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.
• https://www.youtube.com/watch?v=u54bRpbSOgs
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.
2.
3.
Lipid solubility of substance
Channels of appropriate size
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
Process
Energy Source
Example
Simple diffusion
Kinetic energy
Movement of O2 through
phospholipid layer
Facilitated diffusion
Kinetic energy
Movement of glucose into
cells
Osmosis
Kinetic energy
Movement of H20 through
phospholipid bilayer or
AQPs
• https://www.youtube.com/watch?v=ldRZcmppQM8
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
• https://www.youtube.com/watch?v=2-icEADP0J4
Summary of Active Processes
Process
Energy Source
Example
Primary active transport
ATP
Pumping of ions across membranes
Secondary active transport Ion gradient
Movement of polar or charged solutes
across membranes
Exocytosis
ATP
Secretion of hormones and
neurotransmitters
Phagocytosis
ATP
White blood cell phagocytosis
Pinocytosis
ATP
Absorption by intestinal walls
Receptor-mediated
endocytosis
ATP
Hormone 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