Cell Biology – Organelles, Cycles and Division, Signaling …

Organelles, Cycles and Division, Signaling & Techniques

As a sub-discipline of biology, cell biology isconcerned with the study of the structure and function of cells. As such, itcan explain the structure of different types of cells, types of cellcomponents, the metabolic processes of a cell, cell life cycle and signalingpathways to name a few. Here, we shall look at some of the major areas of cellbiology including some of the tools used.

Cell Theory is a basic principle in biologythat was formulated by Thodor Schwann, Matthias Schleiden and Rudolph Virchow.

According to the Cell Theory:

Recently, the theory was modified to include thefollowing ideas:

A cell is a basic unit of life. This simplymeans that a cell is the smallest unit of a living thing. While some organismsare only made up of a single cell (bacteria, yeast etc) others aremulticellular organisms made up of manycells.

While there is a clear difference between unicellular and multicellularorganisms, some organisms may transition from unicellular organisms tomulticellular organisms under certain conditions.

A good example of this isslime mold that tends to transition to a multicellular organism under stressfulconditions. However, they are simply described as being partiallymulticellular. Therefore, the cell is the basic building block of any given organism.

For a multicellular organism, cells are specialized, which means that they havedifferentiated to carry out given functions.

The following are examples of specializedcells:

Sperm Cells - Sperm cells serve to fertilize the female eggto form the embryo.

Red Blood Cells - Red cells contain aprotein molecule known as hemoglobin and serve to transport oxygen to all partsof the body and expel carbon dioxide from the body.

White Blood Cells - There are differenttypes of white blood cells that serve to protect the body from disease causingorganisms.

- Basophils, Lymphocytes, Neutrophils, Monocytes, Eosinophils

Cardiomyocytes - These are cardiac muscle cells that make upthe heart muscle.

Nerve Cells (neurons) - These are cells of thenervous system that transmit information to and from different parts of the body(information is transmitted as electric and chemical signals). See also Sensory Cells.

Any given cell will have three major components.

These include:

Cell Wall

The cell wall is a complex, highly organized structure that defines the shape of a plant cell (it is also found in bacteria, fungi, algae, and archaea). In addition to defining the shape of plant cells, a cell wall has a few other functions that include maintaining the structural integrity of a cell, acting as a line of defense against a variety of external factors as well as hosting various channels, pores and receptors that regulate various functions of a cell. As such, it is a multifunctional structure in plant cells that also contributes to plant growth.

See Plant Biology.

Cell Membrane

Also known as the plasma membrane, the cellmembrane is a bi-lipid membrane layer (it is a double membranous structure)that is also composed of proteins and carbohydrates. This fluid like structureencircles the cell thereby containing the contents of a cell.

It's alsoselectively permeable, which means that it only allows certain materials(nutrients and minerals etc) to pass through to sustain the cell. The cellmembrane also functions to protect the cell and ensure stability.

Nucleus

The nucleus may be described as the largestorganelle of a cell. The nucleus is itself surrounded by a double membrane(nuclear envelope) and contains genetic information (genes) making it thecontrol center of a cell. As such, it controls such activities cell metabolismand reproduction.

Cytoplasm

The cytoplasm is the fluid matrix (jelly-like)found inside the cell (outside the nucleus). Various types oforganelles and minerals (salts) are suspended in this constantly streamingfluid. Apart from containing all the cell organelles, the cytoplasm also helpsmaintain the shape of a cell.

Cell organelles may be described as cellsubunits specialized to carry out given functions within the cell. There aredifferent types of organelles in cells that carry out given functions.

Thefollowing are some of organelles that can be found in a cell (excluding thecell membrane, cytosol and nucleus which are mentioned above):

Mitochondria - The mitochondria are rod-shaped organellesand sites of ATP synthesis. The mitochondria is also surrounded by a doublemembrane (with the inner membrane being highly folded forming the cristae).

Thisorganelle is commonly referred to as a power- generator given that it convertsoxygen and nutrients in to a chemical energy known as ATP (adenosine triphosphate)which provides the energy required for various activities of the cell. Apartfrom being a site for ATP synthesis, the mitochondrion is also involved in theself-destruction of a cell in a process known as apoptosis.

Ribosomes - Found in the cytoplasm and the surface of the rough endoplasmicreticulum, ribosomes are composed of RNA and proteins. They may be described asthe "cell factories" given that they are responsible for the synthesisof protein molecules.

Lysosomes - These are sac-like structures that are surrounded by amembrane (a single membrane). Lysosomes contain digestive enzymes, which areresponsible for breaking down proteins, lipids and nucleic acids. In addition,lysosomes are also involved in the removal of waste molecules as well asrecycling of molecular subunits.

Golgi body - These are flattened structures in a cellresponsible for temporary storage of protein in the cell.

Vacuoles - Vacuoles are also enclosed by a membrane and function to storesuch material as food, water, minerals and waste products among others.

Some of the otherorganelles include:

Cell cycle refers to a sequence in activelydividing cells where the cells pass through several stages before ultimatelydividing.

The stages of cell cycle include:

At GI, the metabolic changes take placepreparing the cell for the division process. At a given point known as therestriction point, the cell is committed to cell division and moves to the nextphase.

S - The S phase involves DNA synthesis. It isduring this phase that the replication of genetic material starts with each ofthe chromosome having two chromatic sisters.

G2 - During this phase, there are metabolicchanges that assemble the necessary cytoplasmic materials for the mitosisprocess and splitting of the mother cell.

M - The M phase is where nuclear division takes placeand followed by the division of the cell.

For most animals, cells may divide by mitosis ormeiosis. While the two processes result in the production of new cells, they aredifferent and produce different daughter cells.

Mitosis is the type of cell division that occursin all somatic cells. These are the types of cells that make up the bodytissues (apart from gametes/sex cells). Therefore, the primary role of mitosisis growth and replacing worn out cells.

Essentially, mitosis results in diploid cellsfrom one cell. Here, the chromosome is copied followed by the separation of thecopies on different sides of the cell before the cell ultimately separates intotwo. In the end, each of the new cells has a copy of the chromosome.

See more on chromosomes.

Mitosis has 5 major phases, which include:

Interphase - Here, the DNA strand is replicated/copied toproduce what is known as a bivalent chromosome (consisting of two chromatids orDNA strands that are replicas of each other). During the interphase stage, thenew strand is attached to the original one at a point known as the centromere.

Prophase - This is the second stage of mitosis. Here, the bivalentchromosomes formed during interphase condense to form tight packages.

Metaphase - This is the third stage where each of the chromosome line upat the center of the cell. The nucleus membrane has already started dissolvingwith each of the mitotic spindles attaching themselves to each of thechromatids. Here, it appears as if the chromatids are being stretched towardseither pole of the cell.

Anaphase - During anaphase, the fourth stage of mitosis, the chromatidsthat had attached to the spindles are separated (the chromatids are separatedfrom their copies) and pulled to either side of the cell. This results in twogroups of monovalent chromosomes.

Telophase - At the end of anaphase, another stage starts where nuclearmembranes start to form around the two formed groups of chromosomes. Thespindle fibers that attached to the chromatids get disassembled. Here, thechromosomes also condense.

Eventually, the cytoplasm divides/splits with a cellmembrane forming on each of the two daughter cells. This process is known ascytokinesis. Each of the new cellshas 46 monovalent chromosomes and has identical genetic information as theother.

In mitosis, it's important that the samegenetic information is copied when forming new cells. This is because the chromosomeshave all the information concerning the function of the cell.

Successfulcopying of information on to the new cells ensures that the new cell functionsproperly. In the event that there is a problem, then the new cell will be unableto perform its function as it should be. This would result in complicationsdepending on the function of the cell.

Unlike mitosis, meiosis produces haploid cells.

Diploid - Two new daughter cells from the original cell with the samenumber of chromosomes.

Haploid - With meiosis (a reductive type of cell division) the resultingcells will have less number of chromosomes.

Stages

Meiosis is also different from mitosis in thatthere are two phases of cell division. These are meiosis I and meiosis II.

Prophase 1 - Here, the homologous chromosomes pair and exchangeDNA form recombinant chromosomes. This stage ends with the spindle fibersstarting to form to attach to the chromosomes.

Metaphase 1 - The bivalent chromosomes arranges double rowhaving attached to the spindlefibers.

Anaphase 1 - The homologous chromosomes (in each bivalent)are separated and move to opposite poles of the cell.

Telophase 1 - With the separation of the chromosomes, anuclear membrane starts to form around the two groups of the chromosomes. Thisis followed by cytokinesis where the cell splits to form two new cells. This isagain followed by meiosis II. Meiosis II follows the same process as meiosis I.However, this halves the number of chromosomes.

* Meiosis is an important process that results ingenetic diversity.

What are the differences between Meiosis and Mitosis?

All cells originate from a single cell (a singlefertilized egg). In cell differentiation, cells become specialized as the bodydevelops. Apart from the single original cell (the fertilized egg), stemcells are also unspecialized. However, under certain conditions, they candifferentiate to become specialized cells that serve a specific function(s).

Although the differentiated somatic cells are different in that they performdifferent functions, they contain the same genome. However, the different typesof cells only express some of these genes, which results in the differencesmorphological and physiologicalbetweenthem.

In cells, signal transductions involve thetransmission of molecular signals. This is particularly from the exterior ofthe cell to its interior for appropriate cell response. Signals (biochemicalchanges) may either come from the environment the cell is in or from othercells that trigger changes.

Cells have receptors on the surface of the cell,which receives the signal prompting a response. For a response to take place,the signal has to be transmitted across the cell membrane.

Some of the common intracellular messengersinclude:

Cell signaling is very important given that ithelps control and maintain the normal physiological processes in the body.Different signaling processes will result in varying responses including celldifferentiation, proliferation of cells as well as metabolism among others.

Cell biology is largely concerned with the study of the structure and functions of cells (morphological and physiological). For this reason, a number of techniques have to be employed.

Some of the main cell biology techniques include:

Cells and tissues can be cultured usingcomplex media. With cells and tissues from more complex organisms, the culturemedia has to be more complex so as to provide the same environment as theenvironment from which the cell/tissue was obtained.

As for the tissue, theculturing process also allows for single cells to be obtained from the tissuein question for more studies.

The culture process requires the following:

Cell culture is an important technique giventhat it allows for only a sample (cells or tissue) to be used to learn moreabout the cells without the need to use the organism as a whole. This alsogives scientists a great opportunity to study the cells under varyingconditions.