Prokaryotic, Eukaryotic and Human Cells

• The smallest unit of protoplasm capable of independent existence is called a cell
• Cells form structural and functional units of all living organisms
• Cells can be grouped into prokaryotic and eukaryotic cells

Prokaryotic Cells

• The genetic material consists of a double-stranded DNA molecule called a nucleoid
• The nucleoid is unenclosed by a membrane
• No organelles are present in the cytoplasm
• Bacteria and archaea are prokaryotic cells

Eukaryotic Cells

• Genetic material exists as chromosomes within the nucleus, surrounded by a nuclear membrane
• The nuclear membrane separates the nuclear contents from the other cellular components
• Organelles are characteristic of the cytoplasm
• Form the structural and functional units of all living organisms (except bacteria and archaea)
• The human body is composed of trillions of eukaryotic cells

Human Cells

• Defined as a small mass of protoplasm enveloped by a cell membrane, containing at least one nucleus
• The protoplasm inside the nucleus is known as karyoplasm
• Remaining protoplasm of the cell is called cytoplasm
• There are more than 200 different types of cells in the human body
• Each living cell produces energy and carries out normal functions
• Each cell synthesizes macromolecules for its own use
• Many cells synthesize macromolecules for export
• Cells can communicate with each other

Shape and Size of Human Cells

• Free cells (e.g., white blood cells and ova) have a spherical or ovoid shape
• Non-free cells' shapes may include cuboidal, columnar, polygonal, pyramidal, cylindrical, fusiform, or irregular
• Cells range from 5 to 50 µm in diameter
• Mature ovum diameter: 120 µm
• Some large neurons diameter: 150 µm

Structure of a Human Cell

• Each cell is enveloped by a cell membrane
• Enclosed within the cell membrane are the nucleus and the cytoplasm

Cell Membrane

• Also called the plasma membrane or plasmalemma
• Envelops the cell and acts as a selective structural barrier between the contents of the cell and the extracellular space
• It is more than just a boundary, it participates in numerous functions
• Thin, so it cannot be seen in tissue sections under a light microscope
• Can be seen clearly in highly magnified electron micrographs
• Average thickness of 7.5 nm
• Has a trilaminar (3-layered) structure
• Consists of two electron-dense lines separated by an electron-lucent central zone
• Each layer is roughly 2.5 nm thick
• Thickness is variable, ranging from 7.5 to 10 nm in different cells
• Composed mainly of phospholipids, proteins, and cholesterol
• Consists of a bimolecular layer of phospholipids (lipid bilayer) with embedded proteins and cholesterol
• The lipid bilayer is fluid in nature and the large protein molecules suspended in it exhibit a mosaic pattern
• The cell membrane structure is known as the fluid mosaic model
• Intracellular membranes, which enclose the nucleus and organelles, also have the same trilaminar structure
• Each phospholipid molecule has a polar (hydrophilic) head and a nonpolar (hydrophobic) tail
• The head of each phospholipid molecule is composed of glycerol conjugated to a nitrogenous compound by a phosphate bridge
• The nitrogenous compound may be choline, serine, or ethanolamine
• The nonpolar tail of each phospholipid molecule is made up of two long chains of fatty acids which are covalently linked to the glycerol component of the polar head
• In the bimolecular layer, the hydrophilic heads of the phospholipid molecules