Review of Cell Structure and Function

Organelles of a Cell

A-Rough Endoplasmic Reticulum: The rough endoplasmic reticulum (RER) is a network of membranes covered with ribosomes. These ribosomes make it look "rough" and are responsible for synthesizing proteins. The RER modifies and folds these proteins into their functional shapes. It also transports these proteins to other parts of the cell, like the Golgi apparatus. Additionally, the RER helps in the production of membranes for the cell.

B-Ribosomes: Ribosomes are tiny structures found either floating freely in the cytosol or attached to the rough endoplasmic reticulum. They are the sites where proteins are synthesized from amino acids. Ribosomes read the genetic information carried by messenger RNA (mRNA) and translate it into protein sequences. These proteins are essential for various cellular functions and structures. Without ribosomes, cells would not be able to produce the proteins needed for survival.

C-Glycogen Granules: Glycogen granules are small clusters of glycogen, a type of stored sugar. These granules serve as energy reserves for the cell. When the cell needs energy, enzymes break down glycogen into glucose molecules. This glucose is then used in cellular respiration to produce ATP, the cell's energy currency. Glycogen granules are particularly abundant in liver and muscle cells.

D-Vesicle: Vesicles are small, membrane-bound sacs that transport substances within the cell. They can carry proteins, lipids, and other molecules to different parts of the cell. Vesicles also help in processes like endocytosis (bringing substances into the cell) and exocytosis (releasing substances outside the cell). They play a crucial role in maintaining the cell’s internal organization and communication.

E-Chromatin: Chromatin is a complex of DNA and proteins found in the nucleus. It contains the genetic information necessary for the cell's functions and reproduction. Chromatin is usually in a relaxed state, allowing genes to be accessed for transcription. During cell division, chromatin condenses into visible chromosomes. This organization helps ensure accurate distribution of genetic material to daughter cells.

F-Nucleolus: The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is produced. It assembles rRNA with proteins to form ribosomal subunits. These subunits are then transported out of the nucleus to the cytoplasm, where they combine to form functional ribosomes. The nucleolus is crucial for protein synthesis in the cell. Without a nucleolus, the cell would not be able to produce ribosomes.

G-Nuclear Envelope: The nuclear envelope is a double membrane that surrounds the nucleus. It protects the genetic material contained within the nucleus. The envelope has nuclear pores that regulate the movement of molecules in and out of the nucleus. It helps maintain the integrity of the nucleus by separating it from the cytoplasm. The nuclear envelope also plays a role in the organization of chromatin.

H-Centrosome: The centrosome is an organelle that serves as the main microtubule organizing center. It plays a critical role in cell division by forming the spindle fibers that separate chromosomes. The centrosome consists of two centrioles surrounded by a dense matrix of proteins. It also helps in organizing the cytoskeleton, providing structural support and facilitating cellular transport. The centrosome ensures proper cell division and distribution of chromosomes.

I-Golgi Apparatus: The Golgi apparatus is a series of flattened membrane-bound sacs. It modifies, sorts, and packages proteins and lipids received from the endoplasmic reticulum. These molecules are then transported to their final destinations, either within or outside the cell. The Golgi apparatus also produces lysosomes, which contain digestive enzymes. It plays a vital role in the secretion and processing of cellular products.

J-Vacuole (e.g., lysosome, food vacuole): Vacuoles are membrane-bound compartments used for storage and transport. Lysosomes are specialized vacuoles containing enzymes that break down waste materials and cellular debris. Food vacuoles store and digest nutrients taken into the cell. Vacuoles also help maintain cell turgor pressure, which is essential for plant cells. They play diverse roles in cellular digestion, waste management, and storage.

K-Smooth Endoplasmic Reticulum (SER): The smooth endoplasmic reticulum (SER) lacks ribosomes and appears smooth. It is involved in the synthesis of lipids, including oils, phospholipids, and steroids. The SER also detoxifies drugs and poisons, especially in liver cells. Additionally, it stores calcium ions, which are crucial for muscle contraction and other cellular functions. The SER plays a versatile role in lipid production and detoxification.

L-Nuclear Pore: Nuclear pores are large protein complexes that span the nuclear envelope. They regulate the movement of molecules between the nucleus and the cytoplasm. Small molecules and ions pass through nuclear pores freely, while larger molecules require active transport. This selective permeability ensures that necessary molecules enter and exit the nucleus efficiently. Nuclear pores are essential for gene expression and cellular function.

M-Nucleoplasm / Nucleus: The nucleoplasm is the semi-fluid substance within the nucleus, also known as the nuclear matrix. It contains the chromatin and the nucleolus, and supports the structure of the nucleus. The nucleus is the control center of the cell, housing the genetic material (DNA). It regulates gene expression, cell growth, and reproduction. The nucleus is essential for maintaining the cell’s genetic information and coordinating cellular activities.

N-Cytosol ('Cytoplasm'): The cytosol is the fluid component of the cytoplasm, excluding organelles. It is the site of many metabolic reactions, including glycolysis and protein synthesis. The cytosol also provides a medium for the organelles to remain suspended and function efficiently. It contains dissolved nutrients, ions, and enzymes necessary for cellular processes. The cytosol plays a critical role in maintaining cell structure and facilitating intracellular communication.

P-Cell Surface Membrane: The cell surface membrane, also known as the plasma membrane, encloses the cell, separating it from its external environment. It is composed of a phospholipid bilayer with embedded proteins. The membrane controls the movement of substances in and out of the cell through selective permeability. It also contains receptors that allow the cell to respond to external signals. The cell surface membrane is vital for maintaining homeostasis and protecting the cell.

Process A - Endocytosis: Endocytosis is the process by which cells engulf external substances, bringing them into the cell. The cell membrane folds around the substance, forming a vesicle that pinches off into the cytoplasm. This process allows cells to intake nutrients, fluids, and other necessary molecules. Endocytosis also plays a role in removing pathogens and debris. It is essential for maintaining cellular function and communication.

Process B - Exocytosis: Exocytosis is the process by which cells expel substances from the cell. Vesicles containing the substances fuse with the cell membrane, releasing their contents into the extracellular space. This process is crucial for secreting hormones, neurotransmitters, and digestive enzymes. Exocytosis also helps in removing waste products from the cell. It is vital for cellular communication and maintaining homeostasis.

Cell Types 

The Cell Cycle

The cell cycle is a series of phases that a cell goes through as it grows and divides. It can be compared to a factory’s production line, ensuring that everything is built correctly and functions smoothly.

1. G1 Phase (Gap 1): This is like the initial planning and stocking phase of a factory. The cell grows in size, produces new organelles, and synthesizes proteins needed for DNA replication. Factory managers check existing machinery and stock up on raw materials and components needed for the upcoming production run.

2. S Phase (Synthesis): During this phase, the cell replicates its DNA so that each new cell will have an identical set of genetic instructions. The factory’s blueprint (DNA) is copied. Workers (enzymes) ensure every instruction (gene) is duplicated so that the new products will have the same specifications as the original.

3. G2 Phase (Gap 2): This phase is like a final inspection before production. The cell continues to grow and produces proteins necessary for cell division. Factory inspectors review the plans and materials, making sure everything is ready for production and that all resources are in place.

4. M Phase (Mitosis): The cell divides its copied DNA and cytoplasm to form two new cells. The production line operates, assembling new products based on the duplicated blueprints. The original factory splits into two identical factories, each with its own complete set of blueprints (DNA) and machinery (organelles).

5. Cytokinesis: This is the final step where the cell's cytoplasm divides, resulting in two separate cells. The newly built products are finalized and packed, ready to function independently as new factories.

Mitosis

Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half, producing four genetically unique daughter cells, essential for sexual reproduction.

Meiosis I:

1. Prophase I: Chromosomes condense, homologous chromosomes pair up and exchange genetic material (crossing over), the nuclear envelope breaks down, and spindle fibers form and attach to the homologous chromosomes. The factory sets up a special production line where custom blueprints are created by combining parts from different original blueprints.

2. Metaphase I: Homologous chromosomes line up in the middle of the cell. The customized components are aligned on the assembly line, ready for unique combinations to be assembled.

3. Anaphase I: Homologous chromosomes are pulled apart to opposite sides of the cell. The assembly line separates the customized components, ensuring each new product gets a unique set of parts.

4. Telophase I: The nuclear envelope re-forms around each set of chromosomes, and the cell divides into two cells (cytokinesis). The unique products are partially assembled and split into two separate production lines for further customization.

Meiosis II:

1. Prophase II: Chromosomes condense, the nuclear envelope breaks down, and spindle fibers form and attach to the chromosomes. The two production lines set up again to finalize the unique products.

2. Metaphase II: Chromosomes line up in the middle of the cell. The final components are aligned on the assembly line for the last stages of customization.

3. Anaphase II: Sister chromatids are pulled apart to opposite sides of the cell. The final assembly line splits the last components, ensuring each new product is complete and unique.

4. Telophase II: The nuclear envelope re-forms around each set of chromosomes, and the cell divides into four genetically unique cells (cytokinesis). The unique products are finalized and packaged, ready to serve their specific purposes, much like new, specialized factories.

By the end of meiosis, the factory has created four new and unique blueprints, each with a different combination of information from the original blueprints. This ensures that the final products (like new cells) are all different and special, contributing to genetic diversity in sexually reproducing organisms.

Homeostasis and Cells

Homeostasis refers to the process by which living organisms maintain a stable internal environment, despite changes in their external surroundings. Cells play a crucial role in maintaining homeostasis by performing various functions that regulate and balance the body's internal conditions. Here's an overview of how cells contribute to homeostasis:

Cell Membrane and Homeostasis

The cell membrane, or plasma membrane, is a selectively permeable barrier that controls the movement of substances in and out of the cell. It plays a vital role in maintaining homeostasis by:

1. Regulating Transport: The cell membrane uses various mechanisms such as passive transport (diffusion and osmosis) and active transport to regulate the entry and exit of ions, nutrients, and waste products. This ensures that the cell maintains the proper balance of these substances.

2. Maintaining Ion Balance: Ion channels and pumps in the cell membrane help maintain the correct concentrations of ions like sodium, potassium, calcium, and chloride. This is essential for nerve impulses, muscle contractions, and overall cellular function.

3. Communication: Receptor proteins on the cell membrane detect chemical signals (hormones, neurotransmitters) from the environment and initiate appropriate cellular responses to maintain homeostasis.

Cellular Respiration and Energy Production

Cells generate energy through the process of cellular respiration, which takes place in the mitochondria. This energy is essential for maintaining homeostasis because it powers cellular activities that regulate internal conditions.

1. ATP Production: Cellular respiration converts glucose and oxygen into adenosine triphosphate (ATP), water, and carbon dioxide. ATP is the primary energy currency of the cell, fueling processes like active transport, protein synthesis, and muscle contraction.

2. Metabolic Balance: Cells adjust their metabolic pathways based on the availability of nutrients and energy demands, ensuring a constant supply of ATP.

Waste Removal and Detoxification

Cells must eliminate waste products and toxins to maintain a stable internal environment. Organelles such as lysosomes and the smooth endoplasmic reticulum (SER) are involved in these processes.

1. Lysosomes: These organelles contain digestive enzymes that break down waste materials, cellular debris, and foreign substances. They help keep the cell clean and free of harmful substances.

2. Smooth Endoplasmic Reticulum: The SER is involved in detoxifying harmful chemicals and drugs, particularly in liver cells. It also metabolizes lipids and stores calcium ions, contributing to cellular homeostasis.

pH Regulation

Cells help maintain the pH balance of the body, which is crucial for enzyme activity and overall metabolic function.

1. Buffer Systems: Cells produce and utilize buffer systems that can neutralize excess acids or bases, ensuring the pH remains within a narrow range.

2. Respiratory and Renal Systems: Cellular activities in the lungs and kidneys regulate the levels of carbon dioxide and bicarbonate in the blood, which are key components in maintaining pH balance.

Temperature Regulation

Cells contribute to the body's temperature regulation by adjusting metabolic rates and through mechanisms like sweating and shivering.

1. Metabolic Heat Production: During cellular respiration, cells produce heat as a byproduct. This heat helps maintain body temperature.

2. Heat Dissipation: Cells in sweat glands produce sweat, which evaporates from the skin's surface, cooling the body down.

Homeostasis Summary

Homeostasis is the result of intricate cellular processes that regulate the internal environment of an organism. The cell membrane's selective permeability, energy production through cellular respiration, waste removal, pH regulation, and temperature control all contribute to maintaining stable conditions essential for survival. Understanding how cells achieve homeostasis highlights their critical role in overall health and function.

Unicellular Organisms

Unicellular organisms are organisms that consist of a single cell, which carries out all the functions necessary for life. Despite their simplicity, these organisms exhibit a wide variety of forms and can thrive in diverse environments. Here is an overview of some common unicellular organisms: Paramecium, Euglena, Amoeba, and extremophiles like thermophiles, halophiles, and acidophiles.

Unicellular Organisms Summary

Unicellular organisms, ranging from Paramecium, Euglena, and Amoeba to extremophiles like thermophiles, halophiles, and acidophiles, exhibit remarkable adaptability and diversity. These organisms play vital roles in ecosystems, contribute to our understanding of life processes, and offer valuable applications in biotechnology and industry. Understanding their structure, function, and adaptations provides insight into the complexity and resilience of life at the microscopic level.