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

A Tour of the Cell

Why Cells Matter

Cells are the fundamental units of life, forming the basis of all living organisms. Understanding cells is essential for grasping how life functions, from the smallest bacteria to complex multicellular organisms like plants and animals.

• Cells are the smallest units that can carry out all life processes.

• All living things are composed of cells, which arise from pre-existing cells.

• Applications: Antibiotics target bacterial cells, saving lives by combating infections.

Biology and Society: Antibiotics and Bacterial Cells

Antibiotics are drugs that specifically target structures unique to bacterial cells, making them effective against infections without harming human cells.

• Penicillin was the first antibiotic discovered (1928).

• Antibiotics bind to bacterial ribosomes or enzymes, leaving human cells unaffected.

• Understanding cell structure is crucial for developing new antibiotics.

• Example: MRSA (Methicillin-resistant Staphylococcus aureus) is a superbug resistant to many antibiotics due to unique proteins that disrupt human cell membranes.

The Microscopic World of Cells

Cellular Organization

Organisms can be single-celled or multicellular, with cells varying greatly in size and complexity.

• Single-celled organisms: Most prokaryotes and protists.

• Multicellular organisms: Plants, animals, most fungi.

• Cell size ranges from small molecules (nanometers) to visible cells (micrometers).

Measurement Equivalents

• 1 meter (m) = 1000 millimeters (mm)

• 1 millimeter (mm) = $10^{-3}$ m

• 1 micrometer (μm) = $10^{-6}$ m

• 1 nanometer (nm) = $10^{-9}$ m

The Two Major Categories of Cells

Cell Theory and Classification

All living things are composed of cells, which are classified into two major types: prokaryotic and eukaryotic.

• Prokaryotic cells: Bacteria and Archaea

• Eukaryotic cells: Protists, plants, fungi, animals

Common Features of All Cells

• Plasma membrane: Thin boundary separating the cell from its environment

• Cytosol: Jelly-like fluid inside the cell

• Chromosomes: Carry genes made of DNA

• Ribosomes: Build proteins according to genetic instructions

Comparing Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	Smaller	Larger
Complexity	Simpler	More complex
Nucleus	Absent	Present
Organelles	Absent	Present (membrane-bound)
Cell Wall	Usually present	Present in plants, fungi; absent in animals
Examples	Bacteria, Archaea	Plants, animals, fungi, protists

Prokaryotic Cell Structure

• Cell wall: Rigid, protects and maintains shape

• Pilli: Short projections for attachment

• Flagella: Long projections for movement

Eukaryotic Cell Structure

• Cytoplasm: Region between nucleus and plasma membrane

• Organelles: Specialized structures for specific functions

• Nucleus: Contains most of the cell's DNA, surrounded by a double membrane

• Plant cells: Have chloroplasts for photosynthesis

• Animal cells: Have lysosomes for digestion

Structure and Function: The Plasma Membrane

Phospholipid Bilayer

The plasma membrane separates the living cell from its surroundings and regulates the movement of substances in and out.

• Composed of phospholipids forming a bilayer

• Each phospholipid has a hydrophilic head and hydrophobic tails

• Contains proteins that regulate traffic and perform other functions

Fluid Mosaic Model

• The membrane is fluid because molecules move freely

• It is a mosaic due to the diversity of embedded proteins

The Process of Science: What Makes a Superbug?

MRSA and Bacterial Resistance

MRSA is a dangerous bacterium resistant to many antibiotics, partly due to proteins that disrupt human cell membranes.

• Observation: PSM proteins form holes in human immune cell membranes

• Hypothesis: MRSA lacking PSM is less deadly

• Experiment: Mice infected with normal MRSA died; those with PSM-deficient MRSA survived more often

• Conclusion: PSM contributes to MRSA's lethality, but other factors are also involved

Cell Surfaces

Plant and Animal Cell Surfaces

• Plant cells: Cell wall made of cellulose, protects, maintains shape, prevents excess water uptake

• Animal cells: Lack cell walls, secrete extracellular matrix (mainly collagen) for support and protection

• Cell junctions: Structures that connect animal cells into tissues for coordinated function

Types of Cell Junctions

• Desmosomes: Anchor cells together

• Gap junctions: Allow communication between cells

• Tight junctions: Prevent leakage between cells

The Nucleus and Ribosomes: Genetic Control of the Cell

Nucleus Structure and Function

• Nucleus: Control center, contains DNA

• Gene: DNA segment coding for a protein

• Chromatin: DNA-protein complex forming chromosomes

• Nucleolus: Site of ribosome assembly

• Nuclear envelope: Double membrane with pores for material exchange

Ribosomes

• Sites of protein synthesis

• Made in the nucleolus, function in cytosol or attached to endoplasmic reticulum

• Proteins made in cytosol remain in the cell; those made on ER are exported or incorporated into membranes

How DNA Directs Protein Production

• DNA is transcribed into messenger RNA (mRNA)

• mRNA exits nucleus, binds to ribosome

• Ribosome translates mRNA into a specific protein

Table: Comparing Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Cytoplasm	Occupies entire interior	Region between nucleus and plasma membrane
Chromosomes	Single circular chromosome in nucleoid region	One or more linear chromosomes in nucleus
Ribosomes	Present, smaller	Present, larger
Cell Wall	Usually present	Present in plants/fungi, absent in animals
Flagella/Pilli	May be present	May be present

Additional info: These notes expand on the provided slides with definitions, examples, and context for key cell biology concepts.