Cell Structure Overview

Cell Structure

• The human body is organized in a hierarchical manner from the atomic level to organ systems.
• Eukaryotic cells have a defined nucleus, while prokaryotic cells lack a true nucleus.
• Cells are composed of cytoplasm, organelles, cytoskeleton, and inclusions.
• A cell's shape is often spherical due to its efficiency and suitability.
• Cells typically range in size from 10-12 micrometers in diameter.
• Cells can vary in size, with some reaching 100 micrometers (e.g., ovum) and others being smaller (e.g., some blood cells at 6 micrometers).
• Cell size is constant within a specific cell type, regardless of the organism's size. This is known as the "Law of constant volume".
• The body begins as a single fertilized egg cell.
• Nerve cells and cardiac muscle cells do not replicate after birth.
• Kidney and liver cells increase in number until reaching a constant level during adulthood.
• Blood and skin cells constantly replenish, but their overall number remains relatively stable.

The Plasma Membrane

• The plasma membrane is a thin barrier approximately 7.5-10 nanometers thick.
• It encloses the cytoplasm and acts as a selective barrier that regulates the passage of substances into and out of the cell.
• The plasma membrane is composed of a phospholipid bilayer, cholesterol, proteins, and oligosaccharides.
• The phospholipid bilayer is arranged in a double layer with hydrophobic tails facing inward and hydrophilic heads facing outward.
• Integral proteins embedded within the membrane carry out a variety of functions, including transport of glucose and amino acids, serving as ion channels for passive diffusion, acting as pumps for ions like calcium, sodium, potassium, and hydrogen, and acting as receptors for hormones and neurotransmitters.
• Peripheral proteins are loosely associated with the membrane surfaces, particularly the inner surface, and are easily extracted.
• Oligosaccharides on the membrane surface associate with integral proteins and lipids to form glycoproteins and glycolipids, respectively. These components create the cell coat or glycocalyx.

Plasma Membrane Transport Processes

• Passive transport does not require energy and includes simple diffusion and facilitated diffusion.
• Active transport requires energy to move substances across the membrane. It can be achieved through ionic pores or carrier proteins.
• Bulk transport is a mechanism for moving large quantities of materials across the membrane and includes endocytosis and exocytosis.