Understanding Cell Membranes: Lipids, Proteins, and Transport, Slides of Biology

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Chapter 7

• Membrane Structure and Function

Overview: Life at the Edge

• The plasma membrane is the boundary that separates the living cell from its surroundings
• The plasma membrane exhibits selective permeability , allowing some substances to cross it more easily than others

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
• Proteins determine most of the membrane’s specific functions

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane
• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids ) or more commonly to proteins (forming glycoproteins )
• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

Transport Proteins

• Transport proteins allow passage of hydrophilic substances across the membrane
• Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
• Channel proteins called aquaporins facilitate the passage of water

• Other transport proteins, called carrier proteins , bind to molecules and change shape to shuttle them across the membrane
• A transport protein is specific for the substance it moves

Water Balance of Cells Without Walls

• Tonicity is the ability of a solution to cause a cell to gain or lose water
• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

• Hypertonic or hypotonic environments create osmotic problems for organisms
• Osmoregulation , the control of water balance, is a necessary adaptation for life in such environments
• The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

• In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis

Facilitated Diffusion: Passive Transport

Aided by Proteins

• In facilitated diffusion , transport proteins speed the passive movement of molecules across the plasma membrane
• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
• Channel proteins include
  • Aquaporins, for facilitated diffusion of water
  • Ion channels that open or close in response to a stimulus ( gated channels )

The Need for Energy in Active Transport

• Active transport moves substances against their concentration gradient
• Active transport requires energy, usually in the form of ATP
• Active transport is performed by specific proteins embedded in the membranes

• Active transport allows cells to maintain concentration gradients that differ from their surroundings
• The sodium-potassium pump is one type of active transport system

Exocytosis

• In exocytosis , transport vesicles migrate to the membrane, fuse with it, and release their contents
• Many secretory cells use exocytosis to export their products

Endocytosis

• In endocytosis , the cell takes in macromolecules by forming vesicles from the plasma membrane
• Endocytosis is a reversal of exocytosis, involving different proteins
• There are three types of endocytosis:
  • Phagocytosis (“cellular eating”)
  • Pinocytosis (“cellular drinking”)
  • Receptor-mediated endocytosis