General Biology Study Notes: The Cell, Microscopy, and Cell Structure (Ch. 4, 24)

THE CELL

Overview of Cell Structure and Function

Cells are the fundamental units of life, performing all necessary functions to sustain living organisms. Understanding cell structure is essential for grasping how life operates at the microscopic level.

• Cell Theory: All living things are composed of cells; cells are the basic unit of structure and function in organisms; all cells arise from pre-existing cells.

• Key Cell Structures: Nucleus, plasma membrane, cytoplasm, ribosomes, organelles (e.g., mitochondria, chloroplasts).

• Cell Types: Prokaryotic (bacteria, archaea) and eukaryotic (plants, animals, fungi, protists).

Example: Human cells are eukaryotic, containing a nucleus and membrane-bound organelles, while bacterial cells are prokaryotic and lack these structures.

VISUALIZING CELLS: MICROSCOPES

Types of Microscopes and Their Uses

Microscopes are essential tools for studying cells and their components. Different types of microscopes allow visualization of structures at varying scales and levels of detail.

• Light Microscope: Uses visible light to observe small, live cells and tissues. Suitable for viewing cell shape, nucleus, and some organelles.

• Fluorescence Microscope: Uses fluorescent dyes to label specific cell components, allowing visualization of proteins, DNA, and other molecules.

• Electron Microscope: Uses beams of electrons for much higher resolution, allowing observation of dead cells and detailed organelle structure.

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

• Transmission Electron Microscope (TEM): Reveals internal cell structures at very high magnification.

Example: Ribosomes are best visualized with an electron microscope due to their small size.

Cell Size and Surface Area to Volume Ratio

Cell size is limited by the need to efficiently exchange materials with the environment. The surface area to volume ratio is a critical factor in determining cell function and efficiency.

• Smaller cells: Have a higher surface area to volume ratio, allowing more efficient exchange of materials.

• Larger cells: May struggle to move materials in and out quickly enough to support life.

Formula:

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Example: As a cell grows, its volume increases faster than its surface area, decreasing the ratio and limiting efficiency.

THE CELL: BASIC UNIT OF LIFE

Essential Cell Structures

All cells share certain basic features necessary for life.

• Nucleus: Contains genetic information (DNA) and directs cellular processes.

• Ribosomes: Make the cell's proteins.

• Plasma Membrane: Regulates what enters and exits the cell; creates a barrier and allows cell communication.

• Cytoplasm: Cellular space where nutrients and organelles are housed.

PROKARYOTES

Structure and Features of Prokaryotic Cells

Prokaryotic cells are simpler than eukaryotic cells and lack membrane-bound organelles. They are typically much smaller and have unique structural features.

• Cell Size: Prokaryotes are about 1-10 µm, much smaller than eukaryotes.

• Cell Wall: Provides structure and rigidity; composition varies between Gram-positive and Gram-negative bacteria.

• Plasma Membrane: Regulates entry and exit of substances.

• Chromosome: Circular DNA located in the nucleoid region; smaller than eukaryotic chromosomes.

• Ribosomes: Smaller than eukaryotic ribosomes; site of protein synthesis.

• Flagella: Used for movement.

• Pili: Used for attachment and DNA transfer.

Example: Escherichia coli is a common prokaryotic cell studied in biology.

Gram-Positive vs. Gram-Negative Bacteria

Bacteria are classified based on their cell wall structure, which affects their response to antibiotics.

Feature	Gram-Positive	Gram-Negative
Cell Wall	Thick peptidoglycan layer	Thin peptidoglycan layer, outer membrane present
Antibiotic Sensitivity	More sensitive to penicillin	Less sensitive; outer membrane blocks antibiotics
Color in Gram Stain	Purple	Pink

Example: Penicillin targets the cell wall of Gram-positive bacteria, making them more susceptible to this antibiotic.

EUKARYOTES

Structure and Features of Eukaryotic Cells

Eukaryotic cells are more complex than prokaryotic cells, containing membrane-bound organelles that compartmentalize cellular functions.

• Nucleus: Stores genetic material and controls cell activities.

• Mitochondria: Powerhouse of the cell; site of ATP production.

• Chloroplasts: Site of photosynthesis in plant cells.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for transport.

• Lysosomes: Digest cellular waste and foreign material.

• Cytoskeleton: Provides structural support, enables movement, and organizes cell contents.

Example: Plant cells contain both mitochondria and chloroplasts, while animal cells only have mitochondria.

THE CYTOSKELETON

Structure and Function of the Cytoskeleton

The cytoskeleton is a network of protein filaments that provides structural support, enables cell movement, and organizes organelles within the cell.

• Microtubules: Hollow tubes that maintain cell shape, enable intracellular transport, and form the mitotic spindle during cell division.

• Microfilaments (Actin Filaments): Thin fibers involved in cell movement, muscle contraction, and maintaining cell shape.

• Intermediate Filaments: Provide mechanical strength and help maintain cell integrity.

Example: The cytoskeleton is essential for processes such as cell division, vesicle transport, and cell motility.

ORGANELLES AND THEIR FUNCTIONS

Major Organelles in Eukaryotic Cells

Each organelle within a eukaryotic cell has a specific function that contributes to the cell's overall operation.

Organelle	Function
Nucleus	Separates the cell from outside environment; selectively permeable; stores genetic information
Chloroplasts	Site of photosynthesis; provides additional support and protection (plants only)
Rough ER	Synthesizes proteins; contains ribosomes
Smooth ER	Synthesizes lipids; detoxifies chemicals
Lysosome	Digests cellular waste and foreign material
Golgi Apparatus	Modifies, sorts, and packages proteins and lipids
Cilia/Flagella	Enable cell movement
Cell Wall	Provides structural support (plants, fungi, some protists)
Mitochondria	Produces ATP; site of cellular respiration
Cytoskeleton	Provides structural support; enables movement
Plasma Membrane	Regulates entry and exit of substances; cell communication

Example: The mitochondria are often called the "powerhouse" of the cell because they generate most of the cell's energy.

ADDITIONAL INFO

• Antibiotics: Many antibiotics target bacterial cell walls or ribosomes, which differ from those in human cells, making them effective against bacteria but not human cells.

• Cell Communication: Cells communicate via chemical signals and direct contact, essential for multicellular organisms.