2.2 Living Processes in Unicellular Organisms
Definition
Single-Cell Nature: Unicellular organisms are living things that consist of only one cell. This single cell carries out all the important processes necessary to keep the organism alive. These processes include eating, breathing, moving, and reproducing. Even though they are tiny, these organisms perform tasks similar to those done by millions of cells in larger organisms.
Self-Sufficiency: Since unicellular organisms are made of just one cell, they are self-sufficient. This means they don’t rely on other cells to perform vital functions. Processes such as respiration, nutrition, movement, and reproduction all take place within this single cell, allowing it to survive on its own.
Examples of Unicellular Organisms
Common Examples: Amoeba sp. and Paramecium sp. are two of the most commonly studied unicellular organisms. They are often used as examples to understand how single-celled organisms function.
Other Examples: Besides Amoeba and Paramecium, there are many other unicellular organisms. These include bacteria, which are microscopic and can be found almost everywhere. Protozoa, some types of algae, and yeasts (a type of fungus) also belong to the group of unicellular organisms.
Respiration
Gas Exchange Method: Unicellular organisms breathe through their cell membrane. Oxygen enters the cell and carbon dioxide leaves the cell by a process called diffusion. Diffusion happens when molecules move from a place where they are in high concentration to a place where they are in low concentration.
Efficiency Factor: Diffusion happens quickly in unicellular organisms because they have a large surface area compared to their volume. This means that gases like oxygen and carbon dioxide can move easily in and out of the cell.
Absence of Specialised Organs: Since these organisms are very small and consist of only one cell, they do not have specialized organs for breathing, like lungs or gills. They depend entirely on diffusion through their cell membrane to exchange gases.
Nutrition
Amoeba Nutrition: Amoeba sp. eats by surrounding its food with its cell membrane, forming a structure called a food vacuole. This process is known as phagocytosis. Amoeba feeds on small organisms such as bacteria and algae.
Food Vacuole Formation: When Amoeba engulfs food, a vacuole forms around it. Inside this food vacuole, digestive enzymes break down the food into smaller molecules.
Digestion and Absorption: Lysosomes, which are small sacs filled with enzymes, fuse with the food vacuole and release digestive enzymes. These enzymes break down the food, and the nutrients are then absorbed into the cytoplasm to provide energy.
Paramecium Nutrition: Paramecium sp. has tiny hair-like structures called cilia that beat in a coordinated manner. These cilia create water currents that direct food particles into a groove called the oral groove or cytostome.
Food Processing: The food particles enter the cytostome, where they are enclosed in a food vacuole. Digestive enzymes break down the food, and any indigestible materials are expelled through the anal pore
Movement
Amoeba Movement: Amoeba sp. moves by extending parts of its cell membrane and cytoplasm, forming temporary projections called pseudopodia. This movement is called amoeboid movement.
Cytoplasmic Flow: The cytoplasm flows into the pseudopodia, causing the cell to move forward in the direction of the extended pseudopodium.
Paramecium Movement: Paramecium sp. moves by beating its cilia in a rhythmic and coordinated manner. The rapid movement of cilia allows Paramecium to rotate and move in a spiral motion through water.
Cilia Coordination: Cilia beat in a coordinated wave-like pattern, which propels the Paramecium forward or backward, depending on the direction of the cilia’s movement.
Reproduction
Binary Fission: Both Amoeba sp. and Paramecium sp. reproduce asexually through a process called binary fission. This occurs when a single parent cell divides into two identical daughter cells.
Process Description: During binary fission, the nucleus divides first, followed by the division of the cytoplasm. This results in the formation of two genetically identical daughter cells.
Spores in Amoeba: In harsh conditions, Amoeba sp. can form spores to protect itself. These spores can survive unfavorable conditions and grow into new Amoeba cells when conditions improve.
Paramecium Conjugation: Paramecium sp. can also reproduce sexually through a process called conjugation. During conjugation, two Paramecia exchange genetic material, which increases genetic diversity.
Growth
Growth Mechanism: Unicellular organisms grow by producing new cytoplasm and cell material. As they consume nutrients, they use the energy to build new cellular components.
Result of Nutrient Assimilation: When nutrients are absorbed and processed, the cell increases in size. Once the cell grows to a certain size, it is ready to divide and reproduce.
Excretion
Waste Removal: Unicellular organisms produce waste as a result of metabolic activities. These waste products include carbon dioxide and ammonia.
Direct Elimination: Waste products diffuse directly through the cell membrane into the surrounding environment, ensuring that the cell remains clean and free from harmful substances.
Osmoregulation
Importance of Osmoregulation: Maintaining the right balance of water is important for unicellular organisms, especially those that live in freshwater environments.
Osmosis Effect: Water tends to enter the cell by osmosis because the cytoplasm is hypertonic (more concentrated than the surrounding water).
Contractile Vacuole Role: The contractile vacuole helps to regulate water balance by collecting and expelling excess water. Without it, the cell could burst due to too much water entering.
Water Expulsion Process: The contractile vacuole fills with water, expands, and then contracts to push the water out of the cell, maintaining balance.
Irritability/Response
Response to Stimuli: Unicellular organisms can sense changes in their environment and respond accordingly. This helps them survive and adapt to their surroundings.
Stimuli Types: Amoeba sp. and Paramecium sp. respond to different types of stimuli such as touch, chemicals, light, and temperature changes.
Directional Movement: When exposed to intense light or other unfavorable conditions, they move away from the source. Conversely, they move toward food sources or favorable conditions.
Paramecium Sensory Role: The cilia in Paramecium not only help with movement but also act as sensory structures that detect changes in the external environment.