Cellular Responses and Nutrition: Metabolism, Movement, and Growth
Cellular Responses
At the molecular level, responses are varied, as all metabolic processes in cells occur as responses to stimuli. Cell behavior can be categorized into three types of responses:
Secretion
Beta cells and other specialized cells have a regulated secretion rate, controlled by specific couriers. Molecules are stored in secretory vesicles until a signal (e.g., calcium levels) triggers their release.
Proliferation and Cell Development
- If a cell receives survival signals, it renews its structures and performs its functions.
- If a cell does not receive appropriate signals, it undergoes apoptosis (programmed cell death).
- If a cell receives proliferation signals, it multiplies and differentiates, giving rise to daughter cells.
Movements
Movement occurs in virtually all cells. The movement of free cells to a stimulus is called taxis or tactismo. Positive taxis is movement towards a stimulus, while negative taxis is movement away. Types of taxis include geotactic, phototactic, and chemotaxis, involving actin filaments and microtubules. Movement types:
1. Cytoplasmic Streaming or Cytosis
In plant cells, actin filaments form tracks, creating currents that spin the cytoplasm.
2. Amoeboid Movements
Free cells without membranes or rigid structures emit pseudopodia (cytoplasmic expansions). Pseudopods are organized from actin filaments in the cortex, polymerizing and depolymerizing to generate movement.
3. Contractile Movements
Cytoplasmic movements in a fixed direction, like in vorticellas or muscle fibers, involve actin and myosin. Myosin filaments move on actin, causing fiber contraction.
4. Cilia and Flagella Movements
Many unicellular organisms use cilia and flagella for movement. Multicellular organisms also use them for movement (e.g., sperm) or to create external currents. Cilia beat back and forth, while flagella have a wave-like motion.
Role of Nutrition
Nutrition encompasses the processes by which a cell obtains materials for building structures and energy for its activities. Organisms are classified based on their source of matter and energy:
Autotrophs
Organisms capable of synthesizing organic molecules from simple inorganic substances. Energy sources include:
- Sunlight (photosynthesis): photolithoautotrophs (plants, algae, some bacteria).
- Oxidation of compounds: chemolithoautotrophs (e.g., Fe, H, S bacteria, nitrifiers).
Heterotrophs
Organisms that use organic matter produced by other organisms for structure and energy (chemoorganotrophs). This includes animals, fungi, protozoa, and many bacteria. Purple non-sulfur bacteria are photoorganotrophs, performing a special type of photosynthesis using organic compounds as electron donors.
Phases of Cellular Nutrition
Nutrient Intake
Transport of substances through the cell membrane.
Intracellular Digestion
Substances enter via pinocytosis into early endosomes, then late endosomes, and finally lysosomes. Hydrolases in lysosomes break down macromolecules. Autophagy degrades cell materials in lysosomes, forming autophagolysosomes. Phagocytosis results in a phagosome that fuses with lysosomes to form phagolysosomes. Simple molecules then exit to the cytoplasm.
Metabolism, Metabolic Processes, and Energy Transfer
Substances from lysosomes or the plasma membrane enter metabolic pathways. Metabolism is the set of chemical processes in a cell. Metabolic intermediates are called metabolites, forming metabolic networks. Reactions are catalyzed by enzymes and fall into two groups:
- Catabolism: Decomposition of complex molecules into simpler ones, releasing energy and providing small molecules for synthesis.
- Anabolism: Synthesis of complex molecules from simpler ones, requiring energy.
These energy reactions are oxidation-reduction reactions. Oxidation is electron loss, and reduction is electron gain. Oxidations release energy (catabolic), while reductions require energy (anabolic). Anabolic and catabolic reactions are coupled using ATP. Catabolism and photosynthesis join ADP and a phosphate group to form ATP (phosphorylation).
Excretion of Waste and Discharge of Substances
Toxic substances (e.g., CO2, NH3) produced during metabolism are pumped out of the cell. Anabolic processes produce organic molecules, some of which are secreted. Secretion often involves packaging in the Golgi apparatus. There are two secretory routes:
- Constitutive secretion: Continuous, automatic secretion without selection.
- Regulated secretion: Specialized cells package substances in secretory vesicles, which remain until a signal triggers exocytosis.