Cell physiology is the biological study of the activities that take place in a cell to keep it alive. The term physiology refers to normal functions in a living organism.
General characteristics
There are two types of cells:
and
. Prokaryotes were the first of the two to develop and do not have a self-contained
Cell nucleus. Their mechanisms are simpler than later-evolved
eukaryotes, which contain a nucleus that envelops the cell's
DNA and some
.
Prokaryotes
Prokaryotes have DNA located in an area called the nucleoid, which is not separated from other parts of the cell by a membrane. There are two domains of prokaryotes:
bacteria and
archaea. Prokaryotes have fewer organelles than eukaryotes. Both have plasma membranes and
(structures that synthesize proteins and float free in
cytoplasm). Two unique characteristics of prokaryotes are fimbriae (finger-like projections on the surface of a cell) and
flagella (threadlike structures that aid movement).
Eukaryotes
Eukaryotes have a nucleus where DNA is contained. They are usually larger than prokaryotes and contain many more organelles. The nucleus, the feature of a eukaryote that distinguishes it from a prokaryote, contains a
nuclear envelope,
nucleolus and
chromatin. In cytoplasm, endoplasmic reticulum (ER) synthesizes membranes and performs other
metabolic activities. There are two types, rough ER (containing ribosomes) and smooth ER (lacking ribosomes). The
Golgi apparatus consists of multiple membranous sacs, responsible for manufacturing and shipping out materials such as
.
are structures that use
to break down substances through
phagocytosis, a process that comprises
endocytosis and
exocytosis. In the
mitochondria, metabolic processes such as cellular respiration occur. The
cytoskeleton is made of
that support the structure of the cell and help the cell move.
Physiological processes
There are different ways through which cells can transport substances across the
cell membrane. The two main pathways are passive transport and
active transport. Passive transport is more direct and does not require the use of the cell's energy. It relies on an area that maintains a high-to-low concentration gradient. Active transport uses adenosine triphosphate (ATP) to transport a substance that moves against its concentration gradient.
Movement of proteins
The pathway for proteins to move in cells starts at the ER.
and proteins are synthesized in the ER, and
are added to make
. Glycoproteins undergo further synthesis in the Golgi apparatus, becoming
. Both glycoproteins and glycolipids are transported into vesicles to the
plasma membrane. The cell releases secretory proteins known as
exocytosis.
Transport of ions
travel across cell membranes through channels, pumps or transporters. In channels, they move down an electrochemical gradient to produce electrical signals. Pumps maintain electrochemical gradients. The main type of pump is the Na/K pump. It moves 3
sodium ions out of a cell and 2
potassium ions into a cell. The process converts one ATP molecule to adenosine diphosphate (ADP) and In a transporter, ions use more than one gradient to produce electrical signals.
Endocytosis in animal cells
Endocytosis is a form of active transport where a cell takes in
, using the plasma membrane, and packages them into vesicles.
Phagocytosis
In phagocytosis, a cell surrounds particles including food particles through an extension of the
, which are located on the plasma membrane. The pseudopods then package the particles in a food
vacuole. The lysosome, which contains
hydrolytic enzymes, then fuses with the food vacuole. Hydrolytic enzymes, also known as digestive enzymes, then digest the particles within the food vacuole.
Pinocytosis
In pinocytosis, a cell takes in ("gulps") extracellular fluid into vesicles, which are formed when plasma membrane surrounds the fluid. The cell can take in any molecule or solute through this process.
Receptor-mediated endocytosis
Receptor-mediated endocytosis is a form of pinocytosis where a cell takes in specific molecules or solutes. Proteins with receptor sites are located on the plasma membrane, binding to specific solutes. The receptor proteins that are attached to the specific solutes go inside coated pits, forming a vesicle. The vesicles then surround the receptors that are attached to the specific solutes, releasing their molecules. Receptor proteins are recycled back to the plasma membrane by the same vesicle.
External links
-
Overview at Medical College of Georgia (archived)
