CELLULAR PHYSIOLOGY PHYSIOLOGY 1 Dr. Tom Madayag

Objectives 1.Describe briefly the basic organization of the cell 2.Describe the cell membrane in terms of structure, chemical composition, properties and functions 3.Name the organelles of the cell and their functions 4.Describe the different transport systems in the cell—simple diffusion, facilitated diffusion, and active transport.

Cell Theory The cell is the smallest structural and functional living unit Organismal functions depend on individual and collective cell function Biochemical activities of cells are dictated by their specific subcellular structures Continuity of life has a cellular basis

Cellular Diversity Over 200 different types of human cells Types differ in size, shape, subcellular components and functions

Generalized Cell All cells have some common structures and functions Human cells have three basic parts: Plasma membrane Cytoplasm/organelles- intracellular fluid containing organelles Nucleus—control center

Cell and its parts

Plasma Membrane Bimolecular layer of lipids and proteins in a constantly changing fluid mosaic Composed of: Lipids Proteins Carbohydrates Separates intracellular fluid (ICF) from extracellular fluid (ECF) Interstitial fluid (IF)= ECF that surrounds cells

Plasma Membrane Functions Define cell boundaries Control interaction with other cells Controls the movement of substances in and out of the cell Referred to as being selectively permeable

Cellular membrane with two layers of fat

Plasma Membrane Lipids The large majority of plasma membrane is lipids 75% phospholipids (lipid bilayer) Phosphate heads: polar and hydrophilic Fatty acid tails: nonpolar and hydrophobic 5% glycolipids Lipids with polar sugar groups on outer membrane surface 20% cholesterol Increases membrane stability and fluidity

Plasma Membrane Proteins Two types of membrane proteins that constitute 50% of the overall weight of plasma membrane Integral (transmembrane proteins) Firmly inserted into the membrane Function is to act as transport proteins (channels and carriers), enzymes or receptors Peripheral Proteins Loosely attached to integral proteins Include filaments on intracellular surface and glycoproteins on extracellular surface Functions are to act as enzymes, motor proteins, cell-to-cell links, provide support on intracellular surface, and form part of the glycocalyx

Functions of Membrane Proteins 1.Transport substances in and out of cell 2.Receptors for signal transduction 3.Attachment to cytoskeleton and extracellular matrix 4.Enzymatic activity 5.Intercellular joining 6.Cell-cell recognition

Transport function of proteins

Receptors for signal transduction

Attachment to the cytoskeleton and extracellular matrix

Enzymatic activity

Intercellular joining (CAMs-cell adhesion molecules)

Cell-cell recognition

Plasma Membrane Junctions Where two cell membranes interact/connect/ “touch” Three types Tight junction Desmosome Gap junction

Membrane Junctions Tight Junctions Prevent fluids and most molecules from moving between cells Where might these be useful in the body?

Tight junctions

Membrane Junctions: DESMOSOMES “Rivets” or “spot-welds” that anchor cells together Where might this be useful in the body

Membrane Junctions: Gap Junctions Transmembrane proteins form pores that allow small molecules to pass from cell to cell For spread of ions between cardiac or smooth muscle cells

Plasma Membrane Transport Plasma membranes are selectively permeable Like a bouncer at a night club Some molecules easily pass through the membrane; others do not

Types of Membrane Transport Passive processes No cellular energy (ATP) required Substance moves down it concentration gradient Active processes Energy (ATP) required Occurs only in living cell membranes

Passive Transport Processes What determines whether or not a substance can passively permeate a membrane? 1.Lipid solubility of substance 2.Channels of appropriate size 3.Carrier proteins

Passive Transport Processes Simple diffusion Carrier-mediated facilitated diffusion Channel-mediated facilitated diffusion Osmosis

Passive Transport Processes: Simple Diffusion Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through the phospholipid bilayer

Passive Transport Processes: Facilitated Diffusion Certain lipophobic molecules (e.g. glucose, amino acids, and ions) use carrier proteins or channel proteins, both of which: Exhibit specificity (selectivity) Are saturable; rate is determined by number of carriers or channels Can be regulated in terms of activity and quantity

Carrier mediated facilitated diffusion & channel mediated facilitated diffusion (picture)

Passive Transport Processes: Osmosis Movement of solvent (water) across a selectively permeable membrane Water diffuses through plasma membranes: Through the lipid bilayer Through water channels called aquaporins (AQPs)

Passive Transport Processes: Osmosis Water concentration is determined by solute concentration because solute particles displace water molecules Osmolarity: the measure of total concentration of solute particles When solutions of different osmolarity are separated by a membrane, osmosis occurs until equilibrium is reached

Importance of Osmosis When osmosis occurs, water enters or leaves a cell Change in cell volume disrupts cell function Cell can shrivel up (become dehydrated) Problem because chemical reactions occur within an aqueous solution Cell can become over saturated with water and lysis (burst), open

Tonicity Tonicity: the ability of a solution to cause a cell to shrink or swell Isotonic: a solution with the same solute concentration as that of cystosol Hypertonic: a solution having greater solute concentration than that of cytosol Hypotonic: a solution having lesser solute concentration than that of cytosol

SUMMARY OF PASSIVE TRANSPORT ProcessEnergy SourceExample Simple diffusionKinetic energyMovement of O2 through phospholipid layer Facilitated diffusionKinetic energyMovement of glucose into cells OsmosisKinetic energyMovement of H20 through phospholipid bilayer or AQPs

Membrane Transport: Active Processes Two types Active transport Vesicular transport Both use ATP to move solutes across a living plasma membrane

Active Transport Requires carrier proteins (solute pumps) Moves solutes against a concentration gradient Types of active transport Primary active transport Secondary active transport

Primary Active Transport Energy from hydrolysis of ATP causes shape change in transport protein so that bound solutes (ions) are “pumped” across membrane

Primary Active Transport Sodium-potassium pump (Na+-K+ ATPase) Located in all plasma membranes Involved in primary and secondary active transport of nutrients and ions Maintains electrochemical gradient essential for functions of muscle and nerve tissues

Na+ K+ pump

Secondary Active Transport Depends on an ion gradient created by primary active transport Energy stored in ionic gradients is used indirectly to drive transport of other solutes

Vesicular Transport Transport of large particles, macromolecules, and fluids across plasma membranes Requires cellular energy (e.g., ATP)

Vesicular Transport Functions: Exocytosis—transport out of cell Endocytosis—transport into cell Transcytosis– transport into, across, and then out of the cell Substance (vesicular) trafficking—transport from one area or organelle in cell to another

Endocytosis and Transcytosis Involve formation of protein-coated vesicles Often receptor mediated, therefore very selective

Endocytosis Example Phagocytosis- pseudopods engulf solids and bring them into the cells’interior Macrophages and some white blood cells

Endocytosis--example Fluid phase endocytosis (pinocytosis)—plasma membrane infolds, bringing extracellular fluid and solutes into the interior of the cell Nutrient absorption in the small intestine

Endocytosis--example Receptor-mediated endocytosis—clathrin-coated pits provide main route for endocytosis and Transcytosis Uptake of enzymes low-density lipoproteins, iron, and insulin

Exocytosis Examples Hormone secretion Neurotranssmitter release Mucus secretion Ejection of wastes

Summary of Active Processes ProcessEnergy SourceExample Primary active transportATPPumping of ions across membranes Secondary active transportIon gradientMovement of polar or charged solutes across membranes ExocytosisATPSecretion of hormones and neurotransmitters PhagocytosisATPWhite blood cell phagocytosis PinocytosisATPAbsorption by intestinal walls Receptor-mediated endocytosis ATPHormone and cholesterol uptake

Cytoplasm Located between plasma membrane and nucleus Cytosol Water with solutes (protein, slats, sugars, etc.) Cytoplasmic organelles Metabolic machinery of cell Inclusions Granules of glycogen or pigments, lipid droplets, vacuoles, and crystals