Eukaryotic Cells: Internal Membranes and Compartmentalization

Concept 4.2: Eukaryotic Cells Have Internal Membranes That Compartmentalize Their Functions

Introduction to Cell Types

Cells are the basic structural and functional units of all living organisms. There are two primary cell types: prokaryotic and eukaryotic. Organisms in the domains Bacteria and Archaea consist of prokaryotic cells, while protists, fungi, animals, and plants are composed of eukaryotic cells.

• Prokaryotic cells: Lack a true nucleus and most internal membrane-bound organelles.

• Eukaryotic cells: Possess a true nucleus and extensive internal membranes that compartmentalize cellular functions.

Comparing Prokaryotic and Eukaryotic Cellsn

Basic Features of All Cells

Despite differences, all cells share certain fundamental features:

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

• Cytosol: A jellylike substance inside the cell in which subcellular components are suspended.

• Chromosomes: Structures carrying genetic information in the form of DNA.

• Ribosomes: Complexes that synthesize proteins according to genetic instructions.

Key Differences Between Prokaryotic and Eukaryotic Cells

• Nucleus: Eukaryotic cells have most of their DNA enclosed within a double-membrane-bound nucleus. Prokaryotic cells have DNA concentrated in a region called the nucleoid, which is not membrane-bound.

• Internal Membranes: Eukaryotic cells contain numerous membrane-bound organelles, allowing compartmentalization of functions. Prokaryotic cells lack these internal compartments.

• Size: Eukaryotic cells are generally much larger (10–100 μm in diameter) than prokaryotic cells (0.1–5 μm in diameter).

Table: Comparison of Prokaryotic and Eukaryotic Cells

Feature	Prokaryotic Cell	Eukaryotic Cell
Nucleus	Absent (DNA in nucleoid)	Present (DNA in nucleus)
Membrane-bound organelles	Absent	Present
Size	0.1–5 μm	10–100 μm
Examples	Bacteria, Archaea	Protists, fungi, plants, animals

Cellular Structures and Their Functions

Plasma Membrane

The plasma membrane is a double layer of phospholipids with embedded proteins, serving as a selective barrier that regulates the passage of substances into and out of the cell.

• Phospholipid bilayer: Provides fluidity and flexibility.

• Proteins: Serve as channels, receptors, and enzymes.

• Carbohydrate side chains: Involved in cell recognition.

Surface Area to Volume Ratio

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

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

• Smaller cells have a higher surface area-to-volume ratio, facilitating efficient exchange of materials.

Formula:

• For a cube of side length s:

$\text{Surface Area} = 6s^2$

$\text{Volume} = s^3$

$\text{Surface Area to Volume Ratio} = \frac{6s^2}{s^3} = \frac{6}{s}$

Table: Geometric Relationships Between Surface Area and Volume

Cube Side (μm)	Surface Area (μm2)	Volume (μm3)	Surface Area/Volume Ratio
1	6	1	6.0
2	24	8	3.0
4	96	64	1.5

Internal Organization of Eukaryotic Cells

Compartmentalization by Internal Membranes

Eukaryotic cells possess an extensive system of internal membranes that divide the cell into compartments, each with specialized functions. These compartments are called organelles.

• Nucleus: Contains most of the cell's DNA and is the site of DNA replication and transcription.

• Endoplasmic Reticulum (ER): Synthesizes proteins and lipids; rough ER is studded with ribosomes, smooth ER lacks ribosomes.

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

• Mitochondria: Sites of cellular respiration and energy (ATP) production.

• Lysosomes: Contain digestive enzymes for breaking down macromolecules.

• Peroxisomes: Carry out oxidation reactions and detoxify harmful substances.

• Cytoskeleton: Provides structural support, cell shape, and facilitates movement.

Example: Generalized Animal Cell

The diagram below (see Figure 4.7 in the source) illustrates the major organelles and their functions in a typical animal cell.

• Nucleus: Contains genetic material.

• Endoplasmic Reticulum: Protein and lipid synthesis.

• Golgi Apparatus: Protein modification and sorting.

• Mitochondria: Energy production.

• Lysosomes: Intracellular digestion.

• Plasma Membrane: Selective barrier.

• Cytoskeleton: Cell structure and movement.

Summary Table: Major Organelles in Eukaryotic Cells

Organelle	Main Function
Nucleus	Stores genetic material; site of DNA replication and transcription
Endoplasmic Reticulum (ER)	Protein and lipid synthesis
Golgi Apparatus	Modification, sorting, and packaging of proteins and lipids
Mitochondria	ATP production via cellular respiration
Lysosomes	Digestion of macromolecules
Peroxisomes	Oxidation reactions; detoxification
Cytoskeleton	Structural support and movement

Conclusion

Eukaryotic cells are distinguished by their internal membrane systems, which allow for compartmentalization of cellular processes. This organization enables eukaryotic cells to carry out complex and specialized functions efficiently, supporting the diversity of life forms in the domain Eukarya.