HAC's Anatomy & Physiology Instructor, Dr. Dan Rovin, Takes Us On A Tour of Cell-Louie
The basic unit of living things is the cell. There are two types of cells. Prokaryotes are cells that lack a nucleus and other membranous organelles and have a nucleoid with a single strand of DNA. Eukaryotes are more complex and have numerous membrane encased organelles including mitochondria and a nucleus with chromosomal DNA. There are two types of eukaryotic cells. Plant cells have a cell wall, a large central vacuole, and chloroplasts (which convert sunlight and carbon dioxide into sugar and oxygen). Animal cells lack a cell wall, have many small vacuoles, and have no chloroplasts. Since they can't make their own sugars these cells must acquire them from plant cells. Human beings are made of somewhere between 76 trillion to 100 trillion cells which all stemmed from the genetic material of mom's egg cell (oocyte) and dad's sperm cell (spermatozoan). The combination of the ovum's 23 chromosomes and the sperm's 23 chromosomes created a unique 46-chromosome zygote that began to copy and copy and copy and .......By now the little ball of cells (blastula) has implanted in the uterus. Once the cells of the blastula have migrated into position differentiation will occur. Over the next 40 weeks, give or take, the cells will divide and become over 220 different types of human cell. Each is part of a collective. Each works to benefit the whole of the body. Each has a purpose, a job, a reason to exist. Each gets its instructions to grow, repair, and divide based on its genetic material found in DNA of the chromosomes. In the end each will die. Ultimately, it is each cell's organelles that allow it to be a working part of the commune so understanding what each organelle does creates a greater understanding for how each cell works.
The cytolemma, or cell membrane, is a phospholipid bilayer that completely surrounds the cell and is impermeable to water. It is riddled with many proteins that can change shape to act as gates and channels to allow entry and exit of materials into and out of the cell. Many membrane proteins act as receptor sites for hormones while others are tightly clustered in cholesterol packed lipid rafts to extend from the matrix of the membrane for adherence to other cells' membrane proteins. Some cells have ciliated membranes and some have microviliated membranes. The sperm cell is the only human cell that is flagellated. The under surface of the cell membrane is supported by a hexagonal spectrin tetramer protein.
The cytoskeleton is what maintains the shape and structure of the cell. It is formed by a scaffolding of polymerized actin filaments and intermediate filaments and is complemented by a network of small hollow tubes called microtubules that are made of the protein tubulin and zipped together with tau protein. Microtubules are used for cell division, vacuole transport, and are the motility of cilia and flagella of the cell membrane. The cytoskeleton is dynamic and in some cells is constantly changing.
Cytoplasm is a mildly viscous cellular fluid comprised of water, salts, proteins, and wastes of metabolism. It is inside the entire cell and is found in all organelles except the nucleus which has its own nucleoplasm.
The nucleus is often the largest intracellular organelle. It is typically centrally located due to its role in governing the internal aspects of the cell. Deoxyribonucleic acid (DNA) is found in the chromatin stored in the nucleus. Ribosomes, used for transport and synthesis of proteins, are manufactured in the nucleolus of the nucleus and are released into the cytosol through pores in the nuclear membrane. During mitosis the chromatin coils up into 23 pairs of chromosomes and the nuclear envelope dissolves. The chromosomes line up along the equator of the cell and duplicate. Once copied the chromosomes will be pulled to opposite ends of the cell and a protein ring will sever the cell membrane creating two new daughter cells from the original. In erythrocytes the nucleus is ejected before the cell is fully matured. This anucleated cell is more pliable allowing it to squeeze through small capillaries. The progenitor cells of skeletal muscles are singly nucleated but as they fuse into their long multinucleated muscle fibers the nuclei are pushed to the periphery such that they don't compete for space with the many myofibrils of actin and myosin.
The endoplasmic reticulum exists in the cell for varying types of synthesis. The rough endoplasmic reticulum (rough ER) is a vast network of folded sacs with ribosomes docked to translocator proteins on the outer surface of the membranes. The added weight of the ribosomes flattens the convolutions of the rough ER which, in many cells, covers an area approximately 50% of the intracellular space from the nucleus to the cell membrane. Ribosomes translate, or assemble, proteins while they are membrane bound. Inside the lumen of the rough ER large proteins are folded, shaped, and packed into vesicles, called vacuoles, for transport to the Golgi bodies. The smooth endoplasmic reticulum (smooth ER) is a smaller network of membranous tubes, also convoluted, that lacks ribosomal attachment. The main role of the smooth ER is the manufacturing of lipids vital for the growth and repair of the membranes of the organelles. The smooth ER in the cells of endocrine glands produce steroidal hormones whereas, in the cells of the liver, it creates glucose from stored glycogen.
In the Golgi apparatus simple proteins (delivered from the rough ER) are combined with other molecules to form complex molecular structures. These are repackaged into vacuoles and shipped out of the cell or stored for later use by the cell. Also important is the formation of lysosomes that will use enzymes to chemically denature harmful substances within the cell.
Adenosine triphosphate (ATP) is an important molecule for metabolic activities within the cell. The stored energy of ATP is harnessed when it is chemically converted to adenosine diphosphate and inorganic phosphate. The ATP is derived from a biproduct of cellular respiration which occurs in the mitochondria of the cell. Mitochondria are large, mobile symbiotes that scavenge the cell for nutrients. They have their own DNA and are found in greater quantities in cells that are highly metabolic. They are not seen in erythrocytes but are found populating most cells, including oocytes.
Numerous small vesicles, or vacuoles, are formed from the membranes of the cytolemma, endoplasmic reticulum, and Golgi apparatus. They are filled with proteins, water, wastes, and other materials for transport between organelles or to the cell membrane for elimination from the cell. The vacuoles are attached to motor proteins called kinesins and dyneins. These proteins undertake the Herculean-task of dragging the vacuoles along the microtubules. Kinesins pull vacuoles one direction towards the cell membrane (plus end) while dyneins compete to draw the vacuoles in the opposite direction (negative end).
Microtubules extend from the centrosome which is a pair of orthogonally bundled rods called centrioles. Centrioles are mainly involved in mitosis. They have a close proximity to the nucleus until they migrate toward the poles of the cell during division.
If the aforementioned presents a challenge and most of the discussion comes across as scientific mumbo-jumbo then creating a simile to compare parts may be for you. Attached herein is the tourism brochure for Cell Louie. I hope that my expression of the working parts of a viable city are easily visualized and that you may transform the world inside of the cell to a world in which you are familiar.
Check Out The Travel Brochure To Cell-Louie
You can keep up with HAC, its staff, faculty and students by following us here, subscribing to our RSS feed or following us on Facebook