Cell Membrane Transport - Transport Across A Membrane - How Do Things Move Across A Cell Membrane — Transcript

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1. 0:00Membrane transport.
2. 0:01So, how do substances move into and out of a cell?
3. 0:04Well, cells have a plasma membrane that acts like a fence and this
4. 0:08plasma membrane is what regulates what moves into and out of a cell.
5. 0:12The membrane is comprised mainly of phospholipid molecules and
6. 0:16has protein molecules scattered throughout its structure.
7. 0:19The phospholipid molecules have a polar hydrophilic water
8. 0:23loving head and two non polar hydrophobic water fearing tails.
9. 0:27The membrane is formed as the heads face out and the
10. 0:30tails face in forming a bilayer as you can see here.
11. 0:34The inner part of the cell called the cytosol is here, and the outside
12. 0:38of the cell is surrounded by a fluid called interstitial fluid.
13. 0:42Substances move into and out of a cell through several
14. 0:45different processes called membrane transport.
15. 0:47There are two main processes, passive transport
16. 0:51processes and active transport processes.
17. 0:54The main difference between the two is that passive
18. 0:57processes do not require energy expenditure
19. 1:00and active processes do require the cells to expend energy.
20. 1:04Let’s start by looking at the passive processes, which
21. 1:08include simple diffusion, facilitated diffusion and osmosis.
22. 1:12Diffusion is the movement of a substance from where it has
23. 1:15a high concentration to where it has a low concentration,
24. 1:18or the tendency of a substance to spread out evenly over a given space.
25. 1:23For instance, when a sugar cube is dissolved in water.
26. 1:26Over time the cube will dissolve and eventually spread
27. 1:29out evenly in the water, until it reaches equilibrium.
28. 1:32Diffusion occurs down a concentration gradient, which is a
29. 1:36difference in concentration of a substance between two areas.
30. 1:41So, the sugar molecules will move from an area of
31. 1:43high concentration to an area of low concentration.
32. 1:47Cellular diffusion is when diffusion of a solute, which is
33. 1:50a dissolved substance, occurs across the plasma membrane
34. 1:54from an area of high concentration to an area of low concentration.
35. 1:59This is dependent on the concentration of a substance in the
36. 2:02interstitial fluid outside the cell, and the cytosol inside the cell.
37. 2:07This can occur through simple diffusion or facilitated diffusion.
38. 2:11Simple diffusion occurs with solutes that are small and non polar.
39. 2:14By being non polar they can move in between phosphoipid molecules that form
40. 2:19the plasma membrane because the interior region of the membrane is non polar.
41. 2:25Some of the materials that move by simple diffusion include
42. 2:28the gases oxygen and carbon dioxide and small fatty acids.
43. 2:32So, if there is a higher concentration of oxygen O2 molecules outside of a cell,
44. 2:37they can move down the concentration gradient, across the membrane without
45. 2:42assistance, and into the cell as long as the concentration gradient exists.
46. 2:47And if there is a higher concentration of carbon dioxide CO2 molecules inside a cell,
47. 2:53they can move across the membrane without assistance,
48. 2:56out of the cell into the interstitial fluid.
49. 2:59So, again, simple diffusion is when non polar
50. 3:02molecules pass directly through a membrane.
51. 3:05The second type of diffusion is facilitated diffusion.
52. 3:08This applies to solutes that are small and either charged or polar.
53. 3:12Because these solutes are polar, the non polar phospholipid bilayer blocks them
54. 3:17from passing through the membrane and into or out of the cell by simple diffusion.
55. 3:22However, they can pass into and out of the cell with the assistance of
56. 3:26plasma membrane proteins through a process called facilitated diffusion.
57. 3:30There are two types of facilitated diffusion, channel
58. 3:34mediated diffusion and carrier mediated diffusion.
59. 3:37The difference between the two is the type of transport
60. 3:40protein used to move the substance across the membrane.
61. 3:44Channel mediated diffusion is when a ion, which is a charged particle where
62. 3:49its total number of electrons does not equal its total number of protons
63. 3:52giving it a positive or negative charge, moves across
64. 3:55the membrane through a water filled protein channel.
65. 3:58Each protein channel is typically specific for one
66. 4:01type of ion, and there are two types of channels,
67. 4:04a leak channel, which is continuously open, and a
68. 4:08gated channel, which only opens due to a stimulus,
69. 4:11and only stays open for a fraction of a second.
70. 4:14So, for a sodium positive ion, it can pass through
71. 4:18a sodium positive leak channel continuously
72. 4:21and a gated sodium positive channel will only open due to a
73. 4:25stimulus to allow the ion to pass through into the cell.
74. 4:29Carrier mediated diffusion involves the movement of polar molecules such as
75. 4:33simple sugars or simple carbohydrates and amino acids across the membrane.
76. 4:38This is accomplished by a carrier protein,
77. 4:40which actually changes shape in the process.
78. 4:43For instance glucose binds to a carrier protein, which changes shape
79. 4:47and moves the glucose molecule to the other side of the membrane.
80. 4:51Now for osmosis.
81. 4:53Osmosis is the passive movement of water through a selectively permeable membrane.
82. 4:58This occurs when there is a difference in concentration
83. 5:01of water on either side of the membrane.
84. 5:03This can happen in one of two ways, water can slip between
85. 5:06the phospholipid molecules that make up the plasma membrane,
86. 5:10or through integral protein water channels that are called aquaporins.
87. 5:14The plasma membrane is not permeable to most solutes,
88. 5:18such as charged, polar and large substances,
89. 5:21so for example, one side of the membrane, the cytosol or
90. 5:24interstatiul fluid, can have more solutes than the other side.
91. 5:28Let's say the interstitial fluid has 3 percent solutes and 97 percent
92. 5:32water, and the cytosol side has 1 percent solutes and 99 percent water.
93. 5:38In this example water will move down its concentration gradient from the 99
94. 5:43percent cytosol side to the 97 percent interstitial side to achieve equilibrium.
95. 5:49Now let’s look at active processes.
96. 5:51Active processes require the use of cellular energy for membrane transport.
97. 5:56There are two types of active processes, active transport and vesicular transport.
98. 6:02Active transport is the movement of a solute against its concentration gradient,
99. 6:07or going from an area of low concentration to a place of higher concentration.
100. 6:11Vesicular transport is the transport of large substances across the plasma
101. 6:17membrane by a vesicle, which is a membrane bound sac filled with materials.
102. 6:22Active transport has two types, primary active
103. 6:25transport and secondary active transport.
104. 6:28In primary active transport cellular protein pumps called ion pumps
105. 6:33move ions across the membrane, against their concentration gradient.
106. 6:38Let’s take a look at an example.
107. 6:40Here we have a membrane along with a sodium
108. 6:42positive, and potassium positive protein pump.
109. 6:45Three sodium ions and ATP bind to the pump.
110. 6:49ATP being like fuel or the form of cellular energy.
111. 6:53In order to release that energy ATP breaks down into ADP and P.
112. 6:59P binds to the pump and the release of energy causes the pump to
113. 7:03change shape are release the sodium positive ions into the interstitial fluid.
114. 7:08Two positive potassium ions bind to the pump and the P
115. 7:12produced by the ATP is released into the cytosol of the cell.
116. 7:16The transport protein, or pump reverts back to its orginal shpe, releasing the
117. 7:22positive potassium ions into the cytosol, and the pump is ready to do its job again.
118. 7:27In secondary active transport a substance is moved against its concentration gradient
119. 7:32by using energy provided by the movement of a
120. 7:36second substance down its concentration gradient.
121. 7:39So, a substance that is moving from a place of high
122. 7:42concentration to low concentration will provide
123. 7:45the energy to move a second substance from a place
124. 7:48of low concentration to high concentration.
125. 7:51There are two types of secondary active transports,
126. 7:54symport, where two substances are moved in the same direction and
127. 7:58antiport, where two substances are moved in opposite directions.
128. 8:02In our example here we have positive sodium ions moving from high concentration
129. 8:08outside of the cell to lower concentration inside of the cell.
130. 8:12A symporter will use the energy created by this to move a glucose
131. 8:17molecule from a low concentration area to a high concentration area.
132. 8:21And here we have an antiporter,
133. 8:23again using the energy from moving positive sodium
134. 8:27ions from high concentration outside of the cell
135. 8:30to lower concentration inside of the cell an H+
136. 8:34is moved up its gradient and outside of the cell.
137. 8:37Vesicular transport involves the transport of larger substances, such as
138. 8:42proteins or large carbohydrate polysaccharides,
139. 8:45across the plasma membrane.
140. 8:47There are two types of vesicular transport, exocytosis and endocytosis.
141. 8:53In exocytosis, materials are secreted from the
142. 8:57cell to the interstitiual fluid outside of the cell.
143. 9:00Typically, membranous vesicles formed by the golgi
144. 9:04apparatus transport materials to the cells membrane.
145. 9:07At the membrane the phospholipid molecules that make up the vesicle
146. 9:12fuse the phospholipid molecules that make up the plasma membrane,
147. 9:16and the contents of the vesicle are released outside of the cell.
148. 9:19The vesicle membrane now becomes part of the plasma membrane.
149. 9:23In endocytosis the plasma membrane kind of traps a substance by folding inward.
150. 9:29The lipid bylayer then fuses to form a vesicle surrounding the substance.
151. 9:33There are three main forms of endocytosis, phagocytosis,
152. 9:38pinocytosis, and receptor mediated endocytosis.
153. 9:43In phagocytosis a large particle is engulfed by the newly
154. 9:47formed vesicle and this vesicle fuses with a lysosome,
155. 9:50which is a membranous vesicle that contains digestive enzymes
156. 9:54that break down the particle into its component molecules.
157. 9:58Phagocytosis is also called cell eating.
158. 10:01Pinocytosis, which is also referred to as cell
159. 10:04drinking, is when the plasma membrane folds inward
160. 10:07and engulfs droplets of interstitial fluid that contain
161. 10:11dissolved solutes that can be used by the cell.
162. 10:14Receptor mediated endocytosis involves using
163. 10:16receptors on the outside of the plasma membrane.
164. 10:19These receptors bind with molecules in the in the interstitial fluid and
165. 10:23the membrane folds enclosing the receptors and the bound molecules,
166. 10:27to form a vesicle for transport within the cell.
167. 10:30One last thing to note, both vesicular transport
168. 10:34processes, endocytosis and exocytosis require energy.
169. 10:39And that be the basics on transport across a membrane.