Cells: The Working Units of Life – Structure, Function, and Diversity

Chapter 4: Cells – The Working Units of Life

Key Concepts Overview

• Cells provide compartments for biochemical reactions.

• Prokaryotic cells do not have a nucleus.

• Eukaryotic cells have a nucleus and other membrane-bound compartments.

• The cytoskeleton provides strength and movement.

• Extracellular structures provide support and protection for cells and tissues.

Concept 4.1: Cells Provide Compartments for Biochemical Reactions

Cell Theory

Cell theory is the foundational principle of modern biology, stating that:

• Cells are the fundamental units of life.

• All organisms are composed of cells.

• All cells arise from preexisting cells.

Implications:

• Studying cell biology is equivalent to studying life itself.

• Life is continuous, tracing back to the evolution of the first living cells.

Surface Area-to-Volume Ratio

Cells are generally small to maintain a favorable surface area-to-volume ratio, which is crucial for efficient exchange of materials.

• The surface area of a cell determines the amount of substances that can enter or leave the cell.

• The volume of a cell determines its metabolic activity per unit of time.

• As cells grow larger, their metabolic needs increase faster than their surface area, making exchange less efficient.

• Some large cells increase surface area by folding their membranes.

Formula:

• For a cube-shaped cell: $\text{Surface Area} = 6a^2$ $\text{Volume} = a^3$ $\text{Surface Area-to-Volume Ratio} = \frac{6a^2}{a^3} = \frac{6}{a}$

Microscopy

Microscopes are essential tools for studying cells due to their small size.

• Light microscopes: Use glass lenses and light; resolution up to 0.2 μm.

• Electron microscopes: Use electron beams; resolution up to 2 nm.

Chemical Analysis of Cells

• Cells can be broken open to make a cell-free extract for chemical analysis.

• Cell structures and macromolecules can be separated by size using a centrifuge.

• The properties of the cell-free extract are similar to those inside the cell.

The Cell Membrane

The cell membrane is a selectively permeable barrier that allows cells to maintain a stable internal environment (homeostasis).

• Important for communication and receiving signals.

• Often has proteins for binding and adhering to adjacent cells.

Concept 4.2: Prokaryotic Cells Do Not Have a Nucleus

Prokaryotic Cell Structure

• Prokaryotes lack membrane-enclosed compartments.

• DNA is located in the nucleoid region.

• The cytoplasm contains cytosol (water and dissolved materials) and ribosomes (sites of protein synthesis).

Cell Wall and Capsule

• Most prokaryotes have a rigid cell wall outside the cell membrane.

• Bacterial cell walls contain peptidoglycan.

• Some bacteria have an additional outer membrane or a slimy layer of polysaccharides called the capsule.

Internal Membranes

• Some bacteria, such as cyanobacteria, have internal membranes for photosynthesis.

Motility Structures

• Some prokaryotes move using flagella, made of the protein flagellin.

• A motor protein anchored to the cell membrane or outer membrane spins each flagellum and drives the cell.

Cytoskeleton

• Some rod-shaped bacteria have a network of helical actin-like protein structures to help maintain their shape.

Concept 4.3: Eukaryotic Cells Have a Nucleus and Other Membrane-Bound Compartments

Eukaryotic Cell Structure

Eukaryotic cells are characterized by the presence of a nucleus and other membrane-bound organelles, each with specific functions.

• Organelles include the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and (in plants) chloroplasts and vacuoles.

• Each organelle plays a specific role in the cell's metabolism, structure, and function.

Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Absent	Present (membrane-bound)
Cell Wall	Usually present (peptidoglycan)	Present in plants (cellulose), fungi (chitin), absent in animals
Size	Generally smaller (1–10 μm)	Generally larger (10–100 μm)
DNA Location	Nucleoid region	Nucleus

Animal vs. Plant Cells

Feature	Animal Cells	Plant Cells
Cell Wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present
Vacuoles	Small or absent	Large central vacuole
Shape	Round or irregular	Rectangular

Major Eukaryotic Organelles and Their Functions

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

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

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Chloroplasts (plants): Site of photosynthesis.

• Vacuoles (plants): Store water, nutrients, and waste products.

Cytoskeleton

• Provides structural support, maintains cell shape, and enables movement.

• Composed of microfilaments (actin), intermediate filaments, and microtubules.

Extracellular Structures

• Provide support and protection for cells and tissues.

• Examples include cell walls in plants and extracellular matrix in animals.

Summary Table: Cell Structures and Functions

Structure	Function
Cell Membrane	Selective barrier; communication; adhesion
Cell Wall	Protection; structural support
Nucleus	Genetic information storage; regulation
Mitochondria	ATP production
Chloroplasts	Photosynthesis (plants)
Ribosomes	Protein synthesis
Golgi Apparatus	Protein and lipid modification and sorting
Lysosomes	Macromolecule digestion
Vacuole	Storage (plants)
Cytoskeleton	Shape, movement, support

Example: Cyanobacteria

• Cyanobacteria are prokaryotes with internal membranes for photosynthesis, demonstrating the diversity of cell structure and function.

Additional info: Academic context and expanded explanations have been added to ensure completeness and clarity for exam preparation.