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THE CELL

INTRODUCTION

  • Definition of Cell

    • The cell is defined as the functional and structural unit of the living body.

    • All living organisms are composed of cells, which exhibit the characteristics of life.

CHARACTERISTICS OF A CELL

  1. Needs nutrition and oxygen

  2. Produces its own energy

    • Necessary for growth, repair, and various activities.

  3. Eliminates waste

    • Removes carbon dioxide and other waste products.

  4. Maintains internal environment

    • Necessary for survival.

  5. Responds to invaders

    • Shows immediate response to the entry of bacteria and other toxins.

  6. Reproduces by division

    • Except for neurons, which do not reproduce.

TISSUES

  • Definition of Tissue

    • A group of cells with similar functions.

  • Major Types of Primary Tissues

    1. Muscle Tissue

    • Types: Skeletal muscle, smooth muscle, cardiac muscle.

    1. Nervous Tissue

    • Types: Neurons and supporting cells.

    1. Epithelial Tissue

    • Types: Squamous, columnar, and cuboidal epithelia.

    1. Connective Tissue

    • Types: Connective tissue proper, cartilage, bone, blood.

ORGANS

  • Definition of Organ

    • A structure formed by two or more primary types of tissues to execute specific functions.

  • Types of Organs

    1. Tubular or Hollow Organs

    2. Compact or Parenchymal Organs

  • Examples of Organs: Brain, heart, lungs, stomach, intestine, liver, gallbladder.

HUMAN BODY SYSTEMS

  • Definition of System

    • A group of organs that work together for specific functions.

  • Major Human Body Systems

    1. Excretory System - Eliminates unwanted substances.

    2. Cardiovascular System - Responsible for transport of substances between organs.

    3. Respiratory System - Supplies oxygen and removes carbon dioxide.

    4. Reproductive System - Involved in species reproduction.

    5. Endocrine System - Regulates growth and maintenance of normal life.

    6. Musculoskeletal System - Provides stability and movement.

    7. Nervous System - Controls locomotion and intellectual functions.

CELL STRUCTURE

  • Components of the Cell

    • Each cell consists of a cell body and a membrane called the cell membrane.

  • Sections of Cell Structure

    1. Cell Membrane

    2. Cytoplasm

    3. Nucleus

CELL MEMBRANE

  • Definition

    • The cell membrane, also known as the plasma membrane, is a protective sheath around the cell body.

  • Functions

    • Separates extracellular fluid (ECF) from intracellular fluid (ICF).

    • Semipermeable, allowing certain substances to cross between ECF and ICF.

  • Thickness: Ranges from 75 to 111 Ångströms.

  • Composition of Cell Membrane

    1. Proteins (55%)

    2. Lipids (40%)

    3. Carbohydrates (5%)

  • Structural Model:

    • The cell membrane is described as a unit membrane composed of a trilaminar structure observed under microscopy.

CELL MEMBRANE STRUCTURE MODELS

  1. Danielli-Davson Model (1935)

    • Proposed as a sandwich of lipids covered by proteins.

  2. Unit Membrane Model (1957)

    • Introduced by J.D. Robertson, emphasizing a trilaminar structure.

  3. Fluid Mosaic Model (1972)

    • Proposed by S.J. Singer and G.L. Nicholson.

    • Describes a fluid membrane with proteins floating in it, rather than a static sandwich.

LIPID LAYERS OF THE CELL MEMBRANE

  • Structure: Bilayered lipid layer.

  • Characteristics: Exists in a liquid state, promoting mobility within the membrane.

  • Types of Lipids

    1. Phospholipids:

    • Composed of hydrophilic heads and hydrophobic tails.

    • Arrangement allows for membrane fluidity.

    1. Cholesterol:

    • Interspersed among phospholipids, enhancing structural integrity.

FUNCTIONS OF LIPID LAYER

  • Acts as a semipermeable barrier.

  • Allows free passage of fat-soluble substances (e.g., O${2}$, CO${2}$, alcohol) while restricting water-soluble substances (e.g., electrolytes, glucose).

PROTEINS OF THE CELL MEMBRANE

  • Categories

    1. Integral Proteins (Transmembrane Proteins)

    • Pass through the membrane completely.

    • Examples: Adhesion proteins, channels, transporters.

    1. Peripheral Proteins

    • Loosely attached to the membrane surfaces.

    • Examples: Cytoskeletal proteins.

  • Functions:

    • Structure and integrity of membrane, facilitation of transport, receptors for signaling, and acting as enzymes.

CARBOHYDRATES OF THE CELL MEMBRANE

  • Carbohydrates' Role

    • Form glycoproteins and glycolipids, contributing to the glycocalyx.

  • Functions:

    1. Negative charge to regulate ion movement.

    2. Cell adhesion properties.

    3. Receptors for hormones.

FUNCTIONS OF THE CELL MEMBRANE

  1. Protective barrier for the cytoplasm and organelles.

  2. Selective permeability to substances.

  3. Nutrient absorption.

  4. Waste excretion.

  5. Gas exchange (oxygen in, carbon dioxide out).

CYTOPLASM

  • Description: Jelly-like material constituting 80% water.

    • Contains cytosol and various organelles.

  • Zones of Cytoplasm

    1. Ectoplasm: Periphery just beneath the cell membrane.

    2. Endoplasm: Inner part between ectoplasm and nucleus.

ORGANELLES IN CYTOPLASM

  • Types of Organelles:

    1. Organelles with Membrane:

    • Examples: Endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, nucleus.

    1. Organelles without Membrane:

    • Examples: Ribosomes, cytoskeleton.

ENDOPLASMIC RETICULUM (ER)

  • Definition: Network of tubular structures with a limiting membrane.

  • Types:

    1. Rough ER: Studded with ribosomes; responsible for protein synthesis.

    2. Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

  • Functions of Rough ER:

    • Protein synthesis, degradation of worn-out organelles.

FUNCTIONS OF SMOOTH ER

  1. Synthesis of non-protein substances (e.g., steroids).

  2. Role in metabolic processes.

  3. Storage and metabolism of calcium.

  4. Detoxification of harmful substances.

GOLGI APPARATUS

  • Description: Membrane-bound organelle for processing proteins.

  • Structure: Composed of 5 to 8 flattened membranous sacs (cisternae).

  • Functions:

    • Processing and packaging of proteins and lipids; often termed the "post office of the cell".

LYSOSOMES

  • Definition: Membrane-bound vesicles that contain hydrolytic enzymes.

  • Types:

    1. Primary Lysosomes: Inactive form.

    2. Secondary Lysosomes: Active form after fusion with phagosomes or endosomes.

  • Functions: Often referred to as the "garbage system" of the cell.

FUNCTIONS OF LYSOSOMES

  1. Degradation of macromolecules through endocytosis.

  2. Removal of worn-out organelles.

  3. Secretion of enzymatic products.

  4. Specific roles in the immune response involving secretory lysosomes.

PEROXISOMES

  • Definition: Membrane-limited vesicles involved in oxidative reactions.

  • Functions:

    1. Breakdown of fatty acids.

    2. Detoxification of hydrogen peroxide.

CENTROSOME AND CENTRIOLES

  • Definition: Organelles facilitating chromosome movement during cell division.

SECRETORY VESICLES

  • Description: Contain secretory substances; formed from ER and processed by the GA.

MITOCHONDRIA

  • Structure: Membrane-bound organelle, oval or rod-shaped; has a bilayer membrane.

  • Function: Produces energy through the oxidation of nutrients and synthesis of ATP.

FUNCTIONS OF MITOCHONDRIA

  1. Energy production: Utilizes digested foods.

  2. ATP synthesis via oxidative phosphorylation.

  3. Regulation of apoptosis.

RIBOSOMES

  • Description: Granular organelles without membranes, made of proteins and RNA.

  • Function: Primary role in protein synthesis.

CYTOSKELETON

  • Description: Network of fibrous structures maintaining cell shape and facilitating movement.

  • Components:

    1. Microtubules: Hollow tubes providing structural support.

    2. Intermediate Filaments: Provide tensile strength.

    3. Microfilaments: Responsible for muscle contraction and cell motility.

NUCLEUS

  • Definition: Prominent organelle containing genetic material; essential for cell regulation.

  • Structure: Enclosed by a double-layered nuclear membrane, containing nucleoplasm, chromatin, and nucleolus.

FUNCTIONS OF NUCLEUS

  1. Control cellular activities: metabolism, protein synthesis, growth, division.

  2. Synthesis and transport of RNA.

  3. Storage and transformation of hereditary information.

Note

  • All cells with nuclei are eukaryotes, while those without are prokaryotes.