Introduction to The Cellular World

Introduction

There are many parts of a cell, they all have specific duties, and are all needed to continue the life of the cell. Some cells exist as single-celled organisms that perform all of the organism's metabolism within a single cell. Such single-celled organisms are called unicellular. Other organisms are made up of many cells, with their cells specialized to perform distinct metabolic functions. One cell within an organism may be adapted for movement, while another cell carries out digestion. The individual cells no longer carry out all life functions, but rather depend on each other.

Many-celled organisms are called multicellular. When a group of cells function together to perform an activity, they form a tissue.

The cells of a human are organized into tissues such as muscle and nerve tissues. Plant tissues include those of the stem and root. Many cells in tissues are linked to each other at contact sites called cell junctions. Cell junctions help maintain differences in the internal environment between adjacent cells, help anchor cells together, and allow cells to communicate with one another by passing small molecules from one cell to another.

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Groups of two or more tissues that function together make up organs. An organ system is a group of organs that work together to carry out major life functions.

Eukariotic Cell Structure

Boundaries and Control

• Plasma Membrane - The plasma membrane is sometimes called the cell membrane, or the cellular membrane. It is the outermost part of the animal cell, and it's purpose is to enclose the cell, and change shape if needed.
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  The cell membrane is capable of allowing materials to enter and exit the cell. Oxygen and nutrients enter, and waste products such as excess water leave. The plasma membrane helps maintain a chemical balance within the cell.

• Cell wall - The cell wall is an added boundary to the cell. It is relatively inflexible, and surrounds the plasma membrane. The cell wall is much thinker than the plasma membrane and is made of different substances in different organisms. The cells of plants, fungi, almost all bacteria, and some protists have cell walls. Animal cells have no cell walls. Plant cells contain cellulose molecules, which form fibers. This fibrous cellulose of plants provides the bulk of the fiber in our diets. Chitin, a nitrogen-containing polysaccharide, makes up the cell walls of fungi.
• Nucleus - The nucleus of the cell is the organelle that manages cell functions in a eukariotic cell. The nucleus contains DNA, the master instructions for building proteins. DNA forms tangles of long strands called chromatin, which is packed into identifiable chromosomes when the cells are ready to reproduce. Also within the nucleus is the nucleolus, a region that produces tiny cell particles that are involved in protein synthesis These particles, called ribosome's, are the sites where the cell assembles enzymes and other proteins according to directions of the DNA. Assembly, Transport, and Storage:
• Cytoplasm - The material that lies outside the nucleus and surrounds the organelles is the cytoplasm, a clear fluid that is a bit thinner than toothpaste gel. It usually constitutes a little more than half the volume of a typical animal cell.
• Endoplasmic Reticulum - The endoplasmic reticulum (ER) is a folded membrane that forms a network of interconnected compartments inside the cell. The ER membranes contain the enzymes for almost all of the cell's lipid synthesis, they serve as the site of lipid synthesis in the cell. The ER functions as the cell's delivery system. Some parts of the ER are studded with ribosome's. In the cell, the sites of protein assembly are the ribosome's.
• Golgi Apparatus - The Golgi apparatus's main purpose is to store materials. The Golgi apparatus is a series of closely stacked, flattened membrane sacs that receives newly synthesized proteins and lipids from the ER and distributes them to the plasma membrane and other cell organelles. Proteins are transferred from the ER to the Golgi apparatus in small, membrane-bound transport packages. These packages, called vesicles, have pinched off from the membrane of the ER and contain proteins. The Golgi apparatus modifies the proteins chemically, then repackages them in new vesicles for their final destination in the cell. They may be incorporated into the cell structures, expelled, or remain stored for later usage.
• Vacuole - A vacuole is a sac of fluid surrounded by a membrane. Vacuoles often store food, enzymes, and other materials needed by cells, and some vacuoles store waste products. A plant cell has one large vacuole that stores water and other substances. Lysosomes - In addition to the assembly and storage of macromolecules, cells also can disassemble things. Lysosomes, organelles that contain digestive enzymes, digest excess or worn out cell parts, food particles, and invading viruses or bacteria.
• Mitochondria - Mitochondria are organelles in which food molecules are broken down to release energy. This energy is then stored in other molecules that can power cell reactions easily. A mitochondrion has an outer membrane and a highly folded inner membrane. As with ER, the folds of the inner membrane provide a large surface area in a small space. Energy is produced on the inner folds.
• Chloroplasts - Chloroplasts transform light energy directly into usable chemical energy and store that energy in food molecules. These foods include sugars and starches. Chloroplasts contain the molecule chlorophyll, a green pigment that traps the energy from sunlight and gives plants their green color. The chloroplast belongs to a group of plant organelles called plastids, which are used for storage. Some plastids store starches or lipids, whereas others contain pigments, molecules that give color.

Structures for Support and Locomotion

The cytoskeleton is a network of thin, fibrous elements that act as a sort of scaffold to provide support for organelles. It also helps maintain cell shape in a manner similar to the way poles maintain the shape of a tent. The cytoskeleton is usually composed of microtubules and microfilaments. Microtubules are thin, hollow cylinders made of protein. Microfilaments are thin, solid protein fibers. Microtubules and microfilaments make up most of the sytoskeleton.

• Cilia - Cilia are only contained in some cells. They are short, numerous, hairlike projections out of the plasma membrane. Cilia tend to occur in large numbers on a cell's surface, and their beating activity is usually coordinated.
• Flagella - Flagella are longer projections that move with a whiplike motion. Cells that have flagella only have one or two per cell. In single-celled organisms, cilia and flagella are the major means of locomotion.

Sperm cells of animals and some plants move by means of flagella. Organisms that contain many cells, including humans, have cilia that move fluids over a cell's surface, rather than moving the cell itself.

Conclusion

In this comprehensive introduction to the cellular world, we have explored the fundamental components and structures that make up eukaryotic cells. Cells, the basic units of life, come in various forms, and their organization varies between single-celled organisms, known as unicellular, and multicellular organisms, where cells specialize in distinct metabolic functions. Cells work together in tissues, which in turn contribute to the formation of organs, and organs collectively create organ systems that carry out vital life functions.

We have delved into the structure of eukaryotic cells, focusing on key components:

1. Plasma Membrane: The outermost boundary of the cell, responsible for enclosing it and regulating the passage of materials in and out.
2. Cell Wall: Found in some organisms, including plants, fungi, and bacteria, this rigid boundary surrounds the plasma membrane.
3. Nucleus: The control center of the cell, containing DNA and the nucleolus, where ribosomes are produced for protein synthesis.
4. Cytoplasm: The clear fluid that surrounds organelles and makes up a significant portion of the cell's volume.
5. Endoplasmic Reticulum (ER): A folded membrane network involved in lipid synthesis and protein transport, with ribosome-studded regions for protein assembly.
6. Golgi Apparatus: Responsible for modifying, packaging, and distributing proteins and lipids synthesized in the ER.
7. Vacuole: A membrane-bound sac used for storing various substances, including food, enzymes, and waste products.
8. Lysosomes: Organelles containing digestive enzymes to break down cellular waste and foreign invaders.
9. Mitochondria: Energy-producing organelles that convert food molecules into usable energy stored in molecules like ATP.
10. Chloroplasts: Organelles in plant cells responsible for converting light energy into chemical energy through photosynthesis.

We also discussed structures for support and locomotion, including the cytoskeleton composed of microtubules and microfilaments, as well as specialized appendages like cilia and flagella that facilitate movement in certain cells.

Understanding the intricate organization and functions of eukaryotic cells is fundamental to comprehending the biology of living organisms. Cells are the building blocks of life, and their diverse structures and capabilities underpin the complexity and diversity of life forms on Earth. Further exploration of cell biology will continue to shed light on the mysteries of life and its processes.