Prokaryotic and Eukaryotic Cell Structure, Morphology, and Function

Cell Morphology and Classification

Morphology

Cell morphology refers to the size, shape, colour, pattern, and outward appearance of an organism. It is a key aspect in analysing and describing cells, especially when distinguishing between prokaryotic and eukaryotic cells.

• Definition: Morphology is the study of the form and structure of organisms and their specific structural features.

• Application: Used to classify and identify microorganisms based on their appearance under a microscope.

Prokaryotic Cell Structure & External Morphology

Prokaryotic cells are characterized by the absence of a membrane-bound nucleus and other organelles typically found in eukaryotic cells.

• No membrane-bound nucleus: Genetic material is found in a nucleoid region.

• Lack of membrane-bound organelles: No mitochondria, endoplasmic reticulum, or Golgi apparatus.

• Cell wall: Most prokaryotes have a rigid cell wall for structural support.

Types of Prokaryotes (by Shape)

Prokaryotes can be classified by their shape, which is a key morphological feature.

• Cocci: Spherical-shaped cells.

• Bacilli: Rod-shaped cells.

• Spirillum: Rigid spiral-shaped cells.

• Spirochete: Flexible spiral-shaped cells.

• Vibrio: Comma-shaped cells.

Motile Meaning

Motility refers to the ability of an organism or cell to move independently using metabolic energy.

• Motile: Able to move on their own, often using structures such as flagella, cilia, or a flexible body shape.

• Importance: Motility allows cells to move toward nutrients or away from harmful substances.

Eukaryotic Cell Structure and Function

Eukaryotes

Eukaryotes include plants, animals, and fungi. Their cells are more complex than prokaryotes, with a nucleus and membrane-bound organelles.

• Nucleus: Contains genetic material enclosed by a nuclear envelope.

• Membrane-bound organelles: Includes mitochondria, endoplasmic reticulum, Golgi apparatus, etc.

Fungi Cells

Fungi are eukaryotic organisms with unique cell wall composition.

• Cell wall: Contains polysaccharide chitin, distinguishing them from plant and animal cells.

• Similarity: Fungi cells are more similar to animal cells than to plant cells.

Animal Cells

Animal cells are heterotrophic, lack a cell wall, and have a flexible structure.

• Heterotrophic: Obtain energy by consuming other organisms.

• Structure: Can be rigid (e.g., calcified bone) or flexible (e.g., neutrophils).

• Shape: May be brick-shaped (epithelial cells) or variable.

Nucleus

The nucleus is a defining feature of eukaryotic cells, serving as the control center for genetic information.

• Membrane-bound: Surrounded by a double membrane (nuclear envelope) with nuclear pores.

• Contents: Contains nucleoplasm, nucleolus, and chromatin.

• Size: Typically about 5 μm in diameter.

Chromatin

Chromatin is the complex of DNA and proteins (mainly histones) that forms chromosomes.

• Heterochromatin: Densely packed, transcriptionally inactive.

• Euchromatin: Loosely packed, transcriptionally active.

• Chromosomes: During cell division, chromatin condenses into visible chromosomes.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system.

• DNA → RNA → Protein: Genetic information is stored as DNA, transcribed into RNA, and translated into protein.

Transcription

Transcription is the process of synthesizing messenger RNA (mRNA) from a DNA template.

• Enzyme involved: RNA polymerase.

• Product: mRNA, which carries genetic information from DNA to the ribosome.

Translation

Translation is the process by which the sequence of an mRNA molecule is used to assemble amino acids into a polypeptide (protein).

• Occurs at: Ribosomes in the cytoplasm.

• tRNA: Transfer RNA brings amino acids to the ribosome according to the mRNA code.

Watson–Crick Base Pairing

Watson–Crick base pairing refers to the specific hydrogen bonding between nucleotide bases in DNA.

• Adenine (A) pairs with Thymine (T) using 2 hydrogen bonds.

• Guanine (G) pairs with Cytosine (C) using 3 hydrogen bonds.

Base	Pairs With	Number of Hydrogen Bonds
Adenine (A)	Thymine (T)	2
Guanine (G)	Cytosine (C)	3

Summary Table: Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present (membrane-bound)
Organelles	Absent	Present (e.g., mitochondria, ER)
Cell Wall	Usually present (peptidoglycan)	Present in plants/fungi (cellulose/chitin), absent in animals
Size	Small (1–10 μm)	Larger (10–100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Key Terms and Definitions

• Prokaryote: An organism whose cells lack a nucleus and other membrane-bound organelles.

• Eukaryote: An organism whose cells contain a nucleus and membrane-bound organelles.

• Chromatin: The complex of DNA and proteins that forms chromosomes within the nucleus.

• Transcription: The synthesis of RNA from a DNA template.

• Translation: The synthesis of protein from an mRNA template.

• Motile: Capable of movement.

Important Equations

• Central Dogma:

$\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$

• Watson–Crick Base Pairing:

$\text{A} = \text{T}$ $\text{G} = \text{C}$

Example

Example: Escherichia coli is a rod-shaped (bacillus) prokaryote that moves using flagella. Human epithelial cells are eukaryotic, have a membrane-bound nucleus, and can be brick-shaped.