Chapter 13, Problem 17STP

Summary Introduction

To propose:

An explanation for difference in evolution between chemosynthetic autotrophs and photosynthetic ones

Introduction: All living organisms need energy to carry out various biological functions. Chemosynthesis refers to biological conversion of nutrients into organic matter through oxidation of inorganic compounds. Autotrophs are organisms that collect energy from sunlight or inorganic substances to make their food.

Answer to Problem 17STP

Chemoautotrophs must have evolved earlier than photoautotrophs because they can synthesize food from inorganic molecules in the absence of sunlight and even oxygen. They are found in regions where sunlight is unavailable. The conditions needed for photosynthesis were found much later after the formation of Earth.

Explanation of Solution

Some autotrophs produce their own food through the process of photosynthesis in which light energy from the Sun is converted to chemical energy for use by the cell. Autotrophs that use the solar energy along with carbon dioxide and water to form glucose and oxygen are called photoautotrophs. Some autotrophs called chemoautotrophs use inorganic substances as sources of energy. In places where sunlight is unavailable some bacteria uses hydrogen sulfide and carbon dioxide to make organic molecules to use as food.

It is found that chemosynthesis may also occur in the absence of oxygen and even in dark regions of oceans where sunlight is not available. Hence scientists have proposed that chemosynthesis is the first type of metabolism that evolved on Earth. This has led way for cellular respiration and photosynthesis to develop later.

Also when Earth was formed about 4.5 billion years ago, the conditions were not congenial for photosynthesis to occur. Chemosynthetic autotrophs may have occurred then. So it can be said that photoautotrophs have evolved much later than chemoautotrophs.