The plasma membrane separates the internal environment of the cell from its external environment.
Function: It regulates the entrance and exit of molecules into and out of the cell.
Terminology: The maintenance of a steady internal environment is called homeostasis.
Structure of the Plasma Membrane
Phospholipid Bilayer:
Basic structure consists of a phospholipid bilayer with embedded proteins.
Components:
Hydrophilic (water-loving) polar heads:
Face towards the inside and outside of the cell (where water is present).
Hydrophobic (water-fearing) nonpolar tails:
Face each other, creating the interior of the membrane (away from water).
Sterols:
Cholesterol found in animal cells helps to stiffen and strengthen the membrane.
Membrane Proteins
Types of Membrane Proteins:
Proteins integrated throughout the membrane:
Peripheral Proteins:
Associated with only one side of the membrane.
Function mainly as enzymes or structural components.
Integral Proteins:
Span the membrane, may protrude from one or both sides.
Embedded within the membrane and capable of lateral movement.
Fluid-Mosaic Model:
Description of the pattern of phospholipids, steroids, and proteins in the membrane which allows movement and flexibility.
Glycolipids and Glycoproteins
Both phospholipids and proteins may have attached carbohydrate chains.
Glycolipids:
Lipids with attached carbohydrates.
Glycoproteins:
Proteins with attached carbohydrates.
Functions of Membrane Proteins
Channel Proteins:
Facilitate the passage of solutes through the membrane.
Allow substances to move across the membrane easily.
Some channel proteins contain gates that must be opened in response to a signal.
Carrier Proteins:
Aid in the transport of a solute by combining with it and assisting in movement across the membrane.
Cell Recognition Proteins:
Type of glycoprotein that helps the body recognize when it is being invaded by pathogens.
Receptor Proteins:
Exhibit a specific shape allowing certain molecules to bind, causing a shape change in the receptor that triggers a cellular response.
Enzymatic Proteins:
Catalyze specific reactions within the cell.
Permeability of the Plasma Membrane
The plasma membrane has a selective permeability feature that regulates the passage of molecules:
Factors affecting permeability:
Size of the molecules.
Nature of the molecule (its polarity and charge).
Diffusion and Osmosis
Diffusion:
Movement of molecules from an area of higher concentration to an area of lower concentration (down a concentration gradient) until equilibrium is reached.
Example: Placing a crystal of dye in water allows for diffusion until the dye is evenly distributed.
Solution Definition:
A solution contains a solute (the solid) and a solvent (the liquid).
Factors Influencing the Rate of Diffusion
Temperature:
As temperature increases, the rate of diffusion also increases.
Pressure:
Electrical currents:
Molecular size:
Osmosis
Definition:
Osmosis is the diffusion of water across a differentially permeable membrane, occurring from higher water concentration to lower water concentration.
Osmotic Pressure:
The pressure that develops in a system due to osmosis.
Higher osmotic pressure leads to greater likelihood of water diffusion in that direction.
Types of Solutions
Isotonic Solution:
The solute concentration is equal inside and outside the cell, resulting in no net movement of water.
Hypotonic Solution:
A solution that has a lower solute concentration than the inside of the cell, causing the cell to gain water.
This can result in cytolysis (lysis of the cell), particularly labeled as hemolysis for red blood cells.
Hypertonic Solution:
A solution that has a higher solute concentration than the inside of the cell, resulting in the cell losing water and undergoing crenation (shriveling).
Transport Mechanisms
Carrier Proteins:
Required for:
Facilitated Transport:
Movement of large or polar molecules (e.g., glucose or amino acids) facilitated by carrier proteins without the need for ATP, occurring down the concentration gradient.
Active Transport:
Carrier proteins (often referred to as pumps) transport molecules against their concentration gradient, typically requiring energy (usually ATP).
Exocytosis
Definition:
Process in which the vesicle fuses with the plasma membrane to secrete substances.
The vesicle membrane becomes part of the plasma membrane.
Examples:
Pancreatic cells that release insulin or enzymes.
Anterior pituitary cells that release growth hormone.
Endocytosis
Definition:
Process where cells take in substances through vesicle formation.
Part of the plasma membrane invaginates to envelop the substance and then pinches off to form an intracellular vesicle.
Types of Endocytosis
Phagocytosis:
Involves the uptake of large particulate matter such as “food” molecules, viruses, or whole cells.
Commonly seen in amoeba and macrophages.
Pinocytosis:
Uptake of liquids and small particles, typical in blood cells and plant root cells.
Receptor-Mediated Endocytosis:
A specialized form of pinocytosis involving membrane pits coated with specific receptor proteins for certain molecules, making it a very efficient and specific uptake method.
Cell Junctions
Certain animal tissues have junctions between cells to facilitate attachment and communication.
Adhesion Junctions:
Connects the cytoskeletal filaments of adjacent cells through adhesion proteins.
Tight Junctions:
Connects plasma membranes of adjacent cells to form impermeable barriers preventing movement between cells.
Gap Junctions:
Plasma membrane channels join cells together, enabling communication and structural strength.
Plant Cell Walls
Plasmodesmata:
Narrow channels that penetrate the cell wall of adjacent plant cells.
Contains strands of cytoplasm that allow for material exchange between cells.
Only water and small solutes can pass freely through these channels.
Cytoplasmic Connection:
These channels help maintain a connected cytoplasm throughout the plant tissue.