Cells: Basic Unit of Life

Definition and Characteristics

Cells are the fundamental units of life, capable of independent existence and reproduction. All living organisms are composed of cells, which may vary in structure and function but share common features.

• Cell Theory: All organisms are made of cells; all cells come from pre-existing cells.

• Cell Lineage: Cells may look different, but share common features and ancestry.

Studying Cells: Microscopy

Types of Microscopes

Microscopes are essential tools for studying cell structure and function. Different types of microscopes provide varying levels of detail.

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses; suitable for viewing live cells.

• Transmission Electron Microscope (TEM): Sends a beam of electrons through a specimen; reveals internal structures of cells.

• Scanning Electron Microscope (SEM): Focuses electrons onto the surface; produces 3D-like images of cell surfaces.

Limitation: Standard light microscopy is too low in resolution to study membrane-enclosed organelles in eukaryotic cells.

Cell Types: Prokaryotic vs. Eukaryotic

Domains and Characteristics

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

• Prokaryotic Cells: Found in domains Bacteria and Archaea. These cells are simpler, smaller, and lack membrane-bound organelles (e.g., nucleus, mitochondria, ER).

• Eukaryotic Cells: Found in domain Eukarya, which includes protists, fungi, plants, and animals. These cells have membrane-enclosed organelles, including a nucleus that houses DNA.

Cell Membrane (Plasma Membrane)

Structure and Function

The cell membrane defines the boundary of the cell and regulates the movement of substances in and out.

• Phospholipid Bilayer: Two layers of phospholipids with hydrophobic tails inside and hydrophilic heads outside.

• Embedded Proteins: Proteins built into the membrane for transport and signaling.

• Protein Channels: Act as tunnels for specific molecules.

• Protein Pumps: Use energy (usually ATP) to move molecules actively.

Cell Size Limits: Determined by metabolic needs and surface area-to-volume ratio. As cells get larger, volume increases faster than surface area, making exchange harder. This is why most cells remain small.

Organelles and Their Functions

Basic Functions of Organelles

• Genetic control of the cell

• Manufacturing, distribution, and breakdown of molecules

• Energy processing

• Structural support, movement, and communication

The Nucleus and Its Parts

• Nucleus: Control center; stores DNA

• Nuclear Envelope: Double membrane surrounding the nucleus

• Nuclear Lamina: Protein layer supporting nuclear shape

• Chromatin: DNA + proteins; genetic material

• Chromosomes: Condensed chromatin, visible during cell division

• Nucleolus: Dense region inside nucleus; ribosome assembly

• Ribosomes: Molecular machines that make proteins

Ribosomes

• Make proteins (inserted into membranes, exported, or used in cytosol)

• Free ribosomes: floating in cytosol

• Bound ribosomes: attached to ER or nuclear envelope

Endomembrane System

The endomembrane system is a group of organelles that work together to make, modify, store, and transport molecules.

• Nuclear Envelope

• Endoplasmic Reticulum (ER): Network for making and moving molecules

• Golgi Apparatus: Packages, sorts, and ships molecules

• Lysosomes: Enzymes that break down molecules

• Vacuoles: Storage sacs

• Plasma Membrane: Outer boundary

Endoplasmic Reticulum (ER)

• Rough ER: Covered with ribosomes; makes proteins for membranes and export

• Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions

Golgi Apparatus

• Modifies products from ER

• Manufactures macromolecules

• Sorts and packages materials into transport vesicles

Lysosomes

• Membranous sac with hydrolytic enzymes

• Break down macromolecules

• Enzymes work best at acidic pH

Vacuoles

• Large vesicles for storage

• Contractile vacuole: pumps out excess water (protists)

• Central vacuole: stores organic compounds and water (plants)

Energy-Related Organelles

Endosymbiont Theory

Explains the origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotic cells.

Mitochondria

• Site of cellular respiration (ATP production)

• Double membrane; inner membrane folded into cristae

• Contains own DNA and ribosomes

Chloroplasts

• Site of photosynthesis (plants and algae)

• Contains green pigment chlorophyll

• Structure: double membrane, thylakoids stacked into grana, stroma (internal fluid)

Peroxisomes

• Specialized metabolic compartments

• Bounded by a single membrane

• Produce hydrogen peroxide and convert it to water

• Detoxify alcohol and harmful compounds

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that maintains cell shape, anchors organelles, and enables movement.

• Microfilaments: Solid rods (~7 nm), made of actin; support cell shape, enable movement

• Intermediate Filaments: Diameter 8-12 nm; support cell shape and anchor organelles

• Microtubules: Hollow tubes (~25 nm), made of tubulin; guide organelle movement, separate chromosomes

• Flagella & Cilia: Microtubule-containing extensions for movement

• Basal Body: Anchors cilium or flagellum

• Dynein: Motor protein driving movement

Cell Wall and Extracellular Structures

Cell Wall

• Extracellular structure distinguishing plant cells from animal cells

• Also found in prokaryotes, fungi, and some protists

• Composed of cellulose fibers embedded in polysaccharides and proteins

Plant Cell Wall Layers

• Primary cell wall: Thin and flexible

• Middle lamella: Thin layer between primary walls of adjacent cells

• Secondary cell wall: Added between plasma membrane and primary wall (in some cells)

Extracellular Matrix (ECM) in Animal Cells

• Covers animal cells; provides structural support

• Composed of glycoproteins (collagen, proteoglycans, fibronectin)

• ECM proteins bind to integrins (receptor proteins in plasma membrane)

Functions:

1. Support

2. Adhesion

3. Movement

4. Regulation

Cell Junctions

Types and Functions

Cell junctions enable neighboring cells to adhere, interact, and communicate through direct physical contact.

• Tight junctions: Bind cells together to form leakproof sheets

• Desmosomes: Anchor cells together

• Gap junctions: Allow passage of ions and small molecules between cells

Plasmodesmata (Plant Cells)

• Channels that perforate plant cell walls

• Allow water, small solutes, and sometimes proteins or RNA to pass directly from one cell to another

Key Equations and Concepts

• Surface Area to Volume Ratio:

As cell size increases, the surface area to volume ratio decreases, limiting efficient exchange of materials.

Summary Table: Major Organelles and Their Functions

Additional info: These notes expand on the original content by providing definitions, examples, and academic context for each topic, ensuring a self-contained study guide suitable for General Biology students.