Cell Biology

Lec.7 Cell Biology

Structure of the cell
A cell is the basic unit of structure and function in a living thing. When cells divide, the hereditary information they contain, as DNA, is passed from cell to cell.
The term protoplasm includes the" living part" of the cell. It can be differentiated into cytoplasm and the nucleus.

Structure of typical animal cell

The cytoplasm
Cytoplasm is homogenous, clear jelly-like materials that fill the cells. The cytoplasm consists of cytosol and the cellular organelles except the nucleus. The cytoplasm plays a mechanical role ,i.e. to maintain the shape, the consistency of the cell and to provide suspension to the organelles. In other words cytoplasm is the home of the cytoskeleton, a network of cytoplasmic filaments that are responsible for the movement of the cell and give the cell its shape.

Cellular Organelles

The endoplasmic reticulum
The endoplasmic reticulum (ER) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER. The hollow portion of the ER tubules is called the lumen. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

Rough ER

The RER is generally a series of connected flattened sacs. Rough ER is named for its rough appearance, which is due to the ribosomes attached to its outer (cytoplasmic) surface. Rough ER lies immediately adjacent to the cell nucleus, and its membrane is continuous with the outer membrane of the nuclear envelope.
Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell (such as protein, hormones, enzymes). The RER also makes phospholipids for cellular membranes. If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER's membrane.

Smooth ER

It is composed of a network of smooth tubules; free of ribosomes . Smooth ER develops from rough ER and is connected directly to it. Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storage of calcium ions. In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells. It also forms vesicles in which large molecules are transported to other parts of the cell .

Golgi apparatus
The Golgi apparatus (also called the Golgi body), is a cellular organelle that packages and sorts proteins and other molecules before they are sent to their final destination. The Golgi apparatus is named for Camillo Golgi, who discovered its presence in cells in 1898.
Structure
The Golgi apparatus consists of a stack of three to twenty slightly curved saccules (cisternae) whose appearance can be compared to a stack of pancakes.






The receiving side of the Golgi apparatus is called the cis face. The opposite side is called the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is the addition of short chains of sugar molecules .These newly-modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations. Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

Function:

Golgi apparatus is responsible for handing the macromolecules that are required for proper cell functioning. It processes and packages these macromolecules for use within the cell or for secretion.

Golgi apparatus modifies proteins that it receives from the RER.

Transport lipids to vital parts of the cell and creates lysosomes.

Some of modifications made inside the Golgi complex include; attaching polysaccharides to proteins, cutting proteins into smaller active fragments, incorporating phosphates on to protein molecules and addition of a sulfate group to molecules.

Other function of Golgi apparatus include the production of glucosaminoglycans which go on to form parts of connective tissues.
Mitochondria
The mitochondrion (plural mitochondria) is a double membrane-bound organelle found in all eukaryotic organisms, although some cells in some organisms may lack them (e.g. red blood cells). The term 'mitochondrion' is derived from a Greek word 'mitos' which means 'thread' and 'chondrion' which means 'granule'. Mitochondria have been described as "the powerhouse of the cell" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of chemical energy.
Mitochondria are typically round to oval in shape and range in size from 0.5 to 10 μm. The number of mitochondria per cell varies widely; for example, in humans, erythrocytes (red blood cells) do not contain any mitochondria, whereas liver cells and muscle cells may contain hundreds or even thousands.





Structure of Mitochondria
Under light microscopic they appear as rods or filaments in cytoplasm. By the electron microscope mitochondrion is a double membrane bound organelle formed of : outer membrane, inner membrane, inter-membrane space , and Matrix .
The outer membrane is a relatively simple phospholipid bilayer, containing protein structures called porins which render it permeable to molecules of about 10 kilodaltons or less (the size of the smallest proteins). Ions, nutrient molecules, ATP, ADP, etc. can pass through the outer membrane with ease.
The inner membrane is more complex in structure than the outer membrane as it contains the complexes of the electron transport chain and the ATP synthetase complex. It is permeable only to oxygen, carbon dioxide and water. The inner membrane is made up of a large number of proteins that play an important role in producing ATP, and also helps in regulating transfer of metabolites across the membrane. The inner membrane has infoldings called the cristae that increase the surface area for the complexes and proteins that aid in the production of ATP.
The intermembrane space is the region between the inner and outer membranes. It has an important role in the primary function of mitochondria, which is oxidative phosphorylation.
The matrix is a complex mixture of enzymes that are important for the synthesis of ATP molecules, special mitochondrial ribosomes, tRNAs and the mitochondrial DNA. Besides these, it has oxygen and carbon dioxide.

Mitochondrial DNA

Mitochondria are independent organelles they have their own DNA and ribosomes. They can replicate and multiply on their own and make their own proteins. They have circular DNA similar to bacteria and replicate by fission. Mutations in the mitochondrial DNA leads to a number of diseases such as:
Young-adult blindness.
Progressive muscular disorders.
Some cases of Alzheimer’s disease.
Type -two diabetes mellitus.

Function of Mitochondria

• The most important function of the mitochondria is to produce energy. The food that we eat is broken into simpler molecules like carbohydrates, fats, etc., in our bodies. These are sent to the mitochondrion where they are further processed to produce charged molecules that combine with oxygen and produce ATP molecules. This entire process is known as oxidative phosphorylation.
• Mitochondria help the cells to maintain proper concentration of calcium ions within the cell.
• The mitochondria also help in building certain parts of blood and hormones like testosterone and estrogen.
• The liver cells mitochondria have enzymes that detoxify ammonia.
• The mitochondria also play important role in the process of programmed cell death.