Cell Membrane: Structure and Function

• The cell membrane is the outer boundary of a cell, separating intracellular and extracellular environments.
• It controls the movement of substances in and out of the cell, facilitates communication with other cells, and provides shape and structure.
• The cell membrane is selectively permeable, fluid, and a mosaic phospholipid bilayer.

Key terms

• Selectively permeable: Allows only certain substances to pass in and out.
• Fluid: Molecules within the membrane can move.
• Mosaic: Composed of many different molecules.
• Bi: Two-layered.
• Phospholipid: A lipid that forms the primary structure of the membrane.
• Membrane: Sheet-like boundary.

Membrane Structure

• The cell membrane consists of two layers of phospholipids.
• Phospholipids are lipids with a phosphate group attached.
• Gases and lipid-soluble molecules can enter the cell through small spaces between phospholipids.
• Cholesterol, embedded in the bilayer, maintains membrane fluidity and increases water permeability.
• Channel proteins are large proteins that act as pores, allowing lipid-soluble molecules to pass through.
• Carrier proteins are large proteins that act as pores and transport large or water-soluble substances in or out of the cell.
• Glycoproteins, proteins with attached carbohydrate groups, enable cells to distinguish between self and non-self.

Cell Transport

• Cell transport involves the movement of substances between the extracellular and intracellular environments.
• Substances move down a concentration gradient, from areas of higher concentration to areas of lower concentration.

Passive Transport

• Passive transport occurs without using ATP energy and follows the concentration gradient.

• Types of passive transport include:

• Diffusion: Particles move directly through the membrane to distribute evenly, such as oxygen, carbon dioxide, and fatty acids.

• Osmosis: A specialized form of diffusion where water moves through the membrane down its concentration gradient.

  • Hypertonic: Higher solute concentration outside the cell, causing water to move out.
  • Isotonic: Equal solute concentration inside and outside the cell, resulting in no net water movement.
  • Hypotonic: Lower solute concentration outside the cell, causing water to move in.

• Facilitated diffusion: Larger or water-soluble molecules move across the membrane with the concentration gradient, aided by proteins.

Active Transport

• Active transport requires ATP energy and can move substances against or with the concentration gradient.
• Carrier proteins transport substances against the concentration gradient (low to high), requiring ATP.
• An example is the sodium-potassium pump.
• Endocytosis: The cell absorbs large particles from the extracellular environment by engulfing them in the cell membrane, forming a vesicle.
  • Phagocytosis: Uptake of solid particles.
  • Pinocytosis: Uptake of liquid particles.
• Exocytosis: Particles inside a vesicle are passed outside the cell.
• The vesicle fuses with the cell membrane, releasing its contents, such as mucus and digestive enzymes.

Factors Affecting Cellular Transport

• Surface area to volume ratio: A larger ratio results in faster diffusion.
• Concentration gradient: A larger difference in concentration results in faster movement.
• Size: Larger substances require carrier proteins, which takes longer.
• Solubility: Water-soluble substances require channel proteins and take longer to transport than lipid-soluble substances.
• Tonicity:
  • Hypertonic: Water moves out of the cell, causing it to shrivel.
  • Isotonic: No net movement of water.
  • Hypotonic: Water moves into the cell, causing it to expand.

Explore the cell membrane, the outer boundary of cells. Learn about its structure as a phospholipid bilayer, its selective permeability, fluidity, and mosaic nature. Understand how this membrane controls substance movement and facilitates cell communication.