Chapter 6: A Tour of the Cell

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. They come in two main types: prokaryotic and eukaryotic. Understanding their similarities and differences is essential for studying biology at the cellular level.

• Cell Theory: All living things are composed of cells, and all cells arise from pre-existing cells.

• Domains of Life: The three domains are Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotic, while Eukarya (including animals, plants, fungi, and protists) are eukaryotic.

Similarities Among All Cells

• Plasma Membrane: All cells are surrounded by a selective barrier called the plasma membrane.

• Cytosol: The semifluid, jellylike substance inside all cells where subcellular components are suspended.

• Ribosomes: Complexes that synthesize proteins.

• Genetic Material: All cells contain DNA (or at least instructions in a nucleic acid form).

• Common Ancestry: All cells are thought to have descended from a common ancestor.

• Basic Metabolic Chemistry: All cells carry out similar types of chemical reactions.

• Self-Replication: Cells are capable of reproducing themselves.

• Response to Environment: Cells can sense and respond to environmental changes.

Comparing Prokaryotic and Eukaryotic Cells

Cells are classified as either prokaryotic or eukaryotic based on their structural characteristics.

Prokaryotic Cells

• No Nucleus: DNA is located in an unbound region called the nucleoid.

• No Membrane-Bound Organelles: Cellular functions occur in the cytoplasm or at the plasma membrane.

• Cell Wall: Most prokaryotes have a rigid cell wall outside the plasma membrane.

• Other Structures: May have fimbriae (attachment), flagella (motility), and a glycocalyx (protective layer).

• Domains: Includes Bacteria and Archaea.

Eukaryotic Cells

• Nucleus: DNA is enclosed within a double-membrane-bound nucleus.

• Membrane-Bound Organelles: Specialized compartments (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).

• Cytoskeleton: Network of protein filaments for structure and movement.

• Larger Size: Eukaryotic cells are generally much larger than prokaryotic cells.

• Compartmentalization: Allows separation of different metabolic reactions.

Key Differences Table

Cell Size and Surface Area-to-Volume Ratio

The size of a cell is limited by the surface area-to-volume ratio. As a cell increases in size, its volume grows faster than its surface area, limiting the rate of exchange with the environment.

• Formula: For a sphere, surface area = , volume = .

• Implication: Larger cells require internal membranes (organelles) to increase surface area for metabolic processes.

Cellular Organelles and Their Functions

Nucleus

The nucleus is the control center of the cell, housing most of the cell's DNA and coordinating activities such as growth and reproduction.

• Nuclear Envelope: Double membrane that encloses the nucleus.

• Chromatin: DNA and associated proteins.

• Nucleolus: Site of ribosome synthesis.

Ribosomes

Ribosomes are molecular machines that synthesize proteins by translating messenger RNA (mRNA).

• Structure: Composed of ribosomal RNA (rRNA) and proteins.

• Types: Free ribosomes (in cytosol) and bound ribosomes (attached to endoplasmic reticulum or nuclear envelope).

Endomembrane System

The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.

• Components: Nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• Function: Regulates protein traffic and performs metabolic functions.

Endoplasmic Reticulum (ER)

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

• Smooth ER: Lacks ribosomes; synthesizes lipids, metabolizes carbohydrates, detoxifies drugs and poisons.

Golgi Apparatus

• Structure: Stacks of flattened membranous sacs (cisternae).

• Function: Modifies, sorts, and packages proteins and lipids for storage or transport out of the cell.

Lysosomes

• Structure: Membranous sacs containing hydrolytic enzymes.

• Function: Digests macromolecules, recycles damaged organelles (autophagy).

Vacuoles

• Types: Central vacuole (plants), food vacuoles, contractile vacuoles (protists).

• Function: Storage, waste disposal, water balance, cell growth.

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria and chloroplasts are the main energy-transforming organelles in eukaryotic cells.

Mitochondria

• Function: Site of cellular respiration; converts glucose and oxygen into ATP (energy).

• Structure: Double membrane; inner membrane folded into cristae to increase surface area.

• Compartments: Intermembrane space and mitochondrial matrix.

Chloroplasts

• Function: Site of photosynthesis in plants and algae; converts solar energy into chemical energy (glucose).

• Structure: Contains thylakoids (stacked into grana) and stroma (internal fluid).

Endosymbiont Theory

• Explanation: Mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own circular DNA, free ribosomes, and ability to grow and reproduce independently within the cell.

Peroxisomes

• Function: Contain enzymes that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide () as a byproduct.

• Role: Breakdown of fatty acids, detoxification of harmful substances.

Non-Organelle Components of the Cell

Cytoskeleton

The cytoskeleton is a network of protein fibers that provides structural support, maintains cell shape, and facilitates movement.

• Components: Microtubules, actin filaments (microfilaments), and intermediate filaments.

• Functions: Cell shape, organelle movement, cell division, intracellular transport.

Cell Walls and Extracellular Structures

Plant Cell Walls

• Function: Protects the cell, maintains shape, prevents excessive water uptake.

• Composition: Cellulose fibers embedded in other polysaccharides and proteins.

• Layers: Primary cell wall, middle lamella, secondary cell wall (in some cells).

Extracellular Matrix (ECM) in Animal Cells

• Function: Provides structural support, segregates tissues, and regulates intercellular communication.

• Components: Glycoproteins (e.g., collagen), proteoglycans, fibronectin, integrins.

Cell Junctions

• Plasmodesmata: Channels between plant cells for transport and communication.

• Tight Junctions: Prevent leakage of extracellular fluid in animal tissues.

• Desmosomes: Anchor cells together in animal tissues.

• Gap Junctions: Allow ions and small molecules to pass between animal cells.

Metabolism and Cellular Processes

Metabolism Overview

Metabolism is the sum of all chemical reactions in the cell. These reactions are organized into metabolic pathways, each consisting of a series of enzyme-catalyzed steps.

• Metabolic Pathways: Series of interconnected reactions that build or break down molecules.

• Energy Harvesting: Pathways maximize energy extraction and minimize energy loss as heat.

• Example Equation (Cellular Respiration):

• Enzyme Catalysis: Each step is catalyzed by a specific enzyme, allowing for regulation and efficiency.

Additional info: The notes reference chapters from Campbell Biology, a standard college-level biology textbook, and cover foundational cell biology topics suitable for General Biology students.