Chapter 6: A Tour of the Cell

Introduction to Cells

The cell is the fundamental unit of structure and function in all living organisms. Understanding cell structure and function is essential for studying biology, as all life processes occur within cells.

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

• Basic Unit: Cells are the smallest units that can carry out all life processes.

• Example: Paramecium is a single-celled organism that performs all necessary functions for life.

Microscopy and Cell Study

Cells are typically too small to be seen with the naked eye, so biologists use microscopes and biochemistry to study them.

• Light Microscopes (LM): Use visible light to magnify specimens up to about 1,000 times. Limited resolution prevents detailed study of organelles.

• Electron Microscopes (EM): Use beams of electrons for much higher resolution.

  • Scanning Electron Microscope (SEM): Provides 3D images of specimen surfaces.

  • Transmission Electron Microscope (TEM): Allows study of internal cell structures.

• Cell Fractionation: Technique that separates cell components to study their functions, often using centrifugation.

Cell Size and Types

Cells vary greatly in size, from small bacteria to large eggs. The size range of cells is determined by their function and metabolic requirements.

• Prokaryotic Cells: Found in domains Bacteria and Archaea. Lack a nucleus and membrane-bound organelles.

• Eukaryotic Cells: Found in domain Eukarya (animals, plants, fungi, protists). Have a nucleus and membrane-bound organelles.

Comparing Prokaryotic and Eukaryotic Cells

All cells share certain features, but prokaryotic and eukaryotic cells have key differences.

• Common Features: Plasma membrane, cytoplasm, DNA, ribosomes.

• Prokaryotic Cells:

  • No nucleus; DNA is in the nucleoid region.

  • No membrane-bound organelles.

  • Smaller size.

• Eukaryotic Cells:

  • DNA enclosed in a double-membrane nucleus.

  • Contains membrane-bound organelles (e.g., mitochondria, ER).

  • Larger size.

Surface Area to Volume Ratio

Cell size is limited by the surface area to volume ratio, which affects the ability to exchange materials with the environment.

• As a cell grows, its volume increases faster than its surface area.

• The plasma membrane must allow efficient passage of oxygen, nutrients, and waste.

• Formula: (for a cube of side s)

Internal Organization of Eukaryotic Cells

Cell Compartments and Organelles

Eukaryotic cells have internal membranes that divide the cell into compartments, allowing specialized functions to occur in different areas.

• Organelles provide distinct environments for specific processes.

• Plant and animal cells share most organelles, but some are unique to each.

The Nucleus and Ribosomes

The nucleus houses genetic material and controls cellular activities. Ribosomes use genetic instructions to synthesize proteins.

• Nucleus:

  • Surrounded by a double membrane (nuclear envelope) with pores for molecular transport.

  • Contains chromatin (DNA and proteins), which condenses into chromosomes during cell division.

  • Nucleolus is the site of ribosomal RNA (rRNA) synthesis.

• Ribosomes:

  • Complexes of rRNA and protein.

  • Free ribosomes (in cytosol) and bound ribosomes (on ER or nuclear envelope).

  • Function: Protein synthesis.

The Endomembrane System

The endomembrane system regulates protein traffic and performs metabolic functions in the cell.

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

• Components are connected directly or via vesicles.

Endoplasmic Reticulum (ER)

• Smooth ER: Synthesizes lipids, detoxifies drugs and poisons.

• Rough ER: Studded with ribosomes; synthesizes glycoproteins, distributes transport vesicles, and produces membranes.

Golgi Apparatus

• Consists of flattened sacs (cisternae).

• Modifies, sorts, and packages products from the ER.

• Manufactures some macromolecules.

Lysosomes

• Membranous sacs containing hydrolytic enzymes for digestion.

• Enzymes work best in acidic environments.

• Functions: Digestion of macromolecules, recycling cell components (autophagy), and breakdown of engulfed particles (phagocytosis).

Vacuoles

• Large vesicles with diverse functions.

• Types:

  • Food vacuoles (formed by phagocytosis)

  • Contractile vacuoles (pump excess water in protists)

  • Central vacuole (in plants, stores ions and contributes to growth)

Energy Conversion Organelles

Mitochondria and Chloroplasts

These organelles transform energy for cellular use.

• Mitochondria: Site of cellular respiration; uses oxygen to generate ATP.

• Chloroplasts: Site of photosynthesis in plants and algae; contains chlorophyll.

Endosymbiont Theory

• Mitochondria and chloroplasts originated from prokaryotic cells engulfed by ancestral eukaryotes.

• Evidence:

  • Double membranes

  • Own circular DNA and ribosomes

  • Reproduce independently within the cell

Mitochondria Structure

• Smooth outer membrane and highly folded inner membrane (cristae).

• Matrix contains enzymes, DNA, and ribosomes.

• Large surface area for ATP synthesis.

Chloroplast Structure

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

• Site of light energy capture and conversion to chemical energy.

The Cytoskeleton

Structure and Function

The cytoskeleton is a network of fibers that organizes cell structures and activities, providing support, movement, and anchorage for organelles.

• Composed of microtubules, microfilaments, and intermediate filaments.

Types of Cytoskeletal Fibers

Microtubules

• Grow from centrosomes (in animal cells, contain centrioles).

• Form cilia and flagella for cell movement.

• Structure of cilia/flagella: 9+2 arrangement of microtubules, anchored by basal bodies, movement driven by dynein motor proteins.

Microfilaments

• Support cell shape, form cortex under plasma membrane.

• Involved in muscle contraction (with myosin), cell crawling (pseudopodia), and cytoplasmic streaming in plants.

Intermediate Filaments

• More permanent structures, maintain cell shape, anchor organelles.

Extracellular Components and Cell Connections

Cell Walls (Plants)

Plant cells have a rigid cell wall made of cellulose, providing protection, shape, and preventing excessive water uptake.

• Other organisms (fungi, some protists, prokaryotes) also have cell walls.

Extracellular Matrix (ECM) in Animal Cells

Animal cells lack cell walls but have an ECM composed of glycoproteins (collagen, proteoglycans, fibronectin).

• ECM proteins bind to integrins in the plasma membrane, influencing cell behavior and gene activity.

• Mechanical signaling through ECM can trigger chemical signals inside the cell.

Cell Junctions

Cells in tissues communicate and adhere through specialized junctions.

• Plasmodesmata (Plants): Channels connecting plant cells, allowing passage of water, solutes, and some macromolecules.

• Tight Junctions (Animals): Prevent leakage of extracellular fluid by pressing membranes together.

• Desmosomes (Animals): Anchor cells together into strong sheets.

• Gap Junctions (Animals): Provide cytoplasmic channels for communication between adjacent cells.

Integration of Cell Components

The cell functions as a coordinated unit, with organelles and structures working together to carry out life processes. For example, a macrophage uses its cytoskeleton, lysosomes, and plasma membrane to engulf and destroy bacteria.

*Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard biology curriculum.*