Cell Specialization All organisms are composed of cells. Unicellular organisms are

composed of a single cell. Multicellular organisms are composed

of many specialized cells. Specialized cells differ in structure

(size, shape...) and function (the role they perform in the

organism). The structural modifications that occur in a

specialized cell equip it to do its job in the organism. An adult

human is composed of approximately 100 trillion cells and has

over 200 different types of specialized cells.

Examples of Specialized Cells

Sperm Cell

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Fat Cells

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White Blood

Cell

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SEM_Lymphocyte.jpg/653px-SEM_Lymphocyte_large.jpg

Macrophage

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The life cycle of a cell consists of a growth phase (interphase)

followed by a division phase (either mitosis or meiosis).

Some cells cycle between interphase and division continuously

through the life of an organism. This allows the body to

produce new cells allowing for growth and maintenance of

tissues. Other cells exit the cell cycle and enter a non-dividing

phase called GO. Depending on environmental signals, they may

reenter the cell cycle or remain in GO permanently.

A cell specializes while in interphase or GO. The process in

which a cell becomes specialized is called differentiation and

occurs when the cell selectively activates or inactivates specific

genes.

The specialized cells of an organism contain the exact same

complement of genes. In humans, this means that each cell type

contains appromixately 30,000 genes. This is because each cell

is the descendent of a single cell (the fertilized egg) that

underwent mitotic cell division to form a multicellular organism

during embryonic development. Cell differences are the result

of differences in gene expression. A gene is expressed

(activated) when it is transcribed and translated into a protein.

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Some genes (such as those responsible for glucose metabolism

or protein synthesis) are expressed in all cells. Other genes

are expressed in a single or select few specialized cells. The

expression of a gene within a particular specialized cell changes

over an individual’s life. For example, all human cells contain a

gene that codes for caesin, the major protein found in

milk. However, the gene is only expressed milk producing cells

of lactating females.

Melanocytes

Obtained from www1.imperial.ac.uk/

Location

Melanocytes are specialized skin cells located in the basal (bottom) layer of the epidermis (the outermost layer of skin).

Function

Melanocytes produce and secrete melanin, the pigment

that gives skin color and protects the DNA in skin cells from UV radiation.

How cell structure relates to function

Melanocytes are rounded cells with long, branch-like extensions called dendrites. The dendrites extend into the lower layers of the epidermis.

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Within each meloanocyte are unique organelles called melanosomes where the production of melanin takes

place. The cell produces mass quanities of an enzyme called tyrosinase that catalyzes the reaction that produces melanin. A defect in the tyrosinase gene or the expression of the gene blocks the production of melanin and results in

albinsm. The biochemical pathway that produces melanin is common to many organisms, thus albinism occurs in many species.

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After the melanin is produced, the dendrites of the melanocytes transfer the melanosome (with the melanin)

to keratinocytes (the pigment storing cells of the skin). The melanosomes are taken in by receptor mediated endocytosis and are deposited over the nucleus of the keratinocytes to protect the DNA from UV radiation.

Differences in skin pigmentation are not due to the activity of the melanocytes, but the keratinocytes. In Caucasians,

lysosomes in the keratinocytes actually degrade some of the melanin. Less pigment means lighter skin. The melanosomes in individuals with darker skin are not degraded.

Differentiation of Melanocytes

Melanocytes differentiate from unspecialized skin cells during embryonic development. Changes in gene expression result in the formation of the melanosomes from components of the SER, RER and Golgi, the activation

of the tyrosinase gene, and modifications to the cytoskeleton that allows for the formation of dendrites.

Melanocytes and the Cell Cycle

Melanocytes exit the cell cycle and enter a non-dividing phase called G0 where they remain permanently. Here,

they perform their specialized role of producing and secreting melanin. As you age, your melanocytes begin to die. Since they don't divide, they slowly decline in number. This is why skin becomes lighter as you age.

Red Blood Cells

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Location

Red blood cells (erythrocytes) are specialized cells of the circulatory system. They are the most common component of blood. A small droplet of blood contains about 5 million erythrocytes. They constitute 40% of a female's blood volume and 45% of a male's blood volume. (Males have more red blood cells than females! This is one of the reason males typically have greater physical stamina then females.)

Function

RBCs carry oxygen from the lungs to body cells.

How cell structure relates to function:

Red blood cells are flexible, flattened, disk shaped cells. They resemble a ball of clay squeezed between the thumb and forefinger. This shape is due to the fact that red blood cells don't have a nucleus! When they differentiate, they lose their nucleus which increases the area available inside the cell for hemoglobin, a large, iron containing protein that acts as the binding site for the oxygen. Each hemoglobin protein can bind four molecules of oxygen. The flattened surface of the red blood cells increases the area through which oxygen can diffuse through the cell membrane. The flexibility of the red blood cell allows it to squeeze through the tiny capillaries of the circulatory system.

Hemoglobin, oxygen binding protein in RBCs

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Differentiation of RBC's

Red blood cells form from undifferentiated cells in the bone marrow throughout your life. Bone marrow is the soft, interior portion of certain bones found in the chest, upper arms, upper legs and hips. The cells located here are undifferentiated, but limited in the type of cell they can become. They can only differentiate into a type of blood cell. They are capable of dividing continuously to allow lost blood cells to be replenished. As RBC's differentiate, changes in gene expression result in the destruction of their nucleus, the production of hemoglobin proteins, and modifications to the cytoskeleton allowing them to take their donut shape. Once differentiation is complete, they are released into the blood stream. You body replaces 2 million red blood cells every second.

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RBC's and the Cell Cycle

Red Blood Cells never divide. Before they are released into circulation, they enter the non-dividing G0 phase and remain here for their entire life span (which is only 120 days). Dead or damaged red blood cells are removed from circulation by the spleen and liver. The iron is recycled back to the bone marrow and used to make more red blood cells. This is not a 100% efficient process. Some iron is excreted and that is why you need iron in your diet.

Sperm Cells

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Location

Sperm cells are specialized cells of the male reproductive system that form

in the testes.

Function:

The function of sperm cells is to deliver the paternal (father) contribution

of chromosomes to the ovum (egg).

How cell structure relates to function

Sperm cells have three distinct regions, the head, midpiece and tail. The

head consists of an acrosome and a nucleus. The acrosome is a

specialized lysosome containing enzymes that digest the protective outer

layers of the ovum allowing the sperm to penetrate and fertilize the

egg. The nucleus contains the father’s contribution of chromosomes for

the soon to be embryo. All cytoplasm surrounding the nucleus is

partitioned off during the maturation process to increase efficiency. The

midpiece is packed with mitochondria that break down glucose (present in

semen) producing ATP to power the movement of the flagella. The tail is

a single flagellum composed of microtubules. The flagellum is the source

of locomotion. Sperm can move about 3 mm per hour and must wave

their flagellum more than 1000 times to move half an inch.

Differentiation Process

Sperm cells differentiate from precursor cells called spermatogonia in the

testes. This begins at puberty and occurs for the rest of life. The

differentiation process begins when spermatogonia undergo meiosis, a

special form of cell division that cuts the chromosome number in

half. After meiosis, changes in gene expression result in the formation of

the acrosome, loss of excess cytoplasm, and formation of a flagellum.

Sperm Cells and the Cell Cycle

Fully differentiated sperm cells do not divide. After spermatogonia

complete meiosis, the resulting cells enter G0, a nondividing phase. Here

they complete their differentiation process. However, the spermatogonia

(precurser cells) divide daily. A normal adult male produces between 50

and 150 million sperm cells each day.

Adipocytes

http://www.biochem.arizona.edu/classes/bioc462

Location

Adipocytes are fat cells located under the skin (subcutaneous fat), above the kidneys, and in the liver and muscles. Men tend to concentrate their fat stores in the

chest, abdomen and buttocks producing an "apple" shape. Women tent to carry their fat stores in the breasts, hips, waist and buttocks, creating a "pear" shape. The

average person has between 25 and 30 million adipocytes. This number can rise to 360 million in seriously obese individuals.

Function

Adipocytes serve three functions. They insulate, provide

cushion, and most importantly store energy in the form of

triglycerides (1 glycerol plus 3 fatty acids).

How Structure relates to Function

Adipocytes resemble tiny plastic bags that are filled with

air (except they are filled with fat. They have a small,

flattened nucleus that sits off center along the plasma

membrane. 85% of the interior is occupied by one large

fat droplet. The surface of the plasma membrane has

receptors for insulin (the hormone that triggers the fat cell

to take up and store fat) and glucogon (the hormone that

induces the fat cell to release fat stores for energy).

Adipocytes and the Cell Cycle

As adipocytes differentiate, they exit the cell cycle and

enter the non-dividing phase (G0). When an individual overeats, their fats store increase two ways. The differentiated adipocytes increase in size as they take in more triglycerides and the number of adipocytes increases

and excessive food intake stimulates the production of more adipocytes from precursor cells.

It is currently under debate whether adipocytes remain in G0 permanently or can reenter the cell cycle. If they

remain in G0 permanently, an individual can only decreases fat stores by decreasing the size of the lipid droplet within each adipocyte, not by reducing the number

of adipocytes. (Once you make a fat cell is stays with you forever!)

However, there is evidence that adipocytes may be

capable of dedifferentiating (reverting back to their precursor form) and reentering the cell cycle. This would mean that people can reduce the number of adipocytes in the body over time and decrease in fat stores occurs by reducing the number and size of the adipocytes.