Cell Structure Notes 2024

The cell structure is the study of the tissues of the body and how these tissues are arranged to constitute organs, with the focus on how cells’ structure and arrangement optimize functions specific to each organ. ell is the structural and functional unit of the organ...

The cell structure is the study of the tissues of the body and how these tissues are arranged to constitute organs, with the focus on how cells’ structure and arrangement optimize functions specific to each organ. ell is the structural and functional unit of the organism and the smallest living parts of the body. All living organisms are formed of cells. Despite their great variation in shape, size and functions, the cells share many common features. The surface of cells is limited by plasma membrane representing the living outer boundary of the cell. In the center of the cell there is the controlling center or the nucleus. The remainder of the cell is the cytoplasm which contains the source of energy together with the various structures that perform all other required functions. Fig. 1 showing different cells. The plasma membrane (PM) The plasma membrane (cell membrane or plasmalemma) is extremely thin, it cannot be seen with the light microscope (LM). With the electron microscope (EM) it appears as two dark lines and one light space in between. Molecular structure: The cell membrane is formed of: 1- Lipid: which is formed mainly of 1 Dr. Manal M. Sayed  a) phospholipids. The phospholipids molecules are arranged into a double layer with their hydrophobic ends directed inward and their hydrophilic polar head outwards. b) Cholesterol inserts among the closely packed phospholipids. Fig. 2 showing the cell membrane structure 2- Protein: There are two types of proteins: a- Integral proteins which may embed completely or partially within the lipid bilayer. b- Peripheral proteins which are associated with the inner lipid bilayer. 3- Carbohydrates: The carbohydrates are conjugated with the protein to form glycoprotein and with lipid to for glycolipids both are present in the outer surface of the cell membrane forming the cell coat. Cell coat or glycocalyx It is a delicate cell surface coating. It is formed of oligosaccharide chains attached to both lipid and protein components of plasma membrane and exposed at the external membrane surface. It has many functions such as protection of the outer surface of plasma membrane, cellular recognition, antigenicity, and filtration. 2 Dr. Manal M. Sayed  The cytoplasm The cytoplasm is an amorphous heterogeneous colloidal substance in which various particles of different sizes are suspended. Cell organoids or organelles Cell organoids (organelles) are living structures present in the cytoplasm and are essential for the life of the cell and perform specific functions inside the cells. The cell organoids include: A - Membranous organoids (which are limited by a membrane), they are rough & smooth endoplasmic reticulum, Golgi apparatus, lysosomes, mitochondria, secretory vesicles, and coated vesicles. B- Non-membranous organoids: as ribosomes, centrosome, microtubules, microfilaments, and intermediate filaments. 3 Dr. Manal M. Sayed  A-The membranous organelles 1-Endoplasmic reticulum The endoplasmic reticulum (ER) is an extensive network of intracellular membranes that are usually anastomose with one another to form a network. Two types of endoplasmic reticulum are found in cells: Rough endoplasmic reticulum (RER) and Smooth endoplasmic reticulum (SER). a) Rough Endoplasmic reticulum (rER) LM: it is highly basophilic due to the presence of ribosomes on its surface. The rER is a membranous organelle which has ribosomes on its membranes and appears as parallel stacks of flattened cisternae. The function: Synthesis of proteins for exports. b) Smooth endoplasmic reticulum (sER): Regions of ER that lack ribosomes make up the smooth endoplasmic reticulum (SER). LM: cells rich in SER are highly acidophilic due to the absence of ribosomes. EM: it appears as membrane-bound tubules &vesicles with no ribosomes on their surfaces. Functions: 1. Lipid synthesis (especially steroids) as in the interstitial cells of the testis 2. Mineral metabolism (especially the formation of HCL) in the gastric glands. 3. Detoxification of toxins as in the liver cell. 4. Muscle contraction: The sarcoplasmic reticulum of striated muscles cells is an example. Fig. 2 showing smooth and rough endoplasmic reticulum. 4 Dr. Manal M. Sayed  2-Golgi complex Golgi complex (Golgi apparatus) is one of the membranous organoids. With LM: it is negative with H&E stain but after silver or osmic impregnation, it appears as dark granules or reticular structures taking different shapes and localizations in different cells. It is perinuclear (around the nucleus) in nerve cells whereas in secretory cells, it lies between the nucleus and the secretory surface, and in liver cells it is scattered in the cytoplasm. With the EM: Golgi complex appears to be formed of membrane bound cisternae (or saccules) as well as small and large vesicles. Fig. 3 showing GolgiFig. 2 showing bodies stained smooth by silverand rough stain and endoplasmic reticulum. structure its electron microscopic Functions of Golgi complex: 1- It is the site for the condensation, packing and modification of proteins. 2- In Golgi complex, carbohydrates are added to proteins to form the proteoglycans which are then packed in the secretory vesicles. 3. Golgi complex is an important site for packing hydrolytic enzymes of primary lysosomes. 5 Dr. Manal M. Sayed  4-Lysosomes Lysosomes are one of the membrane organoids, which are membrane- bound vesicles very rich in hydrolytic enzymes. In electron micrographs: Lysosomes appear as spherical bodies limited by membrane and contain relatively dense materials. Lysosomes originate from the Golgi region where various enzymes are packed. Types of lysosomes 1- Primary lysosome: the newly formed lysosome is homogenous in appearance. 2- Secondary lysosome: when foreign materials unite with the primary lysosomes is heterogenous in appearance. 3- A residual body: After the digestion of the contents of the secondary lysosome and the diffusion of useful material to the cytoplasm, the remaining vacuole is known as a residual body. The residual bodies accumulate with age (lipofuscin pigment). Fig. 4 showing Lysosomes their electron microscopic structure. Functions of lysosomes: 1. The main function of lysosomes is the digestion of materials entering the cell either by phagocytosis or pinocytosis, so they are numerous in phagocytic cells as macrophages and neutrophils. 2. Lysosomes dispose of excess or non-functioning organelles as mitochondria. 3. Lysosomes represent a safe store of digestive enzymes which can be used by the cell in different ways. - In bones, lysosomes are involved in resorption of old bone matrix. - The acrosome of the spermatozoon is a large single lysosome, the contents of which are released on contact with the ovum to make fertilization easy. 6 Dr. Manal M. Sayed  5-Microbodies or peroxisomes Microbodies or peroxisomes are small spherical membrane-limited organelles containing enzymes for various metabolic reactions, especially for oxidation and detoxification and degradation of fatty acids. Fig. 5 showing peroxisomes by electron microscope. 6- Mitochondria Mitochondria are one of the membranous organoids. LM: mitochondria appear in different shapes e.g., spherules, rods, or filaments. EM: the mitochondrion is surrounded by two membranes. The outer is simple, and the inner is thrown into folds called cristae or crests. The space between the outer and the inner membrane is the outer chamber or inter membranous space. Inside the inner membrane is the inner chamber which contains the matrix. The matrix is dense and contains Ca++ particles, DNA, small ribosomes, and enzymes of kreb's cycle. Fig. 6 showing mitochondria their electron microscopic structure. 7 Dr. Manal M. Sayed Function of mitochondria ❑ The most important function of mitochondria is to produce energy, produce ATP molecules. This process is known as oxidative phosphorylation. ❑ Heat production They are numerous in liver cells. B-Non-membranous organoids 1-Ribosomes Ribosomes are non-membranous small particles present in all animal cells in varying amounts depending on their activity in protein synthesis. There are two forms of ribosomes: 1- Attached ribosomes: They are attached to endoplasmic reticulum (rER) and are involved in the formation of secretory proteins. 2- Free ribosomes: present freely in the cytoplasm and are concerned with the formation of cytoplasmic proteins. Each ribosome is formed of two subunits, a small and a large one both formed in the nucleolus. Fig. 7 showing diagram of ribosomes. 2-The cytoskeleton Microtubules, microfilaments, and intermediate filaments are the main components of the cytoskeleton. Function: 1-Determines the shapes of cells. 8 2-Provides structural support for the cells. 2-Plays an important role in the movements of organelles and cytoplasmic vesicles (intra cellular transport). Dr. Manal M. Sayed 3- Allows the movement of entire cells. 5-Cell division a) Microfilaments or actin filaments are 6 to 8 nm in diameter and consist of globular actin molecules polymerized into long filaments. Functions: ❑ Actin filaments are found in great abundance in muscle cells. They integrated with myosin (a motor protein) to permit very forceful contractions. ❑ They are found all non-muscle cells where they play a central role in "cell locomotion", formation of pseudopodia "maintenance of cell shape", "formation of the contractile ring in mitosis", and numerous other activities. Fig. 8 showing diagram of microfilament or actin filaments. b) Intermediate filaments are approximately 10 nm in diameter. They are stable and not contractile. Intermediate filaments are made up of fibrous proteins that are organized into tough, ropelike structures. They have two major structural functions: ❑ They stabilize cell structure. ❑ They resist tension. c) Microtubules are tubular or cylindrical structures with an average diameter of 25nm. They are composed of the protein tubulin arranged in rings stacked end to end making up the tubules. 9 Functions: ❑ Microtubules provide internal support for the cell. Dr. Manal M. Sayed ❑ Microtubules are organized to form dynamic structures such as mitotic spindle of cell division, centrioles, basal bodies and axoneme of cilia and flagella. ❑ Microtubules guide the movement of secretory vesicles and organelles within the cell. Fig. 9 showing diagram of microfilament, intermediate filaments, and microtubules. Cytoplasmic inclusions Cytoplasmic inclusions are substances present in the cytoplasm as result of accumulation of cell products and are not involved in the cellular activities. There are three types of inclusions: A) Stored food: as fat & glycogen B) Pigments, e.g., melanin and hemoglobin. C) Crystals. Modifications of the cell membrane Modifications of the plasma membrane at the Lateral surface I- Intercellular junctions When cells come into contact with one another, specialized junction may form at specific sites on the contacting cell membranes. These junctions have important roles in the differentiation, development, and function of normal cells, tissues, and organs. 10 Dr. Manal M. Sayed  Fig. 9 showing diagram of plasma membrane modifications. Classification 1. Occluding Junctions or Tight junction 2. Adhesive or Anchoring Junctions are sites of strong cell adhesion. i) Zonula adherens ii) Macula adherens (Desmosome) 3. Communicating (Gap) Junctions ❑ Tight junctions (zonula occludens): form a barrier between cells, regulating the passage of substances. The adjacent cell membranes at these junctions appear fused with no intercellular space. Tight junctions prevent the passage of material through the intercellular spaces. ❑ Anchoring Junctions They hold the cell together and provide lateral adhesions between epithelial cells using proteins that link into the cytoskeleton of adjacent cells. Two types of anchoring cell-to-cell junctions can be identified: Zonula adherens which interacts with the actin filaments inside the cell. Macula adherens or desmosome,which interacts with intermediate filaments. The desmosome consists of two cytoplasmic adhesion plaques formed of special proteins and placed on the cytosolic face of the two facing cell membranes. The two plaques are connected by transmembrane linker proteins which extend into the intercellular space and bind the two cells together. In the region of the desmosome, intermediate filaments give support to the junction. Dr. Manal M. Sayed 11  Fig. 10: showing the structure of the desmosome. ❑ Communicating or Gap junction: Most cells are interconnected by junctional channels, so, many small ions that pass freely from one cell to the other. These junctional channels represent regions of low electrical resistance as in between cardiac muscle fibers. 2- Interdigitations: to give a very wide surface area for intercellular passage of material. The best example is the cells of the kidney tubules. 3- Intercellular Canaliculi: Secretory material from adjacent cells is accumulated in channels known as intercellular canaliculi which carry the secretions to ducts. The intercellular canaliculi between liver cells are examples. Dr. Manal M. Sayed Fig. 11 showing interdigitation between cells (a & b) and intercellular canaliculi (c). 12  II-Modifications of the plasma membrane at the apical surface ❑ Cuticle (for protection): a substance rich in protein is deposited on the outer lamina of the plasma membrane in cells exposed to harmful substances. Example, cells lining the urinary bladder and ureter. ❑ Cilia (for transportation): the surface of some cells which are involved in transportation of fluids, particles or cells become modified into ciliated surface e.g., ciliated cells lining the respiratory tract. ❑ Microvilli (for absorption): they are slender processes projecting from the surface of the absorptive cells to increase the surface for absorption. The core of the microvillus is formed of fine microfilaments. III-Modifications of the plasma membrane at the cell base ❑ A) Basal infoldings: Membranous infoldings at the base of certain cells are present to help in increasing the surface of the membrane for effective basal flow of material as in the cells of kidney tubules. ❑ B) Hemidesmosomes: are present at the basal membrane of cells to bind epithelial cells to the underlying basal lamina. Dr. Manal M. Sayed Fig. 12 showing basal infoldings (a) and hemidesmosome (b). Self-assessment. Choose the correct answer. 1. The powerhouse of the cell is a) The SER b) The RER c) Mitochondria d) Golgi bodies 13 2. Modification of proteins occurs in a) The SER b) The ribosome c) Mitochondria d) Golgi bodies 3. Which of the following organelles contains digestive enzymes that breakdown complex molecules? a) Golgi bodies b) Mitochondria c) Lysosomes d) Rough endoplasmic reticulum. 4. Which of these are functions of microfilaments? a) Formation of cellular processes and folds. b) formation of flagella and cilia c) protein secretion d) formation of lysosomes Enumerate A) Functions of smooth endoplasmic reticulum. B) Types of lipids in plasma membrane. C) Functions of Golgi complex. D) Types of lysosomes. E) Non-membranous organelles. True or false: a) Lysosomes originate from Golgi bodies. ( ) b) SER has two membranes. ( ) c) Condensation and modification of proteins is the main functions of Golgi bodies. ( ) d) Rough endoplasmic reticulum is deeply basophilic. ( ) e) Silver stain is used for detection of mitochondria. ( ) 14