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Using a human cell of your choice, describe how organelles work together to make and secrete a protein (LO1. AC 1. 1) An epithelial cell in the thyroid gland, called a thyrocyte forms spherical follicles that produce a protein called thyroglobulin. This is a globular protein that has a functional role in metabolism. The protein is used by the thyroid gland to produce thyroid hormones; thyroxine (T4) is an example of one of the hormones created. Thyroxine is formed by iodine binding to tyrosine residues in thyroglobulin molecules, inside the follicle cell. www. vivo. colostate. edu/hbooks/pathphys/endocrine/thyroid/chem. html) This hormone regulates growth and controls the rate of chemical reactions in the body. This image is from: http://classes. midlandstech. com/carterp/Courses/bio211/chap16/chap16. htm Before the thyroid hormone can be secreted from the thyroid follicle cell, the thyrocyte needs to make the protein thyroglobluin, by different organelles in the cell. There are many organelles inside the cell, however only specific organelles make and secrete thyroglobulin.

The Nucleus (the largest organelle inside the cell) controls all the different organelles within the follicle cell, and instructs each organelle on how to synthesis the protein (thyroglobulin). It also holds the DNA which codes for thyroglobulin, so it knows what protein the cell is producing and how much is needed. The Nucleolus, which is inside the Nucleus, contains the ribosomal RNA that is involved in building proteins and also this organelle is the site where ribosomes are assembled. The ribosome is where Thyroglobulin is made.

This organelle is either found floating free throughout the cell in the cytoplasm or embedded on the rough endoplasmic reticulum. The protein can be formed at both locations. Thyroglobulin uses the rough endoplasmic reticulum as a transport system through the thyrocyte; also this organelle can be a temporary storage for thyroglobulin until it is needed. The protein is then moved to the Golgi complex, this organelle is known to be most commonly found in glandular cells. The Golgi body is where thyroglobulin is modified into a glycoprotein; this is when a carbohydrate is added into the structure of the protein.

This modified thyroglobulin is processed, sorted and packaged into secretory vesicles. These secretory vesicles are then discharged in to the follicle lumen. The thyroid follicle cell traps iodide (active uptake) and it then travels through the cell, on the way through the cell the iodide is oxidized into active iodine by hydrogen peroxide. This reaction is catalyzed by the enzyme thyroid peroxidase (TPO). The iodine is then passed into the colloid lumen. In the colloid the iodine is attached to tyrosine, this is the amino acid that joins together to make up the protein thyroglobulin, forming DIT (T2).

DIT is known as a colloid thyroglobulin. Iodinated tyrosines are joined together to form thyroxine (T4). The thyroglobulin colloid is endocytosed (entering) back into the thyroid follicle cell forming an inward folding. This is then combined with a lysosome, so that the lysosomal enzymes can split thyroxine from the thyroglobulin colloid. Finally the hormone is diffused out of the thyrocyte, through the cell membrane into the bloodstream on its way to the peripheral tissues via the capillary.

Image from: http://wps. aw. com/bc_marieb_happlace_7_oa/42/10969/2808223. cw/index. html



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