Cellular Structures and Active Transport Mechanisms
Active Transport
In this type of transport, substances span the membrane against the concentration gradient. Molecules bind to transport proteins and move against the gradient through the input of energy, often in the form of ATP, from the cell.
Cell Wall
Not all living things have cell walls; they are present only in eukaryotic plant cells. They form channels within the plant called plasmodesmodes. The cell wall is constituted by a middle layer (the outermost layer), which is thin and flexible, and a primary cell wall, a thin wall. Secondary cell walls are thick and rigid.
Cytoplasm
The cytoplasm is the area of the cell between the plasma membrane and the nucleus. We can distinguish the hyaloplasm and organelles.
Hyaloplasm: The space in the cytoplasm excluding the organelles. In the hyaloplasm, there are filaments that structure the cytoskeleton. These include:
- Microtubules: These are the thickest structures. They are cylindrical and hollow. There are two types: labile (structurally unstable) and stable (associated with stable structures).
- Microfilaments: These are finer and maintain the structure of the microvilli.
- Intermediate filaments: These are thick and strong, maintaining the structure of the axons of neurons.
Organelles
The eukaryotic cell is subdivided into membrane-surrounded compartments, which are the organelles. They share the metabolic work. These include:
Mitochondria: Elongated cylindrical organelles. The set of mitochondria in a cell is called the chondriome. Structure: It is bounded by two membranes with distinct features. The external membrane has protein complexes that act as channels for internal penetration. The internal membrane has folds (mitochondrial cristae), is quite impermeable, and has a large number of proteins that develop functions, such as permeases, and complexes that form ATP. Inside is the matrix, rich in enzymes. Function: Its main function is the oxidation of organic matter to obtain ATP (Krebs cycle, fatty acid oxidation, respiratory chain, synthesis, and oxidation of fatty acids, storage of lipids, protein, iron, sodium, potassium).
Chloroplasts: Exclusive to plant cells, their shape is ovoid, and they are green due to chlorophyll. There are about 40 per cell. Structure: They have two membranes. The external membrane is quite permeable, and the inner membrane is less so, because it contains a high number of transport proteins. Inside is the stroma, where there is a set of flattened sacs called thylakoids. In these thylakoids are the photosynthetic pigments. There are two types of thylakoids: grana thylakoids and stroma thylakoids. Function: Its principal function is photosynthesis.
Endoplasmic Reticulum (ER): Present in animal and plant cells. There are two types: rough ER, which has ribosomes and is flattened, and smooth ER, which lacks ribosomes. The space within the ER membrane is called the lumen. Function: Rough ER: protein synthesis by the ribosomes. Smooth ER: lipid synthesis and removal of toxic substances.
Golgi Apparatus: Formed by a set of flattened sacs and secretion vesicles, proximal to the nucleus. These flattened sacs group together, forming the dictyosome. The dictyosome has two faces: “cis” (closer to the endoplasmic reticulum) and “trans” (closer to the plasma membrane). Function: Transport, maturation, glycosylation, and accumulation of proteins and lipids; synthesis of polysaccharides; sorting and specific distribution of substances in vesicles.
Ribosomes: Globular organelles constituted by rRNA. They are free in the cytosol or associated with the rough ER. Structure: A smaller subunit (40S) and a larger subunit (60S). Function: Protein biosynthesis.
Vacuoles: Formed from the endoplasmic reticulum, Golgi apparatus, mitochondria, or plasma membrane. They are large in plant cells and small in animal cells. Function: Storage of salts, lipids, organic acids, proteins, etc. Some cells store special substances such as tannins and pigments. They also transport enzymes and regulate cytoplasmic osmotic pressure.
Centriole: Found in all cells except those of higher plants. Function: Coordination and organization of microtubules in cilia and flagella movements.
Nucleus
The eukaryotic cell separates the DNA from the cytoplasm using a nuclear envelope. The internal environment is the nucleoplasm, containing the nucleolus and chromatin.