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Lesson 9 Multicellular Organisms

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Course: Science 10 [5 cr] - AB Ed copy 1
Book: Lesson 9 Multicellular Organisms
Printed by: Guest user
Date: Sunday, 7 September 2025, 6:46 PM

Table of contents

  Introduction

How do cells work in a multicellular organism?

AS3.1 The cells of a leaf are used for photosynthesis
Multicellular organisms are made of millions of cells, and each cell is part of a system that helps the organism survive. Since each cell is part of a specific system, they are all specialized. These still perform the basic functions of life, but they have other jobs as well that benefit the organism as a whole.  

In Section 3, we will look at plants as an example of a multicellular organism. We will study the specialized cells and how they work together to create the systems they are part of. We will also take a look at the systems to see how they allow for the survival of the plant. 

  Targets


By the end of this section, you will be able to
AS3.2 Plants are multicellular.
AS3.3 Structure of a leaf
AS3.4 Stomata are used for gas exchange in plants.
AS3.5 Celery with xylem and phloem
AS3.6 Plant growing toward light


  1. explain why specialized cells are needed in multicellular organisms
  1. describe how the cells of the leaf system have a variety of specialized structures and functions to support the process of photosynthesis
  1. explain how plants exchange carbon dioxide for oxygen
  1. explain how plants move water and nutrients from the roots to the leaves
  1. explain phototropism and gravitropism and trace the development of theories of phototropism and gravitropism

  Introduction

How do specialized cells work?



A9.1 Types of blood cells
Specialized cells are important to how multicellular organisms survive. They are needed to make up the systems that perform the basic life functions of the organism. For example, a human has many different kinds of specialized cells that make up the circulatory system. Without the circulatory system, oxygen and nutrients would not get to all the organs and tissues in our bodies. 

In this lesson, we will look at the importance of specialized cells and the systems they make up in plants.  

  Targets

By the end of this lesson, you will be able to

        • explain why when a single-celled organism or colony of single-celled organisms reaches a certain size, it requires a multicellular level of organization
        • explain why multicellular organisms need specialized cells, tissues, and systems in plants

          Watch This


        Biological Levels in Biology: The World Tour Ā© YouTube The Amoeba Sisters


        This video gives you an overview of how cells organize themselves into organisms and even ecosystems. It gives you examples of the different levels and what they are used for. 

          Multicellular Organization

        Why go multicellular?



        A9.2 The pygmy marmoset is one of the smallest animals on Earth.
        In the previous lesson, you learned cells need to find the ideal balance between surface area and volume. They need to be large enough to house all their organelles and perform the functions of life but small enough that they can transport materials into and out of the cell efficiently. When a cell gets too large, it divides into two cells to stay within this ideal ratio.

        Multicellular organisms have come up with a way around this limitation. Instead of staying as one cell, they are a grouping of millions of cells that work together. This way they can create systems to transport materials around the organism and do not have to rely on the cell membrane. Most cells within the multicellular organism still find this ideal ratio, but the organism as a whole does not have to.

        Multicellular organisms have many advantages over single-celled organisms:

        • They can grow larger than the surface area to volume ratio allows a single cell to.
        • They can live in a broader range of environments.
        • They can specialize their cells so each cell has a specific job that benefits the entire organism.

        A9.3 The blue whale is the largest animal on Earth.


        Some advantages of being multicellular can become disadvantages depending on the situation. For example, if a few cells do not function properly or become diseased, the multicellular organism still has many healthy cells to perform their tasks. However, if dysfunction spreads to too many cells, such as a group of cancerous cells, the whole organism could suffer. In a single-celled organism, each cell must perform all the life functions, but it relies on itself, and not other cells, to survive.

        To combat some of these disadvantages, there must be some type of organization in multicellular organisms. Just like in a company, the cells must each have a specific job and certain cells must work together to complete their function.

        • Cells that have similar functions create tissues.
        • Tissues that have similar functions create organs.
        • Organs that have similar functions create systems.
        • Systems perform the life functions of the multicellular organism.

        A9.4 Multicellular organization

          Read This

        Please read pages 296 and 297 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the reasons why an organism becomes multicellular and the two systems in a plant. Remember, if you have any questions or you do not understand something, ask your teacher!

          Practice Questions

        Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

        1. Draw a flow chart that shows how cells are organized into systems in a multicellular organism.


        A9.5 Flowchart showing multicellular organization


        1. Name two advantages and two disadvantages to being a multicellular organism.
        Advantages:
        • They can grow larger than the surface area to volume ratio allows a single cell to.
        • They can live in a broader range of environments.
        • They can specialize their cells so each cell has a specific job that benefits the entire organism.
        Disadvantages:
        • The organism must rely on each cell to do its job properly.
        • One cell could cause the whole organism to suffer.

          Plant Systems

        Plants have two main systems.



        A9.6 The shoot system includes the trunk and leaves.
        The shoot system contains everything that is above the ground. This includes
        • stem or trunk
        • leaves or needles
        • buds
        • flowers
        • fruits, vegetables, and nuts
        It also contains swollen roots called tubers. Tubers are often the root vegetables we eat as they store energy for the plant. Some examples of these are potatoes and carrots.

        The root system contains everything below the ground, not including tubers. This includes aerial roots, even though those are above ground. Aerial roots are roots that crawl along the surface of the ground rather than being underground. If you have ever seen an orchid, you may have noticed the aerial roots it has.




        A9.7 The root system

          Did You Know?


        A9.8 Aerial Roots on an orchid

        Aerial roots are used to absorb moisture from the air in environments with a high humidity. In plants like orchids that grow in decomposing trees, these roots are also used to help anchor the plants by grabbing onto the trees and shrubs surrounding the plant.

          Read This

        Please read page 298 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the parts of the shoot and root systems. Remember, if you have any questions or you do not understand something, ask your teacher!

          Practice Questions

        Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

        1. Draw a diagram of a plant that includes all the parts of the root and shoot systems. 

        A9.9 Root and shoot system


          Plant Tissues

        Each plant system is made up of tissues that help the system perform its functions.



        Most of these tissues are present in both systems, but they have slightly different functions depending on the system.

        A9.11 Water Lilly stem
        The dermal tissue is also called the epidermis, and it is the outer layer of cells on all parts of the plants. It is very similar to skin cells in animals. This tissue is normally one cell layer thick, and it has a variety of functions. In woody plants such as trees, the epidermis is the bark. In the shoot system, it

        • protects the plants from disease
        • produces a substance called the cuticle that helps to stop microorganisms from damaging the plant and prevents water loss
        • allows oxygen and carbon dioxide to move into and out of the plant through the stem and leaves

        In the root system, the dermal tissue is responsible for allowing water and nutrients into the cells.

        A9.12 Water lilly stem
        This is the layer below the dermal tissue. It makes up most of the plant and has several important functions. In the shoot system, it

        • provides strength and support to the plant
        • is the location of photosynthesis

        In the root system, it plays an important part in food and water storage.

        The ground tissue is made up of many loosely packed cells. It is important that the cells are loosely packed as that creates space between each cell. This space allows for the quick diffusion of gases such as carbon dioxide and oxygen.

        A9.13 Water Lilly stem
        A vascular tissue is in charge of transporting materials throughout the plant. The xylem vascular tissue is specifically for moving water and dissolved minerals from the roots to the leaves. Each xylem tissue cell starts out as living but is dead by the time it is part of this tissue. The living cells are shaped like small straws and stack up end to end. As they grow, the ends of the cells start to break down, killing the cell. The sides then fuse together and a long tube is created.

        A9.14 Water lilly stem
        The phloem tissue works in a different direction to the xylem. While the xylem transports in one direction, the phloem can transport in many different directions. It takes the sugars that are created through photosynthesis in the leaves and transports them to the rest of the plant. The cells in this tissue are also tube-like in structure, but they remain alive. The ends of the cells have little holes in them to allow for the movement of sugars down the tubes and out into the plant tissues. The cytoplasm of these cells leaks out of the ends and creates a passageway from one to the next.

        The sugars that are transported by the phloem tissue can be used by the cells or stored in the leaves, stem, or roots of the plant.


        A9.10 Root and shoot system

          Read This

        Please read pages 299 and 300 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the different tissues and their functions. Remember, if you have any questions or you do not understand something, ask your teacher!

          Practice Questions

        Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

        1. Complete the chart below about the different plant tissues.

          Tissue Function in Shoot System
          		Function in Root System
          		Location in Plant
          Dermal
          Ground
          Xylem
          Phloem


        Tissue Function in Shoot System
        		Function in Root System
        		Location in Plant
        Dermal
        • protects the plants from disease
        • produces a substance called cuticle that helps to stop microorganisms from damaging the plant and prevents water loss
        • allows oxygen and carbon dioxide to move into and out of the plant through the stem and leaves
        • responsible for allowing water and nutrients into the cells
        		surface of plant
        Ground
        • provides strength and support to the plant
        • is the location of photosynthesis
        • food and water storage
        		 under the epidermis
        Xylem
        • moving water and dissolved minerals from the roots to the leaves
        • moving water and dissolved minerals from the roots to the leaves
        		 within the ground tissue
        Phloem
        • moving sugars from leaves to the rest of the plant, especially to areas where the plant is growing
        • moving sugars from leaves to roots
        		 within the ground tissue

          Specialized Cells

        In order to be specialized, cells only use certain parts of their genetic code.



        A9.15 Sieve tube cells
        Each cell has the exact same DNA as every other cell in that organism; however, it will only use the parts of the DNA that it needs to perform the function assigned to it. Once a cell has become specialized, it will only perform the functions needed to play its part in the tissue it is part of.

        Just like each system has tissues that make it up, each tissue has specialized cells. For example, the phloem contains specialized cells called sieve tube cells. These cells can survive with their cytoplasm extending out past the boundaries of the cell. This allows the sieve tube cells to pass the sugars the phloem is transporting from one cell to the next. All plant cells have the DNA that codes for this function, but only sieve tube cells use this part of the DNA.

        Another example is the growth of root hairs on dermal tissue cells in the root system. The growth of these root hairs increases the surface area of these cells and allows them to absorb more water and dissolved minerals than they otherwise would be able to.

        A9.16 Root hairs on a chilli pepper root

          Did You Know?


        A9.17 Herbal medicine

        Before the development of western science and medicine, many cultures used plants for medicine, food, shelter, and clothing. Many Indigenous cultures saw and continue to see plants as sacred due to their importance in society. In western medicine, plants are often the basis of many drugs.

          Watch This

        Specialized Plant Cells Ā© YouTube Donna Forward 


        This video gives a quick overview of some examples of specialized cells in plants. You will learn about these cells in more detail in the following lessons. This video also gives you microscopic images of the cells.


          Read This

        Please read pages 301 and 302 in your Science 10 textbook. Make sure you take notes on your readings to study from later. You should focus on the examples of specialization in plants. Remember, if you have any questions or you do not understand something, ask your teacher!

          Practice Questions

        Complete the following practice questions to check your understanding of the concept you just learned. Make sure you write complete answers to the practice questions in your notes. After you have checked your answers, make corrections to your responses (where necessary) to study from.

        1. Why are specialized cells important in multicellular organisms?

        Specialized cells allow each cell to have a single function to focus on. This allows the body to build systems and allows the organism to grow much larger than a single cell size. If cells were not specialized, the cells could not work together to form the systems needed for multicellular organisms.

        1. What is one example of a specialized cell?

          You may think of other examples but here are a few:

          1. sieve tube cellsā€”specialized for transporting sugars
          2. root hairsā€”specialized for absorption
          3. small intestine cellsā€”specialized for absorption
          4. xylem cellsā€”specialized for transporting water
          5. red blood cellā€”specialized for transporting oxygen

          Importance of Specialization

        Specialization allows organisms to become multicellular.

        Without specialization, every organism would have to remain a single-celled organism and would have to perform all the basic functions of life. Specialization allows for multicellular organisms to create systems that perform the basic functions of life so each cell can focus on one specific function, rather than all the functions.

        A multicellular organism is organized so specialized cells make up specialized tissues, which then make up specialized organs. These organs then make up specialized systems. Each system helps the organism perform the basic functions of life.

        In the next lesson, we will study the leaf system and how the system supports the process of photosynthesis.
        Ā©OpenStax via Wikimedia Commons
        A9.17 Organization of specialization

          Watch This

        Specialized Cells: Significance and Examples Ā© YouTube Amoeba Sisters


        Watch this video for an overview of the importance of specialized plants and types of plant cells. The video discusses trichomes, which we do not cover in this course. It also covers examples from animals. These specialized animal cells are great examples, but you do not need to memorize them.


        1.5 Assignment

        Unit A Formative Assessment Lesson 9


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        1.5 Assignment