Structural Composition of Cell Notes

Cytoplasm

• Includes all material inside the cell membrane except the nucleus.
• Three main components:
  • Cytosol (intracellular fluid):
    • A semi-gelatinous substance separated from the extracellular fluid (ECF) by the cell membrane.
    • Contains dissolved nutrients, proteins, ions, and waste products.
  • Non-membranous organelles:
    • Cytoskeleton, centrioles, ribosomes.
  • Membranous organelles:
    • Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus.

Cytoskeleton

• A network of protein fibers providing structural support for the cell.
• Made of microtubules, intermediate filaments, and microfilaments.
  • Microfilaments:
    • Smallest element of the cytoskeleton.
    • Involved in muscle contraction.
  • Intermediate filaments:
    • Maintain the structural integrity of the cell.
    • Resist mechanical stresses.
  • Microtubules:
    • Involved in intracellular transport.
    • Allow movement of chromosomes during cell division.
    • Major structural and functional component of cilia and flagella.

Cell Extensions

• Cilia:
  • Tiny hairlike structures projecting from the cell surface.
  • Move substances along the cell surface.
  • Example: Ciliated cells of the respiratory system move mucus.
• Flagella:
  • Long, whiplike structures.
  • Push the cell through fluid.
  • Example: Tail of sperm.
• Microvilli:
  • Tiny, fingerlike extensions.
  • Increase surface area to increase absorption and/or secretion.
  • Found in the lining of the intestine and kidney tubule cells.

Cytoplasmic Organelles

• Organelles are cellular machinery with specific functions vital to maintaining homeostasis.

Nucleus

• Largest organelle.
• DNA:
  • Template for protein synthesis with the help of RNA.
  • Transmits traits from one generation to the next.
• Nuclear envelope:
  • Double-membrane barrier.
  • Outer layer is fused to the endoplasmic reticulum (ER).
  • Controls the movement of substances.
• Nucleolus:
  • Assembles ribosomes.
• Chromatin:
  • When the cell is not dividing, DNA forms chromatin.
  • When the cell is dividing, chromatin coils to form chromosomes.

Endoplasmic Reticulum (ER)

• Continuous network of tubules fused with the nuclear membrane.
• Two varieties: rough ER and smooth ER.
  • Rough Endoplasmic Reticulum (Rough ER):
    • Has ribosomes bound to it.
    • Involved in protein synthesis, transport, and storage of proteins.
  • Smooth Endoplasmic Reticulum (Smooth ER):
    • Involved in lipid, fatty acid, and steroid synthesis.
    • Capable of several detoxification reactions.
    • Stores calcium ions (important for muscle contraction).
    • Lots of smooth ER in liver cells and skeletal muscle.

Ribosomes

• Small, dense granules of RNA and protein.
• Site for protein synthesis.
• Float in the cytoplasm as ‘free ribosomes’ or attach to membranes, forming the rough ER.

Protein Expression in ER

• Ribosome attached to ER.
• Protein extrudes into the lumen of the ER.
• Is folded and inserted into the membrane as it is made.
• Modifications are added (e.g., glycosylation).
• Same proteins are sorted together.
• Move to the Golgi apparatus in transport vesicles.

Golgi Apparatus

• Stack of flattened membranous sacs (cisternae).
• Site of protein modification and maturation.
• Extensive remodeling of carbohydrate chains on proteins.
• Layers of cisternae that are not connected.
• Starts at the cis face (receives vesicles from the ER) and ends at the trans face (vesicles move to final destination).
• Involved in secretion and internal membrane processes.

Mitochondria

• "Powerhouse" of the cell; site for ATP generation.
• Surrounded by a double membrane.
• Inner membrane is folded into cristae, increasing the surface area for ATP production.
• Involved in the metabolism of glucose, fatty acids, and amino acids to form energy (ATP).
• $ATP \rightarrow ADP + P + energy$
• ATP powers most processes.
• Busy cells (e.g., liver cells) have large amounts of mitochondria.
• Ability to replicate themselves due to mitochondrial DNA; example: exercising muscle cells.

Lysosomes

• Membrane sacs filled with enzymes (formed in the Golgi apparatus).
• Act as the digestive system of the cell; break down old or damaged organelles.
• Inappropriate release of lysosomal enzymes:
  • Rheumatoid arthritis: destruction of joint tissue.
  • Tay-Sachs disease: failure to break down glycolipids.

Peroxisomes

• Smaller than lysosomes.
• Degrade long fatty acids.
• Detoxify some harmful materials.
• Degrade hydrogen peroxide (a toxic molecule).
• Convert hydrogen peroxide to water and oxygen.
• Disrupt the normal processes of lipids.

Test Your Understanding

• Endoplasmic reticulum (ER) with attached ribosomes is called rough ER.
• The organelle that serves as the intracellular 'digestive system' is the lysosome.