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 1590s 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 structural and functional unit of life.

  • All cells arise from the division of pre-existing cells.

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

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

Types of Cells: Prokaryotic vs. Eukaryotic

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

  • Contain structures such as plasma membrane, cytoplasm, ribosomes, cell wall, capsule, pili, and flagella.

• Eukaryotic Cells:

  • Have a true nucleus enclosed by a nuclear envelope.

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

  • Examples: Animal cells, plant cells, fungi, and protists.

  • Cells are compartmentalized for specialized functions.

Comparison: Archaea vs. Bacteria

Archaea and Bacteria are both prokaryotes but differ in several key features.

Cellular Structures and Organelles

Prokaryotic Cell Parts

• Capsule: Sticky outer layer for protection and adherence.

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

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

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

• Flagella: Used for motility.

• Ribosomes: Free-floating, site of protein synthesis.

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

• Nucleoid: Region containing the main DNA.

Eukaryotic Cell Parts

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

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; synthesizes and transports proteins.

  • Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Vesicles: Small sacs for transport and storage.

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

• Lysosomes: Contain digestive enzymes for breaking down macromolecules and old cell parts (mainly in animal cells).

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

• Mitochondria: Site of ATP production via aerobic respiration; contains its own DNA.

• Chloroplasts: Site of photosynthesis in plants and algae; contains its own DNA.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) for cell shape, movement, and division.

• Centrosomes/Centrioles: Organize microtubules during cell division (animal cells).

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 apparatus, and transported via vesicles.

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

Energy Organelles: Mitochondria and Chloroplasts

• Mitochondria:

  • Double-membraned organelle; inner membrane forms cristae to increase surface area.

  • Matrix contains enzymes, proteins, ribosomes, and mitochondrial DNA.

  • Site of aerobic respiration and ATP production.

  • Inherited maternally.

• Chloroplasts:

  • Found in plant and algal cells; site of photosynthesis.

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

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

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

Cellular Structure: Cytoskeleton and External Structures

Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that provides structural support, organizes cell contents, and enables movement.

• Microtubules: Hollow tubes for cell shape, transport, and division.

• Microfilaments: Thin filaments for cell movement and muscle contraction.

• Intermediate Filaments: Provide mechanical strength.

• Flagella and Cilia: Extensions for cell motility; composed of microtubules.

• Centrosomes/Centrioles: Organize microtubules during cell division.

Cell Wall and Extracellular Matrix

• Cell Wall:

  • Plant cell walls are made of cellulose (a polysaccharide).

  • Bacterial cell walls are made of peptidoglycan.

  • Provides structural support and protection.

• Extracellular Matrix (ECM):

  • Network of proteins (e.g., collagen) and carbohydrates outside animal cells.

  • Provides structural support, cell adhesion, and communication.

  • Defects in ECM can lead to tissue fragility.

Cell Size and Surface Area

Limitations on Cell Size

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

• As cell volume increases, surface area increases at a slower rate.

• High surface area-to-volume ratio is needed for efficient transport of nutrients and waste.

• Cells may be small, elongated, or have convoluted surfaces (e.g., microvilli) to maximize surface area.

Formulas:

• Volume of a Sphere:

• Surface Area of a Sphere:

• Volume of a Cube:

• Surface Area of a Cube:

• Volume of a Rectangular Solid:

• Volume of a Cylinder:

Cell Membrane Structure and Function

Plasma Membrane Structure

The plasma membrane separates the cell from its environment and compartmentalizes internal structures.

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

• Phospholipids are amphipathic (hydrophilic heads, hydrophobic tails).

• Fluid Mosaic Model: Membrane components move laterally, creating a dynamic structure.

Membrane Proteins and Functions

• Integral Proteins: Span the membrane; involved in transport and communication.

• Peripheral Proteins: Attached to membrane surface; involved in signaling and cell recognition.

• Glycoproteins and Glycolipids: Used for cell recognition and communication.

• Cholesterol: Maintains membrane fluidity and stability in animal cells.

Cellular Transport Mechanisms

Passive Transport

Passive transport moves substances down their concentration gradient without energy input.

• Simple Diffusion: Movement of small, uncharged molecules (e.g., O2, CO2) through the lipid bilayer.

• Osmosis: Diffusion of water across a semipermeable membrane via aquaporins.

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

Active Transport

Active transport moves substances against their concentration gradient using energy (ATP).

• Protein Pumps: e.g., Na+/K+ pump, H+ pump.

• Endocytosis: Uptake of large particles or fluids via vesicle formation (phagocytosis for solids, pinocytosis for liquids).

• Exocytosis: Secretion of substances by vesicle fusion with the plasma membrane.

Osmosis and Tonicity

Osmosis is the movement of water across membranes, influenced by solute concentration.

• 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.

Plant Cell Tonicity and Turgor Pressure

• Central vacuole exerts turgor pressure against the cell wall, maintaining plant structure.

• Loss of turgor pressure leads to wilting (plasmolysis).

Water Potential

Water potential () determines the direction of water movement.

• Formula:

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

• Water potential is affected by solute concentration () and pressure ().

Summary

• Cells are the basic units of life, classified as prokaryotic or eukaryotic.

• Cell structure is closely linked to function, with specialized organelles and membranes.

• Transport across membranes is essential for maintaining homeostasis.

• Surface area-to-volume ratio and compartmentalization are key for cell efficiency.

• Water potential and osmosis are critical for plant cell function and overall cellular health.