The Cell 

Basic living unit of the body. 

Body contains on average 100 trillion cells of which 25% are RBCs. 

All these cells have basic characteristics that are alike but differ in adaptation to their specific functions. 

Specialized Cells of the Human Body

Although there are specialized cells - both in structure and function - within the body, all cells have similarities in their structural organization and metabolic needs (such as maintaining energy levels via conversion of carbohydrate to ATP and using genes to create and maintain proteins).

Here are some of the different types of specialized cells within the human body.

• Nerve Cells: Also called neurons, these cells are in the nervous system and function to process and transmit information (it is hypothesized). They are the core components of the brain, spinal cord, and peripheral nerves. They use chemical synapses that can evoke electrical signals, called action potentials, to relay signals throughout the body.
• Epithelial cells: Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies.
• Exocrine cells: These cells secrete products through ducts, such as mucus, sweat, or digestive enzymes. The products of these cells go directly to the target organ through the ducts. For example, the bile from the gallbladder is carried directly into the duodenum via the bile duct.
• Endocrine cells: These cells are similar to exocrine cells, but secrete their products directly into the bloodstream instead of through a duct. Endocrine cells are found throughout the body but are concentrated in hormone-secreting glands such as the pituitary. The products of the endocrine cells go throughout the body in the bloodstream but act on specific organs by receptors on the cells of the target organs. For example, the hormone estrogen acts specifically on the uterus and breasts of females because there are estrogen receptors in the cells of these target organs.
• Blood Cells: The most common types of blood cells are:
• red blood cells (erythrocytes). The main function of red blood cells is to collect oxygen in the lungs and deliver it through the blood to the body tissues. Gas exchange is carried out by simple diffusion.
• various types of white blood cells (leukocytes). They are produced in the bone marrow and help the body to fight infectious disease and foreign objects in the immune system. White cells are found in the circulatory system, lymphatic system, spleen, and other body tissues.

Major parts of a cell: Nucleus and cytoplasm.

 Cell’s chemical composition 

Substances making up the cell(protoplasm) are composed of five basic substances: ?

Water 

Proteins 

Carbohydrates 

Lipids 

Ions

A cell is made up of an organelles which is  defined as well defined part of a cell or is one of several different types of membrane-enclosed bodies in the cell, each performing a unique function.

The physical structure of the cell

 Endoplasmic reticulum:

o    Structure

§  3D mesh of tubular structures and sheets

§  All throughout the cytosol

§  "Rough" ER is covered in ribosomes, whereas "smooth" ER is not.

o    Function

§  Folding and modification of proteins 

§  Synthesis of phospholipids and steroids

§  Storage of calcium ions in the ER lumen  (sarcoplasmic reticulum)

·         Golgi apparatus

o    Structure:

§  Stacked membraeous cisternae which shed vescicles for transport 

o    Function:

§  Post-translation modification of proteins, including glycosylation, phosphorylation and polymerisation 

§  Prepare proteins for exocytosis

§  Synthesis of glycolipids and sphingomyelin

·         Mitochondria

o    Structure

§  Outer membrane with pores and inner membrane without pores

§  Two separate compartments: intermembrane space or "outer compartment" and matrix space or inner compartment

§  Organisation of the inner membrane into cristae (a comb of many folds) which increases its surface area

o    Function

§  ATP synthesis functions, eg. oxidative phosphorylation

§  Regulatory and synthetic functions (eg, haem synthesis, calcium ion storage, urea cycle, haem and steroid synthesis)

§  Heat production

§  CO₂ production

§  Production of reactive oxygen species

§  Apoptosis

·         Lysosome

o    Structure

§  Membrane-bound compartment in the cell, the last member of the endocytic pathway - equivalent to vacuoles of protozoa

§  Usually have acidic internal pH and contain degradative enzymes

o    Function

§  Terminal destination for endocytosed material

 Nucleus

Outer and inner nuclear membrane with nuclear pores

§  Nucleoplasm separated into heterochromatin and euchromatin

 A nucleolus create RNA and ribosomes

o  Function

§  Concentrates and maintains the genetic contents of the cell, which include the regulation of gene expression and replication.

Control Centre of the cell containing DNA.

DNA: