THE CELL (III) TRANSPORT IN AND OUT OF CELL

Passive diffusion net movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration. Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion.

Diffusion of a dye in a beaker of water

Diffusion across cell membrane across the lipid bilayer via membrane channels (pores) via carrier proteins ( facilitated diffusion )

Diagram of a cell membrane

Structure of a phospholipid, space- filling model (left) and chain model (right). Diagram of a phospholipid bilayer.

Movement of selected molecules across the cell membrane.

The rate of transport depends on : The concentration gradient across the membrane. Temperature. molecular size - small molecules tend to penetrate more rapidly. Polarity of molecules - non-polar molecules pass through readily.  across the lipid bilayer Diffusion across cell membrane

The solute molecules are bound to a specific protein carrier (permease), which carries them across is driven by potential energy of a concentration gradient. e.g. movement pf glucose into red blood cells, which is not inhibited by respiratory inhibitors  Diffusion via carrier proteins ( facilitated diffusion )

Facilitated diffusion through a carrier protein e.g. movement of glucose into a cell down a conc gradient

Facilitated diffusion through a protein carrier

Many small ions and polar molecules, e.g. Na +, K +, diffuse across the membrane readily through some channels The channels are protein in nature and specific to one type of molecule or ion.  Diffusion via membrane channels (pores)

Diffusion through membrane channel

Active transport against a concentration gradient. expenditure of metabolic energy, usually in form of ATP (inhibited by respiratory poison, e.g. cyanide) asymmetric; one direction only.

ADP + Pi ATP Substance Low conc Substance High conc Active Transport Carrier Protein

Cells and tissues carrying out active transport are characterized by : numerous mitochondria A high concentration of ATP A high respiratory rate. Examples of active transport:  Transport of sugars and amino acids across the epithelial cells during absorption in the intestine and kidney.  Sodium pump in the nerve cell.  Active transport of calcium into the sarcoplasmic reticulum in muscle cells

Endocytosis and Exocytosis Endocytosis infolding or extension of the plasma membrane to form a vesicle or vacuole.  Phagocytosis (cell eating)  Pinocytosis (cell drinking)

Phagocytosis (cell eating) Invagination of membrane surrounding the particles. depression then pinched off to form phagocytic vacuole. Lysosomes release their enzymes Useful products are absorbed and undigested matter is expelled by exocytosis.

in liquid form Vesicle are extremely small Pinocytosis (cell drinking)

Endocytosis (pinocytosis) by a capillary endothelial cell. Note the desmosome connection between opposing cell membranes

inhibited by respiratory poison and other metabolic poison Endocytosis is not an alternative process of active transport, but rather a supporting one. It provided a much larger interior interface where passive and active transport are carried out more efficiently than at surface membrane Phagocytosis and pinocytosis are active process and require energy

Exocytosis the reverse of endocytosis two main functions : replace the plasma membrane that have been removed by endocytosis or to add new membrane Provide a route for releasing the molecules synthesized by the cell, e.g. secretion, hormones.

Movement of water across the plasma membrane

Osmosis - This is the passage of solvent (water) molecules from a regions of their high concentration to a region of their low concentration through a differentially permeable membrane

Free energy - is defined as the energy available (without change in temperature) to do work. It depends on the number and activity of the molecules Molecules will move from regions of higher free energy to regions of lower free energy. Chemical potential - of a substance is the free energy per mole of that substance.

Water potential (  ) - It is the chemical potential of water. The water potential of pure water at 25 C and one atmospheric pressure is set arbitrarily as zero. The water potential of a system can be defined as the difference in chemical potential of water in this system and that of pure water at the same temperature and pressure. All solution have lower water potentials than pure water and therefore have negative values of .

The water potential of a plant cell has two components : osmotic potential and pressure potential. water potential = osmotic potential + pressure potential.  =   (  s) +  p

Osmotic potential (   ) / Solute potential (  s) - is defined as the component of water potential that is due to the presence of solute. Solute particles decrease the free energy and hence the chemical potential of solvent molecules. always negative The more concentrated a solution is, the lower is its osmotic potential ( more negative ). Osmotic potential is a measure of the tendency of a solution to pull water into it.

The presence of a solute decreases the water potential of a substance. Thus there is more water per unit of volume in a glass of fresh- water than there is in an equivalent volume of sea-water. In a cell, which has so many organelles and other large molecules, the water flow is generally into the cell.

Pressure potential (  p ) - This is the component of water potential that is due to hydrostatic pressure. This can be regarded as the capacity of the existing hydrostatic pressure of a cell to drive water out of it. The pressure is often a positive inside the living cell, and has a positive value.

 p = 0  s = - 4 MPa  cell = ? Cell at start Placed in distilled water Cell at equilibrium  p = ?  s = ?  cell = ? Then placed in solution of  s = - 3MPa  p = ?  s = ?  cell = ?

 p = +8 MPa  s = - 12 MPa  cell = ?  p = +2 MPa  s = - 14 MPa  cell = ? Cell ACell B Calculate the water potential in cell A and cell B. What is the direction of water flow ? Calculate the  p,  s and  cell at equilibrium.

 cell = -1 MPa Xylem A continuous column of water under tension, i.e.  = -3MPa Water will move from the root cells into the xylem. Explain.

Water relations and cell shape in blood cells

Water relations in a plant cell

Determination of the osmotic potential and water potential

Determination of water potential of a plant tissue Pieces of tissues carefully measured length or weight are placed in a range of solutions of different concentration. The tissue samples are allowed to come to equilibrium with the surrounding solution. The change in size or weight is measured after the tissue reaches equilibrium. In the solution in which no size change ( or weight change ) takes place, the water potential of the cell is equal to that of the solution.

Conc. of sucrose solution / M Volume of distilled water / cm Volume of sucrose solution / cm Table 1 : Dilution table of sucrose solution

Lengths of beetroots strips left in different concentrations of sucrose solution for 24 hours. Molarity of sucrose solution / M Initial length of beetroot strip/ cm Final length of beetroot strip/ cm Percentage change in length / %

Potato tissue takes up water in hypotonic solution ( M) causing it to increase in length. However, the potato tissue releases water in hypertonic solution ( ) causing it to decrease in length. There would be no change in length in isotonic solution (0.27M). WP of potato is equivalent to the WP of 0.27 M sucrose solution

Hypertonic solutions are those in which more solute (and hence lower water potential) is present. Hypotonic solutions are those with less solute (again read as higher water potential). Isotonic solutions have equal (iso-) concentrations of substances. Water potentials are thus equal, although there will still be equal amounts of water movement in and out of the cell, the net flow is zero.

Determination of the mean osmotic potential of the cell sap

Principle WP = OP + PP If PP = 0, then WP = OP PP = 0 > wall pressure drop to 0 > cytoplasm / cell membrane just touches the cell wall, i.e. Incipient plasmolysis

a graded series of solution of different concentration Small pieces of tissues are placed in each solution examined microscopically after equilibrium some of them show signs of plasmolysis when 50 % of cells have plasmolysed. 50 % of cells are unplasmolysed, 50% of cells will have higher OP than the solution and 50% will have lower OP than the solution. Therefore the solution at this point has approximately the same osmotic potential as the median osmotic potential of tissue. Method

Percentage of cells plasmolysed in different concentrations of sucrose solution Molarity of Sucrose solution / M Percentage of plasmolysed cell

% of plasmolysis Conc (M) If all the cells have the same OP, all the cells would become plasmolysed at certain conc. But, OP of cells show variations.

No. of cells Osmotic potential

At 0.2 M, 8% of cells plasmolysed 8 % of cell OP > OP of 0.2M sucrose

At 0.4 M, 78 % of cells plasmolysed 78 % of cell OP > OP of 0.4 M sucrose

At 0.65 M, 100 % of cells plasmolysed 100 % of cell OP > OP of 0.65 M sucrose

At 0.35 M, 50 % of cells plasmolysed 50 % of cell OP > OP of 0.35 M sucrose The mean OP of cells is equivalent to that of 0.35 M sucrose solution

Comment on the statements found in a student’s report In sucrose solutions of 0 M to 0.2 M, below 50 % of cells are plasmolysed. This means that the solution is hypotonic to the cell sap. From the graph, the osmotic potential of cell sap is 0.38 M.