Chapter 5 Solutions — Life Processes

In multicellular organisms, all cells are not in direct contact with the surrounding environment.

Unlike unicellular organisms where the entire body surface is exposed to the environment, multicellular organisms have complex body designs with specialised cells and tissues located deep inside the body.

Simple diffusion is too slow to deliver oxygen to every cell because:

- The distances involved are large — oxygen would need to travel from the body surface to interior cells, which is a very long path.

- The volume of the body is large, so the oxygen demand is much higher than what diffusion alone can supply.

- The rate of diffusion decreases sharply with increasing distance.

Therefore, multicellular organisms need specialised organs (lungs/gills) for taking in oxygen and a circulatory system (blood and heart) to transport it efficiently to all cells.

Organisms use outside raw materials for two main purposes:

1. As a source of energy: Food (carbon-based molecules like glucose) is broken down through respiration to release energy needed for all life processes.

2. As a source of materials for growth and repair: Raw materials are used as building blocks to make new body components (proteins, cell membranes, DNA, etc.) and to repair damaged tissues.

The specific raw materials depend on the organism's mode of nutrition:

- Autotrophs (like green plants) use CO₂, water, and sunlight to synthesise their own food through photosynthesis.

- Heterotrophs (like animals) take in complex food materials from outside and break them down into simpler forms.

In all cases, the external raw materials are needed to provide both the energy and the molecular building blocks required for maintaining the living state.

Autotrophic Nutrition:

- Organisms prepare their own food from simple inorganic substances (CO₂ and H₂O).

- Requires sunlight energy and chlorophyll.

- The process is photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

- Examples: Green plants, some bacteria, and algae.

- Food is synthesised, not ingested.

Heterotrophic Nutrition:

- Organisms cannot prepare their own food and depend on other organisms for nutrition.

- Does not require chlorophyll or sunlight for food preparation.

- Complex food is taken in and broken down by digestive enzymes.

- Examples: Animals, fungi, most bacteria, and non-green plants.

- Food is ingested, absorbed, or decomposed from external sources.

Key difference: Autotrophs convert inorganic molecules into organic food using light energy, while heterotrophs consume ready-made organic food from other organisms.