Cell Biology Overview

Cells

• Basic structural and functional units of all living organisms
• Primary formed by carbon, oxygen, hydrogen, and nitrogen
• All cells are created by cell division. After division, they differentiate into different cell types for their purpose
• Classified by size, shape and function
• Each cell maintains homeostasis at the cellular level
• 2 main types of cells in the human body
  • Sex Cells (Germ Cells)
    • Reproductive cells, reproduce by meiosis
    • Examples: Male sperm, Female oocyte
  • Somatic Cells
    • All body cells except sex cells, reproduce by mitosis

Cell Structure

• Consists of 3 main parts
  • Plasma membrane
    • Sturdy yet flexible outer surface
    • Selective barrier, important in cell communication
  • Cytoplasm
    • Cytosol
    • Organelles
  • Nucleus
    • Cell control center
    • Contains heredity information

Plasma Membrane

• A semipermeable lipid bilayer found outside the cytoplasm of a cell and surround the inner contents
• Transport mechanisms:
  • Simple diffusion
  • Facilitated diffusion
  • Osmosis

Passive Transport

• Simple Diffusion
  • Movement of molecules from a region of high concentration to a low concentration region, down the concentration gradient
  • Membrane may not be needed
  • Factors that speed up the diffusion rate:
    • Increased temperature, light, and small particles
    • Increase in membrane surface area
    • Steeper the concentration gradient

Diffusion Across the Plasma Membrane

• In the body, diffusion occurs across the lipid bilayer
  • Exchange of O2 and CO2 between blood and cells, between blood and air within lungs
  • Absorption of lipid-soluble molecules
  • Release waste from body cells
• Diffusion occurs through pores of channels
  • Examples:
    • Pinocytosis (Cell drinking)
    • Phagocytosis (Cell eating)
    • Receptor-mediated endocytosis
    • Exocytosis

Vesicular Transport

• Endocytosis
  • Tiny droplets of extracellular fluid are taken up
  • The cell engulfs large solid molecules, like bacteria, dust
  • Vesicle is called phagosome
• Receptor-mediated endocytosis
  • A more selective form of phagocytosis or pinocytosis
  • Minimum unnecessary matter is taken
• Exocytosis
  • Materials move out of a cell by fusion with the plasma membrane of vesicular inside the cell
  • Essential in:
    • Secretory cells that secrete digestive enzymes, hormones, mucus or other secretions
    • Nerve cells that release neurotransmitters

Cytoplasm

• Consists of all cellular contents between the plasma membrane and the nucleus
• Components:
  • Cytosol (Intracellular fluid)
    • Site of many chemical reactions
  • Organelles (tiny structures that perform different functions in the cell)

Organelles

• Organelles with Membrane
  • Endoplasmic reticulum
  • Golgi apparatus
  • Mitochondria
  • Lysosomes
  • Peroxisomes
• Non-membranous organelles
  • Centrosome & Centrioles
  • Cytoskeleton
  • Cilia & Flagella
  • Microvilli
  • Ribosome

Endoplasmic Reticulum (ER)

• System of interconnected channels
• Extends from the nuclear membrane throughout the cytoplasm, more than half of membrane surface within the cytoplasm of the cell
• 2 forms of endoplasmic reticulum (ER)
  • Rough ER
    • Covered with ribosomes
    • Functions:
      • Involved in protein synthesis
      • Attach sugar groups to proteins
      • Proteins are bound in vesicles for transport to Golgi apparatus
      • External face synthesizes phospholipids
  • Smooth ER
    • No ribosomes
    • Functions:
      • Involved in the synthesis of fatty acid and steroid
      • Detoxification of drugs/toxic
      • Calcium storage

Mitochondria

• Rod-like organelle surrounded by a double membrane
• The powerhouse of a cell
• Generate ATP to provide energy through aerobic respiration
• A cell may have thousands of mitochondria, depending on its activity

Golgi Apparatus/Complex

• Small flattened membrane and associated vesicles close to the nucleus
• Cupcake shape
• Functions:
  • Synthesis of carbohydrates
  • Package, modifies, and segregates proteins for secretion from the cell, inclusion of lysosomes, and incorporation into the plasma membrane

Lysosome & Peroxisomes

• Lysosome
  • Membrane-enclosed vesicles
  • Form from the Golgi apparatus
  • Contain digestive and hydrolytic enzymes
  • Break down a wide variety of molecules
  • Help recycle worn-out cell structures
  • Removal of pathogens
• Peroxisomes / Microbodies
  • Structurally similar to lysosomes but smaller
  • Contain oxidases- enzymes that can break down fats and other organic compounds
  • Neutralize toxic compounds
  • Abundant in liver and kidney
  • In mitochondria, peroxisomes decompose fatty acids to generate energy for ATP synthesis

Centrosome & Centrioles

• Centrosome
  • Consists of 2 hollow cylinders called centrioles
  • Located near the Golgi apparatus and nucleus
  • Contain a pair of centrioles at the right angle
  • Form mitotic spindle- essential for the movement of chromosomes during cell division
  • Form the basal bodies found at the base of cilia and flagella
• Centrioles
  • Located near the Golgi apparatus and nucleus
  • Contain a pair of centrioles at the right angle
  • Form mitotic spindle- essential for the movement of chromosomes during cell division
  • Form the basal bodies found at the base of cilia and flagella

Microvilli, Cilia & Flagella

• Microvilli
  • Finger-like projections that increase the cell's surface area
  • Found in cells that absorb nutrients
• Cilia
  • Short, hair-like projections
  • Move in a coordinated way to move fluids or particles across the cell's surface
  • Found in cells that line the respiratory tract
  • Help move mucus and trapped debris out of the lungs
• Flagella
  • Similar structure with cilia but much longer
  • Generate forward motion of the entire cell
  • Example: Sperm cells

Gene Mutation

• A permanent alteration in the DNA sequence that makes up a gene
• May occur due to:
  • Errors in DNA replication
  • Exposure to mutagens (e.g., radiation, chemicals)
• Types of gene mutation:
  • Deletion: One or more bases are deleted from the DNA sequence
  • Insertion: One or more bases are added to the DNA sequence
  • Substitution: One base is replaced by another base
• Effects of gene mutation
  • Proteins produced may be beneficial or harmful, or may have no effect at all
  • The effect can be positive, negative or neutral

Protein Package, Processing, & Secretion

• Ribosomes translate mRNA into a polypeptide chain
• The polypeptide chain is then transported to the endoplasmic reticulum (ER)
  • Modification occurs:
    • Folding
    • Attachment of sugar groups
• The modified polypeptide chain is then transported to the Golgi apparatus
  • Further processing occurs:
    • Packing into vesicles
    • Sorting and packaging
• The vesicles containing the protein are then transported to the plasma membrane and released from the cell (Exocytosis)

Cell Cycle

• A cell's life from it forms until it divides consists of a series of phases called cell cycle
• Phases:
  • Interphase
  • Mitosis
  • Cytokinesis (Cytoplasmic division)

Types of Cell Division

• Somatic Cell Division
  • Any cell of the body except sex cells
  • Undergo nuclear division (Mitosis) and cytoplasmic division (Cytokinesis)
  • Produce 2 genetically identically cells- same number and kind of chromosomes as the original cell, i.e. 23 pairs of chromosomes cell (n=46) called Diploid cells
• Reproductive Cell Division
  • Reproductive Cell Division
  • Undergo 2-step division called Meiosis
  • The number of chromosomes in the nucleus is reduced by half (n=23) to become Haploid cells
  • Called Haploid cells

Cell Cycle- Interphase

• Interphase consists of 4 phases:
  • G0 Phase: Resting phase
  • G1 Phase: Organelle duplication and protein synthesis occur
  • S Phase: Cell duplicate centrioles and conduct DNA replication
  • G2 Phase: Cell continues growing to:
    • Makes more organelles
    • Finishes replicating centrioles
    • Synthesis enzymes for cell division
    • Checks DNA and repairs any errors

DNA Replication

• Occurs in S phase
• DNA helix unwound by helicases into single template strand and expose the nitrogenous bases, ATP required
• DNA polymerase read the exposed short segments of nitrogenous bases and assemble the sequence of the new strand with complementary nitrogenous bases with the RNA primers
• DNA ligase joins the short segments together

Mitosis (M Phase)

• Happens in the nucleus
• Functions: Growth, repair, replace, wore out cells
• The replicated chromatin becomes tightly coiled and easier to see under the microscope
• Consists of 4 sub-phases:
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Sub-phases:
  • Early Prophase: Chromatin fibers condense and shorten into chromosomes, centrioles get pushed apart to each pole of the cell by spindle fibers, spindle fibers attached to the centromere on the chromosomes
  • Late Prophase: Nucleoli disappear and the nuclear envelope breaks down
  • Metaphase: The chromatids align on the center of the spindle fibers, attached by their centromeres
  • Anaphase: Centromeres split by enzyme, sister chromatids separated and move toward opposite poles of the cell, chromatids become chromosomes after separated
  • Telophase: Chromosomes are in opposite ends, uncoil and revert to chromatin form, nuclear envelope reform, nucleoli reappear in the identical nuclei, mitotic spindle breaks up
• Following telophase, cytokinesis occurs- the cytosol, intracellular organelles and plasma membrane split to form 2 identical daughter cells

Meiosis

• Sex cells are produced through Meiosis
• Involves 2 cell division instead of one, and produces four genetically unique cells rather than two identical cells
• Sex cells are haploid cells that contain only half of the full set of 46 chromosomes
• Because of haploid and genetically unique, it can combine with another sex cell during fertilization to create offspring with genetic variation
• Detailed process discussed in the reproductive system

Cell Differentiation

• All cells in the human body comes from stem cells, through the processes of mitosis and differentiation- process enables cells to specialize and become mature cells

Tissues

• Groups of similar cells working together to perform a particular function
• 8 types of tissues:
  • Blood
  • Bone
  • Epithelial tissue
  • Cartilage tissue
  • Adipose tissue
  • Connective tissue
  • Muscle tissue
  • Nervous tissue

Epithelial Tissue

• Cover body surface
• Line hollow organs, body cavities and ducts
• Form glands and membranes

Classification of Epithelial Tissue

• Classified by:
  • Number of cell layers
  • Cell shape

Simple Epithelial Tissue

• Location:
  • Found in heart, blood and lymphatic vessels linings
  • Epithelial layer of peritoneum, pleura and pericardium
  • Located at glands, ducts, kidney tubules
  • Makes up secreting portion of thyroid gland and ducts of pancreas
  • Lining of stomach, guts, gall bladder, uterine tubes, collecting ducts of kidney
  • Ciliated: Lining of nasal cavity, trachea and bronchi
  • Non-ciliated: Lining of large ducts, epididymis, and part of male urethra
• Function:
  • Reduce friction
  • Control vessels permeability
  • Perform diffusion
  • Perform secretion
  • Secretion and absorption
  • Secretion, absorption, protection
  • Secrete mucus to trap foreign particles, cilia sweep away mucus for elimination from the body
  • Absorption and protection

Stratified & Transitional Epithelial Tissue

• Location:
  • Found in skin surface, linings of mouth, throat, oesophagus, rectum and vagina, covers tongue
  • Ducts of sweat glands and oesophageal glands
  • Part of male urethra
  • Part of urethra, large excretory ducts
  • Lines several parts of the urinary tract, including bladder
• Function:
  • Protect against abrasion, water loss, ultraviolet radiation, and foreign invasion
  • Form the first line of defense against microbes
  • Protection
  • Limited secretion and absorption
  • Protection and secretion
  • Variable appearance, subject to expansion

Glandular Epithelium

• Found in cells or organs that secret substances for use in other body parts
• Include Endocrine and Exocrine glands
• Endocrine
  • No ducts
  • Secrete directly into blood
  • Secretions are known as hormones
• Exocrine
  • Contact with the surface by means of duct- an epithelial tube that brings secretion to the surface,
  • Example:

Secretions

• Secretions are substances released to the body's surface or into the cavity or lumen of another organ
• Examples include sweat, tears, saliva and digestive enzymes

Membranes in the Body

• Line body cavities and cover the viscera (internal organs)

Cutaneous Membrane (Skin)

• The largest membrane in the body
• Dry

Mucous Membrane (Mucosa)

• Line all body cavities that open to the outside of the body
• Examples include the digestive tract, respiratory tract, and urogenital tract

Serous Membrane (Serosa)

• Internal membrane that covers organs and lines walls of body cavities
• Examples include the pleura, pericardium, and peritoneum

Connective Tissue

• Most abundant tissue and widely distributed
• Found between cells
• Functions:
  • Support and binds structures together
  • Store energy
  • Provide immunity to disease

Loose Connective Tissues

• Fill up space between organs
• Act as cushion and stabilize cells
• Surround and support blood vessels and nerves
• Store lipids
• Provide a route for diffusion of materials
• Examples include areolar, adipose tissue, finely woven reticular fibers

Dense Connective Tissues

• Contain elastic fibers
• Can tolerate cycles of extension and recoil
• Examples include:
  • Lung
  • Vessels
• Connects, provides strength and support
• Examples include:
  • Ligament
  • Tendon