Cell Structure and Function: Study Notes for College Biology

Cell Theory

Fundamental Principles of Cell Theory

Cell theory is a cornerstone of modern biology, describing the basic properties and functions of cells. - All organisms are composed of cells: Cells are the fundamental units of life. - Cells are the smallest living things: They carry out all essential life processes. - Cells arise only from pre-existing cells: This principle emphasizes continuity of life. - Cell size: Prokaryotic cells are typically 1–10 μm in diameter, while eukaryotic cells are 10–100 μm.

Cell Size Limitations

The size of cells is limited by their reliance on diffusion for the transport of substances. - Rate of diffusion is affected by: 1. Surface area available 2. Temperature 3. Concentration gradient 4. Distance Surface area-to-volume ratio is critical; as cells grow, their volume increases faster than their surface area, limiting efficient exchange. Example: Neurons overcome size limitations by being long and narrow.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Living organisms possess two primary cell types: Prokaryotic Cells: - Found in Bacteria and Archaea - Simple structure - DNA located in nucleoid (not membrane-bound) - No membrane-bound organelles Eukaryotic Cells: - Found in all other organisms - Complex structure - DNA housed in a membrane-bound nucleus - Possess membrane-bound organelles

Basic Structural Similarities

Both cell types share: - Nucleoid or nucleus: Location of DNA - Cytoplasm: Semifluid matrix containing organelles and cytosol - Ribosomes: Sites of protein synthesis - Plasma membrane: Phospholipid bilayer

Prokaryotic Cell Structure

Internal and External Features

Prokaryotic cells have distinct internal and external structures. - Internal: Cytoplasm, nucleoid region, ribosomes, microcompartments (e.g., magnetosomes) - External: Cell wall, glycocalyx, appendages (pili, flagella) Example: Magnetosomes are iron oxide crystals used for navigation.

Eukaryotic Cell Structure

Animal and Plant Cells

Eukaryotic cells are more complex and compartmentalized. - Animal cells: Contain plasma membrane and most organelles - Plant cells: Have additional components: cell wall, chloroplasts, specialized vacuoles

Comparison of Animal and Plant Cells

Internal Membranes and the Endomembrane System

Partitioning and Organization

Internal membranes divide the cell into compartments, usually consisting of a double layer of phospholipids.

Endomembrane System Components

The endomembrane system includes: - Nuclear envelope - Endoplasmic reticulum (ER) - Golgi apparatus - Lysosomes - Vacuoles

The Nucleus

Structure and Function

The nucleus is a membrane-bound organelle containing genetic material. - Chromosomes: DNA and proteins - Nucleolus: Site of ribosome assembly

Nuclear Envelope

The nuclear envelope is a double-membrane structure enclosing the nucleus. - Outer membrane: Continuous with ER - Nuclear pores: Passageways for molecules

Endoplasmic Reticulum (ER)

Types and Functions

The ER is a network of tubules and sacs called cisternae. - Smooth ER: Lipid synthesis, carbohydrate metabolism, detoxification - Rough ER: Protein synthesis (glycoproteins), transport via vesicles

Golgi Apparatus

Structure and Function

The Golgi apparatus modifies, sorts, and sends ER products. - Polar structure: Cis face (receiving), trans face (shipping) - Functions: Secretion, processing (proteolysis), protein sorting

Lysosomes

Role in Cellular Digestion

Lysosomes are small organelles in animal cells that break down macromolecules using acid hydrolases. - Autophagy: Recycling worn-out organelles via endocytosis

Vacuoles

Types and Functions

Vacuoles are organelles for storage and regulation. - Central vacuole: Stores water, enzymes, ions in plant cells - Contractile vacuole: Regulates water balance in protists - Food vacuole: Breaks down food in protists

Plasma Membrane

Structure and Functions

The plasma membrane is a phospholipid bilayer that separates the cell from its environment. - Membrane transport: Movement of substances - Cell signaling: Communication via receptors - Cell adhesion: Binding of cells

Semiautonomous Organelles: Mitochondria and Chloroplasts

Definition and Roles

Semiautonomous organelles act independently to some degree. - Mitochondria: Site of cellular respiration, ATP production - Chloroplasts: Site of photosynthesis, found in plant cells

Mitochondria Structure

- Cristae: Folded inner membrane increases surface area - Intermembrane space and matrix: Compartments for metabolic reactions

Chloroplast Structure

- Thylakoid membrane: Forms flattened tubules - Granum: Stack of thylakoids

Cytoskeleton

Types and Functions

The cytoskeleton is a network of protein filaments. - Microtubules: Cell shape, organelle organization, cell division, movement - Intermediate filaments: Mechanical strength, anchorage - Actin filaments: Cell shape, muscle contraction, intracellular movement

Extracellular Structures

Cell Wall in Plants

- Function: Protection, shape maintenance, water regulation - Composition: Cellulose embedded in polysaccharides and proteins

Extracellular Matrix in Animals

- Composition: Collagen, proteoglycans, glycoproteins - Fibronectin and integrins: Bind ECM to plasma membrane

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	1–10 μm	10–100 μm
DNA Location	Nucleoid (no membrane)	Nucleus (membrane-bound)
Organelles	None (except microcompartments)	Membrane-bound organelles
Cell Wall	Present (varies)	Present in plants, absent in animals
Examples	Bacteria, Archaea	Animals, plants, fungi, protists

Surface Area-to-Volume Ratio Table

Cell radius (r)	Surface area (4πr2)	Volume (4/3πr3)	Surface Area / Volume
1 unit	12.57 units2	4.189 units3	3
10 units	1257 units2	4189 units3	0.3

Additional info: The notes have been expanded with academic context and examples to ensure completeness and clarity for exam preparation.