Cell Structure and Function
The Cell: Structure and Vital Functions
The cell is composed of three main elements: the membrane, cytoplasm, and genetic material. It performs three vital functions: nutrition, relationships, and reproduction.
The Plasma Membrane: Cell Boundary and Exchange
The plasma membrane acts as a boundary, limiting the cell’s contents while allowing the exchange of matter and energy with the surrounding environment. It consists of lipids, proteins, and carbohydrates. The plasma membrane has a fluid mosaic structure, allowing lipids and proteins freedom of movement. Its function is to maintain a stable intracellular environment by regulating the passage of water molecules and other substances.
Secretory Membrane: Cell Wall and Glycocalyx
Some cells have a thick shell covering the plasma membrane, known as the secretory membrane. In plant cells, the main component of this membrane is the cell wall, primarily made of cellulose. In animal cells, the secretory membrane’s main component is the glycocalyx, composed of glycoproteins.
Endocytosis: Importing Large Molecules
Endocytosis is the process of transporting large molecules from the environment into the cell. Particles attach to the membrane, which then invaginates. This process requires energy in the form of ATP. There are two types of endocytosis:
- Pinocytosis: Ingestion of cell fluid and dissolved substances through pinocytotic vesicles, which release their contents into the endoplasmic reticulum.
- Phagocytosis: Ingestion of solid and large particles, such as microorganisms and cellular debris. Phagocytosis is used to obtain food from the external environment.
Exocytosis: Exporting Macromolecules
Exocytosis is the mechanism by which macromolecules contained in cytoplasmic vesicles are transported from the cell interior to the plasma membrane for release outside the cell. This requires the vesicle membrane and plasma membrane to merge, creating a pore through which the contents of the cytoplasmic vesicle are discharged.
Transport Across the Membrane: Active Transport
Active transport involves membrane proteins that require energy in the form of ATP to transport molecules across the membrane. This occurs when substances are transported against their electrochemical gradient. Examples include:
- Na+-K+ pump: Transports sodium outside the cell and potassium inside against their electrochemical potential.
- Ca2+ pump: Transports calcium outside the cell.
Cilia and Flagella: Cell Movement
Cilia and Flagella are mobile cytoplasmic structures located on the cell surface. They enable cell movement and, in the case of cilia, create turbulence to attract food.
Mitosis vs. Meiosis: Key Differences
Mitosis
Mitosis is a type of karyokinesis that results in two cells with the same number of chromosomes as the parent cell. Key features include:
- No synapses, crossover, or chiasmata during prophase.
- Sister chromatids separate during anaphase.
- Daughter cells have chromosomes identical to the parent cell.
- Generally occurs in somatic stem cells.
Meiosis
Meiosis involves two karyokineses and two cytokineses, resulting in four cells with half the number of chromosomes as the parent cell. Key features include:
- Synapses and chiasmata during prophase.
- Sister chromatids do not separate during anaphase I but migrate together.
- Daughter cells have chromosomes resulting from genetic recombination.
- Occurs only in cells of gamete mothers and meiospores.
Dark Phase or Calvin-Benson Cycle
The Calvin-Benson cycle occurs in the stroma of chloroplasts and does not require light, so it can occur day or night. The process involves:
- CO2 fixation on a 5-carbon compound.
- Use of ATP and NADPH from the light-dependent reactions to reduce the fixed carbon.
- Regeneration of the 5-carbon compound to restart the cycle.
Each cycle results in a carbon molecule used for organic synthesis in the stroma.