A Tour of the Cell: Structure and Function

Chapter 6: A Tour of the Cell

Introduction to Cell Biology

Cells are the fundamental units of life. All living organisms are composed of cells, which carry out essential functions necessary for survival. Understanding cell structure and function is foundational to the study of biology.

• Cell Theory: All living things are made of cells; cells are the basic units of structure and function in living things; all cells come from pre-existing cells.

• Types of Cells: There are two main types: prokaryotic and eukaryotic cells.

Types of Cells

Prokaryotic vs. Eukaryotic Cells

Cells can be classified based on the presence or absence of a nucleus and membrane-bound organelles.

• Prokaryotic Cells: Lack a true nucleus and membrane-bound organelles. DNA is located in a region called the nucleoid.

• Eukaryotic Cells: Have a true nucleus enclosed by a nuclear envelope and possess membrane-bound organelles.

Key Differences

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present
Organelles	Few, not membrane-bound	Many, membrane-bound
Size	Generally smaller (1-10 μm)	Generally larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Prokaryotic Cell Structure

Bacterial cells are classic examples of prokaryotes. They have a simple structure but are highly efficient.

• Nucleoid: Region containing the cell's DNA, not surrounded by a membrane.

• Ribosomes: Sites of protein synthesis.

• Plasma Membrane: Selectively permeable barrier that encloses the cytoplasm.

• Cell Wall: Provides structural support and shape.

• Glycocalyx: Outer coating that can help with protection and attachment.

• Fimbriae: Hair-like structures for attachment to surfaces.

• Flagella: Tail-like structures used for movement.

Eukaryotic Cell Structure

Eukaryotic cells are more complex and compartmentalized, allowing for specialized functions.

• Nucleus: Contains most of the cell's DNA and is surrounded by a double membrane (nuclear envelope).

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

• Chromatin: DNA and associated proteins found within the nucleus.

• Ribosomes: Can be free in the cytoplasm or bound to the endoplasmic reticulum; synthesize proteins.

• Endoplasmic Reticulum (ER): Network of membranes involved in protein and lipid synthesis.

  • Rough ER: Studded with ribosomes; synthesizes proteins.

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

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain digestive enzymes to break down macromolecules, old organelles, and foreign substances.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Mitochondria: Sites of cellular respiration and ATP production; contain their own DNA.

• Chloroplasts: (in plants and algae) Sites of photosynthesis; contain their own DNA.

• Central Vacuole: (in plant cells) Stores water, nutrients, and waste products; helps maintain turgor pressure.

• Cell Wall: (in plants, fungi, and some protists) Provides structural support and protection.

• Plasmodesmata: Channels between plant cell walls that allow for transport and communication.

Plasma Membrane Structure

The plasma membrane is a dynamic structure that controls the movement of substances into and out of the cell.

• Phospholipid Bilayer: Composed of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Proteins: Embedded in the bilayer; serve as channels, receptors, or enzymes.

• Carbohydrate Side Chains: Attached to proteins and lipids; involved in cell recognition.

Genetic Information and Protein Production

Genetic information stored in DNA is used to produce proteins, which carry out most cellular functions.

• DNA: Located in the nucleus (eukaryotes) or nucleoid (prokaryotes).

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: Ribosomes read mRNA to synthesize proteins.

Endomembrane System

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

• Includes: Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.

• Protein Sorting: Proteins are synthesized in the rough ER, modified in the Golgi apparatus, and transported in vesicles to their destinations.

Lysosomes and Peroxisomes

Lysosomes and peroxisomes are specialized organelles for digestion and detoxification.

• Lysosomes: Digest food particles, old organelles (autophagy), and engulfed pathogens (phagocytosis).

• Peroxisomes: Contain enzymes that break down fatty acids and neutralize toxic compounds.

Energy-Producing Organelles

Mitochondria and chloroplasts are responsible for energy conversion in cells.

• Mitochondria: Site of cellular respiration; convert glucose and oxygen into ATP (energy currency of the cell).

  • Structure: Double membrane, inner membrane folded into cristae, contains its own DNA.

  • Equation for cellular respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

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

  • Structure: Double membrane, internal stacks of thylakoids (grana), contains its own DNA.

  • Equation for photosynthesis: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

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

Structural Elements of Cells

The Cytoskeleton

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

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

• Intermediate Filaments: Rope-like fibers; provide mechanical strength.

• Microfilaments (Actin Filaments): Thin, solid rods; involved in cell movement and muscle contraction.

Motor Proteins

• Motor proteins use ATP to move vesicles and organelles along cytoskeletal fibers.

• Examples include kinesin and dynein (move along microtubules) and myosin (moves along actin filaments).

Cell Junctions and Cell Walls

Cells are often connected to each other and to the extracellular matrix by specialized structures.

• Tight Junctions: Prevent leakage of extracellular fluid between cells.

• Anchoring Junctions (Desmosomes): Fasten cells together into strong sheets.

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

• Cell Walls: Found in plants, fungi, and some protists; provide support and protection.

• Plasmodesmata: Channels in plant cell walls that allow for communication and transport between cells.

From Cells to Tissues

Cells with similar structure and function group together to form tissues, which then combine to form organs and organ systems in multicellular organisms.

• Tissue: A group of similar cells performing a specific function.

• Organ: Structure composed of multiple tissue types working together.

Example: Muscle tissue is made of muscle cells (myocytes) that contract to produce movement.