The Cell - An introduction
The cell is the smallest functional unit within a living organism, which can function independently. It is made up of several types of organelles that allow the cell to function and reproduce. There are two general classes of cells that exist: the self-sustaining simple cells known as prokaryotes (bacteria and archaea) and the more complex dependent cells known as eukaryotes. The eukaryotic cells are generally found in animals, plants, algae, and fungi. For the purpose of this article, the primary focus will be the structure and composition of the animal cell. The major differences between animal and plant cells will be explored as well.
As previously stated, the fundamental components of a cell are its organelles. These organelles are made up of varying combinations of atoms and molecules. The organelles drive different functions of the cell from metabolism, to energy production and subsequently to replication. Cells with particular functions come together to form organs (i.e. lung parenchyma). Organs with interrelated functions work together within a system (i.e. respiratory system). These systems, although of different functions, work in synergy to allow the organism (i.e. human) to survive. Every aspect of a cell is important for it to survive. The article will explore the following cellular components:
- Plasma (Cell) Membrane
- Endoplasmic Reticulum
- Golgi Apparatus
- Vesicles and Lysosomes
- Nuclear Membrane
Plasma (Cell) Membrane
The plasma membrane is the outermost layer of the cell. The main function of the plasma membrane is to protect the cell from its environment. It is often referred to as a fluid mosaic phospholipid bilayer that is hydrophilic externally and internally, but hydrophobic at its core. The hydrophilic property arises from the charged phosphate molecule that forms the head of the phospholipid, and the hydrophobic nature is from the two lipid tails which forms the core. This feature allows the selective permeability of the membrane. For instance, particles that are hydrophilic (e.g. ions) are not able to pass through the hydrophobic core, and those that are hydrophobic (e.g. fats) are repelled from the outer surface. As a result, the cell is able to isolate its internal environment from the external environment.
Some of the phospholipid structures are bonded to cholesterol molecules. These may serve as individual cell markers to allow the body’s immune system to distinguish self from nonself as well as maintaining the consistency of the plasma membrane.
Like any living organism, the cell is not completely self-sufficient and consequently will require nutrients from the external environment and also to export its products to the external environment. The controlled movement of substance is done by protein channels and carrier proteins anchored in the plasma membrane that selectively or generally allow particular particles to enter and leave the cell.
Some protein molecules are tagged with glycogen chains (i.e. glycoproteins) and function as receptor channels that initiate cellular processes. Other proteins are restricted to either the cytosolic (intracellular protein) or extracellular face (extracellular protein) of the membrane, while others span the entire membrane (transmembrane proteins). This is the rationale behind the term “fluid mosaic”, as it refers to the fact that the proteins located in or on the membrane move freely throughout the phospholipid bilayer.
In the case of substances that can neither pass through the membrane or use membrane channels, the plasma membrane has the ability to engulf foreign material in a process known as endocytosis. This process involves recognition of either foreign microorganisms or native substances by receptors on the cell membrane and subsequent folding of that region of the membrane around the intended structure being transported into the cytoplasm. Endocytosis can be further subdivided into three types.
Phagocytosis involves the intake of nonspecific substances (usually solid) into the cell. Pinocytosis involves the intake of specific substances (usually extracellular fluid) into the cell. Receptor-mediated endocytosis involves the specific uptake of certain macromolecules which is controlled by cell surface receptors.
The semi-solid medium that keeps the organelles suspended and nutrients dissolved within the internal cellular environment is the cytoplasm. In addition to the organelles, the cytoplasm also contains microfilaments, microtubules and secretory granules. The microfilaments and microtubules are a part of the cellular architecture that helps give the cell its structure (cytoskeleton) and play a role in cell replication. They also contribute to the formation of cilia and flagella in some cell lines that require motility.
In order for cells to grow and replicate, they must produce the necessary building blocks to achieve this feat. Additionally, some cells – like the β-cells of the pancreas – produce protein based hormones to help maintain homeostasis. This process is achieved by ribosomes. Ribosomes are complex ribonucleic acid based molecules (i.e. ribosomal-ribonucleic acid; r-RNA) that are responsible for translating coded sequences of the messenger-RNA (m-RNA) to proteins. They are made up of a small and a large subunit which coordinate with each other to translate the m-RNA strand. Some ribosomes are membrane bound, while others float freely in the cytoplasm. While free ribosomes synthesize proteins that are used within the cell, the proteins synthesized by bound ribosomes are meant to be exported.
There are clusters of sacs and vesicles that form cisternae (tubules) within the cytoplasm. These structures make the endoplasmic reticulum. There are two types of endoplasmic reticulum: one that has ribosomes bound to its surface - rough endoplasmic reticulum (RER), while the other lacks ribosomes - smooth endoplasmic reticulum (SER).
Another distinguishing feature between rough and smooth endoplasmic reticulum is that rough endoplasmic reticulum is an extension of the nuclear membrane, while the smooth endoplasmic reticulum may either be an independent collection of sacs, or a continuation of the rough endoplasmic reticulum. As previously stated, the rough endoplasmic reticulum stores protein that was synthesized by the ribosomes on its surface. In contrast, the smooth endoplasmic reticulum synthesizes phospholipids, steroids, and lipids, which are subsequently used in steroid based hormone synthesis.
Named after the Italian scientist that discovered it in 1898, Camillio Golgi, this organelle exists within the cytoplasm as a storage center for proteins that will be distributed to other sites. The Golgi apparatus (also referred to as the Golgi complex or Golgi body) is structurally subdivided into cis and trans components. The former represents flattened incoming vesicles from the endoplasmic reticulum that fuse to form cisternae. The trans aspect of the structure is the region from which vesicles bud off to join other vesicles, lysosome or the cell surface (to be exocytosed).
Vesicles and Lysosomes
Some proteins synthesised within the cell are utilized by the cell, while others are intended for export to other areas of the body. To prevent these products from being activated and interacting unintentionally with the cell of origin, they are stored in membrane bound sacs called vesicles. There are three general types of vesicles; exocytotic, lysosomal and secretory vesicles. The exocytotic vesicles contain proteins that will be expelled from the cell via exocytosis. This occurs when the vesicles fuse with the cytoplasmic membrane and expel its contents into the extracellular space. For example, the release of antibodies from activated B-cells during the humoral immune response.
Proteins housed in secretory vesicles are also for extracellular release but require a stimulus; the release of acetylcholine (ACh) from the telodendria of neurons into the synaptic cleft following stimulation by an action potential.
On the other hand, proteases are enzymes designed to digest protein. These are special proteins that are involved in cellular degradation in an apoptotic (programmed cell death) fashion or as part of the defence mechanism against invading pathogens. In either case, these enzymes are stored in the lysosomes for subsequent release. When there is an organelle, cell or microorganism to be digested, a vesicle forms around the substance to be dissolved and subsequently fuses with the lysosome. This is done to prevent unintended damage to other cytoplasmic structures.
Often referred to as the “powerhouse” of the cell, the mitochondria (s. mitochondrion) is an elongated, double membrane structure with numerous cristae within its inner membrane. In addition to the membrane bound ATP synthase proteins that facilitate ATP production, mitochondria are the only organelles that contain their own DNA material and is therefore capable of replication.
The outer membrane that envelops the entire organelle is equipped with prion proteins that allow the selective uptake size of some substances. The inner membrane also has specific proteins such as ATP synthase (makes ATP), cytochrome C (performs oxidation-reduction reactions) and transport proteins (for selective uptake of material into the mitochondrial matrix). The constituents of the intermembrane space (between the inner and outer membranes) are very similar to those in the cytoplasm of the cell.
The matrix is the site at which the citric acid cycle (Krebs cycle - process in ATP formation) occurs. The number of mitochondria found within a particular cell is dependent on its function. For example, cardiac myocytes contain more mitochondria than epithelial cells of the skin because they require more ATP to make them resistant to fatigue.
This is the largest structure within the cell. It is circumscribed by the nuclear envelope and contains a nucleolus, matrix, and most importantly, the hereditary genetic material known as deoxyribonucleic acid (DNA). There is approximately two meters of microscopic genetic material within each cell. This immense volume of DNA is able to be held within the cell by tightly coiling it around histones (proteinaceous scaffold) that are subsequently stacked as chromosomes. However, DNA only exists as chromosomes during the active stages of cellular division. When the cell is in the growth phase, the DNA takes the form of either euchromatin or heterochromatin. DNA that takes the euchromatic form is usually more frequently transcribed and expressed by the cell.
Within the nucleus is a unique region known as the nucleolus. This is an area where DNA that codes for ribosomal RNA (or tandem repeats) is found. The primary function is to make and assimilate r-RNA that will be exported to the cytoplasm to transcribe m-RNA.
There is another selectively permeable membrane that separates the nucleus cellular cytoplasm from the nuclear matrix. This structure is known as the nuclear envelope; like the plasma membrane, it is also made of a lipid bilayer. It is a double layered structure that encircles the nucleolus and the chromatin within the nuclear matrix. The nuclear envelope is continuous with the rough endoplasmic reticulum.
In some areas of the envelope the inner and outer layers merge, forming openings known as nuclear pores. Not only do nuclear pores allow nucleotides and other materials to enter the nucleus, but they also allow m-RNA to leave the nucleus for translation in the cytoplasm.
Differences between Animal and Plant Cells
For completion, it should be noted that animal cells are not the only eukaryotic type of cell that exists. Plant cells are also eukaryotic and have components that are similar to those in animal cells. However, there are some significant differences. While animal cells are more irregular in shape, plant cells are often of a fixed rectangular shape. This fixed form is facilitated by the rigid cellulose-based cell wall that surrounds the plasma membrane of the plant cell; which is also absent in animal cells.
In addition to having mitochondria to produce ATP, plant cells also contain chloroplasts. These structures allow plants to utilize ultraviolet energy in the process of photosynthesis to produce their own food. Finally, while a wide variety of animal cells may be equipped with cilia, these structures are often missing from most types of plant cells.
Cells exist throughout the body and work synergistically to execute their respective functions. These cells undergo mitotic (and in gonads meiotic) transformations in order to sustain the cellular population. When a cell is exposed to a stressful stimulus, it usually makes an attempt to adapt to that environment until the stimulus is removed. Once there is no extenuating damage to the cell, it usually repairs itself and returns to its normal state. However, once the cell is substantially damaged and the injury is irreversible, the cell may undergo programmed cell death - a process known as apoptosis. Apoptosis is a naturally occurring, controlled cell-mediated process where the damaged or worn cell is autophagocytosed. There is another form of cell death that is unplanned and may result in more injury to adjacent cells that is known as cellular necrosis. Here, the death of the cell follows an external agent (i.e. trauma, infections or toxins) that initiates the premature death of the cell.
There are some instances where the genetic material may code a mutation as a result of exposure to a noxious stimulus, inheritance of erroneous coding, or simply due to an error in replication, that “turns off” important house-keeping functions of a cell. This is one of the phenomena observed in malignant cells. There are several hallmark features of cancer cells, including the ability of these cells to spread to distant sites and grow (metastasis), initiate angiogenesis (creation of new blood vessels to enhance their blood supply) and more importantly, the cells are “immortal”. While the cells (albeit difficult) can be killed with several pharmacological, radiological and immunological agents, the innate apoptotic configuration of the cell line is downregulated. Although these cells possess several qualities that would initiate apoptosis in a normal cell line, these cells will grow and reproduce at an uncontrollable rate because they are somehow capable of avoiding programmed cell death.
There are some terms that are specifically associated with changes at the cellular level that are common place in the medical field. As such, they should be appreciated in order to follow discussions around pathological processes:
Hyperplasia refers to the increase in the size of an organ as a result of the increase in the number of cells within the same. For example, in benign prostatic hyperplasia, the number of prostate cells has increased, resulting in an increase in the overall size of the gland. However, the overall size of the cells remains the same.
Hypertrophy on the other hand refers to an increase in the size of the organ following an increase in the size of the constituent cells. Think about the process of left ventricular hypertrophy, where the cardiac myocytes increase in size following chronic increase in total peripheral resistance. Unlike in hyperplasia, however, the number of cells typically remains the same.
Metaplasia is a reversible process in which one mature cell type is replaced by another mature cell type. A good example of this can be found in the distal oesophagus of patients with chronic gastroesophageal disease (i.e. Barrett’s oesophagus). In this case, chronic exposure of the columnar type epithelium to the corrosive gastric acids promote cellular change to the squamous type of cells, which are more robust. When the stimulus is removed, the cell line will return to its previous state. This is not considered as a direct relative to malignant lesions.
Dysplasia speaks to the proliferation of immature cell lines and a decline in the prevalence of the mature cell line resident to that anatomical location. This is observed in cervical intraepithelial neoplasia where the abnormal cell line has not yet invaded the basement membrane. This is considered a precursor to malignant lesions.