Membrane transportMembrane transport

Remember-There is ion gradient across the cell membrane.

Ion Extracellular Intracellular

Na+ 140 mm 10 mm

K+ 4 mm 140 mm

Ca++ 2.5 mm 0.1 Um

Cl- 100 mm 4 mm

Cell membranes maintain their gradients by:2. Preventing ion flux3. Active transport of ions from one side to

the other side of P.M. The movement of materials into and out

of the cells influences the internal concentration of molecules. The ability of cells to move materials in and out can determine whether they will live or die

Transport is either passive or active

PASSIVE TRANSPORT- No energy expenditure

- Passive or inactive because the force moving these molecules or ions is NOT external to them. It comes from KINETIC ENERGY the particles already have.

- They have this motion because they have heat.

- All molecules are in constant motion above a very cold temperature called absolute zero ( -273 C )

- Amount of molecular movement depends on;

1. Temperature 2. Physical properties

- Less heat = Less kinetic energy = Less motionI ) Diffusion ( simple diffusion )- It is the movement of a substance from an area

of its greater concentration to an area of its lesser concentration. This is called down the concentration gradient.

- When we have difference in concentration between 2 regions = Concentration gradient

- The diffusion is driven by the kinetic energy the molecules posses.

- Diffusion occurs when molecules move randomly away from each other in liquid or gas

- The rate of diffusion depends on; 1. Temperature- The higher….the greater 2. Size- Smaller the greater 3. Type of diffusing molecules- Most short distance transport of materials

into and out of cells occur by diffusion.

- When molecules are dispersed evenly, there is no longer any diffusion because there is no further concentration gradient = State of equilibrium

II ) Osmosis- It is the diffusion of water across a

semipermeable membrane( from an area of high concentration of water to a region of low concentration )

- Therefore, osmosis occurs in response to the concentration of solutes dissolved in water.

- In hypertonic solution, water diffuses out of the cell till equilibrium is established.

- If cells lose too much water …Shrivel & shrink

- On the reverse, in hypotonic solution, water will move to the inside…cell swelling

- Many cells function in hypotonic environment such as unicellular fish water organisms. Water diffuses constantly into these organisms

- Since they require lower concentrations of water in the cytosol to function normally, these unicellular organisms must rid themselves of the excess of water that enters by osmosis.

- An example; paramecia. They do this through contractile vacuoles. Such vacuoles collect the excess of water and then contract pumping the water out of the cell.

- Unlike diffusion and osmosis, the pumping action requires energy and thus it is a form of active transport

III )Facilitated diffusion (mediated transport) - Most molecules cannot cross the

membrane by simple diffusion.- This type of diffusion utilizes membrane

protein channels to allow molecules to freely diffuse in and out of the cell

- These channels come into greatest use with small ions ( K+, Na+, Cl- ) = Ion channels.

- The speed of transport is limited by the number of the protein channels available, whereas the speed of diffusion is dependent only on concentration gradient

- These proteins are called; 1. Carrier proteins OR 2. Porters OR 3.Porting system OR 4. Transport system OR

5. Pumps OR6. Translocases- Carrier proteins change shape when

molecules attach to them and this change of proteins enables the molecule to cross the membrane

- The facilitated diffusion can help substances move either into or out of the cell depending on concentration gradient

- An example; facilitated diffusion of glucose transport into the cell.

- Some ion channels are always open. Others have gates that open to allow ions to pass or close to stop their passage.

- This happens in response to conditions in the environment and the cell and these conditions are;

Stretching of the cell membrane Electrical signals Chemicals in cytosol or external

environment- Steps of transport: 1. Recognition of the substance to be transported 2. Translocation: Movement of the substance from one side of the membrane to another

3. Release: to the other side of the membrane4. Recovery: Return of the carrier to the original site.- The carrier proteins are enzyme-like in the

following ways;

1. They are specific

2. Transport can be inhibited by specific

inhibitors.

3. They exhibit saturation

- Diffusion is not saturable and its rate increases with increased concentration.

CHARACTERISTICS OF CARRIERS

2. Change conformation during transport

3. Sometimes, carriers move more than one molecule simultaneously

4. They do not catalyse the reaction

Terminology

2. Uniport = Single molecule moving in one direction

3. Symport = Two molecules move in the same direction simultaneously

4. Antiport = Two molecules move in opposite directions simultaneously

ACTIVE TRANSPORT

- Energy expenditure

- Take molecules up and down their concentration ( against concentration gradient )

- Active transport is limited by the number of protein transporter molecules present

Types:

2. Primary

3. Secondary

- Primary: Using energy through ATP hydrolysis at the cell membrane itself to cause a conformation change that results in the transport of molecules through the protein. The best known is;

NA +-K + pump

- The structure of Na+-K+ pump is a tetramer

- ( of 2 types α2β2 ).- The β subunit is a glycoprotein with the

carbohydrate on the external surface of membrane

- The Na+-K+ pump or Na+-K+ ATPase is an antiport

- This pump is specific for Na+ and can substitute for K+

- Stoichiometry: 3 Na+ getting out and 2 K+ in for every ATP hydrolysis

- The proposed activity of Na+-K+ ATPase (pump )shows the role of ATP in effecting the conformational change of the carrier

STEPS

1. Na+ attaches on the inside of the membrane

2. Protein conformation changes which are due to;

a. Phosphorylation of protein by ATP

b. Affinity of protein to Na decreases

3. Na+ leaves

4. K+ from outside binds

5. K+ de-phosphorylates the protein

6. The protein now goes to its original state

7. K+ now dissociates

- Secondary:

- Using energy to establish a gradient across the cell membrane.

- Then utilizing the gradient to transport the molecule of interest.

- E. coli establishes a proton ( H+ ) gradient across the cell membrane using energy to pump protons out of the cell.

- These protons are coupled to lactose at the lactose permease transmembrane protein

- The lactose permease uses energy of the proton moving down its concentration gradient to transport lactose into the cell

- This coupled transport in the same direction across the membrane is called symport

- E. coli uses similar proton to transport ribose, arabinose and several amino acids

- Another example is Na+-glucose secondary transport

- In this, using Na+-K+ ATPase as the first step generating a strong Na gradient across the cell membrane

- Then, Na+-glucose symport protein uses Na+ gradient to transport glucose into the cell

- This system is used in a novel way in human gut epithelium. These cells take in glucose and Na+ from the intestine and transport them through the blood stream using the concentration action of Na+-glucose symport, glucose permease ( a glucose facilitated diffusion protein and Na+-K+ pumps)

- Another example is Na-amino acid symport

- There are at least 6 proteins of different specificity that employ the mechanism

Active transport-Movement in vesicles-Bulk transport-Endocytosis & exocytosis

- Some molecules as complex proteins are TOO LARGE to cross the cell membrane

- Large molecules, food and others are packed in membrane bound sacs= vesicles and move across the membrane

- Types 1. Endocytosis ( pinocytosis &

phagocytosis )

2. Exocytosis

1.Endocytosis: Begings when a particle contacts the plasma membrane. An invagination of the membrane occurs until the particle is completely wrapped in membrane

2.The wrapped particle is now inside the cell

3.The vesicle can fuse with lysosomes or they release their content inside the cell

A/ Phagocytosis

B/ Pinocytosis

- The difference between the 2 has to do with the size of the molecule.

- Phagocytosis ( cell eating ): Solid material is involved

- Pinocytosis ( cell drinking ) occurs when small particles are in solution are ingested

- Exocytosis: Reverse of endocytosis. Plasma membrane is also involved.These products are packaged in Golgi apparatus which then fuse with the cell membrane and then to the outside. E.g. waste products, mucus