughout the cytoplasm providing support and transport functions.
Rough ER (RER):
Studded with ribosomes; major site for protein synthesis for secretion, membranes, and lysosomes.
Proteins enter the RER, fold, and may undergo chemical modifications like carbohydrate additions.
RER environment promotes quality control; misfolded proteins retained or targeted for breakdown.
Smooth ER (SER):
Lacks ribosomes; synthesizes lipids (e.g., phospholipids), produces lipid-based hormones and steroids, participates in carbohydrate metabolism, detoxifies drugs in liver cells, and stores calcium ions for muscle cell function.
Memory cue: “smooth = synthesis” (especially lipids) and storage, not digestion.
Golgi Apparatus (Golgi Complex)
Modifies, processes, and sorts products received from the ER.
Functions as the cell’s packaging and distribution center, placing finished products into vesicles for specific destinations, including the plasma membrane for secretion.
Cells with high protein secretion needs (e.g., gland cells) have abundant RER and Golgi.
Lysosomes and Digestive Compartments
Contain digestive enzymes to break down old organelles, debris, or ingested materials, functioning optimally in an acidic interior separated from cytosol.
Form when vesicles containing enzymes from the trans Golgi fuse with endocytosis vesicles.
Play a critical role in apoptosis (programmed cell death).
In plants, large central vacuoles are crucial for storage and digestion functions; many protists utilize food vacuoles.
Vacuoles in Plant Cells
Fluid-filled sacs for storing water, food, waste, salts, or pigments.
Plant cells commonly have a large central vacuole maintaining turgor pressure against the cell wall, aiding growth by expansion.
Energy-Related Organelles: Mitochondria and Chloroplasts
Mitochondria
Convert energy from organic molecules into ATP (adenosine triphosphate).
Composed of an outer membrane, an inner membrane with folds (cristae), creating distinct regions: intermembrane space and internal matrix.
Mitochondria contain their own DNA and ribosomes, supporting endosymbiotic theory.
Chloroplasts (Plants and Algae)
Conduct photosynthesis with a double outer membrane and contain chlorophyll, the green pigment.
Internally, thylakoids are stacked as grana, surrounded by stroma.
Like mitochondria, chloroplasts possess their own DNA and ribosomes, offering evidence for endosymbiosis.
Important: Membrane structures support processes like electron transport chains and chemiosmosis, linking structure to function.
Peroxisomes: Specialized Oxidation Compartments
Break down fatty acids and detoxify harmful substances.
Many reactions produce hydrogen peroxide (H2O2) as a byproduct; peroxisomes house enzymes that convert H2O2 into water and oxygen to isolate potentially damaging reactions.
Cytoskeleton: Support, Movement, and Organization
A dynamic network of protein fibers that determine cell shape, facilitate movement, and organize cellular materials.
Microtubules:
Hollow tubes of tubulin; vital for cell division (spindle formation), intracellular transport, and structure of cilia and flagella.
Microfilaments:
Thin rods made of actin; dynamic, allowing for growth/shrinkage, movement, shape changes, and muscle contraction.
Intermediate Filaments:
Provide tensile strength against pulling forces.
Cytoskeletal "tracks" interact with motor proteins to aid vesicle and organelle movement.
Cilia and Flagella (Eukaryotic Cell Movement)
Eukaryotic cilia and flagella are microtubule-based structures enabling locomotion.
Cilia: short and numerous; flagella: longer and fewer.
Structural differences exist between eukaryotic and bacterial flagella, affecting their mechanism of movement.
Cell Walls, Extracellular Matrix, and Cell Junctions
Cell interactions influence their functions.
Cell Walls
Provide support and protection.
Plants: consist of cellulose.
Fungi: made of chitin.
Bacteria: often have peptidoglycan-based walls.
Animal cells lack cell walls.
Extracellular Matrix (ECM) in Animals
Animal cells secrete ECM, composed of proteins and carbohydrates, providing tissue support and enabling cell anchoring and communication.
Cell Junctions and Adhesion
Membrane adhesion proteins form junctions between adjacent animal cells:
Tight Junctions: Seal gaps to prevent leakage.
Desmosomes: Fasten cells together like rivets.
Gap Junctions: Form channels for small molecules/ions passage between cells.
In plants, plasmodesmata connect neighboring cells through cell walls.
Plant Cells vs. Animal Cells: Key Differences
Plant cell features:
Cell wall (cellulose) outside the plasma membrane.
Contains chloroplasts for photosynthesis.
Usually have large central vacuoles dominating cell volume.
Typically lack centrioles.
Exam Focus: Typical Question Patterns
Predict organelle abundance based on cell function (e.g., secretion, detoxification, movement).
Compare prokaryotic and eukaryotic cell structures, inferring functional consequences.
Interpret microscopic images/diagrams to identify organelles by structure.
Common mistakes include:
Mislabeling ribosomes as membrane-bound organelles or placing them within the Golgi.
Confusing smooth and rough ER functions (lipid synthesis vs. protein synthesis).
Viewing organelles as independent rather than part of an ER → Golgi → vesicle trafficking system.
Microscopy, Cell Size, and Surface Area-to-Volume Problem
Cells are microscopic, with size constrained by diffusion and exchange requirements.
As volume increases, the surface area-to-volume ratio decreases, which compromises material exchange efficiency.
Surface Area-to-Volume Ratio (SA:V)
For a sphere:
Volume scales with ; surface area with leading to a decreasing SA:V as radius increases.
Cells enhance surface area through folds or projections (e.g., microvilli).
Strategies to optimize SA:V include:
Using numerous small cells instead of one larger cell.
Adopting flattened or elongated shapes.
Introducing membrane folds or projections.
Bigger isn’t always better: larger cells can store more but are less efficient unless structured appropriately.
Internal Transport and Cytoplasmic Streaming
Within larger cells, materials must be transported efficiently.
Diffusion can be slow across large distances; therefore, cytoskeleton-based transport and cytoplasmic streaming expedite material movement.
Plasma Membrane Structure: The Fluid Mosaic Model
The plasma membrane acts as a selectively permeable barrier regulating transport, communication, recognition, and gradient maintenance.
Membrane Composition
Primarily a phospholipid bilayer:
Phospholipids are amphipathic (hydrophilic heads and hydrophobic tails).
In aqueous environments, they self-assemble into a bilayer due to the hydrophobic effect.
The membrane is semipermeable, primarily composed of phospholipids and proteins.
The Fluid Mosaic Model
Describes the plasma membrane as fluid (lipids and proteins move laterally) and a mosaic (a patchwork of lipids, proteins, and carbohydrates).
Fluidity allows bending (vesicles), clustering, and dynamic responses.
Membrane Proteins
Peripheral Proteins: Loosely associated with bilayer surfaces.
Integral Proteins: Embedded in the bilayer; often amphipathic.
Transmembrane Proteins: Span the bilayer.
Functions include:
Transport (channels, carriers, some ATP-powered pumps).
Receptor proteins (docking sites for signaling molecules).
Enzymes catalyzing reactions at the membrane.
Anchors connecting the cytoskeleton and the ECM.
The lipid bilayer provides the fundamental barrier, while proteins confer specificity.
Carbohydrates on Membranes
Attached to proteins (glycoproteins) or lipids (glycolipids), carbohydrates function in cell recognition, adhesion, and communication.
Their side chains are found only on the extracellular surface, showing the biological importance of membrane orientation.
Cholesterol and Membrane Fluidity
Cholesterol buffers membrane fluidity in animal cells, preventing rigidity at low temperatures and excess fluidity at high temperatures.
Selective Permeability
The hydrophobic interior of the membrane significantly impacts permeability.
Hydrophobic compatibility: Solubility dictates permeability, aligning with the idea that "like dissolves like".
Movements:
Small nonpolar molecules (e.g., ) cross easily.
Small uncharged polar molecules cross slowly.
Ions and large polar molecules require proteins for membrane passage.
Example: Salt Permeability
Salt dissociates into ions (Na+ and Cl−); charged ions strongly interact with water and enter the membrane’s hydrophobic core poorly, necessitating channels or pumps for transport.
Passive Transport: Diffusion, Osmosis, and Facilitated Diffusion
Transport across membranes is driven by gradients.
Passive transport, needing no external energy, moves substances down their gradients.
Factors Influencing Movement
1. Membrane Semipermeability:
2. Size and Charge/Polarity:
Hydrophobic molecules can often pass directly; hydrophilic substances generally require protein assistance.
Diffusion
The net movement of particles from a region of high to low concentration due to random molecular motion continuing even at equilibrium.
Simple Diffusion
Occurs when molecules directly traverse the lipid bilayer, typically for small nonpolar molecules.
Example: If the concentration outside a cell exceeds that inside, diffuses inward until concentrations reach equilibrium.
Facilitated Diffusion
Passive transport utilizing membrane proteins.
Channel Proteins: Create hydrophilic passages; often selective and gated.
Carrier Proteins: Bind solutes and change shape to facilitate transport.
Clarification: Facilitated diffusion does not require ATP as it moves down the gradient.
Example:
When glucose is more concentrated outside than inside, glucose transporters can move it in without ATP.
The transport can reverse based on gradient shifts and transporter regulation.
Osmosis and Aquaporins
Osmosis is the diffusion of water across a selectively permeable membrane, moving from higher free water (lower solute concentration) to lower free water (higher solute concentration).
Water primarily travels through aquaporins—water-specific channels enhancing membrane water permeability.
Tonicity
Describes solutions regarding their effects on water movement and cell volume based on nonpenetrating solutes.
Hypotonic: Lower solute concentration outside than inside → water enters cell.
Hypertonic: Higher solute concentration outside than inside → water exits cell.
Isotonic: Equal solute concentrations → no net water movement.
Plant cells are protected by their cell walls during osmotic changes—if they lose water, the membrane can pull away (plasmolysis); if they gain water, the membrane can press against the wall (turgor).
Osmoregulation Strategies
Organisms maintain water balance through structures and systems like:
Contractile vacuoles (in some freshwater protists to expel excess water).
Kidneys in animals.
Guard cells in plants.
Dialysis
The diffusion of solutes across a selectively permeable membrane, relevant in medical kidney dialysis processes, filters blood based on concentration gradients.
Membranes, Ions, and Polarization
Ions like Na+ and K+ often necessitate membrane proteins for crossing.
Ion movement can lead to polarized membranes, forming the basis for electrical effects pertinent in signaling and transport.
Active Transport and Bulk Transport: Moving Against Gradients
Passive transport cannot create or maintain ion gradients.
Active transport requires energy to move substances against gradients; vesicle-based processes transport large materials.
What Active Transport Involves
Moves substances against concentration gradients (low to high) or ion electrochemical gradients utilizing energy (often ATP).
To maintain gradients despite diffusion tendencies, the cell expends energy.
Primary Active Transport: ATP-Powered Pumps
Directly powered by ATP.
Example: Sodium-Potassium Pump
Moves three Na+ ions out and two K+ ions into the cell, crucial for maintaining gradients vital for numerous cellular processes.
Secondary Active Transport: Using Stored Gradient Energy
Utilizes energy embedded in ion gradients (from primary active transport) to transport other substances.
Symport: Both ions move in the same direction; Antiport: ions move in opposite directions.
Despite not directly using ATP, it qualifies as “active” because one substance moves against its gradient.
Membrane Potential and Electrochemical Gradients
Ion movement involves both concentration differences and electrical forces leading to membrane polarization.
Most cells maintain a negatively charged interior, affecting ion transport through channels and influencing pump work.
Bulk Transport: Endocytosis and Exocytosis
Vesicles are necessary for large particles and macromolecules due to size.
Endocytosis: Engulfs material to form vesicles/vacuoles.
Types:
Pinocytosis: Liquid ingestion.
Phagocytosis: Solid ingestion.
Receptor-Mediated Endocytosis: Selective uptake using receptors; often involves clathrin-coated pits.
Exocytosis: Exports materials via vesicle fusion with the plasma membrane, effectively reversing endocytosis.
Example: If a receptor is defective, uptake through receptor-mediated endocytosis can decrease even when ligands are abundant.
Bulk Flow (Pressure-Driven Transport)
Movement of fluids is propelled by pressure rather than diffusion.
Examples include blood circulation through vessels and fluid transport through plant xylem and phloem.
Integrated System of Organelles: Information Flow, Trafficking, and Coordination
Cells operate as coordinated systems where organelles work together: nucleus, ribosomes, ER, Golgi, vesicles, cytoskeleton, and membrane collaborate to produce proteins and direct their delivery.
From DNA to Protein: Importance of Location
Simplified eukaryotic flow:
Transcription: DNA in the nucleus is converted to RNA.
RNA undergoes processing and exits through nuclear pores.
Translation: Ribosomes synthesize proteins from RNA.
Ribosomes on rough ER produce proteins for secretion/membranes.
Vesicles transport proteins from ER to Golgi.
Golgi modifies, sorts, and ships products to appropriate destinations (membrane, secretion, lysosome, etc.).
This process underscores many structure-function relationships, especially regarding secretion.
Vesicle Trafficking and Cytoskeletal Assistance
Vesicles require guidance; cytoskeletal tracks and motor proteins facilitate targeted delivery of membrane proteins and local secretion.
This dynamic organization is especially vital in large, polarized cells like neurons.
Maintaining Distinct Internal Environments
Cells maintain defined conditions suited to organelle functions:
Lysosomes have an acidic lumen.
Mitochondria uphold gradients across inner membranes.
ER maintains conditions optimum for protein folding/processing.
Active maintenance of these environments arises from intrinsic membrane properties and specific transport proteins.
Endosymbiotic Theory
Proposes mitochondria and chloroplasts originated from prokaryotes that established mutualism with host cells. Evidence includes:
Possession of their own DNA, ribosomes, double membranes, and division resembling prokaryotic processes.
Real-World Connections
Certain antibiotics selectively target prokaryotic structures (like ribosomes) differing from eukaryotic structures, showcasing structure-function relationships.
Genetic disorders or toxins disrupting ER folding or Golgi sorting can lead to widespread dysfunction due to interdependent pathways reliant on accurate protein localization.
Cancer and immune disorders may involve alterations in membrane receptors, signaling, and transport systems.
Example: Organelle Changes Post functional Shift
If a cell adapts to produce significant quantities of a secreted protein (like digestive enzymes), one could forecast:
Increased rough ER (for protein synthesis/initial processing).
Increased Golgi (for modification/packaging).
Heightened vesicle activity near the plasma membrane (for exocytosis).
AP exam emphasizes causal explanations over mere memorized lists.
Exam Focus: Typical Question Patterns
Trace protein trajectory from synthesis to destination detailing organelles involved.
Use evidence supporting the endosymbiotic theory.
Predict consequences at a system level caused by trafficking disruptions (e.g., ER stress, secretion issues).
Common mistakes consist of treating organelles as isolated units, confusing destinations for proteins synthesized on free versus bound ribosomes, and listing evidence for endosymbiosis without elaborating on its implications.