Concept of Cell Structure

Cells can be categorized into unicellular and multicellular organisms
Bacteria, protozoa, and unicellular fungi are examples of unicellular organisms without a nucleus
Human, plants, and animals are multicellular organisms with more than one cell, containing a nucleus and plasma membrane

Red blood cells are unicellular and lack a nucleus
They are typically biconcave in shape and responsible for carrying oxygen in the blood

Neurons are long and thin cells responsible for propagating nerve impulses for signaling
They have subcellular parts attached to the end of the cell

Cells contain four major macromolecules: carbohydrates, proteins, lipids, and nucleic acids
These macromolecules are polymers formed from smaller units called monomers
All cells have a plasma membrane, cytoplasm, ribosomes, and DNA molecules

Prokaryotic cells are unicellular and lack a nucleus and membrane-bound organelles
They have a simple intracellular organization and a rigid cell wall composed of peptidoglycans in bacteria
Components of prokaryotic cells include:
- Cell membrane (regulates entry and exit of materials)
- Cytoplasm (rigid gel-like substance within the cell membrane)
- Ribosomes (70S, responsible for protein synthesis)
- Nucleoid (contains genetic material, single circular bacterial DNA chromosome)
- Plasmids (small circular DNA molecule that can replicate independently)
Prokaryotic cells have protective coating layers: glycocalyx, cell wall, and cell membrane
External structures include sex pili (facilitate genetic material transport), fimbriae (enable bacteria to adhere to surfaces), and flagella (used for locomotion)

Eukaryotic cells are multicellular and contain a nucleus and membrane-bound organelles
They have a simple intracellular organization and are larger in size compared to prokaryotic cells
Components of eukaryotic cells include:
- Cell membrane
- Cytoplasm (occupies everything outside the nucleus, including cytosol, microtubules, and other organelles)
- Ribosomes (80S, composed of ribosomal protein and rRNA)
- Lysosomes (only in animal eukaryotic cells)
- Chloroplasts (only in plant cells, responsible for photosynthesis)
- Peroxisome
- Nucleolus (chromatin)
- Nuclear membranes
- Nuclear pore
- Smooth and rough endoplasmic reticulum
- Mitochondria
- Golgi apparatus
- Centrioles

The cytoskeleton is a three-dimensional structure of protein in the cytoplasm
It regulates cell shape and orientation, providing strength and rigidity, and facilitates organelle movement and cellular processes
The cytoskeleton is composed of three filamentous protein polymers: microfilaments, intermediate filaments, and microtubules
These protein filamentous polymers are involved in muscle contraction, cell signaling, and other cellular processes