6 Biological Membranes and Transport

osmosis, simple diffusion, plasma membrane

Structure of cell membrane

peripheral protein, carbohydrate, cholesterol, lipids, integral proteins

Why is the membrane a mosaic and fluid?

• It is mosaic because it is composed of different components.

• It is fluid because it allows movement while keeping the structure’s basic integrity.

Cholesterol

• a steroid lipid

• Structure of steroids: carbon skeleton of four fused rings

• hydroxyl group at the end makes the head polar and is attracted to phosphate heads (phospholipids)

• nonpolar hydrophobic tail is attracted to the hydrophobic tails of phospholipids

Integral membrane proteins

may have on or more alpha-helices that span the membrane, or they may have beta-sheets that span the mmebrane

Peripheral membrane proteins

• attached to phospholipids or integral proteins

• serves as enzymes or structural elements for the attachment of cytoskeleton’s fibers or as part of cell recognition sites (receptors)

Carbohydrates

• found on the exterior of the plasma membrane and are attached to proteins (glycoproteins) or lipids (glycolipids) 

• may consist of 2-60 monomers, straight or branched

The Extra Cellular Matrix Structure

• composed of collagen (protein) fibers interwoven with proteoglycans (carbohydrate-containing proteins)

Extra Cellular Matrix Function

• holds cells together to form tissues and allows the cells within the tissue to communication with each other

• cell communication happens through cell surface receptors (long distance communication exerted by hormones) or through direct physical contact (desmosomes, gap junction, and tight junctions)  

Simple Diffusion

a substance moves from a high concentration of solute to an area of low concentration of solute until the concentration is equal across a space. passive

What molecules can cross the membrane through simple diffusion?

oxygen, carbon dioxide, steroids

What is the concentration gradient?

It is when the substance goes into the cytoplasm.

Solute Diffusion

• solute moves from high concentration of solute to low concentration of solute

Osmosis

• water moves from high concentration of water to low concentration of water

• causes change in volume

• used to maintain osmotic balance

Tonicity

describes how an extracellular solution can change a cell’s volume by affecting osmosis. A solution’s tonicity often directly correlates with the solution’s osmolarity

Osmolarity

the solution’s total solute concentration

Isotonic Solution

• equal solutes inside and outside of the cell 

• water flows in and out of the cell at the same rate 

• no changes in volume of cell 

Hypertonic Solution

• higher solutes outside compared to inside of the cell 

• water flows out of cell into the solution 

• cell will shrivel and die

Hypotonic Solution

• lowers solutes in solution

• water flows into cell from the solution 

• cell will swell and lyse

How will plant cells react to osmosis?

• Hypotonic: cell swells until cell well opposed it resulting in turgor pressure → ideal

• Isotonic: no movement of water in →cell becomes flaccid → okay 

• Hypertonic: water moves out, cell membrane pulls away from the cell wall resulting in plasmolysis → bad