Chapter 6: A Tour of the Cell

Overview: The Fundamental Units of Life

Cells are the basic structural and functional units of all living organisms. Understanding cell composition, organization, energy processing, and regulation is essential for studying biology.

• All organisms are made of cells: This is the foundation of cell theory.

• The cell is the simplest collection of matter that can be alive: Cells carry out all necessary life processes.

• Cell structure is correlated to cellular function: The shape and internal organization of a cell are related to its role.

• All cells are related by their descent from earlier cells: Cells arise from pre-existing cells.

Cell Theory

Principles of Cell Theory

Cell theory is a cornerstone of modern biology, describing the properties and significance of cells.

• All living things are composed of cells.

• The cell is the basic unit of life.

• All cells arise from pre-existing cells.

Cell Organization and Homeostasis

Cell Structure and Function

Cells maintain homeostasis through compartmentalization and specialized structures.

• Plasma membrane: Surrounds the cell, separating internal contents from the external environment.

• Specialized regions and organelles: Allow for division of labor and efficient functioning.

• Cell size is limited: Due to the surface area to volume ratio, which affects nutrient uptake and waste removal.

Surface Area to Volume Ratio is critical for cell efficiency. As cells grow, volume increases faster than surface area, limiting cell size.

• Small cells have a greater surface area relative to volume, facilitating exchange of materials.

Equation:

• For a sphere:

• For a sphere:

Studying Cells: Microscopy and Fractionation

Microscopy

Microscopy is essential for visualizing cells and their components.

• Light Microscopy (LM): Uses visible light and glass lenses. Can magnify up to 1000x. Suitable for viewing cell shape and some organelles.

• Electron Microscopy (EM): Uses electron beams for much higher resolution (down to 1 nm). Includes:

  • Scanning Electron Microscopy (SEM): Views cell surfaces in 3D.

  • Transmission Electron Microscopy (TEM): Views internal cell structures.

• Fluorescence Microscopy: Uses fluorescent dyes to label specific cell components, allowing for detailed visualization.

Key Parameters:

• Magnification: Ratio of image size to actual size.

• Resolution: Minimum distance between two distinguishable points.

• Contrast: Difference in brightness between parts of the sample.

Cell Fractionation

Cell fractionation separates cellular components for study.

• Homogenization: Breaking cells open.

• Centrifugation: Separates organelles by size and density.

Allows scientists to study the function of individual organelles.

Eukaryotic vs. Prokaryotic Cells

Comparison of Cell Types

Cells are classified as prokaryotic or eukaryotic based on their structure.

Compartments in Eukaryotic Cells

Cell Regions and Organelles

Eukaryotic cells have internal membranes that create compartments for specialized functions.

• Cytoplasm: Region outside the nucleus, contains cytosol and organelles.

• Nucleoplasm: Region inside the nuclear envelope.

• Organelles: Specialized structures such as mitochondria, chloroplasts, endoplasmic reticulum, Golgi apparatus, lysosomes, and vacuoles.

Nucleus and Ribosomes

Nucleus: Information Central

The nucleus stores genetic information and coordinates cellular activities.

• Nuclear envelope: Double membrane with pores for molecular transport.

• Chromatin: DNA and associated proteins; condenses into chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis.

Ribosomes: Protein Factories

Ribosomes synthesize proteins by translating genetic information.

• Free ribosomes: Located in cytosol; produce proteins for use within the cell.

• Bound ribosomes: Attached to endoplasmic reticulum or nuclear envelope; produce proteins for membranes or export.

• Structure: Composed of large and small subunits, made of rRNA and proteins.

Endomembrane System

Components and Functions

The endomembrane system coordinates the synthesis, modification, and transport of cellular materials.

• Nuclear envelope

• Endoplasmic reticulum (ER): Rough ER (protein synthesis), Smooth ER (lipid synthesis, detoxification)

• Golgi apparatus: Modifies, sorts, and packages proteins and lipids

• Lysosomes: Digestive enzymes for breakdown of macromolecules

• Vacuoles: Storage and maintenance of cell turgor (especially in plants)

• Plasma membrane: Selective barrier for transport

Components are connected directly or via vesicle transport.

Energy Converting Organelles

Mitochondria

Mitochondria are the site of aerobic respiration, converting chemical energy in food to ATP.

• Double membrane: Outer and highly folded inner membrane (cristae)

• Matrix: Internal compartment containing enzymes and mitochondrial DNA

• Own DNA and ribosomes: Inherited maternally in humans

Equation for aerobic respiration:

Chloroplasts and Plastids

Chloroplasts are the site of photosynthesis in plants and algae.

• Double membrane

• Thylakoids: Flattened sacs containing chlorophyll, grouped in stacks called grana

• Stroma: Fluid surrounding thylakoids, contains enzymes and DNA

• Other plastids: Amyloplasts (starch storage), chromoplasts (pigment storage)

Equation for photosynthesis:

Endosymbiotic Theory

This theory proposes that mitochondria and chloroplasts originated as free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Evidence: Double membranes, own DNA (circular), ribosomes similar to prokaryotes, autonomous division

• Genetic transfer: Many genes have moved to the host cell nucleus

Cytoskeleton

Components and Functions

The cytoskeleton provides structural support, facilitates movement, and organizes cell contents.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, organelle movement, and cell division

• Microfilaments: Solid rods of actin; involved in muscle contraction, cell movement, and division

• Intermediate filaments: Fibrous proteins; provide mechanical strength and maintain cell shape

Outside the Cell

Cell Walls and Extracellular Structures

Cells produce external structures for protection, support, and communication.

• Prokaryotes: Cell wall (peptidoglycan), capsule (glycocalyx)

• Plants: Cell wall (cellulose), primary and secondary walls

• Fungi: Cell wall (chitin)

• Animals: No cell wall; secrete glycocalyx and extracellular matrix (ECM)

Extracellular Matrix (ECM) in animals:

• Collagen: Main structural protein

• Fibronectin: Glycoprotein connecting ECM to cell surface

• Integrins: Membrane proteins that transmit signals between ECM and cytoplasm

ECM influences cell behavior, communication, and gene expression.

Summary Table: Shared and Unique Organelles in Plant and Animal Cells