Fundamentals of Cell Physiology: Structure and Function

Cells are the basic units of structure and function in the body, with over 100 trillion cells that come in a variety of shapes and sizes. Cell: Basic living unit of structure & function of the body. They are very small (10 -5 m in diameter), highly organized, and each type of cells has a special function.

General Characteristics of All Cells

All cells share certain characteristics: General cell structure & components and general mechanisms for changing nutrients to energy. They deliver end products into their surrounding fluid, and almost all have the ability to reproduce.

Principal Parts of the Cell

The basic structures of cells include a plasma membrane, cytoplasm containing organelles like the nucleus, mitochondria and ribosomes, and the nucleus which houses genetic material. The cell has two major compartments: the nucleus & the cytoplasm.

• Plasma (cell) membrane: Surrounds, holds cell together & gives its form.
• Cytoplasm & organelles: Matrix substance in which chemical reactions occur.
• Nucleus: Contains genetic code that determines which proteins will be manufactured by the cell.

The Plasma (Cell) Membrane

The plasma membrane is selectively permeable and controls what enters and exits the cell. It contains proteins, lipids and carbohydrates. It has a 75 to 111 Å thickness and is not solid; it separates cell’s internal structures from extracellular environment and participates in intracellular communication.

The membrane is composed of: a double layer of phospholipids (hydrophobic/ hydrophilic parts), proteins that span, or partially span the membrane, and negatively charged carbohydrates attached to the outer surface.

Composition of the Cell Membrane

Based on the provided material, the general composition of the cell membrane is as follows:

• Proteins: 55%
• Lipids: 41%
  • Phospholipids: 25%
  • Cholesterol: 12%
  • Glycolipids: 4%
• Carbohydrates: 3%

Cell Membrane Proteins and Their Functions

Proteins are divided into two main categories:

1. Integral proteins: / Internal or intrinsic proteins - span the membrane; transport proteins; provide structural channels or pores.
2. Peripheral proteins: / external or extrinsic proteins - embedded in one side (face) of the membrane; carrier proteins; bind with substances to be transported; include hormone receptors & cell surface antigens.

General functions of cell membrane proteins include providing structural support and transporting molecules across the membrane. They provide enzymatic control of chemical reactions at cellular surface, and some function as receptors for hormones or regulatory molecules.

The Cytoplasm and Nucleus

The cytoplasm contains the major cell organelles & a fluid called cytosol. It is a fluid, jellylike substance b/w cell membrane & nucleus in which organelles are suspended. The cytoplasm and organelles work together to carry out specialized functions and transport materials within the cell.

The nucleus is a double-layered membrane that surrounds the nucleus, composed of protein & lipid molecules. It consists of the following components:

• Nuclear envelope: Supports nucleus & controls passage of materials b/w nucleus & cytoplasm.
• Nucleolus: Dense nonmembranous mass composed of protein & RNA molecules; produces ribosomal RNA for ribosomes.
• Chromatin: Fibrous strands composed of protein & DNA.