Introduction to Anatomy & Physiology: Structural Organization and the Cell

Anatomy vs. Physiology

Definitions and Distinctions

• Anatomy: The study of the structure of body parts and their relationships to one another.

• Physiology: The study of the function of body parts; how they work to carry out life-sustaining activities.

• Example (Liver):

  • Anatomy: Examines the liver's size, shape, location, blood and nerve supply.

  • Physiology: Focuses on bile production, nutrient regulation, and metabolic functions.

Key Principles

Complementarity of Structure and Function

• Structure and function are inseparable; the form of a body part enables its function.

• Examples:

  • Bones are hard, providing support.

  • Heart valves prevent backflow of blood.

  • Thin lung walls allow efficient gas exchange.

Levels of Structural Organization

Hierarchy from Atoms to Organism

• Chemical Level: Atoms and molecules (e.g., water, proteins).

• Cellular Level: Cells, the smallest living units capable of life.

• Tissue Level: Groups of similar cells performing a common function. Four basic types: epithelial, connective, muscle, nervous.

• Organ Level: Structures composed of at least two (usually four) tissue types that perform specific functions (e.g., heart, liver).

• Organ System Level: Organs working closely together for a common purpose (e.g., digestive system).

• Organismal Level: The sum of all structural levels working together to keep us alive.

Subdivisions of Anatomy

Major Branches

• Gross (Macroscopic) Anatomy: Study of large, visible structures.

  • Regional Anatomy: All structures in a particular area of the body.

  • Systemic Anatomy: Study of body systems (e.g., digestive, respiratory).

  • Surface Anatomy: Study of internal structures as they relate to the overlying skin.

• Microscopic Anatomy: Study of structures too small to be seen with the naked eye.

  • Cytology: Study of cells.

  • Histology: Study of tissues.

• Developmental Anatomy: Study of structural changes throughout the lifespan.

  • Embryology: Study of development before birth.

Cells: The Basic Unit of Life

Overview and Structure

• Over 250 different types of human cells, each with unique size, shape, and function.

• All cells share three main parts:

  • Plasma Membrane: Flexible outer boundary.

  • Cytoplasm: Intracellular fluid containing organelles.

  • Nucleus: DNA-containing control center.

Cytoplasm Components

• Cytosol: Fluid portion of cytoplasm.

• Inclusions: Stored substances (e.g., glycogen, pigments).

• Organelles: Specialized structures with specific functions.

  • Membranous Organelles:

    • Endoplasmic Reticulum (ER)

    • Golgi Apparatus

    • Lysosomes

    • Peroxisomes

    • Mitochondria

  • Non-membranous Organelles:

    • Ribosomes

    • Cytoskeleton

    • Centrioles

Major Organelles and Their Functions

• Mitochondria: Powerhouse of the cell; produces ATP via aerobic respiration. Contains its own DNA, RNA, and ribosomes.

• Endoplasmic Reticulum (ER):

  • Rough ER: Studded with ribosomes; site of protein synthesis.

  • Smooth ER: Lacks ribosomes; involved in lipid metabolism and detoxification.

• Golgi Apparatus: Modifies, packages, and directs proteins and lipids from the ER to their destinations via vesicles.

  • Three main pathways for vesicles:

    1. Secretory vesicles (exocytosis)

    2. Membrane vesicles (incorporation into plasma membrane)

    3. Lysosomes (digestive enzymes)

• Peroxisomes: Membranous sacs containing enzymes for detoxification (oxidase, catalase); neutralize free radicals and toxic substances.

• Lysosomes: Spherical organelles containing digestive enzymes; break down waste, cellular debris, and foreign substances.

• Cytoskeleton: Network of protein rods (microfilaments, intermediate filaments, microtubules) providing structural support and enabling movement.

• Centrosomes & Centrioles: Organize microtubules; form the mitotic spindle during cell division; basis for cilia and flagella.

Cellular Extensions

• Cilia: Short, hair-like projections that move substances across the cell surface.

• Flagella: Longer projections that propel the cell (e.g., sperm cell).

• Microvilli: Fingerlike extensions that increase surface area for absorption.

Extracellular Materials

Types and Functions

• Fluids:

  • Interstitial fluid: surrounds cells.

  • Blood plasma: fluid component of blood.

  • Cerebrospinal fluid: surrounds the brain and spinal cord.

• Secretions: Substances such as saliva and mucus produced by cells.

• Extracellular Matrix: A glue-like substance that provides structural support and helps hold cells together.

Summary Table: Levels of Structural Organization

Level	Description	Example
Chemical	Atoms and molecules	Water, proteins
Cellular	Basic living unit	Muscle cell, neuron
Tissue	Group of similar cells	Muscle tissue, nervous tissue
Organ	At least two tissue types	Heart, liver
Organ System	Organs working together	Digestive system
Organismal	All systems combined	Human body

Additional info:

• Understanding the relationship between structure and function is fundamental in anatomy and physiology.

• Cells are the smallest units that can maintain and perpetuate life.

• Organelles within cells perform specialized functions necessary for cell survival and overall body function.

Tissue: The living fabric

• Various tissues

• Epithelial tissues

  • Characteristics of epithelial tissue

  • Classification of epithelial tissue

• Connective tissues

  • Characteristics of connective tissue

  • Structural elements of connective tissue

  • Types of connective tissue

• Individual body cells are specialized

  • Each type performs specific functions that maintain homeostasis

• Tissues

  • Groups of cells similar in structure that perform a common or related function

  • 4 basic tissue types: epithelial, connective, muscle, and nervous tissue

• Histology: study of tissues

Epithelial tissue

Epithelial tissue is a sheet of cells that covers body surfaces or cavities

• 2 main forms

  • Covering and lining epithelia

    • On external and internal surfaces

  • Glandular epithelia

    • Secretory tissue in glands

Characteristics of Epithelial tissue

• Polarity

  • Apical and basal surfaces; the apical surface is often specialized, including microvilli or cilia

• Specialized contacts

  • Tight junctions and desmosomes

• Supported by connective tissues

  • Supported by a basement membrane composed of a basal lamina and a reticular lamina

• Avascular, but innervated

  • Nutrients are supplied by diffusion from blood vessels in the underlying connective tissue

• Regeneration

  • High regeneration capacity

Classification of Epithelia

• Number of cell layers

  • Simple epithelia are a single-layer thick

  • Stratified epithelia are two or more layers thick and are involved in protection

• Shape of cells

  • Squamous: flattened and scale-like

  • Cuboidal: box-like, cube

  • Columnar: tall, column-like

• Cells are flattened with sparse cytoplasm

• Function where rapid diffusion is a priority

  • Kidney, lungs

• 2 special simple squamous epithelia are based on occasions

  • Endothelium: lining of lymphatic vessels, blood vessels, and the heart

  • Mesothelium: serous membranes in the ventral body cavity

• Simple cuboidal epithelium

  • Single layer of cells

  • Involved in secretion and absorption

  • Forms walls of smallest ducts of glands and any kidney tubules 

• Simple Columnar epithelium

  • Single layer of tall, closely packed cells

    • Some cells have microvilli, and some have cilia

    • Some layers contain mucus-secreting goblet cells

  • Involved in absorption and secretion of mucus, enzymes and other substances 

  • Found in digestive tract, gallbladder, ducts of some glands, bronchi, and uterine tubes

  • Ciliated cells move mucus 

• Pseudostratified columnar epithelium

  • Pseudo means false

  • Many cells are ciliated

  • Involved in secretion, particularly of mucus, and in the movement of mucus via ciliary sweeping action

  • Located motley in the upper respiratory tract, ducts of large glands, and tubules in the testes

• Stratified epithelial tissue

  • 2 or more layers

  • New cells regenerate from below

  • Basal cells divide and migrate toward the surface

    • More durable than simple epithelia because protection plays a major role

• Stratified squamous epithelium

  • Most widespread

  • Free surface is squamous, with deeper cuboidal or columnar layers

  • Located in areas of high wear and tear

  • Keratinized cells are found in skin; nonkeratinized cells are found in moist linings

• Stratified epithelial tissues: transitional epithelium

  • Forms lining of hollow urinary organs 

  • Basal layer cells are cuboidal or columnar

  • The ability of cells to change shape when stretched allows for increased flow of urine and, in the case of the bladder, more storage space

    • Stratified cuboidal epithelium

      • Quite rare/typically 2 layers

      • Found in some sweat and mammary glands

    • Stratified columnar epithelium

      • Also very limited in the body

      • Small amounts are found in the pharynx, in the male urethra, and lining of some glandular ducts

      • Only the apical layer is columnar 

• Simple epithelia

  • Squamous; lung and kidney

  • Cuboidal; kidney tubules and the small gland ducts

  • Columnar: digestive tract, bronchi, and uterine tubes

• Stratified epithelia

  • Pseudostratified columnar; upper respiratory tract

  • Squamous; skin

  • Transitional cuboidal and columnar; sweat, mammary glans, and the male urethra 

Glandular Epithelium

• Gland

  • One or more cells that make and secrete a fluid

• Classified by

  • Site of product release

    • Endocrine: internally

    • Exocrine: externally

• Relative number of cells forming the gland

  • Unicellular

  • Multicellular

• Endocrine glands

  • Ductless glands

    • Secretions are not released into a duct; they are released into the surrounding interstitial fluid, which is picked up by the circulatory system

    • They secrete hormones

    • Target organs respond in some characteristic way

• Exocrine glands

  • Secrete products into ducts

  • Secretions are released on body surfaces such as skin or body cavities

  • More numerous than endocrine glands

  • Examples include mucous, sweat, oil and salivary glands

    • Can be

      • Unicellular

      • Multicellular 

• Unicellular exocrine glands

  • The only important unicellular glands are

    • Mucous cells

    • Goblet cells

  • Found in epithelial linings of the intestinal and respiratory tracts

  • All produce mucin, a sugar protein that can dissolve in water to form mucus, a slimy protective, lubricating coating

• Multicellular exocrine glands

  • Multicellular exocrine glands are composed of a duct and a secretory unit

  • Usually surrounded by supportive connective tissue that supplies blood and nerve fibres to glands

  • Connective tissue can form a capsule around the gland and extend into the gland, dividing it into lobes

  • Classified by

    • Structure

    • Mode of secretion

      • Modes of secretion

        • Merocrine

          • Most secrete products by exocytosis as secretions are produced

            • Sweat and pancreas

        • Holocrine

          • Accumulate products within, then rupture

            • Sebaceous oil glands

        • Apocrine

          • Accumulate products within, but only apex ruptures; whether this type exists in humans is controversial

            • Possibly mammary cells??

Connective tissue

• The most abundant and widely distributed of primary tissues

• Major functions:

  • Binding

  • Support

  • Protecting

  • Insulating

  • Storing reserve fuel

  • Transporting substances (blood)

• Characteristics

  • All have a common embryonic origin

    • All come from mesenchyme tissue

  • Have varying degrees of vascularity

  • Cells are suspended/embedded in the ECM

    • Matrix supports cells so they can bear weight, withstand tension, and endure abuse

Structural Elements of Connective Tissue

• All connective tissues have three main elements

  • 1. Ground substance

    • Unstructured gel-like material that fills the space between cells

      • The medium through which solutes diffuse between blood capillaries and cells

    • Components

      • Interstitial fluid

      • Cell adhesion protein “Glue”

      • Proteoglucans (sugar proteins) are made up of a protein core and a large polysaccharide

      • Water is also trapped in varying amounts, affecting the viscosity of the ground substance 

  • 2. Fibers

    • 3 types of fibres provide support

      • Collagen 

        • Strongest and most common

        • Tough, high tensile strength

      • Elastic fibers

        • Networks of long, thin, elastin fibres that allow for stretch and recoil

      • Reticular

        • Short, fine, highly branched collaginous fibres

        • Branching forms networks that offer more “give”

  • 3. Cells

    • “Blast cells”

      • An immature form of cell that actively secretes ground substance and ECM fibres

      • Fibroblasts

        • Found in connective tissue proper

      • Chondroblasts

        • Found in cartilage

      • Oseteoblasts

        • Found in bone

      • Hematopoietic stem cells in bone marrow

    • “Cyte” cells

      • Mature, less active form of “blast” cell that now becomes part of and helps maintain the health of matrix

    • Other cell types in connective tissues

      • Fat cells

      • Mast cells

Connective tissue proper

• Areolar

  • Most widely distributed connective tissue

  • Supports and binds other tissues

  • Universal packing material between other tissues

  • Contains fibroblasts that secrete a loose arrangement of mostly collagen fibres

  • Loose fibres allow for increased ground substance, which can act as a water reservoir by holding more interstitial fluid

  • Macrophages and fat cells are contained in spaces

• Adipose

  • White fat

    • Similar to areolar tissue, but with greater nutrient storage

    • Cells are called adipocytes

    • Scanty matrix

    • Richly vascularized

    • Functions in shock absorption, insulation and energy storage

  • Brown fat

    • Use lipid fuels to heat the bloodstream rather than to produce ATP, as does white fat.

  • Difference

    • White fat is used by mitochondria to create ATP

    • Found in embryos and is not used to create ATP.

• Reticular

  • Resembles areolar tissue, but fibres are thinner reticular fibres

  • Fibroblast cells are reticular cells

    • Secret reticular fibres made up of thin collagen

  • Reticular fibres form a mesh-like stroma that acts as a support for blood cells in lymph nodes, spleen, and bone marrow

• Dense-regular

  • Very high tensile strength can withstand high tension and stretching

  • Closely packed bundles of thick collagen fibers run parallel to direction of pull

    • Fibres appear as white structures 

      • Great resistance to pulling

    • Fibres slightly wavy, so stretch a little

      • Fibroblasts manufacture collagen fibres and ground substance

      • Very few cells and ground substance, mostly fibres

      • Poorly vascularized

• Dense irregular

• Dense elastic

  • Some ligaments are very elastic

  • Also found in walls of many large arteries

    • Arteries need to stretch when  blood enters and recoil to push it out

Connective tissues

• Cartilage

  • Matrix secreted from chondroblasts and chondrocytes

  • Chondrocytews found in cavities called lacunae

    • 80% water with packed collagen fibers and sugar proteins

  • Tough yet flexible material that lacks never fibers

  • Avasular: receives nutrients from the membrane surrounding it (Perichondrium)

    • Perichondrium gives rise to chondroblasts and chondrocytes

• Types of cartilage

  • Hyaline cartilage

    • Must abundant

    • Appears as shiny blueish glass

    • Found at tips of long bones, nose, trachea, larynx and cartilage of the ribs

  • Elastic cartilage

    • Similar to hyaline but with more elastic fibers

    • Found in ears and epiglottis

  • Fiborocartialge 

    • Properties between hyaline and dense regular tissue

    • Strong, so found in areas such as intervertebral discs and knees

Bones

• Carry out multiple function including support, protection and mineral storage and hematopoiesis

• Hard tissue that resist both compression and tension

• Two types of bone

  • Compact bone: outer smooth, and dense

  • Spongy bone: inner, mesh-like boney structure called trabeculae

• Three types of cells

  • Osteoblasts

  • Osteoclasts

  • Osteocytes

• The matrix is a calcified gel-like ground substance that comprises inorganic salts and collagen fibres

Summary:

Tissues – The Living Fabric

• Tissue = groups of similar cells performing a common function.

• Histology = study of tissues.

• 4 basic types:

  1. Epithelial

  2. Connective

  3. Muscle

  4. Nervous

Epithelial Tissue

• Functions: Covers surfaces, lines, cavities,and  forms glands.

• Forms:

  • Covering/lining (surfaces, organs).

  • Glandular (secretion).

• Key Characteristics: polarity (apical/basal), tight junctions & desmosomes, supported by basement membrane, avascular but innervated, high regeneration.

Classification

• By layers:

  • Simple (1 layer → absorption/secretion).

  • Stratified (2+ layers → protection).

• By shape:

  • Squamous (flat, rapid diffusion).

  • Cuboidal (cube, secretion/absorption).

  • Columnar (tall, secretion/absorption, may have goblet cells & cilia).

Types

• Simple squamous: diffusion (lungs, kidneys); special types:

  • Endothelium (blood vessels, heart).

  • Mesothelium (serous membranes).

• Simple cuboidal: ducts, kidney tubules.

• Simple columnar: digestive tract, gallbladder, bronchi, uterine tubes.

• Pseudostratified columnar: secretion, mucus movement (respiratory tract).

• Stratified squamous: most widespread; keratinized (skin), non-keratinized (mouth, esophagus).

• Transitional epithelium: bladder/urinary organs (stretchable).

• Rare: stratified cuboidal (sweat, mammary glands), stratified columnar (male urethra, pharynx).

Glandular Epithelium

• Glands: secrete fluids.

• Endocrine: ductless, release hormones into the blood.

• Exocrine: ducts, secrete onto surfaces/cavities (sweat, oil, mucus).

• Unicellular: goblet & mucous cells (make mucin → mucus).

• Multicellular: duct + secretory unit, supported by connective tissue.

• Modes of secretion:

  • Merocrine (exocytosis, e.g. sweat).

  • Holocrine (rupture, e.g. sebaceous glands).

  • Apocrine (apex ruptures, debated, possible in mammary).

Connective Tissue

• Most abundant & diverse.

• Functions: binding/support, protection, insulation, fuel storage, transport (blood).

• Common traits: mesenchymal origin, varying vascularity, cells in extracellular matrix (ECM).

Structural Elements

1. Ground substance: gel-like medium (fluid, proteins, proteoglycans).

2. Fibres: collagen (strong), elastic (stretch), reticular (support network).

3. Cells:

   • Blast cells (active): fibroblasts (CT proper), chondroblasts (cartilage), osteoblasts (bone), hematopoietic stem cells (blood).

   • Cyte cells (mature, maintenance).

   • Others: fat cells, mast cells, macrophages.

Types of Connective Tissue

1. Connective Tissue Proper

   • Loose:

     • Areolar (universal packing/support).

     • Adipose (white fat = energy, brown fat = heat).

     • Reticular (support in spleen, lymph nodes, marrow).

   • Dense:

     • Regular (tendons/ligaments, parallel collagen, high tensile strength).

     • Irregular (thicker, resists tension in many directions).

     • Elastic (large arteries, ligaments).

2. Cartilage

   • Avascular, flexible, resilient.

   • Hyaline (most abundant, nose, ribs, joints).

   • Elastic (ear, epiglottis).

   • Fibrocartilage (intervertebral discs, knees).

3. Bone

   • Functions: support, protection, mineral storage, hematopoiesis.

   • Compact (dense outer layer).

   • Spongy (trabeculae, inner).

   • Cells: osteoblasts (build), osteocytes (maintain), osteoclasts (break down).

4. Blood

   • Fluid connective tissue transports substances.

Cellular physiology of nerve and muscle outlines

• Plasma membrane

  • Structure of the plasma membrane

  • Functions of the plasma membrane

• Membrane transport

  • Active 

  • Passive

The plasma membrane

• Acts as an active barrier separating intracellular fluid from extracellular fluid

• Selectively permeable, it plays a role in cellular activity by controlling what enters and what leaves the cell

• Allows the cell to respond to changes in the extracellular fluid

• Site of cell-to-cell communication

Structure of the plasma membrane

• Consists of membrane lipids that form a lipid bilayer

• Specialized in membrane proteins that float through fluid membrane, resulting in constantly changing patterns

  • Fluid mosaic

• Surface sugars form glycoalyx

• Membrane structure helps hold cells together through cell junctions

• Lipid bilayer

  • 75% phospholipids

    • Consists of 2 parts

      • Phosphate heads:

        • Polar 

        • Hydrophilic

      • Fatty acid tails

        • Nonpolar

        • Hydrophobic

  • 5% glycolipids (glyco means carbohydrate)

    • Lipids with sugar groups on the outer membrane

  • 20% cholesterol

    • Increase membrane stability

• Membrane proteins

  • Makes up about half the mass of the plasma membrane

  • Allow cell communication with the environment

  • Most have specialized membrane functions

  • Some float freely, and some are tethered to intracellular structures

• 1. Integral proteins

  • Firmly placed in the membrane

  • Most are transmembrane proteins

  • Have both hydrophobic and hydrophilic regions

    • Function as transport proteins, enzymes or receptors

• 2. Peripheral proteins

  • Loosely attached to integral proteins

  • Include filaments on the intracellular surface used for plasma membrane support

    • Function as enzymes, cell-to-cell connections, motor proteins for shape change during cell division and muscle contraction

Functions of Plasma Membrane Proteins

• A. Transport

  • A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute

  • Some transport proteins hydrolyze ATP as an energy source to actively pump substances across the membrane

• B. Receptors for signal transduction

  • A membrane protein exposed to the outside of the cell may have a binding site that fits the shape of a specific chemical messenger, such as a hormone

  • When bound, the chemical messenger may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell

• C. Enzymatic activity

  • A membrane protein may be an enzyme with its active site exposed to substances in the adjacent solution

  • A team of several enzymes in a membrane may catalyze sequential steps of a metabolic pathway, as indicated here

• D. Cell-to-cel recognition

  • Some glycoproteins, bonded to short chains of sugars, which help to make up the glyocalyx serve as identification tags that are specifically recognized by other cells

• E. Attachment to the cytoskeleton and ECM

  • Elements of the cytoskeleton and the ECM 

  • HElps maintain cell shape, fixes the location of certain membrane proteins, and plays a role in cell movement

• F. Cell-to-cell joining

  • Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions

  • Some membrane proteins of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions

Membrane transport

• Substances must constantly move across the plasma membrane 

• The plasma membrane is selectively permeable, allowing only certain molecules to cross 

Passive Transport

• Types

  • Simple diffusion

  • Facilitated diffusion

  • Osmosis

  • Filtration

• All types involve diffusion - natural movement of molecules from areas of high concentration to low concentration

• GRADIENT

• Diffusion

  • Molecules in higher concentration areas collide more, resulting in molecules being scattered to lower concentration areas

  • Speed is affected by 3 factors

    • Concentration

    • Molecular size

    • Temperature

• Simple diffusion

  • Nonpolar, lipid-soluble substances diffuse directly through the phospholipid bilayer

  • Small amounts of very small polar substances, such as water, can even pass

  • NO CARRIER NEEDED IF LIPID SOLUBLE 

• Facilitated diffusion

  • Larger or water-soluble or polar molecules can cross the membrane, but only with assistance

  • Certain hydrophilic molecules are transported passively down the gradient

    • Carrier-mediated: facilitated diffusion

      • Substances bind to protein carriers

      • Lipids are insoluble; too large to pass through channels

      • Carriers are transmembrane integral proteins

    • Chanel-mediated: facilitated diffusion

      • Substances move through water-filled channels

      • Aquous-filled cores are formed by transmembrane proteins

      • Transport molecules, such as ions or water, flow down the gradient

        • Selective due to pore size, charge of a.a. That line channels

        • Water channels are aquaporins

          • Can be inhibited, can show saturation

            • 2 types of channels

              • Leakage channels

              • Gated channels

• Osmosis

  • Diffusion of water from an area of more water to one of less water across a semipermeable membrane

  • Water diffuses across the plasma membrane

    • Through the lipid bilayer 

    • Through specific water channels called aquaporins

  • Flow occurs when the water concentration is different on 2 sides of a membrane

• Osmolarity: Measures the concentration of the total number of solute particles in the solvent

  • Solutions of different osmolarities separated by a membrane permeable to all molecules

    • Diffusion of solutes and osmosis of water occur across the membrane, and the equilibrium of solutes and water is reached

  • Solutions of different osmolarities separated by a membrane permeable only to water

    • Only osmosis will occur until equilibrium is reached

• Water decreases in response to an increase in the number of solute particles in solution

  • Animal cells swell or shrink in response to water movement until equilibrium or rupture

• Tonicity: Ability of a solution to change the shape of a cell bathed by that solution. What is important are nonpenetrating solute particles

  • Isotonic solutions

  • Hypertonic solutions

  • Hypotonic solutions

• Filtration

  • Water and solutes are forced through the membrane or capillary walls by fluid or hydrostatic pressure

  • Selective only by size

Active Transport 

• Primary and secondary

  • Like facilitating diffusion, both require carrier proteins: they combine specifically and reversibly with the substance

  • Unlike facilitated diffusion, solute pumps move substances against concentration gradients

  • Many active transport systems are coupled systems

  • Primary AT vs secondary AT

  • ATP IS USED BECAUSE 

    • Too large for pores and is lipid-insoluble 

    • Moving against the concentration gradient 

• Primary active transport

  • Requires energy directly from ATP hydrolysis

  • Energy from the hydrolysis of ATP causes a change in the shape of the transport protein

  • Shape change causes solutes bound to the protein to be pumped across the membrane

  • The most studied is the sodium-potassium pump

    • An enzyme called Sodium potassium ATPase that pumps sodium out of the cell and potassium back into the cell 

    • Located in all plasma membranes, but is especially active in excitable cells

    • Maintenance of this gradient is challenged by 

      • Slow leakage of potassium and sodium along concentration gradients

      • Stimulation of muscle and nerve cells

• Resting membrane potential

  • Electrical potential energy is produced by the separation of oppositely charged particles across the plasma membrane in all cells

    • The difference in electrical charge between 2 points is referred to as voltage

  • Voltage occurs only at the membrane surface; the rest of the cell is natural

  • RMP is maintained by the sodium-potassium pump

  • Neuron and muscle cells upset the steady rhythm by intentionally opening gated sodium and potassium channels

• Secondary transport

  • Required energy is obtained indirectly from ionic gradients created by primary transport

• Vesciular transport

  • Transports large particles, macromolecules, and fluids across the membrane in membranous sacs called vesicles

  • Required energy is supplied by ATP

  • Vesicular transport processes are

    • Endocytosis: Transport into cells

      • 3 different types of endocytosis

        • Phagocytosis

        • Pinocytosis

        • Receptor-mediated endocytosis

      • For regular endocytosis

        • Large particles enter the cell

        • A vesicle encloses the substances, pinches off and moves into the cytoplasm, where the contents may be digested. It may also traverse the cell to exit to the other side 

      • Special case: Receptor-mediated endocytosis

        • Allows hormones, enzymes and other important macromolecules to be concentrated in the cell 

        • Because receptors are involved, processes can be a very selective process

        • Some pathogens are capable of hijacking a receptor for transport into the cell

    • Exocytosis: Transport out of cells

      • Material is being ejected from the cell

      • Secretion of hormones, neurotransmitters, mucus, and waste

      • Substances that are enclosed in a vesicle move to the PM, fuse with the PM, and then rupture, releasing the contents from the cell.

    • Transcytoiss: Transport into, across, and then out of the cell

    • Vesicular trafficking: Transport from one area or organelle in the cell to another

Summary

Plasma Membrane

• Role: Selectively permeable barrier between intra/extracellular fluid.

  • Controls entry/exit of substances.

  • Site of communication and cell signalling.

• Structure: Fluid Mosaic Model

  • Lipid bilayer: 75% phospholipids (polar heads, nonpolar tails), 5% glycolipids (with sugars), 20% cholesterol (stability).

  • Proteins (50% mass):

    • Integral (often transmembrane; transport, receptors, enzymes).

    • Peripheral (support, enzymes, motor proteins, connections).

  • Glycocalyx: surface sugars, ID tags.

  • Cell junctions: hold cells together.

Functions of Membrane Proteins

1. Transport (channels, pumps, carriers).

2. Receptors (signal transduction).

3. Enzymatic activity (metabolic pathways).

4. Cell recognition (glycoproteins as ID tags).

5. Attachment (to cytoskeleton/ECM for shape & stability).

6. Cell joining (junctions, migration, interactions).

Membrane Transport

Cells must move substances across the membrane.

• Passive = no ATP, down gradient.

• Active = requires ATP, against a gradient or bulk transport.

Passive Transport

• Diffusion: high → low concentration (affected by concentration, size, temperature).

  • Simple diffusion: small nonpolar/lipid-soluble (e.g. O₂, CO₂).

  • Facilitated diffusion: needs proteins.

    • Carrier-mediated (protein carriers).

    • Channel-mediated (ion/water channels → leakage or gated; aquaporins for water).

• Osmosis: diffusion of water via bilayer or aquaporins.

  • Driven by solute concentration differences.

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