Tour of the Cell: Structure and Function of Cellular Components

Chapter 4: Tour of the Cell

Learning Objectives

This chapter introduces the fundamental structures and functions of cells, comparing prokaryotic and eukaryotic cells, and exploring the roles of major organelles. Understanding these concepts is essential for grasping how cells operate as the basic units of life.

• Characteristics of all cells: All cells possess a plasma membrane, cytosol, DNA (chromosomes), and ribosomes.

• Comparison of cell types: Eukaryotic and prokaryotic cells differ in the location of DNA, presence of organelles, and cell size.

• Structure and function of the nucleus: The nucleus houses genetic material and is surrounded by a nuclear envelope.

• Ribosomes, endoplasmic reticulum, and Golgi apparatus: These organelles are involved in protein synthesis, modification, and transport.

• Mitochondria and chloroplasts: Organelles responsible for energy conversion processes.

• Cytoskeleton: Provides structural support and facilitates cell movement.

Cell Structure and Types

Essential Characteristics of a Cell

Cells are the basic units of life, defined by several key features:

• Plasma membrane: A selective barrier that encloses the cell.

• Cytosol: The semifluid substance within the membrane.

• DNA (chromosomes): Genetic material that directs cellular activities.

• Ribosomes: Complexes that synthesize proteins.

Major Types of Cells

• Prokaryotic cells: Lack a nucleus and membrane-bound organelles. Examples: Bacteria and Archaea.

• Eukaryotic cells: Have a nucleus and membrane-bound organelles. Examples: Plants, Animals, Fungi, and Protists.

Comparison of Prokaryotic and Eukaryotic Cells

• Location of DNA: Prokaryotes: nucleoid (not membrane-bound); Eukaryotes: nucleus (membrane-bound).

• Organelles: Eukaryotes have membrane-bound organelles; prokaryotes do not.

• Size: Eukaryotic cells are generally larger than prokaryotic cells.

Nucleus and Ribosomes

Structure and Function of the Nucleus

• Nuclear envelope: Double membrane that encloses the nucleus.

• Nucleolus: Site of ribosome synthesis within the nucleus.

• Chromosomes: DNA molecules associated with proteins.

Ribosomes: Structure and Function

• Function: Protein synthesis.

• Composition: rRNA and proteins.

• Types: Free ribosomes (in cytosol) and bound ribosomes (attached to endoplasmic reticulum or nuclear envelope).

• Lack of membrane: Ribosomes are not true organelles as they are not membrane-bound.

Endomembrane System

Endoplasmic Reticulum (ER)

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

• Rough ER: Studded with ribosomes; synthesizes proteins destined for secretion or for membranes.

• Relationship: Both types form a continuous network within the cell.

Golgi Apparatus

• Function: Modifies, sorts, and ships proteins and lipids received from the ER.

• Structure: Consists of flattened membranous sacs called cisternae.

• Orientation: Has a cis face (receiving side, faces ER) and a trans face (shipping side, faces plasma membrane).

Energy-Transforming Organelles

Mitochondria

• Structure: Double membrane, inner membrane folded into cristae.

• Function: Site of cellular respiration and ATP production.

• Contains: DNA and ribosomes.

Chloroplasts

• Structure: Double membrane, internal stacks of thylakoids (grana), stroma.

• Function: Site of photosynthesis in plants and algae.

• Contains: DNA and ribosomes.

Cytoskeleton

Types and Functions

• Microtubules: Hollow rods that shape the cell, guide organelle movement, and separate chromosomes during cell division.

• Microfilaments: Thin filaments involved in cell movement and muscle contraction.

• Intermediate filaments: Provide mechanical support for the cell and help maintain its shape.

Key Terms

• Cell

• Organelle

• Nucleus

• Nuclear envelope

• Nucleolus

• Ribosome

• Bound ribosome

• Free ribosome

• Rough endoplasmic reticulum

• Smooth endoplasmic reticulum

• Golgi apparatus

• Lysosome

• Mitochondrion

• Chloroplast

• Cytoskeleton

• Microtubule

• Microfilament

• Intermediate filament

Summary Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (nucleoid region)	Present (membrane-bound)
Membrane-bound organelles	Absent	Present
Cell size	Generally smaller	Generally larger
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Example: Protein Synthesis and Trafficking

• Proteins synthesized on the rough ER are transported to the Golgi apparatus for further modification and sorting.

• From the Golgi, proteins may be sent to the plasma membrane, secreted from the cell, or delivered to other organelles.