The anatomy of animal cells
The cell is a basic unit of every live organism. The cells are basic building blocks of the organism, and give bodies their structure, absorb nutrients and create energy. Additionally, every cell contains the genetic material of the entire organism. Cells can make copies of themselves - this is how organisms grow, replace old or defective cells and heal wounds.
Cells are composed of organelles, which are like miniature versions of our organs - units within the cell that perform specific function. These include:
- The nucleus: this contains the cell's genetic material and controls the cell's activity. Operations with the genetic material happen here as well [1, 2, 3, 4, 5, 6, 7].
- The cytoplasm is a jelly-like isotonic substance which fills the inside of the cell. It houses the organelles and provides space for cellular activities such as manufacturing of proteins, metabolism of molecules or transport of molecules [1, 2, 3, 4, 5, 6, 7].
- Mitochondria are organelles responsible for generation of energy (thus referred to as the powerhouse of the cell). Energy is stored in the form of a molecule called adenosine triphosphate (ATP), which is generated by an enzyme ATP synthase and utilises a proton gradient across the mitochondrial membrane. Additionally, mitochondria also play a role in initiating cell death [1, 2, 3, 4, 5, 6, 7].
- The endoplasmic reticulum (ER) is a network of tubules, which is the site of synthesis of proteins and lipids. Here, the mRNA molecule, which is like a blueprint for protein building, is read by ribosomes, and amino acids are pieced together corresponding to the instructions encoded in the mRNA.
- The Golgi apparatus, a series of flattened sacs, modifies, sorts, and packages proteins and lipids for transport, as they exit the endoplasmic reticulum [1, 2, 3, 4, 5, 6, 7].
- Lysosomes break down waste materials and cellular debris into molecules which can be recycled by the cell. A similar structure, peroxisome, metabolises fatty acids by oxidation.
- The cytoskeleton, a network of microtubules, provides structural support to the cell, and plays roles in cellular transport (where molecules are dragged along the microtubule by specialised transport proteins), plasticity and movement of the cell, and cell division.
- The cell membrane (plasma membrane) is a thin lipid layer that surrounds the cytoplasm, providing a protective layer from the environment. The cell membrane also facilitates transport of substances into and out of the cell [1, 2, 3, 4, 5, 6, 7].
What is the Plasma Membrane?
The plasma membrane is arguably the most essential component of a cell. It is the barrier that separates the inside of the cell from the outside environment. However, it is a very dynamic and complex structure, with a diverse array of elements including proteins and lipids [8, 9, 10].
Structurally, the plasma membrane is approximately 4nm thick and primarily consists of a phopsholipid bilayer. This is hydrophobic, meaning it repels water, which created a barrier to the exchange of ions and polar substances between the inside and outside of the cell [10].
Functionally, the plasma membrane is not only a positive barrier, but a very active structure essential for cellular functions. It plays essential roles in transduction of biological signals from cell to cell, as well as import and export of molecules in and out of the cell [11]. For this, it utilises an array of transport systems such as ion channels and lipid flippases [10].
Therefore, the plasma membrane maintains cellular homeostasis, which is a balance of the cell's internal environment [12].
Differences in Plasma Membrane Functions Between Cell Types
The plasma membrane is adapted to the different functions of the different cell types and the type of molecule it interacts with. For example, the plant cell membrane is reinforced with an extra layer of cell wall, and possesses receptors that respond to environmental triggers such as heat, moisture, pressure or mechanical damage [13].
In contrast, bacterial cells have membranes which offer protection (this depends on the environment the bacterium grows in, for example Helicobacter pylori is adapted to low pH in the stomach) and are active in uptake of molecules. Bacterial cell membranes are also optimised for rapid division and secretion of molecules packaged in extracellular vesicles [14].
In animal white blood cells, for example, the plasma membrane is interlaced with numerous receptors that detect pathogens and respond to signals of other immune cells (most notably cytokines, a type of messenger molecule for the immune system) [15]. Cells of the digestive system are better optimised for uptake of nutrients, so their membranes possess a large repertoire of transport proteins.
What can we do with the plasma membrane in therapy?
The purpose of desigining drugs is to get them into cells as effectively as possible. As they have to cross the cell membrane, it is a very important structure to study, and potentially manipulate as well.
One approach is to target membrane proteins, which are accessible on the cell surface and play a crucial role in cellular signaling. By modulating protein-protein, protein-lipid, and protein-nucleic acid interactions, therapeutic effects can be achieved, particularly on cell signalling [16].
Another approach includes the use of nanobodies, which are small derivatives of antibodies. These can be deployed to interact with the plasma membrane receptors and transport proteins, potentially achieving a therapeutic effect, or modulating transport, for example so that the cell can take up a larger molecule [17].
Additionally, as opposed to targeting the membrane itself, drugs can be optimised to cross membranes more effectively. Approaches to achieve so include studying drug-lipid interactions, which can help predict the transport, biodistribution, and efficacy of drugs [18].
Alternatively, drugs can be packaged in liposomes, which are small lipid particles that can interact with the membrane and 'dissolve' the drug in the cell. This is one of the ways how mRNA vaccines are manufactured [19].