Cell Structure and Function

Introduction to Cells

Cells are the fundamental units of life, forming the basis of all living organisms. The development of microscopes in the 16th and 17th centuries led to the discovery of cells and the formulation of cell theory.

• Cell Theory:

  • All living organisms are composed of one or more cells.

  • The cell is the basic structural and functional unit of life.

  • All cells arise from pre-existing cells by division.

  • Cells contain hereditary material (DNA), which is passed to offspring during cell division.

• Key Contributors: Anton van Leeuwenhoek (first observed unicellular organisms), Matthias Schleiden (plants are made of cells).

Types of Cells: Prokaryotic vs. Eukaryotic

Cells are classified into two main types based on their internal structure: 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.

  • Typically smaller and simpler than eukaryotic cells.

• Eukaryotic Cells:

  • Have a true nucleus enclosed by a nuclear envelope.

  • Contain membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum).

  • Examples: Plants, Animals, Fungi, and Protists.

  • Typically larger and more complex than prokaryotic cells.

Comparison of Archaea and Bacteria

Archaea and Bacteria are both prokaryotic, but they have distinct differences.

Prokaryotic Cell Parts

Prokaryotic cells have specialized structures for survival and function.

• Capsule: Sticky outer layer for protection and adherence to surfaces.

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

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

• Pili: Hair-like structures for attachment and DNA exchange.

• Flagella: Whip-like structures for movement.

• Ribosomes: Sites of protein synthesis (free-floating in cytoplasm).

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

• Nucleoid: Region containing the main DNA.

Eukaryotic Cell Parts

Eukaryotic cells contain various organelles that compartmentalize functions.

• Nucleus: Stores DNA and is the site of DNA replication and RNA synthesis.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes proteins.

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

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

• Vesicles: Small sacs for transport and storage of materials.

• Vacuoles: Larger sacs for storage; central vacuole in plants maintains turgor pressure.

• Lysosomes: Contain digestive enzymes to break down waste (mainly in animal cells).

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

• Mitochondria: Site of cellular respiration and ATP production.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Cytoskeleton: Network of protein filaments for cell shape, movement, and division.

Endomembrane System

The endomembrane system coordinates the synthesis, modification, and transport of proteins and lipids within the cell.

• Includes the nuclear envelope, ER, Golgi apparatus, vesicles, lysosomes, and plasma membrane.

• Proteins are synthesized in the rough ER, modified in the Golgi, and transported via vesicles.

• Vesicles can fuse with the plasma membrane to secrete substances (exocytosis) or bring substances in (endocytosis).

Energy Organelles: Mitochondria and Chloroplasts

Mitochondria and chloroplasts are specialized organelles for energy conversion.

• Mitochondria:

  • Site of aerobic respiration; produces ATP.

  • Inner membrane is highly folded (cristae) to increase surface area.

  • Matrix contains enzymes, ribosomes, and mitochondrial DNA.

  • Inherited maternally.

• Chloroplasts:

  • Site of photosynthesis in plants and algae.

  • Contains chlorophyll pigment in thylakoids (stacked as grana).

  • Stroma is the fluid surrounding thylakoids; contains enzymes and DNA.

• Endosymbiotic Theory: Suggests mitochondria and chloroplasts originated from free-living prokaryotes engulfed by ancestral eukaryotes.

The Cytoskeleton

The cytoskeleton provides structural support, organization, and movement for cells.

• Composed of microfilaments, intermediate filaments, and microtubules.

• Functions include maintaining cell shape, enabling movement (e.g., flagella, cilia), and organizing cell division (centrosomes and centrioles).

• Motor proteins use the cytoskeleton as tracks for transporting cellular materials.

Cell Walls and Extracellular Matrix

Cell walls and the extracellular matrix (ECM) provide additional support and protection.

• Plant Cell Walls: Made of cellulose (a polysaccharide).

• Fungal Cell Walls: Made of chitin.

• Bacterial Cell Walls: Made of peptidoglycan.

• Extracellular Matrix (ECM): Found in animal cells; composed of proteins like collagen and glycoproteins, providing structural support and cell signaling.

Cell Size and Surface Area-to-Volume Ratio

Cell size is limited by the need to efficiently exchange materials with the environment.

• As cells grow, volume increases faster than surface area, reducing efficiency of transport.

• Cells maintain high surface area-to-volume ratios by being small, having convoluted shapes, or forming multicellular structures.

Example: Microvilli in the small intestine increase surface area for nutrient absorption.

Cell Membrane Structure and Function

The cell membrane separates the internal environment from the external environment and regulates transport.

• Composed of a phospholipid bilayer with embedded proteins, glycoproteins, and cholesterol (in animal cells).

• Phospholipids are amphipathic, with hydrophilic heads and hydrophobic tails.

• Fluid Mosaic Model: Describes the dynamic and flexible nature of the membrane.

• Membrane proteins serve various functions: transport, enzymatic activity, cell-cell recognition, and attachment.

• Cholesterol modulates membrane fluidity and permeability in animal cells.

Cellular Transport Mechanisms

Cells transport substances across membranes using passive and active mechanisms.

• Passive Transport: Does not require energy; substances move down their concentration gradient.

  • Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).

  • Osmosis: Diffusion of water through aquaporins.

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

• Active Transport: Requires energy (ATP); moves substances against their concentration gradient.

  • Example: Sodium-potassium pump ( pump) maintains ion gradients in neurons.

  • Equation: per ATP hydrolyzed.

• Bulk Transport: Movement of large particles via vesicles (endocytosis and exocytosis).

Osmosis and Water Potential

Osmosis is the movement of water across a semipermeable membrane, influenced by solute concentration and pressure.

• Isotonic Solution: Equal solute concentration inside and outside the cell; no net water movement.

• Hypotonic Solution: Lower solute concentration outside; water enters the cell, causing swelling or lysis.

• Hypertonic Solution: Higher solute concentration outside; water leaves the cell, causing shrinkage or plasmolysis.

Water Potential (): The potential energy of water in a system, determining the direction of water movement.

• Equation:

• = solute potential; = pressure potential

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

Example: Turgor pressure in plant cells maintains structure and prevents wilting.

Additional info:

• Some context and terminology have been expanded for clarity and completeness.

• Equations and tables have been formatted for study purposes.