Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. They carry out essential functions and contain hereditary material passed to offspring.

• Cell Theory: All organisms are composed of one or more cells. Cells are the structural and functional units of life. New cells arise from the division of pre-existing cells.

• Hereditary Material: Cells contain DNA, which is transmitted to offspring during reproduction.

Types of Cells: Prokaryotic vs. Eukaryotic

Cells are classified into two main types based on their structure and complexity: prokaryotic and eukaryotic.

• Prokaryotic Cells: Simpler cells lacking a nucleus and membrane-bound organelles. Examples include Bacteria and Archaea.

• Eukaryotic Cells: More complex cells with a nucleus and various membrane-bound organelles. Found in plants, animals, fungi, and protists.

Comparison of Prokaryotic and Eukaryotic Cells

Prokaryotic Cell Structure

Prokaryotic cells have several key components that enable their survival and function.

• Plasma Membrane: Phospholipid bilayer controlling entry and exit of substances.

• Cell Wall: Provides shape and protection; made of peptidoglycan in bacteria.

• Capsule: Sticky outer layer for protection and adherence.

• Ribosomes: Sites of protein synthesis.

• Nucleoid: Region containing circular DNA.

• Plasmids: Small, circular DNA molecules carrying extra genes.

• Pili and Flagella: Structures for attachment and movement.

Eukaryotic Cell Structure

Eukaryotic cells are compartmentalized by internal membranes, allowing specialized functions within organelles.

• Nucleus: Stores DNA and coordinates activities like growth and reproduction.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; Smooth ER synthesizes lipids and detoxifies chemicals.

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

• Vesicles and Vacuoles: Store and transport substances; central vacuole in plants maintains turgor pressure.

• Lysosomes: Contain digestive enzymes to break down waste.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Mitochondria: Produce ATP via aerobic respiration; contain their own DNA.

• Chloroplasts: Conduct photosynthesis in plants and algae; contain their own DNA.

• Cytoskeleton: Network of proteins providing structural support and enabling movement.

Major Eukaryotic Organelles and Their Functions

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Both organelles have their own DNA, ribosomes, and double membranes.

• Significance: This theory highlights evolutionary relationships and the complexity of eukaryotic cells.

Cell Size and Surface Area-to-Volume Ratio

Cell size is limited by the surface area-to-volume ratio, which affects the efficiency of material exchange.

• Formula for Surface Area and Volume:

• Volume of a Sphere:

• Surface Area of a Sphere:

• Volume of a Cube:

• Surface Area of a Cube:

• Volume of a Rectangular Solid:

• Surface Area of a Rectangular Solid:

• Importance: As cells grow, volume increases faster than surface area, limiting transport efficiency.

• Adaptations: Cells may divide, elongate, or develop structures like microvilli to increase surface area.

Cell Membrane Structure and Function

The cell membrane is a selectively permeable barrier composed of a phospholipid bilayer with embedded proteins, glycoproteins, and cholesterol.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Describes the dynamic arrangement of lipids and proteins in the membrane.

• Membrane Proteins: Serve roles in transport, signaling, cell recognition, and structural support.

• Cholesterol: Maintains membrane fluidity, especially in animal cells.

Functions of Membrane Proteins

Cellular Transport Mechanisms

Cells exchange materials with their environment through various transport mechanisms.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

• Types of Passive Transport:

  • Simple Diffusion: Movement of small, nonpolar molecules through the membrane.

  • Facilitated Diffusion: Movement of larger or charged molecules via membrane proteins.

  • Osmosis: Diffusion of water across a selectively permeable membrane.

• Active Transport: Movement of substances against their concentration gradient using energy (ATP).

• Bulk Transport: Endocytosis (intake) and exocytosis (release) of large particles via vesicles.

Osmosis and Tonicity

Water Potential

Water potential () determines the direction of water movement. It is the sum of solute potential () and pressure potential ().

• Formula:

• Water moves from areas of higher water potential to lower water potential.

• Important in plant water transport and maintaining cell turgor.

Cell Junctions and the Extracellular Matrix

Cells are connected and supported by specialized structures and the extracellular matrix (ECM).

• Cell Junctions: Include tight junctions, gap junctions, and desmosomes for communication and adhesion.

• Extracellular Matrix: Network of proteins (e.g., collagen) providing structural support and regulating cell behavior.

Summary Table: Key Cell Structures

Example Applications

• Small Intestine: Microvilli increase surface area for nutrient absorption.

• Neurons and Muscle Cells: Elongated shapes increase surface area for signaling and contraction.

• Plant Cells: Central vacuole maintains turgor pressure for structural support.

Additional info: Some context and terminology have been expanded for clarity and completeness, including formulas and examples relevant to General Biology.