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

Facilitated Diffusion

when polar or charged molecules diffuse across the plasma membrane with the help of transport proteins (integral). It can be passive or active transport.

Active Transport

substance moves against the concentration gradient. It requires energy or ATP. pump proteins facilitate this. Used to maintain cellular homeostasis. 

Sodium Potassium Pump

• one of the most important pumps in animal cells. It is a primary active transport, antiporter.

• The Na+ moves from low to high concentration (in to out)

• K+ moves from low to high concentration (out to in)

• Another channel protein brings back Na+ with concentration gradient.

Uniporter vs. Symporter vs. Antiporter

Uniporter: one arrow moving substance to each side equally

Symporter: 2 arrows, substance A and B move in same direction 

Antiporter: 2 arrows. Substance A and B move in different directions.

Channel Proteins

forms pores that are open or gated.

hydrophilic

Sodium channels

Carrier Proteins

binds substances which changes protein’s shape moving the molecule across the membrane.

Has slower movement than channel proteins.

Glucose transporter 

Endocytosis

bulk transport inside cell involving vesicles

example is when food vacuole containing bacteria fuzes with lysosomes

exocytosis

bulk transport outside cell involving vesicles. removes undigested material