The cell is the morphological unit of all modern cell types, of which there are two major classifications: eukaryotes (Greek: eu, good or true karyon, kernel or nut), which have a nucleus that encloses their DNA, and prokaryotes (Greek: pro, before), which do not have a nucleus. We will discuss the similarities and differences between these two cell types, as well as viruses, which are non-living protein particles. Further, this article will provide all the relevant facts for your medical education turning attention to a key theme in human physiology: the homeostasis.
Cellular iron homeostasis

Image: “Diagram of cellular iron homeostasis. Generalized to include cell type-specific proteins.” by Neodop. License: CC BY-SA 4.0

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Eukaryotes cells, whether unicellular or multicellular, are vastly more complex than prokaryotes. They are typically 10-100nm in diameter and thus have a thousand to a million times more volume than prokaryotic cells. These cell types are best characterized by their membrane-enclosed organelles. The nucleus contains chromatin, which is a complex of DNA and protein, and the nucleolus (the site of ribosome synthesis). In addition to the nucleus that houses their genetic information, eukaryotes possess an endoplasmic reticulum studded with where the synthesis of many of their cellular components occurs. These components are modified subsequently in the Golgi apparatus. Aerobic respiration takes place in mitochondria or chloroplasts in photosynthetic cells.

Eukaryotes possess other organelles, such as lysosomes and peroxisomes, which perform special functions for the cell. Vacuoles occur more frequently in plant cells than in animal cells, where they serve as storage deposits. The cytosol houses the enzymes of various pathways, for example, those of glycolysis. Also in the cytosol is the cytoskeleton, which is an elaborate array of filaments that serves to give the cell its shape and ability to move.


Prokaryotic cells are the most numerous organisms on the planet. They are also the most widespread, due to their highly adaptive metabolisms that allow them to survive in a great array of different habitats. They have a relatively simple cell structure in comparison to eukaryotes. The vast majority of bacteria are unicellular, with notable exceptions being that some form colonies of independent cells or filaments. Prokaryotes range in size from 1-10nm and morphologically display one of three basic shapes: spheroidal (cocci), rod-like (bacilli), and coiled (spirilla).


Eukaryote vs. Prokaryote

Prokaryotic cells do not possess a cell membrane, although they do have a cell wall. Inside of the cell, their metabolic functions occur regionally, without the compartmentation that is an important feature of eukaryotic cells. Their replication, transcription, and translation material is housed in the cytosol and is circular rather than in the linear form that eukaryotic cells possess.

Average prokaryote cell

Typical prokaryotic cell

The organelles that compartmentalize eukaryotic cells give them a level of complexity that is absent in prokaryotic cells. Bacteria are vastly more efficient than eukaryotes in many respects. They have exploited the advantages of simplicity and miniaturization, and their rapid growth rates permit them to occupy ecological niches in which there may be drastic fluctuations of the available nutrients. It is because of these capabilities that it is erroneous to consider prokaryotes as evolutionarily primitive compared to eukaryotes.

The bacterial flagellum is smaller and simpler in structure and is made up of a protein called flagellin. It is proton driven and capable of rotary movement. Contrast this to the eukaryotic flagella, which are larger, more complex in structure, and are comprised of proteins called tubulin in an apparent 9+2 arrangement. The eukaryotic flagella are ATP-driven, which allows it to perform the bending movement.


Viruses are simpler than cells and cannot be thought of as living because they lack the metabolic repertoire to reproduce without a host cell. They are small protein particles that instead replicate inside of the cells they infect. They range in size from 20-300 nm. Viruses are not capable of movement, nor are they capable of replicating outside of a host cell. They also do not possess a metabolism. When a virus is not inside of its host cell, it exists in a form known as a virion, which is comprised of DNA or RNA enclosed in a protein capsid, or in some cases enveloped in a matrix of lipids derived from the host cell’s membrane.


Image: “Basic structure of viruses. A. nonenveloped virus, B. enveloped virus. 1 – Capside, 2- Nucleic acid, 3 – Capsomer, 4 – Nucleocapsid, 5 – Virion, 6 – Envelope, 7 – Spike (envelope glycoproteins). Examples are the viruses showing icosahedral symmetry.” by Y_tambe. License: CC BY-SA 3.0

A virus is classified as either a DNA or RNA virus, with RNA viruses comprising the vast majority of this group. Their genomes can also be either single-stranded or double-stranded in nature, and are either linear or circular in nature. Some viruses have the unique “ability” to integrate DNA produced by reverse transcription into the host’s genome and are called retroviruses. An important example of a retrovirus is the subgroup of retrovirus named lentivirus, which causes human immunodeficiency virus (HIV).


How is it that your body maintains an average internal temperature around 37°C (98.6°F), regardless of the outside temperature? You might also wonder how the size of your pupils change in response to the level of light in the room, or how your body returns your breathing pattern to normal after exercise. Your body is also capable of modulating your heart rate, blood pressure, and blood glucose levels to respond to ever-changing circumstances through a process called homeostasis (homoios = similar, stasis = standing).

Homeostasis is the ability of an organism to maintain an internal steady state in response to rapidly fluctuating internal and external conditions. This is accomplished by use of control systems comprised of three parts, receptor, control center, and effector.

A receptor is a body structure that detects changes in a variable, which can be either a chemical or process that is regulated. Receptors typically are comprised of nerve cells, which may be in the skin, for example, that relay signals after experiencing a change in the variable, or stimulus (in this case touch or temperature). Another example of a receptor is the retina of your eye, which detects changes in the level of light (the stimulus) that enters your eye.

The control center is the structure that interprets input from the receptor and initiates changes through the effector. Think of the control center as the “go between” for the other two components of the homeostatic system. Most often, the control center is a portion of the nervous system or an endocrine organ.

Calcium regulation

Image: “Overview of calcium regulation” by Mikael Häggström. License: Public Domain

A homeostatic system involving the nervous system provides a relatively quick means of responding to change. For example, your blood pressure must be regulated when you rise from slumber in the morning. Your endocrine system equips you with a more sustained response, generally lasting several hours or even days through the release of hormones. For example, parathyroid hormone regulates your blood calcium levels in a process that is paramount to the function of your muscles and nerve cells.

Sometimes the receptor and control center are housed in the same organ. A good example of this is your pancreas, which acts as a receptor when it detects an increase in blood glucose and also acts as the control center when it releases insulin.

An effector is a structure that brings about changes to alter the stimulus. Most body structures can serve as effectors. The most common effectors are muscles and glands. For example, smooth muscle in the walls of your air passageways (bronchioles). They regulate airflow into and out of the lungs. Glands, such as the pancreas, release hormones (e.g., insulin).

Most of the processes in your body are controlled by negative feedback. In this type of homeostatic control, the resulting action will always be in the opposite direction of the stimulus. In this way, the variable is maintained within a normal level, or what is called its set point.

Example body temperature

A good way to think about negative feedback is on maintaining your body temperature.

  • On a very cold day, a decrease in body temperature is detected by the sensory receptors of the skin, which send nerve impulses to the hypothalamus (a component of the brain). The hypothalamus makes a comparison between sensory inputs to your body’s temperature set point, and then it initiates motor output responses to the blood vessels in your skin to decrease the diameter of the inside opening of the vessels. This effectively decreases the amount of blood circulating to the surface of your body. Less heat is released through the skin. Nerve impulses will also be sent to your skeletal muscles, which will cause shivering, as well as to the smooth muscle associated with hair follicles in the skin, causing “goose bumps.”

    Temperature Regulation

    Temperature regulation

  • On a hot day or when you are exercising, increases in body temperature are detected by both the skin’s sensory receptor and your hypothalamus. The hypothalamus will transmit motor input to the blood vessels in the skin to increase in their size, which effectively allows more blood to be brought to the surface of your body and heat will be released. Nerve impulses are also sent from the hypothalamus to your sweat glands to initiate sweating. Both of these responses will help to cool your body by the loss of heat from its surface. In these examples, regulation occurs through the nervous system.

Positive feedback mechanisms often work in loops, and the initial step of the pathway serves as the stimulus. The end product of the pathway stimulates the activity, instead of turning it off like in negative feedback. An example of positive feedback occurs when a mother breastfeeds her child. The stimulus of her baby suckling promotes the release of hormones that will stimulate the breast to secrete milk.

Popular Exam Questions on Homeostasis

The answers are below the references.

1. Which of the following structures are possessed by eukaryotic cells, but are absent in prokaryotes?

  1. Cytosol
  2. Cell membrane
  3. Cell wall
  4. Membrane-bound vesicles such as the nucleus, Golgi apparatus, and endoplasmic reticulum.
  5. Ribosomes

2. How do viruses reproduce?

  1. They divide by mitosis.
  2. Sexually, by external fertilization
  3. Replication outside the host
  4. Inserting DNA into the host cell
  5. The divide my Meiosis

3. The two body systems that regulate homeostasis are the:

  1. Cardiovascular skeletal systems
  2. Nervous and urinary systems
  3. Respiratory and endocrine systems
  4. Digestive and cardiovascular systems
  5. Nervous and endocrine systems
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