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Unit 2: Cell Structure and Function Lesson 2.3 Structures and Functions of Plant Cells Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It!...

Unit 2: Cell Structure and Function Lesson 2.3 Structures and Functions of Plant Cells Contents Introduction 1 Learning Objectives 2 Warm Up 2 Learn about It! 4 Plant Tissues 4 Meristematic Tissues 5 Apical Meristems 6 Lateral Meristems 6 Vascular Cambium 7 Cork Cambium 7 Simple Permanent Tissues 8 Epidermal Tissues 8 Ground Tissues 10 Complex Permanent Tissues 11 Xylem 12 Phloem 12 Key Points 15 Check Your Understanding 16 Challenge Yourself 17 Bibliography 18 Unit 2: Cell Structure and Function Lesson 2.3 Structures and Functions of Plant Cells Introduction Up to this day, you have probably encountered a variety of plants. You may have seen lots of them on the streets where you walk as you go to your school, or even in your garden where you stay if you want to be at peace with nature. With the variety of plants you have encountered, you have probably differentiated one from another by looking at their colors, shape, or height. We often describe plants as organisms with leaves, stem, roots, flowers, and fruits. We also associate them with beauty and serenity. Plants, as part of nature, provide us essential things like oxygen. They are also used as an alternative medicine to cure diseases like 2.3. Structures and Functions of Plant Cells 1 Unit 2: Cell Structure and Function common colds and cough. These are some of the reasons why we consider them as nature’s gift to us. It is quite unimaginable if plants do not exist on Earth, right? However, have you ever wondered what is inside them? You have learned about cells and known that plants are also composed of cells like animals; but, what do they look like? What kind of cells is each part of a plant composed of? How do plant cells differ in function? Learning Objectives DepEd Competency In this lesson, you should be able to do the Classify different cell types (plant/animal tissues) and specify following: the function(s) of each Differentiate meristematic from (STEM_BIO11/12-iA-C-4). permanent tissue. Determine the functions of each plant tissue. Locate the different types of tissues in plant parts. Warm Up Journey Through Plant Tissues 15 minutes Plant tissues differ in structure and function. These are found in all parts of a plant’s body. Some of these tissues are microscopic, but some can be determined by our naked eye. In this activity, you are going to observe different plant tissues. After observation, you will be asked to describe the different plant tissues you encountered. Materials 1 compound microscope 1 magnifying glass 2 chopping boards out of tree trunks with different sizes 2.3. Structures and Functions of Plant Cells 2 Unit 2: Cell Structure and Function 3 glass slides 1 leaf of oregano and 1 leaf of any kind 1 small bottle of clear nail polish 1 small roll of adhesive tape Procedure 1. Divide the class into three groups. 2. Each group will be assigned to a learning station where students need to work on a certain task. 3. Group one is first assigned to station 1, group two is assigned to station 2, and group three is assigned to station 3. 4. Each group will only be given 5 minutes to do the tasks in each learning station. 5. In station 1, compare and contrast the two chopping boards, and take note of your observation and description. 6. In station 2, observe an oregano leaf. Use a magnifying glass to observe and describe the oregano leaf. Note the distinct characteristics of the oregano leaf. 7. In station 3, make a stomatal imprint. To make this, you need to cover the lower part of the leaf with a clear nail polish. Place the adhesive tape on the lower part of the leaf that is covered with clear nail polish. After at least 2 minutes, carefully remove the adhesive tape from the leaf. Then, place the tape on the microscope glass slide. Observe this under the microscope and take note of the characteristics of the noted cells. 8. After doing the task in your assigned learning station, your group will move to another station to do another task. Group 1 will move to station 2, group 2 will move to station 3, while group 3 will move to station 1. 9. The rotation of groups in the learning station will continue until the group has completed the tasks in all learning stations. 10. Place all your observations in Table 2.3.1. 11. When your group is done in all learning stations, answer the questions that follow. 2.3. Structures and Functions of Plant Cells 3 Unit 2: Cell Structure and Function Observation Table Table 2.3.1. Observation and description of the specimens observed Learning Station Observed Specimen/s Observation 1 2 3 Guide Questions 1. What is the distinct characteristic of an oregano leaf? 2. To what part of the human body can the oregano leaf be compared? 3. What are the commonalities and differences between the chopping boards? 4. Stomata are opening under plants’ leaves. What do you think is their function? 5. How do these structures help plants grow, develop, and survive? Learn about It! Plant Tissues “All organisms are made of cells,” is one of the statements in cell theory which was formulated by Rudolph Virchow. Cells, in a particular part of a body, have specific functions. When a group of cells is performing the same function, they are considered as tissues. There are different types of cells found in an organism’s body. In plants, tissues can be classified as meristematic and permanent tissues. These tissues are generally responsible for growth, support, and transport. Meristematic tissues are found in the growing areas of plants like roots and stems and are made of actively dividing cells. These tissues are classified according to location and type of growth they are responsible for. The major meristematic tissues in plants are apical meristem and lateral meristem. On the other hand, permanent tissues are composed of nondividing cells which can be found in stems, roots, flowers, and leaves. Permanent tissues are classified as simple and complex tissues. Ground tissues (parenchyma, collenchyma, 2.3. Structures and Functions of Plant Cells 4 Unit 2: Cell Structure and Function and sclerenchyma) and epidermis are simple permanent tissues, while xylem and phloem are complex permanent tissues. How do types of meristematic tissues differ from each other? Meristematic Tissues Meristematic tissues are composed of cells that give rise to another set of cells. The cells produced from meristematic tissues can either stay as meristematic cells to continually produce more cells or transform into specialized cells which will become parts of some tissues and organs of a growing plant. Meristematic tissues are mainly responsible for the growth of a plant. They also give rise to essential parts of a growing plant. Apical and lateral meristems are the major meristematic tissues found in plants. These meristems differ in location (as shown in Fig. 2.3.1) and function. Fig. 2.3.1 The blue parts represent the location of the apical meristem, while the red ones represent the location of lateral meristems. 2.3. Structures and Functions of Plant Cells 5 Unit 2: Cell Structure and Function Apical Meristems The height of plants is attributed to the work of apical meristems. These meristems are located on the shoots and roots of the plant. As apical meristems divide to produce new cells, they are also elongating the root and shoot systems by producing the primary plants’ body which includes the dermal, vascular, and ground tissues. Elongation of the root and shoot systems is referred to as primary growth in plants. Apical meristems, located at the root tips (as shown in Fig. 2.3.2) and terminal buds in a plant’s shoots, are continuously dividing to produce primary meristems. Primary meristems are derivatives of apical meristems consisting of protoderm, procambium, and ground meristem which will, later on, give rise to the plants’ three tissue systems-- epidermis, stele (xylem and phloem), and ground tissues, respectively. Fig. 2.3.2 Apical meristems located at the root tips (right) are responsible for the primary growth of plants. The microscopic view of cells (left) shows the apical meristems of Allium cepa. Lateral Meristems A plant does not only increase in height but also increases in diameter. In most plants, growth in diameter and girth is an essential factor for survival because it increases the plants’ rigidity and sturdiness. A growth in diameter and girth is called secondary growth which results from the continuous division of lateral meristems. Vascular cambium 2.3. Structures and Functions of Plant Cells 6 Unit 2: Cell Structure and Function (shown in Fig. 2.3.3) and cork cambium are the lateral meristems found along the length of the plant which causes the increase in diameter and girth of plants. Vascular Cambium Vascular cambium gives rise to secondary xylem and phloem. Inward growth of vascular cambium produces xylem, while an outward growth produces phloem. As vascular cambium grows, layers of secondary xylem are added which becomes wood. When wood develops, plants become sturdy and rigid. Vascular cambium appears as a ring composed of two kinds of cells: ray initials and fusiform initials. Ray initials are composed of xylem and phloem rays that aid in radial transport of water and nutrients in woody stems. On the other hand, fusiform initials are responsible for producing secondary xylem and phloem. Fig. 2.3.3 Vascular cambium in woody dicot stem produces secondary xylem and phloem, while cork cambium produces cork cells. Cork Cambium Cork cambium (shown in Fig. 2.3.3), on the other hand, produces cork that replaces the epidermis of plants as they mature. Cork acts as a protection of plants from damage and disease-causing organisms. Moreover, cork releases suberin as a waxy protective coat to 2.3. Structures and Functions of Plant Cells 7 Unit 2: Cell Structure and Function prevent water loss from the stems. The layers of cork cambium and cork are collectively known as periderm. Many of us know that the outer protective layer of woody plants is bark. It is still correct to refer to the outer protective layer of plants like bark; however, unlike periderm, bark consists of phloem, cork cambium, and cork. Is it possible to grow plants by initiating the growth of their meristems instead of seeds? How will it be done? Simple Permanent Tissues When meristematic tissues differentiate into specialized cells, these cells become permanent tissues that do not have the capability to divide and give rise to a new set of cells. Permanent tissues, as its name suggests, are composed of fully matured cells that do not divide. Generally, these tissues provide support, aid transport of water and minerals, and act as storage of plant food. Permanent tissues in plants are either simple or complex. Simple permanent tissues are composed of one kind of cell, while complex permanent tissues are composed of two or more kinds of cells. Ground and epidermal tissues are considered as simple permanent tissues. On the other hand, xylem and phloem are considered as complex permanent tissues. Epidermal Tissues Epidermis, an outer protective layer in humans and in animals, is also found in plants. This outer protective layer originates from protoderm which is produced by apical meristems in roots and shoots. Epidermis covers the whole body of nonwoody and young woody plants and is protected by a waxy cuticle. The cuticle prevents loss of water and invasion of disease-causing microorganisms. The epidermis in roots has tiny projections called root hairs (shown in Fig. 2.3.4) which help in increasing the absorption capacity of roots. In leaves and in stems, the epidermis has tiny outgrowths called trichomes. These hair-like outgrowths prevent water loss and reflect excess light. When young woody plants mature, the epidermis is replaced by periderm which is produced by cork cambium. 2.3. Structures and Functions of Plant Cells 8 Unit 2: Cell Structure and Function Fig. 2.3.4 Microscopic view of root hairs in a lily plant The lower and upper parts of leaves are protected by the epidermis (as shown in Fig. 2.3.5). The lower epidermis of a leaf has guard cells. Guard cells are specialized structures that regulate the opening and closing of stomata. Stomata are slit-like structures on the lower epidermis of leaves which aids in the exchange of gases between plants and the environment. Fig. 2.3.5 Guard cells are specialized structures on the lower epidermis of leaves. 2.3. Structures and Functions of Plant Cells 9 Unit 2: Cell Structure and Function Why are ground tissues also called fundamental tissues? Ground Tissues Cells that are neither dermal nor vascular are called ground tissues. They are considered as fillers of plants and form the bulk of plants. Ground tissues are usually found between dermal and vascular tissues. The ground tissues found on the exterior of vascular tissues are referred to as cortex. Meanwhile, pith is found at the center of the stem. These simple permanent tissues are generally responsible for photosynthesis, support, and storage. Parenchyma, collenchyma, and sclerenchyma are the kinds of ground tissues found in plants that differ in function and in location. Parenchyma cells (shown in Fig. 2.3.6) are found in all parts of plants. Their structure is the least specialized among other ground tissues. Parenchyma cells have thin and flexible primary walls but lack secondary walls. These cells are mostly responsible for the synthesis and storage of plant food. Parenchyma cells in leaves contain chloroplasts and perform photosynthesis. In stems and in roots, parenchyma cells contain plastids that act as storage of starch, which is a complex sugar found in most plants. Although considered as permanent tissues, parenchyma cells can also divide and differentiate into specialized cells but under certain conditions, such as wound repairing. Fig. 2.3.6 Ground tissues parenchyma (left), collenchyma (center), and sclerenchyma (right) are generally responsible for storage, support, and photosynthesis. In these microscopic pictures, it is noticeable that they differ in structure in terms of primary and secondary walls. 2.3. Structures and Functions of Plant Cells 10 Unit 2: Cell Structure and Function Collenchyma is a type of ground tissue found in the young stems and petioles in plants. Unlike parenchyma, the primary walls of collenchyma are relatively thick although its thickness is uneven. Collenchyma lacks secondary walls and their primary walls do not contain lignin, which is a polymer providing rigidity. This thus provides a furnishing flexible support to immature parts of plants. Collenchyma cells Sclerenchyma is a ground tissue with thick secondary walls. Their secondary walls are strengthened by lignin, thus, making it more rigid compared to collenchyma. Sclerenchyma provides support and rigidity to plants. It has two kinds, namely sclereids and fibers. Sclereids strengthen seed coats and are responsible for gritty-textured flesh of some fruits. On the other hand, fibers are used commercially as components of making rope and flax fibers. Complex Permanent Tissues Plants need a constant supply of food, water, and minerals for their survival. The distribution of food, water, and minerals are acted upon by xylem and phloem. Xylem is considered as water-conducting tissue since its main responsibility is to distribute water and minerals absorbed by the roots. On the other hand, phloem is considered as food-conducting vascular tissue since it distributes sucrose and other inorganic compounds throughout the plant’s body. 2.3. Structures and Functions of Plant Cells 11 Unit 2: Cell Structure and Function Xylem Xylem (shown in Fig. 2.3.7) is composed of two types of conducting cells—tracheids and vessel elements. Tracheids are thin and elongated cells where water passes through. Tracheids, as well as vessel elements, have thin primary walls but thick secondary walls. Their secondary walls have pits that allow the transport of water from one cell to another. The thick secondary walls of tracheids serve as a rigid and strong wall against the tension of water transport. Moreover, vessel elements are thin-walled cells of xylem. Unlike tracheids, vessel elements have perforated plates that allow the transport of water through the vessels. Fig. 2.3.7 Microscopic view of xylem in vascular plants Phloem Sugar and other organic nutrients are distributed throughout the plant’s body by phloem. Phloem is composed of sieve tubes consisting of sieve-tube elements (as shown in Fig. 2.3.8). Sieve-tube elements are cells where nutrients, sucrose, and organic compounds are transported. These cells lack some of the essential cell parts like the nucleus and ribosomes, which makes the transport of substances effective. Furthermore, sieve-tube elements have sieve plates between their end walls. These sieve plates contain pores that regulate the flow of nutrients from one cell to another. Aside from sieve plates, sieve-tube elements also have companion cells, which is believed to be a helpful structure in transporting nutrients through the phloem. 2.3. Structures and Functions of Plant Cells 12 Unit 2: Cell Structure and Function Fig. 2.3.8 Phloem is composed of sieve-tube elements that help in the transport of nutrients throughout the plant’s body. In what direction does food and sucrose flow in the phloem? Why do you say so? Xylem and phloem are located along the stretch of the plant’s body. It extends from the roots to stems to leaves. Xylem and phloem form vascular bundles (shown in Fig. 2.3.9) which arrangement differs in roots and stems. In dicot and monocot roots, vascular bundles are arranged like a stellar. Meanwhile, in monocot and in dicot stems, vascular bundles are scattered and circular, respectively. 2.3. Structures and Functions of Plant Cells 13 Unit 2: Cell Structure and Function Fig. 2.3.9 Xylem and phloem form vascular bundles and are arranged in circular form in dicot stems. The picture shows a microscopic view of vascular bundles in a dicot stem. Did You Know? The gritty texture you feel while eating pears is actually sclerenchyma’s sclereids or stone cells. Sclereids contain lignin which makes it rigid, thus, making the texture of the pear’s flesh gritty. Microscopic view of sclereids in a pear 2.3. Structures and Functions of Plant Cells 14 Unit 2: Cell Structure and Function Key Points ___________________________________________________________________________________________ Plants have two major tissues, namely meristematic and permanent tissues. ○ Meristematic tissues give rise to permanent tissues which will differentiate into specialized cells with certain functions. ○ Permanent tissues are generally responsible for photosynthesis, support, and transport of water and nutrients. Apical and lateral meristems are responsible for primary and secondary growth, respectively. ○ Primary growth is an increase in plant’s height. ○ Secondary growth is an increase in plant’s diameter and girth. Ground tissues are also called fundamental tissues because their functions are essential for the growth and development of plants. ○ Parenchyma is responsible for storage and photosynthesis. ○ Collenchyma and sclerenchyma provide support to the plant’s body. Different types of plant cells ___________________________________________________________________________________________ 2.3. Structures and Functions of Plant Cells 15 Unit 2: Cell Structure and Function Check Your Understanding A. Identify the tissues described in each of the following items. 1. It is a tissue responsible for transporting water throughout the plant’s body. 2. It is a ground tissue that has the thickest secondary walls that provide support to the plant's body. 3. It is a major type of tissue that is mainly responsible for growth in diameter and girth. 4. It is responsible for protecting the plants from water loss. 5. It is where apical meristems are located. 6. It regulates the opening of stomata in the lower epidermis of leaves. 7. It is found adjacent to sieve-tube elements and is responsible for transporting sugar through the phloem. 8. It is a ground tissue that is mainly responsible for photosynthesis and storage. 9. These are tiny outgrowths of epidermal cells in the plant’s roots. 10. It is the waxy coat produced by cork cells in woody stems. B. Write TRUE if the statement is correct. If the statement is incorrect, rewrite the whole sentence with the correct word/phrase. 1. Parenchyma cells have thick and uneven primary walls but lack secondary walls. 2. Sieve-tube elements are cells of xylem that transport water and minerals throughout the plant’s body. 3. Cork cambium produces secondary xylem and phloem in mature woody plants. 4. Periderm replaces the epidermis when woody plants start to mature. 5. Ray initials are composed of xylem and phloem rays that produce secondary vascular tissues. 2.3. Structures and Functions of Plant Cells 16 Unit 2: Cell Structure and Function C. Determine the tissue involved in the following situations and explain your answer. 1. Annual rings determine the age of trees and as they age, they also increase in diameter. 2. When a stalk of celery is soaked in dyed water, the whole stalk and the leaves of celery are colored. 3. A name carved on a 10 feet tree trunk is still on the same spot after 10 years. 4. Budding is one of the techniques to grow plants without using seeds. Buds are transferred to the stem of another plant. 5. Hoya cornosa is an excellent indoor plant. When you drop water on its leaves, water droplets can easily fall down and will not be absorbed by the leaves. Challenge Yourself Read and analyze the situations below, then answer the questions. 1. Drought affects water transport in xylem vessels. During drought, roots cannot absorb water from the soil which results in an increase in tension in the xylem. An increase in tension in the xylem disrupts the flow of water. What could be its possible effects on plants? 2. Stomata are among the plants’ defenses against pathogenic bacteria. However, there are some bacteria that can reverse stomatal closure. Pseudomonas syringae DC3000, a bacterium, can re-open stomata by secreting a chemical effector called coronatine (COR). This mechanism enables this pathogen to enter the plants. If a plant is attacked by Pseudomonas syringae, what could possibly happen to the plant? 3. Root hairs are outgrowth of the epidermis responsible for increasing the absorption rate of water from the soil. On what part of the human body can root hairs be compared? 4. What are the possible effects of a disrupted flow of food in the phloem? 5. Mitosis is a process of dividing cells giving rise to new cells. The four stages of mitosis can be clearly observed in the cells of the growing parts of an organism’s 2.3. Structures and Functions of Plant Cells 17 Unit 2: Cell Structure and Function body with the use of a microscope. If you wanted to see the stages of mitosis in plant cells, what part of the plant should you observe under the microscope? Bibliography Campbell, Neil A. Biology. Frenchs Forest, N.S.W.: Pearson Benjamin Cummings, 2008. Ching, Johnny A., Ching, Charmaine E. Biology. Quezon City, Philippines: St. Bernadette Publishing House Corporation, 2012 Mader, Sylvia S., Michael Windelspecht, and Sylvia S. Mader. Introductory Biology. United States: McGraw-Hill Create, 2014. Miller, Kenneth R., and Joseph S. Levine. Prentice Hall Biology. Upper Saddle River, NJ: Pearson Prentice Hall, 2006. Sabile, Mary Jane G., General Biology 2. Quezon City, Philippines: Phoenix Publishing House, Inc., 2018 2.3. Structures and Functions of Plant Cells 18