1. Cell Membrane Transport

2. Cell Membrane Transport
Interstitial fluid
Surrounds cells
Contained within tissues.
Part of the extra-cellular water compartment
Derived from blood plasma
Contains amino acids, vitamins, hormones, salts, waste products, etc.
Cells must be able to transport materials across the cell membrane
Movement of materials is controlled by the plasma membrane, that is selectively permeable so that it allows only certain materials pass in and out of the cell to maintain differences between ICF and ECF.

3. Mechanisms of transport across cell membrane are:
Passive transport (Diffusion)
Active transport
Bulk transport

4. Factors Affecting the Direction of Transport
The cell membrane is.
Impermeable to most water-soluble substances (substances that dissolve in water)
Closely controls passage of materials in and out of the cell.
Passive Transport versus Active Transport
Passive Transport
Movement of substances through the membrane without the use of energy from the cell is a physical or passive process.
Does not require ATP
Includes simple diffusion, facilitated diffusion, osmosis, and filtration.
Active Transport
Movement of material through the membrane that requires metabolic energy (ATP) is called an active physiological process.
Includes Primary and Secondary Active Transport

5. Driving Forces Acting on Molecules
Driving forces affect the direction of movement of molecules
Gradient
A difference in driving force (chemical or electrical energy) across a cell membrane that tends to push molecules in one direction or another
Always from higher to lower energy if allowed to move spontaneously
There are three types of driving forces
Chemical, Electrical, and Electrochemical

6. Driving Forces Acting on Molecules
Chemical driving force
Difference in energy due to a concentration gradient that causes a molecule to move from high to low concentration
Electrical driving force
Difference in energy due to a separation of charge that acts to move ions from high energy to low energy
Electrochemical driving force
Sum of the chemical and electrical driving forces

7. Simple diffusion Routs of diffusion:
Through lipid bilayer:the rate of diffusion is directly proportional to lipid solubility of substances:
Lipid soluble substances(O2, N2…)
Water molecules :as they are small & have high kinetic energy
Lipid insoluble : if they are small and unchrged

8. Through protein channels:
For transport of ions mainly
They have:
Selective permeability
Gates : their opening and closing are controlled by
Voltage gating :
Ligand gating: acetylecholine receptors

10. Facilitated Diffusion: Passive Transport Through Membrane Proteins
Particles must be helped through the membrane with the use of transmembrane proteins (carriers/transporters, channels/pores).
Requires a concentration gradient
Example
Glucose
Important substance that is lipid insoluble and is too large to pass through membrane pores.
Glucose molecules combine with a protein carrier molecule on the surface of the plasma membrane. The carrier changes shape and releases the glucose inside the cell then returns to its original shape to bring in another glucose on the outside of the membrane.

11. Transport Proteins in Facilitated Diffusion
Carriers
A transmembrane protein that binds to a molecule on one side of the membrane
Conformational change
The carrier “flips” to bring the transported molecule to the other side of the membrane
Transport is limited by the number of carriers available on the membrane.

13. Features of facilitated diffusion
Specificity of the carrier
Competition between similar substances
It’s rate ↑ with concentration gradient upto certain maximum rate
It is more sensitive to temperature

15. Osmosis: Passive Transport of Water Across Membranes
The flow of water across a selectively permeable membrane
Always from an area of high water concentration to an area of low water concentration.
A special case of diffusion of water across a selectively permeable membrane, such as the plasma membrane.
A semi-permeable membrane is freely permeable to water but not to solutes.
It is a very important process because water is found throughout cells and extra-cellular areas of the body.

16. Osmosis depends on A concentration gradient for water
Relative permeability of dissolved solutes
Osmosis occurs when:
There is more water and less solute on one side of the membrane
A high concentration of water or a low concentration of solute
And less water and more solute on the other side
A low concentration of water or a high concentration of solute
The concentration gradient is for water

17. Osmolarity Total solute concentration
Unit is osmole (Osm) or milliosmole (mOsm)
Normal osmolarity (concentration) of body fluids is 300 mOsm
Total solute concentration is 300 milliosmoles per liter
Depends on the total concentration of dissolved solutes
Example
150 mOsm NaCl
Dissolved in water the molecule separated into two particles, so osmolarity is doubled, 300 mOsm

19. Tonicity
It is the osmolaritity of a solution relative to the osmolarity of plasma

20. Tonicity
Refers to the relationships between body cells and the surrounding fluids.
A measure of the ability of a solution to cause a change in cell tone (volume or pressure) by promoting the osmotic flow of water.
Dependent upon concentration and diffusibility of the dissolved solutes
Impermeant solutes
Cannot cross cell membrane
Permeant solutes
Can move across cell membrane and add to the total solutes within cell

21. Types of solutions Iso-tonic Same osmolarity as plasma Hyper-tonic
Solution has greater osmolarity than plasma
Hypo-tonic
Solution has lower osmolarity than plasma

22. Isotonic
A solution that has the same concentration of solute (osmotic pressure) as body fluids.
Fluid surrounding a cell has the same concentration of solute as that inside of the cell.
No osmosis occurs.
Hypotonic
A solution that has a lower concentration of solute (osmotic pressure) than body fluids.
Hypotonic extra-cellular fluid has a lower concentration of solute than the concentration inside cell and causes water to move into the cell following its concentration gradient (more water outside, less inside).
Too much water moving into the cell membrane may cause the cell to burst.

23. Hypertonic
A solution has a higher concentration of solute (osmotic pressure) than the concentration found in body fluids.
Hypertonic extra-cellular fluid will cause water to leave the cell following its concentration gradient (more water inside, less outside) producing a shrunken or crenated cell.

25. Isotonic saline solution
A solution that is .9% saline because the body's red blood cells are .9% salt or NaCl.
Therefore, when an isotonic saline solution is introduced into the body, fluid equilibrium will be maintained.
Remember:
The key to understanding the above terms and process is to understand that hyper and hypo refer to the solute in the solution, not to the water.
Water will move toward the greater amount of solute because the concentration of water there is less.

26. Osmotic Pressure (π)
The membrane is selectively permeable in that it does not allow the solute to pass, it is not permeable to certain molecules, particles, or solute.
Remember that high solute concentration means low water concentration (requires more water to reach equilibrium) and low solute concentration means high water concentration (requires water to leave to reach equilibrium).
Osmosis will continue to occur or the water will continue to move until:
Equilibrium for water is reached so that the concentration of water and solute is equal on each side of the membrane.

27. Water movement changes the volume of water in the container or cell.
Osmotic pressure stops the movement of water.
Osmotic pressure is the amount of pressure required to prevent further water movement.
The ability of osmosis to generate enough pressure to lift a volume of water.
A potential pressure due to the presence of non-diffusible solute particles.
The greater the amount of non-diffusible solute, the greater the gradient attracting water across the membrane and the greater the osmotic pressure produced.
Example
NaCl is a very osmotically active particle because when it dissociates it produces two ions, or double the osmotic activity
Water movement changes the volume of water in the container or cell.

30. Passive Transport: Filtration
Particles forced through a filter or a membrane by hydrostatic pressure.
Hydrostatic pressure
Fluid pressure of the blood generated by the left ventricle
Opposed by osmotically active particles in the blood (plasma proteins).
Example
Blood pressure generated by the heart and blood vessels forces tissue fluid out of tiny openings in the capillary wall and leaving larger particles of blood cells and protein molecules inside the capillary.
Coffee filters work by the pressure from weight of the water above the coffee grounds forcing the flavored water through the filter and leaving the large particles of coffee grounds on the filter paper.
Filtration and osmosis are the major processes in the capillaries of tissues and the kidney.

31. Active Transport Processes
Movement of particles or solutes against a concentration gradient
Requires energy or cellular action with ATP
Primary Active Transport
Direct transport of substances using ATP
Secondary Active Transport
Movement of substances driven by concentration or electrochemical gradients created by Primary Active Transport mechanisms

32. Primary Active Transport
Solute pumping
Pump or protein carrier
An enzyme-like protein carrier that pumps or carries solutes such as ions of sodium, potassium, and calcium, into or out of the cell against their concentration gradients.
ATPase
The enzyme on the protein carrier or pump that catalyzes the breakdown or phosphorylation of ATP producing energy that drives the pump.
This action may require up to 40% of a cell’s supply of ATP

33. Sodium-potassium pump (Na+/K+ ATPase Pump)
Maintains the resting membrane potential of nerve and muscle cells
Sodium
Primary extra-cellular ion that is constantly “leaking” into cells.
Potassium
Primary intracellular ion that is constantly “leaking” out of cells.
The sodium/potassium pump constantly pumps 3 sodium ions out and 2 potassium ions into the cell, maintaining the relative negativity inside the cell.
All cells have a negative charge inside because of this mechanism.

34. Solute Pumping to Maintain the Membrane Potential
Pumps
Transport proteins that use energy from ATP hydrolysis to transport specific molecules against the electrochemical gradient across a membrane
Sodium-Potassium pump (Na+/K+ ATPase Pump)
Transports Na+/K+ ions in opposite directions across cell membranes
Move 3 Na+ ions out of the cell for every 2 K+ ions into cell
Specific for Na+/K+ and unidirectional
Phosphorylation of the pump protein causes a conformational change that turns the binding sites outward to expel Na+
Also decreases affinity for Na+ and increases its affinity for K+
Critical in maintaining resting membrane potential for nerve and muscle impulse conduction

36. Secondary Active Transport
Movement of a molecule that is coupled to the active transport of another molecule
One substance moves down its electrochemical gradient and releases energy in the process
This energy is then used to drive the movement of another substance against its electrochemical gradient

37. Cotransport (Symport)
Movement of 2 substances in the same direction
Example
Sodium-linked glucose transport
Couples the inward flow of sodium with the inward flow of glucose
Sodium movement with its electrochemical gradient releases energy that drives the movement of glucose against its concentration gradient

38. Countertransport (Antiport or Exchange)
Movement of 2 substances in opposite directions
Example
Sodium proton exchange
Couples the inward flow of sodium with the outward flow of protons (H+)
Energy released from the inward flow of sodium along its electrochemical gradient is used to drive the outward flow of protons against its electrochemical gradient

40. Pumps and Leaks
Differences in composition of intra- and extra-cellular fluid are maintained by pumps
Substances are constantly, passively leaking across cell membrane in the opposite direction and at the same rate that they are actively pumped across the cell membrane
Net flux across the cell membrane is zero

42. Bulk transport
Endocytosis:
Exocytosis :

43. Endocytosis
Endocytosis is the process by which cells ingest materials. The cellular membrane folds around the desired materials outside the cell. The ingested particle is trapped within a pouch, vacuole or inside the cytoplasm. Often enzymes from lysosomes are then used to digest the molecules absorbed by this process.

44. Types : pinocytosis and phagocytosis.
In pinocytosis, cells engulf liquid particles (in humans this process occurs in the small intestine, cells there engulf fat droplets)
In phagocytosis, cells engulf solid particles.

45. Exocytosis
Exocytosis is the process by which cells excrete waste and other large molecules from the protoplasm