Biology: Cell Membrane Structure and Functions

Cell Membrane

Structure

• Phospholipid bilayer: two layers of lipid molecules with phosphate heads and fatty acid tails
• Hydrophilic (water-loving) heads face outward, hydrophobic (water-fearing) tails face inward
• Embedded proteins:
  • Integral proteins: span the membrane, involved in transport and signaling
  • Peripheral proteins: attached to the surface, involved in signaling and cell-cell interactions

Functions

• Selective permeability: regulates what enters and leaves the cell
• Cell signaling: receives and transmits signals through embedded proteins
• Cell-cell interactions: facilitates interactions with other cells through adhesion molecules
• Waste removal: aids in the removal of waste products from the cell

Fluid Mosaic Model

• Phospholipid bilayer is fluid, allowing for lateral movement of proteins and lipids
• Embedded proteins are mobile, enabling dynamic interactions and signaling

Transport Across the Cell Membrane

• Passive transport: diffusion, osmosis, and facilitated diffusion
• Active transport: energy-dependent transport of molecules against their concentration gradient

Cell Membrane

Structure

• The phospholipid bilayer is composed of two layers of lipid molecules with phosphate heads and fatty acid tails.
• Hydrophilic (water-loving) heads face outward, while hydrophobic (water-fearing) tails face inward.
• Embedded proteins are classified into two types:
  • Integral proteins that span the membrane, involved in transport and signaling.
  • Peripheral proteins attached to the surface, involved in signaling and cell-cell interactions.

Functions

• The cell membrane exhibits selective permeability, regulating what enters and leaves the cell.
• It plays a crucial role in cell signaling, receiving and transmitting signals through embedded proteins.
• The cell membrane facilitates cell-cell interactions through adhesion molecules.
• It aids in the removal of waste products from the cell.

Fluid Mosaic Model

• The phospholipid bilayer is fluid, allowing for lateral movement of proteins and lipids.
• Embedded proteins are mobile, enabling dynamic interactions and signaling.

Transport Across the Cell Membrane

• Passive transport occurs through diffusion, osmosis, and facilitated diffusion.
• Active transport involves energy-dependent transport of molecules against their concentration gradient.

Movement of Substances across Cell Membrane

• Cell membrane structure: fluid mosaic model, composed of phospholipids and proteins
  • Phospholipid bilayer with hydrophilic heads and hydrophobic tails
  • Proteins interspersed among phospholipid molecules, some attached to surface, others embedded or spanning the bilayer
  • Carbohydrates attached to proteins to form glycoproteins

Structure of Cell Membrane and its Properties

• Phospholipid bilayer:
  • Hydrophilic heads face outwards, hydrophobic tails face inwards
  • Phospholipid molecules can move laterally
• Protein functions:
  • Channel proteins: provide channels for certain substances to move across the cell membrane
  • Carrier proteins: bind to certain substances and transport them to the other side of the cell membrane
  • Receptors: bind to chemical messengers outside cells, triggering cell activities
  • Antigens: glycoproteins for cell recognition
  • Enzymes: speed up chemical reactions

Relationship between Structure, Properties, and Functions of Cell Membrane

• Cell membrane structure related to properties and functions:
  • Phospholipid bilayer makes membrane differentially permeable
  • Small, non-polar molecules can dissolve in and move across phospholipid bilayer
  • Small, polar molecules and ions are transported by channel proteins or carrier proteins
  • Phospholipid molecules can move laterally, making the cell membrane fluid and able to change shape and fuse during phagocytosis and cell division

Factors Affecting Cell Membrane Structure and Permeability

• Temperature: increases permeability, boiling can damage the membrane
• Organic solvents: dissolve phospholipids, increase permeability, and damage the cell membrane

Movement of Substances across Membranes

Diffusion

• Diffusion: net movement of particles from higher to lower concentration
• Factors affecting rate of diffusion:
  • Concentration gradient: steeper gradient increases rate
  • Surface area: larger surface area increases rate
  • Distance: shorter distance increases rate
  • Temperature: higher temperature increases rate
  • Particle size: smaller particles diffuse faster
  • Nature of particles: non-polar substances diffuse faster than polar substances
• Importance of diffusion: enables cells to exchange materials with the environment, involved in absorption of nutrients and gas exchange

Osmosis

• Osmosis: net movement of water molecules from higher to lower water potential across a differentially permeable membrane
• Water potential: pure water has highest water potential (0), solutions have lower water potential (negative value)
• Types of solutions:
  • Hypotonic: higher water potential than the cell
  • Isotonic: same water potential as the cell
  • Hypertonic: lower water potential than the cell
• Changes in animal cells in solutions of different water potentials:
  • Hypotonic: swell and burst
  • Isotonic: no change
  • Hypertonic: shrink and become wrinkled
• Changes in plant cells in solutions of different water potentials:
  • Hypotonic: become turgid
  • Isotonic: no change
  • Hypertonic: become flaccid and plasmolysed
• Importance of osmosis: main mechanism of water entry and exit in cells, involved in water absorption in human intestines and plant roots

Active Transport

• Active transport: movement of substances across the cell membrane from lower to higher concentration using energy
• Process of active transport:
  1. Particle binds to carrier protein
  2. Carrier protein changes shape using energy
  3. Particle is released on the other side of the membrane
• Importance of active transport:
  • Allows uptake of additional useful substances already high in concentration in cells
  • Involved in absorption of nutrients in human small intestine and absorption of minerals from soil into plant roots

Phagocytosis

• Phagocytosis: uptake of large particles into cells by forming vacuoles from the cell membrane
• Process of phagocytosis:
  1. Pseudopodia surround the particle
  2. Cell membrane fuses to form a vacuole
  3. Vacuole is fused with other vacuoles containing digestive enzymes
  4. Particle is broken down into digested products
• Importance of phagocytosis:
  • Allows unicellular organisms to engulf food particles for nutrition
  • Involved in white blood cell engulfment of harmful microorganisms for body defense against diseases

Comparison of Membrane Transport Processes

• Diffusion: passive, no energy needed
• Osmosis: passive, no energy needed
• Active transport: active, energy needed
• Phagocytosis: active, energy needed