Cell Transport: Membrane Structure and Function

Cell Membrane Structure and Function

Membranes allow cells to create and maintain internal environments that differ from external environments.  The structure of cell membranes results in selective permeability; the movement of molecules across them via osmosis, diffusion, and active transport maintains dynamic homeostasis. In eukaryotes, internal membranes partition the cell into specialized regions that allow cell processes to operate with optimal efficiency.  Each compartment or membrane-bound organelle enables localization of chemical reactions. -- The College Board, 2015


Image Credits: "0303 Lipid Bilayer With Various Components" by OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.. Licensed under CC BY 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:0303_Lipid_Bilayer_With_Various_Components.jpg#/media/File:0303_Lipid_Bilayer_With_Various_Components.jpg

Enduring Understanding: Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments.

Key Terminology:

• Fluid mosaic model                                 Hypertonic
• selectively permeable                              Hypotonic
• dynamic homeostasis                               Isotonic
• passive transport                                      solute                                  
• concentration gradient                             solvent
• osmosis                                                    water potential
• diffusion                                                   crenation
• facilitated diffusion                                  lysis
• active transport
• sodium/potassium pump
• proton gradient
• endocytosis
• exocytosis

Key Equations:

From AP --The College Board, 2015 --What you need to understand

Cell membranes separate the internal environment of the cell from the external environment.  The specialized structure of the membrane described in the fluid mosaic model allows the cell to be selectively permeable, with dynamic homeostasis maintained by the constant movement of molecules across the membrane.  

Passive transport does not require the input of metabolic energy because spontaneous movement of molecules occurs from high to low concentrations; examples of passive transport are osmosis, diffusion, and facilitated diffusion.  

Active transport requires metabolic energy and transport proteins to move molecules from low to high concentrations across a membrane.  Active transport establishes concentration gradients vital for dynamic homeostasis, including sodium/potassium pumps in nerve impulse conduction and proton gradients in electron transport chains in photosynthesis and cellular respiration.

The process of endocytosis and exocytosis move large molecules from the external environment to the internal environment and vice versa, respectively.

Essential Knowledge and Supporting Information:

1. Cell membranes separate the internal environment of the cell from the external environment.
2. Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model.

Required Readings: Intro to Cell Membranes; Cell Membranes; Starr & Taggart, p.52-53, 62-63, 76-85; Tonicity

Key Questions:

1. Why is it important to separate the internal and external environments of the cell?
2. Why is the plasma membrane called a "fluid mosaic model?"
3. What is the function of each of the following cell structures:  Phospholipid molecules, Embedded Proteins, Cholesterol, Glycoproteins, Glycolipids
4. What types of molecules can pass freely across the membrane?
5. How do hydrophilic and large polar molecules move across the membrane?
6. How does water move across the membrane?
7. How are the membranes of cell organelles similar to the main outer membrane of the cell?

Essential Experiences and Skills:

Diffusion and Osmosis Lab:

Prior to beginning the diffusion and osmosis lab you need to review the role of cell walls and their role in cell permeability.  Review of plant cell walls.

Unlike plant cells, animal cells and other non-walled cells see the movement of water in or out of the cell is a product of the relative solute concentration on each side of the plasma membrane.  In a hypertonic solution the concentration of the solute is high and the water potential is low compared to the outside (outside of the membrane) solution.  The opposite conditions are true for hypotonic solutions which have lower relative solute concentrations and higher water potential as compared to the outer solution.  In an isotonic solution, equilibrium has been met so water is still moving across the membrane, but there is no net movement in one direction over the other.

Required Equation Practice: Water Potential

Required Readings (Starr & Taggart): p. 74-89 and the Critical thinking questions; Work through the Self-Quiz to make sure you are familiar with the material.  Not sure of an answer??? GOOGLE!!!