🧬Biocode Explorer - Eukaryotic Cell

Eukaryotic Cell

After exploring the structure, composition, and characteristics of prokaryotic cells, it’s time to move on to the more complex cell type: the eukaryotic cell. Unlike prokaryotes, eukaryotic cells have a true nucleus, where their DNA is securely stored, and many membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus, each performing specialized functions to maintain the cell’s integrity and efficiency.

The word eukaryote comes from the Greek words eu- meaning “true” and karyon meaning “nut or kernel,” perfectly describing their well-defined nucleus enclosed by a nuclear membrane.

Eukaryotic cells first appeared on Earth approximately 1.6 to 2.1 billion years ago, long after prokaryotes. This event, known as eukaryogenesis, involved the symbiotic fusion of an archaeon and a bacterium, which led to the formation of the first complex cells with membrane-bound organelles like the nucleus and mitochondria. The rise of eukaryotes was closely linked to, and likely driven by, the increase in atmospheric oxygen during the Proterozoic eon.

Eukaryotic cells include organisms from the kingdoms Protista, Fungi, Plantae, and Animalia. Many are multicellular, and their cells can specialize to perform distinct functions, such as muscle cells, nerve cells, or plant leaf cells. Thanks to their compartmentalized structure, eukaryotic cells can carry out advanced processes like energy production, protein modification, and cell signaling, which paved the way for the evolution of plants, animals, and humans, making them the foundation of all complex life on Earth.

Structure of a Eukaryotic Cell

To understand the structure of a eukaryotic cell, we can look at examples such as a plant cell or an animal cell. Eukaryotic cells are more complex than prokaryotic cells because they have a true nucleus and many membrane-bound organelles that perform specialized functions. By studying a eukaryotic cell, we can learn how its different parts work together to keep the cell alive and functioning.

The structure of a eukaryotic cell is usually explained by dividing it into two parts: the outer structures (such as the plasma membrane and, in plant cells, the cell wall) and the inner structures (such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplasts in plants, lysosomes, and ribosomes). This organization makes it easier to understand how each component plays a role in maintaining life and supporting the activities of the organism.

Illustration by Bio-Code Explorer

An animal cell is a eukaryotic cell that makes up the tissues and organs of animals, including humans. Unlike plant cells, animal cells do not have a cell wall or chloroplasts, but they do have a flexible plasma membrane and many specialized organelles.

The most important feature is the nucleus, which contains the cell’s DNA and controls its activities. Other key organelles include mitochondria (the powerhouse of the cell), endoplasmic reticulum, Golgi apparatus, lysosomes, and ribosomes, all working together to keep the cell alive and functioning.

1. Cell Wall

Structure: The cell wall is a rigid outer covering found outside the plasma membrane. In plants, it is mainly made of cellulose, while in fungi it is made of chitin. It also contains other polysaccharides, proteins, and sometimes lignin to provide extra strength.
Function: The cell wall gives the cell its shape, provides mechanical support, and protects it from physical damage and pathogens. It also prevents the cell from bursting when water enters by osmosis.

2. Plasma Membrane

Structure: The plasma membrane is a flexible, semi-permeable membrane composed of a phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.
Function: It regulates the entry and exit of substances, protects the cell from its surroundings, and facilitates communication between cells.

3. Cytoplasm

Structure: The cytoplasm is a jelly-like substance filling the interior of the cell, composed mainly of water, salts, and organic molecules.
Function: It houses the organelles and is the site of numerous cellular processes, such as metabolic pathways and intracellular transport.

4. Nucleus

Structure: The nucleus is a membrane-bound organelle that contains the cell's genetic material, DNA, organized into chromosomes. It is surrounded by the nuclear envelope, a double membrane with pores.
Function: The nucleus controls cellular activities, such as growth, metabolism, and reproduction, by regulating gene expression. It is also where RNA is synthesized.

5. Mitochondria

Structure: Mitochondria are double-membraned organelles, often referred to as the powerhouse of the cell.
Function: They produce ATP (adenosine triphosphate), the cell's main energy currency, through the process of cellular respiration.

6. Endoplasmic Reticulum (ER)

Structure: The ER is a network of membranous tubules and sacs. It comes in two forms: rough ER (studded with ribosomes) and smooth ER (lacking ribosomes).
Function: The rough ER is involved in protein synthesis and modification, while the smooth ER is responsible for lipid synthesis, detoxification, and calcium storage.

7. Golgi Apparatus

Structure: The Golgi apparatus consists of stacked, flattened membranous sacs known as cisternae.
Function: It modifies, sorts, and packages proteins and lipids for transport to various destinations inside or outside the cell.

8. Ribosomes

Structure: Ribosomes are small, non-membranous organelles made of ribosomal RNA and proteins. They can be free-floating in the cytoplasm or attached to the rough ER.
Function: They are the sites of protein synthesis, translating genetic instructions from mRNA to build proteins.

9. Lysosomes

Structure: Lysosomes are membrane-bound vesicles containing digestive enzymes.
Function: They break down waste materials, cellular debris, and foreign invaders like bacteria, playing a crucial role in the cell’s waste disposal and recycling processes.

10. Centrioles

Structure: Centrioles are cylindrical structures composed of microtubules, usually found in pairs within the centrosome.
Function: They play a key role in cell division by helping to organize the microtubule spindle that separates chromosomes during mitosis.

11. Cytoskeleton

Structure: The cytoskeleton is a network of protein filaments, including microfilaments, intermediate filaments, and microtubules.
Function: It provides structural support, maintains the cell’s shape, facilitates cell movement, and assists in intracellular transport.

12. Vesicles

Structure: Vesicles are small membrane-bound sacs that transport materials within the cell.
Function: They help transport proteins, lipids, and other molecules between different parts of the cell, and can also store and release substances as needed.

13. Peroxisomes

Structure: Peroxisomes are small, single-membrane-bound organelles present in the cytoplasm. They contain a variety of oxidative enzymes, including catalase and oxidases, which help in different metabolic reactions. Unlike mitochondria and chloroplasts, peroxisomes do not have their own DNA or ribosomes; instead, their proteins are synthesized in the cytoplasm and imported into the organelle.
Function: Peroxisomes are mainly involved in breaking down fatty acids through β-oxidation and in detoxifying harmful substances. One of their key roles is converting hydrogen peroxide (H₂O₂), a toxic byproduct of metabolism, into harmless water and oxygen using catalase. In plant cells, specialized peroxisomes (glyoxysomes) help in converting stored lipids into sugars during seed germination.

14. Nucleolus

Structure: The nucleolus is a dense, spherical region inside the nucleus. It is not surrounded by a membrane and is made up mainly of ribosomal RNA (rRNA), DNA, and proteins. It forms around specific chromosomal regions called nucleolar organizer regions (NORs).
Function: The nucleolus is the site of ribosome production. It synthesizes rRNA and combines it with proteins to form the small and large ribosomal subunits. These subunits are then exported to the cytoplasm, where they join together to make functional ribosomes for protein synthesis.

14. Vacuole

Structure: A vacuole is a membrane-bound sac found inside eukaryotic cells. It is surrounded by a single membrane called the tonoplast. Vacuoles are filled with cell sap, which contains water, salts, sugars, and waste products. Plant cells usually have one large central vacuole that can occupy up to 80–90% of the cell’s volume, while animal cells have smaller, temporary vacuoles.
Function: The vacuole helps maintain the cell’s shape and turgor pressure by storing water. It also stores nutrients, pigments, and waste products. In plant cells, vacuoles play a key role in growth (by enlarging as they take in water) and in defense (by storing toxic substances to keep away herbivores). In animal cells, smaller vacuoles assist in processes like endocytosis and exocytosis.

Difference between Prokaryotic & Eukaryotic Cell