A Tour of the Cell – Chapter 6 Study Notes (Campbell Biology)

Chapter 6: A Tour of the Cell

Concept 6.1: Biologists Use Microscopy and Biochemistry to Study Cells

Cells are the fundamental units of life, but they are typically too small to be seen with the naked eye. Scientists use various techniques, including microscopy and biochemistry, to study cell structure and function.

• Microscopy is the use of microscopes to visualize cells and their components.

• Light Microscope (LM): Uses visible light passed through a specimen and glass lenses to magnify images.

• Key Parameters of Microscopy:

  • Magnification: The ratio of an object's image size to its real size.

  • Resolution: The measure of image clarity; the minimum distance between two distinguishable points.

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

• Organelles: Membrane-enclosed structures within eukaryotic cells, often too small to be resolved by standard light microscopy.

Example: Electron microscopes are used to study organelles in greater detail than light microscopes.

Cell Size and Microscopy

Cells vary greatly in size, from small bacteria to large plant and animal cells. The ability to study cells depends on the resolution and magnification of the microscope used.

• Light microscopes can magnify up to about 1,000 times the actual size of a specimen.

• Special techniques can enhance contrast and allow for staining or labeling of cell components.

• Standard light microscopy cannot resolve most organelles.

Additional info: Super-resolution microscopy and electron microscopy allow visualization of smaller structures such as viruses and ribosomes.

Table: Size Range of Cells and Structures

Structure	Approximate Size
Human height	~2 m
Chicken egg	~0.05 m
Most plant and animal cells	10–100 μm
Most bacteria	1–10 μm
Viruses	50–100 nm
Ribosomes	20–30 nm
Proteins	2–10 nm
Small molecules	~1 nm
Atoms	~0.1 nm

Cell Fractionation

Cell fractionation is a technique used to separate cellular components for individual study.

• Process: Cells are broken up (homogenization) and then spun in a centrifuge (differential centrifugation) to separate organelles by size and density.

• Purpose: Allows scientists to study the function and composition of specific organelles.

Example: Isolating mitochondria to study cellular respiration.

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

All cells share certain basic features, but there are key differences between prokaryotic and eukaryotic cells.

• Basic Features of All Cells:

  • Plasma membrane

  • Cytosol (semifluid substance)

  • Chromosomes (carry genes)

  • Ribosomes (make proteins)

Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No; DNA in nucleoid	Yes; DNA in nucleus
Organelles	No membrane-bound organelles	Membrane-bound organelles
Size	Generally smaller	Generally larger
Domains	Bacteria, Archaea	Protists, Fungi, Animals, Plants

Surface Area to Volume Ratio

The surface area to volume ratio is critical for cell function, as it affects the ability to exchange materials with the environment.

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

• Cells must maintain a high surface area to volume ratio for efficient transport of oxygen, nutrients, and waste.

Equation: $\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

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

The nucleus is the control center of the eukaryotic cell, containing most of the cell's genetic material. Ribosomes use this genetic information to synthesize proteins.

• Nucleus: Contains most of the cell's DNA; surrounded by a double membrane called the nuclear envelope.

• Nuclear Envelope: Double membrane with pores that regulate entry and exit of molecules.

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

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

• Ribosomes: Complexes of rRNA and protein; build proteins in the cytosol (free ribosomes) or on the endoplasmic reticulum/nuclear envelope (bound ribosomes).

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

The endomembrane system is a group of interconnected organelles that manage protein synthesis, modification, and transport, as well as other metabolic functions.

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

• These components are either continuous or connected via vesicles.

Endoplasmic Reticulum (ER)

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

• Rough ER: Studded with ribosomes; synthesizes proteins (especially glycoproteins), distributes transport vesicles, and is a membrane factory for the cell.

Golgi Apparatus

• Consists of flattened membranous sacs called cisternae.

• Modifies products of the ER, manufactures certain macromolecules, sorts and packages materials into transport vesicles.

Lysosomes

• Membranous sacs of hydrolytic enzymes that digest macromolecules.

• Enzymes work best in acidic environments.

• Functions include phagocytosis (engulfing food particles) and autophagy (recycling the cell's own organelles).

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Types include food vacuoles, contractile vacuoles (pump excess water out), and central vacuoles (in plant cells, store ions and contribute to cell growth).

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

Mitochondria and chloroplasts are energy-converting organelles found in eukaryotic cells.

• Mitochondria: Sites of cellular respiration; use oxygen to generate ATP.

• Chloroplasts: Found in plants and algae; sites of photosynthesis.

• Peroxisomes: Oxidative organelles that break down fatty acids and detoxify harmful substances.

Endosymbiont Theory

• Mitochondria and chloroplasts originated as prokaryotic cells engulfed by an ancestral eukaryote.

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

Mitochondria Structure

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

• Two compartments: intermembrane space and mitochondrial matrix.

• Cristae increase surface area for ATP synthesis.

Chloroplast Structure

• Contains chlorophyll and enzymes for photosynthesis.

• Thylakoids (stacked into grana) and stroma (internal fluid).

• Member of the plastid family of organelles.

Peroxisomes

• Bound by a single membrane; contain enzymes that transfer hydrogen to oxygen, forming hydrogen peroxide ($H_2O_2$).

• Functions include breakdown of fatty acids and detoxification.

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

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

• Composed of three types of fibers:

  • Microtubules: Thickest; made of tubulin; involved in cell shape, organelle movement, and chromosome separation.

  • Microfilaments (Actin Filaments): Thinnest; made of actin; involved in cell shape, muscle contraction, and cell motility.

  • Intermediate Filaments: Middle diameter; provide structural stability and anchor organelles.

Table: Cytoskeleton Components and Functions

Type	Structure	Main Functions
Microtubules	Hollow tubes (tubulin)	Cell shape, organelle movement, chromosome separation
Microfilaments	Twisted double chain (actin)	Cell shape, muscle contraction, cell motility, cytoplasmic streaming
Intermediate Filaments	Fibrous proteins	Cell shape, anchorage of nucleus/organelles, formation of nuclear lamina

Centrosomes and Centrioles

• Microtubules grow out from the centrosome in animal cells.

• Centrosome contains a pair of centrioles (nine triplets of microtubules arranged in a ring).

Cilia and Flagella

• Microtubule-containing extensions for cell movement.

• Share a common structure: nine doublets of microtubules in a ring, with two singles in the center ("9+2" arrangement).

• Movement driven by the motor protein dynein.

Microfilaments and Motility

• Interact with myosin for muscle contraction and cell movement.

• Enable amoeboid movement and cytoplasmic streaming in plant cells.

Intermediate Filaments

• More permanent than microtubules or microfilaments.

• Support cell shape and fix organelles in place.

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

Cells produce extracellular structures that support and connect them, facilitating communication and coordination.

• Plant Cell Walls: Made of cellulose; protect, maintain shape, and prevent excessive water uptake.

• Multiple layers: primary cell wall, middle lamella (pectins), and sometimes a secondary cell wall.

• Extracellular Matrix (ECM) in Animal Cells: Composed of glycoproteins (collagen, proteoglycans, fibronectin); binds to integrins in the plasma membrane.

• ECM regulates cell behavior and gene activity via mechanical and chemical signaling.

Cell Junctions

• Plasmodesmata (Plants): Channels connecting plant cells, allowing passage of water, solutes, and sometimes proteins/RNA.

• Tight Junctions (Animals): Membranes pressed together to prevent leakage.

• Desmosomes (Animals): Anchoring junctions that fasten cells into strong sheets.

• Gap Junctions (Animals): Channels for cytoplasmic exchange between adjacent cells.

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

Cellular functions depend on the coordinated activity of all cellular components. For example, a macrophage's ability to destroy bacteria involves the cytoskeleton, lysosomes, and plasma membrane working together.

• Integration of structure and function is essential for life.