4-Cell Organelles: Structure and Function

Cell Types

Prokaryotic and Eukaryotic Cells

Cells are classified into two main groups based on the presence or absence of a nucleus and other membrane-bound organelles.

• Prokaryotes: Lack a nucleus; include Archaea and Bacteria.

• Eukaryotes: Possess a nucleus; include Protista, Fungi, Plantae, and Animalia.

Prokaryote	Eukaryote
Archaea, Bacteria	Protista, Fungi, Plantae, Animalia

Prokaryotic vs Eukaryotic Cells

These cell types differ in several key aspects:

	Prokaryotes	Eukaryotes
DNA	Naked, circular	Bound to protein, linear
Organelles	No membrane-bound organelles	Membrane-bound organelles
Ribosomes	70S	80S
Average size	Small (1–5 μm)	Large (10–100 μm)
Reproduction	Binary fission, typically haploid	Mitosis and meiosis, typically diploid

Organelles

Definition and Examples

Organelles are specialized sub-structures within cells that perform specific functions. Prokaryotic cells do not possess membrane-bound organelles, while eukaryotic cells do.

• All cells: Ribosomes, cytoskeleton

• Eukaryotes: Nucleus, endoplasmic reticulum (ER), Golgi complex, mitochondria, peroxisome, centrosome, vacuoles, nucleolus, vesicles

• Specialized cells: Chloroplasts (plants only), lysosomes (animals only)

Plant vs Animal Cells

Comparative Features

Plant Cells	Animal Cells
Have plastids (chloroplast)	Do not have plastids
Have a cell wall (cellulose)	Do not have a cell wall
Large, central vacuole	Small vacuoles (if any)
May have plasmodesmata	Do not have plasmodesmata
No centrioles	Centrioles in centrosome
No cholesterol in membrane	Cholesterol in cell membrane
Store excess glucose as starch	Store excess glucose as glycogen

Types of Organelles

Major Organelles and Their Functions

• Ribosome: Site of protein synthesis

• Nucleus: Stores genetic material

• Golgi complex: Exports secretory products

• Endoplasmic reticulum: Intracellular transport

• Mitochondria: Site of aerobic respiration

• Peroxisome: Digests toxic substances

• Centrosome: Microtubule assembly

• Nucleolus: Ribosome assembly

• Vacuole: Fluid-filled internal cavity

• Vesicle: Temporary storage

• Lysosome (animal cells): Site of material digestion

• Chloroplast (plant cells): Site of photosynthesis

The Nucleus

General Features

The nucleus is a large, membrane-bound organelle found in eukaryotic cells. It contains most of the cell's genetic material and is often referred to as the "brain" of the cell. The nucleus averages about 5 μm in diameter and is the most visible organelle in the cell.

• Genetic material is made up of nucleic acids (DNA and RNA).

• Most cells have a single nucleus, with exceptions such as skeletal muscle cells (multiple nuclei) and mature red blood cells (no nucleus).

Functions of the Nucleus

The nucleus controls all cellular activities through three key functions:

• Replication: Copying of DNA during the cell cycle, essential for cell growth and development.

• Transcription: Production of messenger RNA (mRNA) using DNA as a template.

• Translation: mRNA is transported to the cytoplasm for protein synthesis.

Structure of the Nucleus

• Nuclear membrane: Double-layered structure (inner and outer membranes) that encloses the nucleus.

• Nuclear pore: Complexes that allow selective passage of materials between the nucleus and cytoplasm.

• Nucleolus: Prominent region within the nucleus, site of ribosome assembly, lacks a membrane.

• Chromatin: DNA-protein complex, organized as heterochromatin (condensed, inactive) and euchromatin (threadlike, active).

• Nucleoplasm: Highly viscous liquid surrounding chromatin and nucleolus, contains proteins, nucleotides, and enzymes.

Nuclear Envelope

The nuclear envelope is a double membrane that encloses the nucleus, separating its contents from the cytoplasm. It contains many nuclear pores that regulate the movement of molecules, including mRNA, into and out of the nucleus.

Nuclear Pore

• Composed of 8 subunits forming a ring (15–20 nm diameter).

• Each subunit projects a spoke-like unit into the center, resembling a wheel with 8 spokes.

• Central "plug" inside the pore regulates transport.

Nucleolus

• Made of protein and ribosomal DNA (rDNA).

• Site of RNA transcription and ribosome assembly.

• No membrane.

• Some cells (e.g., Paramecium) have more than one nucleolus.

Number of Nucleoli

• Nucleoli increase in number and size in non-dividing cells, indicating active protein synthesis.

• Nucleoli disappear during cell division and reform afterward.

Chromatin

• Genomic DNA with proteins, found in the nucleus (eukaryotes) or nucleoid (prokaryotes).

• Packages DNA into a smaller volume to fit into the cell.

• DNA is tightly packed around histone proteins in structures called nucleosomes.

• Nucleosomes are further packaged into chromosomes, visible during mitosis.

The Genetic Material

• Genetic material of eukaryotes is deoxyribonucleic acid (DNA).

• Chromatin is made up of DNA and proteins (mainly histones).

• Chromatin organization allows large amounts of DNA to be compacted in the nucleus.

• If stretched, DNA from one human cell would be about 1 m long.

Types of Chromatin

• Heterochromatin: Condensed, inactive form; appears as dense patches, sometimes lining the nuclear membrane.

• Euchromatin: Threadlike, delicate, active form; most abundant in active cells.

Chromosomes

Chromosome condensation is a visible change in nuclear structure during cell division. Interphase chromatin condenses approximately 1000x further to form compact chromosomes, which are easily seen under a microscope.

Nucleoplasm

• Similar to cytoplasm but found in the nucleus; also called nuclear sap.

• Highly viscous liquid surrounding chromatin and nucleolus.

• Contains protein fibers, nucleotides, and enzymes for nuclear functions.

Ribosomes

Definition and Composition

Ribosomes are tiny functional units composed of approximately 60% ribosomal RNA (rRNA) and 40% protein. They are not bound by a membrane and are much smaller than other organelles.

• Found in both prokaryotic and eukaryotic cells.

• Eukaryotic ribosomes are produced and assembled in the nucleolus.

Structure of Ribosomes

• Composed of two subunits: large and small.

• Svedberg (S) values indicate sedimentation rate: eukaryotic ribosomes are 80S (40S + 60S), prokaryotic ribosomes are 70S (30S + 50S).

• Svedberg values are not additive due to dependence on size, shape, and molecular weight.

Location of Ribosomes

• Can be free in the cytosol or membrane-bound (attached to nuclear envelope or endoplasmic reticulum).

• Also found inside organelles such as mitochondria and chloroplasts.

• Eukaryotic ribosomes are slightly larger than prokaryotic ones.

Function of Ribosomes

• Primary function is to synthesize proteins.

• Ribosomal proteins enter the nucleolus and combine with rRNA to form subunits.

• Subunits leave the nucleus and unite in the cytoplasm for protein synthesis.

• When not producing proteins, subunits remain separated.

Free and Bound Ribosomes

• Free ribosomes synthesize proteins for use within the cytosol.

• Bound ribosomes (attached to ER) synthesize proteins for secretion or for use in membranes and organelles.

Endoplasmic Reticulum (ER)

General Structure and Function

The endoplasmic reticulum (ER) is a network of folded membranes forming channels. It is responsible for the synthesis, modification, and transport of proteins and lipids.

• Consists of smooth ER and rough ER.

• Responsible for moving proteins and carbohydrates to the Golgi apparatus, lysosomes, and other locations.

• ER is extensive and continuous with the nuclear envelope.

• Contains many folds; its membrane forms a single sheet enclosing the ER lumen.

• Rough and smooth ER are usually interconnected.

Two Forms of ER

• Rough/Granular ER: Has ribosomes attached; involved in protein synthesis and secretion.

• Smooth/Agranular ER: No ribosomes attached; involved in lipid synthesis, hormone production, carbohydrate metabolism, and detoxification.

Location of ER

The ER is located adjacent to the nucleus and is often seen as an extension of the nuclear membrane. It is closely associated with the Golgi apparatus and other organelles.

Summary Table: Key Organelles and Functions

Organelle	Main Function
Nucleus	Stores genetic material, controls cell activities
Nucleolus	Ribosome assembly
Ribosome	Protein synthesis
Endoplasmic Reticulum	Protein and lipid synthesis, transport
Golgi Complex	Modification and export of secretory products
Mitochondria	Aerobic respiration, energy production
Peroxisome	Digestion of toxic substances
Centrosome	Microtubule assembly
Vacuole	Storage and structural support (plants)
Vesicle	Temporary storage and transport
Lysosome	Material digestion (animals)
Chloroplast	Photosynthesis (plants)

Key Equations and Concepts

• DNA Packaging: DNA wraps around histone proteins to form nucleosomes, which further coil to form chromatin and chromosomes.

• Central Dogma of Molecular Biology:

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

Additional info: The notes cover foundational cell biology topics including cell types, organelle structure and function, and molecular processes relevant to eukaryotic and prokaryotic cells. These concepts are essential for understanding cell organization, genetic material management, and protein synthesis in college-level cell biology.