Cell: Structure and Function

Introduction

The cell is the fundamental unit of life, that forms the basis of all living organisms. From single-celled microorganisms to complex multicellular organisms like humans, cells exhibit remarkable diversity and play crucial roles in maintaining life processes. This article will delve into the fascinating world of cells, exploring their structure, functions, and significance in understanding the complexity of life itself.

Structure of a Cell

Cells come in different shapes, sizes, and types, but they all share certain structural components and organelles. The two primary categories of cells are prokaryotic and eukaryotic cells.

Prokaryotic Cells:

Prokaryotic cells, found in bacteria and archaea, lack a distinct nucleus and membrane-bound organelles. They consist of a cell membrane, cytoplasm, ribosomes, and a single circular DNA molecule located in the nucleoid region.

Eukaryotic Cells:

Eukaryotic cells, present in plants, animals, fungi, and protists, are more complex and contain a nucleus surrounded by a nuclear membrane. They possess membrane-bound organelles, including mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and others. Eukaryotic cells are characterized by their compartmentalization, allowing for specialized functions within different organelles.

Functions of Cells

Cells perform various functions that are essential for the survival and proper functioning of organisms. Here are some key functions:

1. Energy Production:

Cells generate energy through processes such as cellular respiration (in mitochondria) and photosynthesis (in plant cells). Energy production fuels the cellular activities required for growth, metabolism, and reproduction.

2. Protein Synthesis:

Cells utilize DNA as a template to synthesize proteins, which are involved in countless cellular processes. This process occurs in ribosomes, where messenger RNA (mRNA) is translated into proteins.

3. Cell Division:

Cells undergo division to reproduce and grow. Mitosis and meiosis are two types of cell division that ensure proper development, tissue repair, and the formation of gametes for sexual reproduction.

4. Transport and Communication:

Cells maintain homeostasis by regulating the transport of molecules across their membranes. They also communicate with one another through chemical signals, allowing for the coordination and integration of various physiological processes.

Cell Organelles

Within the complex microcosm of a cell, various organelles (which can be considered organs of the cells) act as specialized structures that perform distinct functions, contributing to the overall cellular organization and efficiency. Each organelle has a unique role, working in harmony with others to ensure the cell’s survival and proper functioning. These organelles can be broadly classified into membrane-bound and non-membrane-bound categories based on their structural characteristics. Membrane-bound organelles are further divided into those with a single membrane and those with a double membrane. Understanding the diverse array of organelles and their classifications is crucial to comprehend the intricacies of cellular organization and function.

Membrane-Bound Organelles:

Membrane-bound organelles are surrounded by a lipid bilayer, which separates their internal contents from the cytoplasm. They can be categorized as either single membrane-bound or double membrane-bound organelles.

1. Single Membrane-Bound Organelles:

  • Endoplasmic Reticulum (ER)
  • Golgi Apparatus
  • Lysosomes
  • Peroxisomes
  • Vacuoles (in plant and fungal cells)

2. Double Membrane-Bound Organelles:

  • Nucleus
  • Mitochondria
  • Chloroplasts (in plant cells)

Non-Membrane-Bound Organelles:

Non-membrane-bound organelles, also known as non-membranous or membrane-less organelles, lack a distinct lipid bilayer boundary. Instead, they are formed by dynamic assemblies of proteins and RNA molecules. These organelles play critical roles in various cellular processes, including cellular signalling, protein synthesis, and DNA replication. These are:

  • Ribosomes
  • Nucleolus
  • Centrosomes
  • Cytoskeleton (microtubules, microfilaments, intermediate filaments)

Read structure and function of plasma membrane Plasma Membrane: Structure and Function – Bio Lens

Structure and Functions of Organelles

Endoplasmic Reticulum (ER):

History:

The endoplasmic reticulum (ER) was first observed and described as a “lace-like reticulum” in 1945 by cell biologists Keith Porter, Albert Claude, and Ernest Fullman. They produced the first electron micrograph of a cell, revealing the convoluted structure of the ER. In the late 1940s and early 1950s, Porter and colleagues introduced the term “endoplasmic reticulum” to describe the organelle.

Structure:

The endoplasmic reticulum is a complex network of membranous tubules, flattened sacs called cisternae, and vesicles. It is classified into two distinct regions: the rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER). The RER is studded with ribosomes on its surface, while the SER lacks ribosomes.

Functions:

Rough Endoplasmic Reticulum (RER):

  • Protein Synthesis and Modification: Ribosomes on the RER synthesize proteins that are destined for secretion or integration into cellular membranes. As proteins are synthesized, the RER plays a crucial role in their folding and initial modification, such as adding sugar molecules (glycosylation).
  • Quality Control: The RER also monitors the proper folding of proteins. Misfolded proteins are identified and retained in the ER, preventing their passage to other cellular compartments.

Smooth Endoplasmic Reticulum (SER):

  • Lipid Metabolism: The SER is involved in the synthesis of lipids, including phospholipids and cholesterol. It also participates in the detoxification and metabolism of drugs and toxins.
  • Calcium Storage: The SER acts as a calcium reservoir, regulating calcium levels within the cell and releasing calcium ions when needed for various cellular processes.
  • Steroid Hormone Synthesis: Certain steroid hormones, such as cortisol and estrogen, are synthesized in specialized regions of the SER in specific cells.
  • Drugs detoxification: The SER in the liver cells, detoxify a broad range of organic compounds, such as barbiturates and ethanol.

Golgi Apparatus:

History:

The Golgi apparatus, named after Italian cytologist Camillo Golgi, was discovered in 1897. During his study of neurons using his black reaction staining technique, Golgi observed an “internal reticular apparatus” and named it the Golgi apparatus. This seminal discovery led to the recognition and understanding of this important organelle in the cell. In the past, the Golgi apparatus was also referred to as “dictyosomes.” The term “dictyosome” was commonly used to describe the stacked membrane structures observed within the Golgi apparatus.

Structure:

The Golgi apparatus consists of a series of stacked membranous sacs called cisternae. The cisternae are often curved and vary in shape, forming the cis face (receiving side) and trans face (shipping side) of the Golgi apparatus. The cisternae are interconnected and surrounded by vesicles.

Functions:

  • Protein Modification and Sorting:
  • Protein Processing: The Golgi apparatus receives proteins from the ER and modifies them by adding or removing sugar molecules (glycosylation) or other chemical groups. This post-translational modification helps to determine the protein’s final structure and function.
  • Protein Sorting: The Golgi apparatus sorts proteins into different vesicles based on their destination. Some proteins are packaged for secretion outside the cell, while others are directed to specific organelles or compartments within the cell.
  • Lipid Modification: The Golgi apparatus plays a role in modifying and sorting lipids, such as phospholipids and cholesterol, which are essential for cellular membrane composition and function.

Lysosomes:

History:

Lysosomes were discovered by Belgian biologist Christian de Duve in the 1950s while studying cellular fractions.

Structure:

Lysosomes are membrane-bound organelles filled with digestive enzymes called hydrolases. They have an acidic interior maintained by proton pumps embedded in their membrane.

Functions:

  • Intracellular Digestion: Lysosomes are responsible for breaking down various materials within the cell, including worn-out organelles, cellular debris, and macromolecules. The digestive enzymes in lysosomes degrade these substances into smaller molecules for recycling or disposal.
  • Autophagy: Lysosomes are involved in autophagy, a process that allows cells to recycle their components. They fuse with autophagosomes, which capture damaged organelles or protein aggregates, and break them down to reuse the building blocks.

Vacuoles (in plant and fungal cells):

History:

The term “vacuole” was initially suggested by the renowned French biologist Félix Dujardin to describe the empty spaces observed in contractile vesicles of protozoans. Later on, similar empty spaces were noticed in the leaves and roots of plants. In the late 19th century, a scientist named de Vries proposed the idea that vacuoles were formed from specific precursor structures similar to plastids, known as tonoplasts. This terminology and understanding contributed to the early comprehension of vacuolar structures and their role in cells.

Structure:

Vacuoles are membrane-bound sacs found in plant and fungal cells. In plant cells, the vacuole is a prominent and central organelle, occupying a significant portion of the cell volume. It is surrounded by a single membrane called the tonoplast, which separates its contents from the cytoplasm.

Functions:

  • Storage: Vacuoles function as storage compartments for water, ions, nutrients, and waste materials. They can accumulate various substances, including sugars, pigments, proteins, and toxic metabolic by-products.
  • Turgor Pressure: In plant cells, vacuoles contribute to turgor pressure, the internal pressure exerted by the vacuole against the cell wall. This pressure maintains cell shape, and rigidity, and supports the plant’s overall structure.
  • Removal of toxic substances: Vacuoles can sequester and isolate toxic substances, protecting the rest of the cell from their harmful effects.
  • Reproduction and Growth: During plant development, vacuoles aid in cell expansion and growth by accumulating water and other solutes, causing the cells to enlarge and elongate.
ER, GB, Lysosome and Vacuole together form the Endomembrane system. These organelles are interconnected and work together to form a network of membrane systems that facilitate the movement and processing of proteins, lipids, and other molecules within the cell. They play critical roles in maintaining cellular homeostasis, protein secretion, intracellular transport, and waste disposal.

Peroxisomes:

History:

In 1954, the organelles now known as peroxisomes were initially described as “microbodies” by Rhodin, who observed them in the cytoplasm of proximal tubule cells in the mouse kidney. However, it was not until 1965 that de Duve et al. isolated these microbodies from rat liver and defined them as membrane-bound organelles containing various (per)oxidases and de Duve et al. named these organelles “peroxisomes.”

Structure:

Peroxisomes are single-membrane-bound organelles that contain various enzymes involved in several metabolic reactions, particularly those related to hydrogen peroxide.

Functions:

  • Detoxification: Peroxisomes play a crucial role in detoxifying harmful substances, such as alcohol and other toxins, by breaking them down into less harmful compounds.
  • Lipid Metabolism: Peroxisomes participate in the breakdown of fatty acids through beta-oxidation, generating energy and producing acetyl-CoA molecules for further metabolism.

Nucleus:

History:

The nucleus was first observed by Scottish botanist Robert Brown in 1831. Its significance as the central control center of the cell was later recognized.

Structure:

The nucleus is a membrane-bound organelle that contains the cell’s genetic material, including DNA. It is surrounded by a double membrane called the nuclear envelope, which has nuclear pores that allow the movement of molecules in and out of the nucleus. The nucleus also contains a dense region called the nucleolus.

Functions:

  • Genetic Information Storage and Regulation: The nucleus houses the cell’s genetic material in the form of DNA, which carries the instructions for protein synthesis and cellular functions. It regulates gene expression by controlling the transcription and replication of DNA.
  • Cellular Control Center: The nucleus coordinates various cellular activities, including metabolism, growth, and cell division. It controls the synthesis of RNA and ribosomes, which are essential for protein production.

Mitochondria:

History:

Mitochondria were discovered by German anatomist and physician Richard Altmann in 1886. Their significance as the powerhouse of the cell was later understood.

Structure:

Mitochondria are double-membrane-bound organelles. The outer membrane is smooth, while the inner membrane is highly folded into structures called cristae. The inner membrane encloses the mitochondrial matrix, which contains enzymes, DNA, and ribosomes. Its size ranges from 1 to 4 mm in length. In the inner mitochondrial membrane, F0-F1 particles (Oxysomes or ATPase) are present which are involved in proton movement and ATP synthesis. They divide by binary fission.

Functions:

  • Energy Production: Mitochondria are the primary sites of cellular respiration, where they generate energy in the form of adenosine triphosphate (ATP) through a series of biochemical reactions. This process involves the breakdown of glucose and other molecules to produce ATP, which fuels cellular activities.
  • Metabolism and Synthesis: Mitochondria are involved in various metabolic pathways, including the metabolism of fatty acids and amino acids. They also participate in the synthesis of certain molecules, such as heme and some lipids. Pyruvate (a glycolytic product) enters mitochondria as acetyl CoA and is completely oxidized in aerobic respiration (TCA cycle).
  • Urea cycle: Ammonia produced from amino acid deamination and CO2 produced in the TCA cycle are converted into Urea, a less toxic product.

Chloroplasts (in plant cells):

History:

Although Hugo von Mohl, a German botanist, is often credited with the discovery and detailed description of “Chlorophyll körner” or chloroplast granules in 1837, it’s worth noting that earlier reports of green granules had been published.

Structure:

Chloroplasts are double-membrane-bound organelles found in plant cells and some protists. They have an inner and outer membrane, with a space between them called the intermembrane space. Inside the chloroplast is a fluid-filled region called the stroma, which contains thylakoid membranes arranged in stacks called grana.

Functions:

  • Photosynthesis: Chloroplasts are responsible for photosynthesis, the process by which light energy is converted into chemical energy. They contain chlorophyll and other pigments that capture light energy and use it to synthesize organic molecules, such as glucose, from carbon dioxide and water.
  • Carbon Fixation: Chloroplasts play a crucial role in carbon fixation, where atmospheric carbon dioxide is converted into organic compounds. This process is essential for the synthesis of sugars and other organic molecules that serve as building blocks for plant growth.

Ribosomes:

History:

Ribosomes were discovered by Romanian-American cell biologist George Emil Palade in the 1950s through electron microscopy studies.

Structure:

Ribosomes are non-membrane-bound organelles composed of ribosomal RNA (rRNA) and proteins. They exist as either free ribosomes in the cytoplasm or bound ribosomes attached to the rough endoplasmic reticulum (RER).

Functions:

  • Protein Synthesis: Ribosomes are responsible for protein synthesis, translating the genetic information encoded in messenger RNA (mRNA) into chains of amino acids. They facilitate the assembly of amino acids into polypeptides, which eventually fold into functional proteins.

Nucleolus:

History:

The nucleolus was initially described between 1835 and 1839, but it took nearly a century to discover its association with a specific chromosomal locus, thus establishing it as a cytogenetic entity. Subsequently, nucleoli were isolated for the first time in the 1950s from starfish oocytes, leading to a deeper understanding of their structure and functions.

Structure:

The nucleolus is a non-membrane-bound organelle located within the nucleus of eukaryotic cells. It is composed of proteins, RNA, and DNA.

Functions:

  • Ribosome Biogenesis: The nucleolus plays a crucial role in the production and assembly of ribosomes. It synthesizes rRNA, assembles ribosomal subunits, and exports them to the cytoplasm where they combine to form functional ribosomes.

Centrosomes:

History:

Centrosomes were discovered by Belgian cytologist Edouard Van Beneden in the late 19th century.

Structure:

Centrosomes consist of two centrioles, which are cylindrical structures composed of microtubules. They are surrounded by pericentriolar material that contains various proteins. It is made up of nine peripheral triplets of microtubules (9+0 arrangement of microtubules). A central protein hub is present. The outer microtubule of one triplet is connected with the inner microtubule of the next triplet giving it a cart-wheel appearance.

Functions:

  • Microtubule Organization: Centrosomes are involved in organizing and nucleating microtubules, which are crucial components of the cytoskeleton. They play a significant role in cell division, forming the spindle apparatus that separates chromosomes during mitosis and meiosis.
  • Cell Division: Centrosomes function as the main microtubule organizing centers during cell division. They help establish and maintain the bipolar structure of the mitotic spindle, ensuring proper chromosome segregation.

Cytoskeleton (microtubules, microfilaments, intermediate filaments):

Microtubules:

History:

Microtubules were first observed and described by American cell biologists George Emil Palade and Albert Claude in the 1950s.

Structure:

Microtubules are cylindrical structures composed of alpha and beta-tubulin protein subunits. They form hollow tubes with a diameter of about 25 nanometers. Each microtubule is made up of 13 protofilaments arranged in a circular pattern.

Functions:

  • Cell Shape and Structure: Microtubules provide structural support to the cell and help maintain its shape. They form a scaffold that gives the cell its overall organization and stability.
  • Cell Division: Microtubules play a critical role in cell division. They form the mitotic spindle, a complex structure that separates chromosomes during mitosis and meiosis. Microtubules attach to the chromosomes and exert forces to accurately segregate them into daughter cells.
  • Intracellular Transport: Microtubules serve as tracks for intracellular transport. They facilitate the movement of organelles, vesicles, and other cellular components by interacting with motor proteins, such as kinesin and dynein.

Microfilaments (Actin Filaments):

History:

Microfilaments were first discovered by Belgian cytologist Édouard-Gérard Balbiani in the late 19th century. Their importance in cell function was later recognized.

Structure:

Microfilaments are thin, flexible filaments composed of actin protein subunits. They have a diameter of about 8 nanometers and can exist as individual filaments or bundles.

Functions:

  • Cell Motility: Microfilaments are crucial for cell motility. They form the basis of cellular extensions like lamellipodia and filopodia, which enable cell crawling, migration, and tissue remodelling.
  • Muscle Contraction: Microfilaments, in association with myosin motor proteins, are responsible for muscle contraction. The interaction between actin and myosin generates the force required for muscle fibre contraction.
  • Cell Shape and Support: Microfilaments contribute to the maintenance of cell shape and integrity. They provide mechanical strength and help the cell resist deformation.

Intermediate Filaments:

History:

Intermediate filaments were first described by American cell biologist Keith R. Porter in the 1950s.

Structure:

Intermediate filaments are fibrous proteins that vary in composition depending on the cell type. Unlike microtubules and microfilaments, intermediate filaments do not have a uniform diameter. They form rope-like structures with a diameter of about 10-12 nanometers.

Functions:

  • Structural Support and Stability: Intermediate filaments provide mechanical strength to cells and tissues. They help anchor organelles, maintain cell shape, and resist mechanical stress and stretching.
  • Cell-Cell Adhesion: Intermediate filaments are involved in cell-cell adhesion. They connect with desmosomes, which are specialized junctions between cells, contributing to the integrity and stability of tissues.
  • Nuclear Integrity: Certain intermediate filaments, known as nuclear lamins, are found in the nuclear envelope. They provide support and maintain the shape and stability of the nucleus.

Conclusion

The cell, with its organelles, is life’s basic unit, with a complex organization. The cell membrane controls substance movement. The nucleus stores genetic material, while mitochondria produce energy. Chloroplasts enable photosynthesis in plants. Ribosomes make proteins, the nucleolus helps in this. The endoplasmic reticulum and Golgi process and transport proteins. Lysosomes and peroxisomes handle digestion and detoxification. Centrosomes assist in cell division. The cytoskeleton provides support and motility. Each organelle is crucial for the cell’s survival and functions. Understanding them enhances our biology knowledge, revealing life’s intricacies. Scientists uncover cellular mysteries, advancing our grasp of life.

Read structure and function of plasma membrane Plasma Membrane: Structure and Function – Bio Lens

References:

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6783984/
  • https://link.springer.com/chapter/10.1007/978-981-15-1169-1_1
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3039934/
  • Karp, G., Iwasa, J., & Marshall, W. (2015). Karp’s Cell and Molecular Biology: Concepts and Experiments (8th ed.). Wiley.
  • Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell, Fourth Edition (4th ed.). Garland Science.
  • Md, T. P. D., Earnshaw, W. C., PhD, L. J., & Cmi, J. G. M. P. (2016). Cell Biology (3rd ed.). Elsevier.
  • Harris, H. (2000). The Birth of the Cell (Revised ed.). Yale University Press.
  • https://www.britannica.com/

You Can join us on telegram for updates and quizzes.  Join Telegram group

“Thank you for using our online study materials. We are a self-sustained group of individuals dedicated to creating quality educational resources for students worldwide. However, we rely on your donations to continue our work. If you have found our materials useful, please consider making a contribution to help support our mission. Your support will allow us to continue providing valuable resources to students in need. To donate, please click the DONATE HERE button. Thank you for your support!”

Leave a Comment

Your email address will not be published. Required fields are marked *