Cell Biology Course (Bio-1101) Lect. 5

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Study Notes

Cells have a cell membrane (plasma membrane/plasmalemma) that surrounds the protoplasm.
Protoplasm is composed of cytoplasm and nucleoplasm.
Cytoplasm is located between cell membrane and nuclear envelope.
Cytosol is mostly water, enriched with inorganic and organic constituents.
Organelles are categorized as:
- Non-membrane bound (e.g., cytoskeleton, ribosomes)
- Single-membrane bound (e.g., endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles)
- Double-membrane bound (e.g., nucleus, mitochondria, chloroplasts)
Nucleoplasm is within the nuclear envelope-bound nucleus.
Cytoplasmic inclusions are granules (starch, glycogen) that store energy.
Cytoplasm is mostly water (70-80%). It also contains proteins, carbohydrates, salts, sugars, amino acids, nucleotides, waste products, and other biomolecules.
Organelles are metabolically active units of the cell.

Organelles can be classified into three categories:
- Organelles without a membrane: Ribosomes and cytoskeleton
- Single-membrane bound organelles: Vacuole, lysosome, Golgi apparatus, Endoplasmic reticulum
- Double-membrane bound organelles: Nucleus, mitochondria, chloroplast

The cytoskeleton is composed of three main structures:
- Microtubules: Hollow tubes, 25 nm diameter, composed of α-tubulin and β-tubulin dimers. Maintain cell shape, cell motility, chromosome movements and organelle movement.
- Microfilaments: Two intertwined strands of actin, 7 nm diameter, maintain cell shape, muscle contraction, cell shape changes.
- Intermediate filaments: Fibrous proteins in cables, 8-12 nm diameter. Composed of several different proteins including keratins. Maintain cell shape, anchor nucleus and certain organelles, form nuclear lamina.

Bacterial ribosomes (70S) are composed of 65% rRNA and 35% ribosomal proteins, with a small (30S) and large (50S) subunit.
- 30S subunit has 16S rRNA and 21 proteins
- 50S subunit has 5S, 23S rRNA, and 31 proteins
Eukaryotic ribosomes (80S) have a small (40S) and large (60S) subunit.
- 40S subunit has 18S rRNA and 33 proteins
- 60S subunit has 5S RNA, 28S RNA, a 5.8S rRNA, and 46 proteins
Ribosomes are the site of protein synthesis.
Free ribosomes synthesize cytosolic, cytoskeletal proteins and proteins imported to nucleus, mitochondria and peroxisomes
Bound ribosomes (attached to ER) synthesize proteins for transport through Golgi apparatus. These proteins can enter into membranes, be stored in lysosomes, or be secreted from the cell.

Proteasomes are the major site for the degradation of unwanted, damaged, malformed, or incorrectly folded proteins.
Before a protein enters, it must be tagged with ubiquitin.
Ubiquitination delivers proteins to the proteasome where they are broken down into small peptides that are 7-8 amino acids long.
Proteasomes are 26S nonmembranous cylindrical organelles with a hollow center.
Composed of a 20S core particle with 2 regulatory particles (each 19S).
2 α subunits bind to regulatory particles, and 2 β subunits function as proteolytic enzymes.