BIOL 130 Topic 5: Cell Membrane Structure & Function

• The plasma membrane's structure and function are vital to cell biology (BIOL 130 Topic 5)

Cell membrane

• The cell membrane follows a fluid mosaic model
• It separates internal cell contents from the surrounding environment
• Cytoplasm refers to everything between the plasma membrane and nuclear envelope
• It contains organelles, gel-like cytosol, the cytoskeleton, and various chemicals
• The membrane has a phospholipid bilayer with embedded proteins and carbohydrates

Membrane Components and Functions

• Phospholipids form the main membrane fabric
• Cholesterol is attached between phospholipids and between the two phospholipid layers
• Integral proteins are embedded within the phospholipid layers, possibly penetrating both
• Peripheral proteins are on the inner or outer surface, not embedded
• Carbohydrates are generally attached to proteins

Integral Membrane Proteins

• Proteins span the plasma membrane with one or more alpha-helices or beta-sheets

Membrane Transport

• Selective permeability results in two classes of molecular transport:
  • Passive transport requires no energy input
  • Active transport requires energy, like ATP
• Passive transport includes:
  • Free diffusion
  • Facilitated diffusion
  • Osmosis
• Active transport includes:
  • Primary active transport
  • Secondary active transport

Diffusion

• Diffusion is the movement of molecules from an area of high concentration to low concentration
• The selectively permeable nature of the membrane impacts what molecules can diffuse freely

Facilitated Diffusion

• Facilitated diffusion requires proteins to aid in the movement of larger and electrically charged molecule
• Proteins can:
  • Form channels to allow diffusion
  • Create elaborate structures, like carrier proteins, that pair the diffusion of one molecule to another

Rate of Diffusion

• Several factors impact diffusion rate:
  • Concentration gradient steepness
  • Molecule sizes
  • Temperature
  • Environmental media
  • Solubility of the molecules
  • Surface area of the membrane
  • Distance

Osmosis

• Selectively permeable membranes prevent solute movement which results in osmosis
• Osmosis specifically refers to the diffusion of water across a semi-permeable membrane
• When solutes are blocked, water moves down its concentration gradient

Tonicity

• In biology, tonicity is the comparison of solute concentrations outside and inside the cell:
  • A hypotonic solution has more solute inside the cell
  • An isotonic solution has equal solute concentrations
  • A hypertonic solution has less solute inside the cell

Osmotic Pressure

• Selective permeability causes osmotic pressure, which greatly affects cells
  • Animal cells in hypertonic solution can shrivel (crenation)
  • Animal cells in isotonic solution are ideal
  • Animal cells in hypotonic solution can undergo lysis/cellular explosion

Cell Walls

• Organisms with cell walls use osmotic pressure to maintain structural integrity
• Turgor pressure is the force exerted by water inside the cell pressing against the cell wall
• A hypotonic solution is best for organisms with cell walls ("Goldie-locks" zone)

Active Transport

• Active transport requires energy to move substances in or out of a cell, and it requires ATP
• Active transport moves substances against their concentration gradient
• This process allows cells to establish concentration gradients, useful for doing work

Electrochemical Gradients

• Electrochemical gradients from both concentration and electrical gradients
• This system is the process used by the nervous system for electrical signals
• It is also used in muscle cells for contractions

Types of Active Transport

• Primary active transport directly uses energy (ATP) to move ions across the membrane which results in an electric gradient
• Primary active pumps frequently pair the movement of different ions
• Secondary active transport harnesses the gradients created by primary active transport
  • It is easier to move small molecules
  • Secondary active transport increases cellular ATP output

Endocytosis and Exocytosis

• Endocytosis actively transports substances into a cell
• Exocytosis actively transports substances out of a cell

Endocytosis Types

• There are three classes of endocytosis:
  • Phagocytosis engulfs larger packages, forming vacuoles
  • Pinocytosis engulfs smaller packages, forming vesicles
  • Receptor-mediated endocytosis is driven by activating surface receptors and is highly selective

Exocytosis

• In exocytosis, the vesicle moves to the plasma membrane, attaches, and releases its contents externally
• Nerve cells use this for cellular communication

Explore the cell membrane's structure and function, based on the fluid mosaic model. Learn about phospholipids, cholesterol, and proteins within the bilayer. Understand the roles of integral and peripheral proteins in membrane transport.