Functional System of the Cell

Ingestion by the Cell - Endocytosis


The cell obtains its nutrients and other substances from the surrounding fluid through Diffusion and active transport. Active transport consists of endocytosis, the principle forms are pinocytosis and phagocytosis.


Pinocytosis means the ingestion of small globules of extracellular fluid forming minute vesicles in the cell cytoplasm. The molecules attach to receptors on the membranes, call coated pits, and on the inside of this cell membrane beneath these pits is latticework of fibrillar protien called clathrin and a contractile filament of actin and myosin. After the protein molecules bind with the receptors, the membrane invaginates, and the contractile protiens surround the pit, causing the boarders to close over the attached proteins and form a pinocytotic vesicle.


Phagocytosis is the ingestion of large particles like bacteria, cells, and portions of degenerated tissue. Macrophages and some white cells perform phagocytosis.  
Bacteria attach to antibodies that in turn attach to phagocytic receptors, dragging the bacteria along with them. The intermediation of antibodies is called opsonisation. 


Pinocytic and Phagocytic foreign substances are digested in the cells by lysosomes. Lysosomes attach to the pinocyte or the phagocyte and empty their digestive enzymes into the vesicle. The residual products of digestion are amino acids, glucose, phosphates and so on that can diffuse out of the membrane of the vesicle into the cytoplasm. The undigested substances are called residual bodies, are excreated via a process of exocytosis. 


Synthesis of Cellular Structures


The synthesis of most cell structures begins in the ER. Formed in the ER and then passed on to the Golgi apparatus, where they are further processed before being released into the cytoplasm. 
Granular ER is characterised by large amount of ribosomes attached to the outer surface and is the site of protein formation. Ribosomes synthesise the proteins and extrude them through the wall of the ER to the  interior of the endoplasmic vesicles and tubules, called the endoplasmic matrix.


Proteins entering the ER, enzymes in the ER cause rapid changes, including congregation of carbohydrates to form glycoprotiens. Proteins are oftern cross linked, folded, shortened to form compact molecules. 

Lipids are synthesised in the ER, especially phospholipid and cholesterol, which are incorporated in the lipid bilayer of the ER.
Small ER transport vesicles, continually break off from the smooth reticulum and migrate to the Golgi Apparatus. 


The Golgi apparatus process substances formed in the ER. Substances formed in the ER are transported through the reticulum tubules towards portions of the smooth ER that lie nearest to the Golgi apparatus. Small transport vesicles continuously break away and diffuse to the deepest layer of the Golgi apparatus, and instantly fuse with and empty their contents into the Golgi apparatus. More carbohydrates are added to secretions and reticular secretions are compacted. These compacted secretions are then further compacted till they breakaway from the Golgi apparatus, and diffuse through the cell.


In highly secretory cells, the vesicles formed by the Golgi Apparatus are Secretory vesicles, which diffuse to the cell membrane, fuse with it, and eventually empty their substances to the exterior via exocytosis. 


Extraction of Energy from nutrients  by the Mitochondria.
Cells extract energy from oxygen and one or more of the foodstuffs - carbohydrates, fats and proteins -  that react with oxygen. Carbohydrates are converted into glucose by the digestive tract and the liver before reaching the cell; proteins are converted to amino acids, and fats are converted from fatty acids. These substances react with oxygen, under the influence of enzymes in a directed and controlled way.


Almost all oxidative reactions occur inside the mitochondria, and the energy is released is used to form mainly ATP. ATP is composed of adenine,  pentose sugar ribose and three phosphate radicals. The last two phosphate radicals are connected with the remainder of the molecule by High energy phosphate bonds. 


When ATP releases its energy, a phosphate acid radical is split away, and adenosine diphosphate (ADP) is formed. ADP is then used to convert back to ATP above.


Most of the ATP produced in the cell is formed in  the mitochondria. Glucose is converted to pyruvic acid, a process called glycolysis. <5% of ATP


The pyruvic acid from carbohydrates, the fatty acids from lipids, and amino acids from proteins are all converted to acetyl-Coenzyme A (Acetyl-CoA) in the matrix of the mitochondria. Acetyl-CoA is then converted by the citric acid cycle or the Krebs Cycle. Acetyl-CoA is split into hydrogen and carbon dioxide.  The hydrogen ions are highly reactive and combine with oxygen, releasing a large amount of energy. 


The initial event in the formation of ATP is the removal of the electron from the hydrogen atom, to form a hydrogen ion. The hydrogen ion is then moved through the large globular protein ATP synthetase, which protrude into the mitochondrial membranes, the membranous shelves. ATP synthetase is an enzym that uses the movement of hydrogen ions to convert ADP to ATP when hydrogen combines with oxygen to form water. 


The overall process is the chemosmotic mechanism of ATP formation.


ATP is used for many cellular functions. Three cell functions are promoted:
Membrane transport, as with the sodium potassium pump
synthesis of chemical compounds throughout the cell; and
mechanical work contraction of muscle fibres and cilliary and ameboid motion


Locomotion and Ciliary Movements of the Cells - specialised muscle cells in skeletal, cardiac and smooth muscle 
Other movements are ameboid locomotion and cilliary movement


Ameboid locomotion is the movement of an entire cell in relation to its surroundings. WBC movement throughput tissues




actin and myosin


chemotaxis  
chemotaxic substances
Cilliary movement is a whiplike movement of cillia on surface of cells. respiratory airways and uterine tubes