Cell Biology: Cytoplasm and Cytoskeleton

Cytoplasm

• Cytoplasm is a gel-like solution within cells.
• It contains particles too small to settle and too large to dissolve in water.
• Composed of:
  • Dispersive phase (mostly water: 90%)
  • Dispersed phase (particles in water):
    • 9% organic compounds (proteins, sugars)
    • 1% minerals (salts)
• Functions:
  • Fills cell, gives shape
  • Suspends organelles
  • Site of metabolic reactions
  • Transports materials via cytoplasmic movement (rotation, circulation, pulsation, fountaining)
• Exists in two forms:
  • Sol (semi-liquid, more fluid)
  • Gel (semi-solid, thicker)
• Cytoplasmic flexibility and thickness due to high protein content.

Cytoskeleton

• The cytoskeleton is the internal cellular "skeleton."
• A system of filaments with three types:
  • Intermediate filaments:
    • 8-10 nm diameter
    • Tissue-specific proteins (e.g., keratin)
    • Provide mechanical strength, maintain cell shape, and form the nuclear lamina.
  • Microtubules:
    • ~25 nm diameter
    • Composed of tubulin
    • Form centrioles, mitotic spindle, transport pathways within cells, and cilia/flagella.
    • In non-dividing cells, microtubules cluster in the centrosome.
  • Actin filaments:
    • ~7 nm diameter
    • Composed of actin
    • Support cells and organelles, drive cytoplasmic and organelle movement , and facilitate cell shape changes and muscle contraction.

Membrane-bound Organelles

• Categorized by membrane count:
  • Double membrane:
    • Nucleus: Contains genetic material.
    • Mitochondria: Site of cellular respiration.
    • Chloroplasts: Found in plant cells.
  • Single membrane:
    • Golgi apparatus: Modifies and sorts proteins.
    • Lysosomes: Contain digestive enzymes.
    • Peroxisomes: Break down peroxides.
    • Endoplasmic reticulum (ER): Network of channels, site of protein synthesis (rough ER) and lipid synthesis (smooth ER)
    • Vacuoles:
      • Animal cells: Waste storage
      • Plant cells: Water balance and waste disposal.


Non-membrane bound organelles

• Cell wall: Outer covering of non-animal cells
• Cytoskeleton: Provides cell structure
• Ribosomes: Site of protein synthesis
• Centrosome/Microtubule Organizing Center: Contains centrioles and microtubules important in cell division
• Centrioles: Cylindrical organelles involved in spindle fiber formation

Nucleus

• Structure:
  • Usually one per cell (monokaryocytes)
  • Some cells have two or more nuclei (di/polykaryocytes)
  • Some cells (like red blood cells) lack a nucleus
  • Shape, size depends on cell type and activity
  • Typically centrally located, but can vary
  • Makes up about 10% of cell volume.
• Functional states:
  • Interphase: Resting state, before cell division
  • Mitotic: During cell division
  • Metabolic: Directs metabolic processes in non-dividing cells.
• Internal Components and Functions:
  • Nuclear envelope, membrane protecting nucleus
  • Nucleoplasm/Nuclear matrix, fluid inside
  • Nucleolus site of ribosome production (made of ribosomal RNA subunits from fragments of 5 chromosomes)
  • Chromatin: DNA & proteins, exists in two forms:
    • Heterochromatin (condensed, less active)
    • Euchromatin (dispersed, active RNA production)
  • Nuclear function:
    • DNA replication
    • RNA synthesis (transcription)
    • Ribosome formation

Endoplasmic Reticulum (ER)

• Network of interconnected membranes (cisternae, channels, vesicles)
• Increases internal surface area for cellular processes.

• Facilitates transport and compartmentalization within the cell

• Smooth ER: Lipid synthesis, detoxification

• Rough ER: Protein synthesis, modification, and connection to other organelles.


Ribosomes

• Small structures composed of rRNA & proteins.

• Two types in eukaryotic cells:
  • Free ribosomes (synthesize proteins for the cytosol)

  • Bound ribosomes (attached to ER, synthesize proteins for secretion or modification).

• Ribosomes in mitochondria and chloroplasts are smaller (70S) than those in the cytoplasm (80S)


Mitochondria

• Vary in number, size and shape dependent on cell function
• Two membrane structure (outer and inner) with inner membrane folds (cristae)
• Contain mtDNA, 70S ribosomes, enzymes for ATP production
• Site of aerobic respiration, ATP production (Krebs cycle, electron transport chain)
• Function in cellular energy production and movement within cells.


Golgi Apparatus

• Flattened sacs (cisternae); with vesicles
• Three functional regions: cis, medial, and trans
• Processes proteins and lipids for export
• Adds carbohydrates to molecules (e.g., proteins, lipids)
• Modifies substances received from ER, cell membrane, and endosomes
• Involved in maturation of precursor proteins like proalbumin and preproinsulin.

Lysosomes

• Membrane-bound vesicles for intracellular digestion

• Contain hydrolytic enzymes (acid hydrolases)
• Function in acidic conditions (pH 5)

• Two types:
  • Primary lysosomes (contain inactive enzymes)
  • Secondary lysosomes (contain endocytosed materials and autophagosomes), divided into:
    • Autolysosomes(autolysis & autophagy)
    • Heterolysosomes(formed by fusing with digested material)

• Break down cellular components, recycling their materials

Centrosome

• Structure found near the cell nucleus.
• Composed of centrioles.
• Duplicates during cell division to help organize spindle fibers in cell division.

Plant Cells

• Contain additional components:

  • Plastids (oval-shaped organelles with their DNA and ribosomes)
  • Cell wall: Provides rigidity and protection
  • Vacuoles: Storage of water.



Plastids

• Three types:
  • Chloroplasts (photosynthesis, contain chlorophyll)

  • Chromoplasts (pigments, color)
  • Leucoplasts (storage, store protein, starch, and fats).