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 JunctionsTight 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 substance2. Channels of appropriate size3. 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