A Tour of the Cell: Structure, Function, and Methods of Study

Cells: The Fundamental Units of Life

Introduction to Cell Theory

Cells are the basic unit of structure and function in all living organisms. They can exist as single-celled organisms or as part of multicellular organisms, such as plants and animals. The cell theory states that all living things are composed of cells, and all cells arise from pre-existing cells.

• Unicellular organisms (e.g., Paramecium) consist of a single cell that performs all life functions.

• Multicellular organisms (e.g., plants, animals) are composed of many specialized cells.

• All cells contain DNA as their genetic material.

Cell Theory Key Points

• All living organisms are composed of cells.

• The cell is the simplest collection of matter that can be alive.

• Cells are related by their descent from earlier cells.

• Cells share common features but can differ substantially in structure and function.

Studying Cells: Microscopy and Biochemistry

Microscopy: Visualizing Cells

Cells are typically too small to be seen with the naked eye. Microscopes are essential tools for visualizing cells and their components. There are two main types of microscopy: light microscopy and electron microscopy.

• Light Microscopy (LM): Uses visible light and glass lenses to magnify images of specimens.

• Electron Microscopy (EM): Uses electron beams and magnetic lenses for much higher resolution.

Parameters of Microscopy

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

• Resolution: The minimum distance two points can be separated and still be distinguished as two points.

• Contrast: Differences in intensity between parts of the sample, often enhanced by staining.

Types of Light Microscopy

• Brightfield (unstained and stained): Standard illumination; staining increases contrast but usually kills cells.

• Phase-contrast and Differential Interference Contrast (Nomarski): Enhance contrast in unstained cells.

• Fluorescence Microscopy: Uses fluorescent stains and UV light to visualize specific cell components.

• Confocal and Super-resolution Microscopy: Provide sharper, three-dimensional images.

Electron Microscopy

• Scanning Electron Microscope (SEM): Provides 3D images of specimen surfaces.

• Transmission Electron Microscope (TEM): Reveals internal structures by passing electrons through thin sections.

• Cryo-Electron Microscopy (cryo-EM): Preserves specimens at very low temperatures for imaging in near-native states.

Cell Fractionation

Cell fractionation is a technique used to separate cellular components based on size and density using centrifugation. This allows scientists to study the functions of individual organelles.

• Differential centrifugation: Sequentially spins cell homogenates at increasing speeds to separate nuclei, mitochondria, and ribosomes.

Cell Types: Prokaryotic vs. Eukaryotic Cells

Three Domains of Life

• Bacteria (prokaryotes)

• Archaea (prokaryotes)

• Eukarya (eukaryotes: protists, fungi, animals, plants)

Prokaryotic Cells

• Lack a nucleus; DNA is in an unbound region called the nucleoid.

• No membrane-bound organelles.

• Generally smaller than eukaryotic cells.

Eukaryotic Cells

• DNA is contained within a nucleus bounded by a double membrane.

• Possess membrane-bound organelles (e.g., mitochondria, ER, Golgi apparatus).

• Generally larger and more complex than prokaryotic cells.

Comparison Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Size	Smaller	Larger
DNA location	Nucleoid (unbound)	Nucleus (bound by envelope)
Cell wall	Usually present, no cellulose	Present in plants (cellulose), absent in animals

Internal Organization of Eukaryotic Cells

Compartmentalization by Internal Membranes

Eukaryotic cells have internal membranes that divide the cell into compartments, allowing specialized functions to occur efficiently. The plasma membrane controls the passage of materials in and out of the cell, and the surface area-to-volume ratio limits cell size.

Biological Membranes

• Composed mainly of a double layer of phospholipids and proteins.

• Phospholipids have hydrophilic heads and hydrophobic tails, forming a bilayer.

The Nucleus and Ribosomes

Nucleus

• Contains most of the cell's DNA.

• Enclosed by a double membrane (nuclear envelope) with nuclear pores for molecular transport.

• Contains the nucleolus, the site of ribosomal RNA (rRNA) synthesis.

• DNA is organized into chromosomes, composed of chromatin (DNA + proteins).

Ribosomes

• Complexes of rRNA and protein; sites of protein synthesis.

• Free ribosomes function in the cytosol; bound ribosomes are attached to the rough ER or nuclear envelope.

The Endomembrane System

Components and Functions

• Nuclear envelope

• Endoplasmic reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis, detoxification, Ca2+ storage)

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids

• Lysosomes: Digestive compartments containing hydrolytic enzymes

• Vacuoles: Storage, waste disposal, water balance, and plant cell growth

• Plasma membrane

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins and glycoproteins, distributes transport vesicles, and produces membranes.

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium ions.

Golgi Apparatus

• Consists of flattened sacs (cisternae).

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

Lysosomes

• Membranous sacs of hydrolytic enzymes that digest macromolecules.

• Involved in phagocytosis and autophagy (recycling cell components).

Vacuoles

• Large vesicles with diverse functions: food vacuoles (phagocytosis), contractile vacuoles (water balance), central vacuole (plant cell growth and storage).

Energy Conversion: Mitochondria and Chloroplasts

Mitochondria

• Sites of cellular respiration; generate ATP using oxygen.

• Double membrane structure; inner membrane folded into cristae for increased surface area.

• Contain their own DNA and ribosomes.

Chloroplasts

• Sites of photosynthesis in plants and algae.

• Contain chlorophyll, thylakoids (stacked into grana), and stroma (internal fluid).

• Have their own DNA and ribosomes.

Peroxisomes

• Specialized metabolic compartments; produce hydrogen peroxide and convert it to water.

• Involved in fatty acid breakdown and detoxification (e.g., in liver cells).

Endosymbiont Theory

Mitochondria and chloroplasts are thought to have originated from prokaryotic cells engulfed by ancestral eukaryotes, forming a symbiotic relationship. Evidence includes their double membranes, own DNA, and ribosomes.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of protein fibers that organizes cell structure and activities, providing support, shape, and facilitating movement.

• Microtubules: Thickest; made of tubulin; involved in cell shape, organelle movement, and chromosome separation.

• Microfilaments (Actin Filaments): Thinnest; made of actin; support cell shape, involved in muscle contraction and cell motility.

• Intermediate Filaments: Middle diameter; provide mechanical support and anchor organelles.

Extracellular Components and Cell Junctions

Extracellular Matrix (ECM) in Animal Cells

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

• ECM proteins bind to integrins in the plasma membrane, influencing cell behavior and gene activity.

Cell Walls in Plants

• Provide protection, shape, and prevent excessive water uptake.

• Composed mainly of cellulose.

• Contain multiple layers: primary cell wall, middle lamella, and sometimes a secondary cell wall.

Cell Junctions

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

• Desmosomes: Anchor cells together (animal cells).

• Gap junctions: Allow communication between cells (animal cells).

• Plasmodesmata: Channels between plant cells for transport and communication.

Integration of Cellular Components

No single component of the cell works alone; cellular functions are the result of coordinated activities among organelles, the cytoskeleton, and the plasma membrane. For example, the destruction of bacteria by macrophages involves the cytoskeleton, lysosomes, and plasma membrane working together.