A Tour of the Cell (Chapter 6): Structure and Function of Cells

The Fundamental Units of Life

Introduction to Cells

• All organisms are made of cells, which are the basic units of life.

• The cell is the simplest collection of matter that can be alive.

• All cells are related by their descent from earlier cells.

• Cells can differ substantially from one another but share common features.

Types of Cells: Prokaryotic vs. Eukaryotic

Basic Cell Types

• Prokaryotic cells: Found in Bacteria and Archaea.

• Eukaryotic cells: Found in Protists, Plants, Animals, and Fungi.

Common Features of All Cells

• Plasma membrane

• Semifluid substance called cytosol (cytoplasm)

• Chromosomes (carry genes)

• Ribosomes (make proteins)

Prokaryotic Cells

• No nucleus; DNA is in an unbound region called the nucleoid.

• No membrane-bound organelles.

• Cytoplasm is bound by the plasma membrane.

• Examples: Bacterial chromosome, fimbriae, flagella, cell wall, glycocalyx.

Eukaryotic Cells

• DNA is in a nucleus bounded by a double membrane.

• Contains membrane-bound organelles.

• Cytoplasm is the region between the plasma membrane and nucleus.

• Generally much larger than prokaryotic cells.

Cell Membranes and Surface Area

Plasma Membrane Structure and Function

• The plasma membrane is a selective barrier that allows passage of oxygen, nutrients, and waste to service the cell.

• Composed of a phospholipid bilayer with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

• Proteins and carbohydrate side chains are embedded in the membrane.

Surface Area to Volume Ratio

• Metabolic requirements set upper limits on cell size.

• The surface area to volume ratio is critical for efficient exchange of materials.

Cube Size	Total Surface Area	Total Volume	Surface-to-Volume Ratio
1	6	1	6
5	150	125	1.2
10	750	125	6

The Nucleus and Ribosomes

Nucleus

• Contains most of the cell's DNA.

• Enclosed by the nuclear envelope, a double membrane with pores.

• DNA is organized into chromosomes (DNA + proteins = chromatin).

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

Ribosomes

• Complexes made of rRNA and protein.

• Carry out protein synthesis in two locations:

  • Free ribosomes in the cytosol

  • Bound ribosomes on the endoplasmic reticulum (ER) or nuclear envelope

The Endomembrane System

Components

• Nuclear envelope

• Endoplasmic reticulum (ER)

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

These components are either continuous or connected via transfer by vesicles.

Endoplasmic Reticulum (ER)

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

• Rough ER: Surface studded with ribosomes; produces proteins and membranes, distributes transport vesicles.

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.

• Involved in processes such as phagocytosis and autophagy.

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Types: Food vacuoles, contractile vacuoles, central vacuole (in plants).

Mitochondria and Chloroplasts: Energy Conversion

Mitochondria

• Sites of cellular respiration, a metabolic process that uses oxygen to generate ATP.

Chloroplasts

• Found in plants and algae; sites of photosynthesis, converting light energy to chemical energy.

The Cytoskeleton

Structure and Function

• A network of fibers extending throughout the cytoplasm.

• Organizes the cell's structures and activities, anchoring many organelles.

• Interacts with motor proteins for cell motility; vesicles travel along cytoskeletal tracks.

Types of Cytoskeletal Fibers

• Microtubules: Hollow rods (25 nm diameter), made of tubulin; shape the cell, guide organelle movement, separate chromosomes during cell division.

• Microfilaments (Actin Filaments): Solid rods (7 nm diameter), twisted double chain of actin; support cell shape, form core of microvilli.

• Intermediate Filaments: 8–12 nm diameter; support cell shape, fix organelles in place.

Extracellular Components and Cell Connections

Cell Walls of Plants

• Plant cells have a rigid cell wall made of cellulose, providing structural support.

• Plasmodesmata are channels between plant cells for communication.

Extracellular Matrix (ECM) of Animal Cells

• Composed of glycoproteins such as collagen, proteoglycans, and fibronectin.

• Provides structural support and mediates cell signaling.

Cell Junctions in Animal Cells

• Tight junctions: Prevent leakage of extracellular fluid.

• Desmosomes: Fasten cells together into strong sheets.

• Gap junctions: Provide cytoplasmic channels between adjacent cells for communication.