Chapter 3 of 10 - AP Biology Course
Cell Structure & Function
The cell is the fundamental unit of life. This chapter explores the structural differences between prokaryotic and eukaryotic cells, the roles of key organelles, the endomembrane system, the cytoskeleton, membrane architecture, and the mechanisms cells use to transport materials across their membranes.
Prokaryotic vs Eukaryotic Cells
All cells fall into one of two categories. Prokaryotic cells (bacteria and archaea) are structurally simpler - they lack a membrane-bound nucleus and most internal organelles. Their circular DNA sits in a nucleoid region, and ribosomes (70S) are smaller than those found in eukaryotes. Despite their simplicity, prokaryotes are extraordinarily successful and inhabit virtually every environment on Earth.
Eukaryotic cells (animals, plants, fungi, and protists) possess a true nucleus enclosed by a double membrane (the nuclear envelope) and a complex system of internal compartments. This compartmentalization allows different biochemical processes to occur simultaneously without interfering with each other - oxidative phosphorylation in mitochondria, photosynthesis in chloroplasts, protein processing in the ER and Golgi, and waste digestion in lysosomes.
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
| DNA Shape | Circular, in nucleoid | Linear chromosomes |
| Size | 1-10 micrometers | 10-100 micrometers |
| Ribosomes | 70S (50S + 30S) | 80S (60S + 40S) |
| Organelles | Few or none | Many (ER, Golgi, mitochondria, etc.) |
| Cell Wall | Peptidoglycan (bacteria) | Cellulose (plants), chitin (fungi), or absent |
| Reproduction | Binary fission | Mitosis and meiosis |
Quick Check
Which of the following is found in prokaryotic cells but NOT in animal cells?
Key Eukaryotic Organelles
Eukaryotic cells contain specialized membrane-bound compartments that carry out distinct functions. Understanding each organelle's role is essential for AP Biology.
Nucleus
The nucleus houses the cell's genetic material (DNA organized into chromatin and chromosomes). It is surrounded by a double membrane (nuclear envelope) perforated by nuclear pores that regulate the exchange of materials between the nucleus and cytoplasm. The nucleolus within the nucleus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly.
Endoplasmic Reticulum (ER)
The rough ER is studded with ribosomes and synthesizes secretory proteins and membrane proteins. The smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification (especially in liver cells), and calcium ion storage (in muscle cells as sarcoplasmic reticulum).
Golgi Apparatus
The Golgi modifies, sorts, and packages proteins and lipids received from the ER. It consists of flattened membrane sacs (cisternae) organized into cis (receiving) and trans (shipping) faces. Glycosylation (adding sugar chains) and phosphorylation of proteins occur here.
Mitochondria
Mitochondria are the sites of cellular respiration, generating most of the cell's ATP through oxidative phosphorylation. They have their own circular DNA and 70S ribosomes - evidence supporting the endosymbiotic theory that mitochondria evolved from ancient aerobic bacteria.
Chloroplasts
Found in plant cells and algae, chloroplasts carry out photosynthesis. They contain thylakoid membranes (organized into grana) where the light reactions occur and stroma where the Calvin cycle fixes carbon dioxide into sugars. Like mitochondria, chloroplasts have their own DNA, supporting endosymbiosis.
Lysosomes
Lysosomes are membrane-bound vesicles containing hydrolytic enzymes that digest macromolecules, damaged organelles (autophagy), and engulfed particles (phagocytosis). They maintain an acidic internal pH (around 5) optimal for their enzymes. Lysosomal storage diseases (such as Tay-Sachs) result from defective lysosomal enzymes.
A labeled diagram of a typical animal cell. Key organelles include the nucleus, rough and smooth endoplasmic reticulum, Golgi apparatus, mitochondria, and lysosomes.
The Endomembrane System
The endomembrane system is a network of interconnected membranes that work together to synthesize, modify, package, and transport proteins and lipids. Proteins destined for secretion, membrane insertion, or lysosomal targeting enter this pathway at the rough ER and travel through the system via transport vesicles that bud from one compartment and fuse with the next.
Proteins move through the endomembrane system from synthesis at the rough ER to their final destination.
Rough ER
Ribosomes synthesize proteins into the ER lumen; initial folding and quality control
Transport Vesicles
COPII-coated vesicles bud from ER and carry cargo forward
Golgi Apparatus (cis face)
Receives proteins; begins modification (glycosylation, trimming)
Golgi Apparatus (trans face)
Final sorting and packaging into vesicles for different destinations
Secretory Vesicles
Carry proteins to the plasma membrane for exocytosis
Lysosomes
Receive hydrolytic enzymes tagged with mannose-6-phosphate for intracellular digestion
Fill in the Blank
Proteins synthesized on the rough ER are transported to the Golgi apparatus inside________that bud from the ER membrane.
The Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments that provides structural support, enables cell movement, and facilitates intracellular transport. It consists of three main types of fibers:
Made of actin, these are the thinnest filaments (7 nm). They are involved in cell shape changes, amoeboid movement, cytokinesis (cleavage furrow), and muscle contraction.
A diverse family of fibrous proteins (including keratin) with a diameter of about 8-12 nm. They provide mechanical strength and anchor the nucleus and other organelles in place.
Hollow tubes of tubulin (25 nm diameter). They form the mitotic spindle, maintain cell shape, and serve as tracks for motor proteins (dynein and kinesin) that transport vesicles. They also form the core of cilia and flagella (9+2 arrangement).
Cell Membrane: The Fluid Mosaic Model
The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the cell membrane as a dynamic structure. A phospholipid bilayer forms the basic framework, with hydrophilic heads facing outward and hydrophobic tails facing inward. Embedded within this bilayer are proteins that perform diverse functions.
Integral (transmembrane) proteins span the bilayer and function as channels, carriers, and receptors. Peripheral proteins are loosely attached to the membrane surface and often serve as enzymes or structural anchors. Cholesterol molecules wedge between phospholipids to modulate membrane fluidity - preventing the membrane from becoming too rigid at low temperatures or too fluid at high temperatures.
Cholesterol
Cholesterol is a steroid lipid found in animal cell membranes. It modulates membrane fluidity by preventing tight packing of phospholipid tails at low temperatures and restricting excessive movement at high temperatures.
Formula
C27H46O
Mol. Weight
386.65 g/mol
Membrane Transport
Cells must selectively move substances across their membranes. Transport mechanisms are classified as passive (no energy required, movement down the concentration gradient) or active (requires ATP, movement against the gradient).
Passive Transport
Simple diffusion - small nonpolar molecules (O2, CO2) pass directly through the bilayer. Facilitated diffusion - polar molecules and ions move through channel proteins or carrier proteins. Osmosis - water diffuses through aquaporins or the bilayer from areas of lower solute concentration to higher solute concentration.
Active Transport
The sodium-potassium pump (Na+/K+ ATPase) is a primary active transporter that moves 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed, maintaining the electrochemical gradient essential for nerve impulse transmission. Secondary active transport (cotransport) uses the gradient established by primary pumps to drive movement of other solutes.
Bulk Transport
Endocytosis brings large molecules into the cell by engulfing them in a vesicle formed from the plasma membrane. Types include phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis (specific uptake via coated pits). Exocytosis releases materials by fusing secretory vesicles with the plasma membrane.
Quick Check
Which type of transport requires ATP and moves solutes against their concentration gradient?
Fill in the Blank
The sodium-potassium pump moves 3 Na+ ions out of the cell and________K+ ions into the cell for every ATP molecule hydrolyzed.
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