Cell Membrane

Human Physiology

Lesson 01 : Cell, Organization of Body

Cell Membrane

As its outer boundary, the animal cell has a special structure called the cell or plasma membrane. All of the substances that enter or leave the cell must in some way pass through this membrane.Cell membrane is a thin, pliable elastic structure of 7.5-1 0 nanometres thickness. Main constituents of the plasma membrane are phospholipids, proteins and carbohydrates. The phospholipids, emphatic molecules, are regularly organized in the membrane according to their polarity: two layers of phospholipids form the lipid bilayer with the polar part of the phospholipids pointing to the exterior of the layer and the non-polar phospholipids chains in the interior. Proteins can be found embedded in the lipid bilayer and there are also some carbohydrates bound to proteins and to phospholipids in the outer face of the membrane.

Cellular matrix: The major substance of the cell is known as protoplasm. It is a combination of water and a variety of materials dissolved in the water. Outside the cell nucleus, protoplasm is called cytoplasm. Inside the cell nucleus, protoplasm is called nucleoplasm. Cytoplasm or protoplasm is basically the major substance that fills the cell, also referred to as cytosol meaning cell substance. It is a jelly like material which is eighty percent water in the form of a colloidal suspension.

Cell Organelles: Within the cytoplasm, certain structures are called organelles. These organelles include structures such as the endoplasmic reticulum, ribosome, various kinds of vacuoles, the Golgi apparatus, mitochondria, and centrioles.

Endoplasmic reticulum resembles a circulatory system for the individual cell. It is a network composed of unit (single-thickness) membranes going through cytoplasm onto cell membrane and stores, separates, and serves as cell’s transport system. Rough endoplasmic reticulum has ribosomes attached to transport proteins and helps make membranes, whereas smooth endoplasmic reticulum has no ribosome attached and makes lipids and detoxifies wastes.


Ribosomes are granular particles concerned with protein synthesis. They may be found free, clustered, or attached to the endoplasmic reticulum. Ribosome contain mixture of protein and RNA and are involved in protein synthesis.

Vacuoles are small spaces or cavities within the cytoplasm. These serve functions at the cellular level such as digestion, respiration, excretion, and storage.

Golgi complex is a portion of the endoplasmic reticulum that aids in the final preparation of certain proteins and mucus-like substances and in the movement of these substances. It is best-developed in secretary cells .


Mitochondria Each cell contains few hundred to several thousands mitochondria depending upon its activity. Mitochondria are second largest organelle with unique genetic structure and are referred as the “powerhouses” of the cell. They “recharge” ADP molecules to form ATP molecules. Energy-producing chemical reactions take place on cristae. They have double-layered outer membrane with inner folds called cristae. It controls level of water and other materials in cell and recycles and decomposes proteins, fats, and carbohydrates, and forms urea.


There are ordinarily two centrioles. These organelles play a major role in cell division. These are paired cylindrical organelles near nucleus. It is composed of nine tubes, each with three tubules.
Lysosomes are the cells’ garbage disposal system and also called suicidal bags. They degrade the products of ingestion, such as the bacterium by phagocytosis. After the bacterium is enclosed in a vacuole, vesicles containing lysosomal enzymes fuse with it. The pH becomes more acidic and this activates the enzymes and degrades the bacterium. Lysosomes also degrade worn out cell organelles such as mitochondria. Lysosomes provides intra cellular digestive system that function to digest

  1. damaged cell organelles
  2. food particles ingested by cell and
  3. foreign particles such as bacteria.
Nucleus is controlling centre of cell. DNA constituting ‘gene, is major constituent of nucleus. Gene performs several functions:
  1. Dictates synthesis of different structural and enzymatic protein synthesis.
  2. Promotes cell division by mitosis and meiosis.

Nucleushas nuclearmembrane separating it from the cytoplasm. Within the nucleus is the chromatin material, made up of the proteins and nucleic acid, deoxyribonucleic acid (DNA). At the time of cell division, this chromatin material is aggregated into individual structures known as chromosomes. Each chromosome has a set of specific genes, which determine all of the physical and chemical characteristics of the body, which represent its structure and function. Within the cell is the nucleus sometimes also referred to as the “control centre” which is a membrane bound organelle found in eukaryotic cells? The nucleus also consists of the nucleolus which is visible when cell is not dividing and contains RNA for protein manufacture. The aqueous fluid that fills the nuclear region is called karyolymph or nucleoplasm. In the fluid there are proteins, enzymes and other important substances for the nuclear metabolism.


Last modified: Tuesday, 10 April 2012, 4:37 AM