Transport Across Cell Membranes Flashcards
1
Q
What are the two reasons why phospholipids are important components of cell-surface membranes?
A
The hydrophilic heads of both phospholipid layers point outside of the cell surface membrane attracted by water on both sides.
The hydrophobic tails of both phospholipid layers point to the centre of the cell membrane, repealed by water on both sides.
2
Q
What type of material moves through the phospholipid bilayer?
A
Lipid soluble material moves through the membrane via the phospholipid portion.
3
Q
What are the three functions of phospholipid in the membrane?
A
They allow lipid-soluble substances to enter and leave the cell.
They prevent water-soluble substances from entering and leaving the cell.
They make the membrane flexible and self-sealing.
4
Q
Where are proteins found in the cell surface membrane?
A
They are interspersed.
5
Q
What are the two main ways in which proteins are embedded in the phospholipid bilayer?
A
Proteins that occur in the surface of the bilayer do not extend all the way across it and act to give either mechanical support or act as cell receptors in conjunction with glycolipids.
Other proteins span the phospholipid bilayer completely as either protein carriers or protein channels.
6
Q
What are protein channels?
A
These form water-filled tubes to allow water soluble ions to diffuse across the membrane.
7
Q
What are carrier proteins?
A
These bind to ions or molecules like glucose or amino acids. They then change shape to move the molecules across the membrane.
8
Q
What are the 6 functions of proteins in the membrane?
A
They provide structural support.
Act as channels transporting water-soluble substances across the membrane.
Allow active transport across the membrane through carrier proteins.
Form cell-surface receptors for identifying cells.
Help cells adhere together.
Act as receptors for example, for hormones.
9
Q
Where are cholesterol molecules found?
A
They are found within the phospholipid bilayer of the cell surface membrane.
10
Q
What do cholesterol molecules do?
A
They add strength to the membranes.
11
Q
Why do cholesterol molecules play an important role in preventing the loss of water and dissolved ions from the cell?
A
They are very hydrophobic.
12
Q
What do cholesterol molecules do to the fatty acid tails of the phospholipid?
A
They pull the fatty acid tails together which limits their movement as well as he movement of other molecules without making the membrane as a whole too rigid.
13
Q
What are the 3 functions of cholesterol in the membrane?
A
Reduce lateral movement of other molecules including phospholipids.
Make the membrane less fluid at high temperatures.
Prevent leakage of water and dissolved ions from the cell.
14
Q
What are glycolipids made of?
A
They are made of a carbohydrate that is covalently bonded to a lipid?
15
Q
Where does the carbohydrate end extend to?
A
It extends from the phospholipid bilayer to the watery environment outside of the cell.
16
Q
What does the carbohydrate end of a glycolipid do?
A
It acts as a cell-surface receptor for specific chemicals.
17
Q
What are the 3 functions of glycolipids on the cell surface membrane?
A
They act as recognition sites.
They help maintain the stability of the membrane.
Help cells to attach to one another to form tissues.
18
Q
What are glycoproteins?
A
These are carbohydrate chains that are attached to many proteins on the outer surface of the cell membrane.
19
Q
What are the functions of glycoproteins in the membrane?
A
Act as recognition sites.
Help cells to attach to one another to form tissues.
Allow cells to recognise each other e.g lymphocytes can recognise an organisms own cells.
20
Q
What does the cell surface membrane do?
A
It controls the movement of substances into and out of the cell.
21
Q
Why can most molecules not diffuse freely into the cell?
A
They are not lipid-soluble and so cannot pass through the phospholipid bilayer.
They are too large to pass through protein channels.
They have the same charge as the protein channels and so are repelled.
They are electrically charged and so struggle to get through the non-polar hydrophobic tails in the bilayer.
22
Q
What is the fluid-mosaic model?
A
This is the way in which all the various molecules are combined into the structure of the cell-surface membrane.
23
Q
Why is the arrangement seen as fluid?
A
This is because the individual phospholipid molecules can move relative to one another, giving the membrane a flexible structure that is constantly changing shape.
24
Q
Why is the arrangement seen as mosaic?
A
The proteins in the bilayer vary in shape, size and pattern, like a mosaic.
25
Is diffusion passive or active?
It is passive.
26
What is meant by passive?
This means that the energy comes from the natural, inbuilt motion of particles rather than from an external source.
27
What key 3 things should be noted about particles?
All particles are constantly in motion because of their kinetic energy.
The motion is random and has no set pattern.
The particles are constantly bouncing off each other and other objects.
28
What is diffusion defined as?
This is the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.
29
What molecules can diffuse across the cell membrane?
Small non-polar molecules such as oxygen and carbon dioxide.
30
Why do charged ions and polar molecules not diffuse easily through the phospholipid membrane?
This is because of the hydrophobic nature of the fatty acid tails.
31
What makes the movements of molecules by diffusion easier?
Transmembrane channels and carriers.
32
Is facilitated diffusion and passive or active?
This is passive.
33
Does facilitated diffusion move molecules up or down the diffusion gradient?
It moves down the concentration gradient.
34
What two types of protein are involved in facilitated diffusion?
Protein channels and carrier proteins.
35
True or false: protein channels are selective.
This is true, protein channels are selective and only open when specific water soluble ions are present.
36
What happens when an ion binds to the protein channel?
It changes its shape so one side of the membrane becomes closed and the other side is opened, allowing the ion to pass through.
37
How do protein channels control the ions that enter and exit the cell?
They only open if a specific ion is present.
38
How do carrier proteins work?
They work when a molecule such as glucose that is specific to the protein binds with the protein carrier. This causes it to change shape in such a way that the molecule is released into the membrane.
39
What is osmosis?
The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.
40
What is a solute?
This is any substance that is dissolved in a solvent.
41
What is a solution?
A solvent is made from a combination of solute and solvent?
42
What is water potential measured in?
It is measured in kPa. The units are for pressure.
43
What is water potential?
This is the pressure created by water molecules.
44
How can the water potential be lowered?
If a solute is added to pure water, the water potential will decrease from zero.
45
What must the water potential of a solution always be?
It must always below zero so it will be a negative value.
46
What will happen to the water potential if more solute is added?
It will get lower.
47
What will happen to the water potential if more water is added to the solution?
It will increase.
48
How can the water potential of cells and tissues be determined?
This can be determined by placing the sample in different solutions with different water potentials, where there is no net loss or gain of water, the cell is isotonic and has the same water potential as the solution.
49
Does the partially permeable membrane allow sublet molecules through?
No
50
What happens when a dynamic equilibrium is established?
There is no net movement of water because the water potential on either side of the selectively permeable plasma membrane.
51
What is the water potential of pure water?
0
52
If too much water enters a cell what will happen?
The cell will burst and therefore die.
53
How do animals stop their cells from bursting?
Their cells live in a solution with the same water potential.
54
What happens if the cell is placed in a solution with a lower water potential?
The water will move via osmosis from the cell and into the solution.
55
What three factors affect the rate of diffusion?
Concentration gradient
Thickness of exchange surface
Temperature
56
What is active transport?
This is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.
57
What is ATP used for in active transport?
It is used to directly move molecules. To individually move molecules using a concentration gradient which has already been set up by active transport. This is known as co-transport.
58
What are the four ways that make active transport different to passive forms?
Metabolic energy in the form of ATP is needed.
Substances are moved against a concentration gradient.
Carrier protein molecules that act as protein "pumps" are involved.
The process is very selective; only specific substances are transported.
59
What is meant by the phrase "against a concentration gradient"?
This means that the substance is moved from a low to a high concentration.
60
What is the difference between active transport and facilitated diffusion?
Facilitated diffusion occurs down a concentration gradient, active transport occurs up the concentration gradient.
61
True or false: active transport only moves one molecule at a time?
This is false, whilst normally one molecule or ion is transported at once, sometimes more than one of the same type of molecule or ion may be moved in the same direction at the same time.
62
What is an example of an instance when two different molecules/ions move at the same time but in different directions (one moves into the cell and one is removed for the cell)?
The sodium-potassium pump is an example of this.
63
In the sodium-potassium pump, which ion is removed from the cell?
Sodium ions are actively removed from the cell.
64
What is transported into the cell by the sodium-potassium pump?
Potassium ions are actively taken in from the surroundings.
65
What is the sodium-potassium essential for?
It is essential for many important processes, most notably the creation of a nerve impulse.
66
What type of cell possesses microvilli?
The epithelial cells in the ileum.
67
What is the ileum?
This is the small intestine.
68
What do microvilli do?
They increase the surface area for the insertion of carrier proteins through which diffusion, facilitated diffusion and active transport occur.
69
How does active transport differ from passive forms?
Metabolic energy in the form of ATP is needed.
Substances are moved against a concentration gradient, that is from a low concentration to high concentration.
Carrier protein molecules which actors "pumps" are involved.
The process is very selective, with specific substances being transported.
70
Describe the active transport of a single molecule or ion.
The carrier proteins span across the membrane and bind to the molecule or ion to be transported on one side of it.
The molecule or ion binds to the receptor sites on the carrier protein.
On the inside of the cell/ organelle, ATP binds to the protein causing it to split into ADP and phosphate molecule. As a result, the protein molecule changes shape and opens to the opposite side of the cell membrane.
The molecule or ion is then released to the other side of the membrane.
The phosphate molecule is released from the protein which causes the protein to revert to its original shape, ready for the process to be repeated. The phosphate group them re-combines with the ADP to form ATP during respiration.
71
What is the difference between active transport and facilitated diffusion?
Facilitated diffusion occurs down a concentration gradient, whilst active transport occurs against a concentration gradient.
Facilitated diffusion does not require metabolic energy while active transport does.
72
What is the form of metabolic energy used in active transport?
ATP
73
What is a similarity between active transport and facilitated diffusion?
Both use carrier proteins.
74
Can only one molecule or ion be transported at a time and active transport?
Sometimes more than one molecule or ion may be moved in the same direction at the same time by active transport. Occasionally, the molecule or ion is moved into a cell/organelle at the same time as a different one is being removed.
75
Give an example of an occasion where one molecule is being transported in one direction as another one is transported in the opposite direction.
The sodium-potassium pump.
76
What is actively transported out of the cell in the sodium-potassium pump?
Sodium irons are actually removed from the cell/organelle.
77
What is actively brought into the cell in the sodium-potassium pump?
Potassium ions are actively taken in from the surroundings.
78
What is the sodium-potassium pump essential for?
It is essential to a number of important processes in the organism, including the creation of nerve impulses.
79
What do the epithelial cells lining the ileum possess?
They possess microvilli.
80
What are microvilli?
Microvilli are fingerlike projections on the cell surface membrane about 0.6 micro metres in length.
81
What is collectively known as the "brush border"?
Microvilli
82
Why is the term "brush border" used to describe microvilli?
This is because when viewed under a microscope they look like bristles on a brush.
83
What did the microvilli do?
They increase the surface area for the insertation of carrier proteins through which diffusion, facilitated diffusion and active transport can take place.
84
What is another mechanism to increase the rate of transport across membranes?
Increase the number of protein channels and carrier proteins in any given area of the membrane.
85
What two things are been digested continuously?
Proteins and carbohydrates.
86
What has a greater concentration of glucose and amino acids: the ileum or the blood?
There is normally a greater concentration of glucose and amino acids within the ileum than in the blood.
87
How does glucose move from the ileum to the blood?
The glucose moves down the concentration gradient by facilitated diffusion.
88
What is constantly being removed by cells during respiration?
Glucose
89
What helps to maintain the concentration gradient between the inside of the ileum and the blood?
The continuous circulation of blood which removes glucose from cells.
90
What increases the rate of facilitated diffusion across the epithelial cell surface membrane?
The maintains concentration gradient between the inside of the ileum and the blood.
91
What is the problem with the diffusion of glucose and amino acids in absorption?
The concentrations either side of the intestinal epithelium become equal but this means that not all of the available glucose and amino acids can be absorbed in this way and some of it may pass out the body.
92
What stops the glucose and amino acids being transported out of the body?
Active transport
93
What is the mechanism that absorbs amino acids and glucose from the small intestine?
Co-transport
94
How is the sodium-potassium pump involved in the active transport of glucose and amino acids?
Either glucose or amino acid's are drawn into the cells along with sodium irons that I've been actively transported out by the sodium-potassium pump.
95
Explain the co-transport of glucose molecule.
Sodium ions are actively transported out of epithelial cells, by the sodium-potassium pump, into the blood. This takes place in one type of protein-carrier molecule found in the cell-surface membrane of the epithelial cells.
This maintains a much higher concentration of sodium ions in the lumen of the intestine than inside the epithelial cells.
Sodium ions diffuse into the epithelial cells down this concentration gradient through a different type of protein carrier (co-transport protein) in the cell surface membrane. As the sodium ions diffuse in through the second carrier protein, they carry either amino acid molecules or glucose molecules into the cell with them.
Glucose/amino acids pass into the blood plasma by facilitated diffusion using another type of carrier.
96
Do glucose and amino acid molecules move up or down the concentration gradient in co-transport?
Glucose molecules move against the concentration gradient whilst sodium ions move down the concentration gradient.
97
Is co-transport an indirect or direct form of active transport?
It is indirect.
98
How is this type of co-transport indirect active transport?
This is because it is the sodium ion concentration gradient, rather than the ATP directly, that powers the movement of glucose and amino acids into the cells.
99
What is oral rehydration therapy treatment for?
Diarrhoeal diseases that infect the intestines such as cholera.
100
What are the three causes of diarrhoea?
Damage to the epithelial cells lining the intestine
Loss of microvilli due to toxins
Excessive secretion of water due to toxins, for example the cholera toxin
101
Can diarrhoea be fatal?
Yes, because it creates severe dehydration.
102
What problems does diarrhoea cause?
Insufficient fluid is taken into and/all excessive fluid is lost from the body.
103
Why is just drinking water and insufficient method of treating diarrhoea?
Water is not absorbed from the intestine -indeed, as the case of cholera, water is actually lost from the cells.
Drinking water does not replace the electrolytes that are being lost from the intestinal cells.
104
Why is replacing the water and electrolytes intravenously by a drip a bad idea?
It requires trained personnel and means that the patient is confined to the bed for much of the time.
105
How does a rehydration solution work?
It uses an alternative pathway to absorb sodium ions.
106
What happens to the water potential as the sodium ions are absorbed using the alternative pathway?
The water potential of the cells falls and water enters the cells by osmosis.
107
What does the rehydration solution need to contain?
```
Water
Sodium ions
Glucose
Potassium ions
Other electrolytes
```
108
Why is water incorporated in the rehydration solution?
This is to rehydrate the tissues.
109
What is the purpose of having sodium ions in the rehydration therapy?
This is to replace the sodium ions lost from the epithelium of the intestine and to optimise use of the alternative sodium-glucose carrier proteins.
110
Why is glucose used in the oral rehydration therapy?
This is to stimulate the uptake of sodium ions from the intestine and to provide energy.
111
What is that the purpose of having potassium ions in the oral rehydration therapy?
This is to replace the lost potassium ions and to stimulate appetite.
112
Why are other electrolytes added to the oral rehydration therapy solution?
This is to prevent electrolyte imbalance and a condition called metabolic acidosis.
113
What are electrolytes?
These are ions.
114
What are the other electrolytes that are added to oral rehydration therapies?
Chloride ions and citrate ions.
115
How can you ingredients of the oral rehydration therapy solution be packaged?
The ingredients can be mixed and packaged as a powder and then make the solution made up of boiled water as needed.
116
What is the benefit of having oral rehydration therapy in a powder form?
It is easy to make up and can be administered by people with minimal training.
117
What is the dosage of oral rehydration therapy?
The oral rehydration therapy should be taken regularly and in large amounts throughout the illness.
118
What is the problem with oral rehydration solutions?
Oral rehydration solutions do not prevent or cure diarrhoea. They simply rehydrate and nourish the patient until the diarrhoea is cured by some other means.
119
Is it possible to make an inexpensive home-made rehydration solution when commercial products are not available?
Yes, this can be made up of eight level teaspoons of sugar and one level teaspoon of table salt dissolved in 1 L of boiled water.
120
What was the main problem with early rehydration solutions?
They call side-effects, especially in children.
121
What caused the side-effects in the early rehydration solutions?
The excess sodium.
122
How did scientists change the rehydration solution to reduce the side-effects?
They lowered the sodium content and added more glucose.
123
What was the main problem with adding the extra glucose?
It lowered the water potential in the lumen of the ileum so much that it started to draw out even more water from the epithelial cells making dehydration even worse.
124
What happened with the glucose content was lowered?
It had less of a dehydrating effect but as glucose also acts as a respiratory substrate it reduce the amount of energy being supplied to the patient.
125
How was the problem of too much and too little glucose fixed?
Starch was used in place of some of the glucose.
126
What is starch broken down into?
Starch is broken down steadily by amylase and maltase in the ileum into its glucose monomers.
127
What are the two reasons why rice starch is a popular choice for rehydration therapies?
It is readily available in many parts of the world, especially days like diarrhoeal diseases are common.
It also provides other nutrients like amino acids. Not only are these nutrients nutritionally valuable but they also help the uptake of sodium ions from the ileum.
128
What is the problem with rice flour solutions?
They are very viscous and therefore difficult to swallow.
129
What are the four phases of clinical trials on humans?
Test on 20 to 80 people who are healthy, give a tiny amount to test for side-effects.
Then give to a large number of people 100 to 300 who have the condition to see if the drug works and look at safety issues this can take up to 2 years.
A large scale trial can then be used many patients with the disease are given a dummy drug called the placebo, neither the scientist nor the patients know which one is which- this is known as a double blind trial which takes many years.
If the drug passes all the stages it may be granted a license, but its uses and effects are still monitored over many years to check on any long-term effects.
