Cell Membranes Flashcards
(88 cards)
1
Q
What is the purpose of a membrane?
A
- Separating cell contents from the outside environment
- Separating organelles from cytoplasm (e.g. lysosomes containing digestive enzymes where their optimum pH is low (acidic), and the rest of the cell is neutral).
- Cell recognition and signalling
- Regulating transport of material into and out of cells
- Holding the components of some metabolic pathways in place.
2
Q
What is a phospholipid?
A
Phospholipids consist of:
- A hydrophilic head, made up of a phosphate group and glycerol. As it contains oxygen, it is polar and so water loving.
- A hydrophobic tail, made up of a hydrocarbon chain. It is non polar and so water hating.
- Phospholipids are what make up the cell membrane.
3
Q
Why are polar substances hydrophilic?
A
Anything polar (has a spread of charge) can dissolve in water (which is also polar) so they are water loving.
4
Q
Why are non-polar substance hydrophobic?
A
Molecules with charges that are evenly distributed around the molecules do not easily dissolve or mix with water, and in fact repel water molecules.
5
Q
What happens when phospholipids are mixed with water?
A
If phospholipid molecules are mixed with water, they form a layer at the water surface. The phosphate heads stick into the water, while the fatty acid tails stick up out of the water.
6
Q
What is a phospholipid bilayer?
A
If phospholipid molecules are completely surrounded by water, a bilayer can form. Phosphate heads on each side of the bilayer stick into the water, while the hydrophobic fatty acid tails point towards each other. This means the hydrophobic tails are held away from the water molecules.
-In this state, the phospholipid molecules can move freely, just as fluid molecules do, within the plane of the membrane. Very rarely some may ‘flip-flop’ from one monolayer to the other. The hydrophilic head group cannot easily pass through the hydrophobic region in the middle of the bilayer. This gives the bilayer some stability, despite the fact that the phospholipid molecules are not actually bonded together.
7
Q
Why do phospholipids form a bilayer in plasma membranes?
A
- Phospholipids have a polar phosphate group which are hydrophilic and will face the aqueous solutions.
- The fatty acid tails are non-polar and will move away from an aqueous environment.
- As both the outside of the cell and cytoplasm is aqueous, phospholipids form two layers with the hydrophobic tails facing inwards.
- The phosphate group faces outwards interacting with the aqueous environment.
8
Q
How does the phospholipid bilayer act a membrane?
A
The phospholipid bilayer is the basic structural component of all biological membranes. Essentially, the hydrophobic layer formed by the phospholipid tails creates a barrier to many molecules and separates the cell contents from the outside world. This thin layer of oil is ideal as a boundary in living systems, where most metabolic reactions take place in a water-based environment.
9
Q
Why do membranes need to be flexible?
A
They need to be flexible so are not bonded together.
- Vesicle formation means that membranes to be able to curve and pinch.
- Membranes are still sturdy because hydrophobic tails don’t want to touch water so won’t move very much.
10
Q
How have microscopes allowed the investigation into membranes?
A
You cannot see membranes through the light microscope, but they can be seen using an electron microscope. Here, the membranes appear as two dark ‘tramlines’ (the phospholipid heads) separated by a pale region (the fatty acid tails).
-The electron microscope have revealed that membranes are about 7-10nm thick.
11
Q
Why do membranes need other components to make it functional?
A
A simple phospholipid bilayer would be incapable of performing all the functions of biological membranes. It would also be too fragile to function as a barrier within or around cells. Other components are needed in order to make a functioning biological membrane.
12
Q
What is the specialisation of membranes?
A
The specialisation of cell membranes is a part of the process of differentiation. The number and type of these other components varies according to the function of the particular membrane.
-In a multicellular organism, this means the cell membranes of specialised cells have different properties, as do the membranes of specific organelles.
13
Q
What examples are there of how membranes have specialised in the process of differentiation?
A
- The plasma membranes (cell surface membranes) of the cells in a growing shoot contain receptors that allow them to detect the molecules that regulate growth.
- Muscle cell membranes contain a large number of the channels that allow rapid uptake of glucose to provide energy for muscle contraction.
- The internal membranes of chloroplasts contain chlorophyll and other molecules needed for photosynthesis.
- The plasma membranes of white blood cells contain special proteins that enable the cells to recognise foreign cells and particles.
14
Q
Are cell membranes permeable?
A
(How easy it is for things to pass/diffuse through)
All membranes are permeable to water molecules because water molecules can diffuse through the lipid bilayer. Some membranes are up to 1000 times more permeable to water because they contain aquaporins (protein channels that allow water molecules through them). Cell membranes that are permeable to water and some solutes are described as partially permeable membranes.
15
Q
What are the features of the fluid mosaic model?
A
- A bilayer of phospholipid molecules forming the basic structure.
- Various protein molecules floating in the phospholipid bilayer, some completely freely, some bound to other components or to structures within the cell.
- Some (extrinsic) proteins partially embedded in the bilayer on the inside or the outside face; other (intrinsic) proteins completely spanning the bilayer.
16
Q
What is an extrinsic protein?
A
Partially embedded in the membrane (on the surface)
17
Q
What is an intrinsic protein?
A
Running all the way through the membrane
18
Q
What components are in the fluid mosaic model of the membrane?
A
- Phospholipid bilayer
- Cholesterol
- Channel protein
- Carrier protein
- Glycolipid
- Glycoprotein
- Receptor sites
19
Q
What is a glycolipid?
A
Where phospholipid molecules have a carbohydrate part attached. It acts as a hormone receptor.
20
Q
What is a glycoprotein?
A
Where protein molecules have a carbohydrate part attached.
-It acts as a hormone receptor and can also bind cells together in tissues.
21
Q
What is the role of cholesterol in a membrane?
A
Cholesterol gives the membranes of some eukaryotic cells mechanical stability.
The steroid molecule fits between fatty acid tails and helps make the barrier more complete, so substances like water molecules and ions cannot pass easily and directly through the membrane.
22
Q
What membrane transport systems do membranes have?
A
- Channel proteins
- Carrier proteins
23
Q
What is a channel protein?
A
Channel proteins allow the movement of some substances across the membrane. Molecules of sugar such as glucose are too large and too hydrophilic to pass directly through the phospholipid bilayer. Instead, they enter and leave cells using these protein channels.
They ONLY use diffusion, down a concentration gradient.
24
Q
What is a carrier protein?
A
Carrier proteins actively move some substances across the membrane.
They use active transport, against a concentration gradient, and so need energy from ATP.
25
What is an example of how carrier proteins are used in plants?
- In plant cells, magnesium ions are actively pumped (using ATP energy) into the root hair cells from the surrounding soil, so that the plant ensures a supply of magnesium for the manufacture of chlorophyll.
- When mineral ions are actively transported into root hair cells, they lower the water potential of those cells. This makes water enter by osmosis. Nitrate ions are actively transported into xylem vessels to lower the water potential and cause water uptake from the surrounding root cells.
26
What is the role of receptor sites in a membrane?
- Some receptor sites allow hormones to bind with the cell so that a cell 'response' can be carried out. A cell can respond to a hormone only if it has a receptor for that hormone on its cell surface membrane.
- Cell membrane receptors are also important in allowing drugs to bind, and so affect cell metabolism.
27
What role do glycoproteins and glycolipids have in cell recognition?
Glycoproteins and glycolipids may be involved in cells signalling that they are 'self', to allow recognition by the immune system. Some hormone receptors are glycoproteins and some are glycolipid.
28
What role do enzymes and coenzymes have in membranes for metabolic processes?
Some reactions in photosynthesis take place in membranes inside chloroplasts. Some stages of respiration take place in membranes of mitochondria. Enzymes and coenzymes may be bound to these membranes. The more membrane there is, the more enzymes and coenzymes it can hold.
-This explains why mitochondrial inner membranes are folded to form cristae, and why chloroplasts contain many stacks of membranes called thylakoids.
29
What effect does an increase in temperature have on membrane permeability?
Increasing temperature gives molecules more kinetic energy, so they move faster. This increased movement of phospholipids and other components makes membranes leaky, which allows substances that would not normally do so to enter and leave the cell.
30
What effect does a temperature of below 0°C have on membrane permeability?
Components in the membrane have little energy and so are tightly packed, and rigid.
Channel and carrier protein denature
Membrane become permeable
31
What effect does a temperature of 0-45°C have on membrane permeability?
Phospholipids have energy to move and aren't pack tightly.
| As temperature increases, so does permeability.
32
What effect does a temperature of above 45°C have on membrane permeability?
Bilayer breaks down, and melts.
Channel and carrier proteins denature which disrupts membrane, leading to it becoming leaky.
Membrane becomes more and more permeable.
33
How do organisms living in extremely hot or cold climates adapt to stop the problem of temperature affecting permeability?
Organisms that live in very hot or cold environments need differently adapted molecular components of their membranes, so that their membranes can perform the function needed to maintain life.
-For example, if the organism lived in hot climates it will have a higher cholesterol content to keep it more stable.
34
Cell Communication and Signalling: Why must amoeba be able to detect nutrients around it?
Amoeba (a single-celled organism) must be able to detect nutrient molecules in the water around it. It must then move towards the nutrient and take the molecules into the cell. If the amoeba cannot detect that there are nutrients present, cannot move towards the nutrients, or cannot take in the nutrients it will be unable to survive.
35
Cell Communication and Signalling: How does detection of signals play a part in the survival of multicellular organisms?
In multicellular organisms, the survival of the whole organism requires each cell to play its part. So cells must be able to detect the various internal and external signals used to coordinate and carry out the processes involved in growth, development, movement and excretion. Cells must then be able to carry out reaction or functions in response to the signals. The processes involved in communication between the cells of multicellular organisms are extremely complex.
36
Cell Communication and Signalling: Why do cells need to be able to communicate with each other and their environment?
- Single celled organisms: detection and movement towards nutrients for survival.
- Multicellular organisms: co-ordination between cells for growth, development and movement.
- Detect internal and external signals (hormones)
- Respond to signals and carrying out function/process. Receptors detect the signals.
37
Cell Communication and Signalling: What sensors are capable of receiving signals?
In order to detect signal, cell must have sensors on their surface capable of receiving signals. These sensors are known as receptors. They are often protein molecules or modified protein molecules.
38
Cell Communication and Signalling: What is a target cell?
In multicellular organisms, communication between cells is often mediated by hormones. Hormones are chemical messengers, produced in specific tissues and then released into the organism. Any cell with a receptor for the hormone molecules Is called a target cell.
39
Cell Communication and Signalling: How does a hormone bind with a receptor?
A hormone molecule binds to a receptor on a target cell surface membrane because the two have complementary shapes. Binding od the hormone and receptor causes the target cell to respond in a certain way.
40
Cell Communication and Signalling: How is insulin an example of a hormone receptor?
Insulin is released from beta-cells in islets or Langerhans in the pancreas, in response to increased blood sugar levels.
- Insulin is a protein molecule that attaches to the insulin receptors on the plasma membranes of many cells, including muscle and liver cells.
- When insulin attaches to its receptor, it triggers internal responses in the cell that lead to more glucose channels being present in the plasma membrane. This allows the cell to take up more glucose from the blood, so reducing the blood glucose level.
41
Cell Communication and Signalling: What is the process of insulin production?
- Insulin attaches to the receptor in muscle and liver cells.
- This triggers the cell to produce more glucose channels in the membrane.
- The cells take up the glucose, reducing blood glucose levels.
- This can then either be used in respiration, converted to fatty acids or converted to glycogen to store.
42
Cell Communication and Signalling: How can drugs interfere with receptors?
A number of medicinal drugs have been developed that are complimentary to the shape of a type of receptor molecule. Some drugs are intended to block receptors.
-Some drugs mimic a natural neurotransmitter that some individuals cannot produce - drugs used to treat schizophrenia work in this way.
43
Cell Communication and Signalling: What are beta-blockers?
Beta-blockers are used to prevent heart muscle from increasing the heart rate in people for whom such an increase could be dangerous. Beta-blockers are intended to block epinephrine receptors (adrenaline) so hormone can't fit into receptor to increase heart rate as it is blocked.
44
Cell Communication and Signalling: How are receptors hijacked?
Some viruses can use normal cell receptors and get into the cell. They bind with receptors on the cell's plasma membrane that normally bind to the host's signalling molecules.
45
Cell Communication and Signalling: How is HIV an example of how viruses can hijack receptors?
Human immunodeficiency virus (HIV), which causes AIDS, can infect humans because it can enter the cells of the immune system. It can bind to the CD4 receptor on helper T-lymphocytes. Once HIV enters such a cell, after a period of inactivity it may reproduce inside the cell and eventually destroy it.
46
Cell Communication and Signalling: What is an example of poisons hijacking receptors?
Botulism toxin extracted from the bacterium Clostridium botulinum with receptors in muscle fibres and prevents them from working properly, causing paralysis. This toxin is lethal, but it is used in small quantities in cosmetic surgery under the name BOTOX, to paralyse small muscles in the face and reduce wrinkling of the skin.
47
Cell Communication and Signalling: Why are some toxin molecules more potent and effective than others?
The more complimentary to the receptor they are, the stronger the toxin.
48
Diffusion: How does transport help with survival?
In order to survive, cells need a supply of nutrient molecules. Most cells need a supply of oxygen for aerobic respiration. The reactions in living cells (known collectively as metabolism) generate waste products that must be removed from the cell. Any molecules that need to enter or leave a cell (or enter or leave a membrane-bound organelle) will usually have to cross a membrane to do so.
49
Diffusion: Why do things spread out when in a high concentration?
Molecules in a gas or liquid bump into each other and spread away for where they occur in high concentration. This tendency to even out is called diffusion.
50
Diffusion: How do different type of molecules always diffuse?
Down their concentration gradient. This is a passive process.
51
Diffusion: What is equilibrium?
When diffusion have taken place, molecules are distributed evenly. This does not mean their movement stops - the molecules continue to diffuse around. We refer to this state, where there is no overall movement of molecules in one direction, as equilibrium - there is no net movement.
52
Diffusion: What type of process is diffusion?
Diffusion is a passive process. This means that molecules continue to diffuse down their concentration gradient without using energy from the cell. They have their own kinetic energy.
53
Diffusion: What affects the rate of diffusion?
- Temperature; more KE, faster the rate of movement
- Concentration gradient; the higher it is, the faster the rate
- Distance; the thinner the membrane, the faster the diffusion
- Size of molecule; smaller the molecules, the faster they diffuse
- SA; large SA increases rate (e.g. intestinal epithelial cells/alveoli)
- Stirring; increases the movement of the molecules thus increasing the rate.
54
Diffusion: What molecules can pass through a membrane using simple diffusion?
- Lipid-based molecules
| - Very small molecules and ions
55
Diffusion: How do lipid-based molecules pass through a membrane using simple diffusion?
As the membrane is made of phospholipids, fat-soluble molecules can simply pass through the bilayer. They diffuse down a concentration gradient. Steroid hormones are lipid-based and so diffuse through membranes into cells.
56
Diffusion: How do very small molecules and ions pass through a membrane using simple diffusion?
Carbon dioxide and oxygen molecules are small enough to pass through the bilayer between the phospholipid molecules. Water molecules are very small, some water molecules will pass directly through the membrane even though they are polar.
-Alternatively, water molecules can pass through an aquaporin which is using facilitated diffusion.
57
Diffusion: How do large or charged molecules pass through a membrane using simple diffusion?
Small, charged particles such as sodium ions, or larger molecules such as glucose, cannot pass through the lipid bilayer. Two types of protein molecule are involved in allowing such substances to pass through membranes. Because these proteins allow substances to pass though the membrane, the diffusion of these molecules and ions is known as facilitated diffusion.
58
Diffusion: How do channel proteins aid facilitated diffusion?
Channel proteins basically form pores in the membrane, which are often shaped to allow only one type of ion through. They are often gated meaning they can be opened or closed. As the hormone enters into the receptor, the gate opens allowing substances through.
-Gated sodium ion channel proteins are involved with the working or the nervous system.
59
Diffusion: How do carrier proteins aid facilitated diffusion?
Carrier proteins are shaped so that a specific molecule (e.g. glucose) can fit into them at the membrane surface - when the specific molecule fits, the protein changes shape to allow the molecule through to the other side of the membrane.
-Carrier 'flip-flop' can carry a specific molecule in either direction - the movement depends on the diffusion gradient.
60
Diffusion: How do cells have control over what enters and exits cells and organelle?
Different membranes can have different carrier and channel proteins. This means that cells and organelles have some control over the types of substance that are allowed to pass in or out across the membrane.
61
Diffusion: Why does glucose have to use facilitated diffusion?
Glucose is a product of digestion. It is also a relatively large polar molecule. Once glucose has been digested, it must be absorbed into the bloodstream from the small intestine. Part of the absorption process happens by diffusion.
62
Active Transport: Why might some situations mean that diffusion can't be used?
The needs of cells cannot always be met by the process of diffusion. Sometimes, for a cell to function properly it may need more of a particular substance in the cytoplasm than is present outside the cell.
In other cases, cells may need to move materials in or out of the cell more quickly than simple diffusion allows.
63
Active Transport: What are examples of when diffusion can't be used?
- Magnesium ions are often in a low concentration in soil. Plant cells need magnesium ions to make chlorophyll. The plant cell must be able to move magnesium ions into the cell against a concentration gradient.
- Active transport helps us absorb glucose from our intestines.
64
Active Transport: What is active transport?
Transporting molecules against their concentration gradient, which used energy from ATP.
65
Active Transport: How are carrier proteins involved in active transport?
Some of the carrier proteins found in membranes act as 'pumps'. These proteins are similar to the protein carriers used for facilitated diffusion. They are shaped in a way that the molecule it carried fits on one side of the membrane only.
They carry larger or charged molecules and ions through membranes. There are the molecules and ions that cannot pass through the lipid bilayer by diffusion.
66
Active Transport: How do protein pumps differ from the proteins used in facilitated diffusion?
- They carry specific molecules one way across the membrane.
- In carrying molecules across the membrane, they use metabolic energy in the form of ATP.
- They can carry molecules in the opposite direction to the concentration gradient.
- They carry molecules at a much faster rate than at diffusion.
- Molecules can be accumulated either inside cells or organelles, or outside cells.
67
Active Transport: How do protein pumps ensure one way flow?
The energy used in pumping molecules across membranes by active transport changes the shaped of the carrier protein. The shape change means that the specific molecule to be transported fits into the carrier protein on one side of the membrane only. As the molecule is carried through, the carrier uses energy from ATP. This changes the shape so that the molecule being carried across now leave the carrier protein. The molecule cannot enter the transport protein, because the protein is now a different shape so it will not fit.
68
Active Transport: How is calcium ion movement in muscles an example of active transport?
Muscle fibres can contract only if calcium ions are present. When a muscle is stimulated to contract, calcium ions are released from membrane-bound stores (specialised endoplasmic reticulum), where they are in very high concentration. When the muscle needs to relax again, the calcium ions are pumped rapidly back into the stores by the many calcium ion pumps found on the membrane of the specialised endoplasmic reticulum.
69
Bulk Transport: What is bulk transport?
Using membrane's flexibility, vesicles can be formed to transport substances in bulk. Bulk transport is possible because membranes can easily fuse, separate and 'pinch off'. Like active transport, bulk transport requires energy in the form of ATP. The energy is used to move the membranes around to form the vesicles that are needed, and to move the vesicles around the cell.
70
Bulk Transport: What are examples of bulk transport?
- Hormones; pancreatic cells make insulin in large quantities. The insulin is processed an packaged into vesicles in the Golgi apparatus. These vesicles fuse with the outer membrane and release insulin into the membrane.
- In plant cells, materials required to build the cell wall are carried outside in vesicles.
- Some white blood cells engulf invading microorganisms by forming a vesicle around them. This vesicle then fuses with lysosomes so that the enzymes from the lysosomes can digest the microorganism. Such cells are called phagocytes.
71
Bulk Transport: What is endocytosis?
Bulk transport into the cell
72
Bulk Transport: What is exocytosis?
Bulk transport out of the cell.
73
Bulk Transport: What is phagocytosis?
Bulk transport of solid material
74
Bulk Transport: What is pinocytosis?
Bulk transport of liquid material
75
Water Potential: What is a solute?
A solid dissolved in a liquid
76
Water Potential: What is a solvent?
A liquid that dissolves solids
77
Water Potential: What is a solution?
A liquid containing dissolved solids
78
Water Potential: What affects the diffusion of water?
Water molecules that are free to move around will diffuse from a region where there are a lot of them to a region where there are fewer of them. But if there are any other substances dissolved in the water, this will affect the concentration of 'free' water molecules.
79
Water Potential: What is water potential?
Water potential is a measure of the tendency of water molecules to diffuse from one place to another.
80
Water Potential: How does water move with a water potential value?
Water always moves from a region of high water potential to a region of lower water potential.
-It moves from a region of high concentration of 'free' water molecules to a region of lower concentration.
81
Water Potential: What is the water potential of pure water?
0kPa
| -The more solute added, the water potential value gets more negative.
82
Water Potential: As you add more solute to water, what happens to the water potential?
It will decrease.
- Water particles aren't as free to move around.
- Solute dissolved means there's a lower concentration of water particles.
- Water particles cluster around it so they're not as free to move around.
83
Water Potential: What is osmosis?
Osmosis is a special kind of diffusion. It refers only to the movement of water molecules:
-by diffusion
-across a partially permeable membrane
As with diffusion, net movement of molecules occurs until the concentrations are evened out. So osmosis will occur until the water potential is the same on both sides of the membranes.
84
Water Potential: What are the factors affecting osmosis?
- Temperature
- Thickness of membrane
- Water potential gradient
- Surface area
85
Water Potential: What happens if animal cells are in solutions of high water potential?
Animal cells fill with water and eventually burst open - it is haemolysed.
-Placing animal cells in pure water (solutions with a water potential higher than the cell contents) means there is a water potential gradient from outside to inside the cells. Water molecules will move down the water potential gradient, into the cell by osmosis.
86
Water Potential: What happens if plant cells are in solutions of high water potential?
Plant cells fill with water, the swelling cytoplasm and vacuole will push then membrane against the cell wall. The cell will not burst because the wall will eventually stop the cell getting nay larger. Osmosis will stop at this point and the cell is turgid.
-Placing plant cells in pure water (solutions with a water potential higher than the cell contents) means there is a water potential gradient from outside to inside the cells. Water molecules will move down the water potential gradient, into the cell by osmosis.
87
Water Potential: What happens if animal cells are in solutions of low water potential?
Water moves out of animal cells and the animal cell shrinks, and appears wrinkled - it is crenated, and irreversible.
-Placing animal cells in a concentrated salt or sugar solution (with a water potential lower than the cell contents) means there is a water potential gradient from inside to outside the cell. So water molecules will move out of the cell by osmosis. The cell will shrink.
88
Water Potential: What happens if plant cells are in solutions of low water potential?
Water moves out of the plant cells and the cytoplasm and vacuole will shrink as they lose water, the cell surface membrane will pull away from the cell wall - it is plasmolysed, and is reversible.
-Placing plant cells in a concentrated salt or sugar solution (with a water potential lower than the cell contents) means there is a water potential gradient from inside to outside the cell. So water molecules will move out of the cell by osmosis. The cell will shrink.
