Chapter 7: CELL MEMBRANE STRUCTURE AND FUNCTION

BIOLOGY I

Chapter 7: CELL MEMBRANE STRUCTURE AND FUNCTION Evelyn I. Milian Instructor 2012

BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function

PLASMA MEMBRANE (Cell Membrane or Cytoplasmic Membrane) • The plasma membrane is the cell’s flexible outer limiting barrier that separates the cell’s internal environment from the external (extracellular) environment. • It is present in prokaryotes and eukaryotes. • Main functions of the plasma membrane: 1. Regulation of exchange with the environment. It is a selective barrier that regulates the flow of nutrients into the cell and discharge of wastes out of the cell. 2. Sensitivity to the Environment. It detects changes in the surroundings and plays a role in communication, transmitting signals both among cells and between cells and their external environment. 3. It is involved in energy transfer and chemical reactions.

4. Structural Support. Specialized connections between plasma membranes, or between plasma membranes and extracellular materials, give tissues stability. Evelyn I . Milian - I nst r uct or

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Fluid Mosaic Model of the Plasma Membrane • The “Fluid Mosaic Model” describes the structure of the plasma membrane as a mosaic formed by a phospholipid bilayer with proteins and carbohydrates. The proteins can move laterally, giving fluidity to the plasma membrane. • The phospholipid molecules (made up of two fatty acids joined to glycerol and a phosphate group) are arranged in two layers (a bilayer) or parallel sheets, and are amphipathic molecules—they have a hydrophilic region and a hydrophobic region. • The hydrophilic (“water-loving”) “heads” (phosphate group and glycerol) face outward, and the hydrophobic (water-fearing) “tails” (fatty acids) face inward.

• The eukaryotic cell membrane also has glycolipids (carbohydratelipids), glycoproteins (carbohydrate-proteins) and cholesterol molecules (a type of lipid). Evelyn I . Milian - I nst r uct or

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The eukaryotic plasma membrane has a greater variety of lipids than the prokaryotic membrane. It contains sterols (such as cholesterol) which adds rigidity to the membrane. Because of their larger size, eukaryotic cells have a much lower surface-to-volume ratio than prokaryotic cells. As the volume of cytoplasm enclosed by a membrane increases, the membrane is placed under greater stress. The sterols in the membrane may help it withstand the stress. Evelyn I . Milian - I nst r uct or

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The Fluidity of the Plasma Membrane • Membranes are fluid structures (rather like cooking oil) because most of the membrane lipids and proteins easily rotate and move sideways (laterally) in their own half of the bilayer. • Membrane fluidity is greater when there are more double bonds in the fatty acid tails of the bilayer lipids. • Cholesterol (a steroid lipid) makes the lipid bilayer stronger but reduces fluidity at moderate temperatures. • Because of its fluidity, the lipid bilayer self-seals when torn or punctured.


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The Fluidity of Membranes • Lipids and proteins move laterally in the membrane, but flip-flopping across the membrane is rare. • Unsaturated hydrocarbon tails of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity (unsaturated fatty acids have double bonds). • Cholesterol reduces membrane fluidity at moderate temperatures by reducing phospholipid movement, but at low temperatures it hinders solidification by disrupting the regular packing of phospholipids. Evelyn I . Milian - I nst r uct or

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Selective Permeability of the Plasma Membrane •

Selective permeability (semi-permeability): 

•

The property of the plasma membrane to admit some substances into the cell while excluding others.

Factors determining plasma membrane permeability: 1)

Size of molecules

2)

Solubility of molecules into lipids

3)

Charge on ions

4)

Presence of carrier molecules (transport proteins) Evelyn I . Milian - I nst r uct or

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Arrangement of Membrane Proteins • Integral proteins: 

Extend into or through the lipid bilayer.



Integral transmembrane proteins: • Span the entire lipid bilayer and protrude into both the cytosol and extracellular fluid.

• Peripheral proteins: 

Associated with the inner or outer surface of the membrane.


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Functions of Membrane Proteins: Transport of Ions or Molecules


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Functions of Membrane Proteins: Transport of Ions or Molecules


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Functions of Membrane Proteins: Signal Transduction


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Functions of Membrane Proteins: Enzymatic Activity


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Functions of Membrane Proteins: Cell-Cell Recognition


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Functions of Membrane Proteins: Intercellular Joining and Attachment to Cytoskeleton and Extracellular Matrix (Linkers)


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BIOLOGY I. Chapter 7 – Fig. Cell03.03 Membrane Structure and Function

Functions of Membrane Proteins


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Gradients Across the Plasma Membrane • Concentration gradient: * 

A difference in the concentration of a chemical substance from one place to another, such as from the inside to the outside of the plasma membrane.

• Electrical gradient or potential (membrane potential):* 

A difference in electrical charges between two regions (across the plasma membrane).

• * Both help move substances across the plasma membrane; the combined influence is termed an electrochemical gradient. Evelyn I . Milian - I nst r uct or

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CELLULAR TRANSPORT: Gradients Across the Plasma Membrane

a) Sodium ions and oxygen molecules are more concentrated in the extracellular fluid, whereas potassium ions and carbon dioxide are more concentrated in cytosol. b) Because the inner surface of the plasma membrane of most cells is negative relative to the outer surface, an electrical gradient exists across the membrane.


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TRANSPORT ACROSS THE PLASMA MEMBRANE • Passive Transport 

The movement of a substance across the cell membrane down its concentration gradient, that is, from an area of higher concentration to an area of lower concentration; without expenditure of energy. • Simple diffusion through the lipid bilayer, diffusion through ion membrane channels, facilitated diffusion, osmosis.

• Active Transport 

The movement of a substance across a cell membrane against its concentration gradient, from lower concentration to higher concentration; requiring the expenditure of cellular energy (from ATP, a high-energy molecule). • Group translocation, bulk transport (endocytosis, exocytosis) Evelyn I . Milian - I nst r uct or

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TRANSPORT ACROSS THE PLASMA MEMBRANE: Passive Transport

• Diffusion 

Diffusion is a passive process in which there is a net or greater movement of molecules or ions from a region of high concentration to a region of low concentration until equilibrium is reached (they move down their concentration gradient); no energy is required—it is spontaneous.



Both the solutes, the dissolved substances, and the solvent, the liquid that does the dissolving (such as water in cells), undergo diffusion. Evelyn I . Milian - I nst r uct or

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FIGURE 3.6 – Diffusion. A crystal of dye placed in a cylinder of water dissolves (beginning) and then diffuses from the region of higher dye concentration to regions of lower dye concentration (intermediate). At equilibrium, dye concentration is uniform throughout ,although random movement continues. * This is also called simple diffusion.


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TRANSPORT ACROSS THE PLASMA MEMBRANE: Passive Transport • Diffusion Through the Lipid Bilayer 

Nonpolar, hydrophobic molecules can diffuse across the lipid bilayer; examples are: respiratory gases, some lipids, small alcohols, and ammonia.



It is important for gas exchange, absorption of some nutrients, and excretion of some wastes.

• Diffusion Through Membrane Channels 

Most membrane channels are ion channels, allowing passage of small, inorganic ions which are hydrophilic.



Ion channels are selective and specific and may be gated or open all the time. Evelyn I . Milian - I nst r uct or

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TRANSPORT ACROSS THE PLASMA MEMBRANE: Passive Transport • Facilitated Diffusion 

The spontaneous movement of a substance across the plasma membrane, from an area of higher concentration to an area of lower concentration (down its concentration gradient), mediated by a transmembrane transport protein (permease), but does not require energy (ATP).  Examples of transport proteins are: • Channel proteins such as the aquaporins for water transport. • Carrier proteins such as the glucose transporter. 

In facilitated diffusion, a substance diffuses faster than the physical condition indicates it should.  Molecules and ions that move across membranes by facilitated diffusion include glucose, urea, fructose, galactose, and some vitamins. Evelyn I . Milian - I nst r uct or

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TRANSPORT ACROSS THE PLASMA MEMBRANE: Passive Transport • Facilitated diffusion of glucose across a plasma membrane. 

The transporter (GluT) binds to glucose in the extracellular fluid, changes its shape, and releases glucose into the cytosol.



Facilitated diffusion requires a transporter protein but does not use ATP (energy molecule).


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TRANSPORT ACROSS THE PLASMA MEMBRANE • Osmosis 

The movement (diffusion) of water through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration, (down its concentration gradient) until equilibrium is reached. • Remember that water is the most versatile solvent (dissolving agent or medium); and a solute is a substance that is dissolved in another substance.



Osmotic pressure: The force with which a solvent (such as water) moves from a solution of lower solute concentration to a solution of higher solute concentration. In other words, it is the pressure needed to stop or prevent the flow of water across a membrane. • * The higher the solute concentration, the higher the solution’s osmotic pressure. Evelyn I . Milian - I nst r uct or

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TRANSPORT ACROSS THE PLASMA MEMBRANE: Osmosis •

Two sugar solutions of different concentrations are separated by a selectively permeable membrane, which the solvent (water) can pass through but the solute (sugar) cannot. Water molecules move randomly and may cross through the pores in either direction, but overall, water diffuses from the solution with less concentrated solute to that with more concentrated solute. This transport of water, or osmosis, eventually equalizes the sugar concentrations on both sides of the membrane.


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(a) As the experiment starts, water molecules move from the left arm into the right arm, down the water concentration gradient. (b) After some time, the volume of water in the left arm has decreased and the volume of solution in the right arm has increased. At equilibrium, net osmosis has stopped. (c) If pressure is applied to the solution in the right arm, the starting conditions can be restored. Evelyn I . Milian - I nst r uct or

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TRANSPORT ACROSS THE PLASMA MEMBRANE: Osmosis • Tonicity is the ability of a solution to change the volume of cells by altering their water concentration. In other words, tonicity is the ability of a solution surrounding a cell to cause that cell to gain or lose water. • Isotonic solution 

A medium or solution in which the overall concentration of solutes equals that found inside a cell (iso = equal). If the solution is isotonic to the cell, there is no net movement of water. The cell is also said to be isotonic in relation to the surrounding solution.

• Hypotonic solution 

A medium whose concentration of solutes is lower than that inside the cell (hypo = under, less). If the solution is hypotonic, the cell gains water.

• Hypertonic solution 

A medium having a higher concentration of solutes than inside the cell (hyper = above, more). If the solution is hypertonic, the cell loses water. Evelyn I . Milian - I nst r uct or

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Passive Transport: Osmosis • Tonicity describes the behavior of cells in a fluid environment. • Isotonic solution: its concentration of solutes equals that found inside a cell. • Hypotonic solution: its concentration of solutes is lower than that inside the cell. • Hypertonic solution: its concentration of solutes is higher than that inside the cell. Evelyn I . Milian - I nst r uct or

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FIGURE 3.8 – Tonicity and its effects on red blood cells (RBCs). Tonicity = A measure of the solution’s ability to change the volume of cells by altering their water content. One example of an isotonic solution for RBCs is 0.9% NaCl. Cells placed in an isotonic solution maintain their shape because there is no net water movement into or out of the cell. Cells placed in a hypotonic solution gain water, increase in size and will burst (hemolysis). Cells placed in a hypertonic solution lose water and undergo crenation (or plasmolysis); the cytoplasm shrinks. Evelyn I . Milian - I nst r uct or

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Osmosis, Turgor Pressure and Plasmolysis a)

In hypotonic surroundings, the vacuole of a plant cell fills with water, but the rigid cell walls prevent the cell from expanding. The cells of this healthy begonia plant are turgid. Turgor pressure is the pressure of the cell contents against the cell wall; in plant cells, it is determined by the water content of the vacuole and provides internal support.

b)

When the begonia plant is exposed to a hypertonic solution, its cells become plasmolyzed as they lose water (contraction of cell contents).

c)

The effects of turgor loss are seen during wilting, when leaves and stems droop as a result of cells losing water. The plant eventually dies.


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TRANSPORT ACROSS THE PLASMA MEMBRANE • Active Transport 

The movement of a substance across a cell membrane against its concentration gradient, from lower concentration to higher concentration; requiring the use of cellular energy (from ATP, a high-energy molecule).



Examples of solutes actively transported: ions, amino acids, monosaccharides.


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TRANSPORT ACROSS THE PLASMA MEMBRANE

• Active Transport 

Primary active transport • Energy derived from ATP changes the shape of a transporter protein (“pump”), which pumps a substance across a plasma membrane against its concentration gradient. Example: sodium-potassium pump (Na+/K+).



Secondary active transport (Cotransport) • Energy stored in an ionic concentration gradient is used to drive other substances across the membrane against their own concentration gradients (this transport indirectly uses energy obtained from the hydrolysis of ATP).


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PRIMARY ACTIVE TRANSPORT ACROSS THE PLASMA MEMBRANE

FIGURE 3.11 – The sodium-potassium pump (Na+ /K+ ATPase) expels sodium ions (Na+) and brings potassium ions (K+) into the cell.

Sodium-potassium pumps maintain a low intracellular concentration of sodium ions.


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SECONDARY ACTIVE TRANSPORT ACROSS THE PLASMA MEMBRANE

FIGURE 3.12 – Secondary active transport mechanisms. (a) Antiporters carry two substances across the membrane in opposite directions. (b) Symporters carry two substances across the membrane in the same direction. Evelyn I . Milian - I nst r uct or

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TRANSPORT ACROSS THE PLASMA MEMBRANE: Bulk Transport in Vesicles •

Vesicle: Small, spherical sac that has budded off from an existing membrane.

1) Endocytosis = bringing a substance or particle into cell 

The uptake of large biological molecules and particles into a cell by the formation of a new vesicle from the plasma membrane; a segment of the plasma membrane surrounds the substance, encloses it, and brings it in.

2) Exocytosis = releasing a substance or particle from cell 

Export of substances from a cell through the fusion of cytoplasmic vesicles with the plasma membrane (releasing their contents to the outside of the cell). Evelyn I . Milian - I nst r uct or

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Endocytosis in Animal Cells a) Phagocytosis (“cell eating” by phagocytes): A cell (such as a macrophage or white blood cell of the immune system) ingests and destroys solid particles (for example, microbes or cell debris) by packaging it in a vesicle or vacuole, which is digested by hydrolytic enzymes. b) Pinocytosis (“cell drinking”): The cell “gulps” droplets of extracellular fluid (containing molecules), forming a vesicle around them. Pinocytosis is nonspecific in the substances it transports. c) Receptor-mediated endocytosis: cells take up specific ligands, molecules that bind to specific cell receptors (membrane proteins). Evelyn I . Milian - I nst r uct or

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FIGURE 3.14 – Phagocytosis.

Pseudopods (extensions from the plasma membrane) surround a particle and the membranes fuse to form a vesicle called a phagosome.

* Phagocytosis is a vital defense mechanism that helps protect the body from disease. It is carried out by defensive cells called phagocytes.


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Bulk Transport in Vesicles: Endocytosis: Eating and Drinking by Cells • Phagocytosis. 

A white blood cell ingests and destroys a microbe.


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FIGURE 3.15 – Pinocytosis. The plasma membrane folds inward, forming a pinocytic vesicle.

* Most body cells carry out pinocytosis, the nonselective uptake of tiny droplets of extracellular fluid.


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FIGURE 3.13 – Receptor-mediated endocytosis of a low-density lipoprotein (LDL) particle. • Receptor-mediated endocytosis imports materials that are needed by the cells (for example: lipoproteins, transferrin, some vitamins, antibodies, certain hormones).

• Receptor proteins recognize the molecules needed and a vesicle is formed to take them into the cell.


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TRANSPORT ACROSS THE PLASMA MEMBRANE: Bulk Transport in Vesicles 2) Exocytosis = releasing a substance or particle from a cell. 

Export (secretion) of materials from the cell by fusion of cytoplasmic vesicles with the plasma membrane (releasing their contents to the outside of the cell).



Examples of materials released include: •

digestive enzymes, hormones, neurotransmitters, waste products.


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Exocytosis


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EXOCYTOSIS (Animation)

http://www.biologie.uni-hamburg.de/b-online/library/biology107/bi107vc/fa99/terry/membranes.html Evelyn I . Milian - I nst r uct or

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MOVEMENT OF SUBSTANCES ACROSS CELL MEMBRANES: Bulk Transport in Vesicles Endocytosis and Exocytosis in Eukaryotic Cells • Endocytosis: Process in which substances or particles are taken in by the invagination of the plasma membrane forming a vesicle (or vacuole).

• Exocytosis: Process by which substances or particles are released from a vesicle inside a cell when it fuses with the plasma membrane.


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References •

Audesirk, Teresa; Audesirk, Gerald & Byers, Bruce E. (2005). Biology: Life on Earth. Seventh Edition. Pearson Education, Inc.-Prentice Hall. NJ, USA.

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Brooker, Robert J.; Widmaier, Eric P.; Graham, Linda E.; Stiling, Peter D. (2008). Biology. The McGraw-Hill Companies, Inc. NY, USA.

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Campbell, Neil A.; Reece, Jane B., et al. (2011). Campbell Biology. Ninth Edition. Pearson Education, Inc.-Pearson Benjamin Cummings. CA, USA.

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Ireland, K.A. (2011). Visualizing Human Biology. Second Edition. John Wiley & Sons, Inc. NJ, USA.

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Mader, Sylvia S. (2010). Biology. Tenth Edition. The McGraw-Hill Companies, Inc. NY, USA.

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Martini, Frederic H.; Nath, Judi L. (2009). Fundamentals of Anatomy & Physiology. Eighth Edition. Pearson Education, Inc. – Pearson Benjamin Cummings. CA, USA.

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Solomon, Eldra; Berg, Linda; Martin, Diana W. (2008). Biology. Eighth Edition. Cengage Learning. OH, USA.

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Starr, Cecie. (2008). Biology: Concepts and Applications, Volume I. Thompson Brooks/Cole. OH, USA.

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Tortora, Gerard J.; Derrickson, Bryan. (2006). Principles of Anatomy and Physiology. Eleventh Edition. John Wiley & Sons, Inc. NJ, USA. www.wiley.com/college/apcentral. Evelyn I . Milian - I nst r uct or

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Chapter 7: CELL MEMBRANE STRUCTURE AND FUNCTION

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