Cell Types and Nutrition: Prokaryotes, Eukaryotes, Autotrophs, Heterotrophs

Cell Types

The most primitive cells are called prokaryotes. Bacteria are unicellular prokaryotes. All other living things are composed of eukaryotic cells, which are more complex; in them, the nucleus is well-differentiated.

Eukaryotic organisms, in turn, may include:

• Unicellular organisms such as some fungi, some algae, and protozoa.
• Multicellular organisms such as animals, plants, some fungi, and some algae.

Not all eukaryotic cells are equal. Plant eukaryotic cells differ from animal eukaryotic cells because they present a rigid cellulose wall outside the cell membrane that provides high resistance, and they also contain chloroplasts, where photosynthesis takes place.

The cells of the same organism are not equal. This is because they specialize in different functions.

Autotrophic Nutrition

Plants, algae, and some bacteria perform autotrophic nutrition through the following steps:

• Incorporation of nutrients from the environment. Inorganic molecules are absorbed: water and mineral salts by the roots, and carbon dioxide (CO₂) directly into plants through the leaves.
• Production of organic matter. This process, called photosynthesis, takes place in the chloroplasts of the plant cell, where a substance called chlorophyll captures energy from sunlight. Along with nutrients, this energy is used to produce organic matter such as glucose. As a result of photosynthesis, oxygen (O₂) is released.
• Utilization of organic matter. All plant cells use the simple molecules generated in photosynthesis to regenerate their structure, grow, and so on. These simple molecules are also used in cellular respiration, a process that provides all the energy the plant needs to perform its vital activities.
• In cellular respiration, glucose is degraded in the presence of oxygen to produce H₂O, CO₂, and energy.
• Disposal of waste substances (excretion).

The Importance of Photosynthesis

• During photosynthesis, plants produce oxygen during the day and consume a portion of that oxygen while respiring day and night. As the amount of oxygen produced by photosynthesis is greater than the amount consumed during respiration, there is enough oxygen for other living things in their environment to breathe.
• Plants purify the air through photosynthesis as they convert large amounts of carbon dioxide into organic matter.
• Photosynthetic organisms are producers of organic matter in ecosystems; other living things that depend directly or indirectly on this organic matter are called consumers.

Thus, in ecosystems, the organic matter produced during photosynthesis passes from one living being to another in what are called food chains.

Heterotrophic Nutrition

Living beings that are heterotrophs feed on other living things because they have no capacity for photosynthesis. They incorporate organic molecules as nutrients (sugars, fats, proteins) that their cells metabolize to regenerate structures, grow or reproduce, and obtain energy through cellular respiration.

In heterotrophic nutrition, the following stages can be identified:

• Incorporation of organic matter from the environment. Single-celled heterotrophic organisms capture elements from the environment and enclose them in a digestive vacuole.
• In most animals, which are multicellular organisms, a digestive system is needed to transform the food eaten into simple molecules that the body's cells can use.
• Exchange of gases. In most animals, the respiratory system captures oxygen to be distributed by the circulatory system and collects carbon dioxide to be expelled outside. This process is called external respiration.
• Transport. Absorbed nutrients pass from the gut into the circulatory system, which is responsible for carrying them to different cells.
• Utilization of organic matter (metabolism). Animal cells extract from the circulating fluid the nutrients: simple organic molecules, which come from absorption in the digestive system, and oxygen, which reaches the cells from the respiratory system or from the gastric cavity. Organic nutrients are burned using oxygen during cellular respiration, and consequently, waste products such as carbon dioxide and energy are generated.
• Disposal of waste substances into the environment (excretion).