Anatomy and Physiology Chapter 3: The Cell

Cell Theory  Cells are the building blocks of all plants and animals.  Cells are produced by the division of pre-existing cells.  Cells are the smallest units that perform all vital physiological functions.  Each cell maintains homeostasis at the cellular level.  Homeostasis at the tissue, organ, system, and organism levels reflect the combined and coordinated actions of many cells.

Cells  Basic structural and functional unit of living organisms.  Cytology – study of cell structure and function.  Classes of Cells: –Sex cells – sperm and oocytes (eggs) –Somatic cells – body cells

Plasma Membrane  Defines the extent of the cell and acts as a fragile barrier between the inside and outside of the cell.  Exceedingly thin, but stable, structure composed of a phospholipid bilayer with protein molecules dispersed throughout.  Impermeable to water soluble molecules – semi-permeable.

Phospholipid Bilayer  Polar, phospholipid heads are hydrophilic. –Form inner and outer portions of the membrane that comes in contact with the external cell environment and the cytoplasm of the cell.  Non-polar, fatty acid tails are hydrophobic. –Form the inner portion of the membrane, between two layers of polar phospholipid heads; doesn’t come in contact with any water at all.  Glycoproteins serve as cell surface markers.

Membrane Proteins  Integral –firmly inserted into the lipid membrane. –Act as receptors for hormones and other chemical messengers. –Act as transmembrane proteins which have transport functions such as ion channels / pores or carrier molecules. –Consist of a hydrophobic and hydrophilic region also.

Membrane Proteins  Peripheral –Not embedded in the membrane, but are attached to integral proteins exposed to the surface. –Some are enzymes. –Some lead to cell shape changes associated with cell division. –Some are responsible for muscle cell contraction.

Membrane Protein Functions  Transport of molecules as carrier proteins and channels.  Two types of channels: –“Leaky” – water and ions move through all the time. –Gated – open and close to regulate ion passage.

Membrane Protein Functions  Enzymes  Receptor Cells  Cell adhesion – anchor cell to structure or other cells.  Attachment to cytoskeleton – holds plasma membrane to internal proteins; gives cell a shape.  Recognition – allows body to recognize foreign invaders (glycolipids)

Glycocalyx  “Sticky” carbohydrate rich area of the cell plasma membrane.  Also contains glycoproteins  Binds adjacent cells together.  Lubricates and protects cell membrane.  Locomotion of specialized cells – white blood cells.

Plasma Membrane Functions  Isolation  Sensitivity to changes in cellular environment.  Structural support.  Regulation of exchange with environment (semipermeable / selectivity permeable)

Factors that Effect Membrane Permeability  Size of Particle  Electrical Charge of Particle  Shape of Particle  Lipid Solubility

Passive Transport  No net energy expended.  Occurs down a concentration gradient from an area of high concentration to an area of low concentration without using ATP.  Diffusion –Simple Diffusion – implies no barrier to movement; occurs because of molecular collisions.  Oxygen and carbon dioxide exchange  Water and other small molecules (glucose) –Filtration Diffusion – under pressure; hydrostatic pressure forces water across a membrane, transporting small molecules with it. –Facilitated Diffusion – substance coupled to a carrier molecule; may be an integral membrane protein.  Carriers are highly specific  Saturation limits amount of molecules that can be moved  May be regulated by the binding of cofactors and coenzymes. –Example – glucose + insulin – uptake occurs 7-10X faster.

Factors Affecting Diffusion  Distance – the larger the distance between the molecule and the cell membrane, the longer diffusion takes.  Size of Gradient – the larger the concentration factor, the faster diffusion occurs.  Molecular Size – smaller molecules diffuse faster.  Temperature – increasing temperature increases diffusion rates.  Electrical Forces – like charges repel each other, while opposite charges attract each other.

Tonicity  Ability of a solution to change the tone or shape of cells by altering their internal water volume. –Isotonic  solute in = solute out; no NET movement of water. –Hypertonic  number of solute particles is larger outside the cell than inside the cell; net movement of water is out; cause cell to crenate. –Hypotonic  number of solute particles is smaller outside the cell than inside the cell; net movement is into the cell; causes the cell to burst.

Passive Transport  Osmosis – diffusion of water through a semipermeable membrane from an area of high water content to an area of low water concentration.  Osmotic Pressure – indication of force of water movement into a solution as a result of solute concentration.

Active Transport Processes  Movement of particles using energy (ATP); not concentration dependent. –Solute / Ion Pumps  proteins in the plasma membrane that move ions selectively in or out of the cell  creates a polarized membrane  Sodium – Potassium Pump –Sodium ions and potassium ions must be recaptured and moved back to their original starting points. Each ATP used moves 3 sodium and 2 potassium back.

Active Transport Processes  Bulk Transport – movement of large particles by means of vesicles. –Method:  Molecule comes in contact with the membrane  Membrane surrounds molecule and takes it into the cell.  Membrane pinches off, freeing the vesicle. –Three Types:  Phagocytosis – large particles in  Pinocytosis – liquid particles in  Exocytosis – Moving material out of the cell

Membrane Specializations  Microvilli – minute, finger-like extensions of the plasma membrane that increases the cell surface area; contains a core of actin (protein).  Cilia – hair-like projections that move substances along the surface of the cell.

Membrane Junctions  Tight Junctions – protein molecules fuse together; forming impermeable junctions.  Glycocalyx  “Tongue in groove” fit of adjacent plasma membranes.  Gap Junctions – allow direct passage of chemicals between adjacent cells through hollow cylinders.