Osmosis

Osmosis is a special example of diffusion. It is the diffusion of water through a semipermeable membrane from a more dilute solution to a more concentrated solution.

Note: diffusion and osmosis are passive, i.e. energy from ATP is not used.

A semipermeable membrane is a barrier that permits the passage of some substances but not others; it allows the passage of the solvent molecules but not some of the larger solute molecules.

Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes. The presence of particular solutes stimulates the membrane to open specific channels or trigger active transport mechanisms to allow the passage of those chemicals across the membrane.

Some major examples of osmosis

• Absorption of water by plant roots.

• Reabsorption of water by the proximal and distal convoluted tubules of the nephron.

• Reabsorption of tissue fluid into the venule ends of the blood capillaries.

• Absorption of water by the alimentary canal — stomach, small intestine and the colon.

Osmoregulation Osmoregulation is keeping the concentration of cell cytoplasm or blood at a suitable concentration.

(a) Amoeba, living in freshwater, uses a contractile vacuole to expel the excess water from its cytoplasm. (b) The kidneys maintain the blood at the correct concentration.

Osmosis and Plant Cells (a) Plant Cells in a Less Concentrated Solution

• the plant cells gain water by osmosis.

• the vacuole and cytoplasm increase in volume.

• the cell membrane is pushed harder against the cell wall causing it to stretch a little.

• the plant tissue becomes stiffer, it has swollen slightly and has increased in size and mass.

(b) Plant Cells in a More Concentrated Solution

• the plant cells lose water by osmosis.

• the vacuole and cytoplasm decrease in volume.

• the cell shrinks away from the cell wall.

• shrinkage stops when the cell sap is at the same concentration as the external solution.

• the plant tissue becomes flaccid, it has shrunk slightly and has decreased in size and mass.

Turgor

Turgor is the pressure of the swollen cell contents against the cell wall when the external solution more dilute than the cell sap of the vacuole.

Role of Turgor in Plants

• Mechanical support for soft non-woody tissue, e.g., leaves.

• Change in shape of guard cells forming the stomatal opening between them.

• Enlargement of young immature plant cells to mature size.

Osmosis and Food Preservation

• The food is placed in a high salt or sugar medium.

• The salt or sugar concentration is higher than the cytoplasm of bacteria or fungi.

• Bacteria or fungi, that contaminate the food, will lose water by osmosis and their metabolism will decline.

• Many will die but some bacteria may survive by forming dormant resistant endospores.

• Meat and fish are often preserved in salt.

• Fruit is commonly preserved in sugar as in jam or syrup.

Active Transport

Active transport is the energy demanding transfer of a substance across a cell membrane against its concentration gradient, i.e., from a region where the substance is in lower concentration to where it is in higher concentration.

Special proteins within the cell membrane act as specific protein ‘carriers’. ATP generated by respiration supplies the energy for active transport.

Major Example of Active Transport Reabsorption of glucose, amino acids and salts by the proximal convoluted tubule of the nephron in the kidney.

Mandatory Activity

Demonstration of Osmosis

• Tie a knot at one end of about 20 cm length of semipermeable Visking tubing.

• Half fill the tubing with a strong sucrose (table sugar) solution.

• Seal the open end by tying a knot in it.

• Set up a similar tube with tap water.

• Measure the mass and ‘stiffness’ of both tubes.

• Place both tubes into a beaker of tap water.

• Allow to stand for about 30 minutes.

• Results: the sucrose tube is has increased in size, is much stifferthe external water has decresed in volume; the tap water tube shows no change.

• Conclusion: water passed through the tubing into the sucrose solution by osmosis.

DNA - Structure, Replication, Profiling and Screening

DNA Structure  

DNA - deoxyribonucleic acid

• DNA Structure P P SA = TS P P SC ? GS P P ST = AS P: phosphate S: deoxyribose (5C) sugar Nitrogenous Bases A: adenine T: thymine G: guanine C: cytosine P + S + Base = Nucleotide Nucleotide: subunit of DNA. Adenine and guanine are purines bases Thymine and cytosine are pyrimidines bases Must have a minimum of three pairs of bases in the diagram. Hydrogen bonds link the complementary base pairs. Two between A and T. Three between G and C.

  DNA is the genetic material of all living cells and many viruses.

• The structure of DNA: an alpha double helix of two polynucleotide strands.

• The genetic code is the sequence of bases on one of the strands.

• A gene is a specific sequence of bases which has the information for the formation of a particular protein.

• DNA is self-replicating — it can make an identical copy of itself.

• Replication allows the genetic information to pass faithfully to the next generation.

• Replication occurs during interphase just before mitosis and meiosis.

• The chromosomes contain most of the cell’s DNA.

• DNA is present in mitochondria and chloroplasts.

Textbook Diagram: molecular structure of DNA.

The genetic information is held within the base sequence along a DNA strand.

A gene is a specific section of a DNA strand that contains the information for a particular protein or polypeptide.

Coding Structures

• These are the parts of the DNA that contain vital information for the synthesis of Protein or RNA.

• These coding sequences are present within genes.

• Non-coding Structures.

• These are the parts of the DNA that do not contain critical information for the synthesis of protein or RNA.

• The non-coding sequences are found between genes and within genes.

• These non-coding sequences have been termed ‘junk DNA’ but they do play a role in gene expression, act as spacer material, permit the synthesis of many new proteins and play an important role in evolution.

• Non-coding DNA makes up 95% of human DNA.

• Non-coding DNA within genes are called introns.

DNA Replication

Replication means to make a perfect identical copy.

• Nucleotides are synthesised in huge quantity in the cytoplasm.

• An enzyme unzips the two complementary strands of DNA.

• New complementary nucleotides link to the exposed bases on the separated strands.

• A new complementary strand is built along each ‘old’ strand.

• Two DNAs, identical to the original and each other, are now present.

• Each DNA is ‘half old’ and ‘half new’.

Textbook Diagram: DNA replication.

DNA Profiling