Structure and Function of Eukaryotic Cells

Eukaryotic Cells

Overview of Eukaryotic Cells

Eukaryotic cells are complex cells characterized by the presence of membrane-bound organelles, including a nucleus. They are found in animals, plants, fungi, and protists. The compartmentalization of functions within organelles allows for greater efficiency and specialization.

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Membrane-bound organelles: Specialized structures with unique functions, such as mitochondria, endoplasmic reticulum, and Golgi apparatus.

• Other structures: Cytoskeleton, ribosomes, and extracellular components.

Membrane-Bound Organelles

Organelles of the Endomembrane System

The endomembrane system is a group of organelles involved in the synthesis, modification, and transport of proteins and lipids. These organelles are either continuous with each other or connected via vesicles.

• Nucleus: Stores genetic information and is surrounded by a double membrane called the nuclear envelope.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid synthesis, detoxification, and calcium storage.

• Rough Endoplasmic Reticulum (RER): Studded with ribosomes; synthesizes proteins destined for membranes, secretion, or specific organelles.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for delivery to targeted destinations.

• Lysosomes: Contain hydrolytic enzymes for digestion of macromolecules and recycling of cellular components.

• Vacuoles: Large vesicles for storage and maintenance of cell turgor (especially in plants).

Key Features:

• Membranes of these organelles are derived from the nuclear envelope.

• Transport between organelles is facilitated by vesicles.

Other Membrane-Bound Organelles

• Mitochondria: Sites of cellular respiration and ATP production.

• Peroxisomes: Involved in the breakdown of fatty acids and detoxification of harmful substances.

• Chloroplasts: (In plants and algae) Sites of photosynthesis.

Other Cell Structures

• Ribosomes: Complexes of rRNA and protein; sites of protein synthesis. Found free in cytosol or bound to RER.

• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) that provide structural support, cell shape, and movement.

• Extracellular Structures: Include cell wall (in plants), extracellular matrix (in animals), and other components outside the plasma membrane.

Comparing Animal and Plant Cells

While animal and plant cells share many organelles, they also have unique structures and functions.

Feature	Animal Cell	Plant Cell
Cell Wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present
Central Vacuole	Small or absent	Large, central
Lysosomes	Present	Rare (functionally similar organelles exist)
Shape	Round/irregular	Rectangular/fixed

Prokaryotic vs. Eukaryotic Cells

Prokaryotic cells (bacteria and archaea) lack membrane-bound organelles and a nucleus. Eukaryotic cells have a nucleus and many organelles.

• Structures found in prokaryotic cells:

  • DNA (not enclosed in a nucleus)

  • Ribosomes

  • Cell membrane

  • Cytoplasm

• Structures absent in prokaryotes: Nucleus, membrane-bound organelles

The Nucleus: Information Central

The nucleus is the most prominent organelle in eukaryotic cells and serves as the repository for genetic information.

• Nuclear Envelope: Double membrane that separates the nucleus from the cytoplasm; contains nuclear pores for molecular transport.

• Chromatin: DNA and associated proteins (histones); condenses to form chromosomes during cell division.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Nuclear Lamina: Protein network that maintains nuclear shape.

Ribosomes: Protein Factories

Ribosomes are molecular machines that synthesize proteins by translating messenger RNA (mRNA) into polypeptide chains.

• Free ribosomes: Float in the cytosol; synthesize proteins for use within the cell.

• Bound ribosomes: Attached to the rough ER or nuclear envelope; synthesize proteins for secretion, insertion into membranes, or use in lysosomes.

Translation: The process by which ribosomes convert the nucleotide sequence of mRNA into the amino acid sequence of a protein.

The Endomembrane System

The endomembrane system coordinates the synthesis, modification, and transport of cellular products.

• Nuclear envelope

• Endoplasmic reticulum (ER): Smooth and rough regions with distinct functions.

• Golgi apparatus

• Lysosomes

• Vacuoles

• Plasma membrane

These components are connected directly or via vesicle transport.

Endoplasmic Reticulum (ER)

• Smooth ER: Synthesizes lipids, metabolizes carbohydrates, detoxifies drugs/poisons, stores calcium ions.

• Rough ER: Studded with ribosomes; synthesizes proteins for secretion or membrane insertion.

Golgi Apparatus

• Consists of flattened membranous sacs (cisternae).

• Modifies, sorts, and packages proteins and lipids from the ER.

• Produces certain macromolecules and forms transport vesicles.

Lysosomes

• Membranous sacs containing hydrolytic enzymes for digestion.

• Break down macromolecules, old organelles (autophagy), and engulfed particles (phagocytosis).

• Enzymes function best at acidic pH.

Vacuoles

• Large vesicles derived from the ER and Golgi apparatus.

• Types:

  • Food vacuoles: Formed by phagocytosis.

  • Contractile vacuoles: Expel excess water (in protists).

  • Central vacuole: In plants, stores ions, nutrients, and waste; maintains turgor pressure.

Summary Table: Major Eukaryotic Cell Organelles and Functions

Organelle	Main Function
Nucleus	Stores genetic material; controls cell activities
Ribosome	Protein synthesis
Rough ER	Protein synthesis and modification
Smooth ER	Lipid synthesis, detoxification, calcium storage
Golgi Apparatus	Modification, sorting, and packaging of proteins/lipids
Lysosome	Digestion and recycling of cellular materials
Vacuole	Storage, waste disposal, turgor maintenance (plants)
Mitochondrion	ATP production via cellular respiration
Chloroplast	Photosynthesis (plants and algae)
Peroxisome	Breakdown of fatty acids, detoxification
Cytoskeleton	Structural support, movement, intracellular transport

Key Terms and Definitions

• Organelle: Specialized subunit within a cell with a specific function, usually membrane-bound.

• Vesicle: Small, membrane-bound sac for transport within cells.

• Phagocytosis: Cellular process of engulfing solid particles.

• Autophagy: Process by which cells recycle their own components.

• Turgor Pressure: Pressure exerted by the central vacuole against the cell wall in plants, maintaining rigidity.

Example: Protein Synthesis and Trafficking

1. DNA in the nucleus is transcribed into mRNA.

2. mRNA exits the nucleus via nuclear pores.

3. Ribosomes translate mRNA into protein (translation).

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

5. Final products are delivered to their destinations via vesicles.

Relevant Equations

• Central Dogma of Molecular Biology:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

• Surface Area to Volume Ratio (important for cell size):

$\text{Surface Area to Volume Ratio} = \frac{\text{Surface Area}}{\text{Volume}}$

Additional info: The notes have been expanded to include definitions, examples, and context for clarity and completeness.