Chapter 4 Transport of Substances through Cell Membranes

The concentration gradient of what two fluids are important for muscle contraction?

Extracellular and intracellular

Which ions are in higher concentration and lower concentration in extracellular fluid

Sodium and chloride

Potassium

Which ions are in higher concentration and lower concentration in intracellular fluid

Potassium, phosphates, and proteins

Sodium and chloride

Lipid bilayer

A barrier against movement of water molecules and water-soluble substances between the extracellular and intracellular fluid compartments

Channel proteins

Allow free movement of selected ions or molecules

Carrier proteins

Bind molecules/ions and a conformational change moves the substance through the protein to the other side of the membrane

Diffusion

Random molecular movement of substances, follows concentration gradient and does not require additional energy

Active transport

Moves ions/molecules against the concentration gradient and requires an additional source of energy

Simple diffusion

Through a membrane opening where the molecule moves without interaction with carrier proteins

Facilitated diffusion

Type of movement with the concentration gradient where the molecule interacts with the carrier protein

Lipid solubility

Determines how rapidly a substance diffuses through the lipid bilayer

Aquaporin

Allows water through the lipid bilayer through osmosis

Which ions are readily diffused through the lipid membrane (high lipid solubility)?

O2

N2

CO2

What are the major factors in membrane pore selectivity?

Diameter of pore and electrical charges

Nephrogenic Diabetes Insipidus (NDI)

Mutation in aquaporins

Kidneys fail to respond to vasopressin leading to the excretion of massive amounts of dilute urine

How does sodium have selectivity with its channel?

The channel is lined with negatively charged amino acids that pull dehydrated sodium molecules away from water allowing them to diffuse

Voltage gated channels

Channel that operates due to changes in voltage

Chemical (ligand) gated channel

Channel that responds to an attachment of a molecule

Carrier mediated facilitated diffusion

passive transport process where specific transmembrane proteins help move large or polar molecules across a cell membrane down their concentration gradient without using cellular energy

How does potassium have selectivity with its channel?

Hydrated ions interact with carbonyl oxygens (won’t interact with sodium) which pull off water and allow the ion to pass through

The rate of facilitated diffusion is limited by…

The rate at which the carrier protein can undergo conformational change

GLUT 4

Glucose co-transporter that is activated by insulin and can increase the rate of facilitated diffusion of glucose as much as 10-20x

Found in skeletal muscle

The rate at which substances diffuses inward is proportional to the concentration of molecules on the _______ and vice versa is true

Outside

The rates of diffusion are reliant on what 3 major factors?

Proportional to the concentration difference across the membrane

The membrane electrical potential

Pressure difference across the membrane

How does membrane potential effect rates of diffusion

Even if there is no concentration gradient if there is a large enough membrane potential ions will move

The membrane potential can also reach a point that even if there is a concentration gradient no movement of ions will occur

Osmosis

The natural movement of water across a semi-permeable membrane that is dependent on the level of solutes across either side of the membrane

What is the most abundant substance to diffuse through the cell membrane?

Water

Osmotic pressure of solution

The pressure required to stop osmosis

How is osmotic pressure determined?

Number of particles / volume of fluid

Why do we measure number of particles instead of mass in osmotic pressure?

Particles on average exert the same amount of pressure against the membrane regardless of mass

Osmolality

The concentration of a solution in terms of number of particles per kg of solvent

Measured in osmoles

Osmolarity

Osmolar concentration expressed as osmoles per liter of solution

If a substance dissolves into separate particles it will not change osmolarity since the mass of the substance is the same. (T/F)

False

A solution that has low osmolarity and osmotic pressure will empty out of the stomach _____ than a high osmolarity and osmotic solution

Quicker

Active transport

The movement of molecules against the concentration gradient using ATP

Primary active transport

Energy is derived directly from ATP or some other high-energy phosphate compound

Secondary active transport

Energy is derived from energy that has been stored in the form of a concentration gradient

Sodium-potassium pump

Consist of a carrier protein, large alpha subunit with 3 binding sites for sodium (intracellular) and 2 binding sites for potassium (extracellular) and ATPase activity on the intracellular side

How many sodium ions exits the cells and how many potassium ions enter the cell through the sodium potassium pump?

3 exit 2 enter

The inside of the cell is more _____ than the outside of the cell

Negative

Besides being the main contributor to the cells membrane potential the sodium-potassium pump also aids in the control of…

Osmosis, prevent cell from bursting

SERCA pump

Active transport pump situated within the endoplasmic reticulum that pumps calcium into the ER

Cells that are highly metabolically active (neurons, glandular cells, etc.) expend majority of their energy on…

Pumping of ions to create concentration gradient

What is are some of the main substances transported via secondary active transport?

Glucose

Amino acids

What causes a conformational change in secondary active transport carrier proteins

Binding of the molecules being transported

SGLT-2

Secondary active transport co-transport for glucose and sodium

Primary mechanism by which glucose enters the cell from proximal renal tubules and makes it way to capillaries

Counter transport

Ions bind to the carrier protein and the substance to be transported binds to the other side, the ion and the desired substance travel opposite