Cell Membrane Movement Types

Movement Across Cell Membrane

• The cell membrane plays a crucial role in regulating the movement of substances into and out of the cell, allowing for physiological activities to occur.

Types of Movement

• There are three main types of movement across the cell membrane:
  • Passive transport
  • Active transport
  • Vesicular transport

Passive Transport

• Passive transport occurs along the concentration gradient or electrical gradient without the need for energy.
• It is a downhill movement that does not require energy.
• Passive transport mechanisms include:
  • Diffusion
  • Osmosis

Diffusion

• Diffusion is the transport of substances from an area of high concentration to an area of low concentration.
• There are three types of diffusion:
  • Simple diffusion through lipid layer
  • Simple diffusion through protein layer
  • Facilitated or carrier-mediated diffusion

Simple Diffusion

• Simple diffusion occurs due to the random movement of molecules.
• Molecules collide with each other and the cell membrane, resulting in a net flux of molecules from areas of high concentration to areas of low concentration.
• Respiratory gases are transported across the alveolar membrane by simple diffusion.
• Simple diffusion of lipid-soluble substances through the cell membrane is rapid due to the direct proportional relationship between the rate of diffusion and the solubility of a substance in lipids.
• Examples of lipid-soluble substances include oxygen, nitrogen, carbon dioxide, alcohol, steroid hormones, and weak organic acids and bases.

Simple Diffusion of Water and Other Lipid Insoluble Molecules

• Water and other lipid insoluble substances can pass through the cell membrane with the help of protein channels.
• These channels are highly selective, allowing only specific types of ions to pass through.

Facilitated Diffusion

• Facilitated diffusion is the transport of water-soluble substances with the help of carrier proteins.
• Carrier proteins have specific binding sites for particular substances.
• Examples of substances that cross cell membranes by facilitated diffusion include glucose and most amino acids.

Differences between Simple and Facilitated Diffusion

• Specificity: carrier proteins are highly specific for different molecules.
• Saturation: facilitated diffusion has a saturation point, where the rate of diffusion increases with the concentration gradient until all binding sites are occupied.
• Competition: when two molecules are carried by the same protein, there is competition between the two molecules for transport.

Factors Affecting Net Rate of Diffusion

• Cell membrane permeability: the rate of diffusion is directly proportional to the permeability of the cell membrane.
• Lipid solubility: the rate of diffusion is directly proportional to the lipid solubility of the substance.
• Temperature: the rate of diffusion increases with an increase in temperature.
• Size of the molecules: the rate of simple diffusion is inversely proportional to the size of the molecules.
• Area of the membrane: the net diffusion of the substance is directly proportional to the total area of the membrane.
• Concentration gradient: simple diffusion is directly proportional to the concentration gradient, but facilitated diffusion has limitations beyond a certain level of concentration gradient.
• Electrical potential gradient: net diffusion occurs from inside to outside until the concentration gradient created balances the electrical gradient.
• Pressure gradient: increased pressure on one side of the membrane causes net movement of molecules from the high-pressure side towards the low-pressure side.

Osmosis

• Osmosis is the diffusion of water or other solvent molecules through a semipermeable membrane from a solution containing lower concentration of solutes towards a solution containing higher concentration of solutes.
• Osmotic pressure increases with an increase in solute concentration, resulting in greater water flow into the solution.
• Examples of osmosis include the movement of water from a sodium chloride solution into a solution containing a higher concentration of solutes.

Osmolarity and Tonicity

• Osmolarity: the concentration of osmotically active particles in a solution.
• Tonicity: describes how an extracellular solution can change the volume of a cell by affecting osmosis.
• A solution can be:
  • Isotonic: having the same osmolarity as the cytoplasm.
  • Hypotonic: having a lower osmolarity than the cytoplasm.
  • Hypertonic: having a higher osmolarity than the cytoplasm.