Organelle and Cellular Structure Study Guide

Organelle and Cellular Structure

Definition and Classification of Organelles

Organelles are specialized structures within cells that perform distinct processes necessary for cellular function. They are typically membrane-bound in eukaryotic cells, but some non-membrane-bound structures are also considered organelles.

• Organelle: A subcellular structure with a specific function, often surrounded by a membrane (e.g., nucleus, mitochondria).

• Membrane-bound organelles: Organelles surrounded by lipid bilayer membranes, found mainly in eukaryotic cells.

• Non-membrane-bound organelles: Structures such as ribosomes, which lack a surrounding membrane.

• Example (plant cells only): Chloroplast is found only in plant cells and is responsible for photosynthesis.

• Example (animal cells only): Lysosome is typically found only in animal cells and is involved in digestion of cellular waste.

• Example (plant, animal, and prokaryotic cells): Ribosomes are found in all cell types and are responsible for protein synthesis.

Endosymbiont Theory

The Endosymbiont Theory explains the origin of certain organelles in eukaryotic cells, particularly mitochondria and chloroplasts. It proposes that these organelles originated as free-living prokaryotes that were engulfed by ancestral eukaryotic cells, forming a symbiotic relationship.

• Key Points:

  • Mitochondria and chloroplasts have their own DNA, similar to bacterial DNA.

  • They replicate independently of the cell's nucleus.

  • Both have double membranes, consistent with engulfment.

• Example: Mitochondria are thought to have evolved from aerobic bacteria, while chloroplasts evolved from photosynthetic bacteria.

Cytosol vs. Cytoplasm

The terms cytosol and cytoplasm are often used interchangeably, but they refer to different components of the cell's interior.

• Cytosol: The fluid portion of the cell's interior, excluding organelles and other insoluble components.

• Cytoplasm: The entire contents within the cell membrane, excluding the nucleus; includes cytosol and organelles.

• Comparison: Cytosol is a part of the cytoplasm.

Organelles Shared by Prokaryotes and Eukaryotes

Some cellular components are found in both prokaryotic and eukaryotic cells, reflecting their fundamental roles in cell biology.

• Ribosomes: Present in all cells; responsible for protein synthesis.

• Plasma membrane: Surrounds all cells, controlling the movement of substances in and out.

• Cytoplasm: The internal fluid and structures (excluding the nucleus in eukaryotes).

Organelles Unique to Plant or Animal Cells

Some organelles are unique to either plant or animal cells, reflecting their specialized functions.

• Found in plant cells but not animal cells:

  • Chloroplasts: Site of photosynthesis.

  • Cell wall: Provides structural support; composed mainly of cellulose.

  • Large central vacuole: Stores water, nutrients, and waste products.

• Found in animal cells but not plant cells:

  • Lysosomes: Contain digestive enzymes for breaking down waste.

  • Centrioles: Involved in cell division.

Structural Differences and Similarities: Prokaryotic vs. Eukaryotic DNA

DNA organization and structure differ significantly between prokaryotes and eukaryotes, as well as within organelles such as mitochondria and chloroplasts.

• Prokaryotic DNA:

  • Usually a single, circular DNA molecule located in the nucleoid region.

  • Lacks histone proteins (except in some Archaea).

  • No membrane-bound nucleus.

• Eukaryotic DNA:

  • Multiple, linear chromosomes contained within a membrane-bound nucleus.

  • Associated with histone proteins.

• Mitochondrial and Chloroplast DNA:

  • Circular DNA, similar to prokaryotic DNA.

  • Replicates independently of nuclear DNA.

  • Supports the Endosymbiont Theory.

Comparison Table: Prokaryotic vs. Eukaryotic DNA

Feature	Prokaryotic DNA	Eukaryotic DNA	Mitochondrial/Chloroplast DNA
Shape	Circular	Linear	Circular
Location	Nucleoid (no membrane)	Nucleus (membrane-bound)	Mitochondria/Chloroplasts
Associated Proteins	Few or none (some Archaea have histones)	Histones	Few or none
Number of Copies	Usually one	Multiple	Multiple per organelle

Cellular Components in Prokaryotes and Eukaryotes

Prokaryotic Cellular Components

Prokaryotic cells lack membrane-bound organelles but contain several essential structures.

Organelle/Component	Structure	Function
Nucleoid	Region containing circular DNA, not membrane-bound	Stores genetic information
Ribosomes	Small complexes of RNA and protein (70S type)	Protein synthesis
Cytosol and Cytoplasm	Gel-like fluid inside the cell	Site of metabolic reactions
Fimbriae	Short, hair-like projections	Attachment to surfaces
Plasma Membrane	Phospholipid bilayer	Regulates entry/exit of substances
Cell Wall	Peptidoglycan layer (in bacteria)	Structural support and protection
Glycocalyx	Polysaccharide or protein coating	Protection, adhesion
Flagella	Long, whip-like appendages	Cell movement

Eukaryotic Cellular Components

Eukaryotic cells contain a variety of membrane-bound organelles, each with specialized functions.

Organelle/Component	Structure	Function	Special Features
Nucleus	Double membrane, contains DNA	Stores genetic information, controls cell activities	Contains nucleolus, nuclear pores
Nucleolus	Dense region within nucleus	Ribosome synthesis	Not membrane-bound
Ribosomes	RNA and protein complexes (80S type)	Protein synthesis	Free or bound to ER
Golgi Apparatus	Stack of flattened membranes	Modifies, sorts, and packages proteins	Vesicle formation
Centrosome (includes centrioles)	Microtubule organizing center	Cell division, spindle formation	Centrioles present in animal cells
Lysosomes	Membrane-bound vesicles with enzymes	Digestion of macromolecules	Common in animal cells
Peroxisomes	Membrane-bound vesicles	Breakdown of fatty acids, detoxification	Contains catalase enzyme
Cell Wall	Rigid structure outside plasma membrane	Support and protection	Present in plants, fungi, some protists
Cytoskeleton	Network of protein filaments	Cell shape, movement, transport	Includes microtubules, microfilaments, intermediate filaments
Vacuole	Large membrane-bound sac	Storage of substances	Large central vacuole in plants
Vesicle	Small membrane-bound sac	Transport within cell	Various types
Chloroplast	Double membrane, contains thylakoids	Photosynthesis	Own DNA, found in plants/algae
Mitochondria	Double membrane, inner folds (cristae)	ATP production (cellular respiration)	Own DNA, found in all eukaryotes
Smooth Endoplasmic Reticulum	Network of membranes, lacks ribosomes	Lipid synthesis, detoxification	Continuous with rough ER
Rough Endoplasmic Reticulum	Network of membranes with ribosomes	Protein synthesis and modification	Continuous with nuclear envelope
Plasma Membrane	Phospholipid bilayer with proteins	Regulates entry/exit of substances	Present in all cells

Key Equations and Concepts

• Surface Area to Volume Ratio: Important for understanding cell size limitations.

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

• Endosymbiont Theory Evidence: DNA sequence similarity, double membranes, independent replication.

Summary Table: Organelles in Plant vs. Animal Cells

Organelle	Plant Cells	Animal Cells
Chloroplast	Yes	No
Cell Wall	Yes	No
Large Central Vacuole	Yes	No (small vacuoles may be present)
Lysosome	No (rare)	Yes
Centrioles	No	Yes

Additional info: Some content, such as detailed structure and function of each organelle, was inferred and expanded for completeness and academic clarity.