Anatomy of the Cell

Components of eukaryotic cells

  • Eukaryote: any cell or organism that possesses a clearly defined nucleus
  • Eukaryotic cells have an outer membrane, and the interior of a eukaryotic cell is composed of organelles, the cytoskeleton, and the cytosol
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Plasmalemma (outer membrane)

  • Separates cytoplasm from outside environment, forming a selectively permeable barrier
  • Phospholipid bilayer
    • Hydrophilic heads at outer and inner surfaces
    • Hydrophobic tails at centre
  • Contains integral proteins
  • Can exocytose and endocytose material
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Cytosol

  • Solution of proteins, electrolytes and carbohydrates which the cellular components are embedded

Nuclear envelope

  • Encloses nucleus
  • Inner and outer nuclear membrane with the perinuclear cistern in the middle
    • Outer nuclear membrane is studded with ribosomes
  • Nuclear pores: allow the transport of molecules across the nuclear envelope

Nucleus

  • Contains chromosomes
    • Euchromatin: DNA is more dispersed and undergoing transcription
    • Heterochromatin: DNA highly condensed and not undergoing transcription
  • Site of RNA synthesis
    • mRNA and tRNA in nucleus
    • rRNA in nucleolus

Ribosomes

  • Formed in nucleolus
  • Small subunit binds RNA, large subunit catalyzes peptide bond formation

Endoplasmic reticulum

  • RER: studded with ribosomes, responsible for protein synthesis and initiation of glycoprotein formation
  • SER: involved in continuous processing of proteins from RER and lipid synthesis

Golgi apparatus

  • Composed of a group of flattened, membrane bound cisternae
  • Transport vesicles arrive at Golgi from SER; Golgi modifies and packages them

Cytoskeleton

  • Consits of protein filaments which allow movement of organelles and also movement of the cell itself
  • Microfilaments: composed of actin, involved in cellular movement, cytokinesis, endo and exocytosis etc., 7nm
  • Intermediate filaments: 6 proteins that bind intracellular elements together and to the plasmalemma, >10nm
  • Microtubules: composed of ⍺ and β tubulin, originate from centrosome, polar, dynein and kinesin attach and move along them, 25nm

Cellular transport

  • Endocytosis: membrane invaginates, fuses, newly made endocytic vesicle buds into cell
    • Often receptor mediated
  • Exocytosis: works in reverse to endocytosis
  • Phagocytosis: bacteria binds to cell surface, cell engulfs it to form phagosome, binds with lysosome containing digestive enzymes

Intracellular junctions

  • Occluding junctions: link cells to form a diffusion barrier
    • Also known as tight junctions or zonula occludens
  • Anchoring junctions: provide mechanical strength
    • Adherent junctions: link submembrane actin bundles of adjacent cells via cadherin molecules
    • Desmosomes: link submembrane intermediate filaments of adjacent cells
    • Hemidesmosomes: link submembrane intermediate filaments of cells to extacellular matrix through transmembrane proteins
  • Communicating junctions (gap junctions): allow movement of molecules between cells
    • Each junction is a circular patch studded with pores