Eukaryotic Cell Structure and Function

Eukaryotic Cells

General Characteristics

Eukaryotic cells are a fundamental cell type found in the domain Eukarya. They are distinguished from prokaryotic cells by their larger size, structural complexity, and the presence of membrane-bound organelles.

• Domain: Eukarya

• Size: Typically 10 micrometers to 1 millimeter in diameter

• Complexity: Contain a nucleus, organelles, and internal membranes (lipid bilayers)

• Cellularity: May be single-celled (e.g., yeast) or multicellular (e.g., plants, animals, fungi)

Nucleus

Structure and Function

The nucleus is the defining organelle of eukaryotic cells, responsible for housing genetic material and coordinating cellular activities such as growth, metabolism, and reproduction.

• Enclosed by a double membrane: The nuclear envelope consists of two phospholipid bilayers with nuclear pores for molecular transport.

• Genetic Material: Contains linear DNA organized into multiple chromosomes.

• Nucleolus: Dense region within the nucleus responsible for ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Ribosomes: Partially assembled in the nucleolus before being exported to the cytoplasm.

Example: In human cells, the nucleus contains 46 chromosomes (23 pairs), each composed of linear DNA and associated proteins.

Eukaryotic Cytoplasmic Membrane

Composition and Functions

The cytoplasmic (plasma) membrane is a selectively permeable barrier that encloses the cell, maintaining homeostasis and mediating communication with the environment.

• Phospholipid bilayer: Forms the basic structural framework.

• Proteins: Embedded or associated with the membrane, functioning as channels, receptors, or enzymes.

• Glycolipids and Glycoproteins: Molecules with attached sugar groups (glyco- = "sugar").

  • Adhesins: Mediate cell-cell or cell-surface attachment.

  • Cell markers: Involved in cell recognition and immune response.

• Sterols: (e.g., cholesterol) Provide membrane fluidity and stability.

Organelles

Types and Functions

Organelles are specialized compartments within eukaryotic cells, each performing distinct functions. They are classified as either membranous or non-membranous.

• Membranous organelles: Surrounded by lipid membranes. Examples include:

  • Mitochondria

  • Lysosomes

  • Endoplasmic reticulum and Golgi apparatus (not explicitly listed but commonly included)

• Non-membranous organelles: Not surrounded by membranes. Examples include:

  • Cytoskeleton

  • Centrioles

Mitochondrion

Structure and Function

The mitochondrion is the primary site of ATP (adenosine triphosphate) production in eukaryotic cells, often referred to as the "powerhouse of the cell." It also participates in other metabolic processes.

• Double membrane: Outer and inner membranes create distinct compartments.

• Contains its own DNA: Circular DNA, similar to prokaryotes.

• ATP generation: Through cellular respiration.

• Other metabolic functions: Involved in apoptosis and certain biosynthetic pathways.

Equation: The general equation for aerobic respiration in mitochondria is:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Other Membranous Organelles

Lysosomes and Vesicles

Lysosomes are membrane-bound organelles containing digestive enzymes that break down macromolecules, cellular debris, and foreign substances. Vesicles are small membrane-bound sacs involved in storage, transport, and digestion.

• Lysosome functions:

  • Degradation of imported material

  • Role in apoptosis (programmed cell death)

• Vesicle functions:

  • Storage and transport of substances within the cell

  • Digestion when fused with lysosomes

Example: White blood cells use lysosomes to digest engulfed bacteria.

Ribosomes

Structure and Function

Ribosomes are non-membranous organelles composed of protein and RNA. They are the sites of protein synthesis, translating genetic information from mRNA into polypeptides.

• Location: Free in the cytoplasm or bound to the rough endoplasmic reticulum

• Structure: Larger and more complex than prokaryotic ribosomes

• Function: Translate mRNA into proteins

Equation: General reaction for protein synthesis:

$\text{mRNA} + \text{tRNA} + \text{Amino acids} \xrightarrow{\text{Ribosome}} \text{Polypeptide chain}$

Cytoskeleton and Centrioles

Structure and Function

The cytoskeleton is a network of protein filaments that provides structural support, maintains cell shape, and enables movement. Centrioles are cylindrical structures involved in organizing microtubules during cell division.

• Cytoskeleton components:

  • Microtubules

  • Microfilaments

  • Intermediate filaments (not explicitly listed but commonly included)

• Functions:

  • Provides shape and structural strength

  • Facilitates movement of cell contents and the cell itself (e.g., amoeboid movement)

• Centrioles:

  • Form spindle fibers to separate chromosomes during mitosis and meiosis

  • Sometimes involved in motility (cilia and flagella)

Eukaryotic Locomotion

Mechanisms of Movement

Eukaryotic cells can move using specialized structures such as flagella, cilia, or pseudopods. These structures are composed of cytoskeletal elements and are essential for motility in many unicellular and multicellular organisms.

• Flagella: Long, whip-like structures (one to a few per cell) used for movement; composed of microtubules with a centriole base.

• Cilia: Short, hair-like structures (usually many per cell) that beat in coordinated waves for movement or to move substances across the cell surface.

• Pseudopods: Temporary, foot-like extensions of the cytoplasm used for amoeboid movement; formed by cytoskeletal restructuring.

Example: Amoeba moves using pseudopods; human respiratory tract cells use cilia to move mucus.

Cell Wall and Glycocalyx

Structural Features

Some eukaryotic cells possess a cell wall or a glycocalyx for additional protection and structural support.

• Cell wall: Found in plants (cellulose), fungi (chitin), and some protists; provides rigidity and protection.

• Glycocalyx: A sticky, sugar-rich layer outside the plasma membrane; involved in cell recognition, adhesion, and protection.

Comparison Table: Eukaryotic vs. Prokaryotic Cell Features

Feature	Eukaryotes	Prokaryotes
Nucleus	Present (membrane-bound)	Absent
DNA	Linear, multiple chromosomes	Circular, single chromosome
Organelles	Membrane-bound (e.g., mitochondria)	Absent
Cell Wall	Present in some (composition varies)	Present (peptidoglycan)
Ribosomes	80S (larger)	70S (smaller)
Size	10–100 μm	0.1–5 μm

Selective Toxicity

Basis for Antimicrobial Therapy

The biochemical and structural differences between eukaryotic and prokaryotic cells are exploited in medicine to achieve selective toxicity—the ability of a drug to target pathogens without harming the host.

• Ribosomes: Antibiotics can target bacterial (prokaryotic) ribosomes without affecting eukaryotic ribosomes.

• Cell walls: Many antibiotics target peptidoglycan in bacterial cell walls, which is absent in eukaryotes.

• Other targets: Differences in metabolic pathways, membrane composition, etc.

Example: Penicillin inhibits bacterial cell wall synthesis but has no effect on human cells.