Download Chapter 7 – Cell Membrane Structure and Function

Chapter 7 – Cell Membrane Structure and Function
Cell Membrane
1. Cell membrane is a boundary between cell and its environment. All cells are covered with a thin
covering of a double layer of Phospholipids and associated Proteins present at some places.
2. Phospholipid molecules are amphipathic with one polar and one nonpolar end. Each phospholipid
has a polar (hydrophilic) head and non-polar (hydrophobic) tails. In the double layer the tails face
each other forming a hydrophobic barrier which keeps water dissolved contents inside.
3. Cell membrane is selectively permeable. It allows some molecules to pass through it than others. It
regulates the entry and exit of substances into or outside cell. Nonpolar substances can readily pass
the cell membrane but polar substances use channels present in proteins.
4. Proteins may be Integral – embedded in the lipid double layer and Peripheral – associated at surface
of the lipid double layer. Most of unique functions of cell membrane are due to its proteins.
5. Cell membrane is fluid due to common movements of phospholipid molecules in their layer, lateral
movement of some proteins and flip-flop movement of some phospholipid molecules from one half
of phospholipid bilayer to other half.
Body Fluids: in animal body
Intracellular Fluid = 67%
Extracellular Fluid = 33% = Interstitial Fluid + Plasma
ECF = ISF (80%) + plasma (20%)
Extracellular Matrix: Collagen and Elastin fibers in gel made by glycoproteins and proteoglycans
(mucopolysaccharides)
Membrane Transport: Passive and Active transport; Carrier-mediated Transport
Passive Transport: does not use ATP, uses kinetic energy of molecules. It has 4 types:
1. Simple diffusion:
a. Nonpolar molecules pass through phospholipid layers.
b. Inorganic ions and water move through channel proteins.
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Facilitated Diffusion is the only passive transport to use carrier protein.
Osmosis is net movement of water through semipermeable membrane.
Filtration is mass movement of everything smaller than pore size due to pressure.
Active Transport
Active Transport uses ATP and a carrier protein.
1. Primary Active Transport: uses directly energy of ATP
2. Secondary Active Transport: creates a gradient of an ion with ATP and uses ion gradient to
transport molecule or ion
3. Bulk Transport: Solids and liquids moved in vesicles by using ATP
a. Endocytosis
b. Exocytosis
Diffusion
Diffusion: All fluids (liquids + gases) move from area of higher concentration to area of lower
concentration (concentration gradient). This movement of substances is called Diffusion. It can be
through cell membranes, for example, movement of nonpolar steroids or lipids through
phospholipid bilayer, movement of ions or water through channels in membrane proteins and
movement of O2 and CO2 between lungs and blood or between cells and interstitial fluid.
Diffusion
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Diffusion is movement of one kind of molecules or ions from its high concentration to its low
concentration.
Flux: is the per unit area per unit time movement of solute through membrane.
Net Flux is the difference between fluxes of same solute in both directions.
Non-polar substances diffuse through lipid bilayer, for example, oils.
Small polar molecules and ions move through channels in integral proteins, for example, H2O, Na+,
K+
Large polar molecules cannot diffuse through cell membrane, for example, proteins.
Factors affecting rate of diffusion
Concentration difference - ↑ higher difference
Density of Medium – ↑ from solid  liquid  gas
Size of molecule - ↑ with smaller size
Temperature - ↑ with higher temperature
Surface area of membrane - ↑ higher surface area
Osmosis
Osmosis is always the diffusion of water through cell membrane from its higher concentration
(dilute solution) to its lower concentration (concentrated solution) when the 2 solutions are
separated by semi-permeable membrane.
Diffusion versus Osmosis
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Diffusion
Movement of solute
Membrane not required but can occur
through it
High concentration low concentration
of solute
Osmosis
diffusion of water through semi-permeable
membrane
Cannot occur without cell membrane
High concentration Low concentration of
water
Osmotic Pressure is pressure required to prevent osmosis.
Mole has the quantity of a substance equal to its molecular weight in grams. Atomic weight of
sodium is 23 and atomic weight of Cl is 35.5. Molecular weight of NaCl is 58.5 (23 + 35.5). One
mole of NaCl is 58.5g of NaCl.
Molarity is # of moles of solute / liter of water; 1Molar (M) solution is 1 mole of solute / 1liter
Osmolarity is # of osmoles of solute/L of water; 1 mole of glucose is 1 osmole but 1 mole of NaCl
yields 2 osmoles in water.
Osmotic Flow across a Cell Membrane
Isotonic – when a cell is placed in a medium with equal concentration of a solute, for example,
0.9% NaCl solution or 300mOsm/L. No net osmosis. No change in size of cell.
Hypertonic - When cell is placed in higher concentration medium with > isotonic (5% NaCl or
400mOsm) concentration. Net movement of water is outside the cell, exosmosis. Cell shrinks, and
undergoes Crenation.
Hypotonic - < isotonic (0.1 % NaCl solution or 200mOsm) concentration; net movement of water
inside the cells which swell and may hemolyse to leave ghost cells.
Carrier-Mediated Transport
Facilitated diffusion and Active transport
It obeys all 4 characteristics (Chemical Specificity, Saturation, Competition and Affinity) of ligandbinding site characteristics. Ligand is any molecule or ion that binds to a receptor molecule.
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Facilitated diffusion
Facilitated diffusion is faster than normal diffusion but needs a carrier protein though no ATP
used, for example, absorption of glucose and amino acids in intestine. It is passive transport along
concentration gradient.
Facilitated Diffusion of Glucose: uses a Carrier protein to muscle, liver and adipose cells.
3 Characteristics:
1. Specificity – requires a specific carrier protein
2. Saturation – reaches a transport maximum
3. Competition – for same carrier protein lowers the transport maximum for both.
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Insertion and Removal of Carrier Proteins in cell membrane
1. Insertion of carrier protein in cell membrane when stimulated by insulin or exercise
2. Carrier proteins removed in a vesicle when stimulation stops.
Active Transport
Primary Active Transport uses ATP directly
For example, Ca2+ pump and Na+-K+ ATPase pump
Secondary Active Transport uses ATP to create an ionic gradient; uses the energy of this ionic
gradient to transport another molecule or ion.
2 types of Secondary Active Transport are:
Co-transport – both ion and substance move in same direction, Na+ and amino acid
Counter-transport – ion and substance move in opposite directions, HCO3- ion and Cl- ion in RBC
Vesicular Transport
Needs ligand –binding site characteristics
Endocytosis is the bulk transport into the cell. If solid material including prey is brought in as Food
Vacuole, the process is called Phagocytosis. For example, white blood cells eating bacteria. When
cell brings in liquid bound in a vesicle, the process is Pinocytosis.
Exocytosis: When the cells releases solid or liquid in vesicles, the process is exocytosis. For
example, Cells secrete proteins and hormones in secretory vesicles.
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