Transport and Water Movement

Extracellular fluid

• 1/3 of the body’s fluid is extracellular

• it is in constant motion throughout the body

  • important for nutrient transfer so cells can generate ATP and acquire oxygen

    • difference in ion concentration between external and internal environment requires energy

  • movement is necessary to clear cellular waste like CO2

• nutrients: glucose, fatty acids

• ions: sodium, chloride, bicarbonate

• oxygen, CO2

• waste products

What percent of the body mass is water?

~60%

Intracellular fluid

• nutrients: glucose, fatty acids

• ions: potassium, magnesium, phosphate

• oxygen, CO2

• waste products

Is sodium or potassium found in higher concentrations outside/inside of the cell?

• Na is higher outside

• K is higher inside

• energy is necessary to keep this imbalance and keep cells alive

What general concepts hold true for the internal environment of cells?

• high organic content

• low ionic content

• high potassium relative to sodium

• low chloride

• neutral pH

How is extracellular fluid transported through the body?

1. through the body and the blood circuit

2. from capillaries to intercellular spaces

   • capillaries are never far from cells

How are metabolic end products removed?

• lungs (CO2) → removed using blood to move CO2 to the lungs for exhalation

• kidneys (urea, uric acid, excess of some ions) → carried by blood to kidneys for filtration

• liver and GI tract → exchange of waste through blood

Basic description of membranes

• lipid bilayers

• proteins that penetrate the bilayer

• barrier against water movement

• lipids can freely cross it

• substances that want to cross must either go through the membrane or through a protein

What molecules can and can’t pass through a lipid bilayer?

• small, dissolvable molecules like oxygen and CO2 will pass via diffusion

• larger molecules must pass through membrane proteins

Diffusion

continuous movement of molecules among each other; this continuous movement generates pressure to generate equilibrium

Simple (passive) diffusion

• unassisted movement of molecules through a membrane that occurs without any interaction of carrier proteins

  • ex. oxygen, nitrogen, carbon dioxide, steroids, and alcohols

• can also be through an opening of the membrane (if not lipid soluble)

• direction and rate of flow is concentration dependent

Diffusion through protein pores

• pores are composed by proteins that form open tubes through the membrane

• the diameter of the pore and its charge provides selectivity that allows only certain molecules to pass

• for example, aquaporins allow the passage of water in a single line, while hydrated ions are too large too pass

• direction of flow is concentration dependent

Facilitated diffusion

diffusion through channel proteins that act as gates

Voltage gating

• gate shape responds to changes in electrical potential

  • ex. action potential generation

Chemical of ligand gating

• gate shape responds to changes in the chemical environment

  • ex. acetylcholine gated channels in skeletal muscle

Factors that affect net rate of diffusion

1. rate of diffusion depends on the difference of concentration between the inside and the outside of the cell

   • net movement occurs toward the lower concentration, but there is still movement to the high concentration side

2. electrical potential applied across the membrane when the ion concentration is the same at both sides of the membrane

   • positive polls attract negative charges while negative polls attract positive ones

   • opposing force is created

3. effect of a pressure difference

Osmosis

• net movement of water caused by concentration differences of water

• water tends to move toward higher solute concentrations

Osmotic pressure

• the pressure needed to stop the osmosis is called osmotic pressure

• the osmotic pressure is determined by the number of particles in solution and not by their mass

  • i.e. ten large molecules will exert the same pressure as ten small molecules

Hypotonic solutions

net movement of water into the cell; cell can become enlarged and burst

Hypertonic solution

net movement of water out of the cell; the cell can shrink

Isotonic solution

net movement of water is zero, there is an equal amount of movement into and out of the cell

Water movement will occur if …

there is an osmotic or hydrostatic pressure gradient between body and fluid compartments

Active transport

when a cell membrane moves molecules against a concentration gradient

Primary active transport

• energy is derived from ATP, which is hydrolyzed by ATPase

• a carrier protein that penetrates through the membrane is involved

Sodium-potassium pump

• transports 3 Na+ out for every 2 K+ brought it

• creates a negative voltage inside cells due to more positive charges being pumped out than brought in

• controls the volume of cells

  • intracellular proteins are negatively charged; they attract positively charged ions

  • ions cause osmotic pressure to increase inside the cell

  • water follows

Secondary active transport

• also called co-transport and counter-transport; based on energy stored as a difference in the concentration gradient of molecules

  • i.e. Na+ is mainly outside the cell, diffusion energy can pull substances across the membrane with a coupling protein and Na+

• ATP is not used here, but energy is still used to move substances from low to high concentration

How do massive molecules enter cells?

• endocytosis

  • when the molecule contacts the coated pit, it invaginates and breaks away from the cell

Large molecules enter the cell via …

pinocytosis

Foreign substance digestion

• after a vesicle is formed from something entering the cell, lysosomes attach and empty their hydrolases into it

• digestive vesicles are formed

• lysozyme dissolves bacterial walls

  • acidic pH

  • lysoferrin