A Tour of the Cell: Structure and Function of Cellular Components

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A Tour of the Cell

Introduction to Cells

Cells are the fundamental structural and functional units of all living organisms. They come in two main types: prokaryotic and eukaryotic. Despite their diversity, all cells share several basic features that are essential for life.

Plasma Membrane: All cells are surrounded by a plasma membrane that acts as a selective barrier.
Cytoplasm: The interior of the cell, consisting of a jellylike substance called cytosol.
Genetic Material: All cells contain DNA as their genetic material.
Ribosomes: All cells have ribosomes that synthesize proteins.

Types of Cells

Prokaryotic Cells

Prokaryotic cells are characterized by the absence of a nucleus and other membrane-bound organelles. Their DNA is located in a region called the nucleoid, which is not enclosed by a membrane.

Generally smaller than eukaryotic cells (0.1 – 5 μm in diameter).
Single-celled organisms such as Bacteria and Archaea are prokaryotes.
Most have a peptidoglycan cell wall and may have an additional polysaccharide capsule.
May possess flagella, pili, or fimbriae for movement and attachment.

Example: Escherichia coli (E. coli) is a well-known prokaryotic bacterium.

Eukaryotic Cells

Eukaryotic cells have a true nucleus enclosed by a double membrane and possess various membrane-bound organelles. Their DNA is contained within the nucleus.

Generally larger than prokaryotic cells (10 – 100 μm in diameter).
Include animal, plant, fungal, and protist cells.
Contain specialized organelles such as mitochondria, endoplasmic reticulum, and (in plants) chloroplasts.

Example: Human skin cells (animal) and leaf cells (plant).

Cellular Structures and Their Functions

Plasma Membrane

The plasma membrane forms the boundary of every cell and regulates the movement of substances in and out of the cell. It is essential for maintaining the cell's internal environment.

Composed of a phospholipid bilayer with embedded and attached proteins.
Acts as a selective barrier, allowing the passage of oxygen, nutrients, and waste products.
Each square micrometer of membrane can only allow a limited amount of substances to cross per second.

Key Concept: The fluid mosaic model describes the structure of the plasma membrane as a mosaic of components that gives the membrane a fluid character.

Cytoplasm

The cytoplasm is the region between the plasma membrane and the nuclear envelope. It contains organelles suspended in cytosol, the cytoskeleton, and various dissolved chemicals.

Composed of 70–80% water.
Site of many metabolic reactions, including glycolysis and other biosynthetic pathways.

Nucleus

The nucleus is a prominent organelle in eukaryotic cells that stores genetic information and coordinates cellular activities such as growth, metabolism, and reproduction.

Typically 5 μm in diameter.
Enclosed by a nuclear envelope, a double membrane with nuclear pores for regulated exchange of materials.
The nuclear lamina is a network of protein filaments that supports the nuclear envelope and maintains nuclear shape.

Chromosomes and Chromatin

Within the nucleus, DNA is organized into structures called chromosomes. In non-dividing cells, DNA exists as chromatin, a complex of DNA and proteins.

Each chromosome contains one long DNA molecule associated with proteins.
Humans have 46 chromosomes in most cells; gametes (sex cells) have 23.

Nucleolus

The nucleolus is a dense region within the nucleus responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits.

Plays a key role in protein synthesis by producing rRNA and assembling ribosomes.
The nucleus directs protein synthesis by synthesizing messenger RNA (mRNA) from DNA, which is then translated by ribosomes in the cytoplasm.

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent (DNA in nucleoid)	Present (DNA in nucleus)
Membrane-bound Organelles	Absent	Present
Cell Size	0.1 – 5 μm	10 – 100 μm
Cell Wall	Usually present (peptidoglycan in bacteria)	Present in plants (cellulose), fungi (chitin), absent in animals
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Key Terms and Definitions

Plasma Membrane: The cell's outer boundary, composed of a phospholipid bilayer with proteins.
Cytosol: The fluid component of the cytoplasm.
Nucleoid: Region in prokaryotic cells where DNA is located.
Nucleus: Membrane-bound organelle in eukaryotes containing DNA.
Chromatin: DNA-protein complex that makes up chromosomes.
Nucleolus: Site of rRNA synthesis and ribosome assembly within the nucleus.
Ribosome: Molecular machine that synthesizes proteins from mRNA templates.

Additional info:

Some details about the cytoskeleton, endomembrane system, and organelles (e.g., mitochondria, chloroplasts) are not included in the provided images but are essential for a complete understanding of cell structure. These topics are typically covered in subsequent sections of a cell biology unit.