Cell Structure and Function: Prokaryotic and Eukaryotic Cells, Microscopy, and Organelles

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Microscopy and the Study of Cells

Introduction to Microscopes

Microscopes are essential tools in biology, allowing scientists to visualize structures too small to be seen with the naked eye, such as cells and their components. There are two main types of microscopes used in biological studies: light microscopes and electron microscopes.

Light Microscopes: Use visible light to magnify small objects, typically up to 1000x, suitable for viewing cells and some organelles.
Electron Microscopes: Use beams of electrons for much higher magnification and resolution, allowing visualization of subcellular structures.

Example: The range of the human eye, light microscopes, and electron microscopes covers different scales, from whole organisms down to molecules.

Types of Electron Microscopes

Scanning Electron Microscope (SEM): Visualizes cell surfaces in high detail by scanning the specimen with a focused beam of electrons.
Transmission Electron Microscope (TEM): Visualizes internal cell structures by transmitting electrons through thin sections of the specimen.

Example: SEM images show the external morphology of cells, while TEM images reveal internal organelles and structures.

Cell Types: Prokaryotic and Eukaryotic Cells

Domains of Life

All living organisms are classified into three domains based on cellular structure: Bacteria, Archaea, and Eukarya. The most fundamental distinction is between prokaryotic and eukaryotic cells.

Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. Domains: Bacteria and Archaea.
Eukaryotic Cells: Have a nucleus and various membrane-bound organelles. Domain: Eukarya (includes animals, plants, fungi, and protists).

Domain of Life	Cell Type	Nucleus	Organelles	Cell Size	Cellularity
Bacteria	Prokaryotic	Absent	Absent	Small (~1-10 μm)	Unicellular
Archaea	Prokaryotic	Absent	Absent	Small (~1-10 μm)	Unicellular
Eukarya	Eukaryotic	Present	Present	Large (10-100 μm)	Unicellular or Multicellular

Features of Bacterial (Prokaryotic) Cells

Abundance: Bacteria are the most abundant and diverse organisms on Earth.
DNA: Bacterial DNA is circular and found in a region called the nucleoid.
Ribosomes: Bacteria have small (70S) ribosomes and reproduce by binary fission.

Features of Eukaryotic Cells

Organelles: Eukaryotic cells contain several membrane-bound organelles, including a nucleus.
DNA: Eukaryotic DNA is linear and found within the nucleus.
Ribosomes: Eukaryotes have large (80S) ribosomes and divide by mitosis and cytokinesis.

Comparison: Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells	BOTH	Eukaryotic Cells
No nucleus Smaller size (1-10 μm) Less complex Circular DNA No membrane-bound organelles Cell division: Binary fission	Have a cell membrane Contain major biomolecules: carbohydrates, proteins, nucleic acids, lipids Contain ribosomes	Has a nucleus Larger size (10-100 μm) More complex Linear DNA Has membrane-bound organelles Cell division: Mitosis and cytokinesis

Eukaryotic Cell Organelles

Overview of Eukaryotic Organelles

Eukaryotic cells contain several types of membrane-bound organelles, each with specialized functions. Some organelles are unique to plant or animal cells, while others are shared.

Nucleus: Stores genetic material (DNA) and controls cellular activities.
Mitochondria: Site of cellular respiration and energy (ATP) production.
Chloroplasts: Site of photosynthesis in plant cells.
Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.
Lysosomes: Contain digestive enzymes to break down waste (mainly in animal cells).
Central Vacuole: Large storage organelle in plant cells.
Cell Wall: Provides structural support in plant cells.

Animal vs. Plant Cells

Animal Cells: Contain lysosomes and centrioles; lack a cell wall and chloroplasts.
Plant Cells: Contain chloroplasts, a central vacuole, and a cell wall; lack lysosomes and centrioles.

Ribosomes

Structure and Function

Ribosomes: Non-membranous organelles composed of RNA and proteins; function as molecular machines that build proteins by translating messenger RNA (mRNA).
Location: Ribosomes can be free in the cytoplasm or attached to the rough ER.
Function: Ribosomes synthesize polypeptides (proteins) in all types of cells.

Map of Eukaryotic Cell Organelles

Functional Groupings

Endomembrane System: Includes the nuclear envelope, ER, Golgi apparatus, lysosomes, and vesicles; involved in protein and lipid synthesis, modification, and transport.
Energy-Related Organelles: Mitochondria (all eukaryotes) and chloroplasts (plants and algae) produce cellular energy.
Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support, cell shape, and movement.
Cell Junctions: Structures that connect cells to each other and facilitate communication (e.g., tight junctions, desmosomes, gap junctions in animals; plasmodesmata in plants).

Summary Table: Major Eukaryotic Organelles and Functions

Organelle	Main Function
Nucleus	Stores genetic material, controls cell activities
Mitochondria	Produces ATP via cellular respiration
Chloroplast	Performs photosynthesis (plants/algae)
Rough ER	Protein synthesis and processing
Smooth ER	Lipid synthesis and detoxification
Golgi Apparatus	Modifies, sorts, and ships proteins/lipids
Lysosome	Digests macromolecules and waste (mainly animals)
Central Vacuole	Storage and structural support (plants)
Cell Wall	Provides structural support (plants, fungi, some protists)
Cytoskeleton	Maintains cell shape, enables movement

Key Terms and Definitions

Cell: The basic structural and functional unit of all living organisms.
Organelle: Specialized structure within a cell that performs a specific function.
Prokaryote: An organism whose cells lack a nucleus and membrane-bound organelles.
Eukaryote: An organism whose cells contain a nucleus and membrane-bound organelles.
Ribosome: A complex of RNA and protein that synthesizes proteins.
Cytoskeleton: A network of protein filaments that provides structural support and facilitates movement within the cell.

Additional info:

Prokaryotic ribosomes are 70S, while eukaryotic ribosomes are 80S (Svedberg units, a measure of sedimentation rate).
Binary fission is a form of asexual reproduction in prokaryotes.
Mitosis and cytokinesis are processes of cell division in eukaryotes.
SEM and TEM are both types of electron microscopy, but SEM is for surface imaging and TEM is for internal structures.