Is there a limit on how big a free living unicellular mono-nuclear organism can be?

When I looked for the largest protozoan I've found the name Syringammina fragilissima. According to wikipedia "… Syringammina fragilissima, is among the largest known coenocytes, reaching up to 20 centimetres (7.9 in) in diameter." The catch is it is multi-nucleated organism, not exactly what I had in mind. The reason I am interested in the upper limit of the size of the free living unicellular organism is because I want to have a sense of the limitation of single center has in emitting and receiving information across a large span of space. My interest is not about non-compartmentalization (as in the case of multi-nucleated unicellularity) rather about single center.

Structure of world's largest single cell is reflected at the molecular level

Daniel Chitwood, Ph.D., assistant member, and his research group at the Donald Danforth Plant Science Center's in St. Louis, in collaboration with the laboratory of Neelima Sinha, Ph.D., at the University of California, Davis, are using the world's largest single-celled organism, an aquatic alga called Caulerpa taxifolia, to study the nature of structure and form in plants. They have recently reported the results of their work in the online journal, PLOS Genetics.

"Caulerpa is a unique organism," said Chitwood. "It's a member of the green algae, which are plants. Remarkably, it's a single cell that can grow to a length of six to twelve inches. It independently evolved a form that resembles the organs of land plants. A stolon runs along the surface that the cell is growing on and from the stolon arise leaf-like fronds, and root-like holdfasts, which anchor the cell and absorb phosphorus from the substrate. All of these structures are just one cell."

"For many years, I've been interested in structure and form in plants, especially in tomato, which is the land plant that I've studied most," Chitwood continued. "As you might imagine, finding out what determines structure and form in a complex tomato plant is a challenging goal. It's critical to know how plants grow and develop to provide more tools to improve them and ultimately to make food production more reliable. Multicellularity is an important prerequisite that enables complex architectures in crops. Yet Caulerpa is a plant, too, and independently evolved a land plant-like body plan, but without multicellularity and as a single cell. How does that happen?"

Chitwood and his group reasoned that the structure of Caulerpa might be reflected in the RNA's present in various parts of the cell. (RNA's are the molecular products found when genes are expressed or "turned on.") For example, the frond part of the cell might show different RNA's from the holdfast part of the cell. When performed on Caulerpa, this type of analysis would also provide insights into the distributions of RNA's within single cells, a feat normally difficult to achieve because cells in multicellular organisms are so small.

"The result turned out to be even more interesting than we'd hoped," said Chitwood. "Not only do different parts of the Caulerpa cell show distinctly different RNA's, but there is also some correlation between RNA's that are expressed together within different parts of the Caulerpa cell with those expressed together in the multicellular organs of tomato. Even though the lineage that Caulerpa belongs to probably separated from that giving rise to land plants more than 500 million years ago, in many ways Caulerpa displays patterns of RNA accumulation shared with land plants today."

"Our work on Caulerpa has given me and my team a whole new way of thinking about plant structure and development," Chitwood continued enthusiastically. "It's clear that the basic form we associate with land plants can arise with and without multicellularity. In fact, higher plant cells are connected to each other by means of channels called plasmodesmata, and it has been argued that multicellular land plants exhibit properties similar to single-celled organisms like Caulerpa. What if we could really think of higher plants, like tomato, as one cell instead of multitudes? This idea of thinking of multicellular land plants, like tomato, and giant single-celled algae, like Caulerpa, similarly is supported by our results that demonstrate a shared pattern of RNA accumulation. Frankly, our results have caused us to think about plant structure from an entirely different perspective, which is the most important outcome from this research."

9.4 Signaling in Single-Celled Organisms

In this section, you will explore the following questions:

  • How do single-celled yeasts use cell signaling to communicate with each other?
  • How does quorum sensing allow some bacteria to form biofilms?

Connection for AP ® Courses

Cell signaling allows bacteria to respond to environmental cues, such as nutrient levels and quorum sensing (cell density). Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. For example, budding yeasts often release mating factors that enable them to participate in a process that is similar to sexual reproduction.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP ® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® Exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.D Cells communicate by generating, transmitting and receiving chemical signals.
Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.
Science Practice 1.5 The student can re-express key elements of natural phenomena across multiple representations in the domain.
Learning Objective 3.36 The student is able to describe a model that expresses the key elements of signal transduction pathways by which a signal is converted to a cellular response.
Essential Knowledge 3.D.1 Cell communication processes share common features that reflect a shared evolutionary history.
Science Practice 6.1 The student can justify claims with evidence.
Learning Objective 3.37 The student is able to justify claims based on scientific evidence that changes in signal transduction pathways can alter cellular response.

Teacher Support

Unicellular organisms were assumed to communicate at a very primitive level, but current research reveals the existence more complex signaling systems. Examples of these forms of communication are the formation of biofilms and quorum sensing. Biofilms have received the attention of researchers only recently for several historical and technical reasons. Since the germ theory of disease was established, the interest had been to isolate and characterize pathogens, not to study microorganisms as a community.

It is much easier to grow bacteria as pure cultures than replicate mixed populations biofilms, making the latter difficult to study in the laboratory setting. Such slime layers, previously considered haphazard assemblies of microorganisms, have been found to be highly organized ecosystems. The slime layer is made of extracellular polymers crisscrossed with channels for gases, nutrients, waste exchanges. Microbes attach to the solid substrate in a succession of populations.

Quorum sensing exists both within a same species and across species. It allows microbes to behave as multicellular populations and coordinate responses. One such example is the expression of genes encoding toxins in Staphylococcus aureus. Dr. Bonnie Bassler presents quorum sensing communication in Vibrio harveyi in this Ted Talk. Her enthusiasm and clear explanations make this video a thoroughly engaging experience. This is an opportunity to show a strong female role model in science.

Also available is this video clip: Quorum sensing molecules presented by Dr. Bonnie Bassler:

And an animation on quorum sensing in Vibrio harveyi can be found here.

Further reading: Painter, Kimberley L. et al. (2014). What role does the quorum-sensing accessory gene regulator system play during Staphylococcus aureus bacteremia? Trends in Microbiology 22:676–685

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 3.31][APLO 3.37]

Within-cell signaling allows bacteria to respond to environmental cues, such as nutrient levels. Some single-celled organisms also release molecules to signal to each other.

Signaling in Yeast

Yeasts are eukaryotes (fungi), and the components and processes found in yeast signals are similar to those of cell-surface receptor signals in multicellular organisms. Budding yeasts (Figure 9.16) are able to participate in a process that is similar to sexual reproduction that entails two haploid cells (cells with one-half the normal number of chromosomes) combining to form a diploid cell (a cell with two sets of each chromosome, which is what normal body cells contain). In order to find another haploid yeast cell that is prepared to mate, budding yeasts secrete a signaling molecule called mating factor . When mating factor binds to cell-surface receptors in other yeast cells that are nearby, they stop their normal growth cycles and initiate a cell signaling cascade that includes protein kinases and GTP-binding proteins that are similar to G-proteins.

Signaling in Bacteria

Signaling in bacteria enables bacteria to monitor extracellular conditions, ensure that there are sufficient amounts of nutrients, and ensure that hazardous situations are avoided. There are circumstances, however, when bacteria communicate with each other.

The first evidence of bacterial communication was observed in a bacterium that has a symbiotic relationship with Hawaiian bobtail squid. When the population density of the bacteria reaches a certain level, specific gene expression is initiated, and the bacteria produce bioluminescent proteins that emit light. Because the number of cells present in the environment (cell density) is the determining factor for signaling, bacterial signaling was named quorum sensing . In politics and business, a quorum is the minimum number of members required to be present to vote on an issue.

Quorum sensing uses autoinducers as signaling molecules. Autoinducers are signaling molecules secreted by bacteria to communicate with other bacteria of the same kind. The secreted autoinducers can be small, hydrophobic molecules such as acyl-homoserine lactone (AHL) or larger peptide-based molecules each type of molecule has a different mode of action. When AHL enters target bacteria, it binds to transcription factors, which then switch gene expression on or off (Figure 9.17). The peptide autoinducers stimulate more complicated signaling pathways that include bacterial kinases. The changes in bacteria following exposure to autoinducers can be quite extensive. The pathogenic bacterium Pseudomonas aeruginosa has 616 different genes that respond to autoinducers.

Visual Connection

  1. Autoinducers must bind to receptors to turn on transcription of genes responsible for the production of more autoinducers.
  2. Autoinducers can only act on a different cell. It cannot act on the cell in which it is made.
  3. Autoinducers turn on genes that enable the bacteria to form a biofilm.
  4. The receptor stays in the bacterial cell, but the autoinducers diffuse out.

Some species of bacteria that use quorum sensing form biofilms, complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Bacterial biofilms (Figure 9.18) can sometimes be found on medical equipment when biofilms invade implants such as hip or knee replacements or heart pacemakers, they can cause life-threatening infections.

The ability of certain bacteria to form biofilms has evolved because of a selection of genes that enable cell-cell communication confers an evolutionary advantage. When bacterial colonies form biofilms, they create barriers that prevent toxins and antibacterial drugs from affecting the population living in the biofilm. As a result, these populations are more likely to survive, even in the presence of antibacterial agents. This often means that bacteria living in biofilms have higher fitness than bacteria living on their own.

Science Practice Connection for AP® Courses

Think About It

Why is signaling in multicellular organisms more complicated than signaling in single-celled organisms such as microbes?

Teacher Support

This question is an application of LO 3.36 and Science Practice 1.5 because students are describing and comparing models of signaling pathways in different types of organisms.

Everyday Connection

  1. The squid provides certain nutrients that allow the bacteria to luminesce.
  2. The squid produces the luminescent luciferase enzyme, so bacteria living outside the squid do not luminesce.
  3. The ability to luminesce does not benefit free-living bacteria, so free-living bacteria do not produce luciferase.
  4. Luciferase is toxic to free-living bacteria, so free-living bacteria do not produce this enzyme.

Research on the details of quorum sensing has led to advances in growing bacteria for industrial purposes. Recent discoveries suggest that it may be possible to exploit bacterial signaling pathways to control bacterial growth this process could replace or supplement antibiotics that are no longer effective in certain situations.

Link to Learning

Watch geneticist Bonnie Bassler discuss her discovery of quorum sensing in biofilm bacteria in squid.

  1. Bacteria interact by physical signals among a colony.
  2. Bacterium interact by chemical signals when it is alone.
  3. Bacterium interact by physical signals when it is alone.
  4. Bacteria interact by chemical signals among a colony.

Evolution Connection

The first life on our planet consisted of single-celled prokaryotic organisms that had limited interaction with each other. While some external signaling occurs between different species of single-celled organisms, the majority of signaling within bacteria and yeasts concerns only other members of the same species. The evolution of cellular communication is an absolute necessity for the development of multicellular organisms, and this innovation is thought to have required approximately 2.5 billion years to appear in early life forms.

Yeasts are single-celled eukaryotes, and therefore have a nucleus and organelles characteristic of more complex life forms. Comparisons of the genomes of yeasts, nematode worms, fruit flies, and humans illustrate the evolution of increasingly complex signaling systems that allow for the efficient inner workings that keep humans and other complex life forms functioning correctly.

Kinases are a major component of cellular communication, and studies of these enzymes illustrate the evolutionary connectivity of different species. Yeasts have 130 types of kinases. More complex organisms such as nematode worms and fruit flies have 454 and 239 kinases, respectively. Of the 130 kinase types in yeast, 97 belong to the 55 subfamilies of kinases that are found in other eukaryotic organisms. The only obvious deficiency seen in yeasts is the complete absence of tyrosine kinases. It is hypothesized that phosphorylation of tyrosine residues is needed to control the more sophisticated functions of development, differentiation, and cellular communication used in multicellular organisms.

Because yeasts contain many of the same classes of signaling proteins as humans, these organisms are ideal for studying signaling cascades. Yeasts multiply quickly and are much simpler organisms than humans or other multicellular animals. Therefore, the signaling cascades are also simpler and easier to study, although they contain similar counterparts to human signaling. 2

  1. The tyrosine kinases evolved before yeast diverged from other eukaryotes, but the other fifty-five subfamilies of kinases evolved after yeast diverged.
  2. Fifty-five subfamilies of kinases evolved before yeast diverged from other eukaryotes, but the tyrosine kinases evolved after yeast diverged.
  3. All kinases evolved in yeast, but yeast later lost the tyrosine kinases because they do not need them.
  4. The evolution of tyrosine kinases involved in cellular communication occurred about 2.5 billion years ago.

Link to Learning

Watch this collection of interview clips with biofilm researchers in “What Are Bacterial Biofilms?”

  1. Bacteria often form biofilms in recurrent infections and these may be more antibiotic resistant.
  2. Bacteria rarely form biofilms in recurrent infections, making them more resistant to antibiotics than if they were not in a biofilm.
  3. Bacteria produce biofilms, which behave like a unicellular organism.
  4. Bacteria don't produce biofilms in recurrent infections but become resistant due to repeated exposure to antibiotics.


    G. Manning, G.D. Plowman, T. Hunter, S. Sudarsanam, “Evolution of Protein Kinase Signaling from Yeast to Man,” Trends in Biochemical Sciences 27, no. 10 (2002): 514–520.

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    Key Concepts and Summary

    • Protists are a diverse, polyphyletic group of eukaryotic organisms.
    • Protists may be unicellular or multicellular. They vary in how they get their nutrition, morphology, method of locomotion, and mode of reproduction.
    • Important structures of protists include contractile vacuoles, cilia, flagella, pellicles, and pseudopodia some lack organelles such as mitochondria.
    • Taxonomy of protists is changing rapidly as relationships are reassessed using newer techniques.
    • The protists include important pathogens and parasites.

    Cell Theory

    Scientists once thought that life spontaneously arose from nonliving things. Thanks to experimentation and the invention of the microscope, it is now known that life comes from preexisting life and that cells come from preexisting cells.

    Micrographia Cover

    English scientist Robert Hooke published Micrographia in 1665. In it, he illustrated the smallest complete parts of an organism, which he called cells.

    Photograph by Universal History Archive/Universal Images Group via Getty Images

    In 1665, Robert Hooke published Micrographia, a book filled with drawings and descriptions of the organisms he viewed under the recently invented microscope. The invention of the microscope led to the discovery of the cell by Hooke. While looking at cork, Hooke observed box-shaped structures, which he called &ldquocells&rdquo as they reminded him of the cells, or rooms, in monasteries. This discovery led to the development of the classical cell theory.

    The classical cell theory was proposed by Theodor Schwann in 1839. There are three parts to this theory. The first part states that all organisms are made of cells. The second part states that cells are the basic units of life. These parts were based on a conclusion made by Schwann and Matthias Schleiden in 1838, after comparing their observations of plant and animal cells. The third part, which asserts that cells come from preexisting cells that have multiplied, was described by Rudolf Virchow in 1858, when he stated omnis cellula e cellula (all cells come from cells).

    Since the formation of classical cell theory, technology has improved, allowing for more detailed observations that have led to new discoveries about cells. These findings led to the formation of the modern cell theory, which has three main additions: first, that DNA is passed between cells during cell division second, that the cells of all organisms within a similar species are mostly the same, both structurally and chemically and finally, that energy flow occurs within cells.

    English scientist Robert Hooke published Micrographia in 1665. In it, he illustrated the smallest complete parts of an organism, which he called cells.

    Photograph by Universal History Archive/Universal Images Group via Getty Images

    Is there a limit on how big a free living unicellular mono-nuclear organism can be? - Biology

    Living organisms have evolved a vast array of technologies for taking stock of conditions in their environment. Some of the most familiar and impressive examples come from our five senses. The “detectors” utilized by many organisms are especially notable for their sensitivity (ability to detect “weak” signals) and dynamic range (ability to detect both very weak and very strong signals). Hair cells in the ear can respond to sounds varying over more than 6 orders of magnitude in pressure difference between the detectability threshold (as low as 2 x 10 -10 atmospheres of sound pressure) and the onset of pain (6 x 10 -4 atmospheres of sound pressure). We note as an aside that given that atmospheric pressure is equivalent to pressure due to 10 meters of water, a detection threshold of 2 x 10 -10 atmospheres would result from the mass of a film of only 10 nm thickness, i.e. a few dozen atoms in height. Indeed, it is the enormous dynamic range of our hearing capacity that leads to the use of logarithmic scales (e.g. decibels) for describing sound intensity (which is the square of the change in pressure amplitude). The usage of a logarithmic scale is reminiscent of the Richter scale that permits us to describe the very broad range of energies associated with earthquakes. The usage of the logarithmic scale is also fitting as a result of the Weber-Fechner law that states that the subjective perception of many different kinds of senses, including hearing, is proportional to the logarithm of the stimulus intensity. Specifically, when a sound is a factor of 10 n more intense than some other sound, we say that that sound is 10n decibels more intense. According to this law we perceive as equally different, sounds that differ by the same number of decibels. Some common sound levels, measured in decibel units, are shown in Figure 1. Given the range from 0 to roughly 130, this implies a dazzling 13 orders of magnitude. Besides this wide dynamic range in intensity, the human ear responds to sounds over a range of 3 orders of magnitude in frequency between roughly 20 Hz and 20,000 Hz, while at the same time being able to detect the difference between 440Hz and 441Hz.

    Figure 1: Intensities of common sounds in units of pressure and decibels.

    Similarly impressively, rod photoreceptor cells can register the arrival of a single photon (BNID 100709, for cones a value of ≈100 is observed, BNID 100710). Here again the acute sensitivity is complemented by a dynamic range permitting us to see not only on bright sunny days but also on moonless starry nights with a 10 9 -fold lower illumination intensity. A glance at the night sky in the Northern Hemisphere greets us with a view of the North Star (Polaris). In this case, the average distance between the photons arriving on our retina from that distant light source is roughly a kilometer, demonstrating the extremely feeble light intensity reaching our eyes (as well as how fast the speed of light is….).

    Figure 2: Deflection of a mass-spring system. In the top panel, there is no applied force and the mass moves spontaneously on the frictionless table due to thermal fluctuations. In the lower panel, a force is applied to the mass-spring system by hanging a weight on it. The graphs show the position of the mass as a function of time revealing both the stochastic and deterministic origins of the motion. As shown in the lower panel, in order to have a detectable signal, the mean displacement needs to be above the amplitude of the thermal motions.

    Through studying the observed minimum stimuli detected by cells, we can ask if evolution drove organisms all the way to the limit dictated by physics. To begin to see how the challenge of constructing sensors with high sensitivity and wide dynamic range plays out, we consider the effects of temperature on an idealized tiny frictionless mass-spring system as shown in Figure 2 where the spring is characterized by a spring constant k. The goal is to measure the force applied on the mass. It is critical to understand how noise influences our ability to make this measurement. The mass will be subjected to constant thermal jiggling as a result of collisions with the molecules of the surrounding environment as well as other internal processes within the spring itself. As an extension to the discussion in the vignette on “What is the thermal energy scale and how is it relevant to biology?”, the energy resulting from these collisions equals ½ kBT, where kB is Boltzmann’s constant and T is the temperature in degrees Kelvin. What this means is that the mass will spontaneously jiggle around its equilibrium position as shown in Figure 2, with the deflection x set by the condition that

    As noted above, just like an old-fashioned scale used to measure the weight of fruits or humans, the way we measure the force is by reading out the displacement of the mass. Hence, in order for us to measure the force, the displacement must exceed a threshold set by the thermal jiggling. That is, we can only say that we have measured the force of interest once the displacement exceeds the displacements that arise spontaneously from thermal fluctuations or

    Imposing this constraint results in xmin=(kBT/k) 1/2 , which gives a force limit Fmin=(kkBT) 1/2 . This limit states that we cannot measure smaller forces because the displacements they engender could just as well have come from thermal agitation. One way to overcome these limits is to increase the measurement time (which depends on the spring constant). Many of the most clever tools of modern biophysics such as the optical trap and atomic-force microscope are designed both to overcome and exploit these effects.

    Figure 3: Response of hair cells to mechanical stimulation. (A) Bundle of stereocillia in the cochlea of a bullfrog. (B) Schematic of the experiment showing how the hair bundle is manipulated mechanically by the capillary probe and how the electrical response is measured using an electrode. (C) Microscopy image of cochlear hair cells from a turtle and the capillary probe used to perturb them. (D) Voltage as a function of the bundle displacement for the hair cells shown in part (C). (Adapted from (A) A. J. Hudspeth, Nature, 341:398, 1989. (B) A. J. Hudspeth and D. P. Corey,, Proc. Nat. Acad. Sci., 74:2407, 1977. (C) and (D) A. C. Crawford and R. Fettiplace , J. Physiol. 364:359, 1985.)

    To give a concrete example, we consider the case of the hair cells of the ear. Each such hair cell features a bundle of roughly 30-300 stereocilia as shown in Figure 3. These stereocilia are approximately 10 microns in length (BNID 109301, 109302). These small cellular appendages serve as springs that are responsible for transducing the mechanical stimulus from sound and converting it into electrical signals that can be interpreted by the brain. In response to changes in air pressure, the stereocilia are subjected to displacements that result in the gating of ion channels which leads in turn to further signal transduction. Different stereocilia respond at different frequencies which makes it possible for us to distinguish the melodies of Beethoven’s Fifth Symphony from the cacophony of a car horn. The mechanical properties of the stereocilia are similar to those of the spring that was discussed in the context of Figure 2. By pushing on individual stereocilium with a small glass fiber as shown in Figure 3, it is possible to measure the minimal displacements of the stereocilia that can trigger a detectable change in voltage. Rotation of the hair bundle by only 0.01 degree, corresponding to nanometer-scale displacements at the tip, are sufficient to elicit a voltage response of mV scale (BNID 111036, 111038). How do these numbers compare to those expected from thermal jiggling of the stereocilia? Both the observed Brownian motion as well as simple theoretical estimates using spring models like that described above reveal thermal motions of the stereocilia tips that are several nanometers in size. However, the hearing threshold appears to correspond to displacements of as little as 0.1 nm as seen in Figure 3. This interesting discrepancy actually points to the fact that the hair bundle is active and amplifies input close to its resonance frequency as well as the fact that the stereocilia are coupled, effects not considered in the simple estimate.

    Figure 4: Single-photon response of individual photoreceptors. (A) Experimental setup shows a single rod cell from the retina of a toad in a glass capillary and subjected to a beam of light. (B) Current traces as a function of time for photoreceptor subjected to light pulses in an experiment like that shown in part (A). (Adapted from (A) D. A. Baylor, et al., J. Physiol., 288:589, 1979 (B) F. Rieke and D. A. Baylor, Rev. Mod. Phys., 70:1027, 1998. )

    This same kind of reasoning governs the physical limits for our other senses as well. Namely, there is some intrinsic noise added to the property of the system we are measuring. Hence, to get a “readout” of some input, the resulting output has to be larger than the natural fluctuations of the output variable. For example, the detection and exploitation of energy carried by photons is linked to some of life’s most important processes including photosynthesis and vision. How many photons suffice to result in a change in the physiological state of a cell or organism? In now classic experiments on vision, the electrical currents from individual photoreceptor cells stimulated by light were measured. Figure 4 shows how a beam of light was applied to individual photoreceptors and how electric current traces from such experiments were measured. The experiments revealed two key insights. First, photoreceptors undergo spontaneous firing, even in the absence of light, revealing precisely the kind of noise that real events (i.e. the arrival of a photon) have to compete against. In particular, these currents are thought to result from the spontaneous thermal isomerization of individual rhodopsin molecules as discussed in the vignette on “How many rhodopsin molecules are in a rod cell?”. This isomerization reaction is normally induced by the arrival of a photon and results in the signaling cascade we perceive as vision. Second, examining the quantized nature of the currents emerging from photoreceptors exposed to very weak light demonstrates that such photoreceptors can respond to the arrival of a single photon. This effect is shown explicitly in Figure 4B.

    Another class of parameters that are “measured” with great sensitivity by cells includes the absolute numbers, identities and gradients of different chemical species. This is a key requirement in the process of development where a gradient of morphogen is translated into a recipe for pattern formation. A similar interpretation of molecular gradients is important for motile cells as they navigate the complicated chemical landscape of their watery environment. These impressive feats are not restricted to large and thinking multicellular organisms such as humans. Even individual bacteria can be said to have “knowledge” of their environment as illustrated in the exemplary system of chemotaxis already introduced in the vignette on “What are the absolute numbers of signaling proteins”. That “knowledge” leads to purposeful discriminatory power where even a few molecules of attractant per cell can be detected and amplified and differences in concentrations over a wide dynamic range of about 5 orders of magnitude can be amplified (BNID 109306, 109305). This enables unicellular behaviors in which individual bacteria will swim up a concentration gradient of chemoattractant. To get a sense of the exquisite sensitivity of these systems, Figure 5 gives a simple calculation of the concentrations being measured by a bacterium during the chemotaxis process and estimates the changes in occupancy of a surface receptor that is detecting the gradients. In particular, if we think of chemical detection by membrane-bound protein receptors, the way that the presence of a ligand is read out is by virtue of some change in the occupancy of that receptor. As the figure shows, a small change in concentration of ligand leads to a corresponding change in the occupancy of the receptor. For the case of bacterial chemotaxis, a typical gradient detected by bacteria in a microscopy experiment can be reasoned out as follows. If we consider bacteria swimming roughly 1 mm away from a pipette with 1 mM concentration of chemoattractant, the gradient is of order 10-2 M/m (BNID 111492). Is such a gradient big or small? A single-molecule difference detection threshold can be defined as:

    Interestingly, recent work has demonstrated that bacteria can even detect gradients smaller than this (though an attendant insight is that the quantity actually being “measured” by the cells is actually the gradient of the logarithm of the concentration). As noted above, this small gradient can be measured over a very wide range of absolute concentrations, illustrating both the sensitivity and dynamic range of this process, but also revealing a more nuanced mechanism than the simple occupancy hypothesis described in Figure 5 since like with the hair cell example discussed earlier in the vignette, chemotaxis receptors are adaptive. These same type of arguments arise in the context of development where morphogen gradient interpretation is based upon nucleus-to-nucleus measurement of concentration differences. For example, in the establishment of the anterior-posterior patterning of the fly embryo, neighboring nuclei are “measuring” roughly 500 and 550 molecules per nuclear volume and using that difference to make decisions about developmental fate.

    In summary, evolution pushed cells to detect environmental signals with both exquisite sensitivity and impressive dynamic range. In this process physical limits must be respected but cells find creative solutions. Photoreceptors can detect individual photons, the olfactory system nears the single-molecule detection limit, hair cells can detect pressure differences as small as 10 -9 atm and bacteria can detect gradients that correspond to less than one molecule per cell per cell length, a dazzling display of subtle and beautiful mechanisms.

    How do Protozoa Differ from Bacteria?

    Protozoa can be confused with many other microscopic organisms, most notably bacteria. In general, the size of protozoa significantly differs from bacteria as protozoans are bigger in size and structurally more complex. Bacteria are prokaryotic, which means they do not have membrane encased organelles and lack a defined nucleus.

    Protozoa are eukaryotic and have more advances characteristics in feeding, motility, and overall survival. Recall that the major features of eukaryotes absent from prokaryotes are the presence of membrane-bound organelles and a true nucleus.

    Paramecium (Left). Bacteria (Right)

    What are stem cells, and what do they do?

    Cells in the body have specific purposes, but stem cells are cells that do not yet have a specific role and can become almost any cell that is required.

    Stem cells are undifferentiated cells that can turn into specific cells, as the body needs them.

    Scientists and doctors are interested in stem cells as they help to explain how some functions of the body work, and how they sometimes go wrong.

    Stem cells also show promise for treating some diseases that currently have no cure.

    Stem cells originate from two main sources: adult body tissues and embryos. Scientists are also working on ways to develop stem cells from other cells, using genetic “reprogramming” techniques.

    Adult stem cells

    Share on Pinterest Stem cells can turn into any type of cell before they become differentiated.

    A person’s body contains stem cells throughout their life. The body can use these stem cells whenever it needs them.

    Also called tissue-specific or somatic stem cells, adult stem cells exist throughout the body from the time an embryo develops.

    The cells are in a non-specific state, but they are more specialized than embryonic stem cells. They remain in this state until the body needs them for a specific purpose, say, as skin or muscle cells.

    Day-to-day living means the body is constantly renewing its tissues. In some parts of the body, such as the gut and bone marrow, stem cells regularly divide to produce new body tissues for maintenance and repair.

    Stem cells are present inside different types of tissue. Scientists have found stem cells in tissues, including:

    • the brain
    • bone marrow
    • blood and blood vessels
    • skeletal muscles
    • skin
    • the liver

    However, stem cells can be difficult to find. They can stay non-dividing and non-specific for years until the body summons them to repair or grow new tissue.

    Adult stem cells can divide or self-renew indefinitely. This means they can generate various cell types from the originating organ or even regenerate the original organ, entirely.

    This division and regeneration are how a skin wound heals, or how an organ such as the liver, for example, can repair itself after damage.

    In the past, scientists believed adult stem cells could only differentiate based on their tissue of origin. However, some evidence now suggests that they can differentiate to become other cell types, as well.

    Embryonic stem cells

    From the very earliest stage of pregnancy, after the sperm fertilizes the egg, an embryo forms.

    Around 3–5 days after a sperm fertilizes an egg, the embryo takes the form of a blastocyst or ball of cells.

    The blastocyst contains stem cells and will later implant in the womb. Embryonic stem cells come from a blastocyst that is 4–5 days old.

    When scientists take stem cells from embryos, these are usually extra embryos that result from in vitro fertilization (IVF).

    In IVF clinics, the doctors fertilize several eggs in a test tube, to ensure that at least one survives. They will then implant a limited number of eggs to start a pregnancy.

    When a sperm fertilizes an egg, these cells combine to form a single cell called a zygote.

    This single-celled zygote then starts to divide, forming 2, 4, 8, 16 cells, and so on. Now it is an embryo.

    Soon, and before the embryo implants in the uterus, this mass of around 150–200 cells is the blastocyst. The blastocyst consists of two parts:

    • an outer cell mass that becomes part of the placenta
    • an inner cell mass that will develop into the human body

    The inner cell mass is where embryonic stem cells are found. Scientists call these totipotent cells. The term totipotent refer to the fact that they have total potential to develop into any cell in the body.

    With the right stimulation, the cells can become blood cells, skin cells, and all the other cell types that a body needs.

    In early pregnancy, the blastocyst stage continues for about 5 days before the embryo implants in the uterus, or womb. At this stage, stem cells begin to differentiate.

    Embryonic stem cells can differentiate into more cell types than adult stem cells.

    Mesenchymal stem cells (MSCs)

    MSCs come from the connective tissue or stroma that surrounds the body’s organs and other tissues.

    Scientists have used MSCs to create new body tissues, such as bone, cartilage, and fat cells. They may one day play a role in solving a wide range of health problems.

    Induced pluripotent stem cells (iPS)

    Scientists create these in a lab, using skin cells and other tissue-specific cells. These cells behave in a similar way to embryonic stem cells, so they could be useful for developing a range of therapies.

    However, more research and development is necessary.

    To grow stem cells, scientists first extract samples from adult tissue or an embryo. They then place these cells in a controlled culture where they will divide and reproduce but not specialize further.

    Stem cells that are dividing and reproducing in a controlled culture are called a stem-cell line.

    Researchers manage and share stem-cell lines for different purposes. They can stimulate the stem cells to specialize in a particular way. This process is known as directed differentiation.

    Until now, it has been easier to grow large numbers of embryonic stem cells than adult stem cells. However, scientists are making progress with both cell types.

    Researchers categorize stem cells, according to their potential to differentiate into other types of cells.

    Embryonic stem cells are the most potent, as their job is to become every type of cell in the body.

    The full classification includes:

    Totipotent: These stem cells can differentiate into all possible cell types. The first few cells that appear as the zygote starts to divide are totipotent.

    Pluripotent: These cells can turn into almost any cell. Cells from the early embryo are pluripotent.

    Multipotent: These cells can differentiate into a closely related family of cells. Adult hematopoietic stem cells, for example, can become red and white blood cells or platelets.

    Oligopotent: These can differentiate into a few different cell types. Adult lymphoid or myeloid stem cells can do this.

    Unipotent: These can only produce cells of one kind, which is their own type. However, they are still stem cells because they can renew themselves. Examples include adult muscle stem cells.

    Embryonic stem cells are considered pluripotent instead of totipotent because they cannot become part of the extra-embryonic membranes or the placenta.

    Stem cells themselves do not serve any single purpose but are important for several reasons.

    First, with the right stimulation, many stem cells can take on the role of any type of cell, and they can regenerate damaged tissue, under the right conditions.

    This potential could save lives or repair wounds and tissue damage in people after an illness or injury. Scientists see many possible uses for stem cells.

    Tissue regeneration

    Tissue regeneration is probably the most important use of stem cells.

    Until now, a person who needed a new kidney, for example, had to wait for a donor and then undergo a transplant.

    There is a shortage of donor organs but, by instructing stem cells to differentiate in a certain way, scientists could use them to grow a specific tissue type or organ.

    As an example, doctors have already used stem cells from just beneath the skin’s surface to make new skin tissue. They can then repair a severe burn or another injury by grafting this tissue onto the damaged skin, and new skin will grow back.

    Cardiovascular disease treatment

    In 2013, a team of researchers from Massachusetts General Hospital reported in PNAS Early Edition that they had created blood vessels in laboratory mice, using human stem cells.

    Within 2 weeks of implanting the stem cells, networks of blood-perfused vessels had formed. The quality of these new blood vessels was as good as the nearby natural ones.

    The authors hoped that this type of technique could eventually help to treat people with cardiovascular and vascular diseases.

    Brain disease treatment

    Doctors may one day be able to use replacement cells and tissues to treat brain diseases, such as Parkinson’s and Alzheimer’s.

    In Parkinson’s, for example, damage to brain cells leads to uncontrolled muscle movements. Scientists could use stem cells to replenish the damaged brain tissue. This could bring back the specialized brain cells that stop the uncontrolled muscle movements.

    Researchers have already tried differentiating embryonic stem cells into these types of cells, so treatments are promising.

    Cell deficiency therapy

    Scientists hope one day to be able to develop healthy heart cells in a laboratory that they can transplant into people with heart disease.

    These new cells could repair heart damage by repopulating the heart with healthy tissue.

    Similarly, people with type I diabetes could receive pancreatic cells to replace the insulin-producing cells that their own immune systems have lost or destroyed.

    The only current therapy is a pancreatic transplant, and very few pancreases are available for transplant.

    Blood disease treatments

    Doctors now routinely use adult hematopoietic stem cells to treat diseases, such as leukemia, sickle cell anemia, and other immunodeficiency problems.

    Hematopoietic stem cells occur in blood and bone marrow and can produce all blood cell types, including red blood cells that carry oxygen and white blood cells that fight disease.

    People can donate stem cells to help a loved one, or possibly for their own use in the future.

    Donations can come from the following sources:

    Bone marrow: These cells are taken under a general anesthetic, usually from the hip or pelvic bone. Technicians then isolate the stem cells from the bone marrow for storage or donation.

    Peripheral stem cells: A person receives several injections that cause their bone marrow to release stem cells into the blood. Next, blood is removed from the body, a machine separates out the stem cells, and doctors return the blood to the body.

    Umbilical cord blood: Stem cells can be harvested from the umbilical cord after delivery, with no harm to the baby. Some people donate the cord blood, and others store it.

    This harvesting of stem cells can be expensive, but the advantages for future needs include:

    The smallest protist

    © Hervé Moreau, Laboratoire Arago

    The smallest known free-living eukaryotes are marine picoplankton, of which the best-studied is Ostreococcus tauri. These organisms are so small (about 1 micrometer in diameter) that they are near the limit of resolution of ordinary light microscopes. O. tauri has a single chloroplast and a single mitochondrion. Despite their size, these organisms are extremely important contributors to the productivity of the oceans.

    M-J. Chrétiennot-Dinet, C. Courties, A. Vaquer, J. Neveux, H. Claustre, J. Lautier and M. C. Machado .
    A new marine picoeucaryote: Ostreococcus tauri gen et sp. nov. (Chlorophyta, Prasinophyceae).
    Phycologia vol. 34, pages 285-292 ( 1995 ).