Cells

Introduction to Cells

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

• Definition: A cell is the smallest structural and functional unit of an organism, typically microscopic and consisting of cytoplasm and a nucleus enclosed in a membrane.

• Cell Theory: All living things are composed of one or more cells; the cell is the basic unit of life; all cells arise from pre-existing cells.

Characteristics of Life

Defining Features of Living Organisms

Living organisms share several key characteristics that distinguish them from non-living matter.

• Separation from Environment: Organisms maintain boundaries to separate their internal environment from the external environment.

• Homeostasis: The ability to maintain a stable internal environment despite changes in external conditions.

• Composed of Cells: All living things are made up of one or more cells.

• Metabolism: The process of obtaining and using energy and nutrients to sustain life.

• Genetic Information: Living things store and use information, primarily in the form of DNA.

• Reproduction: The ability to produce offspring and pass on genetic information.

What is a Cell?

Different Meanings of 'Cell'

The term 'cell' can refer to various concepts, but in biology, it specifically refers to the basic unit of life. (See humorous cartoon and images in the original material for other meanings.)

• Biological Cell: The smallest unit of life, capable of independent existence and reproduction.

• Other Meanings: The word 'cell' can also refer to prison cells, battery cells, or even fictional characters, but these are not relevant to biology.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells are classified into two main types based on their structural characteristics: prokaryotic and eukaryotic.

• Prokaryotic Cells: Lack a nucleus and membrane-bound organelles. DNA is located in a region called the nucleoid. Examples: Bacteria and Archaea.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles. Examples: Animals, plants, fungi, and protists.

Cell Size and Surface Area-to-Volume Ratio

Importance of Cell Size

Cell size is limited by the surface area-to-volume ratio, which affects the ability of the cell to exchange materials with its environment.

• Surface Area: The total area of the cell's plasma membrane.

• Volume: The space occupied by the cell's internal contents.

• Surface Area-to-Volume Ratio: As a cell grows, its volume increases faster than its surface area, limiting the efficiency of material exchange.

Conclusion: Smaller cells have a higher surface area-to-volume ratio, which facilitates efficient exchange of materials.

Prokaryotic Cell Structure

Key Features of Prokaryotic Cells

Prokaryotic cells have a simpler structure compared to eukaryotic cells.

• Plasma Membrane: Phospholipid bilayer that encloses the cell.

• Cell Wall: Provides shape and protection; composed of peptidoglycan in bacteria, and unique polymers in archaea.

• Cytoplasm: Gel-like substance inside the cell.

• Nucleoid: Region where DNA is located (not membrane-bound).

• Ribosomes: Sites of protein synthesis.

• Flagella: Tail-like structures used for movement (composed of flagellin in bacteria).

Eukaryotic Cell Structure

Key Features of Eukaryotic Cells

Eukaryotic cells are more complex and contain various membrane-bound organelles.

• Plasma Membrane: Selectively permeable barrier that maintains homeostasis.

• Nucleus: Contains DNA; surrounded by a nuclear envelope with nuclear pores.

• Endomembrane System: Includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles.

• Ribosomes: Free in cytoplasm or attached to rough ER; sites of protein synthesis.

• Mitochondria: Site of cellular respiration and ATP production; contains its own DNA.

• Chloroplasts: Found in plants and some protists; site of photosynthesis.

• Vacuoles: Storage organelles, especially large in plant cells.

• Cytoskeleton: Network of protein filaments (microfilaments, intermediate filaments, microtubules) that provide structure and facilitate movement.

Plasma Membrane Structure

Fluid Mosaic Model

The plasma membrane is a dynamic structure composed of a phospholipid bilayer with embedded proteins, carbohydrates, and cholesterol.

• Phospholipid Bilayer: Hydrophilic heads face outward; hydrophobic tails face inward.

• Proteins: Serve as channels, receptors, and enzymes.

• Carbohydrates: Attached to proteins and lipids; involved in cell recognition.

Endomembrane System

Components and Functions

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins.

• Endoplasmic Reticulum (ER):

  • Rough ER (RER): Studded with ribosomes; synthesizes and modifies proteins.

  • Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies chemicals, and stores calcium ions.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

• Lysosomes: Contain digestive enzymes to break down macromolecules, old organelles, and foreign substances.

• Vesicles: Small membrane-bound sacs that transport materials within the cell.

Mitochondria and Chloroplasts

Energy Conversion Organelles

Mitochondria and chloroplasts are specialized organelles involved in energy transformation.

• Mitochondria: Site of aerobic respiration; converts glucose and oxygen into ATP (adenosine triphosphate).

• Chloroplasts: Found in plants and some protists; site of photosynthesis, converting light energy into chemical energy.

• Both organelles contain their own DNA and ribosomes, supporting the endosymbiotic theory.

Cytoskeleton

Structural Support and Movement

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement.

• Microfilaments (Actin Filaments): Smallest diameter; involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical support and maintain cell integrity.

• Microtubules: Largest diameter; form the internal skeleton, facilitate intracellular transport, and are involved in cell division.

Cell Walls and Extracellular Structures

Support and Protection

Many cells have external structures that provide support, protection, and facilitate communication.

• Cell Wall: Found in plants (cellulose), fungi (chitin), and bacteria (peptidoglycan); provides rigidity and protection.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides structural support and mediates cell signaling.

Cell Junctions

Communication and Adhesion

Cells interact with each other through specialized junctions that allow communication and adhesion.

• Tight Junctions: Seal adjacent cells together to prevent leakage of molecules.

• Desmosomes: Rivet-like structures that fasten cells together.

• Gap Junctions: Channels that allow direct communication between animal cells.

• Plasmodesmata: Channels between plant cells that allow the passage of molecules and communication.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Example: Escherichia coli is a prokaryotic bacterium, while a human skin cell is a eukaryotic cell.