📚Study Guide: Cell Structure and Function
Unit 2: Cell Structure and Function
Cells are the fundamental units of life, and understanding their structure and function is critical for success on the AP Biology exam. This unit explores the differences between prokaryotic and eukaryotic cells, the structure and function of membrane-bound organelles, and the critically important structure of the plasma membrane. The fluid mosaic model describes the membrane as a dynamic, semi-permeable barrier composed of phospholipids, proteins, cholesterol, and carbohydrates. Students must understand how the hydrophobic core of the membrane creates a selective barrier that regulates the passage of substances. The endomembrane system—including the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane—demonstrates how organelles work collaboratively to synthesize, modify, transport, and degrade proteins and lipids. Additionally, this unit covers the cytoskeleton's role in maintaining cell shape, facilitating movement, and enabling intracellular transport. On the AP exam, this unit frequently appears in free-response questions asking students to explain how a specific cellular structure contributes to a biological process, or to predict the effect of disrupting a specific organelle's function.
Key Concepts
- Prokaryotic vs. Eukaryotic Cells: Prokaryotes (bacteria, archaea) lack a nucleus and membrane-bound organelles. Eukaryotes (plants, animals, fungi, protists) have a nucleus and organelles. Both have ribosomes, cytoplasm, plasma membrane, and genetic material.
- Fluid Mosaic Model: The plasma membrane consists of a phospholipid bilayer with embedded proteins (integral and peripheral), cholesterol (for stability), and carbohydrate chains (for cell recognition). It is fluid because phospholipids and proteins can move laterally.
- Selective Permeability: Small, nonpolar molecules (O2, CO2) cross freely. Polar molecules and ions require transport proteins. Large molecules cross via endocytosis or exocytosis.
- Endomembrane System: The nuclear envelope houses DNA. Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies. The Golgi modifies, sorts, and packages proteins. Lysosomes contain hydrolytic enzymes for digestion.
- Mitochondria and Chloroplasts: Mitochondria perform cellular respiration (produce ATP). Chloroplasts perform photosynthesis. Both have double membranes and their own DNA, supporting the endosymbiotic theory.
- Cytoskeleton: Microfilaments (actin) enable muscle contraction and cell movement. Intermediate filaments provide structural support. Microtubules (tubulin) form cilia, flagella, and the mitotic spindle.
Vocabulary
- Organelle: A specialized subunit within a cell that has a specific function, usually membrane-bound in eukaryotes.
- Phospholipid Bilayer: A double layer of phospholipid molecules that forms the core structure of cellular membranes; hydrophilic heads face water, hydrophobic tails face inward.
- Integral Protein: A protein embedded within the hydrophobic core of the membrane, often spanning the entire bilayer (transmembrane protein).
- Endosymbiotic Theory: The theory that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.
- Lysosome: A membrane-bound organelle containing hydrolytic enzymes that break down waste materials, cellular debris, and foreign invaders.
- Peroxisome: An organelle that carries out oxidation reactions producing hydrogen peroxide (H2O2) and then converts it to water.
Processes and Diagrams to Know
- Fluid Mosaic Model Diagram: Be able to label phospholipids, integral proteins, peripheral proteins, cholesterol, and glycoproteins/glycolipids.
- Endomembrane System Pathway: Nucleus -> Rough ER -> Transport Vesicle -> Golgi -> Secretory Vesicle -> Plasma Membrane (exocytosis).
- Prokaryotic Cell Structure: Label cell wall, capsule, flagella, pili, nucleoid region, ribosomes, and plasma membrane.
Experimental Designs
- Cell Fractionation: Using centrifugation to separate cellular components by size and density to study organelle function.
- Membrane Permeability: Investigating how temperature, pH, or solvents affect membrane integrity using beetroot pigment leakage.
Common Mistakes
- Confusing Plant and Animal Cells: Plant cells have cell walls, chloroplasts, and large central vacuoles. Animal cells have centrioles and lysosomes (usually).
- Thinking All Membranes Are Identical: Membrane composition varies by cell type and organelle. The inner mitochondrial membrane has different proteins than the plasma membrane.
- Confusing Rough and Smooth ER: Rough ER has ribosomes and makes proteins; Smooth ER lacks ribosomes and makes lipids/detoxifies.
- Assuming Prokaryotes Lack All Complexity: Prokaryotes can have complex internal membrane systems and perform sophisticated metabolic processes despite lacking organelles.
AP Exam Strategies
- Use Comparative Language: When comparing cell types, explicitly state "Unlike prokaryotes, eukaryotes..." to earn comparison points.
- Explain Directionality: In transport questions, specify the direction of movement (into or out of the cell) and the mechanism (diffusion, osmosis, active transport).
- Link Structure to Function: For every organelle mentioned, connect its structural features to its biological role.
- Draw Diagrams in FRQs: If a question asks you to sketch, label clearly. A well-labeled diagram can earn points even if your explanation is weak.
Real-World Applications
- Antibiotic Development: Many antibiotics target prokaryotic-specific structures like ribosomes (70S) or cell walls, leaving human (80S ribosome) cells unharmed.
- Cystic Fibrosis: A mutation in the CFTR chloride channel protein disrupts ion transport across epithelial membranes, demonstrating the importance of membrane proteins.
- Biofuel Production: Understanding chloroplast function helps engineers optimize algae for lipid production used in biodiesel.