cell organisation cytoplasm and organelles 1.3

The Cell: Cytoplasm and its Components

Overview of Cytoplasm

• Cytoplasm Definition:
  • Everything that sits inside the cell membrane, outside of the nucleus.
• Two Main Parts:
  1. Cytosol: Watery, viscous fluid.
  2. Cellular Organelles: Specialized structures within a cell that carry out specific functions.

Structure of the Cytoplasm

• Components:
  • Mitochondrion
  • Plasma Membrane
  • Smooth Endoplasmic Reticulum (SER)
  • Rough Endoplasmic Reticulum (RER)
  • Centriole
  • Lysosome
  • Vacuole
  • Nuclear Membrane
  • Nucleus
  • Nucleolus
  • Chromatin
  • Ribosomes
  • Golgi Apparatus
  • Microfilaments
  • Intermediate Filament
  • Microtubule
  • Glycogen

The Cytosol

• Definition: Intracellular fluid that fills the cell.
• Composition and Consistency:
  • Approximately 55% of total cell volume.
  • Varies in composition:
    • 75-90% water
    • 10-25% dissolved solutes including:
      • Ions
      • Glucose
      • Amino Acids
      • Fatty Acids
      • Proteins
      • Lipids
      • ATP
      • Waste Products
      • Organic Molecules

Cellular Organelles

• Definition: Specialized structures within a cell that carry out specific functions.
• Examples:
  • Ribosomes: Site of protein synthesis.
  • Endoplasmic Reticulum: Comprises smooth (SER) and rough (RER) ER.
  • Golgi Apparatus: Modifies and packages proteins.
  • Mitochondria: Produce energy in the form of ATP.
  • Lysosomes: Involved in breakdown of waste.
  • Vesicles: Transport substances.
  • Centriole: Involved in cell division.
  • Cytoskeleton: Provides structural support and facilitates movement.

Ribosomes

• Function: Synthesis of proteins.
• Structure: Composed of RNA and proteins.
  • Consist of two subunits:
    1. Large subunit
    2. Small subunit
• Formation: Ribosomes are formed in the nucleolus.
• Location: Can be found either:
  1. Attached to the Rough Endoplasmic Reticulum (RER)
  2. Floating free in the cytosol.

Endoplasmic Reticulum (ER)

• Definition: A network of membranous sheets and channels extending throughout the cytoplasm.
• Connection: Continuous with the nuclear envelope.
• Types:
  1. Rough Endoplasmic Reticulum (RER):
     • Studded with ribosomes.
     • Function: Structural modification of proteins.
  2. Smooth Endoplasmic Reticulum (SER):
     • Lacks ribosomes, contains unique enzymes.
     • Function: Synthesis of lipids and carbohydrates.

Golgi Apparatus

• Function: Modifies and packages proteins; acts as the cell’s packaging and distribution center.
• Structure: Composed of stacks of 3-20 flattened membranous sacs called cisternae.
  • Surfaces:
    • Entry: Facing the RER (cis cisternae).
    • Exit: Facing the plasma membrane (trans cisternae).
• Modifications: Involves the addition/removal of sugar molecules, sulfate groups, and phosphate groups.

Golgi Complex Process

1. Protein synthesized in ribosome is folded in the RER and packaged into a vesicle.
2. Vesicle shuttles to the Golgi (cis).
3. Vesicle fuses with the Golgi complex which delivers the protein into the lumen.
4. Specific enzymes modify the protein.
5. Modified protein is shuttled into the next cisterna via a transfer vesicle.
6. The protein passes through the Golgi complex and exits via the trans cisterna in a vesicle.

Vesicles/Vacuoles

• Function: Transport and delivery of substances.
• Structure: Membrane-bound sacs containing:
  • Water
  • Proteins/enzymes/hormones
  • Wastes
• Vacuoles: A type of vesicle specifically containing water/waste, continuously formed, remade, and remodelled from membranes of other organelles.

Mitochondria

• Function: Production of ATP through cellular respiration.
• Structure:
  • Composed of a double membrane (inner and outer).
  • Inner membrane forms numerous inward folds (cristae).
  • The inner matrix contains enzymes, ribosomes, and mitochondrial DNA (mtDNA).
• Number Variation:
  • Varies by cell type (e.g., red blood cells have 0 mitochondria; muscle cells have hundreds to thousands).
• Characteristics:
  • Contain mitochondrial DNA (mtDNA).
  • Can replicate independently.
  • Synthesize mitochondrial proteins.

Lysosomes

• Function: Breakdown of damaged organelles, pathogens, and organic compounds.
• Structure: Membrane-bound vesicles containing approximately 50 digestive enzymes.
• Capabilities:
  • Break down proteins, nucleic acids, carbohydrates, and lipids.
  • Degrade materials taken up from outside the cell.
  • Recycle obsolete components of the cell itself.
  • Optimal function at acidic conditions (pH ~5).

The Cytoskeleton

• Functions:
  • Supports the plasma membrane and gives the cell shape.
  • Supports the positioning and movement of organelles.
  • Enables cell movement.
• Structure: Protein framework consisting of filaments or slender tubes throughout the cell, characterized by:
  • Rigid, filamentous protein chains.
  • Highly labile - continuously remodelled.

Components of the Cytoskeleton

1. Microfilaments:

   • Narrow diameter (~7nm).
   • Composed of actin.
   • Filament structure resembles a double helix.
   • Characteristics: Highly dynamic; assembly and disassembly occur quickly.
   • Functions: Strength, structural support, facilitate cell movement, involvement in muscle contraction and cell division.

2. Intermediate Filaments:

   • Intermediate diameter (~8-11nm).
   • Varies in protein composition among cell types.
   • Composed of multiple strands of protein wound together.
   • More permanent structure, not rapidly assembled/disassembled.
   • Functions: Strength, structural support, movement of materials through the cytoplasm, anchoring organelles in place.

3. Microtubules:

   • Large diameter (~25nm).
   • Composed of hollow tubes of tubulin.
   • Highly dynamic; can grow/shrink quickly.
   • Functions: Strength, structural support (resists compression), movement of organelles, involved in cell division.

Centrioles

• Function: Support the spatial arrangement of the cell and are involved in cell division.
• Structure: Cylindrical structures composed of cytoskeletal microtubules.
• Formation of Centrosome: Two centrioles oriented at right angles, surrounded by a dense mass of proteins.
• Importance: Crucial for cell division; cells lacking centrioles (e.g., red blood cells, neurons) cannot divide.

Centrosome and Cell Division

• During Cell Division (Mitosis):
  • Centrioles are involved in the movement of DNA strands.
  • Essential role in the formation of the mitotic spindle, which segregates duplicated chromosomes.
  • Ensure equal distribution of DNA to each daughter cell by moving to opposite ends of the cell, pulling on duplicated chromosomes.

Projections from Cell Surface: Microvilli, Cilia, and Flagella

Microvilli

• Description: Finger-like extensions of the plasma membrane.
• Function: Increase surface area to aid absorption.
• Location: Found prominently in the digestive tract.

Cilia

• Description: Hair-like projections from the cell surface.
• Function: Move substances along the cell surface.
• Location: Commonly found in the respiratory tract.

Flagella

• Description: Similar in structure to cilia but much longer and usually singular.
• Function: Move the entire cell (locomotion).
• Location: Present in sperm cells (the only human cell with this structure).

Summary: Differences Between Microvilli, Cilia, and Flagella

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