Cell Membrane and Organelles

Here is a detailed explanation of the cell membrane and organelles found within a cell:

1. Cell Membrane

• Thickness: 100 Å (Angstrom)
• Each layer: 25 – 30 Å
• Lipid Composition:
  • Phospholipid: Comprises 55% – The primary component of the cell membrane.
  • Cholesterol: Comprises 25 – 30% – Maintains membrane stability, regulates viscosity, and flexibility.
    • Note: The cholesterol content in the liver cell membrane is 40%, higher than that of other cell membranes. It has the highest cholesterol content among cell membranes.
  • Glycolipid: Comprises 18% – Plays a role in cell recognition and adhesion.
  • Hydrophobic fatty acids: Comprises 2% – A building block of phospholipids.
• Protein Composition:
  • Integral proteins: Comprises 70% – Connects two phospholipid layers, responsible for transport, binding, and signal transduction.
  • Peripheral proteins: Associated with the outer protruding ends of integral proteins via hydrophobic interactions or electrostatic forces. Play a role in enzymatic activity, cell cytoskeleton, and cell adhesion.
• Protein Network:
  • Shape: Hexagonal
  • Sides: Spectrin
  • Corners:
    • Type 1: Actin and band 4.1
    • Type 2: Ankyrin
• Organelles that synthesize new membranes:
  • RER (Rough Endoplasmic Reticulum): Synthesizes proteins and phospholipids for the cell membrane.
  • SER (Smooth Endoplasmic Reticulum): Synthesizes lipids and steroids.

2. Organelles

• Ribosome:
  • Prokaryote:
    • Small: 1 rRNA 16S + 21 proteins
    • Large: 2 rRNA 5S, 23S + 34 proteins
  • Eukaryote:
    • Small: 1 rRNA 18S + 33 proteins
    • Large: 2 rRNA 5S and 28S bound to 5.8S + 49 proteins
• Endoplasmic Reticulum (ER):
  • RER:
    • P/L (Protein/Lipid ratio): 1 < 2, ~2
    • Cholesterol: 6%
    • Phosphatidyl choline: 55%
  • SER:
    • P/L: 1 < 2, ~2
    • Cholesterol: 10%
    • Phosphatidyl choline: 55%
  • Note:
    • Pancreatic cells: Only have RER
    • Muscle cells: Only have SER
    • Liver cells: RER/SER ~ 1
    • Function of SER in testes: Synthesizes steroid hormones from cholesterol.
• Nuclear membrane:
  • Nuclear pores:
    • Diameter: 9 nm
    • Length: 15 nm
    • Density: 11 pores/µm2 (in mammals)
    • Note: Mammals synthesize 3000 – 4000 nuclear pores per cell.
  • Lamina: A protein network 10 – 20 nm thick, located inside the nuclear membrane.
• Mitochondria:
  • Outer membrane:
    • P/L: 1
    • Phosphatidyl choline: 2.5 times higher than the cell membrane
    • Cholesterol: 10%
    • Cholesterol: = 1/6 compared to red blood cell membrane
  • Inner membrane:
    • P/L: 3
  • Note: Mammalian liver cells have 1000 – 1500 mitochondria.
• Golgi Apparatus:
  • Thickness of each sac: 150 Å
  • P/L: Decreases from cis to trans (~2 at cis)
  • Note: New membrane synthesizing organelles are RER, SER, and Golgi.
• Centrioles:
  • Diameter: 150 nm
  • Length: 300 – 500 nm
  • Wall: 9 protein plates
  • Note: Two centrioles are always perpendicular to each other.
• Microtubules:
  • Diameter: 24 nm
  • Side wall: 5 nm
  • Wall: Usually has 13 protein strands (10 – 14)
• Microfilaments:
  • Diameter: 8 nm
  • Twist pitch: 72 nm
  • Actin G: Spherical shape
  • Actin F: Filamentous shape
• Intermediate filaments:
  • Size: 10 nm
  • Note: Thicker and shorter than microfilaments.
• Lysosomes:
  • P/L: 1
  • Cholesterol: = 1/2 compared to the cell membrane
  • pH: <= 4.8
• Peroxisomes:
  • Size: 0.15 – 1.7 µm
  • Membrane thickness: 6 – 8 nm
• Chloroplasts:
  • Number of membrane layers: 3
  • pH of chloroplast lumen: 8
• Perinuclear space:
  • Thickness: 10 – 15 nm

3. Other Concepts

• Mediated transport: For example: Glucose transport across the red blood cell membrane.
• Physiological phagocytosis: For example: The resorption of tadpole tails.
• Cytoskeleton: Formed by 3 organelles: Microtubules, microfilaments, and intermediate filaments.
• Glycerin: Does not decrease permeability when exposed to anesthetics.
• Ca2+: Permeability does not increase when binding with water.
• Ribosomal RNA gene: Located near the ends of chromosomes 13, 14, 15, 21, 22.
• Photosynthetic pigments: Have wavelengths of 680 nm and 700 nm.

Note:

• The numbers regarding size, ratio, density, etc., can vary depending on species, cell type, and environmental conditions.
• The knowledge of cell membrane and organelles is vast, this article only covers the basic principles. You should consult textbooks and specialized materials to understand it more comprehensively.

Hope this information is helpful!