Cell Biology: Structure, Function, and Metabolism
The Cell: Structural and Functional Unit of Life
The cell is the structural and functional unit of living things because all living beings are formed by one or more cells and because it is the smallest unit of matter that can carry out the basic functions of living things. Each cell is derived from another existing cell.
The Cell from a Structural Viewpoint
Cells can be grouped into:
- Eukaryotes: Have DNA in a nucleus separated from the rest of the cytoplasm by a double membrane. Animal and plant cells are eukaryotic.
- Prokaryotes: Have DNA but not separated from the cytoplasm by a membrane, instead, it is scattered in the cytoplasm. Bacteria are prokaryotic cells.
Plasma Membrane
The plasma membrane is the limit of the cell. It controls the exchange of substances between the external and internal cell. It is formed by a lipid bilayer in which proteins are interspersed.
Cytoplasm
The cytoplasm is divided into organelles called compartments. These compartments are surrounded by membranes. The solution in which they are embedded is called hyaloplasm.
Non-Membranous Structures
- Cytoskeleton: A committee of protein filaments that form a lattice distributed in the hyaloplasm. It shapes the cell and is responsible for its movements.
- Centrosome: A zone close to the nucleus from where the cytoskeletal filaments emerge. In animal cells, it contains the centrioles.
Membranous Structures
A bundle of flattened tubes tied together and spread throughout the cytoplasm.
- Smooth Endoplasmic Reticulum (SER): A tubular-like structure without ribosomes. It synthesizes membrane lipids.
- Rough Endoplasmic Reticulum (RER): Covered externally by ribosomes, membranous structures involved in protein synthesis.
- Golgi Apparatus: A committee of membranous vesicles. Its function is to store macromolecules synthesized inside the vesicles to expel them to the outside (secretion).
- Lysosomes: Membranous vesicles containing digestive enzymes (digestion).
- Mitochondria: Surrounded by a double membrane delimiting an outer space called the matrix. The internal membrane extends into the interior of the matrix, forming ridges. It contains small molecules of DNA and is responsible for energy production.
- Nucleus: Isolated from the cytoplasm by a nuclear membrane. The nuclear membrane is perforated, allowing the exchange of substances. Inside the nucleus are embedded:
- Chromatin: Formed by fibrils, each consisting of a molecule of DNA. It controls and regulates the vital functions of the cell.
- Nucleoli: One or more areas of granular appearance in which ribosomes are formed.
Plant Cell
Plant cells lack centrioles and possess unique organelles and structures such as chloroplasts, cell wall, and large vacuoles.
- Cell Wall: Formed mainly by cellulose, it protects the cell and maintains its shape.
- Vacuoles: Can occupy up to 90% of the cell. They perform storage functions.
- Chloroplasts: Conduct photosynthesis. They have a double membrane delimiting the interior space that contains formations called thylakoids, whose membranes contain chlorophyll.
The Cell as a Functional Unit
Chemical reactions occur in cells, which are called metabolism. There are two metabolic processes:
- Anabolism: Fabrication of their own food from substances or chemical nutrients incorporating from the environment, requiring energy from the sun and storing it in chemical compounds.
- Catabolism: Compounds are broken down into simpler chemical components, releasing the energy contained in their links. This energy is used for movement and transportation of nutrients through the membrane.
ATP
ATP is the most common intermediary between metabolic processes that release energy (photosynthesis and respiration). A molecule of ATP is formed by a nucleotide, a nitrogenous base (adenine), a sugar (ribose), and a phosphate group of three links called “high energy” bonds.
Cellular Exchange of Matter with the Environment
- Autotrophic: Cells that make their own organic matter from inorganic nutrients such as CO2 and H2O.
- Heterotrophic Nutrition: The cell needs to incorporate organic matter produced by other organisms from the environment.
Membrane Transport
The plasma membrane lets some substances pass through and prevents the passage of others. Depending on the size, passage occurs by different mechanisms:
Small Molecules
Traverse through the plasma membrane.
- Diffusion Mechanisms: When separated by a permeable membrane, water and solutes move up to an intermediate concentration (crossing from the side of higher concentration towards the lower).
- Osmosis: A semi-permeable membrane allows only water to pass through. Water will move from the more dilute solution (hypotonic) to the more concentrated solution (hypertonic) until both are equal (isotonic).
- Active Transport: Small molecules are transported in the opposite direction to the concentration gradient, that is, from where their concentration is lower to where it is higher. This requires ATP energy.