Cell Structure and Function: Study Notes for General Biology

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Cell Structure and Function

Introduction

Cells are the fundamental units of life, forming the basis of all living organisms. Understanding cell structure and function is essential for grasping how life operates at the microscopic level.

Biomolecules in Cells

Major Classes of Biomolecules

Nucleic Acids: Store and transmit genetic information (DNA, RNA).
Proteins: Perform a wide range of functions including catalysis (enzymes), structure, and transport.
Carbohydrates: Provide energy and structural support (e.g., cellulose in plants).
Lipids: Form membranes, store energy, and act as signaling molecules.

Example: The plasma membrane is primarily composed of phospholipids and proteins.

History of Cells and Cell Theory

Discovery of Cells

Robert Hooke (1665): First observed cells in cork tissue.
Anton Leeuwenhoek (1674): Discovered unicellular organisms.
Matthias Schleiden (1838): Proposed that plants are made of cells.
Theodor Schwann (1839): Proposed that animals are made of cells.

Cell Theory

All living organisms are made of one or more cells.
The cell is the structural and functional unit of all organisms.
All living cells arise by division of pre-existing cells.
Cells contain hereditary material passed to offspring during division.

Prokaryotic Cells

Characteristics

Lack a nucleus and membrane-bound organelles.
Components: plasma membrane, cytoplasm, genetic material (nucleoid), ribosomes.
Types: Bacteria and Archaea.

Comparison: Archaea vs. Bacteria vs. Eukarya

Characteristic	Archaea	Bacteria	Eukarya
Membrane lipids with branched hydrocarbons	✔
Chromosomes are circular	✔	✔
Lacks nuclear envelope	✔	✔
Lacks membrane-bound organelles	✔	✔
Methionine initiator for protein synthesis	✔		✔
Lack peptidoglycan in cell wall	✔		✔
Growth not inhibited by streptomycin/chloramphenicol	✔		✔
Histones associated with DNA	✔		✔
Several types of RNA polymerase	✔		✔

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: Surface appendages for attachment or DNA exchange.
Flagella: For movement.
Ribosomes: Sites of protein synthesis.
Plasmids: Small DNA molecules with extra genes.
Nucleoid: Region containing the main DNA.

Eukaryotic Cells

Characteristics

Have a true nucleus and membrane-bound organelles.
Compartmentalize functions for efficiency and specialization.
Can be autotrophic (plant-like) or heterotrophic (animal-like).

Types of Eukaryotic Cells

Plant Cells: Have chloroplasts, cell wall, and large central vacuole.
Animal Cells: Have centrioles, lysosomes, and unique junctions.
Fungal and Protist Cells: Share features with both plant and animal cells.

Cell Differentiation

Cells in multicellular organisms specialize for different functions due to gene expression.
Example: Neurons and epithelial cells arise from the same progenitor but express different genes.

Venn Diagram: Cell Structures

All Cells: Plasma membrane, ribosomes, cytoplasm, DNA/RNA.
Plant Cells: Chloroplasts, plasmodesmata, large central vacuole.
Animal Cells: Centrioles, gap junctions, lysosomes, cilia, microvilli.
Prokaryotes: Nucleoid, pili, capsule, plasmids.

The Endomembrane System

Overview

The endomembrane system is a network of membranes within eukaryotic cells that coordinates the synthesis, modification, and transport of proteins and lipids.

Key Components

Nucleus: Stores DNA, site of DNA replication and transcription. The nucleolus makes ribosomes.
Ribosomes: Sites of protein synthesis. Free ribosomes make cytoplasmic proteins; bound ribosomes (on rough ER) make membrane and secretory proteins.
Rough Endoplasmic Reticulum (ER): Studded with ribosomes; synthesizes proteins for membranes or export.
Smooth ER: Synthesizes lipids, detoxifies chemicals, stores calcium ions.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery.
Vesicles: Membranous sacs for storage and transport.
Lysosomes: Contain digestive enzymes; break down waste (animal cells only).
Peroxisomes: Break down fatty acids and detoxify harmful substances.
Vacuoles: Large vesicles for storage; central vacuole in plants maintains turgor pressure.

Protein Trafficking

Proteins synthesized in the rough ER are packaged into vesicles, sent to the Golgi apparatus for modification, and then delivered to their final destinations.

Energy Organelles

Mitochondria

Site of aerobic respiration; produces ATP.
Inner membrane is folded into cristae to increase surface area.
Matrix contains enzymes, ribosomes, and mitochondrial DNA.
Mitochondria are inherited maternally and have their own DNA.

Chloroplasts

Site of photosynthesis in plants and algae.
Contain chlorophyll pigment for capturing light energy.
Internal structures: thylakoids (stacked into grana), surrounded by stroma.
Contain their own DNA and ribosomes.

Endosymbiotic Theory

Suggests mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.
Evidence: Both have double membranes, their own DNA, and ribosomes.

The Cytoskeleton

Structure and Function

Network of protein filaments (microtubules, microfilaments, intermediate filaments).
Provides structural support, organizes cell contents, and enables movement.
Motor proteins move along cytoskeletal tracks to transport materials.
Cilia and flagella are extensions for cell movement.
Centrosomes (with centrioles) organize microtubules during cell division in animal cells.

External Cellular Structures

Cell Walls

Rigid structures outside the plasma membrane.
Plant cell walls: made of cellulose.
Fungal cell walls: made of chitin.
Bacterial cell walls: made of peptidoglycan.

Example: Cellulose microfibrils provide strength to plant cell walls.

Summary Table: Key Cell Structures

Structure	Prokaryotes	Plant Cells	Animal Cells
Nucleus		✔	✔
Chloroplasts		✔
Cell Wall	✔	✔
Mitochondria		✔	✔
Lysosomes			✔
Central Vacuole		✔
Ribosomes	✔	✔	✔

Additional info: These notes cover the essential concepts of cell structure and function, including the differences between prokaryotic and eukaryotic cells, the roles of organelles, and the importance of cellular compartmentalization.