• 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
    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.

    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

    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 se mitochondria and ribosomes and state their roles in the life of the cell.
    When 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.

    4.1.2 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: Differencied 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 placein mesosomeswhich 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 I not enclosed in a double membraned chloroplast

    4.4.1. 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.

    4.3.1Nucleus

    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 whichconsist 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.

    4.3.2 Endoplasmic reticulum (ER)


    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.

    4.3.4 Mitochondria


    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.

    4.3.6 Lysosomes


    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’

    4.3.7 Ribosomes


    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.

    4.3.8 Centrioles


    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.

    4.3.9 Vacuole


    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. The diagram below shows the 3D structures which would be visible in ultrastructure of a plant cell.
    Identify the parts labeled in this plant cell and:
    a. State one function for 1, 2, 3, 7, and 10
    b. What are parts 4 and 5 made of?
    c. What are two functions of the cytoskeleton?

    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.

    4.4.4. 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.