Page 15 - 84 human physiolofy part-1

Cell Physiology

• 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.

• 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 (To see this in action please click here).
• 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.

Cell Size

Cells are the smallest living units within our body, but play a big role in making our body function
properly. Many cells never have a large increase in size after they are first formed from a parental cell.
Typical stem cells reproduce, double in size, then reproduce again. Most Cytosolic contents such as the
endomembrane system and the cytoplasm easily scale to larger sizes in larger cells. If a cell becomes
too large, the normal cellular amount of DNA may not be adequate to keep the cell supplied with RNA.
Large cells often replicate their chromosomes to an abnormally high amount or become multinucleated.
Large cells that are primarily for nutrient storage can have a smooth surface membrane, but
metabolically active large cells often have some sort of folding of the cell surface membrane in order to
increase the surface area available for transport functions.

Cellular Organization

Several different molecules interact to form organelles with our body. Each type of organelle has a
specific function. Organelles perform the vital functions that keep our cells alive.

Cell Membranes

The boundary of the cell, sometimes called the plasma membrane, separates internal metabolic
events from the external environment and controls the movement of materials into and out of the cell.
This membrane is very selective about what it allows to pass through; this characteristic is referred to
as "selective permeability." For example, it allows oxygen and nutrients to enter the cell while keeping
toxins and waste products out. The plasma membrane is a double phospholipid membrane, or a lipid
bilayer, with the nonpolar hydrophobic tails pointing toward the inside of the membrane and the polar
hydrophilic heads forming the inner and outer surfaces of the membrane.

Protein and Cholesterol

Proteins and cholesterol molecules are scattered throughout the flexible phospholipid membrane.
Peripheral proteins attach loosely to the inner or outer surface of the plasma membrane. Integral
proteins lie across the membrane, extending from inside to outside. A variety of proteins are scattered
throughout the flexible matrix of phospholipid molecules, somewhat like icebergs floating in the ocean,

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