Cell size - surface area to volume ratio

Cells are the basic building blocks of tissues in every living thing. The basis of the cell is the phospholipid bilayer which makes up the cell membrane. Phospholipids have a hydrophilic head and hydrophobic tail. The hydrophilic phosphate regions of the phospholipids are oriented toward the aqueous outside of the cell and the inside of the cell, while the hydrophobic fatty acid tails face each other on the inside of the membrane. Proteins and other molecules are also embedded in the phospholipid bilayer which add to its structural integrity and serve functions that require access to both the inside of the cell, such as transport or cell signaling. These proteins are able to move around freely throughout the membrane.

No matter the size of the organism, from mosquito to elephant, the cells maintain the same size; that is, larger organisms do not have larger cells than smaller organisms, they just have more cells. This is due to the surface area to volume ratio of a cell. Mathematically, on a very small scale, the squared area function is larger than the cubed volume function. In the graph, you can see how the x² function is larger than the x³ function when x < 1. Since transport between the inside and outside of a cell is hugely important for its function, the increased surface area compared to the volume of a small cell allows more effective exchange of nutrients and waste, heat regulation, and release of various molecules. Make sure to know basic surface area and volume formulas for calculations involving the surface area to volume ratio.

Membrane Permeability

The structure of the phospholipid bilayer allows for selective permeability; that is, only certain molecules can enter and exit the cell.

Passive transport is the ability for molecules to move through the cell membrane without the addition of energy powered simply by a concentration gradient (difference in concentration of the molecule between the inside and outside of the cell) and the molecule’s ability to pass through the membrane. Small, nonpolar molecules such as N₂, O₂, and CO₂ are able to pass through the cell membrane through simple diffusion. Larger or polar molecules like glucose and water enter the cell through protein channels through facilitated diffusion, another form of passive transport. In the case of water, these protein channels are known as aquaporins.

Active transport is the movement of matter across the cell membrane with the use of energy in the form of ATP. The sodium potassium pump is a classic example of active transport in which a transport protein embedded in the cell membrane is needed for movement of molecules. Energy is needed for this protein to function as the pumping goes against the concentration gradient of the ions and instead creates a larger concentration gradient.

Endocytosis is a generalization for the forms of active transport that bring matter into the cell. Phagocytosis, pinocytosis, and receptor-mediated endocytosis are all forms of active transport, meaning that they need energy to function. In all three types of endocytosis, a small vesicle is made as a small portion of the cell membrane closes in around matter outside the cell. As this vesicle is pinches off from the rest of the cell membrane, the extracellular matter is now inside the cell, surrounded by a phospholipid bilayer and able to move throughout the cell. Phagocytosis generally refers to bringing solids into the cell while pinocytosis refers to bringing liquids into the cell. Receptor-mediated endocytosis performs the same functions as the other forms of endocytosis but works through a process of cell signaling when ligands signal receptor proteins within the cell membrane to bring extracellular matter into the cell.

Exocytosis is the opposite of endocytosis. Exocytosis removes matter from the inside of a cell by bringing a vesicle to the cell membrane, then opening up and fusing to the cell membrane so that the matter in the vesicle can leave the cell.

Tonicity

Tonicity is a measure of the water potential of two solutions separated by a semipermeable membrane, like the inside and outside of the cell, that is permeable to water but not to the solute. Hypertonic, isotonic, and hypertonic refer to the level of solute in the surrounding of a cell, such as in the blood

Hypotonic: when the concentration of solute is too low outside the cell/too high inside the cell, water rushes in to balance the concentrations. This is preferable in plant cells that become turgid due to the presence of a cell wall, but animal cells lyse (burst) in this condition

Isotonic: When the concentration of solute is balanced inside and outside the cell, so the water molecules maintain a dynamic equilibrium and no shift is noticeable. This will cause a plant cell to become flaccid but is preferable in an animal cell which remains normal

Hypertonic: When the concentration of solute is too high outside the cell/too low inside the cell, water rushes out to balance the concentrations. This will cause a plant cell to plasmolyze when the cell membrane rips off the inside of the cell wall, and an animal cell will become shriveled.

Mitochondria and endosymbiont theory (origins of compartmentalization)

The presence of membrane bound organelles within a cell is quite an evolutionary achievement. Today, most scientists believe in the endosymbiont theory which resulted in the creation of prokaryotic cells. In this theory, a prokaryotic organism essentially phagocytized another prokaryotic cell which continued to live within the larger cell. An important piece of evidence for this theory is that mitochondria have their own DNA which is passed from mother to child. While the importance of this DNA is still being researched, it shows that membrane-bound organelles likely once existed as their own organisms.