Current Cell Theory states that:
- All organisms are composed of cells
- Cells can come only from preexisting cells
First and foremost, we need to understand how our cells operate. Following is a general simplified eukaryote cell diagram showing its principal components.
The Cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing and is often called the building block of life. Adult human beings are constituted from the ensemble of about 100 trillion (1014 ) cells; they communicate and exchange molecules among each other through miriad different pathways and using elegant, sophisticated and, at times, extremely complex physico-chemical properties to accomplish the miracle we call life.
The plasma membrane is the structure that surrounds the cell. It controls the cell’s interactions with other cells including communication, identification, and protection. The cellular membrane also controls the passage of materials in and out of cell. This principle of selective permeability means that some things are allowed in (or out) while other substances are completely excluded (or kept in). In this sense, everything on the inside of the phospholipid bilayer is intracellular, while everything outside this barrier is extracellular.
The plasma membrane is composed of several macromolecules, including a variety of lipids, proteins, and carbohydrates that serve as channels and transporters, in and out:
- Selective and controlled permeability of molecules and ions
- Receptors -communication between cells and activation of biochemical cascades in the intracellular environment for metabolic, adaptation and regulation purposes-.
Lipids constitute 90 to 99% of the plasma membrane, depending on the shape and function of the cell. The primary lipid is a phospholipid.
Phospholipids are special lipids that are hydrophobic on one end and hydrophilic on the other. Thus, the most energetically favorable arrangement is a double layer with the hydrophilic “heads” on the outside and the hydrophobic “tails” facing each other on the inside.
The second major lipid in the cellular membrane is cholesterol. Cholesterol affects the fluidity of the membrane; the less cholesterol in the membrane the more rigid the membrane, while the higher the concentration of cholesterol the more fluid the membrane. Some cells require great flexibility (like red blood cells) while other cells require more rigid shapes.
Glycolipid constitute about 5% of the lipids and are found only on the extracellular side. These special lipids contribute to the glycocalyx of the cell. The glycocalyx functions to allow cellular adhesion as well protection from harmful extracellular substances.
Although proteins only constitute about 2% of the molecules found in plasma membrane, they are essential for the two major functions of the cellular membrane: sensitivity and selective permeability. There are two major types of proteins associated with the cellular membrane, transmembrane proteins that pass completely through the membrane, and peripheral proteins, that are loosely associated with the lipids or proteins of the cellular membrane. Peripheral proteins often serve as anchors and intracellular signaling molecules. Most transmembrane proteins are glycoproteins with the sugar part extending into the external environment and forming part of the glycocalyx. These proteins form two major classes of membrane proteins: transport proteins and receptors.
Membrane Protein functions
The proteins in the membrane enable the membrane to perform its functions. Receptors in the membrane receive information from the environment and convert it into signaling molecules inside the cell. Receptors also identify the cell as “self” and enable the cells to adhere to each other or surrounding structures.
Other proteins act as “doors” and “windows” in the lipid bilayer to allow solutes to enter or leave the cell. These proteins are called pores, channels, carriers, and pumps.
A definition and function of what is found inside the cell, after we have briefly toured the plasma membrane follows :
- Cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.
The contents of the cell within the cell membrane, excluding the cell nucleus and other organelles are referred to as the cytoplasm. Many metabolic pathways occur in the cytosol; others are contained within organelles. The cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, its structure and properties within cells is not well understood. The concentrations of ions such as sodium and potassium are different in the cytosol than in the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation and cell signaling. The cytosol also contains large amounts of macromolecules, which can alter how molecules behave, through macromolecular crowding.
There are several types of organelles in a cell. Some are unitary like the Nucleus and Golgi Apparatus , while others (suchas mitochondria, peroxisomes and lysosomes) can be numerous (hundreds to thousands).
- Nucleus: A cell’s information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell’s chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell’s DNA from various molecules that could accidentally damage its structure or interfere with its processing.
During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled.
- Mitochondria: The Power-Plants. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. Mitochondria play a critical role in generating energy in the eukaryotic cell. Mitochondria generate the cell’s energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP.
Mitochondria multiply by splitting in two (Binary Fission, strikingly similar to some microorganisms…. ). Respiration occurs in the cell mitochondria. Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products. The reactions involved in respiration are catabolic reactions, which break large molecules into smaller-ones, releasing energy in the process as weak so-called “high-energy” bonds are replaced by stronger bonds in the products. Respiration is one of the key ways a cell gains useful energy to fuel cellular activity. Cellular respiration is considered an exothermic redox reaction.
- Endoplasmic reticulum (ER): It is the transport network for molecules targeted for certain modifications (glycosilation being one) and tagged so they can reach specific destinations, once they are released . It also displays functions of sorting mechanism, before transport of proteins reach their target final destination is as compared to molecules that float freely in the cytoplasm.
Proper folding of proteins is performed here by chaperone proteins.The ER has two forms: the rough ER, which has ribosomes on its surface and secretes proteins into the cytoplasm, and the smooth ER, which lacks them. Smooth ER, specializes in lipid, phospholipid and steroid synthesis and plays a role in calcium sequestration and release.
- Golgi apparatus: Packages the macromolecules such as proteins and lipids that are synthesized by the cell. It is a complex network, containing cisterns (flattened membrane saccules) stacked on top of each other in layers and perform protein maturation, sorting and packaging. Post-translational modifications that started in the ER are continued here.
From the Golgi apparatus, the proteins are transported by vesicles to various targets in the cells: lysosomes , the plasma membrane, and secretory vesicles that release their contents into the extracellular space (exocytosis). Protein transport can either proceed continuously (constitutive), or it can be regulated by chemical signals. The decision regarding which pathway a protein will take and whether its transport will be constitutive or regulated depends on the signal sequences or signal structures that proteins carry with them like address labels . In addition to proteins the Golgi apparatus also transports membrane lipids to their targets.
- Ribosomes: The ribosome is a large complex of RNA and protein molecules. They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesize proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum).
Protein synthesis requires the assistance of two other kinds of RNA molecules in addition to rRNA. Messenger RNA (mRNA) provides the template of instructions from the cellular DNA for building a specific protein. Transfer RNA (tRNA) brings the protein building blocks, amino acids, to the ribosome. There are three adjacent tRNA binding sites on a ribosome: the aminoacyl binding site for a tRNA molecule attached to the next amino acid in the protein, the peptidyl binding site for the central tRNA molecule containing the growing peptide chain, and an exit binding site to discharge used tRNA molecules from the ribosome.
Once the protein backbone amino acids are polymerized, the ribosome releases the protein and it is transported to the Golgi apparatus in . There, the proteins are completed and released inside or outside the cell. Ribosomes are very efficient organelles. A single ribosome in a eukaryotic cell can add 120 amino acids to a protein chain in about a minute.
- Lysosomes and Peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system.
- Centrosome: The cytoskeleton organizer. The centrosome produces the microtubules of a cell – a key component of the cytoskeleton-. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells.
- Vacuoles: Store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane.
- MicroVili: they are extensions of the cytosol shaped as microscopic needle like projections protruding from the surface of the cell with the main purpose of increasing the latter’s area, in order to enhance absorption and secretion as well as cellular adhesion capabilities. Not all kind of cells express these appendages, but cochlea cells ( inner ear) do have them, as well as small intestine cells.
Finally, we find an extracellular structure with vital functions:
The Extracellular Matrix EM:
The extracellular matrix can be defined as a dynamic group of macromolecules placed around the majority of the cells in the body to form an ordered three-dimensional framework.
All cells in solid tissue are surrounded by Extra Cellular Matrix, whether connecting by very fine fibers, called fibrils, and by cell adhesion molecules (CAM) with one another, or as a foundational base where cells reside.
Various important properties of the cell surface as well as intracellular functions depend on proteins that protrude from the plasma membrane and anchor into the ECM.
Common functions are Cell adhesion, cell to cell communication, and differentiation.
The extracellular matrix has three major components:
- Highly viscous proteoglycans ( heparan sulfate, keratan sulfate , chondroitin sulfate ), which cushion cells
- Insoluble collagen fibers, which provide strength and resilience
- Soluble multiadhesive extracellular matrix proteins ( fibronectin , laminin ), which bind proteoglycans and collagen fibers to receptors on the cell surface
Different combinations of these components tailor the matrix for different functions depending on the amount of strength (for instance, the remarkably high tensile strenght in muscle tendons), cushioning (for example, cartilage tissue i joints) and adhesion required. All extracellular matrix components are synthesized intracellularly and secreted out via exocytosis.
Tissues are made up of cells, still, a surprisingly significant part of tissue volume is extracellular space, which is largely filled by an intricate network of macromolecules constituting what is referred to as the Extracellular Matrix. This matrix is composed of a variety of proteins and Polysaccharides that are secreted locally and assembled into an organized meshwork in direct association with the surface of the cell that produced them, to achieve specific structural, connective or functional purposes.
Variations in the relative amounts of the different types of matrix macromolecules and the way in which they are organized in the EM give rise to an amazing diversity of forms, each adapted to the functional requirements of the particular tissue. The matrix can become calcified to form remarkably hard structures like those of bones, or it can adopt the converging parallelly tied string structure that confers tendons their high tensile strength capability.