Makindo Medical Notes

Cell structure

Cell Membrane

• Lipid Bilayer: A dynamic, 7.5 nm–thick barrier composed of two layers of phospholipids with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails oriented inward.
• Selective Barrier: Separates the internal contents of the cell from the interstitial space, controlling the passage of substances.
• Fluid Mosaic Model: Embedded proteins and lipids can move laterally within the bilayer, allowing for flexibility and dynamic interactions.
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Membrane Components

• Ion Channels: Proteins that form pores for selective ion passage across the membrane.
• Transmembrane Transport Proteins: Facilitate the movement of molecules and ions via active or passive transport mechanisms.

Adhesion Molecules

• These molecules are key to cell–cell recognition and adhesion, and their expression can be upregulated by cytokines such as interleukin-1 (IL-1) and tumor necrosis factor (TNF).
• L-Selectins: Expressed on neutrophils, they bind to mucin-like molecules (e.g., GlyCam-1) on endothelial cells.
• E-Selectins and Other Families: Include adhesion proteins such as LFA-1, MAC-1, and VLA-4, which mediate leukocyte-endothelial interactions.

Cell Regeneration

• Labile Cells: Continuously dividing cells (e.g., skin, mucous membranes, lymphoid tissues) that can fully regenerate damage.
• Stable Cells: Cells that do not actively divide once fully differentiated (e.g., connective tissue, liver, pancreas) but can regenerate in response to injury.
• Permanent Cells: Cells that have lost the capacity for division (e.g., neurons, cardiac muscle), with damaged areas typically replaced by fibrous tissue.

Cell Cycle and Checkpoints

• The cell cycle is a regulated series of events that leads to cell division. Its phases include:
  • G0 Phase: A quiescent state for cells not actively dividing.
  • G1 Phase (First Gap): The cell grows and prepares for DNA synthesis.
  • S Phase (Synthesis): DNA replication occurs.
  • G2 Phase (Second Gap): The cell continues to grow and repairs any DNA errors.
  • M Phase (Mitosis): Division of the nucleus and cell, resulting in two daughter cells.
• Checkpoints:
  • The G1 checkpoint verifies that conditions are favorable and that the DNA is undamaged before progressing to the S phase.
  • The G2 checkpoint ensures complete DNA replication and repairs any damage before mitosis begins.
  • Passage through the restriction point in G1 commits the cell to enter the cycle; if conditions are unsuitable, the cell may enter G0.

Cell Cycle Control and Cancer Drugs

• Cyclins: A family of at least 15 proteins whose levels fluctuate during the cell cycle.
• Cyclin-Dependent Kinases (CDKs): At least 9 serine/threonine kinases that, when bound to cyclins, drive cell cycle progression.
• Kinase Inhibitory Proteins (KIPs): These proteins bind to and inhibit CDKs, serving as regulatory brakes.
• Certain anticancer drugs target specific phases of the cell cycle. For example, agents like methotrexate and 5-fluorouracil interfere with the S phase (DNA synthesis), while vincristine and vinblastine disrupt mitosis during the M phase.

Overview of the Human Cell Nucleus

The nucleus is the command center of eukaryotic cells, where genetic information is stored, organized, and expressed. It controls protein synthesis and cell division.

• Nuclear Envelope: A double-membrane structure with nuclear pores that regulates the exchange of materials with the cytoplasm.
• Chromatin: A complex of DNA and proteins that condenses into chromosomes during cell division. In interphase, it exists in a relaxed form to allow gene expression.
• Nucleolus: A dense, non-membrane-bound structure responsible for synthesizing ribosomal RNA and assembling ribosomes.
• Nuclear Lamina: A supportive network of fibrous proteins lining the inner nuclear membrane that helps maintain nuclear shape and regulates DNA replication.

Cytoplasmic Organelles

• Cytoplasm: The gel-like substance that fills the cell, hosting a variety of metabolic pathways including glycolysis, protein synthesis, and fatty acid synthesis.
• Mitochondria: Organelles responsible for aerobic respiration. They carry out fatty acid β-oxidation, produce acetyl-CoA, and fuel the Krebs cycle to generate ATP.
• Golgi Bodies: Stacks of flattened membranous sacs that modify, sort, and package proteins and lipids for transport.
• Smooth Endoplasmic Reticulum (SER): A network involved in lipid synthesis, detoxification, and calcium storage.
• Lysosomes: Organelles containing digestive enzymes that break down waste materials, damaged organelles, and foreign substances.
• Peroxisomes: Small organelles that metabolize fatty acids and detoxify harmful substances.
• Ribosomes: The protein factories of the cell, found both free in the cytoplasm and bound to the rough endoplasmic reticulum, where they translate mRNA into polypeptides.

Conclusion

This comprehensive overview covers key aspects of cellular structure and regulation, from the fluid, dynamic cell membrane and its embedded proteins to the intricacies of cell cycle control and the nuclear powerhouse that houses genetic information. Understanding these components is essential for appreciating how cells function, how they maintain homeostasis, and how disruptions in these processes can lead to disease.