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HELPFUL REVIEWS FOR BENCHMARK ORGANIC COMPOUND OVERVIEW: Organic compounds are compounds that contain one or more atoms of carbon covalently bonded to atoms of other elements, most notably hydrogen, oxygen, phosphorus, nitrogen, or sulfur. However, not all carbon-containing compounds are classified are organic. Inorganic carbon compounds include carbon dioxide, carbon monoxide, metal cyanides, carbonates and bicarbonates of metal ions, and a few others. Many organic compounds are important for living organisms. Examples of organic compounds can be found in carbohydrates (which include polysaccharides), lipids, nucleic acids (including DNA and RNA as polymers), vitamins, enzymes, and amino acids (monomer building blocks of proteins and peptides). Polysaccharides include starches in animals and animals, as well as cellulose in plants. DIGESTIVE SYSTEM REVIEW: In the digestive system, both physical and chemical changes occur as food digests. When food first enters the digestive system in the mouth, it changes physically through the process of chewing. Food then physically changes into smaller pieces that are easy to swallow. Most of the physical change (mechanical digestion) is done by chewing. Saliva mixes with food to moisten it making it easier to swallow. Digestive enzymes are present in saliva beginning the process of chemically changing the food (chemical digestion). Food passes from the mouth to the stomach through the esophagus. The walls of the esophagus produce mucus, which lubricates the food making it easier to move. Next, food enters the stomach where the breakdown of protein in food begins. The muscles in the stomach churn the food contributing to mechanical digestion (physical change). Food is also mixing with digestive juice containing mucus, pepsin (an enzyme that digests proteins) and hydrochloric acid. These help to break food down chemically. As food leaves the stomach and passes into the small intestine, the food becomes a thick liquid called chyme. Muscles move food through the small intestine by peristalsis. Most chemical digestion occurs in the small intestine. Food mixes with several digestive juices produced by the small intestine, liver and pancreas. Once food is digested, it absorbs into the bloodstream in the small intestine. This is where nutrients pass through all body parts. When food enters the small intestine, the pancreas releases enzymes that help digest starch, proteins and fats. The liver produces bile, which helps break apart large clusters of fat into tiny droplets making it easier for enzymes to digest. The last part of the digestive system is the large intestine. Undigested food entering the large intestine contains lots of water and minerals. After the water and minerals are reabsorbed by the large intestine, feces remain as waste until it is eliminated from the body. As food is digested, larger molecules break down chemically into smaller molecules with the help of digestive enzymes. WORK OVERVIEW: Work might happen when an object moves a certain distance under force. Work here means mechanical work, and it is measured in units called joules. Note that the force and distance here have to be in the same directions. That is, the force should be in the direction in which the object moves. Work is linked with certain quantity of energy change, and does not change based on how the work is finished. For example, there are different ways to accomplish work, such as making use of inclined planes, ramps or simple machines. However, when the force required is reduced by these versatile methods, the other factor in the Work formula, distance, does increase. Thus the total amount of energy or work remains the same. Friction happens between the contacting surface and a moving object. This acts as a resister to prevent the relative motion between the surface and the object. When we move an object, the direction of this friction force is opposite to the direction where we push the object. Hence this resisting force needs to be overcome in order to successfully move the object. Forces affect motion. There are lots of examples you may already notice in your daily life. The process of seedling growth demonstrates a greater upward force than the gravity, which is working in the opposite direction. This is the reason that the plants can break through the soil. The turgor pressure of the water in the cytoplasm must balance the force from outside the cell. Plants can stand upright and still due to the effect of this turgor pressure. The combined pressure is so strong that it could break other hard objects, such as concrete. Plants have the ability to sense the gravity and grow in the opposite direction of gravity. A demonstration example is if plants are turned over, they still can recognize the downward pull of gravity. This makes them grow in a geotropic way so their stems go upward and roots go downward. WATERSHED OVERVIEW: A watershed, also known as a drainage basin, is the area of land that funnels all of its surface water and groundwater into a body of water, such as a stream, river, lake, aquifer or ocean. Areas of high ground separate watersheds from each other. On the large scale, the area of land bordered by the Rocky Mountains on the west and the Appalachian Mountains on the east is a watershed that drains all of its water into the Gulf of Mexico. Large watersheds, such as this one, are made up of thousands of smaller watersheds. Harris County has seven different watersheds, which help to drain all of the county’s surface water into the Gulf of Mexico. The city of Houston is located within the Brazos Watershed. All of the surface water in Houston eventually drains into the Brazos River, which then drains its water into the Gulf of Mexico. The water that is held underneath Earth’s surface is called groundwater. Surface water can become groundwater by infiltrating, or sinking, below the surface. Groundwater can be stored in and move through layers of rock known as aquifers. A rock layer is a good aquifer if it has high porosity and high permeability. High porosity means that there are many pores, or spaces, in the rock where water can reside. High permeability means that these pores are well connected, allowing water to flow through the rock. For example, a cupful of gravel has high porosity and high permeability, so water will flow through it quickly. Relative to gravel, a cupful of sand has lower porosity and permeability, and therefore, water will flow through it more slowly than through gravel. One of the ways we get drinking water is by drilling wells into aquifers and pumping the water to the surface. Unfortunately, we can deplete our drinking water supplies if we pump water out faster than rain can infiltrate the ground and replenish it. During summers in Texas, we can experience droughts severe enough that our aquifers cannot be replenished fast enough. In order to maintain groundwater during droughts, local governments try to regulate usage by limiting the amount of water we pump out. Also, when we pump out too much water, the pore spaces in the rocks, which are used to hold water, shrink in size. This shrinkage leads to subsidence, or the sinking of Earth’s surface. If Earth’s surface sinks, our buildings and homes can be damaged (for example, cracked foundations). Human activities can also lead to the pollution of Earth’s groundwater and surface waters. Water pollution is either point source or non-point source. If water pollution is point source, we can trace back to the source of pollution (for example, a factory that is releasing chemicals directly into a river). If water pollution is non-point source, there is not an apparent single source of pollution. Examples of non-point source pollution include runoff of the following pollutants into storm drains: fertilizer, pesticide, herbicide, pet waste, and oil leakage from cars. Water pollution can impact our health as well as the health of the environment. If there are chemicals in our drinking water, they can lead to illness. Similarly, polluted waters can hurt the environment. Fertilizers that are applied on farms can be washed into our streams and rivers. If too much fertilizer is applied, eventually these contaminants drain into larger bodies of water (lakes and oceans). In order to ensure we have enough clean drinking water for future generations, it is important that we use our water in a sustainable manner and that we stop water pollution. Due to the connected processes of weathering, erosion, and deposition, Earth’s surface is constantly changing. Weathering is the process of rock breaking down into smaller pieces. Erosion moves weathered pieces of rock to another place. After material has been eroded and is no longer being moved, it is deposited in a new location. In addition, once this material has been eroded, fresh rock is exposed, which can then also be weathered. These processes not only shape Earth’s surface (i.e. topography), but also influence soil characteristics. Soil is made up of broken down rocks, produced by weathering and erosion, as well as of organic matter produced by organisms. Weathering is the process of either chemically or physically breaking down rock. If a rock is chemically weathered, it is chemically altered or dissolved. Air and water are agents of chemical weathering. When carbon dioxide in the air dissolves in rainwater, it acts as a mild acid and degrades the rocks that it contacts. Chemical weathering can weaken rocks and make them more susceptible to physical weathering, but these two processes do not always occur together. If a rock is physically weathered, it is broken into smaller pieces through mechanical processes. For instance, the movement of the crust due to plate tectonics can cause fractures in rocks, breaking them into smaller parts. Frost wedging is another physical weathering process where water seeps into cracks in rock, then freezes and expands, which makes the cracks larger. As this process is repeated over a winter or over years, the cracks get bigger and bigger until the rock breaks into multiple pieces. In addition, organisms cause both chemical and physical weathering. Bacteria and algae produce acid, and when they live in cracks in rocks, this acid acts as a chemical weathering agent. Trees and animals can act as physical weathering agents. Tree roots can grow into rocks and break them apart. We can see this when we walk over sidewalks that have been cracked by the trees growing next to them. Animals can also break rocks through burrowing into them. Once rocks have been broken into smaller pieces, or sediment, either through chemical and/or physical weathering, they can then be moved to a new location. Erosion is the process of moving rock or sediment from one area to another. There are several different forces that can act as agents of erosion. These include water, wind, ice, and gravity. The strength of water, wind, and ice varies. The more powerful these erosional agents, the bigger the pieces of rocks they move. For example, a slight breeze can blow dirt and sand around, but hurricane-force winds can move boulders. Typically, water in rivers moves sediment from upstream to downstream and the water usually deposits that sediment into bays, gulfs, or oceans. Wind moves sediment along Earth’s surface and is an important erosional agent in flat areas, such as in deserts or on the plains. Ice is one of the most powerful erosional agents. A glacier can drag large boulders under its ice and deposit the boulders hundreds to thousands of miles away. Gravity moves rocks and sediments downhill. Landslides and mudslides are erosional events where rocks and sediments are moved from a high elevation to a low elevation (due to gravity). Eroded materials are deposited when gravity overcomes the force of the erosional agent. For example, in a river, sediments deposit when the force of the moving water can no longer overcome the force of gravity. BIODIVERSITY OVERVIEW: On the microscopic scale, there are also environmental variations that differentiate types of habitats. Microhabitats exist in gardens and in the spaces between rocks, for example. A variety of organisms live in these microhabitats, where they interact with different biotic and non-biotic factors (resources) such as water, food, and shelter. Biodiversity is a shortened term for biological diversity, and it can be observed and measured on a large scale — such as in the ocean or a national park or on a smaller scale, such as in a microhabitat. In a certain geographical region, the variety of life and the intricate interactions between them indicate the level of diversity present in an area. A variety of genes, different kinds of species, the ecosystems and the relationship between all of these factors form biodiversity. The stability of the ecosystem is tightly linked to its diversity. Simply put, the lower the biodiversity of an ecosystem, then the more vulnerable it is to environmental change and imbalance. If an ecosystem has a higher biodiversity, which means more genetic diversity, more varieties of species and so on, then the ecosystem can withstand more environmental stress. Thus, if certain genes or species are lost permanently, then the biodiversity is reduced. With the same environmental stress, this ecosystem will have less ability to withstand. The biodiversity of an ecosystem depends on those factors that limit the ability of the habitat to sustain a population. SUCCESSION OVERVIEW: Following a major disturbance, such as natural disaster, a progression of re-building occurs. Weeds, small insects, and other pioneers will move into the disturbed area first. This literally lays the foundation for other species to move into the area, and the progress continue. This is referred to as ecological succession. There are also different kinds of successions. Generally, if the development happens on a bare site, which has not been occupied before, it is considered as primary succession. A succession which begins on the rock exposed by geologic activity would be this kind. On the other hand, if the community develops and proceeds on a site from which another community was removed, it is secondary succession, as it develops on the existing soil. DICHOTOMOUS KEYS OVERVIEW: Biologists have identified more than 1.5 million different species on Earth. This is only a fraction of what scientists believe the total number could be — anywhere from 5 million to 100 million. Because of this abundance and diversity, scientists organize species with similar characteristics into groups based on their structure, function, and relationships. This is known as taxonomy or taxonomic classification. Organisms can be classified into groups based on their cellular structure, whether they have defined nuclei or not (eukaryotes versus prokaryotes) or if their entire body is made up of one cell (unicellular) or many cells (multicellular). Scientists can also look at how organisms function to help classify them. For example, organisms that make their own food are known as autotrophs, while organisms that need to consume other organisms in order to get the nutrients they need to survive are known as heterotrophs. How organisms reproduce is another way we can separate organisms into smaller groups. There are two main types of reproduction: asexual, in which one parent passes copies of its genes to its offspring, or sexual, in which two parents combine their genes and pass it on to their offspring. ADAPTATIONS AND NATURAL SELECTION OVERVIEW: Through natural selection, organisms in a population that are best adapted to their local environment increase in frequency relative to less well-adapted organisms over a number of generations. This difference in survival and reproduction is not due to chance. The genetic variation within a population leads to some individuals surviving and reproducing more successfully than others. An example of natural selection can be seen with the Galapagos finches. A few million years ago, one species of finch migrated to the rocky Galapagos from the mainland of Central or South America. From this one migrant species would come many species of finch evolving from the single ancestor. The ancestral finch was a ground-dwelling, seedeating finch. Selection pressures exerted by the ecological niches pushed the populations in various directions. On various islands, finch species have become adapted for various diets, including seeds, insects, flowers, leaves, etc. This process has resulted in many types of finches that differ mainly in their sizes and the shape of their beaks. Selective breeding is the process of breeding plants and animals for particular genetic traits. Breeding is done intentionally in attempts to produce offspring with desirable characteristics or improved traits. Because selective breeding often times uses inbreeding techniques, it decreases the genetic variety in the gene pool. Through this process, however, animals can be bred for domestication. Human are able to breed animals for various domestic reasons such as food production or commodities (wool, cotton, etc.), help with types of work or transportation, scientific research, or for pets. The internal structures of many organisms have adaptations that allow specific functions. The gills in fish are respiratory organs that extract dissolved oxygen from water, and excrete carbon dioxide. When a fish breathes, it takes in water and forces the water through the gill openings so that it passes over the gills to the outside. Gases are exchanged through the thin walls in gills allowing blood to carry oxygen to other parts of the body. Carbon dioxide passes from the blood through the thin gill tissue into the water. BODY SYSTEMS OVERVIEW: The human body is made up of several organ systems that work together as one unit. Each system depends on the others, either directly or indirectly, allowing the body to function normally and maintain homeostasis. The circulatory system consists of the heart, blood, and blood vessels. Its primary function is to transport needed substances, such as oxygen, to cells and tissues, and carry waste products away from cells and tissues. Blood carries oxygen from the lungs to the body cells and also transports the glucose cells use to produce energy. The circulatory system also picks up waste from cells such as carbon dioxide. When CO2 passes from cells into the blood, the circulatory system carries it to the lungs where it is exhaled. The circulatory system also transports disease-fighting blood cells helping the body systems restore homeostasis. The primary function of the respiratory system is to supply the blood with oxygen in order for the blood to deliver oxygen to all parts of the body. Through breathing, the body inhales oxygen and exhales carbon dioxide. The respiratory system is specifically designed to maintain the gas exchange relationship between the body and the environment. Through inhaling, oxygen and other gases pass through the mouth and nose, through the pharynx, trachea, and branching into the left and right bronchi. This exchange system keeps the cell well supplied with oxygen and removes carbon dioxide wastes from the body. The skeletal system, made up of all the bones in the body, has several functions. The skeleton is the framework of the body as it supports the softer tissues and provides points of attachment for most skeletal muscles. It provides protection for many of the body’s internal organs, reducing risk of injury to them. For example the vertebrae protect the spinal cord, the skull protects the brain, and the rib cage protects the heart and lungs. The skeletal system assists with movement of the body. Skeletal muscles are attached to bones, therefore when these muscles contract, they cause the bones to move. Bone tissues store several minerals such as calcium and phosphorus. Bones release these minerals into the blood as needed by the body. The muscular system comprises over 600 muscles categorized into one of three types: skeletal, smooth, and cardiac muscle. Muscles help the body with movement, both internally and externally. Skeletal muscles are attached to bones and are voluntary and aid in motility. The inside of many internal organs, such as the walls of the stomach and blood vessels, contain smooth muscles, which are involuntary. Cardiac muscles are striated and involuntary. These muscles do not tire and can contract repeatedly. The muscular system also provides the body with strength, shock absorption, maintaining shape and posture, and heat production. It helps with respiration and plays a role in the digestive process by peristalsis allowing food to move through the digestive system. The digestive system has three main functions. It first breaks down food into smaller molecules that the cells of the body can use. Then, the molecules are absorbed into the blood and carried throughout the body. Finally, wastes are eliminated from the body. Digestion is the process by which the body breaks down food into smaller nutrient molecules. In mechanical digestion, food is physically broken down into smaller pieces, such as chewing by the teeth. In chemical digestion, chemicals produced by the body break foods into smaller molecules. For example, digestive enzymes in the mouth and stomach break down larger compounds into their smaller components. After food is digested, the molecules are transported throughout the body. Absorption is the process by which nutrients pass through the walls of the digestive system into the blood (such as in the small intestine). Materials that are not absorbed are eliminated from the body as waste. The excretory system is the system in the body that collects wastes produced by cells and removes wastes from the body. It also eliminates unused by-products excreted from cells, eradicates harmful chemical build-ups, and maintains a steady, balanced chemical concentration. Waste is excreted through a number of different ways. Carbon dioxide is removed from the body through the lungs. Sweat is excreted through sweat glands in the skin. The liver helps break down certain wastes before they can be excreted. For example, urea, which comes from the breakdown of proteins, is produced by the liver. It converts part of the hemoglobin molecule from old blood cells into substances such as bile, which break down fats during digestion. The kidneys, one of the major organs of the excretory system, help filter wastes from the blood, forming the liquid waste urine. The large intestine not only removes solid waste but collects water from the waste that can be reused. The function of the reproductive system is to produce new individuals of the same type through reproduction. Unlike other organ systems, the reproductive system has significant differences for males and females. The male reproductive system is responsible for the production of the male gamete, or sperm, for fertilization of the ovum and for production of the hormone testosterone. The function of the female reproductive system is to produce eggs, and, if an egg is fertilized, to nourish a developing embryo until birth. The female system also produces hormones such as estrogen, which triggers the development of some adult female characteristics. The integumentary system consists of the skin, hair, nails, nerves, and glands acting as a barrier to protect the body from the outside world. It also functions to retain necessary body fluids, maintain steady body temperature, eliminate wastes, and protect against disease. As it works with other body systems, the integumentary system helps maintain homeostasis in order for the body to function properly. The skin, the largest and one of the most important organs of the body, has several functions including thermoregulation, detecting sensations such as touch, pressure, temperature, production of vitamin D, and healing from cuts and burns. The nervous system has two divisions that work together – the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord and is the control center of the body. The peripheral nervous system consists of all the nerves located outside of the central nervous system. The brain is the part of the central nervous system that controls most functions in the body. The spinal cord is the column of nerve tissue that links the brain to most of the nerves in the peripheral nervous system. The peripheral nervous system consists of a network of nerves that branch out from the central nervous system and function to connect it to the rest of the body. Impulses from this network of nerves travel through the spinal cord to get to the brain. The brain directs a response that usually travels from the brain, through the spinal cord, and then to the peripheral nervous system. The endocrine system is a collection of glands that secrete different hormones for the various functions and chemical reactions that occur in the body. The main function of the endocrine system is to maintain a stable internal environment, or homeostasis. It regulates metabolism, growth, development, and tissue functions. The hypothalamus is a collection of specialized cells located in the lower central part of the brain and is the main link between the endocrine system and the nervous system. It controls the pituitary gland by stimulating or suppressing the hormone secretions. The pituitary gland is one of the most important parts of the endocrine system. For example, one pituitary hormone signals the thyroid gland to produce hormones. Other pituitary hormones control body activities directly. Growth hormone regulates growth from infancy to adulthood. Another pituitary hormone directs the kidneys to regulate the amount of water in the blood. STRUCTURE AND FUNCTION IN PLANT AND ANIMAL CELLS OVERVIEW: Cells are the structural and functional units common to all living organisms. A cell is the smallest unit of life that is classified as a living thing. Some organisms are unicellular, meaning they consist of only a single cell. Most bacteria are unicellular. Other organisms, including humans, are multicellular, consisting of many cells. For example, humans have about 100 trillion cells. There are two distinct types of cells: prokaryotic cells (e.g. bacterial cells) and eukaryotic cells (e.g. plant or animal cells). The main difference between the two is a well-defined nucleus surrounded by a membranous nuclear envelope that is present in only eukaryotic cells. Both types of cells share many common features. The genetic information is stored in genes. Proteins serve as the main structural material. Ribosomes are used to synthesize proteins. And a cell membrane controls what substances enter and leave the cell. The primary difference between the two types is that prokaryotes lack a true nucleus. Multicellular organisms, such as plants and animals, have various levels of organization within them. Although individual cells can perform specific functions, they become dependent on one another and can work together for the good of the entire organism. The first level, cells, is the simplest level as cells are the basic structural and functional units in living things. Examples include blood cells, bone cells, root cells, etc. The next level, tissues, are made up of cells that are similar in structure and function and which work together to perform a specific activity. Humans, for example, have four basic tissues: connective, epithelial, muscle, and nerve. Organs are made up of tissues that work together to perform a specific activity. Examples of this level are the heart, brain, skin, etc. The fourth level is the organ systems. These are groups of two or more organs that work together to perform a specific function for the organism. Examples in the human body include the circulatory, nervous, skeletal, muscular, integumentary, endocrine, digestive, immune, reproductive, excretory, and respiratory systems. The final level of organization is the organism. Organisms are entire living things that carry out all basic life processes. Organisms take in materials, release energy from food, release wastes, grow, respond to the environment, and reproduce. Although an organism can be made up of only one cell such as bacteria, most organisms, including plants and animals, are usually made up of organ systems performing these functions. The levels of organization from smallest to largest are: cells → tissues → organs → organ systems → organisms The cell membrane is an important structure present in all cells. It has many functions including regulating the entry and exit of molecules in and out of the cell, maintaining the boundaries of the cell, supporting its contents, and maintaining proper cell to cell contact. The structure of the cell membrane includes a phospholipid bilayer. The hydrophilic parts are on the outside and the hydrophobic parts point inwards towards each other. This prevents the entry of polar solutes and is one of the main factors responsible for regulating what enters and exits the cell. Integral membrane proteins present in the cell membrane structure have many important functions. They serve as receptors for the cell and can act as carriers for active transport of substances in and out of the cell. The cell wall is a structure that is present only in plant cells which surrounds the cell membrane. This is a special characteristic that helps distinguish plant from animal cells. The cell wall is composed of polysaccharides including cellulose, lignin, protein, certain lipids, and water for example. The cell wall has many functions for a plant cell. It determines the shape of the cell, helping monitor the rate and direction of growth of the cell. Since it is rigid in nature, the cell wall provides strength and support to plant cells. Cell walls maintain turgor pressure, giving the plant rigidity. It is semi-permeable, allowing exchange of substances in and out of the cell. The cell wall also provides protection for the cell, and is the first line of defense for the cell from pathogens or harmful microorganisms. The nucleus is a highly specialized spherical structure that is the control center of a eukaryotic cell. The nucleus controls the hereditary characteristics of an organism and is responsible for protein synthesis, cell division, growth, and differentiation. Proteins and RNA are stored in the nucleolus. Ribosomes are produced in the nucleolus as well. As the control center of the cell, the nucleus stores all the chromosomal DNA of an organism. The region between the cell membrane and the nucleus of a cell is the cytoplasm. It is a gel-like substance which is part of the cell and holds important organelles. The cytoplasm is the site for many biochemical reactions necessary for maintaining life. It is the place where cell expansion and growth take place. Cell reproduction, protein synthesis, glycolysis, and cytokinesis are some of the vital functions that are carried out in the cytoplasm. The cell has many organelles present in the cytoplasm that have specific vital functions. The mitochondria are rod-shaped organelles made up of a double layered membrane. The primary function of mitochondria is the production of energy in the form of ATP and the regulation of cellular metabolism. Chloroplasts are organelles found only in plant cells that carry out the process of photosynthesis in which light energy is converted into chemical energy. Chloroplasts contain chlorophyll, giving plants a green color. They are present in each and every part of the plant including stems and even fruits. They also play an important role in storage of energy and synthesis of metabolic substances. Vacuoles are large, round, water-filled sacs in the cytoplasm that are the storage areas of the cell. Plant cells usually have one large vacuole taking up a large amount of space. At times, it can occupy more than 90% of the plant cell space. It is selectively permeable and helps maintain the pH and ionic concentration of the cell by regulating what travels in and out of the vacuole. Vacuoles store food and other materials needed by the cell. Most of the water in plant cells is stored in vacuoles. A single cell is a system, made up of many individual parts that work together, much the same way that an entire organism is a system made up of many structures with specific functions. A cell’s organelles and other structures work to carry out functions necessary to sustain life. A cell requires various substances and energy for growth, as does any living organism. The cell uses many different chemical reactions to survive. These processes create a large amount of products that the cell needs to remove. This waste includes byproducts of reactions, toxic substances, and many of the parts of the cell that break down over time and need to be eliminated. Lysosomes are organelles that help break down part of the cell that are worn out and help get rid of waste products that are created by different parts of the cell. They contain a number of different enzymes that break down waste, similar to enzymes in the digestive systems of organisms. The cell has specialized pumps that use energy to move the smaller amounts of waste product out of the cell. Exocytosis through the plasma membrane allows the cell to get rid of large quantities of waste products all at once. The process in which a cell removes waste is similar to the processes of the excretory systems of many organisms. CELL THEORY BACKGROUND: The cell theory is an explanation of the relationship between cells and living organisms. It states that all living organisms are composed of cells, cells are the basic unit of structure and function in living things, and that cells arise from pre-existing cells. This theory holds true for all living things, unicellular or multicellular. Modern variations of the theory include the ideas that energy flow occurs within cells, hereditary information (DNA) is passed on from cell to cell, and all cells have the same basic chemical composition. To carry out their day to day functions, cells require energy. The ultimate source of this energy is the sun. Some organisms can trap energy directly from the sun, storing it away in the bonds of organic molecules such as glucose through photosynthesis, and organisms which are capable of photosynthesis are called autotrophs. Organisms which are not capable of photosynthesis are called heterotrophs, and must acquire their energycontaining organic molecules through their diet instead. To convert the energy stored in organic molecules into a form that is usable, both autotrophs and heterotrophs must take large molecules and break them down, and then recapture the energy released in the process and store it the bonds of smaller, easier to use molecules. INHERITED TRAITS OVERVIEW: Students have previously been introduced to the basic concepts of heredity by examining and being aware of observable traits, such as eye color in humans or shapes of leaves in plants. Such shared characteristics are different from learned behaviors, such as table manners or learning a language. Students have likely also explored the basic concept of a cell and that it contains a nucleus. They may even be aware that each human cell has 46 chromosomes, with all of a person’s DNA organized into two sets of 23 chromosomes. Chromosomes contain the DNA for these traits and that traits, such as eye color, are passed from one generation to the next by each parent contributing a set of chromosomes to an offspring. This is why children look similar to their parents. Furthermore, which set of chromosomes gets inherited from each parent is random. This is why siblings born from separate pregnancies look similar but not identical, and why identical twins are just that, because they actually do both carry the same inherited sets of chromosomes. Essentially, the DNA provides the instructions or recipe for “building” an offspring, using the blueprint provided by the combination of the two individual parents. Heredity is not merely observed within single species, however. Mapping the human genome, as well as that of other species, has provided insight into how different species are related to each other. Not only have mammals inherited traits such as mammary glands and hair from a common ancestor, for example, but also about 75% of known human disease genes have a recognizable match in the genome of fruit flies. This infers that humans and fruit flies also share some common ancestry. A genotype is the genetic makeup of an organism, while the phenotype is a description of how that genotype is expressed in the organism’s morphology and physiology. Furthermore, a genotype for a trait often includes two variations that are referred to as a dominant allele and a recessive allele. When both a dominant allele and a recessive allele are present for a trait, the dominant allele will mask the recessive allele’s expression of the trait. Only when two copies of the recessive allele are present – one from each parent – is the recessive form expressed. This concept is especially easy to understand when examining phenotypic traits that are controlled by single genes. The ability to roll your tongue or the presence of a Widow’s Peak hairline are examples of dominant expression of traits that scientists believe are controlled by a single gene. If a person’s DNA that controls hairline shape contains both the dominant allele (Widow’s peak hairline) and the recessive allele (straight hairline), or a heterozygous state, then the person’s phenotype will show a Widow’s Peak hairline. Individuals who have two recessive alleles, or a recessive homozygous state, for the trait will have a straight hairline. However, not all traits are controlled by single genes. Most inherited traits are controlled by a combination of multiple genes. This fact makes genetic research especially complicated when trying to figure out how defects and risks for disease are configured into a person’s genotype. Surveying generational data, ongoing work on mapping genomes, and other studies continue to further our understanding of how heredity works and how medical professionals can predict, and possibly curb, health risks. Wild animal and plant populations, of course, also demonstrate how traits are inherited. There are eight genetic lineages of felines, for example. Lions, leopards, panthers, servals, cheetahs, pumas, and mountain lions, all share genetic traits inherited from a common ancestor. Genetic similarities can easily be observed between cat species, including teeth, nose, hair, feet, and tail characteristics. Wild populations can suffer, however, when their numbers are reduced. Inbreeding can occur, which results in low genetic variation and often causes what are typically recessive, deleterious traits to show up in the phenotypes in successive generations of offspring. Such growing homozygosity in recessive traits is observable in the Florida Panther, for example. Abnormal phenotypic traits include kinked tails and severe birth defects. The principles of inheritance are also studied and applied in domestication of wild species. Artificial selection, or selective breeding, has produced a variety of livestock breeds and plant types that boost human population survival and growth, while pet breeds provide comfort and companionship. For hundreds of generations, humans have bred together two individuals with desirable traits in order to enhance those desirable traits in their offspring. Unfortunately, also due to the principles of inheritance and the nature of chromosomes and their contained DNA, not all of the desirable traits can be teased from non-desirable traits. A hybrid plant that may produce a high seed yield may also have a higher vulnerability to disease, for example. Desirable traits in a dog breed may also be accompanied by a higher risk of hip dysplasia. ASEXUAL VS. SEXUAL REPRODUCTION OVERVIEW: In asexual reproduction there is only one parent and the offspring looks uniform (exactly like the parent). In sexual reproduction there are two parents and the offspring look diverse (different) because they carry characteristics from both parents. © 2012, Rice University, All Rights Reserved. While every attempt is made to ensure student-friendly educational website visits, Rice University is not responsible for the content of third-party websites.