11
Cells And Biochemistry
Cells are the building blocks of all life, and in this respect insects are no different from other animals. Each individual cell is a little self-contained biological machine with its own particular role to play and tasks to carry out in order to keep the entire insect healthy and functional. Cellular processes involve interactions between individual molecules—in the realm of biochemistry.
11.1 • Structure of a typical cell
11.6 • Insects in cellular research
A butterfly alighting on our hand is a treat, but this behavior usually means it is interested in consuming the salts and other minerals contained in our sweat to meet certain cellular requirements.
structure of a typical cell
Cells are the self-contained, microscopic structures from which insects’ and other animals’ bodies are built. They are differentiated into many types, each with its own “job” to carry out.
We think of insects as small organisms, but compared to single-celled organisms such as amoebae, insects are very large and very complex. The cells that make up animal bodies are, on average, the same size, regardless of the actual size of the whole animal. Each gram of animal tissue contains about 1 billion cells, so a honeybee weighing one tenth of a gram has some 100 million cells in its body. The heaviest of all living insects, the Giant Weta (Deinacrida heteracantha) of New Zealand, can weigh 2½ ounces (75g), which means its body contains about 75 billion cells.
Although a cell is self-contained and to some extent self-supporting, it is part of a community of similar cells, which work together, forming the distinct organs and systems within an insect’s body. Most cells have the means to replicate themselves, allowing for growth in the larval stages and some level of tissue repair and regrowth in all stages.
The cell membrane is formed by a double layer of phospholipid molecules, plus other molecules involved in detecting and transporting nutrients and other substances into and out of the cell.
Cells vary in structure according to their function—nearly all of them are quite specialized in one way or another. However, most have at least some commonalities. The contents of a cell are known as its cytoplasm. This is contained within a flexible cell membrane. The membrane is made of layers of phospholipid molecules. It is semipermeable—certain molecules can pass through it, but most cannot. As they work, cells consume oxygen and release carbon dioxide, and these gases can diffuse through the membrane. Larger molecules tend to pass through at specific points where protein-based structures (channels or transporters) are embedded in the membrane. Molecules may also be absorbed into the cell by being engulfed by an inward fold of the membrane.
The acute and fast-acting sensory systems of damselflies are formed from various different highly specialized cell types.
Insects’ body cells are fluid-filled, and the cell membrane’s control over fluid balance enables insects to survive even in arid environments.
As well as its fluid component (the cytosol), the cytoplasm includes a number of smaller structures called organelles—they are the working parts of the cell, like organs within an organism. The number and type of organelles varies according to the type of cell. Their roles are described here.
receptors
The cell membrane often also has molecules on its outer surface that bind chemically to hormones, neurotransmitters, and other molecules. They do not enter the cell but cause a change in the cell’s state. For example, a neurotransmitter binding to a neuron (nerve cell) causes electrically charged ions to enter the cell, and this electrical charge then passes along the cell.
cell organelles
Under the microscope, several distinct structures are visible inside a typical animal cell. Some are large and prominent, while others can only be made out under the highest magnification, but all have important functions.
The most prominent structure inside a cell is its nucleus, which looks like a dark circular blob. This organelle is the cell’s control center, determining and overseeing its other activities. The cell’s genetic material—its paired chromosomes—reside within the nucleus. Chromosomes are strands of DNA that hold all the instructions telling the cell which proteins to build. When a cell divides, the process begins with the chromosomes in the nucleus being duplicated, a process that also relies on another type of organelle—the centriole. A distinct darker area within the nucleus, the nucleolus, is the building site for ribosomes, another type of organelle, which is directly involved in actually building proteins.
The membrane of the nucleus is combined with another membranous structure called endoplasmic reticulum (ER). This membrane comes in two forms—rough ER, which has ribosomes bound to it and is involved with protein-building, and smooth ER, which lacks ribosomes and builds fat molecules from free fatty acids. There are also ribosomes free in the cytoplasm. The Golgi apparatus is another membranous structure, which is involved in fine modifications to proteins built by the ribosomes.
Mitochondria are relatively large, oval organelles found within the cytoplasm. Their role is the creation of the energy-providing molecule ATP, through the metabolism of oxygen and glucose. They contain their own DNA (known as mitochondrial DNA or mtDNA), and can replicate themselves using this DNA.
Growing larvae need to replicate their body cells rapidly.
A typical cell contains several task-specific structures (organelles), which can be discerned under a powerful microscope.
The cytoplasm may also contain fat stores, in membrane-bound vesicles. Water-soluble molecules are stored in fluid-filled pockets called vacuoles. The lysosome is a particular kind of vesicle (sac) that contains enzymes. These enzymes help break down waste products and obsolete remnants of spent organelles, converting them into molecules small enough to pass out of the cell through its membrane.
numbers of organelles
Cells vary in terms of how many organelles of each type they contain. Cells with a high rate of energy consumption contain more mitochondria than average. Sperm cells, for example, contain a cluster of mitochondria near the tail end, to power its swimming movement. Some of the longest muscle cells have multiple nuclei, as do some types of large cells carried in the hemolymph and involved with the immune response.
Under the scanning electron microscope, the ribosomes attached to the folds of rough endoplasmic reticulum are visible.
cell replication
For single-celled organisms, splitting into two is the means of reproduction. Higher organisms reproduce sexually, but this process, as well as ordinary bodily growth, also begins with cell division.
Meiosis is the process by which sperm and egg cells form, each carrying 50 percent of the parental genes but in different combinations.
All insects begin their lives as a single fertilized cell, so the number of cell divisions that they go through during their lives is astronomical. The rate of division in the earliest life of an embryo is extremely rapid—in fruit fly embryos, one cell has become 6,000 within about three hours of fertilization.
The first stages of normal cell division involve the entire cell becoming larger, its organelles moving into particular parts of the cell (and sometimes duplicating), and the creation of a copy of all of the chromosome pairs inside its nucleus. It also makes a duplicate of its centrosome, the organelle that will separate the two sets of chromosomes. This process is known as interphase.
The second phase, mitosis, involves the two sets of chromosomes shortening and becoming aligned within the nucleus, and at the same time the two centrosomes form a spindle-shaped structure around the nucleus. This spindle pulls apart the two sets of chromosomes. Once they are separated, the chromosomes reassume their normal elongated, disorganized form, and the nuclear membrane forms separate seals around each set, forming two nuclei in place of one. As the two nuclei stretch and part, the entire cell membrane is also stretched and begins to pinch inward in the middle, eventually forming two separate cells.
As cells divide, they may also begin to differentiate, from generalized stem cells into those adapted for a specific function. Differentiation is gradual, stem cells becoming fully specialized cells via one or more intermediate cell types, or precursor cells. The first precursors to neurons, for example, are neuroblasts, which then become ganglion mother cells and then ganglion cells. These ganglion cells differentiate into either neurons or glial cells (another cell type found in the nervous system).
meiosis
The process of cell division is different when the cells being formed are the gametes or sex cells—the ova and the sperm. Each ovum and each sperm should contain only one chromosome from each pair, and when fertilization occurs a full chromosome set is formed by the combination of each gamete’s half-set. Cell division from the gametes’ precursor cells therefore involves an additional stage whereby each precursor cell divides twice, producing four daughter cells (ova or sperm), each with a half-set of chromosomes. This process is called meiosis. Another important part of meiosis is the “crossing over” that occurs in the earliest stages, when the tangle of chromosomes in the nucleus of a precursor are lined up in their matched pairs. During this sorting, both chromosomes in a pair will break at certain points (chiasmata) and the broken-off sections will join the other member of the pair at the same point. Although pairs of chromosomes are made of the same genes in the same sequence, they may have different “versions”—alleles—of these genes. Crossing over results in different combinations of the alleles in each sperm and egg cell.
The developing embryo of a fruit fly, showing body-segmentation: This stage is about 8 hours after fertilization.
immunology
Insects protect themselves from disease through behavior, and by the integrity of their body structure. If infectious agents like bacteria do find their way into the body, an immune system response occurs.
A phagocytic cell engulfs a bacterium, forming a phagosome. An enzyme-filled lysosome combines with the phagosome, forming a phagolysosome in which the bacterium is broken down. The products of its breakdown are then released (exocytosis).
Any adult or larval insect’s body can potentially be invaded by disease-causing viruses, bacteria, fungi, protozoa, and, in some cases, parasites and the eggs of parasitoid insects. The first line of defense, the cuticle, has its vulnerabilities, such as the spiracles that let air into the body, and any injury to the cuticle creates a significant new vulnerability. Pathogens could also be eaten accidentally, or even introduced during copulation.
Insect hemolymph contains specialized cells that detect and respond to foreign bodies. They deal with small objects such as bacterial cells by engulfing them (phagocytosis), and then breaking them down. They form aggregations around larger objects, such as eggs injected into the insect’s body by parasitoid wasps, in an attempt to fully encapsulate it. These cells are called hemocytes, and come in three main types, of which the most numerous are phagocytic plasmatocytes. As well as engulfing pathogens, these cells release signaling molecules, which alert other hemocytes to the danger. The other two types are crystal cells, which release molecules that attack pathogens, and lamellocytes, which are involved in the encapsulating process.
As well as an army of hemocytes, the insect produces pathogen-attacking proteins in its fat-body, which are released into the hemolymph. These protein molecules attack pathogens such as fungi and certain types of bacteria. They are also involved in forming clots in the hemolymph at the sites of wounds.
Insects do not have specialized cells in their hemolymph that provide an acquired immune response. In vertebrates, this function is carried out by B-lymphocytes and T-lymphocytes, and these cells help ensure that some infectious diseases, if successfully fought off, will never take hold a second time. However, insects do show increased resistance to a pathogen that they have encountered before. Studies on honeybees show that the enhanced response can even be passed on from a queen to her offspring. Biologists have not yet established how this response is achieved.
disease in insects
We tend to think of insects more as carriers of disease than sufferers, but insects are susceptible to many kinds of diseases and, in common with other animals, their immune response may not be up to the task. Biologists studying insect pathology have developed strains of bacteria, viruses, and fungi that can be used to kill off outbreaks of commercially damaging insects—for example, conservationists have used the Japanese fungus Entomophaga maimaiga in North America to control the non-native Gypsy Moth, whose larvae can be very damaging to native tree species. Gypsy Moths have also been successfully controlled in some areas through use of a species-specific virus, which kills the caterpillars. Viral particles are then shed from the dead caterpillars onto nearby foliage.
This fly has died from a fungal infection. The fungal mycelia erupting from between the abdominal segments will release airborne spores that infect new hosts.
specialized cell types
All the cells in an insect’s body have been derived from generalized stem cells and are adapted to meet a particular function. Some adaptations are particularly extreme.
Probably the most distinctive type of cell that insects’ (and other sexually reproducing organisms’) bodies produce is the sperm cell, with its long, fast-moving tail, or flagellum. This projection enables the sperm to swim, through an arrangement of sliding microtubules inside the flagellum, which create the rippling bends that propel it along. Studies on mosquito sperm have shown that the motion of the flagellum is accelerated in the presence of certain chemical signals—effectively, the sperm cell has a sense of smell, thanks to chemical receptor molecules.
The neuron or nerve cell is another distinctive, elongated cell type, with an extended axon along which nerve impulses travel to the next neuron (see Chapter 3). However, they are only part of the story when it comes to the nervous system. There are also very large numbers of glial cells in both the central and the peripheral nervous system. They come in several types, including star-shaped cells that take up unused neurotransmitters around synapses, and sheathing cells that provide protection for the delicate axons.
The fat-body contains cells called adipocytes, used for storage of fats. These round cells hold their stores in the form of droplets and can expand to a great size when necessary—for example, in larvae during their final instars, to store fuel that will be consumed during the process of metamorphosis.
The neuron, or nerve cell, has a fine, branching structure to transmit electrical impulses from dendrites to the axon’s terminal buttons. From there, the impulse passes to the next neuron and thus around the body.
Muscle cells (myocytes) have the ability to shorten (contract) and lengthen again when they relax. They contain fibers of two different types of protein (actin and myosin), which break their chemical bonds to slide over each other when the muscle contracts. Muscle myocytes have an elongated structure and a very high supply of mitochondria to fuel their energetic activity.
Chafer beetle larvae build large fat-stores over their growth period, which can last up to five years.
single-cell glands
The epidermis of an insect contains large, specialized secretory cells that function as exocrine glands, meaning they secrete their product outward, rather than inside the body. Exocrine glands produce various different chemical compounds, including the pheromones with which insects may attract a mate, and unpleasant-smelling or bad-tasting substances intended to discourage predators. Although insects may have many exocrine glands with this defensive function, it is typically only those closest to the site of stimulation that will emit the secretion.
Adipose (fat-storing) cells can form from different precursor types. They are filled by one or many fat globules.
insects in cellular research
Because of their small size, and how easy it is to keep and breed them, insects are popular study animals in laboratories. We owe much of our understanding of cell biology to insect subjects.
An insect’s head and compound eye: Scanning electron microscopes can produce amazingly detailed close-up images of insect body parts and even individual cell organelles.
Studying cells of any kind became possible with the invention of the microscope. The first cells were observed under a light microscope in the 17th century. This device uses several lenses to magnify the image, and material to be viewed is placed in a thin layer on a glass slide, lit up from below. The use of staining chemicals reveals structures within the cell more clearly (for example, mitochondria are not visible at all without some kind of staining).
The electron microscope is a much more powerful device for visualizing tiny details at high magnification and resolution. First invented in the early 20th century and revised and refined many times since, this device uses a beam of electrons rather than visible light to generate its images. Through electron microscopy of cells, the detailed structure of organelles can be studied. Although cell research has concentrated primarily on plant and vertebrate cells, since the 1970s new methods have been developed to prepare insect cells for microscopy, and our understanding of insect cell biology has made great advances.
Lineages of identical, cloned cells in the laboratory are used frequently for studying cellular biochemistry. Using identical cells ensures that experiments can be repeated reliably, without any outside variables being introduced as a result of genetic differences between the cells used. Several lineages of cells derived from insects are used in biochemistry laboratories for work such as using cells to build particular types of proteins and studying their response to different viruses.
The larval form of the Fall Armyworm Moth—the species from which the widely used cell lineage Sf9 was derived: The cells’ many uses include study on why cells “self-destruct” at a predetermined age.
The lineage Sf9, derived from ovary tissue taken from the Fall Armyworm Moth (Spodoptera frugiperda), is used in labs for a wide variety of purposes, including development of influenza vaccines, a study of how cell function changes in low-gravity conditions, and investigations into the genes that control apoptosis (programmed cell death), which is involved in aging processes.
hidden history
One of the most interesting revelations to come from investigations into insect cell biology concerns the traits that are still shared between them and us. For example, the molecular processes that guide the development of egg and sperm cells have been found to be largely the same in insects and mammals. This indicates that this process came into being more than 500 million years ago.