Introduction to Zoology and Cell Biology

Definitions of Zoology

• Zoology is the branch of biology focused on the science of animals
• The term originates from the Ancient Greek words "zo" meaning animal, and "ology" meaning the study of knowledge

The Cell

• All living things consist of cells
• A cell represents the most basic structural and functional unit in organisms
• Cells perform functions necessary to sustain life
• Cytology examines the structure/function of body cells
• Typically, a cell includes cytoplasm, protoplasm and a plasma membrane
• Protoplasm is simply cytoplasm combined with the nucleus

Cytoplasm

• Cytoplasm fills the space between the plasma membrane and nucleus
• Cytoplasm is a gel-like fluid containing organelles, inclusions, and the cytosol
• Organelles/inclusions are structures embedded in the cytoplasm and perform jobs for the cell
• Cytosol is fluid in the cytoplasm holding the organelles
• Cytoplasm contains sugars, amino acids, and proteins

Cell Organelles

• Organelles are small, living structures within the cytoplasm
• Organelles perform specific functions for the cell
• Organelles are present in almost all eukaryotic cells

Membranous Organelles

• These organelles are surrounded by a membrane
• Mitochondria
• Endoplasmic reticulum
• Golgi apparatus
• Lysosomes
• Peroxisomes
• Proteosomes

Non-Membranous Organelles

• These organelles are not surrounded by a membrane
• Ribosomes
• Cytoskeleton
• Centrioles

Endoplasmic Reticulum

• The endoplasmic reticulum is a complex network within the cytoplasm
• It consists of interconnected channels called cisternae
• Tubules connect cisternae together
• Tubules/cisternae branch out giving a network like appearance
• Cells contain two forms of endoplasmic reticulum (ER): rough (R.E.R) , and smooth (S.E.R)

Rough Endoplasmic Reticulum (RER)

• The name "rough" or "granular" comes from ribosomes on its surface
• RER is made up of parallel, flattened sacs covered with ribosomes
• RER connects with the outer surface of the nuclear envelope
• High protein synthesis occurs in cells with more RER, such as pancreatic and plasma cells
• RER facilitates the synthesis, storage, and transport of proteins

Smooth Endoplasmic Reticulum (SER)

• The name is smooth or agranular because it has no ribosomes surface
• Cisternae in SER are more tubular/branching
• Cisternae are thought to connect with the RER
• It synthesizes lipids and steroid hormones like testosterone and estrogen

Functions of the Endoplasmic Reticulum

• Synthesizes steroids and other lipids
• Synthesizes proteins
• Produces phospholipids and proteins of the plasma membrane
• Manufactures cell membranes
• Detoxifies alcohol and other drugs, particularly in liver and kidney cells

Ribosomes

• Ribosomes are small and oval, without a membrane
• Each ribosome has a large and small subunit
• Proteins and ribosomal RNA (rRNA) are contained within ribosomes
• Large and small subunits exist independently in the nucleus
• The subunits then leave the nucleus and join in the cytoplasm as free ribosomes or attached to the RER
• The main ribosome function is to synthesize proteins

Golgi Apparatus

• The Golgi apparatus is known as the Golgi complex, or Golgi body
• Camillo Golgi, an Italian biologist, discovered it in 1897 in nerve cells
• The Golgi apparatus consists of 3-20 flattened, membranous sacs (cisternae) and transfers vesicles
• The Golgi has two faces: convex (cis/immature) and concave (trans/mature)
• Some vesicles become lysosomes
• Some vesicles migrate to the plasma membrane and fuse into it
• Some vesicles become secretory and store a protein product for future release
• It receives newly synthesized proteins from RER
• The Golgi apparatus sorts, cuts, splices, and packages proteins in membrane-bound vesicles

Lysosomes

• Membranous spherical vesicles created by the Golgi complex
• Digestive (hydrolytic) enzymes are contained in it
• High phagocytic activity in cells increased the number of lysosomes, like macrophages
• The main function is to digest macromolecules and assist in defense against invading organisms
• These can kill bacteria and viruses

Peroxisomes (Microbodies)

• These resemble lysosomes, but with different enzymes
• Catalase and oxidase (oxidative enzymes) are present
• Peroxisomes are in all cells, mainly liver and kidney
• Functions:
  • Oxidizes long chained of fatty acids and transforms it to Acetyl CoA + H2O2
  • Hydrogen peroxide detoxifies harmful substances, e.g. alcohol
  • Protects cells from hydrogen peroxide (H2O2)

Proteasomes

• Hollow and cylindrical organelles contain protease (proteolytic enzymes)
• Degrades damaged proteins by breaking them into amino acids and small peptides

Mitochondria

• Present in all eukaryotic cells
• Vary in their size, number and shape, some are spherical and some are rod-like.
• Also called the "powerhouse" since it produces energy via ATP synthesis
• It is surrounded by double membrane.
  • The inner membrane has folds (cristae)
  • The space between cristae is called the matrix
  • The matrix contains ribosomes and enzymes for ATP synthesis, as well as small circular mitochondrial DNA (mtDNA)

The Cytoskeleton

• It is a network of protein filaments existing throughout the entire cytosol
• It maintains cell shape, cell content, organizes, and assists in cell movement
• It consists of:
  • Microtubules
  • Thin filaments (microfilaments)
  • Intermediate filaments

Centrioles

• Occur as non-membranous organelles in most animal cells
• Near the nucleus is where you find cylindrical structures called centrioles in the centrosome)
• Each centriole has nine clusters of three microtubules (triplets) oriented in a circle
• Cell division is an important function of centrioles

Nucleus

• The nucleus is the most distinct and largest component
• Cellular functions and cell divisions are controlled by it
• The nucleus is made of:
  • Nuclear envelope: The double (inner and outer) nuclear envelopes contain nuclear pores
  • Nucleolus: Each nucleus contains generally one or two nucleoli, which contain proteins
  • Nucleoplasm: semifluid containing: enzymes, RNA, and DNA
  • Chromatin: Two types euchromatin and heterochromatin

Plasma Membrane

• The limiting membrane that surrounds/separates the cell from the extracellular environment
• Under an electron microscope (E.M.) there appears to be two dark, parallel lines around the cell
• It is formed chemically of proteins, lipids, and carbohydrates

Plasma Membrane Composition

• The lipid bilayer forms 35% of the membrane
  • Phosphate heads are polar (hydrophilic)
  • Tails are non-polar (hydrophobic)
• Cholesterol molecules reside in the inner cytoplasmic region
• Glycolipids exist solely in the outer layer and face the extracellular area

Plasma Membrane Proteins

• Proteins account for 55% of the plasma membrane
• The major component of the membrane
• They do not form a continues layer, yet globular masses floating in lipid bilayer
• Integral (intrinsic): Partially or entirely embedded in the lipid layer, act as channels
• Peripheral (extrinsic): Proteins present on both cell membrane surfaces, act as enzymes

Membrane Protein Functions

• Receptors, enzymes, channel proteins (gates), cell-identity markers, cell-adhesion molecules are functions of proteins

Plasma Membrane Carbohydrates

• Make up 10% of the plasma membrane
• Exist on the outer cell membrane surface
• They usually link with proteins (glycoproteins) or lipids (glycolipids)
• The Glycocalyx is made from glycoproteins and glycolipids (cell coating)
• Glycocalyx allows cells to bind to other cells and serves as a marker

Plasma Membrane Functions

• Protection of cell contents
• Separating cells from their environment and from each other
• Regulating cell to cell interaction
• Controlling movements of substances into and out of the cell (selective permeability)
• Exocytosis: a release of materials to the extracellular space
• Endocytosis: the materials going inside the cell

Cell Surface Extensions

• Microvilli
• Cilia and Flagella
• Pseudopods

Microvilli

• The extensions of plasma membrane
• Serve to increase the cell surface area
• Specialized absorption cells develop best
• This arrangement yields 15-40x more absorptive surface area
• Densely packed microvilli manifest as a brush border
• Internally, they contain bundles of protein filament called actin

Cilia

• These are short hair-like processes extending from a cell's surface
• Non-motile cilia are found on a cell
  • Used as an antenna for sensing nearby conditions
  • Sensory function in inner ear, retina and nasal cavity
• Motile cilia occur less widely, mostly in the respiratory and uterine tubes
  • They more substances across cell surfaces in one direction

Cilia Structure

• The axoneme is the structured core of the cilia facilitating ciliary movement
• Microtubules create a 9+2 structure
  • Two microtubules are in the center
  • Nine pairs of microtubules surround them as a ring

Flagella

• Flagella are whip-like structures with an identical axoneme to that of cilia
  • Much longer than cilia
  • Movement is more undulating / snake-like
  • Flagella move entire cells
• In humans, the only example is the tail of perm cells

Pseudopodia

• Temporary extensions that make up the plasma membrane
• Involved in cell movement and engulfing particles

Intercellular Junctions

• Connection or contact between the neighboring cells to the extracellular matrix
• Also known as a membrane junction
• They are classified into three types:
  • Tight (occluding) junction
  • Anchoring (adhering) junction
  • Gap (communicating) junction

Types of Animal Cell Junctions

• Tight junctions:
  • Consist of rows of proteins running parallel and prevent leaks between adjoining cells
• Adhering junctions:
  • Mass of interconnected proteins welded together, either between cells or attaching cells to the ECM
• Gap junctions:
  • Cylindrical clusters of proteins across plasma membranes of cells
  • Channels which are opened and closed, are often paired

Occluding Junction (Tight Junction)

• Borders of two cells are fused around their perimeter
• This resembles a continuous belt or zonula occludens
• It prevents molecules from passing through the epithelium

Anchoring (Adherence) Junction

• Attach cells (and their cytoskeleton) to each other
• Attach cells to the extracellular matrix
• Provides mechanical support

Desmosome

• Connects intermediate filaments between neighboring cells and are discrete
• They are not encircling the cells, they are discrete disc-like plaques scattered over the lateral surface of the cells with identical one at the surface of neighboring cells.

Hemidesmosome

• Connects intermediate filaments of a cell with the extracellular matrix
• In epithelium, hemidesmosomes secure it to the basal lamina

Gap Junction

• Gap junctions permit the exchange of electrical and/or chemical signals between cells
• These are composed of proteins forming channels for small molecules to pass through

Membrane Transport

• Passive Transport
  • Diffusion and Simple diffusion
  • Osmosis and Facilitated diffusion
  • Filtration
• Active transport
  • Primary and Secondary active transport
  • Co- transport and Counter transport
  • Endocytosis and Exocytosis
  • Pinocytosis (cell drinking) and Phagocytosis (cell eating)

Diffusion

• Substance transport across a semipermeable membrane along its electrochemical gradient
  • From high to low concentration, pressure, or voltage
  • No energy is needed
  • Movement is to equalize concentration

Types of Diffusion

• Simple Diffusion: Diffusion independent of carrier proteins
  • Transports lipid-soluble substances passively through the phospholipid bilayer, e.g. O2 & CO2
  • Permits small, water-soluble substance to transport through protein channels, e.g. Na+, K+, Cl-
• Facilitated (carrier) diffusion: needs a carrier protein(permease)
  • Transports of large lipid-insoluble molecules (e.g., glucose, amino acids)
  • Each carrier is specific for certain substances

Process of Facilitated Diffusion

• Protein binds with molecule
• Protein shape changes
• Molecule moves across membrane

Osmosis

• Diffusion of water (solvent) across a membrane
• Movement is from an area with a high concentration to an area with low concentration of solute
• Osmotic pressure is what stops osmosis from occurring
• Transports lipid, water and glucose

Filtration (bulk flow)

• Diffusion of fluid through a membrane due to hydrostatic pressure

Active Transport

• Movement of a substance through a membrane against the electrochemical gradient
  • Movement from low to high concentration or electrical/pressure gradient
  • Requires energy

Primary Active Transport

• Active transport of a substance by a protein pump
• The pump contains ATPase activity
  • This pump is able to hydrolyze ATP and extract the energy directly

Primary Active Transport Examples

• Sodium-potassium (Na-K) pump:
  • Exists in the membrane of all body cells
  • It pumps three Na+ ions out, and two K+ ions into the cell
• Calcium pump: Exists in the cell membrane, mitochondria membrane & in the sarcoplasmic reticulum

Secondary Active Transport

• Transport of a substance (active) secondary to transporting another substance (passive)
• Kinetic energy generated during passive passage is extracted by the active substance

Secondary Active Transport Types

• Co-transport:
  • Two substances are transported in the same direction
  • Example: sodium-glucose transport in renal tubules
• Counter transport:
  • Two substances are transported in opposite directions
  • Example: Na+ - H+ transport in renal tubules

Endocytosis & Exocytosis

• Endocytosis: Transporting macromolecules, such as proteins or bacteria, inside the cell
  • Pinocytosis (cell drinking)
  • Phagocytosis (cell eating)
• Exocytosis (cell excretion): Transporting macromolecules to the outside of the cell

Definitions of Histology

• "Histology" originates from Greek; "histo” (web or tissue) and “logos” (study).
• The study of tissues and organizing the body, also called microscopic anatomy

Aims of the Histology Course

• Understanding normal cell structure
• Knowing normal structure of different tissues
• Learning to identify them under a microscope
• Recognizing the functional and clinical correlation

Histology Divisions

• Histopathology: Microscopic examination of tissue appearance to observe appearance of diseased cells.
• Histochemistry: Study of chemical substances and components within tissues.
• Immunohistochemistry (IHC): Detecting antigens in sections of intact biological tissue.
• Cytology: Study of the structures and functions of body cells.

Levels of Organization

• Cell → Tissue → Organ → System → Human body

Definitions of Tissues

• A group of similar cells with the same specialized function and embryonic origin
• Tissue, made of cells and extracellular matrix, forms an organ and performs a discrete function

Basic Types of Tissues

• Epithelial
• Connective
• Muscle
• Nervous