Cell Structure and Function: Study Notes for General Biology (chapter 4)

Cell Structure and Function

Microscopy

Microscopy is essential for studying cells and their components, as most cellular structures are too small to be seen with the naked eye. Different types of microscopes are used depending on the specimen and the level of detail required.

• Light Microscopy: Uses visible light to view specimens. Suitable for observing living cells and general cell structure.

• Types of Light Microscopy: Includes brightfield, phase-contrast, and fluorescence microscopy. Each type enhances contrast or highlights specific cell components.

• Electron Microscopy: Uses a beam of electrons for much higher resolution than light microscopy. Cannot be used to view living cells.

• Scanning Electron Microscopy (SEM): Provides detailed 3D images of cell surface structures.

• Transmission Electron Microscopy (TEM): Reveals internal cell structures in 2D, useful for studying organelles.

• Application Example: To observe amoeboid movement (dynamic cell behavior), a light microscope is preferred because it allows viewing of living cells.

What is a Cell?

The cell is the fundamental unit of structure and function in all living organisms. It is the smallest collection of matter that can be considered alive.

• Unicellular Organisms: Consist of a single cell (e.g., bacteria, amoeba).

• Multicellular Organisms: Composed of many cells organized into tissues, organs, and organ systems (e.g., plants, animals).

Cellular Components

Plasma Membrane

The plasma membrane surrounds every cell, acting as a selective barrier that regulates the movement of substances in and out.

• Structure: Composed of a phospholipid bilayer with embedded proteins.

• Properties: Selectively permeable, allowing only certain molecules to pass.

• Key Regions: Hydrophilic (water-attracting) heads face outward; hydrophobic (water-repelling) tails face inward.

Cytosol and DNA

The cytosol is the fluid component inside the cell where organelles are suspended. DNA contains the genetic instructions for cell function.

• Cytosol: Site of many metabolic reactions.

• DNA: Organized into chromosomes; location differs between prokaryotes and eukaryotes.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their internal organization.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (DNA in nucleoid)	Present (DNA enclosed in nucleus)
Membrane-bound Organelles	Absent	Present (e.g., mitochondria, ER)
Cell Wall	Chemically complex (e.g., peptidoglycan in bacteria)	Chemically simple (e.g., cellulose in plants, chitin in fungi)
Organization	Usually unicellular	Often multicellular

Major Eukaryotic Cell Organelles

Eukaryotic cells contain specialized organelles, each with distinct functions.

• Nucleus: Stores genetic material (DNA); site of transcription.

• Ribosomes: Complexes of RNA and protein; site of protein synthesis. Found free in cytosol or bound to rough ER.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes and processes proteins.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs, and stores calcium ions.

• Golgi Apparatus: Stacks of membranous sacs; modifies, sorts, and packages proteins and lipids from the ER.

• Mitochondrion: Site of cellular respiration; converts chemical energy in food to ATP. Contains its own DNA.

• Chloroplast (plants and algae): Site of photosynthesis; converts solar energy to chemical energy. Contains its own DNA.

• Peroxisome: Contains enzymes for oxidation reactions; breaks down fatty acids and detoxifies harmful substances.

• Centrosome: Region containing centrioles; organizes microtubules during cell division (animal cells).

Cytoskeleton Components

The cytoskeleton provides structural support, facilitates cell movement, and organizes organelles.

• Microfilaments (Actin Filaments): Twisted strands of actin; involved in cell movement, muscle contraction, and cytokinesis.

• Intermediate Filaments: Rope-like proteins (e.g., keratin); provide mechanical strength and maintain cell shape.

• Microtubules: Hollow tubes of tubulin; guide vesicle movement, form cilia and flagella, and organize chromosomes during cell division.

Extracellular Components and Junctions

Cells interact with their environment and neighboring cells through extracellular structures and junctions.

• Extracellular Matrix (ECM): Network of proteins (e.g., collagen, proteoglycans) outside animal cells; provides structural support and regulates cell behavior.

• Cell Wall: Rigid structure outside the plasma membrane in plants, fungi, and some protists; composed of cellulose (plants) or chitin (fungi).

• Plasmodesmata: Channels between plant cells; allow passage of water, ions, and small molecules.

• Gap Junctions: Channels between animal cells; permit direct communication and transfer of small molecules.

• Tight Junctions: Seal adjacent animal cells together; prevent leakage of extracellular fluid.

• Desmosomes: Anchor animal cells together; provide mechanical strength.

Endosymbiont Theory

The endosymbiont theory explains the origin of mitochondria and chloroplasts in eukaryotic cells.

• Key Points: Mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Supporting Evidence:

  • Both organelles have double membranes.

  • Contain their own circular DNA, similar to prokaryotes.

  • Reproduce independently within the cell by binary fission.

  • Have ribosomes similar to those of prokaryotes.

Summary Table: Comparison of Cell Junctions

Junction Type	Structure	Function	Location
Tight Junction	Proteins seal membranes	Prevent fluid leakage	Animal cells (epithelia)
Desmosome	Intermediate filaments anchor	Mechanical strength	Animal cells
Gap Junction	Pores formed by proteins	Cell-cell communication	Animal cells
Plasmodesmata	Membrane-lined channels	Transport between cells	Plant cells

Key Equations and Concepts

• Surface Area to Volume Ratio: Limits cell size and influences transport efficiency. $\text{Surface Area} = 6s^2$ $\text{Volume} = s^3$ Where $s$ is the length of a side of a cube-shaped cell.

• Fluid Mosaic Model: Describes the structure of the plasma membrane as a mosaic of proteins floating in or on the fluid lipid bilayer.

Example: To study the movement of organelles within a living cell, use phase-contrast or fluorescence light microscopy. For detailed internal structure, use TEM.

Additional info: Some content was inferred and expanded for completeness, including the summary tables and equations.