chapter 7: cell membrane structure and function

Chapter 7:

CELL MEMBRANE

STRUCTURE AND FUNCTION

Evelyn I. Milian

Instructor

2012

BIOLOGY I

BIOLOGY I. Chapter 7 – Cell Membrane Structure and Function

Evelyn I . Mil ian - Instructor 2

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.



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 (carbohydrate-lipids), glycoproteins (carbohydrate-proteins) and cholesterol molecules (a type of lipid).



• 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.



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.


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.

8 Evelyn I . Mil ian - Instructor



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)



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.



Functions of Membrane Proteins:

Transport of Ions or Molecules



Functions of Membrane Proteins: Signal Transduction



Functions of Membrane Proteins: Enzymatic Activity



Functions of Membrane Proteins: Cell-Cell Recognition



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


Functions of Membrane Proteins


Fig. 03.03



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.


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.




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)



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.



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.




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.




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.



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).





• 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.


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.




(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.



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.



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.



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.


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.






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.





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).



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.



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.




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).


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).



44

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.

Evelyn I . Mil ian - Instructor


Bulk Transport in Vesicles:

Endocytosis: Eating and Drinking by Cells

• Phagocytosis.

A white blood cell ingests and destroys a microbe.




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.

Evelyn I . Mil ian - Instructor





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.


Exocytosis




EXOCYTOSIS (Animation)

http://www.biologie.uni-hamburg.de/b-online/library/biology107/bi107vc/fa99/terry/membranes.html



MOVEMENT OF SUBSTANCES

ACROSS CELL MEMBRANES:


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.


References

• Audesirk, Teresa; Audesirk, Gerald & Byers, Bruce E. (2005). Biology: Life on Earth.

Seventh Edition. Pearson Education, Inc.-Prentice Hall. NJ, USA.

• Brooker, Robert J.; Widmaier, Eric P.; Graham, Linda E.; Stiling, Peter D. (2008). Biology.

The McGraw-Hill Companies, Inc. NY, USA.

• Campbell, Neil A.; Reece, Jane B., et al. (2011). Campbell Biology. Ninth Edition. Pearson

Education, Inc.-Pearson Benjamin Cummings. CA, USA.

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

NJ, USA.

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

• Martini, Frederic H.; Nath, Judi L. (2009). Fundamentals of Anatomy & Physiology. Eighth

Edition. Pearson Education, Inc. – Pearson Benjamin Cummings. CA, USA.

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

Learning. OH, USA.

• Starr, Cecie. (2008). Biology: Concepts and Applications, Volume I. Thompson

Brooks/Cole. OH, USA.

• Tortora, Gerard J.; Derrickson, Bryan. (2006). Principles of Anatomy and Physiology.

Eleventh Edition. John Wiley & Sons, Inc. NJ, USA. www.wiley.com/college/apcentral.


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