Which of the cell types commonly found in mammals has the greatest number of mitochondria?

Which of the cell types commonly found in mammals has the greatest number of mitochondria?

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This is basically a fun question, inspired by this answer on So, which cell type will have the greatest number of mitochondria? Obviously, I am talking about wild type, healthy individuals and am looking for a rule of thumb. I realize that the number can change under different conditions/ between different individuals.

Off the top of my head, I would say that it's sperm cells given the strenuous exercise they need to perform and their MT-packed axonemes but perhaps there are better contenders?

I am primarily interested in human cell types but answers dealing with other mammals are welcome.

It's commonly believed that it's muscle cells that have the largest amount of mitochondria, and for good reason. Muscle cells are continually used to move the body, so they have the most mitochondria because of the large energy requirement. If you want to be more specific, the muscle cells of a marathon runner have even more since the muscle cells involved in running need to be able to do a lot of aerobic respiration. Another example is the cells in the heart; the amount of mitochondria makes up 40% of the cell.

The truth is, retinal cells with photoreceptors most likely have the most (80% of the cell volume) since they need to reset the potassium/sodium pump to keep the action potential in the nerves the same.

If you're curious, the cells with the least amount of mitochondria are skin cells and erythrocytes (red blood cells) since skin is just a protective barrier and red blood cells don't need to exert any effort or energy to move.


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General structure of an animal cell

Animal cells have a number of organelles and structures that perform specific functions for the cell. The huge variety of cells that have evolved to fulfill different purposes do not always have all the same organelles or structures, but in general terms, these are some of the structures you can expect to find in animal cells:

Plasma membrane

The plasma membrane is a porous membrane that surrounds an animal cell. It is responsible for regulating what moves in and out of a cell. The plasma membrane is made from a double layer of lipids. Extra compounds such as proteins and carbohydrates are embedded into the lipid membrane and perform roles such as receiving cellular signals and creating channels through the membrane.


The cells of animals and plants almost always have a ‘true’ nucleus. A nucleus consists of a nuclear envelope, chromatin, and a nucleolus.

The nuclear envelope is made from two membranes and encapsulates the contents of the nucleus. The double membrane has numerous pores to allow substances to move in and out of the nucleus.

Inside the nuclear envelope, the majority of the nucleus is filled with chromatin. Chromatin contains the majority of a cell’s DNA and condenses down to chromosomes as a cell divides. The nucleolus is the center core of the nucleus and produces organelles called ribosomes.


The cytoplasm is the internal area of an animal cell that isn’t occupied by an organelle or nucleus. It consists of a jelly-like substance called ‘cytosol’ and allows organelles and cellular substances to move around the cell as needed.

Endoplasmic reticulum (ER)

The endoplasmic reticulum is a network of membranes found within almost all eukaryotic cells. The membranes are connected to the membrane of the cell’s nucleus and are important for many cellular processes such as protein production and the metabolism of lipids and carbohydrates.

The endoplasmic reticulum includes both the smooth ER and the rough ER. The smooth ER is a smooth membrane and has no ribosomes, whereas the rough ER has ribosomes that are used to produce proteins.


Mitochondria are one of the most important of all organelles. They are the site of cellular respiration – the process that breaks down sugars and other compounds into cellular energy. It is in the mitochondria where oxygen is used and CO₂ is produced as a byproduct of respiration.

Golgi apparatus

The golgi apparatus (or golgi body) is another set of membranes found within the cell but is not attached to the nucleus of the cell. It serves many important functions including modifying proteins and lipids and transporting cellular substances out of the cell.


Ribosomes are involved in the process of creating proteins. They can be either attached to the endoplasmic reticulum or floating freely in the cell’s cytoplasm.


These small organelles perform a number of functions regarding the digestion of compounds such as fats, amino acids, and sugars. They also produce hydrogen peroxide and convert it to water.


A lysosome is the waste disposal unit of the cell. They are another small organelle and contain a range of enzymes that allow them to digest molecules such as lipids, carbohydrates, and proteins.


Centrosomes are involved in cell division and the production of flagella and cilia. They consist of two centrioles that are the main hub for a cell’s microtubules. As the nuclear envelope breaks down during cell division, microtubules interact with the cell’s chromosomes and prepares them for cellular division.


Villi are needle-like growths that extend from the plasma membrane of a cell. For some cells, such as the cells along the wall of intestines, it is important to be able to rapidly exchange substances with their surrounding environment. Villi increase the rate of exchange of materials between cells and their environment by increasing the surface area of the plasma membrane. This increases the space available for material to move in and out of the cell.


Movement is particularly important for certain animal cells. Sperm cells, for example, live for the sole purpose of traveling to an egg and fertilizing it. Flagella (plural of flagellum) provide the mechanical ability for cells to move under their own power. A flagellum is a long, thin extension of the plasma membrane and is driven by a cellular engine made from proteins.

Energy Conversion

The original energy source for nearly the entire biosphere is the nuclear reactions within the sun. From plants and other autotrophs, to single celled protozoa and the largest mammals, the energy to sustain life is derived from the sun through a series of energy conversions.

In autotrophs, solar radiation is first used to generate high-energy electrons, which are then used to pump protons against their concentration gradient, creating a proton-motive force across a membrane. The potential energy in such an electrochemical gradient is channeled to generate ATP, which, in turn, facilitates the formation of complex macromolecules. Nutrients created by autotrophs are consumed by heterotrophs, digested and then metabolized within their cells. The chemical bond energy in these molecules behaves like a storage system for the energy initially harnessed from the sun. When nutrients are oxidized, the bond energy is released – both as ATP and as high-energy electrons. In a process that parallels the initial reactions within chloroplasts, these electrons are used to gradually create an electrochemical gradient that, once again, powers the formation of ATP. ATP is repeatedly generated and utilized to sustain the living processes of the organism.

Energy from the sun, therefore, is transmuted from one form to another, as the energy in electrons, the potential energy in proton gradients and the bond energy of macromolecules.


Some examples of species that belong to the genus Aquaspirillum include A. fasciculus, A. peregrinum, A. serpens, A. polymorphum, A. itersonii, A. magnetotacticum, A. dispar, and A. bengal.

Unlike Oceanospirillum, Aquaspirillum are commonly found in freshwater habitats. They have been identified in ditches, canal water, as well as sewage among other stagnant water bodies. Some of the species have also been found in a number of other habitats including streams, pond water, and some soil samples.

Given that members of the genus Aquaspirillum reside in freshwater habitats, they do not grow under saline conditions (they have low salt tolerance and cannot grow in culture with 3 percent sodium chloride and above).

While they are also aerobic organisms, like Oceanospirillum, they have a relatively higher G+C content. Based on molecular studies, the G+C content of these organisms has been shown to range from 50 to 65 mol%. This, along with the fact that the majority of species in this group are found in freshwater habitats is some of the factors used to group these organisms in a different genus.

Like many other bacteria, Aquaspirillum bacteria are very small in size, ranging from 0.2 to 1.4 in diameter. Although the majority of the species in the group are aerobic (with some of the species being microaerobic) and use respiratory metabolism, a few of the species have been shown to be capable of growing anaerobically (using nitrate). As such, they are capable of nitrogen fixation where nitrogenase is used to reduce nitrogen sources. However, this only occurs under microaerobic conditions.

The cons of the genetically modified food isn’t 100% safe because it violate the law of natural growth. What is genetically Modified food? Genetically Modifi.

The main purpose of this book is to defend the thesis that it has been discovered in science that in a cell biochemical systems are “irreducibly complex”, me.

Biologists seek to find the answers to further and better understand the processes that occur in living systems. The first step to the scientific method is t.

Cows are being fed genetically modified crops which hinders the quality of the meat, and the milk produced by cattle.“ The US Department of Agriculture appro.

A population of microorganisms can rapidly evolve to changes due to their short generation time, quick division rate and large population (Haney, Janice. 200.

Yes, atavism and fossils do suggest theories of evolution and how we have come from one organism however they are subjective and have many faults. DNA is acc.

But in addition to the eukaryotic and prokaryotic cells, we have viruses—they are not regarded as life in the scientific community, however, they are importa.

Each rainforest differs in the animals it inhabits. Some of the animals that can be found are insects, fish, birds, reptiles, amphibians, and mammals (EL, 20.

Autotrophs can gain their energy from non-living systems like the sun for example. Most plants are consider autotrophs because they gain their energy from th.

Viruses have the ability to infect animals, plants, bacteria and archaea. Although there are millions of strains of viruses, in modern science, about 5000 vi.

3.3 Eukaryotic Cells

At this point, it should be clear that eukaryotic cells have a more complex structure than do prokaryotic cells. Organelles allow for various functions to occur in the cell at the same time. Before discussing the functions of organelles within a eukaryotic cell, let us first examine two important components of the cell: the plasma membrane and the cytoplasm.

Visual Connection

What structures does a plant cell have that an animal cell does not have? What structures does an animal cell have that a plant cell does not have?

The Plasma Membrane

Like prokaryotes, eukaryotic cells have a plasma membrane (Figure 3.8) made up of a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule composed of two fatty acid chains, a glycerol backbone, and a phosphate group. The plasma membrane regulates the passage of some substances, such as organic molecules, ions, and water, preventing the passage of some to maintain internal conditions, while actively bringing in or removing others. Other compounds move passively across the membrane.

The plasma membranes of cells that specialize in absorption are folded into fingerlike projections called microvilli (singular = microvillus). This folding increases the surface area of the plasma membrane. Such cells are typically found lining the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form matching the function of a structure.

People with celiac disease have an immune response to gluten, which is a protein found in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.

The Cytoplasm

The cytoplasm comprises the contents of a cell between the plasma membrane and the nuclear envelope (a structure to be discussed shortly). It is made up of organelles suspended in the gel-like cytosol , the cytoskeleton, and various chemicals (Figure 3.7). Even though the cytoplasm consists of 70 to 80 percent water, it has a semi-solid consistency, which comes from the proteins within it. However, proteins are not the only organic molecules found in the cytoplasm. Glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, fatty acids, and derivatives of glycerol are found there too. Ions of sodium, potassium, calcium, and many other elements are also dissolved in the cytoplasm. Many metabolic reactions, including protein synthesis, take place in the cytoplasm.

The Cytoskeleton

If you were to remove all the organelles from a cell, would the plasma membrane and the cytoplasm be the only components left? No. Within the cytoplasm, there would still be ions and organic molecules, plus a network of protein fibers that helps to maintain the shape of the cell, secures certain organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently. Collectively, this network of protein fibers is known as the cytoskeleton . There are three types of fibers within the cytoskeleton: microfilaments, also known as actin filaments, intermediate filaments, and microtubules (Figure 3.9).

Microfilaments are the thinnest of the cytoskeletal fibers and function in moving cellular components, for example, during cell division. They also maintain the structure of microvilli, the extensive folding of the plasma membrane found in cells dedicated to absorption. These components are also common in muscle cells and are responsible for muscle cell contraction. Intermediate filaments are of intermediate diameter and have structural functions, such as maintaining the shape of the cell and anchoring organelles. Keratin, the compound that strengthens hair and nails, forms one type of intermediate filament. Microtubules are the thickest of the cytoskeletal fibers. These are hollow tubes that can dissolve and reform quickly. Microtubules guide organelle movement and are the structures that pull chromosomes to their poles during cell division. They are also the structural components of flagella and cilia. In cilia and flagella, the microtubules are organized as a circle of nine double microtubules on the outside and two microtubules in the center.

The centrosome is a region near the nucleus of animal cells that functions as a microtubule-organizing center. It contains a pair of centrioles, two structures that lie perpendicular to each other. Each centriole is a cylinder of nine triplets of microtubules.

The centrosome replicates itself before a cell divides, and the centrioles play a role in pulling the duplicated chromosomes to opposite ends of the dividing cell. However, the exact function of the centrioles in cell division is not clear, since cells that have the centrioles removed can still divide, and plant cells, which lack centrioles, are capable of cell division.

Flagella and Cilia

Flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to move an entire cell, (for example, sperm, Euglena). When present, the cell has just one flagellum or a few flagella. When cilia (singular = cilium) are present, however, they are many in number and extend along the entire surface of the plasma membrane. They are short, hair-like structures that are used to move entire cells (such as paramecium) or move substances along the outer surface of the cell (for example, the cilia of cells lining the fallopian tubes that move the ovum toward the uterus, or cilia lining the cells of the respiratory tract that move particulate matter toward the throat that mucus has trapped).

The Endomembrane System

The endomembrane system (endo = within) is a group of membranes and organelles (Figure 3.13) in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell (Figure 3.7). The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromatin and directs the synthesis of ribosomes and proteins. Let us look at it in more detail (Figure 3.10).

The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 3.10). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material, and proteins. This combination of DNA and proteins is called chromatin. In eukaryotes, chromosomes are linear structures. Every species has a specific number of chromosomes in the nucleus of its body cells. For example, in humans, the chromosome number is 46, whereas in fruit flies, the chromosome number is eight.

Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes resemble an unwound, jumbled bunch of threads.

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus, called the nucleolus (plural = nucleoli), aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported through the nuclear pores into the cytoplasm.

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) (Figure 3.13) is a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids. However, these two functions are performed in separate areas of the endoplasmic reticulum: the rough endoplasmic reticulum and the smooth endoplasmic reticulum, respectively.

The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope.

The ribosomes synthesize proteins while attached to the ER, resulting in transfer of their newly synthesized proteins into the lumen of the RER where they undergo modifications such as folding or addition of sugars. The RER also makes phospholipids for cell membranes.

If the phospholipids or modified proteins are not destined to stay in the RER, they will be packaged within vesicles and transported from the RER by budding from the membrane (Figure 3.13). Since the RER is engaged in modifying proteins that will be secreted from the cell, it is abundant in cells that secrete proteins, such as the liver.

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (see Figure 3.7). The SER’s functions include synthesis of carbohydrates, lipids (including phospholipids), and steroid hormones detoxification of medications and poisons alcohol metabolism and storage of calcium ions.

The Golgi Apparatus

We have already mentioned that vesicles can bud from the ER, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles need to be sorted, packaged, and tagged so that they wind up in the right place. The sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs (Figure 3.11).

The Golgi apparatus has a receiving face near the endoplasmic reticulum and a releasing face on the side away from the ER, toward the cell membrane. The transport vesicles that form from the ER travel to the receiving face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications. The most frequent modification is the addition of short chains of sugar molecules. The newly modified proteins and lipids are then tagged with small molecular groups to enable them to be routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into vesicles that bud from the opposite face of the Golgi. While some of these vesicles, transport vesicles, deposit their contents into other parts of the cell where they will be used, others, secretory vesicles, fuse with the plasma membrane and release their contents outside the cell.

The amount of Golgi in different cell types again illustrates that form follows function within cells. Cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundant number of Golgi.

In plant cells, the Golgi has an additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell.


In animal cells, the lysosomes are the cell’s “garbage disposal.” Digestive enzymes within the lysosomes aid the breakdown of proteins, polysaccharides, lipids, nucleic acids, and even worn-out organelles. In single-celled eukaryotes, lysosomes are important for digestion of the food they ingest and the recycling of organelles. These enzymes are active at a much lower pH (more acidic) than those located in the cytoplasm. Many reactions that take place in the cytoplasm could not occur at a low pH, thus the advantage of compartmentalizing the eukaryotic cell into organelles is apparent.

Lysosomes also use their hydrolytic enzymes to destroy disease-causing organisms that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen (Figure 3.12).

Vesicles and Vacuoles

Vesicles and vacuoles are membrane-bound sacs that function in storage and transport. Vacuoles are somewhat larger than vesicles, and the membrane of a vacuole does not fuse with the membranes of other cellular components. Vesicles can fuse with other membranes within the cell system. Additionally, enzymes within plant vacuoles can break down macromolecules.

Visual Connection

Why does the cis face of the Golgi not face the plasma membrane?


Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, free ribosomes appear as either clusters or single tiny dots floating freely in the cytoplasm. Ribosomes may be attached to either the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum (Figure 3.7). Electron microscopy has shown that ribosomes consist of large and small subunits. Ribosomes are enzyme complexes that are responsible for protein synthesis.

Because protein synthesis is essential for all cells, ribosomes are found in practically every cell, although they are smaller in prokaryotic cells. They are particularly abundant in immature red blood cells for the synthesis of hemoglobin, which functions in the transport of oxygen throughout the body.


Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. The formation of ATP from the breakdown of glucose is known as cellular respiration. Mitochondria are oval-shaped, double-membrane organelles (Figure 3.14) that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae, which increase the surface area of the inner membrane. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria because muscle cells need a lot of energy to contract.


Peroxisomes are small, round organelles enclosed by single membranes. They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. Alcohol is detoxified by peroxisomes in liver cells. A byproduct of these oxidation reactions is hydrogen peroxide, H2O2, which is contained within the peroxisomes to prevent the chemical from causing damage to cellular components outside of the organelle. Hydrogen peroxide is safely broken down by peroxisomal enzymes into water and oxygen.

Animal Cells versus Plant Cells

Despite their fundamental similarities, there are some striking differences between animal and plant cells (see Table 3.1). Animal cells have centrioles, centrosomes (discussed under the cytoskeleton), and lysosomes, whereas plant cells do not. Plant cells have a cell wall, chloroplasts, plasmodesmata, and plastids used for storage, and a large central vacuole, whereas animal cells do not.

The Cell Wall

In Figure 3.7b, the diagram of a plant cell, you see a structure external to the plasma membrane called the cell wall. The cell wall is a rigid covering that protects the cell, provides structural support, and gives shape to the cell. Fungal and protist cells also have cell walls.

While the chief component of prokaryotic cell walls is peptidoglycan, the major organic molecule in the plant cell wall is cellulose, a polysaccharide made up of long, straight chains of glucose units. When nutritional information refers to dietary fiber, it is referring to the cellulose content of food.


Like mitochondria, chloroplasts also have their own DNA and ribosomes. Chloroplasts function in photosynthesis and can be found in eukaryotic cells such as plants and algae. In photosynthesis, carbon dioxide, water, and light energy are used to make glucose and oxygen. This is the major difference between plants and animals: Plants (autotrophs) are able to make their own food, like glucose, whereas animals (heterotrophs) must rely on other organisms for their organic compounds or food source.

Like mitochondria, chloroplasts have outer and inner membranes, but within the space enclosed by a chloroplast’s inner membrane is a set of interconnected and stacked, fluid-filled membrane sacs called thylakoids (Figure 3.15). Each stack of thylakoids is called a granum (plural = grana). The fluid enclosed by the inner membrane and surrounding the grana is called the stroma.

The chloroplasts contain a green pigment called chlorophyll, which captures the energy of sunlight for photosynthesis. Like plant cells, photosynthetic protists also have chloroplasts. Some bacteria also perform photosynthesis, but they do not have chloroplasts. Their photosynthetic pigments are located in the thylakoid membrane within the cell itself.

Evolution Connection


We have mentioned that both mitochondria and chloroplasts contain DNA and ribosomes. Have you wondered why? Strong evidence points to endosymbiosis as the explanation.

Symbiosis is a relationship in which organisms from two separate species live in close association and typically exhibit specific adaptations to each other. Endosymbiosis (endo-= within) is a relationship in which one organism lives inside the other. Endosymbiotic relationships abound in nature. Microbes that produce vitamin K live inside the human gut. This relationship is beneficial for us because we are unable to synthesize vitamin K. It is also beneficial for the microbes because they are protected from other organisms and are provided a stable habitat and abundant food by living within the large intestine.

Scientists have long noticed that bacteria, mitochondria, and chloroplasts are similar in size. We also know that mitochondria and chloroplasts have DNA and ribosomes, just as bacteria do. Scientists believe that host cells and bacteria formed a mutually beneficial endosymbiotic relationship when the host cells ingested aerobic bacteria and cyanobacteria but did not destroy them. Through evolution, these ingested bacteria became more specialized in their functions, with the aerobic bacteria becoming mitochondria and the photosynthetic bacteria becoming chloroplasts.

The Central Vacuole

Previously, we mentioned vacuoles as essential components of plant cells. If you look at Figure 3.7, you will see that plant cells each have a large, central vacuole that occupies most of the cell. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. In plant cells, the liquid inside the central vacuole provides turgor pressure, which is the outward pressure caused by the fluid inside the cell. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That is because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm and into the soil. As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the cell walls of a plant results in the wilted appearance. Additionally, this fluid has a very bitter taste, which discourages consumption by insects and animals. The central vacuole also functions to store proteins in developing seed cells.

Extracellular Matrix of Animal Cells

Most animal cells release materials into the extracellular space. The primary components of these materials are glycoproteins and the protein collagen. Collectively, these materials are called the extracellular matrix (Figure 3.16). Not only does the extracellular matrix hold the cells together to form a tissue, but it also allows the cells within the tissue to communicate with each other.

Blood clotting provides an example of the role of the extracellular matrix in cell communication. When the cells lining a blood vessel are damaged, they display a protein receptor called tissue factor. When tissue factor binds with another factor in the extracellular matrix, it causes platelets to adhere to the wall of the damaged blood vessel, stimulates adjacent smooth muscle cells in the blood vessel to contract (thus constricting the blood vessel), and initiates a series of steps that stimulate the platelets to produce clotting factors.

Intercellular Junctions

Cells can also communicate with each other by direct contact, referred to as intercellular junctions. There are some differences in the ways that plant and animal cells do this. Plasmodesmata (singular = plasmodesma) are junctions between plant cells, whereas animal cell contacts include tight and gap junctions, and desmosomes.

In general, long stretches of the plasma membranes of neighboring plant cells cannot touch one another because they are separated by the cell walls surrounding each cell. Plasmodesmata are numerous channels that pass between the cell walls of adjacent plant cells, connecting their cytoplasm and enabling signal molecules and nutrients to be transported from cell to cell (Figure 3.17a).

A tight junction is a watertight seal between two adjacent animal cells (Figure 3.17b). Proteins hold the cells tightly against each other. This tight adhesion prevents materials from leaking between the cells. Tight junctions are typically found in the epithelial tissue that lines internal organs and cavities, and composes most of the skin. For example, the tight junctions of the epithelial cells lining the urinary bladder prevent urine from leaking into the extracellular space.

Also found only in animal cells are desmosomes , which act like spot welds between adjacent epithelial cells (Figure 3.17c). They keep cells together in a sheet-like formation in organs and tissues that stretch, like the skin, heart, and muscles.

Gap junctions in animal cells are like plasmodesmata in plant cells in that they are channels between adjacent cells that allow for the transport of ions, nutrients, and other substances that enable cells to communicate (Figure 3.17d). Structurally, however, gap junctions and plasmodesmata differ.

Cell Component Function Present in Prokaryotes? Present in Animal Cells? Present in Plant Cells?
Plasma membrane Separates cell from external environment controls passage of organic molecules, ions, water, oxygen, and wastes into and out of the cell Yes Yes Yes
Cytoplasm Provides structure to cell site of many metabolic reactions medium in which organelles are found Yes Yes Yes
Nucleoid Location of DNA Yes No No
Nucleus Cell organelle that houses DNA and directs synthesis of ribosomes and proteins No Yes Yes
Ribosomes Protein synthesis Yes Yes Yes
Mitochondria ATP production/cellular respiration No Yes Yes
Peroxisomes Oxidizes and breaks down fatty acids and amino acids, and detoxifies poisons No Yes Yes
Vesicles and vacuoles Storage and transport digestive function in plant cells No Yes Yes
Centrosome Unspecified role in cell division in animal cells organizing center of microtubules in animal cells No Yes No
Lysosomes Digestion of macromolecules recycling of worn-out organelles No Yes No
Cell wall Protection, structural support and maintenance of cell shape Yes, primarily peptidoglycan in bacteria but not Archaea No Yes, primarily cellulose
Chloroplasts Photosynthesis No No Yes
Endoplasmic reticulum Modifies proteins and synthesizes lipids No Yes Yes
Golgi apparatus Modifies, sorts, tags, packages, and distributes lipids and proteins No Yes Yes
Cytoskeleton Maintains cell’s shape, secures organelles in specific positions, allows cytoplasm and vesicles to move within the cell, and enables unicellular organisms to move independently Yes Yes Yes
Flagella Cellular locomotion Some Some No, except for some plant sperm.
Cilia Cellular locomotion, movement of particles along extracellular surface of plasma membrane, and filtration No Some No

This table provides the components of prokaryotic and eukaryotic cells and their respective functions.

Skin Cells

Science Photo Library/Getty Images

The skin is composed of a layer of epithelial tissue (epidermis) that is supported by a layer of connective tissue (dermis) and an underlying subcutaneous layer. The outermost layer of the skin is composed of flat, squamous epithelial cells that are closely packed together. The skin covers a wide range of roles. It protects internal structures of the body from damage, prevents dehydration, acts as a barrier against germs, stores fat, and produces vitamins and hormones.

Among the Excavata are the diplomonads, which include the intestinal parasite, Giardia lamblia. Until recently, these protists were believed to lack mitochondria. Mitochondrial remnant organelles, called mitosomes, have since been identified in diplomonads, but these mitosomes are essentially nonfunctional. Diplomonads exist in anaerobic environments and use alternative pathways, such as glycolysis, to generate energy. Each diplomonad cell has two identical nuclei and uses several flagella for locomotion.

Every winter, European robins in the northern part of the continent migrate hundreds of kilometers south to the Mediterranean. It’s a navigational feat made possible by magnetoreception—specifically, the birds’ ability to detect the direction of the Earth’s magnetic field. But early attempts to explain this sixth sense, including the proposal that birds rely on internal magnetite crystals, failed to garner experimental support.

By the late 1990s, the problem had caught the eye of Thorsten Ritz, then a graduate student working on quantum effects in photosynthesis under the supervision of the late biophysicist Klaus Schulten at the University of Illinois at Urbana-Champaign. He became particularly interested in cryptochrome, a light-sensitive protein found in the retinas of birds for which there’s now “good evidence” of a role in magnetoreception, says Ritz, who has since moved to the University of California, Irvine. So in 2000, focusing on this protein and building on Schulten’s earlier theoretical work, Ritz, Schulten, and another Illinois colleague published what would come to be known as the radical-pair model to explain how magnetoreception might operate. 14

The researchers proposed that reactions in the cryptochrome protein generate a pair of radicals—molecules that each have a lone electron. The behavior of those electrons, which can be quantumly entangled with each other, is sensitive to the alignment of weak magnetic fields such as the Earth’s. Changes in the alignment of this pair relative to the magnetic field could theoretically trigger downstream chemical reactions, allowing the information to be somehow transmitted to the brain. (See illustration.)

The hypothesis generated a handful of predictions that Ritz went on to test in collaboration with the biologists who first described magnetoreception in robins, Roswitha and Wolfgang Wiltschko. In a study published in 2004, for example, the team exposed robins to magnetic fields oscillating at frequencies and angles that the model predicted would disrupt the radical pair’s sensitivity to the Earth’s magnetic field—and effectively knocked out the birds’ ability to navigate. 15

The idea has taken off since then, with growing theoretical support. And two 2018 studies of the molecular properties and expression patterns of one version of cryptochrome, Cry4, point to the protein as a likely candidate magnetoreceptor in zebra finches 16 and European robins. 17

More work is needed to determine whether or not avian magnetoreception really works this way, and to reveal if entanglement between the electrons of the radical pair is important. Scientists also don’t fully understand how cryptochrome could communicate magnetic field information to the brain, says Ritz. Meanwhile, his group is focused on mutagenesis experiments, which could help unravel cryptochrome’s magnetosensitivity. Last fall, University of Oxford chemist Peter Hore and biologist Henrik Mouritsen of the University of Oldenburg in Germany won European funding for QuantumBirds, a project with similar aims.

Now you’re not considered completely mad if you say you’re studying quantum mechanics in biology. It’s just considered a little bit wacky.

Magnetoreception isn’t the only puzzle in animal sensory biology that’s generated interest among quantum physicists another scientifically mysterious sense that researchers hope to help crack is olfaction. The traditional theory—that odorant molecules fit into protein receptors on olfactory neurons to trigger smells—faces the challenge that some molecules with almost identical shapes have completely different odors, while others with different stereochemistry smell alike.

In the mid-1990s, University College London (UCL) biophysicist Luca Turin, now a respected perfume critic, proposed that olfactory receptors might be sensitive not just to shape, but to the frequencies of vibrating bonds in odorant molecules. 18 He argued that when an odorant binds to a receptor, if its bonds are vibrating at a certain frequency they can facilitate the quantum tunneling of electrons within that receptor. This transfer of electrons, according to his model, triggers a signaling cascade in the olfactory neuron that ultimately sends an impulse to the brain.

Experimental evidence for the idea is still elusive, says Jenny Brookes, a UCL physicist who has formulated the problem mathematically to show that it’s theoretically feasible. “But that’s partly why it’s quite exciting.” In recent years, researchers have looked for isotope effects similar to the ones found in enzyme function. If tunneling plays a substantial role, odorant molecules containing heavier hydrogen isotopes should smell different from normal versions due to the lower vibration frequencies of their bonds.

The findings are mixed. In 2013, Turin’s group reported that humans can distinguish between odorants containing different isotopes. 19 Two years later, other researchers failed to reproduce the results and called the theory “implausible.” 20 But the idea didn’t go out of fashion. In 2016, another team reported that honey bees can differentiate odors with different isotopes, 21 while a recent theoretical study presents a suite of new predictions to help test the model’s validity. 22

Theoretical work is also driving interest in quantum biological explanations with far less experimental support. For example, some researchers have speculated that the coherence effects posited to play a role in photosynthesis could also contribute to such widespread biological phenomena as vision and cellular respiration. Others have suggested that proton tunneling could promote spontaneous mutations in DNA, although theoretical work by Al-Khalili and colleagues suggest this isn’t terribly likely, at least for the adenine-thymine base pairs they modeled. 23

Perhaps the most extreme extension of quantum physics to the animal kingdom is the idea that weird quantum effects might play a role in the human brain. University of California, Santa Barbara, physicist Matthew Fisher has argued that neurons possess molecular machinery capable of behaving like a quantum computer, which instead of using bits of 0s or 1s operates with qubits, units of information that can have states of both 0 and 1 simultaneously. 24

The brain’s qubits, Fisher proposed, are encoded in the states of phosphate ions inside Posner molecules, clusters of phosphate and calcium found in bone and possibly within certain cells’ mitochondria. Recent theoretical work by his team argues that the states of phosphate ions in different Posner molecules could be entangled with one another for hours or even days, and may therefore be able to perform rapid and complex computations. 25 Fisher recently received funding to set up an international collaboration, called QuBrain, to look for these effects experimentally. Many neuroscientists have expressed skepticism that the project will turn up positive results.