Life-sustaining processes and survival of species

Transcription

1 CHAPTER 4 Life-sustaining processes and survival of species WHAT IS A CELL? In this chapter, you will discover the characteristics that define a living organism. Then, you will look at the structure and functions of plant and animal cells. Finally, you will learn about different reproductive mechanisms in plants and animals. The world of cells is fascinating. Even though they re very small, cells are the basis of all living organisms. Look at the nucleus of just one of your cells: it contains all of your genetic information. What s more, that information came from your parents and has been passed down from generation to generation. Some living organisms, like bacteria, are made up of only one cell. Other living organisms have many, many cells. The human body, for example, contains between billion and billion cells. Each cell has microscopic components that have specific functions: respiration, nutrition, reproduction, etc. Each of a living organism s cells also has a specific role. For example, some cells contribute to the reproduction of a species, like the cells in the frog eggs below. The cell is the base unit of all living organisms. It is the smallest unit of life. The living 112 world

2 4.1 Characteristics of living things What characterises a living thing? How can we tell that a rock is not alive, but a blade of grass is? What are the characteristics of living things? All forms of life, despite their great diversity, share certain characteristics. DEFINITION Characteristics of living things are the characteristics shared by all living organisms. If one of these seven characteristics is absent, it is not a living organism. 1 It has a complex, cell-based structure. It has exchanges with its environment (nutrition, respiration, etc.). 2 It reacts to stimuli (sound, light, heat, smells, tastes, hunger, thirst, etc.) It grows and develops. It requires energy. 4 6 It reproduces. It adapts to changes in its environment. The end of a theory 1861 FRANCE In the mid-19th century, people believed in a very old theory: spontaneous generation. According to this theory, some living things, like flies, came from non-living things, like garbage. However, Louis Pasteur used scientific experiments to show that living things always come from seeds or eggs. His work put an end to the theory of spontaneous generation. Louis Pasteur ( ) Chapter Life-sustaining processes and survival of species

3 Evolution, p. 99 Physical and behavioural adaptations, p. 103 Demonstrating the characteristics of living things The following table shows how the seven characteristics of living things apply to two species: the vervet (a monkey) and the dandelion (a plant). SPECIES Plant and animal cells, p. 117 Asexual or sexual reproduction, p. 126 CHARACTERISTIC OF A LIVING THING 1. It has a complex, cell-based structure. All parts of a living organism are made of cells or of substances made by cells. 2. It reacts to stimuli (sound, light, heat, smells, tastes, hunger, thirst, etc.). Stimuli are events that cause a reaction. 3. It has exchanges with its environment (nutrition, respiration, etc.). Living things make exchanges in order to ensure their growth and survival. 4. It grows and develops. Living organisms grow a lot at the start of their life. This growth results in an increase in the number of cells. 5. It requires energy. Living things need energy to move, grow, reproduce and do the tasks needed for survival. 6. It reproduces. Living organisms reproduce in order to prevent their species from disappearing and replace individuals that have died. 7. It adapts to changes in its environment. If its environment changes, a species has to adapt or die off. Adaptation can take thousands of years. Vervet Made up of animal cells. Reacts to a group member s warning call by fleeing. Consumes water, plants and small animals. Expels fecal matter and urine. Inhales oxygen and exhales carbon dioxide. Develops in its mother s womb for 160 days. Lives for around 30 years. Uses energy from its food. Reproduces sexually. Generally has only one baby per pregnancy. Adapted to stand upright, like other primates. This allows it to see farther to detect danger. Dandelion Made up of plant cells. Reacts to light by growing in that direction (phototropism). Absorbs carbon dioxide from the air. Draws the water and minerals it needs from the soil. Expels oxygen and water vapour into the air. Grows from a seed, which develops into a plant with yellow flowers, then produces seeds with white fluff. Uses energy from the sun. Reproduces sexually or asexually. The flower produces seeds (future plants). Adapted to reproduce in large numbers, due to seeds that are easily spread by the wind. All living things share seven characteristics. These characteristics are expressed differently from one species to another. The living 114 world

4 ACTIVITIES 1. Underline the things that describe a living organism. A board made of maple A little girl A mushroom A robot A rock A star A tomato plant An ant An apple tree 2. Complete these sentences. a) Living things reproduce to replace individuals that have died. b) Living things need energy to move, grow and perform the tasks necessary for survival. c) Living things react to stimuli. d) Living things are made up of cells. e) Living things must adapt to changes in their environment. f) Living things grow and develop. g) Living things have exchanges with their environment. h) If one characteristic of living things is absent, it is not alive. 3. Indicate which of the characteristics of living things applies best to each statement. a) Frogs croak in the springtime because that s when they reproduce. 6 b) Beans grow a few millimetres a day. 4 c) The pupil of the human eye gets bigger in the dark. 2 and 7 d) The chlorophyll in plants absorbs light used to perform photosynthesis. 3 and 5 e) Giraffes necks allow them to eat leaves from the branches of tall trees. 3, 5 and 7 Chapter Life-sustaining processes and survival of species

5 4. Show that maples are living organisms by using the seven characteristics of living things. Sample answer: 1 Maples are made up of many cells. 2 They react to light by growing in that direction (phototropism). 3 4 They pull carbon dioxide from the air. They seek water and minerals from the soil. They expel oxygen and water vapour into the air. They grow and mature. 5 They use energy from the sun. 6 They release seeds ( helicopters ) to reproduce. 7 They can produce more seeds if their survival is threatened. 5. Which characteristic of living things is shown in each image? a) Characteristic 4: the butterfly grows and develops. b) Characteristic 5: the koala uses energy from its food. c) Characteristic 2: these animals are reacting to a stimulus (meeting each other). The living 116 world

6 4.2 Plant and animal cells The cell is the base unit of all living organisms. How are cells made? Are they all identical? How do they work? What are a cell s vital functions? For a living thing to survive, its cells need to do certain functions. DEFINITION A cell s vital functions are the things it needs to do to survive: feeding, digesting, producing energy through respiration, creating useful substances, eliminating waste, multiplying and dying. Structure and functions of an animal cell Let s look at the main components of an animal cell and the role that each of them plays. COMPONENT ROLE Cell membrane Cytoplasm Mitochondrion Nuclear membrane Nucleus Cell membrane Cytoplasm Surrounds and protects the cell. Controls exchanges between the inside and the outside of the cell. A jellylike substance that contains parts called organelles. Organelles are sort of like the organs in a body: they are responsible for digestion, respiration, creating useful substances, etc. Many substances travel through the cytoplasm: water, oxygen, nutrients, waste, etc. Mitochondrion Uses cellular respiration to produce the energy that the cell needs to function. Nuclear membrane Thin membrane around the nucleus. Controls exchanges between the nucleus and the cytoplasm. Chromosome Nucleus Controls the cell s activities. You can think of it as the brain of the cell. Chromosome Contains millions of genes. Contains hereditary information. Each cellular component plays an important role for the cell and the organism as a whole. Chapter Life-sustaining processes and survival of species

7 Structure and functions of a plant cell There are differences between plant cells and animal cells, just like there are differences between plants and animals. Here is a diagram of a plant cell that highlights the differences between the two types of cells. The words in green show components that are unique to plant cells. Plant cells have the same components as animal cells. They have a nucleus (containing chromosomes), a nuclear membrane, a cell membrane and cytoplasm (containing mitochondria). Chloroplasts contain chlorophyll, which gives plants their green colour. Chlorophyll captures sunlight used by plants to create their own food (photosynthesis). In plants, the cell membrane is surrounded by a thick cell wall. This wall contains a lot of cellulose, which gives plants their stifness. Nuclear membrane Cell membrane Nucleus Cell wall Chloroplast Vacuole Cytoplasm Vacuoles are a type of container that stores food and waste. In plant cells, the central vacuole can make up 80% to 90% of the cell s volume. In animal cells, vacuoles are smaller and are often called vesicles. Generally, plant cells look like animal cells. However, in plant cells, you can see chloroplasts, cell walls and a large vacuole. Why aren t all cells identical? In a living organism, cells can have a wide variety of forms and functions. For example, nervous cells, or neurons, are responsible for transmitting information throughout the body. That s why they have extensions that carry messages, the same way electric wires do. In total, the human body has around 200 different types of cells, including muscle, skin and stomach cells. However, these cells still share some basic elements, like cytoplasm, nuclei, etc. The living 118 world

8 ACTIVITIES 1. What am I? a) I m the organelle that produces energy for the cell. Mitochondrion. b) I give plants their stiffness. Cell wall. OR Cellulose. c) We are responsible for passing down hereditary characteristics. d) I am the base unit of all living organisms. Chromosomes. Cell. 2. a) Here is a plant cell. Complete the diagram by connecting each component to its image. Vacuole Chloroplast Cytoplasm Nucleus Cell wall Cell membrane Nuclear membrane Mitochondrion b) Here is an animal cell. Complete the diagram by connecting each component to its image. Cell membrane Cytoplasm Nucleus Mitochondrion Nuclear membrane c) Underline the parts of the plant cell shown in a) that are not found in the animal cell shown in b). Chapter Life-sustaining processes and survival of species

9 3. Cells have four main functions: 1. Respiration. 2. Nutrition. 3. Waste elimination. 4. Reproduction. Indicate the function described in each example. a) The cell membrane absorbs glucose (sugar). 2 b) A cell divides into two identical cells. 4 c) The mitochondria transform oxygen and nutrients into carbon dioxide, water and energy Link each function (on the left) to the cell component that performs it (on the right). a) It surrounds a plant s cell membrane. Nuclear membrane b) It controls exchanges between the nucleus and the organelles of the cytoplasm. c) These organelles produce the energy that the cell needs to function. Cell wall Vacuole d) It s a jellylike substance that contains many different components called organelles. e) It s the green substance that allows plants to absorb sunlight. Mitochondria Cytoplasm f) In animals, it stores fats. In plants, it stores nutrients and waste. Chlorophyll g) It controls the cell s activities and contains hereditary information. h) It controls exchanges between the inside and the outside of the cell. Cellular membrane Nucleus The living 120 world

10 4.3 Plant and animal cells, p Cellular components visible under a microscope Have you ever looked at cells under a microscope? Their appearance can be surprising to people who have only ever seen drawings. What are the main cellular components visible under a microscope? The light microscopes that we use in science and technology class have lenses allowing for magnification that usually varies from 400X to 2000X. What can you see with this type of microscope? DEFINITION The four main cellular components visible under a light microscope are the cellular membrane, the cytoplasm, the nucleus and the nuclear membrane. In plant cells, we can also see the cell wall, the vacuole and the chloroplasts. Images visible under a microscope Here are some examples of the images you might see under a light microscope. Plant cells Animal cells Cell wall Chloroplast Vacuole This photo shows plant cells that come from a Canadian waterweed leaf (aquatic plant). This photo shows animal cells that come from the inside of a human cheek (oral epithelium). The parts that are visible under a microscope are indicated. Nucleus Nuclear membrane Cytoplasm Cell membrane Some cell parts are not visible under a light microscope. You need another type of microscope (like an electron microscope) to see them. Chapter Life-sustaining processes and survival of species

11 How to use a microscope? A microscope is an instrument that lets us see objects that are too small to see with the naked eye. To look at an object under a microscope, it needs to be placed on a slide and covered with a coverslip (small, thin piece of glass). The slide is placed on a mechanical stage or held in place with stage clips. Slide Stage clip MICROSCOPE PARTS AND THEIR FUNCTIONS PART Eyepiece Arm Objective lenses Revolving nosepiece Stage FUNCTION Lens through which you look at the object. It generally has a 10X magnification. MICROSCOPE PARTS AND Piece that links the top and bottom of the microscope and allows the microscope THEIR FUNCTIONS to be carried. Lenses that allow you to magnify the image of the object you re looking at. Each lens has a number that indicates its magnification. The most common magnifications are 4X, 10X, 45X and 100X. Some microscopes have only one objective lens that allows you to zoom in and out. Cylindrical part that holds the objective lenses. Turn the nosepiece to change lenses. Platform that holds the slides. Mechanical stage Light source Diaphragm Coarse adjustment knob Fine adjustment knob Mechanical system that allows you to move the slides horizontally along the stage. Some microscopes have stage clips pieces that hold the slide against the stage. System that illuminates the object being observed. Slide that allows you to adjust the lighting of the object being observed. Some microscopes have a disc underneath the stage that serves the same purpose as a diaphragm. Knob that moves the stage up and down to move the slide closer or farther away. Its quick movement allows you to roughly focus on the image of the object being observed. Knob that moves the stage up and down. Its slow movement allows you to more precisely focus on the image of the object being observed. The living 122 world

12 How to use a microscope? (continued) To observe an object under a microscope, we need to start by using the lowest level of magnification, increasing it only if necessary. ONE METHOD FOR OBSERVING SAMPLES UNDER A MICROSCOPE 1. Plug in the microscope and turn on the light source. 2. Place the slide on the stage. Hold it in place using the mechanical stage or the stage clips. 3. Centre the object being observed over the opening in the stage. 4. Select the weakest magnification by turning the revolving nosepiece or the zoom wheel. 5. Set the diaphragm or disc opening to one-half. 6. Note which direction you need to turn the coarse adjustment knob to move the stage closer or farther away. Head Revolving nosepiece Objective lenses Stage Diaphragm Light source Eyepiece Arm Mechanical stage Coarse adjustment knob Fine adjustment knob Base 7. Move the object being observed as close as possible to the objective lens, making sure the two don t touch. To do so, look at the microscope so that the stage is at eye level and check the distance between the slide and the objective lens. 8. Look through the eyepiece and slowly move the stage away using the coarse adjustment knob. Stop when the image appears. 9. Adjust the image by slightly moving the stage towards or away from you using the fine adjustment knob. Stop when the image becomes clear. 10. If the image isn t clear, start over from step Increase or decrease the lighting using the diaphragm or disc, if needed. 12. If necessary, adjust the magnification by changing lenses. Repeat steps 9 through 11 to obtain a clear image. Cells under a microscope 1665 ENGLAND Robert Hooke ( ), an English scientist, discovered the cell using a rudimentary microscope that he invented. He is one of the first people to build a microscope containing an objective lens, a lens and an eyepiece. He used this device to observe insects, bird feathers, sponges and slices of cork. Drawings of his observations appeared in a book called Micrographia, published in A replica of Hooke s microscope Chapter Life-sustaining processes and survival of species

13 ACTIVITIES 1. Name the type of cell shown and the components that are indicated. a) Type of cell: Plant cell. Nucleus. Cell wall. Chloroplast. Vacuole. Cell membrane. b) Type of cell: Animal cell. Cell membrane. Nucleus. Cytoplasm. Nuclear membrane. 2. Describe the roles of the following components by completing the sentences. Use the words in the list provided. If needed, see page 117. Cell Cell Cytoplasm Exchanges Hereditary information Hereditary information Nucleus Nucleus Organelles Outside of the cell a) The cell membrane. It surrounds and protects the cell. It controls exchanges with the outside of the cell. b) The nucleus. It controls the cell s activities. It contains hereditary information (chromosomes). c) Cytoplasm. This jellylike substance gives the cell its shape. It contains organelles. d) Nuclear membrane. It surrounds and protects the hereditary information contained in the nucleus. It controls exchanges between the nucleus and the cytoplasm. The living 124 world

14 3. In each box, write the letter corresponding to the microscope part in the picture. Here are the steps for using a light microscope. A B C D E F G H I J K L 1. Plug in the microscope and turn on the light source F. 2. Place the slide on the stage D. Hold it in place using the mechanical stage or I the stage clips. 3. Centre the object being observed over the opening in the stage. 4. Select the weakest magnification by turning the revolving nosepiece B or the zoom wheel. 5. Set the diaphragm E or disc opening to one-half. 6. Note which direction you need to turn the coarse adjustment knob J to move the stage closer or farther away C. 7. Move the object being observed as close as possible to the objective lens making sure they don t touch. To do so, look at the microscope so that the stage is at eye level and check the distance between the slide and the lens. 8. Look through the eyepiece G and slowly move the stage away using the coarse adjustment knob. Stop when the image appears. 9. Adjust the image by slightly moving the stage towards or away from you using the fine adjustment knob K. Stop when the image becomes clear. 10. If the image isn t clear, start over from step Increase or decrease the lighting using the diaphragm or disc, if needed. 12. If necessary, adjust the magnification by changing lenses. Repeat steps 9 through 11 to obtain a clear image. Chapter Life-sustaining processes and survival of species

15 Evolution, p Asexual or sexual reproduction All living organisms reproduce. It s one of their characteristics. Do you think that plants and animals reproduce the same way? What is asexual reproduction? Clones exist in nature. They are the product of asexual reproduction. DEFINITION Asexual reproduction is a method of reproduction in which a single parent produces a copy of itself that then splits off. Advantages This is a quick and easy method of reproduction, since it doesn t require a partner. Only one individual acts as a parent. Disadvantages All descendants are identical to the parent. From a genetic standpoint, they are clones (identical copies). If all the individuals in a species are identical, they have less chance of survival if their environment changes, because no individuals have advantages. The species adaptation, evolution and survival may be compromised. An example of asexual reproduction You may have already seen someone cut part of a plant stem off and place it in water so it develops roots. This procedure, called cutting, is an example of asexual reproduction. To start, place a stem containing some of the plant s leaves in water. After some time, roots will develop. When there are enough roots, place the shoot in earth. You will then have a new plant that is genetically identical to the parent plant. Some vegetables can reproduce asexually. Other living organisms also reproduce in this way. The living 126 world

16 4.4.2 What is sexual reproduction? As you probably already know, reproduction often requires a male and a female individual. DEFINITION Sexual reproduction is a type of reproduction that requires the fusion of two specialized cells (gametes): one male cell and one female cell. Advantages Descendants are not identical to their parents. They have some traits from one parent and some from the other. This diversity makes it easier for a species to adapt to its environment. Disadvantages This method of reproduction is fairly complex. Organisms need to find a partner, produce specialized cells for reproduction and ensure that the cells meet. This requires more energy than asexual reproduction does. An example of sexual reproduction Of course, humans reproduce sexually. Human beings are created from the meeting between a spermatozoon (a male sex cell) and an ovum (a female sex cell). Babies are not entirely identical to their parents. They have some traits from their mother and some traits from their father. Almost all plants, animals and fungi reproduce sexually. Chapter Life-sustaining processes and survival of species

17 ACTIVITIES 1. Fill in the table below by answering yes or no to each question. CHARACTERISTIC SEXUAL REPRODUCTION ASEXUAL REPRODUCTION a) Requires a partner? Yes. No. b) Allows for diversity of individuals? Yes. No. c) Requires male and female cells? Yes. No. 2. Indicate whether the following situations are examples of sexual or asexual reproduction. a) Strawberry plants produce stolons (also known as runners ), which are stems that grow under the ground and form new plants that are identical to the parent plant. b) A horse and a mare must mate in order to produce offspring. c) The female pickerel frog spawns thousands of eggs, which are then fertilized by the male frog. d) Some fungi produce hyphae, which are thin filaments that resemble roots. The hyphae turn into new fungi that are identical to the first. e) For pears to grow, the wind must carry the pollen from one pear tree to the next. Asexual reproduction. Sexual reproduction. Sexual reproduction. Asexual reproduction. Sexual reproduction. 3. True or false? If a statement is false, correct it. a) Sexual reproduction results in offspring that are identical to the parents. False. Asexual reproduction results in clones. OR Sexual reproduction results in offspring that are different from the parents. b) Asexual reproduction allows for a lot of adaptation. False. Sexual reproduction favours adaptation. OR Asexual reproduction does not favour adaptation. The living 128 world

18 c) Asexual reproduction is quick and fairly simple. True. d) Sexual reproduction does not require a partner. False. Sexual reproduction requires a male and a female partner. 4. Complete the following text, using the words provided. Adapting Asexual reproduction Female Identical Male Meet Partner Reproduction Specialized Species Sexual reproduction happens with the meeting of two cells: a male sex cell and a female sex cell. This method of reproduction has the advantage of producing offspring that are not identical to their parents. This diversity provides a species with a better chance of adapting to its environment. However, this method of reproduction is fairly complicated. Organisms need to find a partner, produce specialized cells for reproduction and ensure that the cells meet. This requires more energy than asexual reproduction does. 5. Duckweed, a type of aquatic plant, reproduces in two different ways. Indicate the method of reproduction described in each case. a) In the summer, duckweed reproduces very quickly. It can cover an entire pond, even if a number of species are feeding on it. In the summer, duckweed uses asexual reproduction. b) In the fall, when conditions become tougher, duckweed produces male and female flowers. In the fall, duckweed uses sexual reproduction. Chapter Life-sustaining processes and survival of species

19 4.5 Asexual or sexual reproduction, p Reproductive mechanisms in plants How do apple trees and cacti reproduce? Are there male and female trees? How can seeds come to grow so far away from their parent plant? What is asexual reproduction in plants? Flowering and seed plants can reproduce in many different ways. They can reproduce sexually or asexually, depending on their composition. DEFINITION In plants, asexual reproduction can happen naturally or artificially through cutting or layering. It can also happen through a special organ, like a bulb (e.g., tulips) or a tuber (e.g., potatoes). Cutting Natural cutting: one part of the plant (stem, leaves or roots) detaches and takes root. Artificial cutting: one part of the plant is cut off in order to make it grow roots. Layering Natural layering: a stalk takes root in the soil without detaching from the plant. Artificial layering: the plant stalk is buried in the ground so it takes root. Reproduction through bulbs A bulb is a sort of bud that ensures life in slow motion. It is made up of smaller bulbs called bulbils, which can each form a new plant. Reproduction through tubers A tuber is a round stem. Each of its buds can turn into a new stem. The Jardins de Métis 1926 CANADA Today, the Jardins de Métis in Gaspésie are home to some 3000 species and varieties of plants. Elsie Reford, who was born in Ontario in 1872, was behind the idea for the gardens. In 1918, her uncle, Lord Mount Stephen, left her his large property. She started to garden in it in For over 30 years, she designed and developed gardens herself. She died in Montréal at age 95. Elsie Reford ( ) The living 130 world

20 Some types of asexual reproduction There are many examples of asexual reproduction in plants. Here are some of them. The Barbary fig s stalks look like miniature rackets. (This cactus leaves are actually spines.) If one of these rackets falls down, it can take root and form a new cactus. Cutting Layering The spider plant s stalks grow away from the parent plant. If one of these stalks touches the soil, it takes root and eventually detaches from its parent plant altogether. Tulips can reproduce asexually, using bulbs. Planting a bulb Each of a potato s eyes can form a new stalk. Planting a tuber As these examples show, there are many ways for plants to reproduce asexually. Chapter Life-sustaining processes and survival of species

21 4.5.2 What is sexual reproduction in plants? It is well known that bees travelling from one flower to another aid in plant reproduction. Here, we will see how and why that happens. DEFINITION Sexual reproduction is the main way that plants reproduce. This varies depending on the type of plant: for example, ferns use spores, evergreens use pinecones, and flowering plants use flowers. With flowering plants, sexual reproduction happens in several stages. (You can look at the diagram on the next page to get a better idea of what s happening in the following descriptions.) 3. Fertilization 4. Fructification 5. Germination Germination is the development of the embryo contained in the seed: it grows roots and a stalk. 1. Flowering Flowering is the development of flowers, which contain male organs (stamens) and female organs (pistils). 2. Pollination Pollination is the transportation of pollen (which contains male cells) from the anther in the stamen to the stigma in the pistil (which contains female cells). Fertilization is the union of a male sex cell (contained in a grain of pollen) and a female sex cell (or ovule). Fructification involves the formation of a fruit, which contains seeds. SEED-BEARING PLANTS Most plants reproduce using seeds. If the seeds are contained in a relatively fleshy fruit, they come from a flowering plant. Most plant species are flowering plants. If the seed is not protected by a fruit, it comes from an evergreen. NON-SEED-BEARING PLANTS Algae are plants that live in the water and humid places. They float or attach themselves to the soil. Mosses are land plants. They have absorbent hairs that act like roots. They reproduce through spores. Ferns and horsetails are also land plants that reproduce through spores, but their process is more complicated than that of moss. The living 132 world

22 A flower s male organ is called a stamen. The stamen is composed of a filament, which is a long stem, and an anther, which sits at the end. An anther is a small structure covered in pollen. Pollen is the equivalent of sperm in animals. Parts of a flower Before we move too far away from the descriptions of flowering plant reproduction, let s look at how flowers are made. Stamen Anther Filament Pollen Peduncle Stigma Sepal Style Ovules Ovary Pistil Petal A flower s female organ is called a pistil. The pistil is composed of an ovary, which contains ovules, and a style, which is a long tube that connects the ovary to the stigma (the entrance to the pistil). The bright colours in petals often serve to attract insects. Sepals and the peduncle act as a support for the flower. The flower in this example has both male and female parts. However, in some species, there are male flowers and female flowers. Flowers contain a flowering plant s sexual organs. They are the reason plants can reproduce sexually. Why do some flowers close at night? Some flowers close at night to protect against dew, which could prevent the pollen from being transported by insects. Other flowers close to protect themselves from cold or bad weather. Still others close to protect themselves from nocturnal insects. On the other hand, flowers that count on bats or nocturnal butterflies to pollinate often close during the day. Chapter Life-sustaining processes and survival of species

23 Stages of sexual reproduction in flowering plants Here are some images that show each stage of the sexual reproduction process in flowering plants. 1 Flowering 2 Pollination Flowering is the stage in which plants produce flowers. It is triggered by environmental factors, like temperature or the length of the day. In order for fertilization to happen, first there needs to be pollination. Plants count on outside forces to transport pollen from the anther to the stigma. Wind, water and some animals (like insects, bats and birds) act as pollinators. 3 Fertilization 4 Fructification The grains of pollen that reach the stigma stick to it. They form a long tube that goes down into the ovary. A male pollen cell then goes down the tube and meets the ovule (the female cell). Fertilization happens. When a flower is fertilized, one or more seeds form in the flower s ovary. In apple trees, these seeds are called pips. A seed is made up of a plant embryo, a food reserve and a protective shell. While the seed is growing, the ovary turns into a fruit. In apple trees, the ovary becomes an apple. While the ovary is turning into a fruit, the peduncle becomes the fruit s stem. The end of the flower s stigma remains visible as a calyx: the small nub at the bottom of a fruit. The living 134 world

24 Stages of sexual reproduction in flowering plants (continued) 5 Germination Once the fruit has formed, the seeds inside will germinate if they end up in the right environment. Therefore, seeds need to be spread around in order to have better chances of developing. The wind spreads dandelion seeds. Water transports coconut seeds. Some seeds are transported by animals, which eat the fruit and expel the seeds in their excrement, far away from the adult plant. When the bean seed sprouts, two small parts appear: the radicle, which will turn into roots, and the plumule, which will become a stalk and leaves. Cotyledon Stalk and leaves Cotyledon The cotyledon acts as a food reserve for the young plant until its leaves are developed enough to use the energy from the sun. After that, the cotyledon disappears. Plumule Radicle Roots Some seeds, like corn seeds, have only one cotyledon. Others, like bean and peanut seeds, have two. This illustration shows only one of the bean s cotyledons. Germination of a bean seed As you have just learned, sexual reproduction is much more complex than asexual reproduction. Chapter Life-sustaining processes and survival of species

25 Life cycle of a flowering plant Here are the different life stages of a flowering plant, using the apple tree as an example. A seed germinates. Apples fall to the ground. The plant grows. The fertilized flowers form fruits. The plant, now an apple tree, flowers. This diagram shows that flowering plants have a life cycle that goes from seed to fruit. Do fruits ripen after they re picked? Some fruits do ripen after they re picked. Others do not. Bananas, apples and peaches all continue to ripen after they ve been picked. On the other hand, pineapples, strawberries, cherries and citrus fruits do not continue ripening. Fruits that do continue to ripen release ethylene, a gas that sets off the ripening process once the fruit has fallen from the tree. This is an adaptation that is used to attract animals so that they will spread seeds. The living 136 world

26 ACTIVITIES 1. Link each image (on the left) to the asexual reproduction method it represents (on the right). a) Cutting b) Layering c) Reproduction through bulbs Reproduction through tubers 2. Identify the type of reproduction being used in the following situations. a) To grow garlic, you plant bulbs. Reproduction through bulbs. b) When the leaves fall off of a crassula (a type of succulent), they grow roots and turn into a new plant near the parent plant. Cutting. c) Dandelions produce yellow flowers that turn into seeds with white fluff after a few days. Sexual reproduction. d) Samuel offers Sandra a plant. After a few months, she notices that the stalks have replanted themselves in the pot. Layering. e) The Jerusalem artichoke is a root vegetable that can be replanted in the ground. Reproduction through tubers. Chapter Life-sustaining processes and survival of species

27 3. Complete the diagram below. Stigma Stamen Anther Pollen Style Ovules Pistil Filament Ovary Petal Peduncle Sepal 4. a) What is the flower s female reproductive organ called? Pistil. b) What is its male reproductive organ called? Stamen. c) Do all flowers have both male and female organs? Explain your answer. No. In some species, there are male flowers and female flowers. 5. In order for flowering plants to reproduce, pollen needs to travel from one flower to another. Plants can t travel from one place to the next, so they have different adaptations to attract pollinators. a) Name two pollinators. Two of the following answers: Animals (insects, bats, birds), wind and water. b) Give two examples of pollination that uses a pollinator. Bees forage in flowers and transport pollen. Sample answer: Rain can carry grains of pollen from the top of the tree towards flowers lower down. The living 138 world

28 6. Complete the diagram below, then answer the questions. Germination Flowering Fructification Stages of sexual reproduction in flowering plants Pollination Fertilization a) What role does pollination play? Transporting pollen from the male organ to the female organ. b) What is fertilization? It is the union of a male sex cell and a female sex cell. c) What is the difference between fructification and germination? Fructification involves the formation of a fruit, which contains seeds. Germination is the development of the embryo contained in the seed, including the growth of roots and a stalk. 7. Name the flowering plant part described in each statement. a) Holds all of the flower s sexual organs. Flower. b) Becomes fruit after fertilization. Ovary. c) Contains grains of pollen. Stamen. OR Anther. d) Acts as a food reserve for the young plant. Cotyledon. e) The flower s female organ. Pistil. Chapter Life-sustaining processes and survival of species

29 4.6 Asexual or sexual reproduction, p Reproductive mechanisms in animals How do animals reproduce? Do they reproduce the same way as plants do? Let s take a look. What is sexual reproduction in animals? As in plants, animal sexual reproduction requires the union of a male sex cell and a female sex cell. DEFINITION Sexual reproduction is the main way that animals reproduce. This reproductive method is very complex: a female and a male must produce mature sex cells at the same time, they must meet in the same place and they must be compatible. Reproductive period This is the time of year reserved for mating. In mammals, this is called rutting season. In fish, it is called spawning season. Fertilization This is the union of a male sex cell (spermatozoon) and a female sex cell (ovum). Fertilization can happen outside of the female s body. This is called external fertilization. Fertilization can also happen inside the female s body. This is called internal fertilization. Development Oviparous: Describes animals (like birds) whose eggs develop completely outside the female s body. Substances are not exchanged between the mother and the embryo. Viviparous: Describes animals (like mammals) whose fertilized eggs develop inside the female s body. The embryo is connected to the mother, who provides it with everything needed for development. Ovoviviparous: Describes animals (like some reptiles) whose eggs develop inside the female s body until they are ready to hatch. Substances are not exchanged between the mother and the embryo. The living 140 world

30 Reproductive period and fertilization Let s look at reproduction in white-tailed deer and in frogs. ANIMAL White-tailed deer Frog REPRODUCTIVE PERIOD TYPE OF FERTILIZATION OTHER ANIMALS THAT FERTILIZE IN THE SAME WAY Rutting season happens in the fall. Internal fertilization. After mating, a spermatozoon fertilizes an ovum inside the female s body. Reptiles (crocodiles, snakes, etc.), birds (ducks, ostriches, etc.) and mammals (monkeys, humans, etc.). Reproductive season is in the spring. Frogs don t really mate: the male triggers egg-laying by climbing on the female s back. External fertilization. The female lays her eggs in a moist area, so that they don t dry out. At the same time, the male releases sperm and the spermatozoa fertilize the eggs. External fertilization requires a large number of eggs and spermatozoa, so that there is a better chance of them meeting in the water. Most fish (salmon, perch, etc.), mollusks (mussels, snails, etc.) and amphibians (frogs). These two examples show that internal fertilization and external fertilization are very different. Why are eggs oval-shaped? Eggs are rounded to make laying them easier. However, if they were perfectly round, they would be more likely to roll out of the nest. Their oval shape means that they roll in circles, eventually ending up back in the same place. Chapter Life-sustaining processes and survival of species

31 How do offspring grow? Once the egg has been fertilized, the offspring can develop in three different ways. Oviparous In oviparous animals, the egg develops completely outside the female s body. Everything the embryo needs is contained in the egg. Substances are not exchanged between the mother and the embryo. Generally, amphibians, fish, reptiles and birds are oviparous. In some cases, eggs are protected or incubated by the parents. In other cases, they are left alone. Viviparous In viviparous animals, fertilized eggs develop inside the female s body. The embryo is connected to the mother, who provides it with everything needed for development. Many substances are exchanged between the mother and the embryo. Mammals, including humans, are viviparous. (This photo shows a five-week-old human embryo.) Ovoviviparous Ovoviviparous animals develop in the same way as oviparous animals. The egg is completely independent from the female. However, the female still keeps the egg inside her body until it is ready to hatch. Some reptiles (like garter snakes, boas and vipers) and fish (like sharks) are ovoviviparous. The egg (fertilized ovum) can grow inside (viviparous and ovoviviparous) or outside (oviparous) the mother s body. It can be dependent (viviparous) on the mother or independent (oviparous and ovoviviparous). The living 142 world

32 Life cycle of an insect Among animals, insects have a unique life cycle. The following diagram shows the different life stages of a monarch butterfly. 1 First, an adult female lays an egg on a leaf. She can lay between 200 and 400 eggs. Next, the male fertilizes the eggs through external fertilization The egg hatches 3 to 12 days later, depending on temperature. A larva emerges from the egg. It is a caterpillar. The caterpillar grows quickly and molts (changes skin) 4 or 5 times. 4 The pupa (or chrysalis) becomes a butterfly. 4 This series of transformations is called metamorphosis. Most insects pass through stages that are similar to metamorphosis. 3 3 After molting for the last time, the caterpillar weaves a cocoon on a branch. The cocoon forms a hard, green shell. The caterpillar becomes a pupa (or chrysalis). This stage lasts 14 days. This diagram shows that animals like the monarch butterfly have a life cycle that takes them from egg to adult. Are there more than two sexes in the animal kingdom? Living organisms do not have a third sex. However, there are animals that can be male and female at the same time (like snails) and others that can change their sex over their lifetime (like oysters). Others, like hydrozoans (see photo), even reproduce asexually, like plants, through budding. Chapter Life-sustaining processes and survival of species

33 ACTIVITIES 1. Indicate whether the following animals use internal or external fertilization. a) Wood frogs External. b) Hairy woodpeckers Internal. c) Bats Internal. d) Salmon External. e) Rattlesnakes Internal. f) Donkeys Internal. g) Sea urchins External. h) Cats Internal. 2. Indicate the roles of the male and the female for each type of fertilization. TYPE OF FERTILIZATION MALE ROLE FEMALE ROLE Internal fertilization Produce sperm. Fertilize the ovum. Produce ova. External fertilization Produce sperm. Fertilize eggs. Produce eggs. Lay eggs. 3. What is rutting season? It s the time of year when mammals reproduce. 4. Why do animals that use external fertilization need to produce so many sex cells? These animals need to produce a large number of ova and spermatozoa in order to increase the chances of them meeting. 5. Name the male sex cell and the female sex cell in animals. The male sex cell is called a spermatozoon and the female sex cell is called an ovum. The living 144 world

34 6. What is the difference between an oviparous and an ovoviviparous animal? In both cases, the embryo develops in an egg that provides it with all of the nutrients it needs. In oviparous animals, the egg develops completely outside the female s body. In ovoviviparous animals, the egg stays inside the female s body until it is ready to hatch. 7. Once the female has been fertilized, the offspring can develop in one of three ways. a) Name these three types of development in the left-hand column. b) Write the name of each animal species in the row that corresponds to its development. Boa Cow Dog Eagle Frog Human Salmon Shark Viper DEVELOPMENT SPECIES Oviparous. Eagle. Frog. Salmon. Viviparous. Cow. Dog. Human. Ovoviviparous. Boa. Shark. Viper. 8. Complete the following text on the mole s development. In moles (a type of mammal), rutting season usually happens in the spring. Fertilization is internal, which means that the sperm fertilizes the egg inside of the female. During its development, the embryo remains connected to the female. There are constant exchanges between the mother and the embryo. The female carries between two and five young. Gestation lasts for four to six weeks. Chapter Life-sustaining processes and survival of species

35 Chapter 4 Review 1. What am I? I am the base unit of all living organisms and the smallest unit of life. Cell. 2. A computer is not alive. However, it has some of the characteristics of living things. List three characteristics that prove that computers are not alive. Computers are not made of cells. They do not grow or develop. They do not reproduce. 3. Describe three differences between plant cells and animal cells. Plant cells have the same components as animal cells, plus a cell wall, chloroplasts and a large vacuole that stores food and waste. 4. Fill in the crossword. Horizontal 2. Part of the cell that controls the cell s activities. 3. Cell components that produce energy. 5. Method of fertilization that happens outside of the female s body. 11. Animals that develop in the female s body. 12. Acts as a food reserve for the young plant. 15. Rounded stalk whose buds can form a new stalk. 18. Part of a seed that becomes roots after germination. 19. Stage of the reproductive cycle when plants produce flowers. 20. Animals that develop in an egg inside the female s body. Vertical 1. Union of a male sex cell and a female sex cell. 4. Method of fertilization that happens inside the female s body. 6. Method of reproduction that requires only one individual. 7. Reproductive period in mammals. 8. Method of reproduction that allows for more adaptation to changes in the species environment. 9. Part of a seed that becomes the stalk and leaves after germination. 10. Type of reproduction where a stalk takes root in the soil without detaching from the parent plant. 13. Animals that develop in an egg outside the female s body. The living 146 world

36 Vertical (continued) 14. Reproductive period in fish. 16. Procedure that involves cutting off part of the plant so that it grows roots. 17. A sort of bud that ensures life in slow motion. It divides to make new plants in the spring. 1 F 2 N U C L E U S R 4 3 M I T O C H O N D R I A I 6 N E X T E R N A L A T R S I S E U E 9 10 Z E R T X P L A X N T U L A T U A 11 V I V I P A R O U S Y I A 13 L N 14 L M E 12 C O T Y L E D O N G S 15 T U B E R N V P 16 L I 17 I 18 R A D I C L E N B P W U G U A N T 19 F L O W E R I N G I T B O N I U G N 20 O V O V I V I P A R O U S G Chapter Life-sustaining processes and survival of species