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Life
Douglas Wilkin, Ph.D.Jean Brainard, Ph.D.
Jessica Harwood
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Printed: August 5, 2015
AUTHORSDouglas Wilkin, Ph.D.Jean Brainard, Ph.D.Jessica Harwood
EDITORSKathy KnoxChelsea HerndonLeiha ChaissonCora JamesDouglas Wilkin, Ph.D.
CONTRIBUTORSDoris Kraus, Ph.D.Niamh Gray-WilsonJean Brainard, Ph.D.Sarah JohnsonJane WillanCorliss Karasov
www.ck12.org Chapter 1. Life
CHAPTER 1 LifeCHAPTER OUTLINE
1.1 The Characteristics of Life
1.2 A Review of Cell Biology
1.3 Organization of the Human Body
1.4 Homeostasis
1.5 DNA Structure and Replication
1.6 DNA to RNA to Protein; Central Dogma of Molecular Biology
1.7 Cell Transport
1.8 Diffusion and Osmosis
1.9 References
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1.1 The Characteristics of Life
HS-LS1-3
Plan and conduct an investigation to provide evidence that feedback mechanisms maintain homeostasis. [Clari-fication Statement: Examples of investigations could include heart rate response to exercise, stomate response tomoisture and temperature, and root development in response to water levels.]
My Learning Goals
• I can describe all the characteristics living things possess.• I can use the characteristics of life to determine if a thing is living or not.• I can explain why internal stability is necessary in living things.
What do a bacterium and a whale have in common?
Do they share characteristics with us? All living organisms, from the smallest bacterium to the largest whale, sharecertain characteristics of life. Without these characteristics, there is no life.
Characteristics of Life
Look at the duck decoy in the Figure 1.1. It looks very similar to a real duck. Of course, real ducks are livingthings. What about the decoy duck? It looks like a duck, but it is actually made of wood. The decoy duck doesn’thave all the characteristics of a living thing. What characteristics set the real ducks apart from the decoy duck? Whatare the characteristics of living things?
To be classified as a living thing, an object must have all six of the following characteristics:
1. It responds to the environment.2. It grows and develops.3. It produces offspring.4. It maintains homeostasis.5. It has complex chemistry.6. It consists of cells.
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FIGURE 1.1This duck decoy looks like it’s alive. Iteven fools real ducks. Why isn’t it a livingthing?
FIGURE 1.2Is fire alive? Fire can grow. Fire needs fuel and oxygen. But fire is nota form of life, although it shares a few traits with some living things. Howcan you distinguish between non-living and living things?
Response to the Environment
All living things detect changes in their environment and respond to them. What happens if you step on a rock?Nothing; the rock doesn’t respond because it isn’t alive. But what if you think you are stepping on a rock andactually step on a turtle shell? The turtle is likely to respond by moving—it may even snap at you!
Plants also respond to their environment. Plant roots always grow downward because specialized cells in root capsdetect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from astimulus in the environment. Growing toward gravity is called geotropism . Plants also exhibit phototropism, orgrowing toward a light source. This response is controlled by a plant growth hormone called auxin . As shown inthe Figure below , auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bendtoward the light.
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FIGURE 1.3Phototropism response
Growth and Development
Why do you eat everyday? To get energy. Energy is the ability to do work. Without energy, you could not do any"work." Though not doing any "work" may sound nice, the "work" fueled by energy includes everyday activities,such as walking, writing, and thinking. But you are not the only one who needs energy. In order to grow andreproduce and carry out the other process of life, all living organisms need energy. But where does this energy comefrom?
The source of energy differs for each type of living thing. In your body, the source of energy is the food you eat.Here is how animals, plants, and fungi obtain their energy:
• All animals must eat in order to obtain energy. Animals also eat to obtain building materials.• Plants don’t eat. Instead, they use energy from the sun to make their "food" through the process of photosyn-
thesis.• Mushrooms and other fungi obtain energy from other organisms. That’s why you often see fungi growing on
a fallen tree; the rotting tree is their source of energy ( Figure below ).
Since plants harvest energy from the sun and other organisms get their energy from plants, nearly all the energy ofliving things initially comes from the sun.
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FIGURE 1.4Fungi breaking down a rotting log
All living things grow and develop. For example, a plant seed may look like a lifeless pebble, but under the rightconditions it will grow and develop into a plant. Animals also grow and develop. Look at the animals in the Figure1.5. How will the tadpoles change as they grow and develop into adult frogs?
FIGURE 1.5Tadpoles go through many changes to become adult frogs.
Living Things Are Made of Cells
If you zoom in very close on a leaf of a plant, or on the skin on your hand, or a drop of blood, you will find cells,you will find cells ( Figure below ). Cells are the smallest structural and functional unit of living organisms. Mostcells are so small that they are usually visible only through a microscope. Some organisms, like bacteria, planktonthat live in the ocean, or the Paramecium shown in Figure below are made of just one cell. Other organisms havemillions, billions, or trillions of cells.
All cells share at least some structures. The nucleus is clearly visible in the blood cells ( Figure below ). The nucleuscan be described as the "information center," containing the instructions ( DNA) for making all the proteins in a
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cell, as well as how much of each protein to make. The nucleus is also the main distinguishing feature between thetwo general categories of cell. Although the cells of different organisms are built differently, they all have certaingeneral functions. Every cell must get energy from food, be able to grow and divide, and respond to its environment.
FIGURE 1.6Nuclei of reptilian blood cells
Reproduction
All living things reproduce to make the next generation. Organisms that do not reproduce will go extinct. As aresult, there are no species that do not reproduce ( Figure below ). Some organisms reproduce asexually ( asexualreproduction), especially single-celled organisms, and make identical copies of themselves. Other organismsreproduce sexually ( sexual reproduction), combining genetic information from two parents to make geneticallyunique offspring. Reproducing may be as simple as a single cell dividing to form two daughter cells. Generally,however, it is much more complicated. Nonetheless, whether a living thing is a huge whale or a microscopicbacterium, it is capable of reproduction.
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FIGURE 1.7Like all living things cats reproduce tomake a new generation of cats
Living Things Maintain Stable Internal Conditions
When you are cold, what does your body do to keep warm? You shiver to warm up your body. When you are toowarm, you sweat to release heat. When any living organism gets thrown off balance, its body or cells help it returnto normal. In other words, living organisms have the ability to keep a stable internal environment. Maintaininga balance inside the body or cells of organisms is known as homeostasis. Like us, many animals have evolvedbehaviors that control their internal temperature. A lizard may stretch out on a sunny rock to increase its internaltemperature, and a bird may fluff its feathers to stay warm ( Figure below ).
FIGURE 1.8A bird fluffs its feathers to stay warm andto maintain homeostasis
Complex Chemistry
All living things—even the simplest life forms—have complex chemistry. Living things consist of large, complexmolecules, and they also undergo many complicated chemical changes to stay alive. Complex chemistry is neededto carry out all the functions of life.
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Cells
All forms of life are built of at least one cell. A cell is the basic unit of the structure and function of living things.Living things may appear very different from one another on the outside, but their cells are very similar. Comparethe human cells on the left in Figure 1.9 and onion cells on the right in Figure 1.9. How are they similar? If youclick on the animation titled Inside a Cell at the link below, you can look inside a cell and see its internal structures.http://bio-alive.com/animations/cell-biology.htm
FIGURE 1.9Human Cells (left). Onion Cells (right). If you looked at cells under a microscope, this is what you might see.
Summary
• All living things detect changes in their environment and respond to them.• All living things grow and develop.• All living things are capable of reproduction, the process by which living things give rise to offspring.• All living things are able to maintain a constant internal environment through homeostasis.• All living things have complex chemistry.• All forms of life are built of cells. A cell is the basic unit of the structure and function of living things.
Making Connections
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MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/5714
Explore More
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Non-Majors Biology→ Search: Defining Biology
1. What does "biology" encompass?2. What characteristics define life?3. Define metabolism.4. Are viruses living? Explain your answer.
Vocabulary
• Asexual reproduction: reproduction that does not involve transfer of genes between organisms. It results ingenetically identical organisms.
• Homeostasis: maintaining internal balance inside individual cells and within the entire organism• Sexual reproduction: Reproduction that involves recombining genetic material from two organisms to pro-
duce offspring that have genetic variance from their parents.• Tropism: turning toward or away from a stimulus in the environment.
Review
1. List all the characteristics of life and explain why each characteristic is necessary for life.2. Describe a real life example of how you maintain homeostasis every day.3. What happens if homeostasis is disrupted in a living organism?4. Is flame a living thing? Use the claims, evidence, reasoning format to make a claim and justify your reasoning
with evidence.5. Assume that you found an object that looks like a dead twig. You wonder if it might be a stick insect. How
could you determine if it is a living thing?
s
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1.2 A Review of Cell Biology
• Explain how cells are observed.• Define cell. Describe the general role of a cell.• State the three main parts of the cell theory.• Summarize the structure-function relationship of a cell.• Explain the levels of organization in an organism.
What are you made of?
Cells make up all living things, including your own body. This picture shows a typical group of cells. But not allcells look alike. Cells can differ in shape and sizes. And the different shapes usually means different functions.
Introduction to Cells
A cell is the smallest structural and functional unit of an organism. Some organisms, like bacteria, consist of onlyone cell. Big organisms, like humans, consist of trillions of cells. Compare a human to a banana. On the outside,they look very different, but if you look close enough you’ll see that their cells are actually very similar.
Observing Cells
Most cells are so small that you cannot see them without the help of a microscope. It was not until 1665 that Englishscientist Robert Hooke invented a basic light microscope and observed cells for the first time. You may use lightmicroscopes in the classroom. You can use a light microscope to see cells ( Figure 1.10). But many structures in thecell are too small to see with a light microscope. So, what do you do if you want to see the tiny structures inside ofcells?
In the 1950s, scientists developed more powerful microscopes. A light microscope sends a beam of light througha specimen, or the object you are studying. A more powerful microscope, called an electron microscope, passes a
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FIGURE 1.10The outline of onion cells are visible under a light microscope.
beam of electrons through the specimen. Sending electrons through a cell allows us to see its smallest parts, eventhe parts inside the cell ( Figure 1.11). Without electron microscopes, we would not know what the inside of a celllooked like.
FIGURE 1.11An electron microscope allows scientists to see much more detail than alight microscope, as with this sample of pollen.
Cell Theory
In 1858, after using microscopes much better than Hooke’s first microscope, Rudolf Virchow developed the hypoth-esis that cells only come from other cells. For example, bacteria, which are single-celled organisms, divide in half(after they grow some) to make new bacteria. In the same way, your body makes new cells by dividing the cells youalready have. In all cases, cells only come from cells that have existed before. This idea led to the development ofone of the most important theories in biology, the cell theory.
Cell theory states that:
1. All organisms are composed of cells.2. Cells are alive and the basic living units of organization in all organisms.3. All cells come from other cells.
As with other scientific theories, many hundreds, if not thousands, of experiments support the cell theory. SinceVirchow created the theory, no evidence has ever been identified to contradict it.
Specialized Cells
Although cells share many of the same features and structures, they also can be very different ( Figure 1.12). Eachcell in your body is designed for a specific task. In other words, the cell’s function is partly based on the cell’sstructure. For example:
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• Red blood cells are shaped with a pocket that traps oxygen and brings it to other body cells.• Nerve cells are long and stringy in order to form a line of communication with other nerve cells, like a wire.
Because of this shape, they can quickly send signals, such as the feeling of touching a hot stove, to your brain.• Skin cells are flat and fit tightly together to protect your body.
As you can see, cells are shaped in ways that help them do their jobs. Multicellular (many-celled) organisms havemany types of specialized cells in their bodies.
FIGURE 1.12Red blood cells (left) are specialized tocarry oxygen in the blood. Neurons (cen-ter ) are shaped to conduct electrical im-pulses to many other nerve cells. Theseepidermal cells (right) make up the “skin”of plants. Note how the cells fit tightlytogether.
How many different types of cells are there?
There are many different types of cells. For example, in you there are blood cells and skin cells and bone cells andeven bacteria. Here we have drawings of bacteria and human cells. Can you tell which depicts various types ofbacteria? However, all cells - whether from bacteria, human, or any other organism - will be one of two generaltypes. In fact, all cells other than bacteria will be one type, and bacterial cells will be the other. And it all dependson how the cell stores its DNA.
Prokaryotic cells are cells without a nucleus. The DNA in prokaryotic cells is in the cytoplasm rather than enclosedwithin a nuclear membrane. Prokaryotic cells are found in single-celled organisms, such as bacteria, like the oneshown in Figure below . Organisms with prokaryotic cells are called prokaryotes. They were the first type oforganisms to evolve and are still the most common organisms today.
FIGURE 1.13
Eukaryotic cells are cells that contain a nucleus. A typical eukaryotic cell is shown in Figure below . Eukaryoticcells are usually larger than prokaryotic cells, and they are found mainly in multicellular organisms. Organisms with
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eukaryotic cells are called eukaryotes, and they range from fungi to people.
Eukaryotic cells also contain other organelles besides the nucleus. An organelle is a structure within the cytoplasmthat performs a specific job in the cell. Organelles called mitochondria, for example, provide energy to the cell,and organelles called vacuoles store substances in the cell. Organelles allow eukaryotic cells to carry out morefunctions than prokaryotic cells can. This allows eukaryotic cells to have greater cell specificity than prokaryoticcells. Ribosomes, the organelle where proteins are made, are the only organelles in prokaryotic cells.
FIGURE 1.14
Review of cellular Organelles
MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/166097
Summary
• Cells were first observed under a light microscope, but today’s electron microscopes allow scientists to take acloser look at the inside of cells.
• Cell theory says that:
– All organisms are composed of cells.– Cells are alive and the basic living units of organization in all organisms.– All cells come from other cells.
• Cells are organized into tissues, which are organized into organs, which are organized into organ systems,which are organized to create the whole organism.
• Prokaryotic cells are cells without a nucleus.• Eukaryotic cells are cells that contain a nucleus.• Eukaryotic cells have other organelles besides the nucleus. The only organelles in a prokaryotic cell are
ribosomes.
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Explore More
Use the sliding bar to zoom in on this animation to get an idea of the relative sizes of your cells.
• Cell Size and Scale - The University of Utah at http://learn.genetics.utah.edu/content/begin/cells/scale/
1. What is the average size of a grain of salt?2. How big is an amoeba proteus? How big is a paramecium?3. How big is a skin cell? How big is a red blood cell? Can you think of any problems that might exist if this
relationship was reversed? Explain your thinking fully.4. How big is an E. coli bacterium? How big is a mitochondrion?5. Are all cells the same size?
Review
1. What type of microscope would be best for studying the structures found inside of cells?2. What are the three basic parts of the cell theory?3. According the cell theory, can you create a cell by combining molecules in a laboratory? Why or why not?4. Give an example of a specialized cell
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1.3 Organization of the Human Body
My Learning Goals
• I can explain how systems of cells work together at different levels to make up an organism.• I can give examples of how internal systems of organs function to help maintain homeostasis.• I can compare and contrast internal systems in multicellular organisms and unicellular organi sms.
Why be organized?
It can be said organization leads to efficiency. And in you, cells are organized into tissues, which are organized intoorgans, which are organized into organ systems, which form you. And it can be said that the human body is a veryorganized and efficient system.
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Organization of Cells
Biological organization exists at all levels in organisms. It can be seen at the smallest level, in the moleculesthat made up such things as DNA and proteins, to the largest level, in an organism such as a blue whale, the largestmammal on Earth. Similarly, single celled prokaryotes and eukaryotes show order in the way their cells are arranged.Single-celled organisms such as an amoeba are free-floating and independent-living. Their single-celled "bodies" areable to carry out all the processes of life, such as metabolism and respiration, without help from other cells. Somesingle-celled organisms, such as bacteria, can group together and form a biofilm. A biofilm is a large grouping ofmany bacteria that sticks to a surface and makes a protective coating over itself. Biofilms can show similarities tomulticellular organisms. Division of labor is the process in which one group of cells does one job (such as makingthe "glue" that sticks the biofilm to the surface), while another group of cells does another job (such as taking innutrients). Multicellular organisms carry out their life processes through division of labor. They have specializedcells that do specific jobs. However, biofilms are not considered multicellular organisms and are instead calledcolonial organisms. The difference between a multicellular organism and a colonial organism is that individualorganisms from a colony or biofilm can, if separated, survive on their own, while cells from a multicellular organism(e.g., liver cells) cannot.
FIGURE 1.15Volvox algae colony
Colonial Organisms
Colonial organisms were probably one of the first evolutionary steps towards multicellular organisms. Algae of thegenus Volvox are an example of the border between colonial organisms and multicellular organisms. Each Volvox isa colonial organism. It is made up of between 1,000 to 3,000 photosynthetic algae that are grouped together into ahollow sphere. The sphere has a distinct front and back end. The cells have eyespots, which are more developed inthe cells near the front. This enables the colony to swim towards light.
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Origin of Multicellularity
The oldest known multicellular organism is a red algae Bangiomorpha pubescens, fossils of which were found in1.2 billion-year-old rock. As the first organisms were single-celled, these organisms had to evolve into multicellularorganisms.
Scientists think that multicellularity arose from cooperation between many organisms of the same species. The Colo-nial Theory proposes that this cooperation led to the development of a multicellular organism. Many examples ofcooperation between organisms in nature have been observed. For example, a certain species of amoeba (a single-celled protist) groups together during times of food shortage and forms a colony that moves as one to a new location.Some of these amoebas then become slightly differentiated from each other. Most scientists accept that the ColonialTheory explains how multicellular organisms evolved.
Multicellular organisms are organisms that are made up of more than one type of cell and have specialized cellsthat are grouped together to carry out specialized functions. Most life that you can see without a microscope ismulticellular. As discussed earlier, the cells of a multicellular organism would not survive as independent cells. Thebody of a multicellular organism, such as a tree or a cat, exhibits organization at several levels: tissues, organs, andorgan systems. Similar cells are grouped into tissues, groups of tissues make up organs, and organs with a similarfunction are grouped into an organ system.
Organization of Your Body: Cells, Tissues, Organs
Cells are grouped together to carry out specific functions. A group of cells that work together form a tissue. Yourbody has four main types of tissues, as do the bodies of other animals. These tissues make up all structures andcontents of your body. An example of each tissue type is pictured in the Figure 1.16.
Groups of Tissues Form Organs
A single tissue alone cannot do all the jobs that are needed to keep you alive and healthy. Two or more tissuesworking together can do a lot more. An organ is a structure made of two or more tissues that work together. Theheart ( Figure 1.17) is made up of the four types of tissues.
Groups of Organs Form Organ Systems
Your heart pumps blood around your body. But how does your heart get blood to and from every cell in your body?Your heart is connected to blood vessels such as veins and arteries. Organs that work together form an organ system.Together, your heart, blood, and blood vessels form your cardiovascular system.
What other organ systems can you think of?
Organ Systems Work Together
Your body’s 12 organ systems are shown below ( Table 1.1). Your organ systems do not work alone in your body.They must all be able to work together.
For example, one of the most important functions of organ systems is to provide cells with oxygen and nutrients andto remove toxic waste products such as carbon dioxide. A number of organ systems, including the cardiovascularand respiratory systems, all work together to do this.
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FIGURE 1.16Your body has four main types of tissue: nervous tissue, epithelial tissue, connective tissue, and muscle tissue.They are found throughout your body.
FIGURE 1.17The four different tissue types work to-gether in the heart as they do in the otherorgans.
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TABLE 1.1: Major Organ Systems of the Human Body
Organ System Major Tissues and Organs FunctionCardiovascular Heart; blood vessels; blood Transports oxygen, hormones, and
nutrients to the body cells. Moveswastes and carbon dioxide awayfrom cells.
Lymphatic Lymph nodes; lymph vessels Defend against infection and dis-ease, moves lymph between tissuesand the blood stream.
Digestive Esophagus; stomach; small intes-tine; large intestine
Digests foods and absorbs nutrients,minerals, vitamins, and water.
Endocrine Pituitary gland, hypothalamus;adrenal glands; ovaries; testes
Produces hormones that communi-cate between cells.
Integumentary Skin, hair, nails Provides protection from injury andwater loss, physical defense againstinfection by microorganisms, andtemperature control.
Muscular Cardiac (heart) muscle; skeletalmuscle; smooth muscle; tendons
Involved in movement and heat pro-duction.
Nervous Brain, spinal cord; nerves Collects, transfers, and processesinformation.
Reproductive Female: uterus; vagina; fallopiantubes; ovariesMale: penis; testes; seminal vesi-cles
Produces gametes (sex cells) andsex hormones.
Respiratory Trachea, larynx, pharynx, lungs Brings air to sites where gas ex-change can occur between the bloodand cells (around body) or bloodand air (lungs).
Skeletal Bones, cartilage; ligaments Supports and protects soft tissues ofbody; produces blood cells; storesminerals.
Urinary Kidneys; urinary bladder Removes extra water, salts, andwaste products from blood andbody; controls pH; controls waterand salt balance.
Immune Bone marrow; spleen; white bloodcells
Defends against diseases.
Summary
• The levels of organization in organisms include: cells, tissues, organs, and organ systems.• Single-celled organisms are able to carry out all the processes of life without help from other cells.• Multicellular organisms carry out their life processes through division of labor. They have specialized cells
that do specific jobs.• The Colonial Theory proposes that cooperation among cells of the same species led to the development of a
multicellular organism.• Multicellular organisms, depending on their complexity, may be organized from cells to tissues, organs, and
organ systems.
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Explore More
Use the resources below to answer the following questions.
• Human Body Plan at http://vimeo.com/37349968 (2:28)
MEDIAClick image to the left or use the URL below.URL: http://www.ck12.org/flx/render/embeddedobject/57507
Review
1. What are the four levels of organization in an organism?2. What features distinguish a colonial organism from a multi-cellular organism?3. What is the difference between an organ and an organ system?4. If a tissue that makes up an organ is not functioning correctly how could that affect the organism?
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1.4 Homeostasis
.
HS-LS1-3
Plan and conduct an investigation to provide evidence for how feedback mechanisms maintain homeostasis. (Clar-ification statement: Examples of investigations could include heart rate response to exercise, stomate response tomoisture and temperature, and root development in response to water levels.)
My Learning Goals
• I can explain how feedback is used by organisms to help maintain homeostasis.• I can compare and contrast negative and positive feedback.• I can use cause and effect to predict consequence when homeostasis is unstable in an organism.• I can plan and conduct an investigation to explore how organisms respond when homeostasis is disrupted.
What happens if stability is disrupted?
Remove one stone and the whole arch collapses. The same is true for the human body. All the systems work togetherto maintain stability or homeostasis. Disrupt one system, and the whole body may be affected.
Homeostasis
All of the organs and organ systems of the human body work together like a well-oiled machine. This is becausethey are closely regulated by the nervous and endocrine systems. The nervous system controls virtually all bodyactivities, and the endocrine system secretes hormones that regulate these activities. Functioning together, the organsystems supply body cells with all the substances they need and eliminate their wastes. They also keep temperature,pH, and other conditions at just the right levels to support life processes.
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Maintaining Homeostasis
The process in which organ systems work to maintain a stable internal environment is called homeostasis. Keepinga stable internal environment requires constant adjustments. Here are just three of the many ways that human organsystems help the body maintain homeostasis:
• Respiratory system: A high concentration of carbon dioxide in the blood triggers faster breathing. The lungsexhale more frequently, which removes carbon dioxide from the body more quickly.
• Excretory system: A low level of water in the blood triggers retention of water by the kidneys. The kidneysproduce more concentrated urine, so less water is lost from the body.
• Endocrine system: A high concentration of sugar in the blood triggers secretion of insulin by an endocrinegland called the pancreas. Insulin is a hormone that helps cells absorb sugar from the blood.
So how does your body maintain homeostasis? The regulation of your internal environment is done primarily throughnegative feedback. Negative feedback is a response to a stimulus that keeps a variable close to a set value ( Figure1.18). Essentially, it "shuts off" or "turns on" a system when it varies from a set value.
For example, your body has an internal thermostat. During a winter day, in your house a thermostat senses thetemperature in a room and responds by turning on or off the heater. Your body acts in much the same way. Whenbody temperature rises, receptors in the skin and the brain sense the temperature change. The temperature changetriggers a command from the brain. This command can cause several responses. If you are too hot, the skin makessweat and blood vessels near the skin surface dilate. This response helps decrease body temperature.
Another example of negative feedback has to do with blood glucose levels. When glucose (sugar) levels in the bloodare too high, the pancreas secretes insulin to stimulate the absorption of glucose and the conversion of glucose intoglycogen, which is stored in the liver. As blood glucose levels decrease, less insulin is produced. When glucoselevels are too low, another hormone called glucagon is produced, which causes the liver to convert glycogen backto glucose.
FIGURE 1.18Feedback Regulation. If a raise in bodytemperature (stimulus) is detected (recep-tor), a signal will cause the brain to main-tain homeostasis (response). Once thebody temperature returns to normal, neg-ative feedback will cause the response toend. This sequence of stimulus-receptor-signal-response is used throughout thebody to maintain homeostasis.
Positive Feedback
Some processes in the body are regulated by positive feedback. Positive feedback is when a response to an eventincreases the likelihood of the event to continue. An example of positive feedback is milk production in nursingmothers. As the baby drinks her mother’s milk, the hormone prolactin, a chemical signal, is released. The more the
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baby suckles, the more prolactin is released, which causes more milk to be produced. Other examples of positivefeedback include contractions during childbirth. When constrictions in the uterus push a baby into the birth canal,additional contractions occur.
Failure of Homeostasis
Many homeostatic mechanisms such as these work continuously to maintain stable conditions in the human body.Sometimes, however, the mechanisms fail. When they do, cells may not get everything they need, or toxic wastesmay accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease or even death.
Summary
• All of the organ systems of the body work together to maintain homeostasis of the organism.• If homeostasis fails, death or disease may result.
Review
1. What is homeostasis?2. When homeostasis is disrupted what can organisms do to gain internal stability? Give specific examples.3. Describe how one of the human organ systems helps maintain homeostasis.4. A house has several systems, such as the electrical system, plumbing system, and heating and cooling system.
In what ways are the systems of a house similar to human body systems?
Vocabulary
• negative feedback: A response to a stimulus that keeps a variable close to a set value. Essentially, it "shutsoff" or "turns on" a system when it varies from a set value.
• positive feedback: response to an event increases the likelihood of the event to continue• homeostasis: The process in which organ systems work to maintain a stable internal environment.
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1.5 DNA Structure and Replication
HS-LS1-1
Construct an explanation based on evidence for how the structure of DNA determines the structure of proteins whichcarry out the essential functions of life through systems of specialized cells.
My Learning Goals
• I can describe the structure of DNA.• I can explain why DNA is important for all living things.• I can decode a single sided segment of DNA to determine the complimentary base pairs.• I can describe the purpose of replication.
What large organic molecule has a spiral shape, and may be the most important molecule to life?
Here’s a hint: molecules like this one determine who you are. They contain genetic information that controls yourcharacteristics. They determine your eye color , facial features, and other physical attributes. What molecule is it?You probably answered “DNA.” Today, it is commonly known that DNA is the genetic material. For a long time,scientists knew such molecules existed. They were aware that genetic information was contained within organicmolecules. However, they didn’t know which type of molecules play this role. In fact, for many decades, scientiststhought that proteins were the molecules that carry genetic information.
DNA and RNA
All cells come from other cells. How does a cell know to divide and become two cells , then four cells, and so on?Does this cell have instructions? What are those instructions and what do they really do? What happens when thoseinstructions don’t work properly? Are the “instructions” the genetic material? Though today it seems completelyobvious that deoxyribonucleic acid, or DNA , is the genetic material, this was not always known.
Practically everything a cell does, be it a liver cell, a skin cell, or a bone cell, it does because of proteins . It isyour proteins that make a bone cell act like a bone cell, a liver cell act like a liver cell, or a skin cell act like a skincell. It is the proteins that perform the functions of the cell, and of course, many of those functions are specific forthe bone cell, liver cell, skin cell, or any other type of cell. In other words, it is the proteins that give an organismits traits. We know that it is your proteins that make you tall or short, have light or dark skin, or have brown or blue
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eyes. But what tells those proteins how to act? It is the structure of the protein that determines its function. And it isthe order and type of amino acids that determine the structure of the protein. And that order and type of amino acidsthat make up the protein are determined by your DNA sequence.
The relatively large chromosomes that never leave the nucleus are made of DNA. And, as proteins are made onthe ribosomes in the cytoplasm, how does the information encoded in the DNA get to the site of protein synthesis ? That’s where RNA comes into this three-player act.
DNA→ RNA → Protein
That’s known as the central dogma of molecular biology . It states that “DNA makes RNA makes protein.”Really it means that the genetic information within DNA is used to make smaller molecules of RNA, whichleave the nucleus and then the genetic information in RNA is used to assemble amino acids into proteins. Butthis process does start with DNA
.
DNA winds into the familiar double helix configuration. However, it is the order of the four bases (adenine,guanine, cytosine and thymine) that provide the genetic information/instructions.
Base-Pairing
The bases in DNA do not pair randomly. When Erwin Chargaff looked closely at the bases in DNA, he noticedthat the percentage of adenine (A) in the DNA always equaled the percentage of thymine (T) , and the percentageof guanine (G) always equaled the percentage of cytosine (C) . Watson and Crick’s model explained this result bysuggesting that A always pairs with T, and G always pairs with C in the DNA helix. Therefore A and T, and G andC, are "complementary bases," or bases that always pair together, known as a base-pair . The base-pairing rules statethat A will always bind to T, and G will always bind to C. For example, if one DNA strand reads ATGCCAGT, theother strand will be made up of the complementary bases: TACGGTCA.
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"0322 DNA Nucleotides" by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/,Jun 19, 2013.. Licensed under CC BY 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:0322_DNA_Nucleotides.jpg#/media/File:0322_DNA_Nucleotides.jpg
Why Does DNA copy itself?
You consist of a great many cells , but like all other organisms, you started life as a single cell. How did youdevelop from a single cell into an organism with trillions of cells? The answer is cell division. After cells grow totheir maximum size, they divide into two new cells. These new cells are small at first, but they grow quickly andeventually divide and produce more new cells. This process keeps repeating in a continuous cycle. Your DNA needsto copy itself every time a new cell is created. The new cell needs to have DNA exactly like the rest of your cells. Otherwise, that cell might malfunction. That’s why it’s important that the process of copying DNA, called DNA
replication , is very accurate.
DNA Replication
DNA must replicate (copy) itself so that each resulting cell after mitosis and cell division has the same DNA as theparent cell. DNA replication occurs during the S phase of the cell cycle , before mitosis and cell division . Thebase pairing rules are crucial for the process of replication. DNA replication occurs when DNA is copied to form anidentical molecule of DNA.
The general steps involved in DNA replication are as follows:
1. The DNA helix unwinds like a zipper as the bonds between the base pairs are broken. The enzyme DNAHelicase is involved in breaking these bonds.
2. The two single strands of DNA then each serve as a template for a new stand to be created. Using DNA as atemplate means that on the new strand, the bases are placed in the correct order because of the base pairingrules. As a template strand is read, the new strand is created. If ATGCCA is on the "template strand," thenTACGGT will be on the new DNA strand. The enzyme DNA Polymerase reads the template and builds thenew strand of DNA.
3. The new set of nucleotides then join together to form a new strand of DNA. The process results in two DNAmolecules, each with one old strand and one new strand of DNA.
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This process is known as semiconservative replication because one strand is conserved (kept the same) in each newDNA molecule ( Figure below ).
FIGURE 1.19
DNA replication occurs when the DNA strands “unzip,” and the original strands of DNA serve as a template for newnucleotides to join and form a new strand.
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Summary
• All cells contain DNA.• DNA is the genetic material.• The central dogma of molecular biology states that DNA makes RNA and RNA makes protein.• DNA stores the genetic information of the cell in the sequence of its 4 bases: adenine, thymine, guanine, and
cytosine.• During DNA replication, the DNA helix unwinds and the two single strands of DNA then each serve as a
template for a new stand to be created.• DNA replication is semi-conservative: the new DNA molecule consists of half of the parent DNA molecule.
Review
1. Describe three reasons cells must be able to replicate their DNA.2. Summarize a list of the three steps involved in DNA replication.3. Explain why DNA replication is sometimes called semiconservative.4. What would happen to an organism if its cells replicated too frequently?5. What would happen to an organism if its cells lost their ability to replicate?6. How is DNA replication a component of the characteristics of life?7. What elements compose the chemical structure of DNA?8. If one DNA strand reads CCGTAATGCAT, what will be the sequence of the complementary strand?
Vocabulary
• amino acid : Small molecule that is a building block of proteins; the monomer of a polypeptide• central dogma of molecular biology : A framework for understanding the transfer of sequence information
between sequential information-carrying biopolymers DNA, RNA and protein.• ribonucleic acid (RNA): Single-stranded nucleic acid; involved in protein synthesis .• ribosome : A non-membrane bound organelle inside all cells ; site of protein synthesis (translation).• deoxyribonucleic acid ( DNA ) : Double-stranded nucleic acid that composes genes and chromosomes ; the
hereditary material .• double helix: The double spiral shape of the DNA molecule; resembles a spiral staircase• base-pair: Bases in DNA that complement each other and are linked by hydrogen bonds. (A to T, C to G)
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1.6 DNA to RNA to Protein; Central Dogma ofMolecular Biology
My Learning Goals
• I can explain where proteins come from.• I can describe the role of RNA and the ribosome in protein synthesis.• I can use the Central Dogma of Molecular Biology to explain the role of DNA.• I can model the protein synthesis process.• I can connect the helical structure of DNA to its function.• Using evidence, I can explain how the structure of DNA relates to its function as the genetic material.
Is it always DNA to RNA to proteins?
The central dogma of molecular biology. Coined by Francis Crick. And in his own words, "I called this idea thecentral dogma, for two reasons, I suspect. I had already used the obvious word hypothesis in the sequence hypothesis,and in addition I wanted to suggest that this new assumption was more central and more powerful."
Central Dogma of Molecular Biology
Your DNA, or deoxyribonucleic acid, contains the genes that determine who you are. How can this organic moleculecontrol your characteristics? DNA contains instructions for all the proteins your body makes. Proteins, in turn,determine the structure and function of all your cells. What determines a protein’s structure? It begins with thesequence of amino acids that make up the protein. Instructions for making proteins with the correct sequence ofamino acids are encoded in DNA.
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DNA is found in chromosomes. In eukaryotic cells, chromosomes always remain in the nucleus, but proteins aremade at ribosomes in the cytoplasm. How do the instructions in DNA get to the site of protein synthesis outsidethe nucleus? Another type of nucleic acid is responsible. This nucleic acid is RNA, or ribonucleic acid. RNA is asmall molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in thenucleus to a ribosome in the cytoplasm and then helps assemble the protein.
How does a cell use the information in its DNA ?
Transcription is “DNA→ RNA .” To transcribe means "to paraphrase or summarize in writing." The informationin DNA is transcribed - or summarized - into a smaller version - RNA - that can be used by the cell. This processis called transcription. Only certain specific "segments" - genes - in the DNA are transcribed at any one time. RNAis similar to DNA but it is only one sided and it contains the nucleotide uracil, rather than thymine.
In other words, transcription is the transfer of the genetic instructions from DNA to RNA. During transcription, acomplementary copy of RNA is made. Whereas in DNA replication both strands of the DNA double helix are usedas templates, in transcription only one strand is needed. An enzyme “reads” a template strand of DNA, known asthe coding strand, to synthesize the complementary RNA strand.
In short: DNA→ RNA→ Protein
Translation is the second part of the central dogma of molecular biology: RNA→ Protein. The process of readingthe mRNA code in the ribosome to make a protein is called translation. The mRNA is translated from the languageof nucleic acids (nucleotides) to the language of proteins (amino acids).
Figure below shows how this happens. After mRNA leaves the nucleus, it moves to a ribosome. The ribosome readsthe sequence of the mRNA and other molecules bring amino acids to the ribosome in the correct sequence.
FIGURE 1.20Translation of the mRNA to a chain of amino acids occurs at a ribosome. Notice the growing amino acid chainattached to the ribosome. What are the roles of proteins in an organism?
Bonds form between adjacent amino acids as they are brought one by one to the ribosome, forming a polypep-tide chain. The chain of amino acids keeps growing until a stop signal is reached. The polypeptide chain will beshaped into a protein. To see how this happens, go the link below.
An overview of protein synthesis can be viewed below or at https://www.youtube.com/watch?v=41_Ne5mS2ls
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After a polypeptide (a chain of amino acids is called a polypeptide), chain is synthesized, it may undergo additionalprocesses. For example, it may assume a folded shape due to interactions among its amino acids. It may also bindwith other polypeptides or with different types of molecules, such as lipids or carbohydrates. Many proteins travelto the Golgi apparatus to be modified for the specific job they will do.
Summary
• The central dogma of molecular biology states that DNA contains instructions for making a protein, which arecopied by RNA.
• RNA then uses the instructions to make a protein.• In short: DNA→ RNA→ Protein, or DNA to RNA to Protein.• Transcription occurs in the nucleus when DNA is copied to RNA.• Translation is the RNA→ Protein part of the central dogma.• Translation occurs at a ribosome.
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Explore More
• What Makes a Firefly Glow? at http://learn.genetics.utah.edu/content/begin/dna/firefly/ .
DNA to Protein at http://www.concord.org/activities/dna-protein .
Review
•1. What happens during transcription?2. What happens during translation?3. Describe the steps of the central dogma of molecular biology.4. Compare and contrast transcription and translation.5. What is the role of protein in a cell?6. What aspects of DNA allow it to code for many different proteins?7. Use evidence to explain how the structure of DNA relates to its function as the genetic material.
Vocabulary
• mRNA : molecule that carriers the instructions from the DNA to the rest of the cell; messenger RNA• Polypeptide : Chains of amino acids. Proteins are made up of one or more polypeptide molecules.
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• Protein Synthesis :Process in which a protein is made, consisting of transcription of DNA to RNA in thenucleus and translation of RNA to a protein at a ribosome in the cytoplasm.
• RNA : (ribonucleic acid): Single-stranded nucleic acid that transcribes and translates the genetic code in DNAto make proteins, among other functions.
• Transcription : First of two steps of protein synthesis in which RNA makes a copy of the genetic code inDNA in the nucleus of a cell.
• Translation : Second of two steps of protein synthesis in which the genetic code in RNA is read and aminoacids are joined together to form a protein at a ribosome in the cytoplasm.
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1.7 Cell Transport
My Learning Goals
• I can explain how the structure of the cell membrane helps it to regulate homeostasis in living things.
How is a cell membrane like a castle wall?
The walls of a castle, like the cell membrane, are designed to keep out dangerous things. Whether you’re concernedabout an enemy army or a disease-causing bacteria, you don’t want to allow everything to enter! However, in orderto survive, there are some things that the cell (or the castle) does need to let in.
Introduction to Cell Transport
Cells are found in all different types of environments, and these environments are constantly changing. For example,one-celled organisms, like bacteria, can be found on your skin, in the ground, or in all different types of water.Therefore, cells need a way to protect themselves. This job is done by the cell membrane, which is also known asthe plasma membrane.
Controlling the Cell Contents
The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can passthrough the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as
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the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaininga stable internal environment. For example, if your body cells have a temperature of 98.6◦F, and it is freezing outside,your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop with the outsidetemperature.
How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell?The composition of the cell membrane helps to control what can pass through it.
Composition of the Cell Membrane
Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer ofphospholipids (a "bilayer") and proteins ( Figure below). Recall that phospholipids, being lipids, do not mix withwater. It is this quality that allows them to form the outside barrier of the cell.
A single phospholipid molecule has two parts:
1. A head that is hydrophilic, or water-loving.2. A tail that is hydrophobic, or water-fearing.
FIGURE 1.21The cell membrane is made up of a phos-pholipid bilayer, two layers of phospholipidmolecules.
There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving, and theywant to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing,hydrophobic tails face each other in the middle of the cell membrane, because water is not found in this space. Thephospholipid bilayer allows the cell to stay intact in a water-based environment.
An interesting quality of the plasma membrane is that it is very "fluid" and constantly moving, like a soap bubble.Due to the composition of the cell membrane, small molecules such as oxygen and carbon dioxide can pass freelythrough the membrane, but other molecules cannot easily pass through the plasma membrane. These molecules needassistance to get across the membrane. That assistance will come in the form of transport proteins.
The passage of a substance through a cell membrane is called transport. There are two basic ways that transport canoccur: passive transport and active transport. For a good video introduction to passive and active transport, click onthis link: http://www.youtube.com/watch?v=kfy92hdaAH0 .
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Passive Transport
Passive transport occurs when a substance passes through the cell membrane without needing any energy to passthrough. This happens when a substance moves from an area where it is more concentrated to an area where it is lessconcentrated. Concentration is the number of particles of a substance in a given volume. Let’s say you dissolvea teaspoon of salt in a cup of water. Then you dissolve two teaspoons of salt in another cup of water. The secondsolution will have a higher concentration of salt.
Why does passive transport require no energy? A substance naturally moves from an area of higher to lowerconcentration. This is known as moving down the concentration gradient. The process is called diffusion. It’sa little like a ball rolling down a hill. The ball naturally rolls from a higher to lower position without any addedenergy. You can see diffusion if you place a few drops of food coloring in a pan of water. Even without shakingor stirring, the food coloring gradually spreads throughout the water in the pan. Some substances can also diffusethrough a cell membrane. This can occur in two ways: simple diffusion or facilitated diffusion.
Simple Diffusion
Simple diffusion occurs when a substance diffuses through a cell membrane without any help from other molecules.The substance simply passes through tiny spaces in the membrane. It moves from the side of the membrane whereit is more concentrated to the side where it is less concentrated. You can see how this happens in Figure below .
FIGURE 1.22Simple diffusion of molecules (blue) fromoutside to inside a cell membrane.
Facilitated Diffusion
Hydrophilic molecules and very large molecules can’t pass through the cell membrane by simple diffusion. Theyneed help to pass through the membrane. The help is provided by proteins called transport proteins. This process
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is known as facilitated diffusion.
There are two types of transport proteins: channel proteins and carrier proteins. They work in different ways. Youcan see how they work in Figure below.
• A channel protein forms a tiny hole called a pore in the cell membrane. This allows water or hydrophilicmolecules to bypass the hydrophobic interior of the membrane.
• A carrier protein binds with a diffusing molecule. This causes the carrier protein to change shape. As it does,it carries the molecule across the membrane. This allows large molecules to pass through the cell membrane.
FIGURE 1.23Transport proteins
Summary
• The cell membrane is selectively permeable, meaning only some molecules can get through.• The cell membrane is made of a double layer of phospholipids, each with a hydrophilic (water-loving) head
and a hydrophobic (water-fearing) tail.
Explore More
• Membrane tutorial at http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf
1. Can proteins in the plasma membrane move around the membrane? Why is this characteristic beneficial to thecell?
2. What are five functions of the membrane in cells?3. What types of lipids are found in plasma membranes? What characteristics do these types of lipids share?
Review
1. Why is the plasma membrane considered selectively permeable? Why is this important?2. Explain the composition of the cell membrane.3. Explain the arrangement of phospholipids in the membrane.4. How is passive transport different from active transport?5. What are three types of passive transport? What do these all have in common?6. What does the body use iodine for? What kind of transport is necessary to transport this molecule into a cell?7. What happens to the receptor complex in receptor mediated endocytosis?
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8. Describe the structure of the cell membrane.9. Compare and contrast active and passive transport.
10. How does the cell membrane help to regulate homeostasis?
Vocabulary
• Cell membrane: Also called the plasma membrane. A semipermeable membrane that controls what entersand leaves a cell. it is composed of two layer of phospholipids.
• Concentration: The amount of a substance in relation to the total volume.• Concentration gradient: During diffusion, molecules are said to flow down their concentration gradient,
flowing from an area of high concentration to an area of low concentration. This is a natural process and doesnot require energy
• Facilitated diffusion: When energy is required to force substances across cell membrane because it is goingagainst the concentration gradient.
• Passive transport occurs when a substance passes through the cell membrane without needing any energy topass through. This happens when a substance moves from an area where it is more concentrated to an areawhere it is less concentrated.
• Phospholipid: the type of lipids that makes up cell membranes. It has two regions. Semipermeable: Or selec-tively permeable, which means that only some molecules can pass through the membrane.
– A head that is hydrophilic, or water-loving.– A tail that is hydrophobic, or water-fearing.
• Transport proteins: proteins within the cell membrane that assist with facilitated diffusion. They requireenergy to work.
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1.8 Diffusion and Osmosis
My Learning Goals
• I can explain how osmosis and diffusion are used to maintain homeostasis in living things.• I can explain how plants and other organisms use cells to help regulate their internal water balance.• I can model the process of diffusion.• I can explain how diffusion and osmosis are both different and similar.• I can explain how the cell membrane regulates movement into and out of cells.
What happens if you put a few drops of food coloring in water? Over time, the molecules of color spread outthrough the rest of the water. When the molecules are evenly spread throughout the space, the water will become aneven color. This process of molecules moving from an area where there are lots of molecules to an area where thereare fewer molecules is known as diffusion.
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Diffusion
Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is themovement of molecules from an area where there is a higher concentration (larger amount) of the substance toan area where there is a lower concentration (lower amount) of the substance ( Figure below). The amount ofa substance in relation to the total volume is the concentration. During diffusion, molecules are said to flowdown their concentration gradient, flowing from an area of high concentration to an area of low concentration.This is a natural process and does not require energy. Diffusion can occur across a semipermeable membrane,such as the cell membrane, as long as a concentration gradient exists. Molecules will continue to flow in this manneruntil equilibrium is reached. At equilibrium, there is no longer an area of high concentration or low concentration.
FIGURE 1.24
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Saltwater Fish vs. Freshwater Fish?
Fish cells, like all cells, have semi-permeable membranes. Eventually, the concentration of "stuff" on either side ofthem will even out. A fish that lives in salt water will have somewhat salty water inside itself. Put it in the freshwater,and the freshwater will, through osmosis, enter the fish, causing its cells to swell, and the fish will die. What willhappen to a freshwater fish in the ocean?
Osmosis
Imagine you have a cup that has 100ml water, and you add 15g of table sugar to the water. The sugar dissolves andthe mixture that is now in the cup is made up of a solute (the sugar) that is dissolved in the solvent (the water). Themixture of a solute in a solvent is called a solution.
Imagine now that you have a second cup with 100ml of water, and you add 45 grams of table sugar to the water. Justlike the first cup, the sugar is the solute, and the water is the solvent. But now you have two mixtures of differentsolute concentrations. In comparing two solutions of unequal solute concentration, the solution with the highersolute concentration is hypertonic, and the solution with the lower solute concentration is hypotonic. Solutions ofequal solute concentration are isotonic. The first sugar solution is hypotonic to the second solution. The secondsugar solution is hypertonic to the first.
You now add the two solutions to a beaker that has been divided by a selectively permeable membrane, with poresthat are too small for the sugar molecules to pass through, but are big enough for the water molecules to pass through.The hypertonic solution is on one side of the membrane and the hypotonic solution on the other. The hypertonicsolution has a lower water concentration than the hypotonic solution, so a concentration gradient of water now existsacross the membrane. Water molecules will move from the side of higher water concentration to the side of lowerconcentration until both solutions are isotonic. At this point, equilibrium is reached.
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Osmosis is the diffusion of water molecules across a selectively permeable membrane from an area of higherconcentration to an area of lower concentration. Water moves into and out of cells by osmosis. If a cell is in ahypertonic solution, the solution has a lower water concentration than the cell cytosol, and water moves out of thecell until both solutions are isotonic. Cells placed in a hypotonic solution will take in water across their membraneuntil both the external solution and the cytosol are isotonic.
A cell that does not have a rigid cell wall, such as a red blood cell, will swell and lyse (burst) when placed in ahypotonic solution. Cells with a cell wall will swell when placed in a hypotonic solution, but once the cell is turgid(firm), the tough cell wall prevents any more water from entering the cell. When placed in a hypertonic solution, acell without a cell wall will lose water to the environment, shrivel, and probably die. In a hypertonic solution, a cellwith a cell wall will lose water too. The plasma membrane pulls away from the cell wall as it shrivels, a processcalled plasmolysis. Animal cells tend to do best in an isotonic environment, plant cells tend to do best in a hypotonicenvironment. This is demonstrated in Figure 1.25. A video of osmosis in plants is at https://youtu.be/zVvHn6Sj9PQ
FIGURE 1.25Unless an animal cell (such as the redblood cell in the top panel) has an adap-tation that allows it to alter the osmoticuptake of water, it will lose too muchwater and shrivel up in a hypertonic en-vironment. If placed in a hypotonic solu-tion, water molecules will enter the cell,causing it to swell and burst. Plant cells(bottom panel) become plasmolyzed in ahypertonic solution, but tend to do best ina hypotonic environment. Water is storedin the central vacuole of the plant cell.
Osmotic Pressure
When water moves into a cell by osmosis, osmotic pressure may build up inside the cell. If a cell has a cell wall, thewall helps maintain the cell’s water balance. Osmotic pressure is the main cause of support in many plants. When aplant cell is in a hypotonic environment, the osmotic entry of water raises the turgor pressure exerted against the cellwall until the pressure prevents more water from coming into the cell. At this point the plant cell is turgid ( Figure1.26). The effects of osmotic pressures on plant cells are shown below.
The action of osmosis can be very harmful to organisms, especially ones without cell walls. For example, if asaltwater fish (whose cells are isotonic with seawater), is placed in fresh water, its cells will take on excess water,lyse, and the fish will die. Another example of a harmful osmotic effect is the use of table salt to kill slugs and snails.
Diffusion and osmosis are discussed at http://www.youtube.com/watch?v=aubZU0iWtgI (18:59).
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FIGURE 1.26The central vacuoles of the plant cells inthis image are full of water, so the cellsare turgid.
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Using osmosis to regulate Homeostasis
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Controlling Osmosis
Organisms that live in a hypotonic environment such as freshwater, need a way to prevent their cells from takingin too much water by osmosis. A contractile vacuole is a type of vacuole that removes excess water from a cell.Freshwater protists, such as the paramecium shown in Figure 1.27, have a contractile vacuole. The vacuole issurrounded by several canals, which absorb water by osmosis from the cytoplasm. After the canals fill with water,the water is pumped into the vacuole. When the vacuole is full, it pushes the water out of the cell through a pore.
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FIGURE 1.27The contractile vacuole is the star-likestructure within the paramecia.
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Summary
• Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.• The diffusion of water across a membrane because of a difference in concentration is called osmosis.• In comparing two solutions of unequal solute concentration, the solution with the higher solute concentration is
hypertonic, and the solution with the lower concentration is hypotonic. Solutions of equal solute concentrationare isotonic.
Explore More
• Osmosis at http://www.youtube.com/watch?v=7-QJ-UUX0iY (5:07)
Review
1. What is osmosis?2. If you add salt to the surface of a cell what does it do to the water in the cell?3. Describe the process of diffusion using the word concentration gradient.4. How does osmosis and diffusion help organisms maintain homeostasis?5. How can a hypotonic solution cause a cell to rupture? Describe this process as specifically as you can. You
may want to draw a diagram to help explain.6. Do water molecules leave or enter a cell in an isotonic solution? You may want to draw a diagram to help
explain.
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7. If a plant cell is placed in a solution and the cell shrivels up, what type of solution was it placed in? How doyou know?
Vocabulary
• Cell membrane: Also called the plasma membrane. A semipermeable membrane that controls what entersand leaves a cell.
• Concentration: The amount of a substance in relation to the total volume.• Concentration gradient: During diffusion, molecules are said to flow down their concentration gradient,
flowing from an area of high concentration to an area of low concentration. This is a natural process and doesnot require energy
• Diffusion: The movement of molecules from an area where there is a higher concentration (larger amount) ofthe substance to an area where there is a lower concentration (lower amount) of the substance
• Equilibrium: Water molecules will move from the side of higher water concentration to the side of lowerconcentration until both solutions have equal concentrations.
• Hypertonic: In comparing two solutions of unequal solute concentration, the solution with the higher soluteconcentration is hypertonic.
• Hypotonic: In comparing two solutions of unequal solute concentration, the solution with lower soluteconcentration is hypotonic.
• Osmosis: The diffusion of water molecules across a selectively permeable membrane from an area of higherconcentration to an area of lower concentration.
• Osmotic pressure: The main cause of support in many plants. Plant cells fill with as water and providestructural support.
• Solute: The substance being dissolved.• Solvent: The substance that is dissolving the solute. Often water.• Solution: The mixture of a solute in a solvent.
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www.ck12.org Chapter 1. Life
1.9 References
1. Peter Shanks (Flickr:BotheredByBees). A duck decoy looks like it’s alive, but it doesn’t have all the characteristics of a living thing . CC BY 2.0
2. . Phototropism response.3. . Phototropism response.4. . Fungi breaking down a rotting log.5. . Fungi breaking down a rotting log.6. Tadpole: Dan Century; Frog: Gábor Kovács. Tadpoles go through visible changes that show growth and dev
elopment, a characteristic of life . CC BY 2.07. . Nuclei of reptilian blood cells.8. . Nuclei of reptilian blood cells.9. . Like all living things, cats reproduce to make a new generation of cats.
10. . Like all living things, cats reproduce to make a new generation of cats.11. . A bird fluffs its feathers to stay warm and to maintain homeostasis.12. . A bird fluffs its feathers to stay warm and to maintain homeostasis.13. Human cells: Image copyright Sebastian Kaulitzki, 2014; Onion cells: Umberto Salvagnin. Humans and o
nions look very different, but when comparing the cells, you might notice some similarities . Human cells:Used under license from Shutterstock.com; Onion cells: CC BY 2.0
14. Image copyright Jubal Harshaw, 2014. The outline of onion cells are visible under a light microscope . Usedunder license from Shutterstock.com
15. Dartmouth Electron Microscope Facility. An electron microscope image of pollen . Public Domain16. Bruce Wetzel and Harry Schaefer/National Cancer Institute; Mike Seyfang; Umberto Salvagnin. Picture of r
ed blood cells, neurons, and epidermal cells . Public Domain; CC BY 2.0; CC BY 2.017. . A diagram of a typical prokaryotic cell and its structure.18. . A diagram of a typical prokaryotic cell and its structure.19. . A diagram of the parts of a typical eukaryotic cell.20. . A diagram of the parts of a typical eukaryotic cell.21. . Volvox algae colony.22. . Volvox algae colony.23. Boxer: U.S. Army (Flickr:familymwr); Illustrations: Laura Guerin. The four main types of tissue are nervou
s tissue, epithelial tissue, connective tissue, and muscle tissue . Boxer: CC BY 2.0; Illustrations: CC BY-NC3.0
24. Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist. Illustration of how the four tissue typeswork together in the heart . CC BY 2.5
25. Megan Totah. Negative feedback regulation is used to regulate the temperature of the body . CC BY-NC 3.026. . Diagram of DNA replication.27. LadyofHats. The composition of the cell membrane . CC BY-NC 3.028. . In simple diffusion, small hydrophobic molecules squeeze through lipid molecules.29. . Transport proteins can help facilitate diffusion of large molecules and hydrophilic molecules.30. . Diagram of diffusion across a cell membrane.31. . Diagram of diffusion across a cell membrane.32. Mariana Ruiz Villarreal (User:LadyofHats/Wikimedia Commons). illustrates how animal and plant cells c
hange in different solution types . Public Domain33. Flickr:fickleandfreckled. A photo of turgid plant cells . CC BY 2.034. Image copyright Lebendkulturen.de, 2014. A photo that shows the contractile vacuole within paramecia .
Used under license from Shutterstock.com
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