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 basic units of life. All living organisms are composed of cells, and all cells arise from preexisting cells.

• Cells are the fundamental units of life.

• All organisms are composed of cells.

• All cells come from preexisting cells.

Implications:

• Studying cell biology is equivalent to studying life itself.

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

Cell Size and 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 and metabolic activity.

• Surface area determines the amount of substances that can enter or leave the cell.

• Volume determines the cell's metabolic activity per unit of time.

• As cells grow larger, volume increases faster than surface area, limiting resource exchange.

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

Formula:

• For a cube-shaped cell:

Microscopy: Tools for Studying Cells

Microscopes are essential for visualizing 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

Cell analysis often involves breaking cells open to make cell-free extracts, which can be studied to understand cellular composition and reactions.

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

• Properties of cell-free extracts are similar to those inside living cells.

The Cell Membrane

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

• Contains proteins for binding and adhering to adjacent cells.

• Important for signal reception and communication.

Concept 4.2: Prokaryotic Cells Do Not Have a Nucleus

Prokaryotic Cell Structure

Prokaryotic cells are characterized by the absence of a nucleus and other membrane-bound organelles.

• Enclosed by a cell membrane.

• DNA located in the nucleoid region.

• Cytoplasm contains cytosol and suspended particles.

• Ribosomes are the sites of protein synthesis.

Cell Wall and Capsule

Most prokaryotes have a rigid cell wall outside the cell membrane, providing structural support and protection.

• Bacterial cell walls contain peptidoglycan.

• Some bacteria have an additional outer membrane or a polysaccharide capsule for extra protection.

Internal Membranes

Some bacteria, such as cyanobacteria, possess internal membranes that contain molecules necessary for photosynthesis.

Motility: Flagella

Some prokaryotes move using flagella, which are protein structures anchored to the cell membrane or outer membrane.

• Flagella are made of the protein flagellin.

• Motor proteins rotate the flagellum, propelling the cell.

Cytoskeleton in Prokaryotes

Some rod-shaped bacteria have a network of actin-like protein structures that help maintain cell shape.

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

Eukaryotic Cell Structure

Eukaryotic cells are distinguished by the presence of a nucleus and various membrane-bound organelles, each with specialized functions.

• Cell membrane, cytoplasm, and ribosomes are present.

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

Animal vs. Plant Cells

Both animal and plant cells share many organelles, but plant cells have additional structures such as chloroplasts, a central vacuole, and a cell wall.

• Animal cells: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, centrioles.

• Plant cells: Nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplasts, central vacuole, cell wall.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Additional info:

• Some details about the cytoskeleton and organelle functions were expanded for clarity.

• Tables were inferred and constructed based on standard textbook comparisons.