◄UNIT 3: MICROSCOPYUNIT 5: DIVERSITY OF SPECIALIZED TISSUES►

UNIT 4: CELL STRUCTURE AND SPECIALIZATION

  • UNIT 4: CELL STRUCTURE AND SPECIALIZATION

    UNIT 4: CELL STRUCTURE AND SPECIALIZATION
    Key Unit Competence
    Describe the structure and function of cells in an organism.
    Learning objectives
    By the end of this unit, I should be able to:
    – Identify plant and animal cell structures visible under a light microscope.
    – State functions of cell structures as seen under an electron microscope.
    – Describe the nature of artefacts.
    – State the importance of freeze fracturing for examining membrane structure.
    – Explain how cell organelles can be isolated by cell fractionation.
    – List the functions of cell membranes.
    – Describe the fluid mosaic structure of cell membranes.
    – Explain the role of the different components of a cell membrane.
    – Explain cell specialization as the differentiation of a cell or process to do a
    particular function.
    – Interpret charts and micrographs to relate the structure of specialized cells to
    their functions.
    – Prepare, observe and draw diagrams for specimens on temporary slides for:
    Wandering Jew, in plants and cheek cells under a light microscope.
    – Distinguish between ultra-structures of plant cells and animal cells.
    – Compare ultra-structures of prokaryotic and eukaryotic cells
    – Show resilience and be aware of artefacts when preparing temporary slides.

    – Appreciate the importance of cell specialization in multicellular organisms

    Introductory Activity
    Using addition resources to your textbook available in your school such as the
    books from the school library and search further information from the internet:
    1. Differentiate between prokaryotic and eukaryotic cells.
    2. By using charts for the two cells, identify different organelles of eukaryotic

    cell that may perform functions similar to those of a prokaryotic cell.

    4.1. Ultra-structure of a cell
    Activities 4.1
    1. Observe the chart given for Ultra structure of a cell and identify parts that
    are easily recognizable when compared with a photomicrograph form a
    light microscope.
    2. Identify the mitochondria and ribosomes and state their roles in the life of
    the cell.
    Cytology is the study of the structure and function of cells. A Cell is the basic unit of 
    life. All living organisms are made up of cells.
    Living organisms are classified into:
    – Unicellular organisms are made of only one cell, such as bacteria,
    – Multicellular organisms are animals and plants composed of many cells. In
    multicellular organisms, cells divide and then undergo differentiation or

    specialisation for specific functions. 

    Cell theory.
    The cell theory states that all living organisms are made up of cells, and cells are the
    basic unit of structure function in all living organisms.
    The main principles of cell theory are based on the following ideas.
    – All known living organisms are made up of one or more cells,
    – All cells come from pre-existing cells by division
    – Cells contain the hereditary information that is passed from cell to cell during
    cell division.
    – Metabolism takes place in cells

    – Given suitable conditions, cells are capable of independent existence

    When a cell is viewed under light microscope, the most obvious features observed
    are the very large nucleus and a clear cytoplasm surrounded by a cell membrane.
    However, under electron microscope, it is possible to identify a range of organelles
    in plant and animal cells. Ultrastructure is the detailed of cell as revealed by the

    electron microscope.

    Similarities between animal cell and plant cell
    – Both have a cell membrane, a cytoplasm and a nucleus.
    – Both animal and plant cells have mitochondria, Golgi apparatus, Reticulum
    endoplasmic, lysosome, big ribosomes (80S), peroxisome, microtubules. 

    Table 4.1: The differences between animal and plant cell

    Self-assessment 4.1
    1. What structures do both animal and plant cells have in common?
    2. State any five principles of the cell theory.
    3. Give the major difference between a plant and animal cell. Which organelles

    does this difference relate to?

    4.2. Prokaryotic cells
    Activities 4.2
    Under microscope, observe mounted slides of bacteria, and plant cells. Draw and

    label the parts that are common in both plant and bacterial specimens

    A typical bacterial cell has a cell surface membrane enclosing the cytoplasm that
    contains enzymes, ribosomes and food granules. The membrane is surrounded by
    the cell wall and this may in turn be enclosed in a capsule. A bacterial cell lacks high
    level of organization compared to animal or plant cell. It has no Golgi apparatus
    or endoplasmic reticulum. The genetic material is a single strand of DNA usually
    coiled up into the center of the cell to form a nucleoid. This nucleoid has no double
    membraned nuclear envelope so is often described as an ‘ill-defined nucleus’.

    – Some bacterial cells contain plasmids with additional DNA.
    – Respiration generally takes place in mesosomes which is an in-folding of the
    cell surface membrane but lack mitochondria
    – Photosynthesizing bacterial cells such as cyanobacteria (blue green
    algae) have a special form of chlorophyll but it is not enclosed in a double

    membraned chloroplast

    Comparison between prokaryotic and eukaryotic cells

    Table 4.2 Comparison between prokaryotic and eukaryotic cells


    Self-assessment 4.2
    Organisms such as bacteria are known as prokaryotes.
    1. Which structure in a bacterial cell resembles a nucleus?

    2. How does it differ from the nucleus of eukaryotic cells?

    4.3. Cell organelles
    Activities 4.3
    By using iodine solution, methylene blue, a piece of onion leaf, a scalpel, forceps,
    light microscope, slides and cover slips, clean cotton wool bud, and onion bulbs.
    Observe cells from onion epidermis under light microscope.
    Observation of a plant cell
    – Add a drop of diluted iodine solution on the slide.
    – Remove a transparent layer of onion epidermis from the inner side that you
    will mount on the slide and add iodine solution.
    – Cover your preparation with a cover-slip and mount it on the stage.
    – Observe the preparation under the low power and thereafter under high
    magnification.
    Why did you use iodine solution in this experiment?

    What main parts of a plant cell are easily observed from a light microscope?

    Observe animal cells from mouth cheek epithelium
    – By using a clean cotton wool bud, wipe over inside of your cheek.
    – Smear cells over surface of a clean grass microscope slide containing a drop
    of methylene blue stain
    – Carefully put the cover-slip on the preparation and mount it on the stage to
    observe.

    Draw both plant and animal cell and label the cell wall, nucleus and vacuole\

    The cell nucleus contains nearly all the cell’s DNA with the coded instructions for
    making proteins and other important molecules. The nucleus is surrounded by a
    double nuclear envelope, which allow materials to move into and out of the nucleus
    through nuclear pores. The granules found in the nucleus are called chromatin which
    consist of DNA bound to protein. When a cell divides, the chromatin condenses into
    chromosomes containing the genetic information. The nucleus contains a dense

    spherical structure called nucleolus in which assembly of ribosomes occurs

    The ER consists of a series of flattened membrane-bound sacs called cisternae. The
    rough ER is surrounded with ribosomes. The rough ER transports proteins made on
    attached ribosomes. The smooth ER is made of tubular cavities lacks ribosomes, and
    it involves in synthesis of lipids that the cell needs. The number and distribution of
    the ER relates to the functions of the cell; glandular cells are seen to have several RER
    for synthesis of hormones and enzymes. Examples include liver cells, plasma cells,

    and pancreatic cells.

    4.3.3. Golgi apparatus 


    The Golgi apparatus is a stack of membrane-bound, flattened sacs, which receives
    proteins from the ER and modifies them. It may add sugar molecules to them to form
    glycoproteins or lipids to form glycolipids. The Golgi apparatus then packages the
    modified substances into vesicles that can be transported to their final destinations

    throughout the cell or outside of the cell by exocytosis.


    Mitochondrion have two membranes separated by a fluid-filled intermembrane
    space. The inner membrane is highly folded to form cristae that plays a big role in

    aerobic respiration. The central part of the mitochondrion is called matrix. 

    The mitochondria are the site where Adenosine triphosphate (ATP) is produced

    during aerobic respiration.

    4.3.5. Chloroplasts


    Chloroplasts are the site of photosynthesis in plant cells. These are found in plant
    cells and in cells of some protoctists. They also have two membranes separated
    by a fluid-filled space, circular DNA as in mitochondria. The inner membrane is
    continuous, with thylakoids. A stalk of thylakoids is called a granum (plural:

    grana). Chlorophyll molecules are present on the thylakoid membranes.

    These are spherical sacs surrounded by a single membrane. They contain powerful
    digestive enzymes. Their role is to break down materials such as worn out cell
    organelles, and destroy foreign microorganisms that enter the body. In acrosome,
    lysosomes help the sperm to penetrate the egg by breaking down the material
    surrounding the egg. Lysosomes are also involved in autolysis, breakdown of dead
    tissues or harmful objects inside the cell. Therefore, lysosomes are referred to as

    ‘suicide bags’

    Ribosomes appear as dark granules in the cytoplasm and are not surrounded
    by a membrane. They have the same size as those found attached to the rough
    endoplasmic reticulum- about 20nm in diameter and known 80S. Free ribosomes
    make proteins that are as enzymes or in other forms in the cytoplasm. Ribosomes

    are made in a region of the nucleus called the nucleolus.

    \

    Centrioles are small tubes of protein fibers called microtubules which have many
    roles including moving chromosomes during nuclear division. Animal cells have
    structures called centrioles which consist of two groups of nine triple microtubules.

    Centrioles form an anchor point for microtubules during cell division.

    A vacuole is a saclike structure that stores materials such as water, salts, proteins, and
    carbohydrates. In many plant cells there is a single and large central vacuole filled
    with liquid. The pressure in the cells of central vacuole makes it possible for plants
    to support heavy structures like leaves and flowers. Some animals and unicellular
    organisms contain contractile vacuoles which contract to pump excess water out

    of the cell.

    Self-assessment 4.3
    1. Explain why muscle cells contain several mitochondria compared to fat
    storage cells
    2. What kind of information is contained in chromosomes?
    3. Describe the functions of the endoplasmic reticulum, Golgi apparatus,
    chloroplasts, mitochondria and nucleus in the cell.
    4. Consider the 3D structures which would be visible in ultrastructure of a
    plant cell.
    a. Identify their parts and label them

    b. State one function for each part 

    4.4. Membrane structure
    Activity 4.4
    Learners mix a portion of cooking vegetable oil with water and shake the mixture
    vigorously and leave it to settle. Note the way water and oil are distributed within

    the mixture and suggest a possible explanation for your observation.

    Cell membranes cover surfaces of every cell. Some organelles in cytoplasm are
    enveloped by membranes. The cell membranes ultrastructure is not easily visible
    under a light microscope but is studied by electron microscopes, freeze structuring
    and other modern techniques which reveal complex structures
    A detailed study of a cell membrane reveals that it is 7-8nm wide and is made of a
    phospholipid bilayer.
    – Lipid component makes up 45% protein and 10% carbohydrate. Most of the
    lipids are phospholipids
    – Each molecule of phospholipid consists of a hydrophobic tail of two fatty acids
    and a hydrophilic phosphate head. They arrange themselves in phospholipids
    bilayer with their tails pointing inward away from the water both inside and

    outside the cell

    \
    In 1972, Jonathan singer and Garth Nicolson proposed the fluid mosaic model of the
    cell membrane structure. This was done after realizing that membranes must have a
    complex structure to carry out a variety of activities. In their model;
    – Individual protein molecules shift and move on a fluid bilayer of phospholipids;
    some spanning the width of the membrane (intrinsic proteins), others confined
    to the outer or inner surface (extrinsic protein)
    – Protein molecules are variable in structure and function but they all contribute

    to the mechanical strength of membranes


    The membrane is referred to as;
    – A fluid because it appears to have the properties of a fluid rather than a solid as
    the major constituent, lipids and proteins move about the structure
    – Mosaic because protein and lipid components form a pattern of parches model
    4.4.1. Properties of the cell membrane
    – It is mainly made of lipids, proteins and carbohydrates.
    – It is semi-permeable or partially permeable to allow some substances to pass 
    through but prevents others to cross depending on their size, charges and
    polarity.
    – It is positively charged outside and negatively charged inside and has a
    hydrophilic pole and a hydrophobic pole
    – It is a bilayered sensitive and flexible.It has inorganic ions and its proteins and
    lipids may be mobile and contains different types of enzymes and coenzymes.
    – It is perforated of pores and recognizes chemicals messengers including

    hormones and neurotransmitters.

    4.4.2. Roles of different components of cell membrane
    a. Cholesterol
    – Gives the membranes of some eukaryotic cells the mechanical stability.
    – It fits between fatty acid tails and helps make the barrier more complete,
    so substances like water molecules and ions cannot pass easily and directly

    through the membrane.

    b. Channel proteins
    – Allow the movement of some substances across the membrane.
    – Large molecules like glucose enter and leave the cell using these protein
                channels.
    c. Carrier proteins
    – Actively move some substances across the cell membrane. For example,
    magnesium and other mineral ions are actively pumped into the roots hair
    cells from the surrounding soil.
    – Nitrate ions are actively transported into xylem vessels of plants
    d. Receptor sites
    – Allow hormones to bind with the cell so that a cell response can be carried out.
    – Glycoproteins and glycolipids may be involved in cells signaling and they allow
    the immune system to recognize foreign objects to the cells.
    – Some hormone receptors are glycoprotein and some are glycolipid.
    e. Enzymes and coenzymes
    – Some reactions including metabolic processes in photosynthesis take place in
    membranes of chloroplasts.
    – Some stages of respiration take place in membranes of mitochondria, where
    Enzymes and coenzymes may be bound to these membranes.
    – The more membrane there is, the more enzymes and coenzymes it can hold
    and this helps to explain why mitochondrial inner membranes are folded to
    form cristae, and why chloroplasts contain many stacks of membranes called

    thylakoids.

    4.4.3. Functions of a cell surface membrane
    – The cell membrane acts as a selective barrier at the surface of the cell, and
    controls the exchange between the cell and its environment.
     Glycoproteins and glycolipids are involved in the cell protection, the process
    by which cell adhesions are brought about and in the cell recognition.
    – Receptor sites for hormones and neurotransmitters
    – Transmission of nerve impulses
    – Insulation of nerves to improve transmission speeds.
    Internal membranes:
    – Act as reaction surfaces
    – Act as an intra cellular transport system
    – Providing separate intra cellular compartment, isolating different chemical

    reactions as in organelles.

    Self-assessment 4.4
    1. What is meant by the fluid mosaic model of the cell membrane?
    2. State at least three properties of the cell membrane.
    3. Describe at least 4 types of the proteins in the cell membrane and their
    roles.

    4. What is a partially permeable membrane?

    5. What do the words hydrophilic and hydrophobic mean?
    6. The diagram below shows the structure of a cell membrane. Study it carefully
    and answer the following questions.
    a. Name parts labelled A, B, C and D and give the function of the part B.
    b. What types of molecule are likely to be involved in?
    i. Cell signaling and recognition
    ii. Allowing small charged molecules to pass through the cell membrane
    iii. Site metabolic reactions
    7. What is the difference between rough and smooth endoplasmic reticulum?
    8. Describe the role of cytoskeleton
    9. The photograph in the figure below shows an organelle of the living cell.

    a. Name this organelle.
    b. What is the function of this organelle?

    c. In which ways is this organelle similar to a chloroplast?

    4.5. Specialized cells
    Activity 4.5
    By using the diagrams below, relate the structure of specialized cells to their

    functions.


    Differentiation refers to the changes occurring in cells of a multicellular organism so
    that each different type of cell becomes specialized to perform a specific function.
    In animals, the first type of cells in the developing embryo is stem cells. These are
    unspecialized cells that go on to form all the different types of cells in adult. Cell can
    differentiate in many ways, with changes to the shape of the cell, the number of

    particular organelles and the content of the cell.

    4.5.1. Specialized animal cells and their functions

    4.5.1.1. Red blood cells


    All blood cells are produced from undifferentiated stem cells in the bone marrow
    but the cells destined to become erythrocytes (red blood cells) lose their nucleus,
    mitochondria, Golgi apparatus and rough endoplasmic reticulum. They are packed
    full of the protein called haemoglobin. The shape of this cells change so that they

    become biconcave discs, and they are then able to transport Oxygen in the body.


    Sperm cells are specialized to fertilize the egg. Its specialization involves many
    changes in shape and organelles content.

    By shape: the sperm cells are very small, long and thin to help them to move easily,
    and they have a flagellum which helps them to move up the uterine tract towards

    the egg.

    By organelles content: sperm cells contain numerous mitochondria which
    generate much energy for their movement. Their acrosome has specialized
    lysosomes containing many enzymes that are released on the outside of the egg.
    These enzymes lyse the wall of the egg, and facilitate the sperm nucleus to penetrate
    easily. In content, the sperm cell nucleus contains the half number of chromosomes
    of the germ cell in order to fulfil its role as a gamete of fertilizing the egg.
    Did you know: As a sperm fuses with an ovum to form a zygote which grows into
    an individual, in the same way: a man maries a woman to form a couple which will

    produce children and form a family.

    Nerve cells also known as neurons are specialized cells to carry nervous impulses
    in the body. These signals between neurons occur via specialized connections
    called synapses. Specialized animal cells have different functions. Some of them are

    summarized in the following table.

    Table 4.3: Specialized animal cells and their functions.



    4.5.2. Specialized plant cells and their functions

    4.5.2.1. Root hair cells 


    The root hair cells have hair-like projection from their surface out into the soil. This
    increase the surface area of root available to absorb water and minerals from the

    soil. 

    4.5.2.2. Palisade cells

    Palisade cells are in leaves, right below the upper epidermis. They are vertically
    elongated, a different shape from the spongy mesophyll cells beneath them in
    the leaf. Their large numbers of chloroplasts allow them have several chloroplasts
    used in photosynthesis.
    Parenchyma cells
    Parenchyma is composed of relatively simple and undifferentiated parenchyma
    cells. They function in storage, photosynthesis. In most plants, metabolic activity
    such as cell division, respiration, and photosynthesis occurs in these cells because
    they retain their active cytoplasm. .
    4.5.2.3. Guard cells


    Guard cells are cells surrounding each stoma. Guard cells are specialized cells in the
    epidermis of leaves, stems and other organs that are used to control gas exchange.
    They are produced in pairs with a gap between them that forms a stomatapore.
    Guard cells have the following feature:
    – Un even thick walls
    – Possess chloroplasts; they are the epidermal cell that have chloroplasts an

    adaptive feature in controlling pore opening.

    Self-assessment 4.5
    1. Explain why differentiation to produce erythrocytes involves a change in
    shape.
    2. Red blood cells cannot divide as they have no nucleus. State two other
    biological processes that red blood cells cannot carry out.
    3. Describe how the following are specialized for their roles:
    a. Neutrophil
    b. Sperm cell
    c. Root hair cell
    4. Explain why photosynthesis is carried out in palisade mesophyll more than
    in spongy mesophyll.
    5. In what kinds of organisms is cell specialization pronounced characteristic?

    6. Discuss the advantages of cell specialization in living things

    End of unit assessment 4
    Section A. Multiple choice questions
    1. Which organelle converts the chemical energy in food into a form that cells can
    use?
    a. Chromosome
    b. Chloroplast
    c. Nucleus
    d. Mitochondrion
    2. The cell membranes are constructed mainly of:
    a. Carbohydrate gates
    b. Protein pumps
    c. Lipid bilayer
    d. Free-moving proteins
    3. In many cells, the structure that controls the cell’s activities is the:
    a. Nucleus
    b. Nucleolus
    c. Cell membrane
    d. Organelle
    4. Despite differences in size and shape, all cells have cytoplasm and a
    a. Cell wall
    b. Cell membrane
    c. Mitochondria
    d. Nucleus
    5. If a cell of an organism contains a nucleus, the organism is a (an)
    a. Plant
    b. Eukaryote
    c. Animal

    d. Prokaryote

    6. Match each part of the cell (left column) to corresponding statement (right
    column):
    Nucleus                                              controls movement of substances in and out of the cell
    Mitochondrion                               where photosynthesis takes place
    Chloroplast                                      where aerobic respiration takes place
    Smooth ER                                       controls the activity of the cell
    Ribosomes                                        where lipids including steroids are made
    Section B: Questions with short answers
    1. How does a cell membrane differ from a cell wall?
    2. Name the structures that animal and plant cells have in common, those found in
    only plant cells, and those found only in animal cells.
    3. List:
    a. Three organelles each lacking a boundary membrane
    b. Three organelles each bounded by a single membrane
    c. Three organelles each bounded by two membranes (an envelope)
    4. Identify each cell structure or organelle from its description below.
    a. Manufactures lysosomes and ribosomes
    b. Site of protein synthesis
    c. Can bud off vesicles which form the Golgi body
    d. Can transport newly synthesized protein round the cell
    e. Manufactures ATP in animal and plant cells
    f. Controls the activity of the cell, because it contains the DNA
    g. Carries out photosynthesis
    h. Can act as a starting point for the growth of spindle microtubules during
    cell division
    i. Contains chromatin
    j. Partially permeable barrier only about 7 nm thick
    k. Organelle about 25 nm in diameter

    l. Which two organelles other than the nucleus contain their own DNA

    Section C: Essay questions
    1. Describe the structure and function of the cell membrane and cell wall.
    2. Describe the basic structure of the cell membrane.
    3. Explain two common characteristics of chloroplasts and mitochondria.

    Consider both function and membrane structure. 

    4. The diagram below shows the structure of a liver cell as seen using an electron

    microscope.

    a. Name the parts labelled A, B, C and D.
    b. The magnification of the diagram above is x12 000. Calculate the actual
    length of the mitochondrion labelled M, giving your answer in µm. Show
    your working.
    c. Explain the advantage to have a division of labor between different cells in

    the body.




    ◄UNIT 3: MICROSCOPYUNIT 5: DIVERSITY OF SPECIALIZED TISSUES►