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A body cavity is a space created in an organism which houses organs. It is lined with a layer of cells and is filled with fluid, to protect the organs from damage as the organism moves around. Body cavities form during development, as solid masses of tissue fold inward on themselves, creating pockets in which the organs develop. An example of a body cavity in humans would be the cranial cavity, which houses the brain.
A coelom is a special type of body cavity derived from the mesoderm, or middle layer of germ cells present in an embryo. Some organisms, like sponges, have no body cavities. Others, like segmented worms, have many body cavities, one present in each segment. Organisms with three distinct germ layers which form a body cavity are known as coelomates. Humans are coelomates, as we have a distinct coelom which forms during embryogenesis. The various body cavities and organs which occupy them are discussed below.
Carbohydrates are macromolecules with which most consumers are somewhat familiar. To lose weight, some individuals adhere to “low-carb” diets. Athletes, in contrast, often “carb-load” before important competitions to ensure that they have sufficient energy to compete at a high level. Carbohydrates are, in fact, an essential part of our diet grains, fruits, and vegetables are all natural sources of carbohydrates. Carbohydrates provide energy to the body, particularly through glucose, a simple sugar. Carbohydrates also have other important functions in humans, animals, and plants.
Carbohydrates can be represented by the formula (CH2O)n, where n is the number of carbon atoms in the molecule. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. Carbohydrates are classified into three subtypes: monosaccharides, disaccharides, and polysaccharides.
Monosaccharides (mono- = “one” sacchar- = “sweet”) are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbon atoms usually ranges from three to six. Most monosaccharide names end with the suffix -ose. Depending on the number of carbon atoms in the sugar, they may be known as trioses (three carbon atoms), pentoses (five carbon atoms), and hexoses (six carbon atoms).
Monosaccharides may exist as a linear chain or as ring-shaped molecules in aqueous solutions, they are usually found in the ring form.
The chemical formula for glucose is C6H12O6. In most living species, glucose is an important source of energy. During cellular respiration, energy is released from glucose, and that energy is used to help make adenosine triphosphate (ATP). Plants synthesize glucose using carbon dioxide and water by the process of photosynthesis, and the glucose, in turn, is used for the energy requirements of the plant. The excess synthesized glucose is often stored as starch that is broken down by other organisms that feed on plants.
Galactose (part of lactose, or milk sugar) and fructose (found in fruit) are other common monosaccharides. Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of differing arrangements of atoms in the carbon chain (Figure 3).
Figure 3. Glucose, galactose, and fructose are isomeric monosaccharides, meaning that they have the same chemical formula but slightly different structures.
Disaccharides (di- = “two”) form when two monosaccharides undergo a dehydration reaction (a reaction in which the removal of a water molecule occurs). During this process, the hydroxyl group (–OH) of one monosaccharide combines with a hydrogen atom of another monosaccharide, releasing a molecule of water (H2O) and forming a covalent bond between atoms in the two sugar molecules.
Common disaccharides include lactose, maltose, and sucrose. Lactose is a disaccharide consisting of the monomers glucose and galactose. It is found naturally in milk. Maltose, or malt sugar, is a disaccharide formed from a dehydration reaction between two glucose molecules. The most common disaccharide is sucrose, or table sugar, which is composed of the monomers glucose and fructose.
A long chain of monosaccharides linked by covalent bonds is known as a polysaccharide (poly- = “many”). The chain may be branched or unbranched, and it may contain different types of monosaccharides. Polysaccharides may be very large molecules. Starch, glycogen, cellulose, and chitin are examples of polysaccharides.
Starch is the stored form of sugars in plants and is made up of amylose and amylopectin (both polymers of glucose). Plants are able to synthesize glucose, and the excess glucose is stored as starch in different plant parts, including roots and seeds. The starch that is consumed by animals is broken down into smaller molecules, such as glucose. The cells can then absorb the glucose.
Glycogen is the storage form of glucose in humans and other vertebrates, and is made up of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever glucose levels decrease, glycogen is broken down to release glucose.
Cellulose is one of the most abundant natural biopolymers. The cell walls of plants are mostly made of cellulose, which provides structural support to the cell. Wood and paper are mostly cellulosic in nature. Cellulose is made up of glucose monomers that are linked by bonds between particular carbon atoms in the glucose molecule.
Every other glucose monomer in cellulose is flipped over and packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength—which is so important to plant cells. Cellulose passing through our digestive system is called dietary fiber. While the glucose-glucose bonds in cellulose cannot be broken down by human digestive enzymes, herbivores such as cows, buffalos, and horses are able to digest grass that is rich in cellulose and use it as a food source. In these animals, certain species of bacteria reside in the rumen (part of the digestive system of herbivores) and secrete the enzyme cellulase. The appendix also contains bacteria that break down cellulose, giving it an important role in the digestive systems of ruminants. Cellulases can break down cellulose into glucose monomers that can be used as an energy source by the animal.
Carbohydrates serve other functions in different animals. Arthropods, such as insects, spiders, and crabs, have an outer skeleton, called the exoskeleton, which protects their internal body parts. This exoskeleton is made of the biological macromolecule chitin, which is a nitrogenous carbohydrate. It is made of repeating units of a modified sugar containing nitrogen.
Thus, through differences in molecular structure, carbohydrates are able to serve the very different functions of energy storage (starch and glycogen) and structural support and protection (cellulose and chitin) (Figure 4).
Figure 4. Although their structures and functions differ, all polysaccharide carbohydrates are made up of monosaccharides and have the chemical formula (CH2O)n.
Obesity is a worldwide health concern, and many diseases, such as diabetes and heart disease, are becoming more prevalent because of obesity. This is one of the reasons why registered dietitians are increasingly sought after for advice. Registered dietitians help plan food and nutrition programs for individuals in various settings. They often work with patients in health-care facilities, designing nutrition plans to prevent and treat diseases. For example, dietitians may teach a patient with diabetes how to manage blood-sugar levels by eating the correct types and amounts of carbohydrates. Dietitians may also work in nursing homes, schools, and private practices.
To become a registered dietitian, one needs to earn at least a bachelor’s degree in dietetics, nutrition, food technology, or a related field. In addition, registered dietitians must complete a supervised internship program and pass a national exam. Those who pursue careers in dietetics take courses in nutrition, chemistry, biochemistry, biology, microbiology, and human physiology. Dietitians must become experts in the chemistry and functions of food (proteins, carbohydrates, and fats).
Humans have five vital organs that are essential for survival. These are the brain, heart, kidneys, liver and lungs.
The human brain is the body's control center, receiving and sending signals to other organs through the nervous system and through secreted hormones. It is responsible for our thoughts, feelings, memory storage and general perception of the world.
The human heart is a responsible for pumping blood throughout our body.
The job of the kidneys is to remove waste and extra fluid from the blood. The kidneys take urea out of the blood and combine it with water and other substances to make urine.
The liver has many functions, including detoxifying of harmful chemicals, breakdown of drugs, filtering of blood, secretion of bile and production of blood-clotting proteins.
The lungs are responsible for removing oxygen from the air we breathe and transferring it to our blood where it can be sent to our cells. The lungs also remove carbon dioxide, which we exhale.
Kidney – Internal Anatomy
Externally, the kidneys are surrounded by three layers, illustrated in (Figure 3). The kidney has three regions: Outer renal cortex, Inner renal medulla and Renal pelvis.
Figure 3. The internal structure of the kidney is shown. (credit: modification of work by NCI)
- Renal Cortex: In a dissected kidney, it is easy to identify the cortex it appears lighter in color compared to the rest of the kidney. The renal cortex is granular due to the presence of nephrons—the functional unit of the kidney. Some nephrons have a short loop of Henle that does not dip beyond the cortex. These nephrons are called cortical nephrons. About 15 percent of nephrons have long loops of Henle that extend deep into the medulla and are called juxtamedullary nephrons.
- Renal Medulla: The medulla consists of multiple pyramidal tissue masses, called the renal pyramids. In between the pyramids are spaces called renal columns through which the blood vessels pass. The tips of the pyramids, called renal papillae, point toward the renal pelvis. There are, on average, eight renal pyramids in each kidney.
- Renal Pelvis: The renal pelvis leads to the ureter on the outside of the kidney. On the inside of the kidney, the renal pelvis branches out into two or three extensions called the major calyces, which further branch into the minor calyces. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.
- The renal hilum is the entry and exit site for structures servicing the kidneys: vessels, nerves, lymphatics, and ureters. The medial-facing hila are tucked into the sweeping convex outline of the cortex. Emerging from the hilum is the renal pelvis, which is formed from the major and minor calyxes in the kidney. The smooth muscle in the renal pelvis funnels urine via peristalsis into the ureter. The renal arteries form directly from the descending aorta, whereas the renal veins return cleansed blood directly to the inferior vena cava.
Reporting Scientific Work
Whether scientific research is basic science or applied science, scientists must share their findings for other researchers to expand and build upon their discoveries. Communication and collaboration within and between sub disciplines of science are key to the advancement of knowledge in science. For this reason, an important aspect of a scientist’s work is disseminating results and communicating with peers. Scientists can share results by presenting them at a scientific meeting or conference, but this approach can reach only the limited few who are present. Instead, most scientists present their results in peer-reviewed articles that are published in scientific journals. Peer-reviewed articles are scientific papers that are reviewed, usually anonymously by a scientist’s colleagues, or peers. These colleagues are qualified individuals, often experts in the same research area, who judge whether or not the scientist’s work is suitable for publication. The process of peer review helps to ensure that the research described in a scientific paper or grant proposal is original, significant, logical, and thorough. Grant proposals, which are requests for research funding, are also subject to peer review. Scientists publish their work so other scientists can reproduce their experiments under similar or different conditions to expand on the findings. The experimental results must be consistent with the findings of other scientists.
There are many journals and the popular press that do not use a peer-review system. A large number of online open-access journals, journals with articles available without cost, are now available many of which use rigorous peer-review systems, but some of which do not. Results of any studies published in these forums without peer review are not reliable and should not form the basis for other scientific work. In one exception, journals may allow a researcher to cite a personal communication from another researcher about unpublished results with the cited author’s permission.
1: Main Body - Biology
As you have learned, information flow in an organism takes place from DNA to RNA to protein. DNA dictates the structure of mRNA in a process known as transcription , and RNA dictates the structure of protein in a process known as translation . This is known as the Central Dogma of Life.
Does the Central Dogma always apply?
Scientists are always experimenting and exploring within their current understanding of the world. As they learn and discover new things, their ideas and understanding change to reflect the new evidence they have before them.
With modern research, it is becoming clear that some aspects of the central dogma are not entirely accurate. In order to flesh out our understanding, current research is focusing on investigating the function of non-coding RNA. Although this molecules does not follow the central dogma it still has a functional role in the cell.
Identify the central dogma of life
As you have learned, information flow in an organism takes place from DNA to RNA to protein:
- DNA is transcribed to RNA via complementary base pairing rules (but with U instead of T in the transcript)
- The RNA transcript, specifically mRNA, is then translated to an amino acid polypeptide
- Final folding and modifications of the polypeptide lead to functional proteins that actually do things in cells
This is known as the Central Dogma of Life, which holds true for all organisms.
Figure 1. Click for a larger image. Instructions on DNA are transcribed onto messenger RNA. Ribosomes are able to read the genetic information inscribed on a strand of messenger RNA and use this information to string amino acids together into a protein.
The biology of fats in the body
When you have your cholesterol checked, the doctor typically gives you levels of three fats found in the blood: LDL, HDL and triglycerides. But did you know your body contains thousands of other types of fats, or lipids?
In human plasma alone, researchers have identified some 600 different types relevant to our health. Many lipids are associated with diseases--diabetes, stroke, cancer, arthritis, Alzheimer's disease, to name a few. But our bodies also need a certain amount of fat to function, and we can't make it from scratch.
Researchers funded by the National Institutes of Health are studying lipids to learn more about normal and abnormal biology. Chew on these findings the next time you ponder the fate of the fat in a French fry.
Triglycerides, cholesterol and other essential fatty acids--the scientific term for fats the body can't make on its own--store energy, insulate us and protect our vital organs. They act as messengers, helping proteins do their jobs. They also start chemical reactions involved in growth, immune function, reproduction and other aspects of basic metabolism.
The cycle of making, breaking, storing and mobilizing fats is at the core of how humans and all animals regulate their energy. An imbalance in any step can result in disease, including heart disease and diabetes. For instance, having too many triglycerides in our bloodstream raises our risk of clogged arteries, which can lead to heart attack and stroke.
Fats help the body stockpile certain nutrients as well. The so-called "fat-soluble" vitamins--A, D, E and K--are stored in the liver and in fatty tissues.
Using a quantitative and systematic approach to study lipids, researchers have classified lipids into eight main categories. Cholesterol belongs to the "sterol" group, and triglycerides are "glycerolipids." Another category, "phospholipids," includes the hundreds of lipids that constitute the cell membrane and allow cells to send and receive signals.
Breaking It Down
The main type of fat we consume, triglycerides are especially suited for energy storage because they pack more than twice as much energy as carbohydrates or proteins. Once triglycerides have been broken down during digestion, they are shipped out to cells through the bloodstream. Some of the fat gets used for energy right away. The rest is stored inside cells in blobs called lipid droplets.
When we need extra energy--for instance, when we exercise--our bodies use enzymes called lipases to break down the stored triglycerides. The cell's power plants, mitochondria, can then create more of the body's main energy source: adenosine triphosphate, or ATP.
Recent research also has helped explain the workings of a lipid called an omega-3 fatty acid -- the active ingredient in cod liver oil, which has been touted for decades as a treatment for eczema, arthritis and heart disease. Two types of these lipids blocked the activity of a protein called COX, which assists in converting an omega-6 fatty acid into pain-signaling prostaglandin molecules. These molecules are involved in inflammation, which is a common element of many diseases, so omega-3 fatty acids could have tremendous therapeutic potential.
This knowledge is just the tip of the fat-filled iceberg. We've already have learned a lot about lipids, but much more remains to be discovered.