All living organisms are composed of cells. Cells are responsible for all
anatomical and physiological features of all body systems. Different cell types can vary
greatly in shape and size but they all have a common structure and similar components.
A typical cell is enclosed in a plasma membrane and contains a nucleus and a cytoplasm.
The plasma membrane serves as the cell’s boundary, controlling the traffic of substances in and out
of the cell. It is also the site of communication between the cell and its environment.
The membrane consists mainly of 2 layers of phospholipids, with their hydrophilic
heads - the phosphate groups - facing the aqueous environments inside and
outside the cell; and their hydrophobic fatty acid tails facing in together.
Other membrane lipids include: cholesterol, which is essential to membrane structure and fluidity;
and glycolipids, which maintain membrane stability and facilitate cell to cell interactions.
The lipid membrane is dotted with membrane proteins, of which there are 2 types:
- integral, or transmembrane, proteins, which span across the membrane, some passing through multiple
times. Some transmembrane proteins have a small carbohydrate chain on the outside of the cell.
- and peripheral proteins, which attach to the membrane on the inside.
A peripheral protein typically functions together with an integral protein.
Membrane proteins fulfill a variety of functions: - As receptors (or receptor-associated proteins),
they receive messages from outside the cell. For example, a non-steroid hormone must bind
to a membrane receptor and act via several other membrane proteins to activate a cellular response.
Each receptor is specific to a certain molecule. - As ion channels or transport proteins, they help
move charged particles and large uncharged polar molecules across the cell membrane.
- As adhesion molecules, they help cells adhere to each other and to the extracellular matrix.
- As enzymes, they catalyze reactions that are required outside the cell
but in the vicinity of the cell membrane. - Transmembrane glycoproteins also serve as
surface antigens, determining the cell’s identity. On top of the cell membrane, some cells have
surface extensions that carry out specialized functions. Examples include microvilli that
increase surface area in the small intestine, cilia that move mucus in the respiratory tract,
and flagella that are responsible for the movements of sperm cells.
The nucleus contains genetic material, the DNA, and is where DNA replication, and
transcription - the major step of gene expression - take place. Most cells have 1 nucleus, with the
exception of red blood cells which have none, and some other cells that have multiple nuclei.
The nuclear envelope surrounding the nucleus consists of 2 membranes: inner and outer,
each of which is a phospholipid bilayer. The envelope is dotted with nuclear pores - protein
complexes that provide controlled passage between the nucleus and cytoplasm.
Chromosomes are strands of DNA wrapped around proteins. Under a light microscope,
chromosomes are only visible during cell divisions, when they are highly condensed.
Instead, the most prominent feature of the nucleus is the nucleolus - the area around the clusters of
ribosomal RNA genes. This is where ribosomal RNAs are made and ribosomes are assembled.
Ribosomes then move to the cytoplasm to fulfill their function in protein synthesis.
The cytoplasm includes a gel-like liquid called cytosol, various organelles and cytoskeleton.
The endoplasmic reticulum (ER), Golgi apparatus, and vesicles constitute the intracellular membrane
system. The ER is a network of connected flattened sacs called cisternae. Its membrane is continuous
with the outer nuclear membrane. Part of the ER appears “rough” as it is covered with ribosomes.
This is where the synthesis of secretory and transmembrane proteins takes place.
These proteins have a signal sequence within their amino terminus, which, as soon as it emerges from
the ribosome, targets the RNA-ribosome complex to the ER membrane where translation continues.
The emerging polypeptide enters the ER membrane as it is being translated. Transmembrane proteins,
identified by the presence of a hydrophobic stretch, stay in ER membrane, while secretory
proteins are released into the ER lumen. The “smooth” part of the ER synthesizes lipids and
lipid components of cell membranes. As lipids are produced, they are inserted into the ER membrane.
Membrane proteins, lipids, and secretory proteins are then packaged into vesicles to
be transported to the Golgi, where proteins undergo post-translational modifications.
Vesicles pinch off from ER membranes, travel to Golgi apparatus, fuse with Golgi membranes,
and release their content. The Golgi is a stack of separated cisternae,
each contains a set of enzymes responsible for a certain step in protein maturation. Similar
vesicles transport lipids and proteins from one cisterna to another; and ultimately to their
destinations - the plasma membrane, lysosomes, or storage vesicles. The destination of a protein
is typically determined by a signal sequence, acting as an address tag, within the protein.
The ER is also a major site for metabolism and storage of calcium, whose release
is a trigger for many cellular processes. Lysosomes are vesicles containing hydrolases
that breakdown macromolecules into their building units, which can then be recycled.
The enzymes are activated by the acidic environment within lysosomes. In white
blood cells, lysosomes digest phagocytized bacteria and play a role in immune response.
Mitochondria are best known as the cell’s powerhouses. This is where
energy is extracted from food compounds and stored in energy-rich molecules.
A mitochondrion has 2 membranes. The inner membrane has multiple folds called cristae.
Two of the three main steps of cellular respiration occur in the mitochondria:
citric acid cycle in the matrix, and oxidative phosphorylation on the cristae.
Cytoskeleton is a network of protein filaments that fulfill a variety of functions.
There are 3 types of filaments: microfilaments, intermediate filaments, and microtubules.
- Microfilaments are made of the protein actin. They enable muscle contraction,
provide support for microvilli, produce cell movements, and play a role in cell division.
- Intermediate filaments are made of different proteins in different cells.
Their roles are mostly supportive. - Microtubules are large tubes of
13 proto-filaments, each is a long chain of tubulin dimers.
A centriole is a short cylinder of 9 triplets of microtubules. A cell typically has 2 centrioles
lying perpendicular to each other forming a structure called centrosome. Centrosome
serves as a microtubule organizing center, from which microtubules grow out into the cytoplasm.
In non-dividing cells, microtubule network holds organelles in place.
During cell division, they form the mitotic spindle that guides chromosome movements.
Microtubules are also responsible for the movements of cilia and flagella.