A Tour of the Cell: Structure, Function, and Microscopy

Chapter 4: A Tour of the Cell

Overview: The Fundamental Units of Life

All living organisms are composed of cells, which are the basic units of structure and function in biology. Despite their diversity, all cells share certain fundamental features.

• Cell: The simplest collection of matter that can be alive.

• All cells are related by their descent from earlier cells.

• Cells can differ substantially from one another but share common features.

Microscopy and Cell Study

Concept 4.1: Biologists Use Microscopes and Biochemistry to Study Cells

Cells are generally too small to be seen by the unaided eye, so microscopes are essential tools for cell biology. Microscopy allows scientists to observe cell structure and function in detail.

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images of cells.

• Micrograph: An image taken using a microscope.

Parameters of Microscopy

• Magnification: The ratio of an object's image size to its real size.

• Resolution: The measure of image clarity; the minimum distance between two distinguishable points.

• Contrast: The difference in brightness between the light and dark areas of an image.

• Light microscopes can magnify up to about 1,000 times the size of the specimen.

• Staining or labeling samples increases contrast and enables visualization of cell components.

• Subcellular structures, such as organelles, are often too small to be resolved by light microscopy.

Advances in Microscopy

• Fluorescent markers improve visualization of specific molecules or structures.

• Confocal and super-resolution microscopy provide sharper images and higher resolution (as small as 10–20 μm).

• Cryo-electron microscopy (cryo-TEM): Preserves specimens at extremely low temperatures, allowing visualization of protein structures in their native environment.

Types of Light Microscopy

• Bright field: Involves staining; used to observe dead cells.

• Simple phase contrast: No staining; used to observe live cells.

• Differential interference contrast (Nomarski): No staining; higher resolution than simple phase contrast; used for live cells.

• Fluorescence microscopy and confocal microscopy for enhanced detail and specific labeling.

Types of Electron Microscopy

• Scanning Electron Microscope (SEM): Focuses a beam of electrons onto the surface of a specimen, producing 3D images.

• Transmission Electron Microscope (TEM): Focuses a beam of electrons through a specimen to reveal internal structures.

• Electron microscopes use electron beams and magnetic lenses, require a vacuum, and provide higher resolution than light microscopes.

Cell Fractionation

Cell fractionation is a technique that breaks up cells and separates their components using centrifugation. This allows scientists to study the function of organelles and correlate cell structure with function.

• Components separate based on size and density.

• Combines biochemistry (study of metabolism) and cytology (study of cell structure).

Cell Structure: Prokaryotic vs. Eukaryotic Cells

Concept 4.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Cells are classified as either prokaryotic or eukaryotic, with distinct structural differences.

• Prokaryotic cells: Found in Bacteria and Archaea; lack a nucleus and membrane-bound organelles.

• Eukaryotic cells: Found in protists, fungi, animals, and plants; have a nucleus and membrane-bound organelles.

Common Features of All Cells

• Plasma membrane: A selective barrier composed of a phospholipid bilayer.

• Cytosol: Semi-fluid substance within the cell.

• Chromosomes: Carry genetic information (DNA).

• Ribosomes: Synthesize proteins.

Comparing Prokaryotic and Eukaryotic Cells

• Prokaryotic cells: DNA is in an unbound region called the nucleoid; no membrane-bound organelles.

• Eukaryotic cells: DNA is in a membrane-bound nucleus; contain membrane-bound organelles.

• Both types have cytoplasm bound by the plasma membrane.

• Eukaryotic cells are generally larger (10–100 μm) than prokaryotic cells (1–5 μm).

Plasma Membrane and Surface Area

The plasma membrane controls the passage of oxygen, nutrients, and waste. The surface area-to-volume ratio is critical for efficient cellular metabolism.

• As cell size increases, volume grows faster than surface area.

• Smaller cells have a greater surface area-to-volume ratio, facilitating exchange with the environment.

The Nucleus and Ribosomes

Concept 4.3: The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

The nucleus contains most of the cell's DNA and is the control center of the cell. Ribosomes are the sites of protein synthesis.

• Nucleus: Surrounded by a double membrane (nuclear envelope) with nuclear pores for molecular exchange.

• Nuclear lamina: Protein filaments that maintain nuclear shape.

• Chromosomes: DNA molecules associated with proteins; together called chromatin.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis within the nucleus.

• Ribosomes: Complexes of rRNA and protein; can be free in the cytosol or bound to the endoplasmic reticulum (ER) or nuclear envelope.

The Endomembrane System

Concept 4.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions

The endomembrane system includes several organelles that work together to modify, package, and transport proteins and lipids.

• Nuclear envelope

• Endoplasmic reticulum (ER): Rough (RER) and smooth (SER) regions

• Golgi apparatus

• Lysosomes (animal cells only)

• Vacuoles (plant and fungal cells)

• Plasma membrane

• Components are connected directly or via transfer vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Rough ER (RER): Studded with ribosomes; modifies proteins (e.g., glycoprotein synthesis), produces membranes, and transports proteins via vesicles.

Golgi Apparatus

• Consists of flattened sacs called cisternae.

• Modifies ER products, manufactures macromolecules, sorts and packages materials into vesicles.

Lysosomes

• Membranous sacs of hydrolytic enzymes that digest macromolecules in animal cells.

• Engulf food particles by phagocytosis and recycle cellular components by autophagy.

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Contractile vacuoles pump excess water out of cells (in protists).

• Central vacuoles in plants store ions and organic compounds, and may have lysosomal functions.

Energy-Transforming Organelles

Concept 4.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

Mitochondria and chloroplasts are the main energy-transforming organelles in eukaryotic cells.

• Mitochondria: Sites of cellular respiration; use oxygen to generate ATP.

• Chloroplasts: Sites of photosynthesis in plants and algae.

Endosymbiont Theory

• Mitochondria and chloroplasts share similarities with bacteria (double membranes, circular DNA, 70S ribosomes, independent division).

• Theory: Early eukaryotic cells engulfed prokaryotic cells, forming a symbiotic relationship.

Mitochondria Structure

• Smooth outer membrane and folded inner membrane (cristae).

• Two compartments: intermembrane space and mitochondrial matrix.

• Site of metabolic steps of cellular respiration and ATP synthesis.

Chloroplast Structure

• Contain chlorophyll and enzymes for photosynthesis.

• Structure: thylakoids (stacked into grana), stroma (internal fluid).

• Member of the plastid family of organelles.

Peroxisomes

• Specialized metabolic compartments bounded by a single membrane.

• Produce hydrogen peroxide and convert it to water; detoxify harmful substances (e.g., in liver cells).

The Cytoskeleton

Concept 4.6: The Cytoskeleton Is a Network of Fibers That Organizes Structures and Activities in the Cell

The cytoskeleton provides structural support, organization, and motility for the cell.

• Composed of three main types of fibers: microtubules, microfilaments (actin filaments), and intermediate filaments.

• Dynamic structure; interacts with motor proteins for movement of organelles and vesicles.

Microtubules

• Hollow rods made of tubulin; largest cytoskeletal fibers.

• Functions: maintain cell shape, guide organelle movement, separate chromosomes during cell division, form cilia and flagella.

• Grow from the centrosome (microtubule-organizing center) in animal cells, which contains a pair of centrioles.

Cilia and Flagella

• Microtubule-containing extensions for cell motility.

• Cilia: numerous, short; flagella: few, long.

• Structure: "9+2" arrangement of microtubules, anchored by a basal body; movement driven by dynein motor proteins.

Microfilaments (Actin Filaments)

• Thin rods built from actin subunits; bear tension and resist pulling forces.

• Form the core of microvilli in intestinal cells.

• Interact with myosin for muscle contraction, amoeboid movement, cytoplasmic streaming, and cytokinesis.

Intermediate Filaments

• Intermediate in size; only in some animal cells (e.g., vertebrates).

• Reinforce cell shape, anchor organelles, and provide mechanical support.

• More permanent than microtubules or microfilaments.

Extracellular Components and Cell Junctions

Concept 4.7: Extracellular Components and Intercellular Junctions Help Coordinate Cellular Activities

Cells synthesize and secrete materials outside the plasma membrane, which are essential for structure and communication.

Plant Cell Walls

• Distinguish plant cells from animal cells; provide protection, shape, and prevent excessive water uptake.

• Composed of cellulose microfibrils in a matrix of polysaccharides and proteins.

• Layers: primary cell wall (thin, flexible), middle lamella (between adjacent cells), secondary cell wall (in some cells, between plasma membrane and primary wall).

Extracellular Matrix (ECM) of Animal Cells

• Composed of glycoproteins (collagen, proteoglycans, fibronectin).

• ECM proteins bind to integrins (receptor proteins) in the plasma membrane.

• Functions: support, adhesion, movement, and regulation.

Intercellular Junctions

• Plasmodesmata (plants): Channels that allow water, solutes, and some macromolecules to pass between cells.

• Tight junctions (animals): Prevent leakage of extracellular fluid.

• Desmosomes (animals): Fasten cells together into strong sheets.

• Gap junctions (animals): Provide cytoplasmic channels between adjacent cells for communication.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Membrane-bound organelles	Absent	Present
Cell size	1–5 μm	10–100 μm
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Equations

• Surface Area of a Cube: $SA = 6s^2$

• Volume of a Cube: $V = s^3$

• Surface Area to Volume Ratio: $\text{Ratio} = \frac{6s^2}{s^3} = \frac{6}{s}$

Additional info: These notes are based on "Campbell Biology in Focus, Chapter 4: A Tour of the Cell" and are suitable for General Biology college students preparing for exams.