Chapter 6: A Tour of the Cell

Introduction to Cell Biology

The cell is the fundamental unit of structure and function in all living organisms. Understanding cells is essential for comprehending the complexity of life, as all biological processes originate at the cellular level. Cells are classified as either prokaryotic or eukaryotic, each with distinct structural features and functions.

Concept 6.1: Biologists Use Microscopes and Biochemistry to Study Cells

Microscopy

• Microscopes are essential tools for visualizing cells, which are typically too small to be seen by the naked eye.

• Light microscopes (LM) use visible light and glass lenses to magnify specimens up to about 1,000 times, but their resolution is limited for viewing organelles.

• Key parameters of microscopy:

  • Magnification: Ratio of image size to actual size.

  • Resolution: Clarity of the image; the minimum distance between two distinguishable points.

  • Contrast: Visible differences in brightness between parts of the sample.

• Electron microscopes (EM) provide much higher resolution:

  • Scanning Electron Microscopes (SEM): Visualize cell surfaces in 3D.

  • Transmission Electron Microscopes (TEM): Visualize internal cell structures.

• Recent advances include fluorescent labeling, confocal microscopy, and cryo-electron microscopy (cryo-EM), which allow for detailed visualization of cellular components.

Cell Fractionation

• Cell fractionation is a technique that separates cellular components using centrifugation, allowing scientists to study the function of individual organelles.

• This method helps correlate cell structure with function by isolating nuclei, mitochondria, ribosomes, and other organelles.

Concept 6.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Prokaryotic vs. Eukaryotic Cells

• All cells share basic features: plasma membrane, cytosol, chromosomes, and ribosomes.

• Prokaryotic cells (Bacteria and Archaea):

  • No nucleus; DNA is in the nucleoid region.

  • No membrane-bound organelles.

  • Generally smaller and simpler.

• Eukaryotic cells (Protists, fungi, animals, plants):

  • DNA enclosed in a nucleus with a double membrane.

  • Contain membrane-bound organelles.

  • Larger and more complex.

• Cell size is limited by the surface area-to-volume ratio, which affects the efficiency of material exchange.

Internal Membranes and Compartmentalization

• Eukaryotic cells have internal membranes that create compartments (organelles), allowing specialized functions to occur efficiently within the same cell.

• Biological membranes are primarily composed of a phospholipid bilayer with embedded proteins.

Concept 6.3: The Eukaryotic Cell’s Genetic Instructions Are Housed in the Nucleus and Carried Out by the Ribosomes

The Nucleus

• The nucleus contains most of the cell’s genetic material (DNA) and is surrounded by a double-membrane nuclear envelope.

• Nuclear pores regulate the movement of molecules between the nucleus and cytoplasm.

• DNA is organized into chromosomes, which are composed of chromatin (DNA and proteins).

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

Ribosomes

• Ribosomes are complexes of rRNA and protein that synthesize proteins.

• They can be free in the cytosol or bound to the endoplasmic reticulum (ER) or nuclear envelope.

Concept 6.4: The Endomembrane System Regulates Protein Traffic and Performs Metabolic Functions

Components of the Endomembrane System

• The endomembrane system includes the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vacuoles, and plasma membrane.

• These components are either continuous or connected by vesicle transfer.

Endoplasmic Reticulum (ER)

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

• Smooth ER: Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium ions.

Golgi Apparatus

• Consists of flattened sacs (cisternae); modifies, sorts, and packages products from the ER for secretion or delivery to other organelles.

Lysosomes

• Membranous sacs containing hydrolytic enzymes for digesting macromolecules.

• Involved in phagocytosis (engulfing food particles) and autophagy (recycling cellular components).

Vacuoles

• Large vesicles with diverse functions: food storage, water regulation (contractile vacuoles), and storage of ions and other substances (central vacuole in plants).

Concept 6.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

Mitochondria

• Sites of cellular respiration, generating ATP from organic molecules using oxygen.

• Have a double membrane; inner membrane forms cristae, increasing surface area for ATP synthesis.

• Contain their own DNA and ribosomes, supporting the endosymbiont theory of their evolutionary origin.

Chloroplasts

• Found in plants and algae; sites of photosynthesis.

• Contain chlorophyll, thylakoids (stacked into grana), and stroma (internal fluid).

• Also support the endosymbiont theory.

Peroxisomes

• Specialized organelles that carry out oxidation reactions, producing hydrogen peroxide as a byproduct.

• Involved in fatty acid breakdown and detoxification of harmful substances.

Concept 6.6: The Cytoskeleton Is a Network of Fibers That Organizes Structures and Activities in the Cell

Components of the Cytoskeleton

• Microtubules: Hollow rods made of tubulin; maintain cell shape, guide organelle movement, and separate chromosomes during cell division.

• Microfilaments (Actin Filaments): Twisted double chains of actin; support cell shape, enable cell movement, and are involved in muscle contraction and cytoplasmic streaming.

• Intermediate Filaments: Fibrous proteins; provide structural support and anchor organelles.

Centrosomes and Centrioles

• Microtubules originate from the centrosome in animal cells, which contains a pair of centrioles.

Cilia and Flagella

• Microtubule-containing extensions that enable cell movement.

• Cilia are numerous and short; flagella are few and long.

• Both have a "9+2" arrangement of microtubules and are powered by the motor protein dynein.

Concept 6.7: Extracellular Components and Connections Between Cells Help Coordinate Cellular Activities

Cell Walls of Plants

• Plant cell walls provide structural support, protection, and prevent excessive water uptake.

• Composed mainly of cellulose, with primary, secondary, and middle lamella layers.

Extracellular Matrix (ECM) of Animal Cells

• Animal cells are surrounded by an ECM made of glycoproteins (collagen, proteoglycans, fibronectin).

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

Cell Junctions

• Cells in tissues interact through specialized junctions:

  • Tight junctions: Prevent leakage of extracellular fluid.

  • Desmosomes: Anchor cells together into strong sheets.

  • Gap junctions: Allow communication by permitting the passage of ions and small molecules.

• In plants, plasmodesmata connect cells, allowing transport of water and small molecules.

Concept 6.8: A Cell Is Greater Than the Sum of Its Parts

Cellular functions are highly integrated, with organelles and structures working together to maintain life. For example, the destruction of bacteria by a macrophage involves coordination between the cytoskeleton, lysosomes, and plasma membrane.