Cytoplasm, Cell Organelles, and Cytoskeleton Structure and Function

Include the following:

1.5 Cytoplasm and Organelles

• The cytoplasm is defined as everything located inside a cell with the exception of the nucleus.

• It is a feature found in both animal and plant cells.

• Physical Properties and Location:

  • It consists of jelly-like compounds positioned between the cell membrane and the nucleus.

  • It is primarily composed of organic and inorganic substances.

  • The cytoplasm is a fundamental cell component in which all other organelles reside.

  • It is colorless and contains approximately $80\%$water along with various nutrients.

  • It exhibits the properties of both viscous and elastic matter.

• Structure of Cytoplasm:

  • Cytosol: A gel-like substance that includes other matter.

  • Cell Organelles: Smaller cell-like bodies that are bound by separate membranes.

  • Cytoplasmic Inclusions: Materials contained within the cytoplasmic matrix.

  • Insoluble Molecules: Molecules that store energy but are not surrounded by any layers.

• Cytoplasmic Streaming:

  • This is a process that allows materials to move inside cells.

  • It is made possible by the elastic nature of the cytoplasm.

  • It aids in the distribution and movement of nutrients throughout the cell.

• Metabolic and Biological Roles:

  • The cytoplasm is the site for the majority of reactions occurring within cells.

  • It houses enzymes responsible for controlling metabolic activities.

  • It is where essential cellular and enzymatic reactions take place, such as cellular respiration and the translation of $mRNA$to proteins.

  • It serves as a buffer, protecting genetic material and organelles from damage due to collisions or changes in the $pH$of the cytosol.

Golgi Apparatus (Golgi Complex / Golgi Body)

• The Golgi apparatus is the primary organelle in eukaryotic cells responsible for packaging macromolecules into vesicles for transport to their site of action.

• Structure:

  • It is described as an elastomeric organelle composed of a set of flattened, stacked pouches known as cisternae.

  • The structure is pleomorphic (occurring in various forms), typically appearing as cisternae, vesicles, and tubules.

  • Cisternae: The smallest component, consisting of sac-like, flattened structures placed in bundles.

  • Tubules: Branched structures that extend from the edges of the cisternae.

  • Vesicles: Spherical components classified into three categories: secretory vesicles and clathrin-coated vesicles.

• Presence: It is located in the cytoplasm of both animal and plant cells.

• Functions:

  • Principal involvement in transporting, changing, and packaging proteins and lipids to specific locations.

  • Crucial for directing proteins and lipids to their correct destinations.

  • Involved in the synthesis of different organelles, including cell membranes and lysosomes.

Mitochondria

• Mitochondria are organelles characterized by double membranes, responsible for the production and storage of fuel for the cell.

• Main Purpose: The oxidation of various substances to release energy in the form of $ATP$(Adenosine Triphosphate).

• Structure:

  • Outer Membrane: A smooth outer layer.

  • Inner Membrane: Characterized by finger-like and fold-like structures known as the cristae.

  • Mitochondrial Interior: Contains various enzymes, coenzymes, and components for multiple metabolic cycles.

  • Pores: Exist within the membrane for the transport of substances, $ATP$, and phosphate molecules.

  • Matrix: A layer within the membranes that houses enzymes required for metabolic processes like the Krebs cycle.

• Functions:

  • Formation of $ATP$Energy is essential for the function of all cell organelles.

  • Assisting in balancing $Ca^+$ions within cells.

  • Assisting in the process of apoptosis (programmed cell death).

  • Incorporation of different blood-related hormones and blood components.

  • In the liver, mitochondria help to detoxify ammonia.

Chloroplast

• Chloroplasts are a type of plastid involved in photosynthesis in plants and algae.

• Chlorophyll: A vital pigment within the chloroplast that traps sunlight to produce glucose.

• Structure:

  • Double-membraned structure containing its own $DNA$, which it inherits from the previous chloroplast.

  • Typically lens-shaped; the number and shape can vary depending on the cell size.

  • Membrane System: Composed of an outer membrane, an inner membrane, and a thylakoid membrane.

  • Stroma: A gel-like matrix contained within the inner membrane.

  • Both the inner and outer membranes are porous to allow material transport.

  • The stroma contains $DNA$, starch granules, proteins, and chloroplast-ribosomes.

• Functions:

  • Serves as the primary center for both light-dependent and light-independent photosynthesis reactions.

  • Involved in the control of photorespiration via various proteins found in chlorophyll.

Endoplasmic Reticulum (ER)

• The name is derived from "Endo" (inside) and "Plasm" (cytoplasm), while "Reticulum" is Latin for "net."

• It is a plasma membrane that forms inside a cell, folding inwards to create an inner space called the lumen.

• The lumen is continuous and connected to the perinuclear area.

• The ER is the largest network of membranous, fluid-filled canals in the cell.

• Types of ER:

  • Rough Endoplasmic Reticulum (RER): Composed of cisternae, tubules, and vesicles. It is found throughout the cell and is involved in protein production.

  • Smooth Endoplasmic Reticulum (SER): Acts as a storage organelle associated with the production and use of steroids and lipids, as well as detoxification.

• Structural Forms:

  • Cisternae: Flattened, unbranched sac-like structures that can be stacked.

  • Vesicles: Spherical structures used to carry proteins throughout the cell.

  • Tubules: Branched tubular structures that connect the cisternae to the vesicles.

• Functions:

  • Hosts enzymes required for numerous metabolic processes.

  • The surface area is vital for diffusion, osmosis, and active transport.

  • Synthesis of lipids, including cholesterol and steroids.

  • Rough ER allows for the modification of polypeptides emerging from ribosomes to create the secondary and tertiary structures of proteins.

  • Production of various membrane proteins.

  • Plays a critical role in preparing nuclear envelopes following cell division.

Vacuole

• Vacuoles are membrane-bound organelles that vary in size across different species.

• Structure:

  • Enclosed by a membrane called the tonoplast.

  • Contains fluid composed of inorganic substances (like water) and organic materials (like nutrients and enzymes).

  • Formed via the fusion of various types of vesicles, making them structurally similar to vesicles.

• Functions:

  • Acts as a storage facility for nutrients and waste substances to protect the cell from toxicity.

  • Essential for homeostasis; regulates the $pH$ equilibrium in cells through the inflow and expulsion of $H^+$ions.

  • Contains enzymes required for various metabolic processes.

Ribosomes

• Ribosomes are ribonucleoproteins containing equal portions of RNA and proteins, along with components needed for protein synthesis.

• Location:

  • Prokaryotes: Found floating freely.

  • Eukaryotes: Found either floating freely or attached to the endoplasmic reticulum.

• Structure:

  • Composed of two subunits.

  • Prokaryotic Ribosomes ($70S$): Consist of a larger $50S$ subunit and a smaller $30S$subunit.

  • Eukaryotic Ribosomes$80S$): Consist of a larger $60S$ subunit and a smaller $40S$subunit.

• Lifespan: Ribosomes have a short lifespan; after protein synthesis, the subunits break up and are either recycled or degraded.

• Functions:

  • The primary site of biochemical protein synthesis in all living organisms.

  • They arrange amino acids in the specific order indicated by $tRNA$.

Plasmodesmata

• Plasmodesmata are tiny passages or channels that allow for communication and the transfer of materials between cells.

• Structure:

  • Typically $10^3$ to $10^5$Plasmodesmata link two adjacent cells.

  • They are approximately $50-60\,nm$in size.

  • Three Layers:

    1. Plasma Membranes: Continuous with the cell's plasma membrane and composed of the same phospholipid bilayer.

    2. Cytoplasmic Sleeves: In continuous contact with the cytosol, permitting substance exchange.

    3. Desmotubule: A part of the endoplasmic reticulum that creates a connection between cells and facilitates molecule transport.

• Functions:

  • The principal location for intercellular communication.

  • Facilitates the transfer of molecules such as proteins, $RNA$, and viral genomes.

1.6 Cytoskeleton

• The cytoskeleton is a complex, dynamic network of interlinking protein filaments located in the cytoplasm of all cells, including bacteria and archaea.

• It extends from the cell nucleus to the cell membrane and uses similar proteins across various organisms.

• Eukaryotic Components:

  • Microfilaments

  • Intermediate filaments

  • Microtubules

  • All three components can undergo rapid growth or disassembly based on the cell's needs.

• Primary Functions:

  • Provides the cell with its shape and mechanical resistance to deformation.

  • Enables cell migration by contracting and deforming the cell and its environment.

  • Involvement in cell signaling pathways and the uptake of extracellular material.

  • Segregation of chromosomes during cell division.

  • Facilitation of intracellular transport of vesicles and organelles.

  • Acts as a template for cell wall construction.

  • Forms specialized cellular structures, including cilia, flagella, lamelliopodia, and podosomes.

• Variability: The behavior, structure, and function of the cytoskeleton can vary significantly depending on the specific organism and cell type.