Chapter 3.

Chapter 3

How rapidly do cells divide?
All healthy cells have regular rates of dividing. For example, certain bacterial cells divide once every 20 minutes. Frog embryo cells divide in about an hour, cells lining your intestine take about 48 hours to divide, and liver cells divide once every 200 days.

Cells and cancer Cell division is essential for body growth and repair. What happens if cells begin to multiply and spread in an uncontrolled way? That is what happens in the bodies of people with cancer. Cancer cells divide at a far greater rate than normal cells and they spread to other parts of the body.

Cells and Cancer Cancer has been referred to as “mitosis gone wild”. As the abnormal cancer cells continue to multiply, they spread to other parts of the body and damage them. Cancer is one of the leading causes of death in Canada today.

Cells and Cancer Cancer can affect many parts of the body and may be caused by many different factors. Factors that can produce cancer are called carcinogens. They include some types of chemicals, radiation, inherited (genetic) factors, certain viruses, and repeated damage to the body.

Stop and Think: Name 3 things you already knew about cancer before you came to class today. List 2 questions you’ve had about cancer. What is one thing you’ve learned already in this class about cancer?

Cells and cancer People working in some jobs may be exposed to particular types of carcinogens. For example, people working in industries using asbestos have had high rates of lung cancer because they inhaled find particles of this substance over long periods of time. Some farmworkers have had high rates of cancer after improperly using certain pesticides.

Cells and Cancer Some cancers can be prevented by changing lifestyle habits to reduce exposure to carcinogens. One example is the link between smoking and lung cancer. Smokers are far more likely to die of lung cancer than non smokers and they can reduce this risk by not smoking.

Cells and cancer Another example is the connection between sun exposure and skin cancer. People who spend many hours in the sun without protective clothing or sunblock have a high risk of developing skin cancer, which is caused by ultraviolet radiation from the sun.

Cells and cancer Although prevention is better than cure, there are some treatments that can slow or stop the spread of cancer in patients who already have the disease. The techniques consist of destroying the cancerous cells while leaving normal cells intact.

Cells and cancer This can be done by chemicals (chemotherapy) or by radiation treatment- using high-energy particles to kill cells. These treatments are most successful if the cancer is diagnosed in an early stage, before the abnormal cells have spread widely through the body.

Cells and cancer New techniques may give better methods of curing cancer in the future. One method is gene therapy, which alters genes that case cells to divide and produce cancer. Alternative therapies focus on ways to boost the body’s own natural immune system.

Specialized cells Although multicellular organisms grow from single cells that repeatedly divide, not all cells are the same. Like instruments in a band, different cells have different appearances and perform different jobs. They are said to be specialized for particular tasks.

Stop and Think: Write down at least 5 types of specialized cells found in the human body. Pair up with a classmate, and see which cells you thought of. Be ready to share your answers!

Specialized cells For example, your muscle cells are shaped to move parts of your body, and your skin cells are built to protect your body from the sun’s rays. Humans have about a hundred different types of cells, each with their own special structure and function.

Specialized cells In the previous slide, you saw nerve cells, muscle cells, red blood cells, and onion skin cells. Each has a different shape relating to its function. For example, red blood cells are disk-shaped because they carry oxygen in the bloodstream. Their shape gives them a large surface area to pick up large amounts of oxygen and carry it to its destination.

The advantages of being multicellular
Imagine you are a microscopic, unicellular organism. Your whole body is one cell. This one cell must carry out all the functions needed to keep you alive. It must be able to move, obtain food, reproduce, and respond to the environment.

There are many living organisms that are only one cell. What disadvantages do you think they have, compared with multicellular organisms?

We already know one disadvantage: unicellular organisms can’t grow very big. Also, because they need to take in all the materials they need through their cell membranes, most unicellular organisms can only live in watery, food-rich surroundings.

Multicellular organisms have several advantages compared to unicellular living things. They can live in a wide variety of environments. They are able to grow very large, like a whale or a Douglas fir tree.

Multicellular animals can obtain their energy from a wide variety of foods. Their bodies are more complex. By specializing in particular functions, each cell in a multicellular organism can work much more efficiently than the cell of a unicellular organism, which must do all the jobs.

In multicellular organisms, specialized cells of a similar kind work closely together, and are usually found grouped closely together in the body. Groups of specialized cells, in turn, work in harmony with other groups.

Stop and Think Would you rather be a unicellular organism, or a multicellular organism? Do you think there are advantages to being unicellular?

Cells working together
Most cells of a multicellular organism such as a horse or a tree (or yourself!) are not in direct contact with the outside environment. How do these cells get the things they need?

Cells working together
To think about this problem imagine that all the cells of an animal or plant are organized into different systems. Each system has a particular function to perform. For example, one system carries oxygen throughout the body to every cell. Other systems make sure that each cell receives food, and so on.

How cells are organized
We already know that many animals and plants are made up of trillions of cells. How are all of these cells organized so that our bodies can function properly?

First, cells with the same structure and function are grouped into tissues (like muscles, nerves, and skin), in the same way that students are grouped into grades and classes at school.

Groups of tissues form organs (like the heart, stomach, and liver), just like different classes of students form a school.

The organs work together in organ systems (like the digestive system, for example), just like schools are grouped together in a school district. This arrangement of cells, tissues, organs and systems form levels of organization in living things. Each level can be studied on its own, or in relation to the levels above or below it.

Tissues Tissues are a group of similar cells. Onion skin is a tissue made of sheets of similar, thin, tightly-packed cells. These specialized skin cells form a layer that covers and protects the onion.

Muscle Tissue Epithelial tissue Nerve Tissue Connective Tissue

Organs Eating a juicy apple or a plate of french fries wouldn’t be possible without your organs. Organs are distinct structures in the body that perform particular functions. Eating involves many organs: your eyes (to see the food), your brain (to co-ordinate your actions), and your mouth and stomach (to digest the food).

Organs Each one of your organs is made of several tissues working together. For example, your stomach is made up of 4 types of tissue (muscle, epithelial, connective, and nerve). Other examples of organs in your body are the lungs, the heart, and the kidneys. Plants have organs, too. Plant organs include roots, stems, leaves and flowers.

Stop and Think: List the 4 levels of organization. Can you think of another analogy to explain the organizational system, instead of comparing it to a school system?

Organ Systems So far, we’ve learned that organs work together just as cells and tissues do. Organs form organ systems to perform activities that help the body function as a whole. For example, your stomach is part of a group of organs that form your digestive system. Other organs in their system include your tongue, pancreas, and small intestine.

Organ systems in plants
Why don’t plants have muscles? The function of muscles is movement, which is essential to animals because we need to find food. Plants don’t need to move, because they can produce food through photosynthesis.

Because of the differences in how plants and animals survive, plants have fewer types of tissues and organ systems than animals have. For example, plants don’t need “sense organs” (like eyes and ears) to find their food. They also don’t need a digestive system to break down large pieces of food, nor do they need a nervous system to co-ordinate movement.

You might be wondering what plants do need. Plants have two main organ systems: a root system below ground, and a shoot system (the stems and leaves) above the ground. The root system exists to obtain water and minerals from the soil and to anchor the plant in the ground. The shoot system makes food for the plant. Sometimes, flowering plants produce a third system for reproduction. The main organs of the reproductive system are the flowers.

Connecting the systems
The plant’s tissues are responsible for transporting the nutrients. Inside the plant, two types of tissues, called vascular tissues, connect the root system and the shoot system. Phloem tissue transports sugar manufactured in the leaf to the rest of the plant. Xylem tissue conducts water and minerals absorbed by the root cells to every cell in the plant. Don’t forget that all cells need food and water, plus oxygen to carry out their functions. Phloem and xylem tissues are found together along the length of the plant stems and roots.

From root to leaf One way to understand how the different levels or organization (cells, tissues, organs, and systems) work together is to follow the path of water through a plant. Most plants need a large supply of water, to make sugars in the process of photosynthesis. If you examine the structure of a root system, you will see that its growing tips are covered with fine root hairs. These “hairs” are in fact, extensions of single epidermal cells. When the concentration of water in the soil is greater than the concentration of water in the root cells, water enters the root hairs by osmosis. Getting water for the plant is a process of the first level of organization-the cells.

From root to leaf From the root hairs, water passes from cell to cell by osmosis until it reaches the xylem tissue. The tube-shaped cells making up xylem tissue have thick walls with hole in their ends. Stacked end to end, they form bundles of hollow vessels similar to drinking straws. Water can flow easily through these vessels. As more water enters the root hairs, it creates pressure that pushes water up the plant through the xylem tissue- the second level of organization.

From root to leaf Water is transported by xylem tissue into the stems and the leaves. Leaves are the plant’s food producing organs- the third level of organization. Don’t forget that photosynthesis makes sugars from water, carbon dioxide, and sunlight. Most photosynthesis takes place in a layer of cells in the leaf that are filled with chloroplasts. These cells are called palisade cells. You may have wondered why leaves are flat and thin. This shape provides a large surface area to absorb sunlight and it makes it easy for gases to diffuse into the leaf cells.

From root to leaf Did you notice the tiny openings on the underside of the leaf? These openings are called stomata, and they allow air to enter the leaf, supplying the oxygen the cells need for respiration and the carbon dioxide they need for photosynthesis. Spaces between leaf cells allow the air to flow around each cell. Surrounding each stoma are guard cells, which can expand to close off the stoma.

From root to leaf Unlike our blood, water doesn’t continually circulate through the plant. Instead, water leaves the plant through the open stomata. The loss of water from a plant is called transpiration. The loss of water is not a problem as long as it is replaced by more water that enters the plant through the roots.

Pushing and Pulling If all of the tissues of a plant were to magically disappear, leaving only the water in them behind, you would see an outline of the plant in a web-like network of water. There is no break in the water system. Fine columns of water connect every cell, from the leaves to the roots. The network extends even beyond the root hairs- it connects root hairs to channels of water in the soil.

Pushing and pulling According to particle theory, individual water particles are held together by bonds of attraction, which make the plant’s water network behave as a single unit. Water drawn in to the root hairs by osmosis pushes slender water columns up the plant. At the same time, water lost from the leaves by transpiration pulls water up the xylem tissues all the way from the roots. Both of these actions- pushing and pulling- are necessary to raise the water up to the top of very tall trees. In this way, trees can transport water without having a pumping organ similar to the human heart.

Organ adaptations in plants
Like animals, plants have adaptations that help them grow and survive in different environments. You can see some of these adaptations in the structure of roots, stems, and leaves. For example, many plants growing in deserts have small, fleshy leaves with a heavy wax coating that helps reduce water loss. Cactus spines are, in fact, narrow, waxy leaves. To compensate for their reduced leaf area, cacti carry out photosynthesis in their stems. The leaves of coniferous (cone-bearing) trees such as pines and other evergreens are also adapted to dry conditions. The needle like shape of the leaf reduces evaporation from the surface of the leaf.

Organ adaptations in plants
Plants that grow in water, such as water lilies, could have a problem obtaining the air they need to survive. To ensure their underwater roots obtain the oxygen they need for cell respiration, the root tissues of these plants have large air spaces in them. Still other plants have roots in the air. Orchids grow high above the ground on the branches of trees in tropical forests. Their root tissues are specially adapted to absorb moisture from the warm air.

Organ systems in humans
Every cell in the body needs a steady supply of food and oxygen to give it energy. Three different organ systems must work together to make this possible. Do you know what they are?

Organ systems in humans digestive system
Food first enters the body through the mouth, then passes in to the stomach and the intestine. It is broken down along the way into small, soluble particles that can be used by cells. Unused food is expelled from the body as waste.

Organ systems in humans- respiratory system
The person in this photo is doing yoga, which requires long, deep breaths. Breathing in (inhalation) fills her lungs with oxygen containing air. Breathing out (exhalation) rids her body of waste carbon dioxide. The organs involved in this gas exchange form the respiratory system.

Organ systems in humans- circulatory system
The digestive system puts food into the intestine and the respiratory system puts oxygen into the lungs. How do particles of food and oxygen eventually get from these systems to cells in the toes, the brain, and other parts of the body? A third system transports particles of food and oxygen. The circulatory system consists of the heart, blood, and blood vessels. This system circulates blood around the body, delivering food particles, dissolved gasses, and other materials to every cell and carrying away cell wastes.

A tale of two systems To connect all the individual cells throughout your body with the air around you, the respiratory system and the circulatory system work together. The function of the respiratory system is to exchange oxygen and carbon dioxide, while the circulatory system transports those gases throughout the body. How do the gases pass from one system to the other? Look for an answer where the two systems come into closest contact- among the tissues of the lungs.

A tale of two systems- respiratory system
After air enters the nose, it passes to the lungs through a series of smaller and smaller tubes. The trachea (windpipe) is about 20 mm in diameter. It divides into a right and left bronchus, which are each about 12mm in diameter. Each bronchus tube branches into thousands of small, narrow bronchioles, which diameters of 0.5mm. Finally, the bronchioles divide and end in millions of tiny air sacs called alveoli, which are only 0.2mm in diameter.

A tale of two systems- circulatory system
The circulatory system also involves a series of tubes- the blood vessels. Like the air tubes of the respiratory system, blood vessels branch and divide into smaller and smaller channels. The three main types of blood vessels are: Arteries, which have thick, muscular walls for carrying blood under pressure. Veins, which have thinner walls compared to arteries, and valves to prevent the blood from flowing backward. Capillaries are hair-thin vessels. Their walls are made of epithelial tissue only one cell layer thick.

A Tale of two systems Each alveolus in your lungs is surrounded by a web of capillaries. It is here that gases are exchanged. Oxygen and carbon dioxide pass back and forth between the air in the alveoli (respiratory system) and the blood in the capillaries (circulatory system). Oxygen passes from the alveoli into the capillaries by diffusion. The air in your alveoli has the same composition as the air in the atmosphere (20%). This is much higher than the concentration of oxygen found in your blood. The oxygen first dissolves in a thin film of moisture covering the walls of the alveoli, then it diffuses from the alveoli through the thin capillary walls into the bloodstream.

A tale of two systems Carbon dioxide diffuses in the opposite direction. Air normally contains only a very low concentration of carbon dioxide (about 00.3 percent). Blood in the capillaries carries all the dissolved carbon dioxide collected from cells throughout the body. Don’t forget that carbon dioxide is a waste product of cellular respiration. This gas therefore moves from the capillaries into the alveoli. When you exhale, you release the carbon dioxide and water vapour into the air.

Did you know? Many seals, with lungs no bigger than a human adult’s, can easily stay underwater without breathing for 20 minutes or more. Before they dive underwater, they exhale. How is this possible?

Did you know? The answer to this problem lies in how the seal’s blood and circulatory system function. All the oxygen a seal needs while underwater is stored in its blood and muscle tissue, rather than in its lungs. To be able to store this large amount of oxygen, a seal has about one and a half times to twice as much blood in its body as other mammals of similar size. When seals dive underwater, their heartbeats slow down immediately from about 100 beats per minute to as little as 10 beats a minute. Blood flow to some parts of their bodies, like the kidneys and the muscles, stops completely. Seals are also able to tolerate large amounts of carbon dioxide in their blood.

Getting food to body cells
We’ve already learned that your bloodstream takes oxygen from your lungs, and that it carries food particles from your digestive system. The transfer of food from the digestive system to the circulatory system takes place in the inner lining of the small intestine. Covering the surface of this lining are millions of tiny, fingerlike projections called villi (singular villus). Each villus contains a network of capillaries. Dissolved food particles pass from the intestine into the capillaries by a process called absorption. The food particles are now small enough to enter your body’s cells to supply them with the food they need.

Getting food to body cells
Do you see any similarities between the villi in the intestine and alveoli in the lungs? Like alveoli, villi have thin walls through which particles can pass into the circulatory system. Both alveoli and villi consist of tiny particles, and both occur in huge numbers. The arrangement greatly increases the surface area that is in contact with capillaries, without taking up a large amount of space in the body.

It’s all under control When you feel tired, you know you need to sleep. When you are cold, you know to turn up the heat or put on warmer clothing. When you are feeling hungry, you know it’s time for a snack. In these and other ways, you respond to changing external conditions and make appropriate adjustments. Your body systems also make constant adjustments to maintain a stable internal environment for your cells. This process is known as homeostasis. It occurs automatically, usually without you even being aware of it.

It’s all under control For example, no matter whether it is hot or cold outside, the inside of our bodies remains at an amazingly constant temperature of 37 degrees Celsius all year. A change in body temperature of as little as 0.5 degrees Celsius can make us feel either feverish or chilled. How is this steady body temperature maintained?

It’s all under control Nearly 90% of your body heat is lost through the skin. Most of the rest of your body heat is lost through your lungs. When you get cold, you may shiver. Your shaking muscles generate heat. You may also get goosebumps, which are produced by the contraction of small muscles in the skin that make your hairs stand on end. In animals with a thick coat of hair, and in our hairier prehistoric ancestors, fluffing up the body hair helps reduce heat loss by improving insulation.

It’s all under control Do you get flushed and red after exercise? This happens because tiny blood vessels in your skin expand. This increases blood flow near the body surface where heat can be lost to the outside. Sweating helps cool your body as the moisture evaporates from your skin surface. Your body’s responses to stimuli are co-ordinated by the nervous system (the brain, the spinal cord, and nerves), and the endocrine system (a set of glands that produce hormones). Diet, exercise, drugs, injury, and disease can affect body systems and disrupt homeostasis.

It’s all under control Information from temperature receptors in your skin goes to the heat-regulating center of your brain, called the hypothalamus. Responding to this information, the brain sends nerve signals to your muscles, skin, and blood vessels. Working together, your muscles, skin, and blood vessels adjust your blood flow and muscle activity, causing your body to increase its heat production or reduce its heat loss.

Body systems and your health
A healthy heart and blood vessels are essential to good health. Without this, your blood would not circulate, and your cells would not get the supplies of food and oxygen they need. Moving substances around the body is a necessary job in all multicellular organisms. In a unicellular organism, materials are exchanged between the cell and the environment. In humans, our blood transports substances. About 8 percent of an adult’s body weight is blood. The main components of blood are plasma, red blood cells, white blood cells, and platelets.

Body systems and your health
Plasma makes up 55% of your blood. It is the liquid portion of the blood, and it transports most of the carbon dioxide produced during cellular respiration. Red blood cells make up 44% of your blood. They are specialized to carry oxygen. Red blood cells contain an iron-rich chemical called hemoglobin, which attracts oxygen. This allows the blood to carry much more oxygen than it would normally. White blood cells make up less than 1% of your blood. They defend your body against disease and infection. Platelets make up less than 1% of your blood. They cause the blood to clot at the site of wounds to prevent blood loss.

Blood pressure Doctors measure a patient’s blood pressure because it indicates several things about the health of the circulatory system. For example, if someone has lost a lot of blood due to injury, the pressure of blood will be lower. If someone has a higher than usual heart rate, their blood pressure will be higher because the blood is being pushed quickly through the arteries. If someone has partially clogged arteries, their blood pressure will be higher than someone with open arteries.

You are what you eat Different foods contain different combinations of substances that your body needs. Substances that provide energy and materials for cell development are referred to as nutrients. Carbohydrates are the cells’ main source of quick energy. Fats are another source of energy, but unlike carbohydrates, they can be stored by the body. When you eat more food than the body needs for an activity, the extra is stored in your tissue as fat. Proteins are essential for growth and repair of the body’s tissues.

You and your body Like any complex structure, your body needs proper care to function properly. To maintain healthy organs and systems, everyone has the same essential needs: clean air and water, nutritious foods, exercise, and sleep. Clean air means oxygen for your cells, which use the oxygen to produce energy. A balanced diet provides your cells with the food materials they need for growth and activities. Lack of essential materials makes the body grow weaker, while too much of some substances such as fats, sugars, and salt can make more work for some systems.

You and your body Exercise is important because it helps the body process food and oxygen more efficiently. A healthy heart and lungs help carry materials to the cells and get rid of wastes. Strong muscles help protect the body from injury. Not only do healthy lifestyle habits make you feel better, they help your body resist diseases. Your immune system attacks and destroys invading germs and helps break down harmful materials in your body. If you do get a cold, a disease, or an injury, you are likely to recover faster if you are healthy to begin with.

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