CE Biology-A Summary

Part 2-The Cell 

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There are many biochemical reactions taking place in a living cell, which can then exhibit its life processes.

Metabolism refers to all of the chemical reactions taking place in cells.

The metabolic rate is the overall rate of chemical reactions of an organism.

Catabolism: ¡§breaking down¡¨ reactions in cells

Anabolism: ¡§building up¡¨ reactions in cells

In order for biochemical reactions take place at a temperature that the cell can survive in, enzymes are involved.

Enzyme: an enzyme is a protein that increases (catalyses) the rate of a biochemical reaction.

Substrate: a substrate is a molecule, which after combining with an enzyme is converted to a product.

Active site: the active site of an enzyme is the specific region which has a distinctive shape to which the substrate(s) may bind to form an enzyme-substrate complex.

The enzyme-substrate complex then splits up, releasing product(s) and the enzyme.

Properties of enzymes:

1.      They are made of protein so they are easily denatured by heat and change in pH.

2.      They can speed up both the forward and backward reactions of a reversible reaction.

3.      They are very specific in action ¡V ¡§lock and key¡¨ hypothesis~ Each enzyme (key) acts on only one or a few substrate(s) (lock) that can fit into its active site, and it therefore catalyses only one kind of reaction.

4.      They are not used up and changed in the reaction.

5.      They are required in small quantities only as they remain unchanged at the end of the catalytic reaction and so can be used again.

6.      Their action is affected by different external factors: temperature, pH, enzyme concentration, substrate concentration and presence of inhibitors.

Effect of temperature on enzyme activities:

0^oC-5^oC the reaction rate is very low because the enzymes are inactive at this temperature range.

5^oC-37^oC as temperature rises, the kinetic energy of both enzyme and substrate molecules increases. Their chance of colliding with one another increases, more enzyme-substrate complexes and products will be formed, and thus the reaction rate increases.

At 37^oC optimum temperature ¡V the reaction rate is maximum.

37^oC-60^oC as temperature rises, more and more enzyme molecules are denatured and the rate of reaction appears to fall.

Effect of pH on enzyme activities:

Enzymes work best in their optimum pH. A change in the pH value alters the shape and thus the activity of enzymes.

For metabolism to occur, some raw materials are required to be taken into the cell while some of the products are to be passed out, across the cell membrane.

Diffusion - is the random movement of molecules due to their kinetic energy. Net movement of substances from a higher concentration to a lower concentration. It is the random movement of particles along their concentration gradient. No net movement ¡V diffusion in each direction continues at the same rate.

Diffusion rate is faster when temperature increases, concentration gradient increases and the size of particles decreases.

Osmosis - is a special form of simple diffusion involving movement of water from a higher to lower water potential through a selectively permeable membrane.

Water potential is the amount of free energy possessed by water molecules. Pure water has the highest water potential and has a value of zero. Dissolved particles tend to lower the free energy of water. Therefore solutions have lower and negative water potential.

When comparing the water potentials of two solutions separated by a selectively permeable membrane.

(a)    The one with higher water potential is called a hypotonic solution.

(b)   The one with lower water potential is called a hypertonic solution.

(c)    If both solutions have the same water potential, they are known as isotonic solution.

 Active transport ¡V is the movement of a substance across the cell membrane from a lower to a higher concentration, i.e. against the concentration gradient. Metabolic energy is required for the process. Diffusing substances require help to cross the membrane, and this comes from the specific carrier molecules. Since specific carrier is involved, active transport has a selective manner.

Importance

Diffusion: moving oxygen and food molecules into cells, carbon dioxide and other waste molecules out of cells. Moving substances within cell.

Osmosis: moving water into and out of cells and across living tissue.

Active transport: moving necessary materials and wastes across cell membranes if diffusion is too slow or if there are no concentration gradient or if it do not allow net diffusion.

Cell division

Cell is the basic unit of life. All organisms are made up of cells. Some are unicellular while others are multi-cellular. In multi-cellular organisms, there is a division of labour among different cells.