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Objectives

At the completion of this unit, students will be able to:

1. Describe the structure and functions of a cell
2. Discuss the process of cell division i.e. mitosis and meiosis.
3. Briefly discuss the importance of mitosis & meiosis.
4. Classify the tissues of the body on the basis of structure, location and function into the following four major types.
• Epithelial tissue
• Connective tissue
• Muscle tissue
• Nervous tissue

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Introduction to Cell

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• The cell is the smallest living unit of structure and function in the body, that combines to form tissues.
• Each cell can utilize nutrients, eliminate waste products, and reproduce to sustain life.
• Two types of cell; Prokaryotes & Eukaryotes.
• The discovery of cells by English scientist Robert Hooke in 1665.
• The nearly 100 trillion cells in an average adult can be classified into about 200 different cell types.
• Cell biology or cytology is the study of cellular structure and function.

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Describe the structure and functions of a cell

Objective 01

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• The human cell is structurally organized into three primary components: 
• Plasma membrane
• Cytoplasm
• Nucleus

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Plasma membrane

• The plasma membrane forms the cell’s flexible outer surface, separating the internal environment from the external environment.
• It acts as a selective barrier.
• Regulating the flow of materials into and out of the cell, helping maintain the appropriate environment for cellular activities.
• Facilitates communication among cells and with the external environment.

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Structure of the Plasma Membrane

• The plasma membrane’s basic structural framework is the lipid bilayer, composed of two back-to-back layers of three types of lipid molecules: phospholipids, cholesterol and glycolipids.
• Phospholipids: make up about 75% of the membrane lipids, contain phosphorus.
• Cholesterol: Account for approximately 20% of the membrane lipids, A steroid with an attached hydroxyl (oh) group.
• Glycolipids: Constitute about 5% of the membrane lipids, Lipids with attached carbohydrate groups.

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Cytoplasm

• The cytoplasm includes all cellular contents between the plasma membrane and the nucleus.
• It consists of two components cytosol & organelles.
• Cytosol: (intracellular fluid) the fluid portion containing water, dissolved solutes, and suspended particles.
• Organelles: such as the cytoskeleton, ribosomes, endoplasmic reticulum, golgi complex, lysosomes, peroxisomes, and mitochondria, each with specific shape & functions.

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• The cytoskeleton is a network of protein filaments extending throughout the cytosol.
• Maintains cell shape, supports organelles, and enables movement.
• Composed of three types filaments:
1. Microfilaments (thinnest)
2. Intermediate filaments
3. Microtubules (thickest)

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1. Microfilaments

• Structure:
• Thinnest elements of the cytoskeleton.
• Composed of actin and myosin proteins.
• Located mostly at the cell’s edge.
• Functions:
• Generate movement (muscle contraction, cell division, locomotion).
• Provide mechanical support.
• Anchor the cytoskeleton to plasma membrane proteins.
• Support microvilli, increasing surface area for absorption.

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2. Intermediate Filaments

• Structure:
• Thicker than microfilaments, thinner than microtubules.
• Made of various proteins (e.g., keratin).
• Functions:
• Provide mechanical strength to the cell.
• Stabilize organelle positions, such as the nucleus.
• Help in cell-to-cell attachment.

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3. Microtubules

• Structure:
• Largest cytoskeletal components.
• Hollow tubes made of tubulin protein.
• Originate from the centrosome and extend toward the cell periphery.
• Functions:
• Determine cell shape.
• Enable organelle movement (e.g., vesicles, chromosomes during division).
• Support cilia and flagella, aiding in movement.

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Centrosome

• A structure near the nucleus. Main microtubule-organizing center of the cell, it plays a key role in cell division.
• Components of the Centrosome
• Centrioles
• A pair of cylindrical structures
• Each centriole has nine clusters of three microtubules (triplets) in a circular pattern
• Pericentriolar Material
• Surrounds the centrioles

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• Functions of the Centrosome
• Organizes the mitotic spindle during cell division
• Helps in the formation of cilia & flagella
• Regulates microtubule growth, essential for maintaining cell shape and intracellular transport.

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Cilia & Flagella

• Cilia & Flagella are motile projections of the cell's surface that extending from the plasma membrane.
• Composed of microtubules.
• Play a key role in cell movement & movement of substances.
• Both have a common framework called the axoneme.

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• Cilia
• Short, hair-like & numerous
• Move in a wave-like, coordinated motion
• Function: Moves substances along the cell surface.
• Example: Cilia in the respiratory tract sweep mucus & dust out of airways.
• Flagella
• Longer, whip-like & fewer in number
• Moves in a propelling (whip-like) motion
• Function: Moves the entire cell
• Example: Sperm cell flagellum helps in swimming toward the egg

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Ribosomes

• Tiny, spherical organelles made of ribosomal RNA (rRNA) & proteins.
• Function; as the sites of protein synthesis.
• Composed of two subunits:
1. Small subunit
2. Large subunit
• Subunits join together during translation.

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• Ribosomes found in two main locations: Free ribosomes & Bound ribosomes

Free ribosomes:

• Suspended in the cytosol
• Synthesize proteins that function within the cytosol

Bound ribosomes:

• Attached to the rough ER (RER) & nuclear envelope
• Synthesize proteins for:
• Plasma membrane insertion
• Secretion outside the cell
• Incorporation into lysosomes

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Endoplasmic Reticulum (ER)

• The endoplasmic reticulum (ER) is a large network of membranes, shaped like flat sacs or tubes.
• It spreads throughout the cytoplasm and is connected to the nuclear envelope.
• Two types of endoplasmic reticulum:
1. Rough ER
2. Smooth ER

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Rough Endoplasmic Reticulum (RER)

Structure:

• Has ribosomes on its surface, Appears “rough” under a microscope

Functions: Synthesizes proteins for:

• Secretion outside the cell
• Insertion into the plasma membrane
• Use in organelles like lysosomes
• Modifies and folds newly made proteins

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Smooth Endoplasmic Reticulum (SER)

• Structure:
• Lacks ribosomes, has a more tubular structure
• Functions:
• Synthesizes lipids (phospholipids & steroids)
• Detoxifies drugs & harmful substances (especially in liver cells)
• Stores & regulates calcium ions (important for muscle contraction & other cellular functions).
• Together, they ensure proper synthesis, modification, and transport of essential molecules.

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Golgi Complex (Golgi Apparatus)

• Membrane-bound organelle made of flattened sacs called cisternae
• Located near the nucleus.
• Functions as the cell’s "post office“.
• Modifies, sorts, and packages proteins & lipids for transport to their final destinations.
• Three main regions:
1. Cis face: Receives proteins & lipids from the rough ER
2. Medial cisternae: enzymatically modifies molecules (glycosylation, phosphorylation)
3. Trans face: Packages & sends products in vesicles for export.

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Functions of Golgi Complex

• Processes & modifies proteins and lipids
• Produces different types of vesicles:
1. Secretory vesicles – Deliver proteins to the plasma membrane for exocytosis.
2. Membrane vesicles – Contribute to the plasma membrane itself.
3. Transport vesicles – Carry enzymes to lysosomes.

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Lysosomes

• Lysosomes are membrane-bound organelles that contain strong digestive enzymes. These enzymes work best in an acidic environment, which is maintained by proton pumps.
• They are formed from the Golgi complex and act as the cell’s recycling centers. Lysosomes digest materials brought in by endocytosis (like bacteria or debris) and recycle damaged organelles through autophagy.
• They also help in autolysis, where the cell destroys itself when damaged or during development.
• Lysosomes are essential for cellular homeostasis by breaking down macromolecules, removing waste, and recycling useful parts to keep the cell healthy and efficient.

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Peroxisomes

• Peroxisomes are small, membrane-bound organelles that carry enzymes for important metabolic reactions.
• They are self-replicating and form from existing peroxisomes, unlike lysosomes which come from the Golgi complex.
• They detoxify harmful substances like hydrogen peroxide (H₂O₂ (a byproduct of cellular metabolism)) using the enzyme catalase, which breaks it down into water and oxygen to preventing cellular damage.
• Peroxisomes also help in:
• Oxidation of fatty acids (a process that generates energy)
• Synthesis of special lipids, such as those needed for myelin sheaths in nerve cells
• They are especially found in liver and kidney cells, where they detoxify alcohol and other harmful chemicals.
• Peroxisomes are vital for maintaining cellular health and metabolic balance.

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Mitochondria

• Mitochondria are double-membrane-bound organelles known as the "powerhouses" of the cell because they produce most of the cell’s ATP, which provides energy for various cellular activities.
• They are usually oval or round in shape, and their number varies based on the energy needs of the cell.
• Outer membrane: smooth and acts as a barrier between the cytosol and the inner environment of the mitochondrion.
• Inner membrane: membrane is highly folded into structures called cristae, to increase surface area for biochemical reactions.
• Matrix: The space inside the inner membrane with enzymes, mitochondrial DNA, and ribosomes, helping mitochondria replicate and make their own proteins.

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• Mitochondria carry out aerobic respiration, which includes:
• Citric acid cycle (Krebs cycle)
• Electron transport chain
• These reactions break down glucose using oxygen to produce ATP.
• They also help in:
• Apoptosis (programmed cell death)
• Calcium storage
• Metabolism regulation
• Mitochondria are unique because they have their own DNA (inherited from the mother) and can replicate independently.
• Their vital roles make them essential for the survival and proper functioning of the cell.

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The Nucleus

• The nucleus is a large, membrane-bound organelle, that controls cell activities by housing most of the cell’s genetic material in the form of DNA and directing protein synthesis. It is essential for cell growth, division, and function.
• Components of the nucleus:
• Nuclear Envelope
• Nucleoplasm
• Chromatin
• Nucleolus

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1. Nuclear Envelope:The nucleus is enclosed by a double membrane, which separates it from the cytoplasm. It has pores that control the exchange of RNA and proteins between the nucleus and cytoplasm.
2. Nucleoplasm: A gel-like substance that supports the diffusion of ions, molecules, and enzymes needed for nuclear functions.
3. Chromatin: DNA in the nucleus is arranged as chromatin, a mix of DNA and proteins. During cell division, it condenses into chromosomes to ensure accurate genetic replication and distribution.
4. Nucleolus: The nucleolus is a dense structure inside the nucleus where ribosomal RNA (rRNA) is made and ribosome subunits are assembled.

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Functions of the Nucleus

1. Protection and Storage of DNA: The nucleus keeps the cell’s DNA safe and protected, maintains its stability, and prevents it from getting damaged.
2. Control of Gene Expression: The nucleus controls which genes are activated and when, helping in the production of proteins that are essential for the cell.
3. Coordination of Cell Activities: The nucleus gives instructions for the formation of proteins and RNA, which help regulate cell growth, metabolism, and division.

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4. Ribosome Production (in the Nucleolus): Inside the nucleus, there's a part called the nucleolus that produces rRNA and assembles parts of ribosomes. These ribosomes later move to the cytoplasm to help in protein synthesis.
1. The nucleus is the most essential part of the cell—it not only organizes cell activities but also accurately passes on genetic information when cells divide.

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Discuss the process of cell division i.e. mitosis and meiosis

Objective 02

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• Most cells in the human body undergo cell division, there are two main types:
• Somatic Cell Division
• Reproductive Cell Division

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Somatic Cell Division

• Somatic cell division, also referred to as mitosis, is the process by which a single parent cell divides into two genetically identical daughter cells.
• Essential for growth, repair, and tissue maintenance
• Phases of Somatic Cell Division (Mitosis)
1. Interphase
2. M Phase

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Interphase - Preparation for Division

• G1 Phase (Growth 1): Cell grows & performs normal functions
• S Phase (Synthesis): DNA replication occurs
• G2 Phase (Growth 2): Cell prepares for mitosis, checks for errors

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M Phase (Mitosis + Cytokinesis)

• Mitosis - nuclear division & consists of four stages:
• Prophase:
• Chromatin condenses into chromosomes
• Nuclear envelope dissolves
• Spindle fibers form
• Metaphase:
• Chromosomes align at the metaphase plate
• Anaphase:
• Sister chromatids separate & move to opposite poles
• Telophase:
• Chromosomes decondense into chromatin
• Nuclear envelopes reform

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• Cytokinesis - Cytoplasmic Division
• Begins in late anaphase or telophase
• Cleavage furrow forms, dividing cytoplasm
• Results in two genetically identical daughter cells

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Reproductive Cell Division

• Reproductive cell division, or meiosis, is the process by which gametes (sperm and egg cells) are produced.
• Reduces chromosome number from diploid (2n) to haploid (n).
• Ensures genetic diversity through recombination & independent assortment.
• Two Stages of Meiosis with distinct stages.
1. Meiosis I
2. Meiosis II

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Meiosis I - (Reduction Division) – Separates homologous chromosomes

• Prophase I:
• Homologous chromosomes pair up (synapsis).
• Crossing-over occurs, increasing genetic diversity.
• Nuclear envelope dissolves, spindle fibers form.
• Metaphase I:
• Homologous chromosome pairs align randomly at the metaphase plate (independent assortment).
• Anaphase I:
• Homologous chromosomes are pulled apart to opposite poles.
• Sister chromatids remain attached.
• Telophase I & Cytokinesis:
• Cytoplasm divides, forming two haploid cells (n).

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Meiosis II - (Equational Division) – Separates sister chromatids

• Prophase II:
• Chromosomes condense, spindle fibers form.
• Nuclear envelope dissolves (if reformed in Telophase I).
• Metaphase II:
• Chromosomes align at the metaphase plate.
• Anaphase II:
• Sister chromatids separate and move to opposite poles.
• Telophase II & Cytokinesis:
• Nuclear envelopes reform, cytoplasm divides.
• Four haploid gametes (n) are produced.

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Briefly discuss the importance of mitosis & meiosis

Objective 03

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Importance of Mitosis

• Mitosis is a fundamental biological process for growth, tissue repair, and asexual reproduction in multicellular organisms.
• It allows cells to divide and produce identical copies, ensuring proper development (e.g., a child’s height increase or wound healing).
• This process maintains genetic consistency because daughter cells receive exact DNA copies.
• Mitosis also enables asexual reproduction in and single-celled organisms like bacteria.

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Importance of Meiosis

• Meiosis is essential for sexual reproduction. It produces gametes (sperm and egg) with half the normal chromosomes (23 instead of 46), so fertilization restores the correct number.
• This process creates genetic diversity through crossing-over and random assortment, making each offspring unique. Such variation helps species adapt to environmental changes and drives evolution.

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Classify the tissues of the body on the basis of structure, location and function into the following four major types

Objective 04

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• A tissue is a group of cells that usually have a common origin in an embryo and function together to carry out specialized activities.

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• Histology (Histo = tissue; -logy = study of) is the science that deals with the study of tissues.

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• Body tissues can be classified into four basic types according to their structure and function.
1. Epithelial tissue
2. Connective tissue
3. Muscular tissue
4. Nervous tissue

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• Epithelial tissue covers body surfaces and lines hollow organs, body cavities, and ducts (e.g., skin, digestive tract). it also forms glands.
• This tissue allows the body to interact with both its internal and external environments.
• Epithelial tissues are classified based on two features:
1. Arrangement of cells in layers
2. Shape of the cells

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• Arrangement of cells in layers: The cells are arranged in one or more layers depending on function:
1. Simple epithelium
2. Pseudostratified epithelium (pseudo- _ false)
3. Stratified epithelium (stratum _ layer)

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Arrangement of cells in layers

• a. Simple Epithelium:
• One single layer of cells
• Functions: diffusion, osmosis, filtration, secretion, absorption
• Secretion = releasing substances like mucus, sweat, enzymes
• Absorption = taking in fluids or digested food from the intestine

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Arrangement of cells in layers

• b. Pseudostratified Epithelium:
• Looks like many layers because nuclei are at different levels
• Actually one layer; all cells touch the basement membrane
• Some cells reach the surface and may have cilia
• Goblet cells release mucus

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Arrangement of cells in layers

• c. Stratified Epithelium:
• Two or more layers of cells
• Protects tissues under it
• Found where there is wear and tear

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Cell Shapes

• a. Squamous Cells:
• Flat and thin
• Allow fast passage of substances
• b. Cuboidal Cells:
• Cube or hexagon shape (equal height and width)
• May have microvilli
• Function: secretion or absorption

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Cell Shapes

• c. Columnar Cells:
• Taller than they are wide
• Protect tissues
• Apical surface may have cilia or microvilli
• Also help in secretion and absorption
• d. Transitional Cells:
• Can change shape: squamous ↔ cuboidal
• Found in organs like the urinary bladder that stretch and shrink

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• When we combine the two characteristics (arrangements of layers and cell shapes), we come up with the following types of epithelial tissues:

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I. Simple epithelium

A. Simple squamous epithelium

B. Simple cuboidal epithelium

C. Simple columnar epithelium (nonciliated and ciliated)

D. Pseudostratified columnar epithelium (nonciliated and ciliated)

II. Stratified epithelium

A. Stratified squamous epithelium (keratinized, when surface cells are dead and become hardened, and nonkeratinized, when surface cells remain alive)

B. Stratified cuboidal epithelium*

C. Stratified columnar epithelium*

D. Transitional epithelium

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Reference

• Tortora, Gerard J. and Bryan Derrickson. Principles of Anatomy & Physiology. 14th ed., Wiley, 2014.

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If you have any…!

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Questions…?

or

Confusion…?

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Good luck

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