Page 14 - 84 human physiolofy part-1

Chapter 2

2 CELL PHYSIOLOGY

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Cell Structure and Function

What is a Cell?

C ells are the microscopic fundamental units of all living things. Every living thing has cells:

bacteria, protozoans, fungi, plants, and animals are the main groups (Kingdoms) of living things.
Some organisms are made up of just one cell (e.g. bacteria and protozoans), but animals, including
human beings, are multicellular. An adult human body is composed of about 100 trillion cells! Each
cell has basic requirements to sustain it, and the body's organ systems are largely built around
providing the many trillions of cells with those basic needs (such as oxygen, food, and waste removal).

There are about 200 different kinds of specialized cells in the human body. When many identical
cells are organized together it is called a tissue (such as muscle tissue, nervous tissue, etc). Various
tissues organized together for a common purpose are called organs (e.g. the stomach is an organ, and so
is the skin, the brain, and the uterus).

Ideas about cell structure have changed considerably over the years. Early biologists saw cells as
simple membranous sacs containing fluid and a few floating particles. Today's biologists know that
cells are infinitely more complex than this. Therefore, a strong knowledge of the various cellular
organelles and their functions is important to any physiologist. If a person's cells are healthy, then that
person is healthy. All physiological processes, growth and development, and disease can be described
at the cellular level.

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. They are the core components of the brain, spinal cord and
peripheral nerves. They use chemical and electrical synapses 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.

14 | Human Physiology