Cell Structure & Function

Overview of Cells

Cells are the fundamental units of life, and their internal organization is essential for proper function. The structure of each organelle within a cell is closely related to its specific role, and the cell membrane is crucial for maintaining homeostasis by regulating the movement of substances.

• Prokaryotic vs. Eukaryotic Cells: Prokaryotic cells lack a nucleus and membrane-bound organelles, while eukaryotic cells possess both. Compartmentalization in eukaryotes allows for specialized functions.

• Cell Size Limits: Cells have both upper and lower size limits due to surface area-to-volume ratio constraints, which affect material exchange.

• Surface Area-to-Volume Ratio: Higher ratios facilitate efficient exchange of materials; as cells grow, this ratio decreases, limiting cell size.

• Microscopy: Light microscopes use visible light to view cells, while electron microscopes provide higher resolution using electron beams.

Cell Components

Each organelle within a cell has a distinct structure and function, contributing to the cell's overall operation.

• Nucleus: Enclosed by a nuclear envelope, it contains chromatin and the nucleolus, which produces ribosomes. Controls protein synthesis.

• Ribosomes: Free ribosomes synthesize proteins for the cytosol; bound ribosomes (on rough ER) produce proteins for membranes or export.

• Endomembrane System: Includes the endoplasmic reticulum (smooth and rough), Golgi apparatus (cis and trans faces), lysosomes, vacuoles, and transport vesicles. Responsible for synthesis, modification, and transport of cellular products.

• Mitochondria & Chloroplasts: Mitochondria perform cellular respiration; chloroplasts conduct photosynthesis. Both are semiautonomous, with their own DNA and double membranes.

• Cytoskeleton: Network of microtubules, microfilaments, and intermediate filaments that maintain cell shape, organization, and movement.

• Cell Surfaces & Junctions: Plant cells have cell walls and plasmodesmata; animal cells have extracellular matrix (ECM) and junctions like gap junctions.

Cell Structure Summary Table

This table summarizes the main organelles and their functions.

Membrane Transport & Cell Signaling

Membrane Structure

The cell membrane is a dynamic structure composed of phospholipids, proteins, and carbohydrates. Its fluidity and selective permeability are essential for cellular function.

• Phospholipids: Amphipathic molecules forming a bilayer; hydrophilic heads face outward, hydrophobic tails inward.

• Fluid Mosaic Model: Membrane proteins and lipids move laterally; composition affects fluidity.

• Proteins & Carbohydrates: Integral and peripheral proteins serve as transporters, receptors, and enzymes; glycoproteins aid in cell recognition.

Traffic Across Membranes

Cells regulate the movement of substances through various transport mechanisms, maintaining homeostasis and responding to environmental changes.

• Selective Permeability: Only certain molecules can cross the membrane, depending on size, polarity, and transport proteins.

• Diffusion: Passive movement of molecules from high to low concentration; driven by kinetic energy.

• Osmosis: Diffusion of water across a semi-permeable membrane; direction depends on solute concentration.

• Tonicity: Hypertonic, hypotonic, and isotonic solutions affect cell volume and water movement.

• Transport Proteins: Facilitate diffusion (channels, carriers) and active transport (pumps).

• Membrane Potential: Voltage across the membrane due to ion distribution.

• Active Transport: Requires energy (ATP) to move substances against their concentration gradient (e.g., sodium-potassium pump).

• Bulk Transport: Endocytosis and exocytosis move large molecules or particles.

Water Potential Equation

Water potential determines the direction of water movement in plant and animal cells.

• Equation:

• Solute Potential (): Effect of dissolved solutes; always negative.

• Pressure Potential (): Physical pressure on the solution.

• Application: Used to predict water movement and calculate isotonic points in lab experiments.

Endosymbiotic Theory & Origin of Eukaryotes

Endosymbiotic Theory

The endosymbiotic theory explains the origin of eukaryotic cells as a result of symbiosis between ancestral prokaryotes.

• Key Evidence: Mitochondria and chloroplasts have double membranes, circular DNA, ribosomes, and reproduce independently within cells.

• Historical Context: Theory proposed by Lynn Margulis; supported by molecular and structural data.

• Implications: Explains the evolution of complex cells and the diversity of life.

Experimental Design & Lab Applications

Diffusion & Osmosis Lab

Students investigate the rate of osmosis and diffusion using model systems, collect quantitative data, and interpret results using water potential concepts.

• Model Systems: Dialysis tubing, potato cores, or cells.

• Data Analysis: Graph changes in mass, percent change, or molarity to identify solute potential or isotonic point.

• Experimental Design: Justify choices and explain sources of error.

• Biological Connections: Relate molecular movement to plant turgor, cell volume regulation, dehydration, and saltwater adaptation.

Practice & Application

Quiz and FRQ Practice

Students are encouraged to use learning objectives for practice problems and free response questions, focusing on key concepts such as membrane transport, organelle function, surface area-to-volume ratio, and cell structure.

• Practice Problems: Calculating surface area-to-volume ratios, predicting water movement, and explaining endosymbiotic theory.

• FRQ Scenarios: Experimental design, data interpretation, and connecting structure to function.

Vocabulary Reference

Key terms for Unit 2 include: cell wall, chloroplast, chromatin, chromosome, cytoplasm, cytoskeleton, cytosol, electron microscope, endomembrane system, endoplasmic reticulum, eukaryotic cell, extracellular matrix, gap junction, glycoprotein, Golgi apparatus, light microscope, lysosome, mitochondrion, nuclear envelope, nucleoid, nucleolus, nucleus, organelle, phagocytosis, plasma membrane, plasmodesmata, prokaryotic cell, ribosome, rough ER, smooth ER, transport vesicle, vacuole, vesicle, active transport, amphipathic molecule, aquaporin, concentration gradient, cotransport, diffusion, electrochemical gradient, endocytosis, exocytosis, facilitated diffusion, flaccid, fluid mosaic model, gated channel, glycoprotein, hypertonic, hypotonic, integral protein, ion channel, isotonic, ligand, membrane potential, osmoregulation, osmosis, passive transport, peripheral protein, phagocytosis, pinocytosis, plasmolysis, proton pump, receptor-mediated endocytosis, selective permeability, sodium-potassium pump, tonicity, transport protein, turgid, aerobic, autotrophs, chemoautotrophs, cyanobacteria, endosymbiosis, heterotroph.