Plasma Membrane and Cytoplasm: Structure and Function
Plasma Membrane
The plasma membrane, also known as the cell membrane, is the first layer that surrounds the cell, separating its internal components from the external environment. It is a dynamic structure that allows for changes in cell shape and movement.
Structure
The plasma membrane has a consistent structure across all cells and is closely associated with cytoplasmic organelles. According to the fluid mosaic model proposed by Nicholson, the membrane is primarily composed of a double layer of lipids.
Composition:
- 52% Proteins
- 40% Lipids
- 8% Carbohydrates
The lipid bilayer is mainly composed of phospholipids (the most abundant), cholesterol, and glycolipids. The polar heads of these molecules are arranged towards the aqueous environments inside and outside the cell, while the nonpolar tails face each other within the bilayer. This arrangement allows the membrane to spontaneously form a closed structure.
Proteins are embedded within the membrane, with their polar regions interacting with the aqueous environment and their nonpolar regions interacting with the lipid bilayer. Based on their arrangement, proteins are classified as:
- Integral proteins: Partially or wholly embedded within the bilayer, possessing lipophilic regions. Transmembrane proteins span the entire bilayer.
- Peripheral proteins: Located on the surface of the membrane, interacting with the polar heads of lipids or integral proteins.
The plasma membrane is a dynamic structure, with its components constantly moving laterally. This fluidity allows for self-repair and adaptation to changing conditions.
The outer surface of the plasma membrane is covered by the glycocalyx, a layer of oligosaccharides attached to glycolipids and glycoproteins. The glycocalyx plays a role in cell recognition and adhesion.
Function
The plasma membrane maintains the stability of the intracellular environment by acting as a selectively permeable barrier. It regulates the passage of substances into and out of the cell.
Key functions of membrane proteins:
- Support and structure
- Regulation of transport
- Maintenance of ion gradients and membrane potential
- Endocytosis and exocytosis
- Cell recognition through the glycocalyx
Transport Across the Membrane
Passive Transport:
- Simple diffusion: Small, nonpolar molecules move across the membrane down their concentration gradient.
- Facilitated diffusion: Small, polar molecules require the assistance of transmembrane proteins (channels or permeases) to cross the membrane.
Active Transport:
Membrane proteins use energy (ATP) to transport molecules against their concentration gradient.
The membrane potential, a difference in electrical charge across the membrane, allows for the passive transport of cations and plays a role in cell signaling.
Endocytosis:
Cells engulf macromolecules by forming vesicles from the plasma membrane.
- Pinocytosis: Ingestion of fluids and dissolved substances.
- Phagocytosis: Ingestion of large particles.
Extracellular Matrix
The extracellular matrix (ECM) is a network of proteins and carbohydrates secreted by cells in animal tissues. It provides structural support, fills spaces between cells, and contributes to tissue integrity.
Composition:
- Collagen: Fibrous protein providing strength and structure.
- Elastin: Fibrous protein providing elasticity.
- Fibronectin: Glycoprotein promoting cell adhesion.
- Proteoglycans: Hydrated glycoproteins providing resistance to compression.
Function:
- Holds tissues together and forms organs.
- Provides consistency and elasticity to tissues.
- Allows for diffusion of substances.
Plant Cell Wall
The plant cell wall is a rigid outer layer surrounding plant cells. Its primary component is cellulose, arranged in layers to form an exoskeleton that persists even after cell death.
The cell wall is composed of a network of cellulose fibers embedded in a matrix. This matrix can be impregnated with substances like lignin (for stiffness), suberin and cutin (for waterproofing), and calcium carbonate and silica (for rigidity).
Function:
- Provides shape and support to the cell.
- Withstands internal pressure from the cytoplasm.
Cytoplasm
The cytoplasm is the cellular space between the plasma membrane and the nucleus.
Cytosol
The cytosol is the aqueous portion of the cytoplasm, containing a high concentration of dissolved molecules. It is the site of protein synthesis, numerous metabolic reactions, and houses the cytoskeleton.
Cytoskeleton
The cytoskeleton is a network of protein filaments and tubules that provides structural support and facilitates cell movement and organelle organization.
Function:
- Maintains cell shape.
- Enables cell movement.
- Transports organelles.
- Organizes cellular components.