Cell Types and Plasma Membrane Structure

Cell Types

Eukaryotic Cells

Eukaryotic cells (found in animals and plants) are characteristic of the cells of organisms. Their structure is evolved and complex.

Prokaryotic Cells

Prokaryotic cells are very simple and primitive and have few structures inside.

Differences Between Prokaryotic and Eukaryotic Cells

• Size: Eukaryotes are typically 10µm and larger, while prokaryotes are around 1µm.
• Nucleus: Prokaryotes have no true nucleus (genetic material is not isolated from the cytoplasm by a nuclear membrane), while eukaryotes have a true nucleus.
• Organelles: Cellular organelles (such as the Golgi apparatus, mitochondria, etc.) are present in eukaryotic cells but not in prokaryotic cells.
• Cell Wall: Prokaryotes have a strong cell wall (often containing peptidoglycan). Eukaryotic plant cells have a cell wall (cellulose), but animal eukaryotes do not.

Differences Between Eukaryotic Animal and Plant Cells

• Cell Wall: Present in plant cells, absent in animal cells.
• Centrioles: Present in animal cells, generally absent in plant cells (except lower plants).
• Nucleus Position: Often central in animal cells, often peripheral in plant cells due to a large central vacuole.
• Vacuoles: Plant cells typically have one large central vacuole; animal cells have smaller, temporary vacuoles.
• Chloroplasts: Present in plant cells, absent in animal cells.

Plasma Membrane

The plasma membrane is found in all cells and surrounds most cellular organelles, except centrioles and the cytoskeleton. Ribosomes are not membrane-bound.

Its structure is approximately 50% phospholipids (forming a lipid bilayer) and 50% proteins. It has a thickness of about 5-10 nm.

The Fluid Mosaic Model

In the fluid mosaic model, the phospholipids are in constant motion, and proteins float within the membrane, which are also in motion. It is also asymmetric, and proteins are often found on the external surface of the outer membrane.

In animal cells only, the glycocalyx is present (a set of oligosaccharides associated with proteins and phospholipids).

Functions of the Plasma Membrane

1. Selective Permeability: All membranes prevent or allow the passage of only certain substances. They are capable of allowing nutrient input and the output of waste products.
2. Receptor Function: The glycocalyx and proteins present on its external side are capable of capturing substances needed by the cell (e.g., hormones).
3. Immunological Function: The proteins and glycocalyx identify cells carrying necessary substances and help avoid them being destroyed by the immune system (leukocytes).