Question

1. Why do ions have a difficult time getting through plasma membranes despite their small size? How are ions transported through biological membranes?
   
2. Where does the cell get energy for active transport processes?
   
3. Describe the position of the transition state on a vertical energy scale, from low to high, relative to the position of the reactants and products, for both endergonic and exergonic reactions. How do enzymes affect the energy of the reactants, product, and transition state.
   
4. Do you think that the E_A for ATP hydrolysis is relatively low or high? Explain your reasoning.

5. Explain in your own words the difference between a spontaneous reaction and one that occurs instantaneously, and what causes this difference.

6. Nearly all organisms on Earth carry out some form of glycolysis. How does this fact support or not support the assertion that glycolysis is one of the oldest metabolic pathways?

7. What is the primary goal of glycolysis and the citric acid cycle (think about what is being produced and what it is used for)?
   
8. Why is it important to have regulatory control over metabolic pathways like glycolysis?

Answer 1

1. Ions have specific charge, so are hydrophilic. So, they cannot associate with the hydrophobic membrane components and do not cross the plasma membranes easily.

Ions require special channels or transporters for transport across plasma membranes.

2. Active transportation of components occurs against the concentration gradient which requires pumps. The energy required for such transportation is obtained from ATP.

3. The transition state is always at a higher energy level compared to the reactants or products, irrespective of whether the reactions are endergonic or exergonic.

Enzymes do not change the free energy of the reactants or products, they only reduce the activation energy required to reach the transition state.

4. ATP is an unstable molecule and its hydrolysis is an exergonic reaction. So, the energy of activation of ATP is low.

5. Instantaneous reactions are those which occurs immediately and quickly.

Spontaneous reactions are those which has a negative delta G value and releases energy. It maynot take place immediately and fast similar to instantaneous reactions, since the energy of activation may be high.

6. Glycolysis is a universal pathway in organisms which proves the fact that it evolved during earlier times.

If glycolysis evolution would have evolved late, it would not have been universal. It existed from primitive times along with other carbohydrate metabolic pathways.

7. Glucose is converted into pyruvate with the production of ATP and NADH as reducing equivalents. The energy in NADH is harvested to form ATP in the Electron Transport Chain.

Kreb's TCA cycle produces reduced equivalents in the form of NADH and FADH2, which is again converted to ATP in the ETC.

8. Regulatory control of metabolic pathways are required to maintain a constant set of conditions within cells referred to as homeostasis. The metabolic pathways are generally regulated in response to signals and requirements of the cell for metabolites.

These pathways are regulated by increasing or decreasing the activity of enzymes in response to signals and cellular requirements.

For example, the enzymes involved in the irreversible steps of glycolysis, i.e, hexokinase, phosphofructokinase and pyruvate kinase responds to the increase or decreased metabolite concentrations, and thus regulates the overall pathway.

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