Lecture 2 - passive movements through CM

Provide a brief overview of the sugar cube experiment

1. place coloured sugar cube in water

2. cube dissolves, establishes a steep conc. gradient with sugar and dye

   1. sugar and dye conc. are high near cube and negligible elsewhere

3. time passes, sugar and dye spread in solution

4. eventually, sugar and dye m/ are spread evenly

What happened during the sugar cube experiment?

• a concentration gradient between sugar and its surroundings was established

• there’s a net movement of sugar m/ b/w regions from high to low conc. due to diffusion

What does diffusion drive?

passive movements

What are the requirements for diffusion?

1. there must be a concentration gradient

2. random motions of molecules (BROWNIAN motion)

what is diffusion?

random movement of m/ from two regions with high to low concentration

simple diffusion

• a m/ can move via a cell membrane without any help

when does net diffusion end?

• ends when dynamic eq’m is established

• movements can happen at eq’m, but no net distribution can occur as m/ are already equally distributed

when will net diffusion be fastest across a cell membrane?

• When difference between number of m/ inside and outside cell is large 

  • As rate of diffusion is driven by concentration gradient 

  • Greater concentration difference = steeper gradient which causes m/ to move at a faster net rate

• not b, when the difference in the number of molecules between the inside and outside of the cell is small

  • as the concentration on both sides become closer to equal, rate of net diffusion slows down until dynamic equilibrium is reached

What are the factors affecting rate of net diffusion?

• diffusion is proportional to:

  • D(diffusion constant of gas) 

  • SA of tissue (A)

  • Partial pressure (P) difference of the gas on each side of the tissues 

• diffusion is inversely proportional to:

  • Thickness of tissue (x) 

    • J = S x A x (Pout-Pin)/x

describe the variables in fick’s law of diffusion

• flux(J) = number of molecules passing through a cross-sectional area in a certain period of time 

• Flux is proportional to conc. Gradient 

• Fick’s law of diffusion 

  • Thus, J = -D ( delta C)/ (delta X) 

  • Where D = diffusion coefficient and can include -ve sign to denote diffusion always occurs DOWN a conc. Gradient 

net diffusion

• there’s a difference in concentration (delta C) over a certain distance (delta X) 

  • Delta C / delta X 

  • Delta C is the concentration difference between the 2 compartments 

How much oxygen is able to diffuse through the respiratory membrane per minute under normal resting conditions?

Diffusion capacity of oxygen = 21 ml/minute/mmHg

Plung-Pblood= 11mmHg

• Pulmonologists use a variation of Fick’s law where

J= D x A x Pout-Pin/x

Is rewritten as: 

J = diffusion capacity x (Pout-Pin )

• Variables:

  • Diffusion capacity of oxygen = 21 ml/minute/mmHg

  • Plung-Pblood= 11mmHg

  • So…..

  • 231 ml of oxygen diffuses through respiratory membrane per minute at rest

how would gas exchange be affected in patients with pulmonary edema?

• Gas exchange is impaired in pulmonary edema

• as excess fluid fills the alveoli and interstitial spaces, increasing the diffusion distance and causing ventilation-perfusion

facilitated diffusion

• the movement of a m/ through the CM with the help of a channel or carrier protein embedded in the membrane 

• Conc. gradient drives this movement typically enables the movement of ions

movement of potassium via CM

• More conc. In cytosol and have tendency to move out of cell 

  • remember: ions always move from high to low conc areas

• Leaves behind trapped anions 

• Membrane potential becomes negative 

describe the movement of sodium in the cm

• More conc. Outside than inside cell 

• As they enter into cell, they neutralize excessive negative charge in cytosol and membrane potential becomes positive 

describe the movement of chloride in the cm

• Cross membrane together with a permeable cation (usually K+) 

• As CL enters, it tends to make membrane potential more negative 

what influences cell physiology in terms of membranes?

• membrane mediated process

• ex. chxm and electrical process influence our ability to engage in physiological processes

• especially activity of several types of ion channels

What is the difference between net diffusion and dynamic equilibrium? Explain the processes.