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

Chapter 4: A Tour of the Cell

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. Understanding cell structure and function is essential for studying biology.

• Cell Theory: All living things are composed of cells, and all cells arise from pre-existing cells.

• Unicellular vs. Multicellular: Organisms may be single-celled (e.g., most prokaryotes, protists) or multicellular (e.g., plants, animals, fungi).

• Importance: Every cell in your body was formed by division from a previously existing cell.

Why Cells Matter

Cells provide structure, function, and organization to living organisms. Without cellular infrastructure, organisms would collapse, much like a building without support.

• Cytoskeleton: Provides mechanical support and maintains cell shape.

• Example: A collapsed building illustrates the importance of cellular infrastructure.

Biology and Society: Antibiotics and Bacterial Cells

Antibiotics: Targeting Bacterial Cells

Antibiotics are drugs designed to kill or disable infectious bacteria without harming human cells.

• Specificity: Most antibiotics target structures unique to bacterial cells, such as cell walls or ribosomes.

• Research: Scientists exploit these differences to develop new antibiotics.

• Application: Antibiotic resistance is a growing concern, prompting ongoing research for new drugs.

The Microscopic World of Cells

Cell Types and Domains of Life

Cells are classified into two major types: prokaryotic and eukaryotic. All life is grouped into three domains: Bacteria, Archaea, and Eukarya.

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

• Eukaryotic Cells: Found in Eukarya (protists, plants, fungi, animals); possess a nucleus and membrane-bound organelles.

Comparing Prokaryotic and Eukaryotic Cells

Prokaryotic and eukaryotic cells differ in structure and complexity.

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	No true nucleus; DNA in nucleoid region	True nucleus with nuclear envelope
Organelles	Few, non-membrane-bound	Many, membrane-bound
Size	Generally smaller	Generally larger
Domains	Bacteria, Archaea	Eukarya

Additional info: The nucleoid region in prokaryotes is not surrounded by a membrane, unlike the nucleus in eukaryotes.

Cell Structure: Animal and Plant Cells

Animal Cell Components

• Nucleus: Contains genetic material (DNA).

• Ribosomes: Sites of protein synthesis.

• Cytoskeleton: Provides structure and movement.

• Plasma Membrane: Controls entry and exit of substances.

• Mitochondrion: Site of cellular respiration and ATP production.

• Endoplasmic Reticulum (ER): Rough ER (with ribosomes) and Smooth ER (lipid synthesis).

• Golgi Apparatus: Modifies, sorts, and ships proteins.

• Lysosome: Contains digestive enzymes.

Plant Cell Components

• Nucleus, Ribosomes, Cytoskeleton, Plasma Membrane, Mitochondrion, ER, Golgi Apparatus: As in animal cells.

• Chloroplast: Site of photosynthesis.

• Cell Wall: Made of cellulose; provides support and protection.

• Central Vacuole: Stores nutrients, water, and waste; maintains cell turgor.

• Channels between cells: Plasmodesmata allow communication.

Membrane Structure and Function

Plasma Membrane

The plasma membrane separates the cell from its environment and regulates the movement of substances.

• Phospholipid Bilayer: Composed of two layers of phospholipids.

• Fluid Mosaic Model: Membrane is fluid, with proteins embedded like a mosaic.

Cell Surfaces

• Plant Cells: Cell wall made of cellulose; maintains shape and prevents excess water uptake.

• Animal Cells: Lack cell walls; secrete extracellular matrix for support and cell connection.

• Cell Junctions: Structures that connect animal cells into tissues.

Nucleus and Ribosomes: Genetic Control

Nucleus

The nucleus is the control center, housing DNA and directing cellular activities.

• Chromatin: DNA and proteins forming fibers; each fiber is a chromosome.

• Nucleolus: Site of ribosome component synthesis.

• Nuclear Envelope: Double membrane with pores for material exchange.

Ribosomes

• Function: Protein synthesis.

• Location: Free in cytoplasm or bound to rough ER.

• Process: Ribosomes translate messenger RNA (mRNA) into proteins.

Central Dogma:

• DNA → RNA → Protein

$\text{DNA} \xrightarrow{\text{transcription}} \text{mRNA} \xrightarrow{\text{translation}} \text{Protein}$

Endomembrane System: Manufacturing and Distribution

Components

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

These organelles are connected directly or via vesicles.

Endoplasmic Reticulum (ER)

• Rough ER: Studded with ribosomes; synthesizes proteins and membranes.

• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies drugs.

Golgi Apparatus

• Receives, modifies, stores, and ships products from the ER.

• Proteins are packaged into vesicles for transport.

Lysosomes

• Membrane-bound sacs containing digestive enzymes.

• Break down macromolecules, destroy bacteria, recycle organelles, and sculpt tissues during development.

• Lysosomal Storage Diseases: Genetic disorders affecting lysosome function.

Vacuoles

• Food Vacuoles: Store nutrients.

• Contractile Vacuoles: Pump out excess water in protists.

• Central Vacuole (plants): Stores nutrients, water, pigments, and toxins.

Chloroplasts and Mitochondria: Cellular Energy

Chloroplasts

• Found in plants and algae; site of photosynthesis.

• Convert light energy into chemical energy (sugar).

$\text{6 CO}_2 + \text{6 H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + \text{6 O}_2$

Mitochondria

• Found in almost all eukaryotic cells; site of cellular respiration.

• Produce ATP from food molecules.

$\text{C}_6\text{H}_{12}\text{O}_6 + \text{6 O}_2 \rightarrow \text{6 CO}_2 + \text{6 H}_2\text{O} + \text{ATP}$

Note: Both plant and animal cells have mitochondria; only plant cells have chloroplasts.

Endosymbiotic Theory

• Chloroplasts and mitochondria contain their own DNA and ribosomes.

• They reproduce independently within the cell.

• Evidence suggests they evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

Cytoskeleton: Cell Shape and Movement

Structure and Function

• Network of protein fibers (microtubules, intermediate filaments, microfilaments).

• Provides support, maintains shape, anchors organelles, and enables movement.

Cilia and Flagella

• Flagella: Long, whip-like structures; usually singular; propel cells (e.g., sperm).

• Cilia: Short, numerous; move in coordinated waves; propel cells or move substances (e.g., tracheal lining).

• Defects in cilia/flagella can cause infertility and respiratory problems.

Evolution Connection: Bacterial Resistance in Humans

Natural Selection and Disease Resistance

• Persistent diseases (e.g., cholera) can drive natural selection for resistance in human populations.

• Genetic mutations may confer increased resistance.

• Understanding these mutations can inform new antibiotic development.

Major Themes Illustrated

• Structure and Function: Cell structures are specialized for their functions.

• Information Flow: DNA directs cellular activities via RNA and proteins.

• Energy and Matter Pathways: Cells transform energy and matter (photosynthesis, respiration).

• Interactions: Organelles and cells interact within biological systems.

• Evolution: Cellular features and resistance evolve over time.