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Chapter 3 Notes (10/24/08)

Intro to Medical Science

Chapter 3 Notes

 

I.  Why is it important to study the body on a cellular level?

            1. Activities essential for life occur there.

            2. Disease processes originate there.

 

Cell – is the smallest unit of structure and function that life can be broken into.

Cytology – is the study of cells.

 

A generalized animal cell = can be broken down into 4 principal parts:

            1. Plasma membrane – the outer covering, limiting membrane that seperates the cells internal parts from the external parts and outside environment.

            2. Cytoplasm – the substance that surrounds organelles and is located between the nucleus and the plasma membrane.

            3. Organelles – permanent structures with characteristic morphology that is highly specialized for specific cellular activities.

            4.  Inclusions – The secretions and storage products of cells.

 

A. Plasma Membrane

= very thin structure seperates the inside stuff from the outside stuff.

= measures about 4.5 nm (bilayer about 10 nm)

 

B. Chemistry and Structure      

= consists of phospholipids (lipids and phosphorus) and proteins

= lesser amounts of cholesterol (lipids), glycolipids (carbs and lipids), and carbs (oligosaccharides)

= Refer to the diagram illustrated during class.

= The layer is “Dynamic” which means they can move sideways, switch places within their own row but rarely between rows.

= The layer is self-sealing automatically

 

C. Plasma Membrane Proteins (PMP’s) – classified into 2 categories

1.  Integral proteins = embedded in the lipid bilayer = some are close to the outer layer, some are closer to the inner layer, and some penetrate completely

= since the membrane is fluid and flexible – proteins move from one area to another like Icebergs floating in the ocean.

= subunits of some integral proteins that have channels.  Main purpose is to transport materials in and out of cell.

= others are bound by branching chains of oligosaccharides together they provide receptor sites that enables the cell to recognize the following:

            1. others of its kind to make tissues.

            2. foreign cells that are potential hazards

            3. attaches hormones, nutrients, and other chemicals

            4. RBC’s also have receptors to prevent clumping (Type II diabetes is believed to be faulty cell receptor sites)

 

2. Peripheral proteins – loosly bound to surface and easily detached

=not much known about them

= thought to be enzymes that catalyze cell reactions

=believed to assume a role in changes in membrane shape during cell division, locomotion, and ingestion.

 

Fluid Mosaic Model = is the current accepted model of the plasma membrane

=the presence of cholesterol makes the membrane less flexible and permeable.

= Glycolipids mediate cell-to-cell recognition and communication, participate in growth and development, and may be infection sites for several kinds of viruses and bacteria.

 

Physiology –

1.      Provides a flexible boundary that encloses the cellular contents and separates them from the outside.

2.      Facilitates contact with other cells or with foreign cells or substances

3.      Provides receptors for chemicals like hormones, neurotransmitters, enzymes, nutrients, and antibodies.

4.      Mediates the entrance and exit of materials – this ability to control what enters and leaves the cell is call selective permeability.

 

What affects the passage through the membrane –

1.      Size of molecules (water and amino acids pass readily with out expenditure of energy)

2.      Solubility of lipids – substances that can dissolve easily in lipids pass easily (oxygen, carbon dioxide, and steroid hormones are examples)

3.      Charge of Ion’s – the charge of an ion will determine how easily it is transported

a.       The protein portion of membrane is capable of ionization

b.      If the ion charge is opposite then it will attach to membrane and pass thru

c.       If the ion charge is the same it will be repelled and passage restricted

 

Presence of Carrier molecules = some integral proteins are called carrier molecules capable of transport across the membrane regardless of size, ability to dissolve in lipids, or membrane charge.

 

Movement of materials across the Plasma Membrane –

1.      Substances are essential for sustaining life have to get into the cell

2.      Substances that are harmful must get out of the cell.

 

There are two main categories of movement:

1.      Passive Transport = movement across the membrane with out expenditure of energy.  It depends on kinetic energy of the molecules.  They will move down a concentration gradient.

Types of passive transport:

a.       Diffusion – net movement of molecules from an area of high concentration to an area of low concentration.  It moves from where there is a lot to where there is very few, until it reaches equilibrium. (ex. Dye in water, oxygen from the blood to cells, and carbon dioxide from cells to blood)

b.      Facilitated diffusion – happens with the help of integral proteins acting as carriers.  (ex. Glucose – carrier makes the sugar soluble)  This type is faster and depends on concentration and the amount of proteins and how quickly they combine.  Greatly enhanced by insulin (produced by pancreas) Low blood sugar levels by getting it out of blood and into cells.

c.       Osmosis – net movement of water pass through integral proteins.  Water moves from an area of high concentration to an area of low concentration.  This passage generates pressure called osmotic pressure.  So the less solutes dissolved the less pressure. (Solute is the substance being dissolved, and solvent is the substance doing the dissolving) Types of Osmosis:

                                                                           i.      Isotonic solution – where the water molecules and solute molecules are equal on both sides of the membrane and is at equilibrium.

                                                                         ii.      Hypotonic solution – a cell placed in a solution where it has a low concentration of solutes than in the cell (causes the cell to swell or burst) RBC’s bursting = hemolysis, distilled water is highly hypotonic

                                                                        iii.      Hypertonic solution – the cell has a low concentration of solutes than the solution (causes the cell to shrink) RBC’s = crenation, bulk flow is the movement of large molecules or particles in the same direction.

d.      Filtration – movement of solvents like wastes and dissolved substances like sugar across the membrane by gravity or hydrostatic pressure.  (Kidney’s – where blood pressure forces urea and water out of the cells, into kidneys through the thin walled vessels)  Urea is a waste product generated by metabolism in cells (amino acids to ammonia and carbon dioxide)

e.       Dialysis – separation of small molecules from large ones by diffusion.  Hook up a bag with a high concentration solution in order to create a concentration gradient.  This forces waste products (toxins) out of the cell and replaces them with nutrients the cell can metabolize.

2.      Active Transport = movement of molecules across the membrane that requires energy to move them against the concentration gradient.  (uses ATP)

Types of Active Transport:

a.       Endocytosis – where a portion of the cell encloses a molecule and brings it in with the use of pseudopodia.  (include diagram from class.)

b.      Exocytosis – opposite of endocytosis.

c.       Phagocytosis – “cell eating” project6ions called pseudopods engulf the molecules breaks it down forming a phagocytic vesicle which contains enzymes from lysosomes (WBC’s – destroying bacteria)

d.      Pinocytosis – “cell drinking” takes in liquids instead of solids, minute droplets of liquid attach to membrane then sucked in. 
      (include diagram from class)
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