1. What is extracellular digestion?
Extracellular digestion is the process in which food is broken down into useful molecules that can be internalized by the cell. It is carried out in the extracellular space; that is, outside the cell. In extracellular digestion, cells secret substances that break down large molecules into smaller ones in the external environment. Later, the cell benefits from these products of digestion.
2. What is intracellular digestion?
Intracellular digestion, or cellular digestion, is the process in which large molecules, from outside or from a cell’s own metabolism, are broken down into smaller molecules within the cell. Products and wastes of intracellular digestion are either used by the cell or excreted.
Intracellular digestion is classified into two types: heterophagic intracellular digestion and autophagic intracellular digestion.
3. What is the main cell organelle involved in cell digestion? What properties of that organelle allow it to carry out its task?
The organelles responsible for intracellular digestion are lysosomes. Lysosomes are vesicles that contain digestive enzymes capable of breaking down large molecules into smaller ones. These vesicles fuse with others carrying the material to be digested and, as a result, digestion takes place.
- Cell Digestion Review - Image Diversity: lysosomes
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Types of Intracellular Digestion
4. What is heterophagic intracellular digestion? How is this process accomplished?
Heterophagic intracellular digestion is the breaking down into smaller substances of external substances brought into the cell via pinocytosis or phagocytosis. Phagosomes or pinosomes fuse with lysosomes to make digestive vacuoles. Within the digestive vacuoles, the molecules to be digested are hydrolyzed. The products of digestion either cross the membrane and reach the cytoplasm or remain inside the vacuoles. Vacuoles containing residues from digestion are called residual bodies. Via exocytosis, they fuse with the plasma membrane and release their “waste” externally.
5. What is autophagic intracellular digestion? Why is this type of intracellular digestion intensified in an organism experiencing starvation?
Autophagic intracellular digestion is the internal cellular digestion of waste and residual materials. In general, it is carried out by lysosomes.
Autophagic intracellular digestion is intensified in situations of starvation because under such conditions, the cell tries to obtain the nutrients necessary to stay alive from its own material components.
The Enzymatic Action of Lysosomes
6. What are some examples of biological processes in which lysosomic enzymes play a fundamental role?
The remodelation of bone tissue, the action of acrosomes in sperm cells and the elimination of the tail of a tadpole are examples of biological processes in which lysosomal enzymes are key factors.
Bone is a tissue made of a matrix containing osteoblasts (osteoblasts are the secretory cells of the bone matrix), osteocytes (mature bone cells) and osteoclasts (the remodeling cells). Osteoclasts are responsible for the continual replacement of the bone tissue, since their lysosomal enzymes digest the bone matrix.
The acrosome of a sperm cell is responsible for the perforation of the egg cell membrane, during the fertilization process, due to the action of enzymes it carries. The acrosome, located at the back end of the sperm cell, is a specialized region of the Golgi apparatus that accumulates a large amount of digestive enzymes.
In tadpoles the tail disappears while the organism develops into an adult frog. This tissue destruction consists of the digestion of the cells and extracellular materials of the tail and is carried out by lysosomes and their enzymes. The complete digestion of a cell by its own mechanisms is called autolysis, and is a type of apoptosis (cell suicide).