Question

For the following cell organelles, describe the structure and function of each: nucleus, nucleolus, ribosomes, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, centrioles, and mitochondria. (pages $52-56$ )

Short answer

Nucleus: The control center of the cell, containing DNA and regulating gene expression. It has a double membrane and controls cell activities. Nucleolus: A small, dense structure within the nucleus, responsible for ribosome synthesis for protein production. Ribosomes: Small, round structures composed of RNA and proteins, which are the site of protein synthesis. Endoplasmic Reticulum: A continuous membrane system with two types: rough ER (protein synthesis) and smooth ER (lipid synthesis and detoxification). Golgi Apparatus: A stack of flattened, membrane-bound sacs involved in the modification, sorting, and packaging of proteins and lipids. Lysosomes: Small, spherical organelles containing hydrolytic enzymes involved in the breakdown and recycling of cellular components. Centrioles: Cylindrical structures composed of microtubules, involved in cell division through the formation of spindle fibers. Mitochondria: Double-membrane organelles responsible for generating ATP, the primary source of cellular energy, and regulating cell metabolism and apoptosis.

Step-by-step solution

Introduction to Cell Organelles

Organelles are specialized structures within cells that perform specific functions. In this exercise, we will describe the structure and function of the following organelles: nucleus, nucleolus, ribosomes, endoplasmic reticulum (rough and smooth), Golgi apparatus, lysosomes, centrioles, and mitochondria.

Nucleus

The nucleus is the control center of the cell, containing the cell's DNA and regulating gene expression. The structure of the nucleus consists of a double membrane, called the nuclear envelope, which encloses the nucleoplasm. The nuclear envelope contains pores for communication between the nucleus and the cytoplasm. Function: The nucleus controls cell activities by regulating gene expression. It also stores the cell's genetic material (DNA) and plays a role in cell division.

Nucleolus

The nucleolus is a small, dense structure within the nucleus. Its primary function is to synthesize ribosomes, which are then transported out of the nucleus. Function: The nucleolus is responsible for ribosome synthesis, an essential process for protein production within the cell.

Ribosomes

Ribosomes are small, round structures that are composed of RNA and proteins. They can be found free-floating in the cytoplasm or attached to the endoplasmic reticulum. Function: Ribosomes are the site of protein synthesis, as they translate the genetic code from mRNA into specific amino acids chains, resulting in the formation of proteins.

Endoplasmic Reticulum (Rough and Smooth)

The endoplasmic reticulum (ER) is a continuous membrane system that consists of two types: rough ER and smooth ER. The surface of the rough ER is coated with ribosomes, giving it its rough appearance. In contrast, smooth ER does not have ribosomes attached to its surface. Function: Rough ER is involved in protein synthesis and processing, as ribosomes attached to it translate mRNA into proteins. Smooth ER is associated with lipid synthesis and detoxification processes.

Golgi Apparatus

The Golgi apparatus is a stack of flattened, membrane-bound sacs called cisternae. It is involved in the modification, sorting, and packaging of proteins and lipids synthesized by the rough ER and smooth ER. Function: The Golgi apparatus modifies, sorts, and packages proteins and lipids, before sending them to their appropriate destinations within or outside the cell.

Lysosomes

Lysosomes are small, spherical organelles that contain hydrolytic enzymes. These enzymes enable lysosomes to break down various biomolecules, such as proteins, lipids, and nucleic acids. Function: Lysosomes are involved in the degradation and recycling of cellular components. They break down damaged or unwanted organelles and macromolecules, allowing their components to be reused or expelled from the cell.

Centrioles

Centrioles are cylindrical structures composed of nine sets of triplet microtubules, arranged in a circular pattern. They are typically found in pairs and located near the nucleus. Function: Centrioles are involved in cell division, specifically in the formation of spindle fibers that help separate chromosomes during mitosis and meiosis.

Mitochondria

Mitochondria are double-membrane organelles with a smooth outer membrane and a folded inner membrane, called cristae. The space between the inner and outer membranes is called the intermembrane space, while the space enclosed by the inner membrane is called the matrix. Function: Mitochondria are the powerhouse of the cell, as they generate ATP (adenosine triphosphate), the primary source of cellular energy, through the process of oxidative phosphorylation. They also play a role in regulating the cell's metabolism and apoptosis (cell death).

Key concepts

Nucleus

The nucleus is often referred to as the control center of the cell. It is central to the functions and structure of eukaryotic cells, housing the majority of the cell's genetic material in the form of DNA. Structurally, the nucleus is encased in a double-membraned nuclear envelope which contains numerous pores to facilitate material exchange between the nucleus and cytoplasm.
The primary role of the nucleus involves regulating gene expression, essentially turning genes on and off as needed. This process determines which proteins are synthesized and impacts the cell's overall behavior and function. The nucleus also stores DNA, ensuring it is accurately replicated and distributed during cell division. This orchestration of cellular activities enables the cell to perform complex processes and adapt to its environment.
The presence of the nucleolus within the nucleus enhances its function by focusing on ribosomal RNA synthesis, crucial for producing ribosomes.

Ribosomes

Ribosomes are tiny but indispensable organelles found throughout the cytoplasm and on the surface of rough endoplasmic reticulum (ER). They are composed of RNA and specific proteins and play a critical role in protein synthesis.
The process of synthesizing proteins begins when ribosomes bind to mRNA (messenger RNA) molecules. The ribosome reads the sequence of the mRNA and translates it into a specific sequence of amino acids, creating polypeptides that fold into functional proteins.

• Ribosomes free in the cytoplasm typically produce proteins that function within the cytosol itself.
• Ribosomes attached to the rough ER are involved in synthesizing proteins destined for secretion or for various membranes.

Through this vital function, ribosomes ensure the cell can build new proteins crucial for cell structure and function.

Endoplasmic Reticulum

The endoplasmic reticulum (ER) is a vast network of membranes found within eukaryotic cells. It comes in two forms—rough ER and smooth ER. The rough ER is studded with ribosomes, giving it a characteristic rough appearance, and specializes in protein synthesis and folding. Proteins synthesized here are primarily those bound for the cell membrane, lysosomes, or for secretion.
Conversely, the smooth ER lacks ribosomes and performs a variety of functions, which include lipid synthesis and detoxification. It is critical in the synthesis of lipids like cholesterol and phospholipids, which are essential components of cellular membranes, and in breaking down toxins that could harm the cell.

• Rough ER: Protein synthesis and processing.
• Smooth ER: Lipid synthesis and detoxification.

This division of labor between rough and smooth ER supports the cell’s intricate and diverse biochemical tasks.

Mitochondria

Often termed the powerhouses of the cell, mitochondria are dynamic structures enveloped by two membranes. Their primary role involves generating ATP (adenosine triphosphate) through oxidative phosphorylation, providing energy required for various cellular activities. The inner membrane is extensively folded into cristae, which increase the surface area for energy production processes.
Mitochondria are unique among organelles because they have their own DNA, allowing them to produce some of their proteins independently of the nuclear DNA. They also play a role in regulating the cell's metabolism, calcium storage, and the initiation of apoptosis (programmed cell death).

• Energy Production: Through cellular respiration, they convert nutrients into ATP.
• Metabolic Regulation: Influencing significant metabolic pathways.

Thus, mitochondria provide the necessary energy and regulatory functions that maintain cellular health and activity.

Golgi Apparatus

The Golgi apparatus is often described as the cell's packaging and distribution center. It consists of a series of stacked, membrane-bound sacs called cisternae. This structure is crucial in modifying, sorting, and packaging proteins and lipids received from the rough and smooth ER.
Once proteins and lipids reach the Golgi apparatus, they undergo further processing which may include glycosylation (addition of sugar molecules) and other modifications. These proteins and lipids are then sorted based on their destination, whether they are to be sent to the cell surface, incorporated into lysosomes, or secreted outside the cell.

• Modification: Proteins and lipids are chemically altered.
• Sorting and Packaging: Preparation for delivery within or outside the cell.

The Golgi apparatus ensures cellular products are properly modified, accurately sorted, and delivered to the right destination, fully functioning as integral parts of cellular life.