2.1.5 Membranes (includes practical skills) Flashcards
1
Q
What are the 5 functions of the membrane?
A
- Formation of partially permeable barriers
- Compartmentalisation
- Sites of enzyme attachment and there fore of chemical reactions
- Maintenance of concentration gradients
- • Sites of cell signalling
2
Q
what is a partially permeable barrier?
A
partially permeable means that only specific molecules/ions may pass through the membrane, others may not, giving control over what may pass into/out of a cell or organelle.
3
Q
where to partially ermeable barriers come in handy?
A
between the cell and its environment, or between organelles and the cytoplasm, and within organelles
4
Q
what does partially permebale mean?
A
a membrane which allows some substances through but not others
5
Q
what is compartmentalisation?
A
membranes define organelles within eukaryotic cells by forming their boundaries. These membrane‐bound organelles can then function as specialised compartments, effective in carrying out specific roles.
6
Q
give two examples of compartmentalisation
A
- transport vesicles carry proteins from RER to Golgi
- the membrane of a lysosome keeps the hydrolytic enzymes separate from other parts of the cell so that these are not accidently digested
7
Q
why is compartmentalisation important?
A
Within each organelle, the relevant substrates and enzymes can be concentrated and pH can be optimised, giving higher reaction rates.
8
Q
why is membranes being sites of enzyme attachment important? give it in an example form
A
membrane of mitochondria is folded into cristae so there is more surface area for attachment of ATP synthase proteins and hence a higher rate of ATP synthesis.
9
Q
why is it important for the maintenance of concentration gradients?
A
the presence of a partially permeable membrane means that a concentration gradient can be set up and maintained across it, potentially giving faster rates of diffusion
10
Q
give an example of a concentration gradient
A
chemiosmosis
11
Q
what is cell signalling?
A
A complex system of intercellular communication. This is when chemicals binf to complementary receptors casuing a direct or cascade of events in a cell.
12
Q
How does cell signalling work
A
Plasma (i.e. cell surface) membranes contain receptors (typically made from glycoprotein), which have a binding site of complementary shape to a signalling molecule (e.g. a hormone or cytokine) which has been released by another cell. When the signalling molecule binds to its complementary receptor (found only in the plasma membrane of appropriate target cells), this triggers a change in the tertiary structure (3D shape) of the receptor. This in turn will result in a response (a change in activity) in the target cell, e.g. a change in metabolism due to enzyme activation, or changes to gene expression.
13
Q
how do medicinal and r3ecreational drugs work?
A
Some receptors in plasma membranes act as sites where (medicinal or recreational) drugs can bind.
14
Q
What is the model of membranes called?
A
the fluid mosaic model
15
Q
why is the fluic mosaic model called a model?
A
The term model is used to describe a simplified representation of a complex structure or process
16
Q
whats the aim of using a model?
A
The aim of a model is to enable clear communication of the key features of the structure or process, such that they are easier to understand.
17
Q
what is the fluid mosaic model?
A
The fluid mosaic model is a representation of the key features known about biological membrane structure. The model helps us to visualise the different components found in a biological membrane and how they are organised
18
Q
how thick is a membrane?
A
7nm
19
Q
what is included in the fluid mosaic model?
A
A phospholipid bilayer from the basis of any biological membrane. Embedded in the bilayer are the other components: cholesterol, glycolipids, glycoproteins and proteins.
20
Q
what does the term fluid refer to in the fluid mosaic model?
A
The term fluid refers to the idea that most of the membrane components, especially the phospholipids, are not in fixed positions but rather are free to move past one another; the membrane is essentially in liquid phase
21
Q
what does the term mosaic mean in the fluid mosaic model?
A
The term mosaic refers to the overall appearance of the membrane from above. large proteins are embedded, with what appears to be random distribution, in the bilayer of phospholipids. The phospholipids appear to fill in all the spaces between the proteins
22
Q
what is the role of a phospholipid in a membrane? 3
A
Forms a bilayer that functions as a barrier with the property of partial permeability;
Due to hydrophobic fatty acid tails, prevent passage of ions (e.g. Na+) and polar
molecules (e.g. glucose)
Allow passage of molecules that are lipid‐soluble (e.g. steroid hormones) and/or very small (e.g. water).
23
Q
what is the role of cholesterol in the bilayer? 4
A
Slots between fatty acid tails of phospholipids in the bilayer;
Due to largely hydrophobic nature, increases effectiveness of the bilayer as a barrier to ions and polar molecules
Regulates the fluidity (degree of movement) of phospholipid molecules within the bilayer to keep this optimal despite temperature fluctuations
Increases mechanical strength of the membrane.
24
Q
what is the role of glycolipids in the bilayer? 2
A
Carbohydrate component (always projecting to the outside of the cell) can act as a self‐antigen, with a role in recognition of self‐cells by the immune system
Carbohydrate component is sticky hence plays a role in cell‐to‐cell adhesion
25
what is the role of proteins in the bilayer? 3
May be cylindrical in shape, functioning as channel proteins providing a pore through the membrane that allows passage of specific ions or small polar molecules by facilitated diffusion
May function as carrier proteins, changing shape to transport specific ions or small polar molecules by active transport across the membrane
May functions as enzymes, e.g. ATP synthase in the inner mitochondrial membrane makes ATP from ADP and Pi.
26
what is the role of glycoprotein in the bilayer? 3
Act as receptors for cell signalling, with a complementary binding site for a specific signalling molecule (e.g. hormone, cytokine, neurotransmitter or drug)
Carbohydrate component (always projecting to the outside of the cell) can act as a self‐antigen, with a role in recognition of self‐cells by the immune system
Carbohydrate component is sticky hence plays a role in cell‐to‐cell adhesion.
27
what are the effects on membrane structure due to temperature increase? 2
Increased kinetic energy (KE) of phospholipids (PLs) means they move around more, so bilayer becomes more fluid and hence more/larger gaps appear between PLs, resulting into increased permeability
Membrane proteins denature, resulting in gaps opening up around them;
If respiratory enzymes denature, ATP production decreases.
28
what are the effects on membrane function due to temperature increase? 4
Increased gaps in the membrane increase permeability to larger/ hydrophilic molecules and to ions, which pass through the gaps by diffusion, and increased permeability
Diffusion rates increase due to increased KE of the molecules/ions
Facilitated diffusion and active transport cannot occur if carrier proteins have denatured
Active transport and bulk transport cannot occur if insufficient ATP is available.
29
what are the effects on membrane structure due to temperature decrease? 2
Decreased KE of PLs means they move around less (packing more closely together), so bilayer becomes less fluid and hence less/smaller gaps occur between PLs
Decreased KE decreases respiration rate, so ATP production decreases.
30
what are the effects on membrane function due to temperature decrease? 3
Decreased gaps in the membrane decrease permeability even to molecules which usually can diffuse through
Diffusion and facilitated diffusion rates decrease due to decreased KE of the molecules/ions
Active transport and bulk transport cannot occur if insufficient ATP is available.
31
what are the effects on membrane structure due to freezing?
If temperature decreases so much that the membrane and water around it freeze solid, ice crystals form which penetrate into the membrane
32
what are the effects on membrane function due to freezing? 2
Whilst the membrane and surrounding water are frozen solid, the membrane will have zero permeability and diffusion rates will be zero
BUT if temperature later increases and the frozen membrane thaws, large holes (punctured by ice crystals) appear, causing a massive increase in permeability that results in the leakage of cytoplasm out of cells.
33
what are the effects on membrane structure due to organic substances such as ethanol? 2
Non‐polar solvent molecules interact with the hydrophobic fatty acid tails of PLs, inserting themselves into the bilayer and disrupting its regular structure
If the solvent is present in significant quantities, the PLs dissolve into the solvent, resulting in the bilayer completely dispersing.
34
what are the effects on membrane function due to organic substances such as ethanol?
Gaps appear in the bilayer as its structure is disrupted, resulting in increased fluidity and permeability to larger/ hydrophilic molecules and to ions, which pass through the gaps by diffusion
If the bilayer is dispersing due to PLs dissolving into the solvent, there will be leakage of cytoplasm out of cells.
35
what are the effects of detergents on membranes?
Detergent molecules have similar properties to phospholipids (i.e. hydrophilic heads and hydrophobic tails), but have a conical rather than a cylindrical shape. If detergent is applied to a membrane, this causes the phospholipids to leave the bilayer and associate with the detergent molecules in spherical structures called micelles.
The hydrophilic part of the detergent molecule is attracted to/by the phosphate heads of the phospholipids and water molecules; the hydrophobic part interacts with the fatty acids tails of the phospholipids, repelling/repelled by water molecules.
Consequently, the membrane structure is significantly disrupted, losing its partial permeability properties and eventually ceasing to exist as a coherent structure altogether.
Molecules that were previously held inside organelles or inside the cell (due to being too large and/or too hydrophilic to pass through the membrane) are now released.
36
give an example of when detergents are useful
This phenomenon is useful in the context of DNA purification from cells: detergent is used to disrupt the plasma membranes and nuclear envelopes of cells, releasing the DNA molecules that were previously held in the nuclei of the cells (being too large and hydrophilic to cross these membranes directly by diffusion, and too large to pass through nuclear pores).
37
what is passive diffusion, and what are the transport methods?
Passive transport processes simply make use of the kinetic energy that the molecules/ions already have; they do not require an input of energy from respiration
diffusion, facilitated diffusion and osmosis.
38
what is simple diffusion?
Diffusion is the passive movement of molecules/ions from a region of high concentration to a region of low concentration, down a concentration gradient.
39
what molecules can passs through the membrane and how? 2
Very small molecules (e.g. oxygen, carbon dioxide and water) can diffuse directly through a membrane (by being small enough to fit through the gaps between phospholipids
larger molecules that are hydrophobic (e.g. steroid hormones) and hence soluble in the hydrophobic interior of the bilayer.
40
give an example of simple diffusion in an animal
diffusion of oxygen molecules from the air space in an alveolus into the red blood cells (passing through the squamous epithelial cells in the alveolar wall and then through the endothelium of the capillary wall). The concentration of oxygen molecules in the alveolus is much higher than that in the red blood cells, hence the oxygen molecules move passively, down their concentration gradient, into the red blood cells. They will have to cross at least five plasma membranes before they reach the haemoglobin in the cytoplasm of a red blood cell.
41
give an example of simple diffusion in a plant
the movement of carbon dioxide into a leaf, through open stomata. The concentration of carbon dioxide in the atmospheric air is higher than that in the air spaces within the leaf. Hence the carbon dioxide molecules move into the leaf passively, down their concentration gradient. Once in the air spaces of the leaf, the carbon dioxide will then continue to move by diffusion, entering the mesophyll cells (through their plasma membranes) and then finally the chloroplasts (where the carbon dioxide is used in photosynthesis.
42
how can rates of reaction be increased? 4
increasing the steepness of the concentration gradient
increasing the surface area
decreasing the diffusion distance
increasing the temperature
43
how does increasing the surface area increase rxn?
as there will be more area across which the molecules can diffuse
44
how does increasing the temperature increase rxn? but why is this not wise?
kinetic energy and rate of movement of the molecules increase.
however, increasing the temperature is not generally a suitable strategy to increase diffusion rates in living organisms, since the higher temperature will have other (less beneficial) consequences e.g. denaturation of enzymes.
45
what is facilitated diffusion?
Facilitated diffusion is the passive movement of water‐soluble molecules/ions across a membrane from a region of high to low concentration, down their concentration gradient, BUT requiring a specific channel or carrier protein.
46
What molecules require facilitated diffusion?
water‐soluble molecules (polar), and ions are not able to pass directly through the phospholipid bilayer, unless they are so very small [water molecules] that they can fit through gaps between phospholipids, the larger molecules cannot fit through the small gaps
47
why cant polar molecules or ions pass through the bilayer?
The interior of the bilayer is comprised of hydrophobic fatty acid tails and cholesterol: only molecules that are hydrophobic themselves and hence soluble in hydrophobic core of the membrane can pass though. Partially permeable
48
why cant large hydrophilic molecules pass through the bilayer?
too hydrophilic to dissolve in the fatty acids tails that make up the hydrophobic core of the bilayer, whilst also being too large to fit through any gaps between phospholipids.
49
what is used in facilitated diffusion?
transport protein ( intrinsic protein, embedded in the bilayer).
Either a channel protein or carrier protein.
These tend to be highly specific to one type of ion or molecule
50
give an example of facilitated diffusion
a sodium ion channel will only allow facilitated diffusion of sodium ions through the membrane
51
what is a channel protein? Are they open?
Channel proteins are cylindrical in shape and act as pores (filled with water) through which ions can pass passively, down their concentration gradient.
Some channels always open however others are ‘gated meaning that they are closed by default and only open in response to a specific stimulus
52
what is a carrier protein? How do they change shape?
Carrier proteins allow the binding of molecules/ions on one side of the membrane; the protein then changes shape bringing the molecules/ions to the other side of the membrane where they are released.
The type of carrier protein used for facilitated diffusion will change shape spontaneously when the molecules/ions bind, without the need for any energy to be supplied by ATP. Hence facilitated diffusion is always a passive process (no ATP required) regardless of whether it uses a channel or a carrier protein.
53
how do carrier proteins change shape in facilitated diffusion?
The type of carrier protein used for facilitated diffusion will change shape spontaneously when the molecules/ions bind, without the need for any energy to be supplied by ATP. Hence facilitated diffusion is always a passive process (no ATP required) regardless of whether it uses a channel or a carrier protein.
54
what is an exmaple of a channel protein stimulus?
a change in voltage or the binding of a specific molecule such as a neurotransmitter.
55
Draw the simple diffusion graph
look in notes
56
Draw the facilitated diffusion graph
look in notes
57
What is an active process? and what is included?
Some transport processes require ATP as an immediate source of energy. At the site of an active transport process, the hydrolysis of ATP releases the energy required in order for the transport process to occur.
active transport and bulk transport
58
What is active transport?
Active transport is the movement of molecules/ions across a membrane from a region of low to high concentration against the concentration gradient, this requires energy from ATP and a specific carrier protein.
59
give two examples of active transport, one is plants and one in animals.
Root hair cells of plants actively transport mineral ions into their cytoplasm from the soil water.
Neurones actively transport Na+ ions out of their cytoplasm and K+ ions into their
60
How does active transport work?
The energy released when ATP is hydrolysed is used to trigger a change in the shape of the specific carrier protein that is used to bring the molecules/ions across the membrane. Unlike in facilitated diffusion (where the binding of the molecules/ions to the carrier is sufficient to trigger the necessary change in shape), the carrier proteins used in active transport will not change shape unless made to do so by the energy from ATP hydrolysis
61
what is bulk transport?
the active process in which large molecules are moved into the cell by endocytosis or out the cell by exocytosis
62
what is endocytosis?
Endocytosis is a bulk transport process in which the plasma membrane engulfs a large solid particle (e.g. a bacterium) or portion of fluid; the engulfed material is brought into the cytoplasm, enclosed in membrane to form a phagosome; this process requires energy from ATP.
63
what are the two types of endocytosis?
phagocytosis means that a large solid particle is engulfed by the plasma membrane
pinocytosis means that a portion of liquid is engulfed.
64
What is the process of phagocytosis? Use the example of a neutrophil white blood cell engulfing a pathogen
1. The initial trigger for phagocytosis is the binding of markers (e.g. antibodies) attached to the pathogen's surface to complementary receptors in the plasma membrane of the neutrophil
2. The plasma membrane of the neutrophil begins to wrap around the pathogen, pushed by the cytoskeleton (requiring lots of ATP).
3. The plasma membrane now completely surrounds the pathogen (which has been engulfed).
4. The pathogen is now contained in a phagosome as the membrane around it pinches off from the PM. A lysosome may then fuse with the phagosome, forming a phagolysosome.
5. The hydrolytic enzymes from the lysosome are now in contact with the pathogen and digest it. Useful products, e.g. amino acids, pass into the cytoplasm for use by the neutrophil. (Undigested material may be egested from the cell by exocytosis.)
65
what is exocytosis?
Exocytosis is a bulk transport process in which the contents of a vesicle are secreted from the cell when the membrane of the vesicle fuses with the plasma membrane; this requires energy from ATP
66
when is exocytosis used?
secreting extracellular enzymes, antibodies and specific glycoproteins
67
what are the steps of exocytosis?
1. A secretory vesicle containing (glyco)proteins moves through the cytoplasm towards the PM, using microtubules as tracks (and requiring energy from ATP).
2. The vesicle membrane fuses with the plasma membrane, i.e. the vesicle 'docks' with the membrane
3. The (glyco)proteins are released to the outside of the cell by exocytosis.
4. The components of the vesicle membrane are now part of the PM.
68
what is osmosis
the passive net movement of water, down a water potential gradient, from a region of high water potential to a region of low water potential through a partially permeable membrane.
69
does osmosis require energy?
no, its passive
70
what is the symbol for water potential?
Ψ (psi)
71
A solution with low solute concentration has high water potential. What will water do?
water will tend to move out of this solution.
72
A solution with high solute concentration has low water potential. What will water do?
water will tend to move into this solution
73
what is water potential measure in?
KPa
74
what is water potential?
a measaure of the quantity of water compared to solutes, measured as the pressure created by the water molecules
75
what has the highest possible water potential?
pure water (with no dissolved solutes), 0 kPa.
76
what are the water potentials of cells with solutes dissolved in solution?
Solutions containing dissolved solutes have negative water potentials
a dilute solution of glucose might has a water potential of ‐200 kPa, whereas a more concentrated solution would have a lower (more negative) water potential, e.g. ‐850 kPa.
77
how will water move through cells
Water will move from cell‐to‐cell through tissues, from the area with highest water potential into the area with lowest water potential
78
can water move freely though the bilayer with ions?
water molecules can freely pass through this membrane (i.e. it is permeable to water) but that the solutes dissolved in the water (e.g. salt ions or sugar molecules) cannot pass through.
79
what is a solution of equal water potential called?
isotonic
80
Animal cells in an isotonic solution
solution of equal water potential to its cytoplasm
no water potential gradient
no net movement of water by osmosis
81
what is a hypertonic solution?
the outside solution is has a higher concentration of solutes than the cytoplasm
the outside solution a lower water potential potential (more negative)
82
what is a hypotonic solution?
the outside solution is has a lower concentration of solutes than the cytoplasm.
the outside solution a higher water potential potential (less negative)
83
what occurs if you place an animal cell in a hypotonic solution? (higher water potential outside)
There is a higher water potential outside the cell than inside, so there will be a net movement of water by osmosis into the cell, down the water potential gradient, through the partially permeable cell surface membrane. The volume and hence pressure of the cytoplasm begins to increase, however the cell surface membrane alone is not strong enough to withstand this increase; there is no cell wall to prevent further increase in volume and so the membrane ruptures (tears) and the cytoplasm leaks out into the surrounding solution. The cell has lysed, which is irreversible and results in the immediate death of the cell.
84
what occurs if you place an animal cell in a hypertonic solution? (lower water potential outside)
There is a lower water potential outside the cell than inside, so there will be a net movement of water by osmosis out of the cell, down the water potential gradient, through the partially permeable cell surface membrane. The volume of the cytoplasm will decrease hence the cell shrinks (get smaller); this also results in the cell surface membrane being pulled inwards, giving a ruffled/crinkled appearance. The cell has become crenated, which may be reversible if the water potential is later increased
85
what happens if you place a plant cell in a isotonic solution?
At this isotonic point, there is no water potential gradient, no difference in water potential, hence no net movement of water by osmosis.
86
what occurs if you place a plant cell in a hypotonic solution (higher water potential outside)
There is a higher water potential outside the cell than inside, so there will be a net movement of water by osmosis into the cell, down the water potential gradient, through the partially permeable cell surface membrane. The volume of the cytoplasm begins to increase, however the cell wall is not able to expand significantly, resulting in an increase in the pressure of cytoplasm against the cell wall. The cell wall is very strong, so is able to withstand this increase in pressure, such that the plant cell will NOT undergo lysis. Instead of bursting (as an animal cell would), the plant cell will swell and becomes turgid (stiffened) or is said to have cell turgor. Cell turgor is important in the leaves of a plant, as it enables them to be held up to receive maximum sunlight.
87
what occurs if you place a plant cell in a hypertonic solution (lower water potential outside)
There is a lower water potential outside the cell than inside, so there will be a net movement of water by osmosis out of the cell, down the water potential gradient, through the partially permeable cell surface membrane. The volume of the cytoplasm will decrease and there is a decrease in the pressure of the cytoplasm, such that it no longer pushes against the cell wall;
the cell is now described as flaccid (soft) or as having lost turgor. If the cells in a plant’s leaf lose turgor and become flaccid, the leave wilts, such that it is no longer held up to the sunlight. However, loss of turgor is reversible if the cells later regain water by osmosis.
If water loss from the cell (by osmosis) continues, due to there being a very steep water potential gradient, the cell surface membrane is pulled inwards as the cytoplasm decreases in volume; the cell surface membrane rips away from the cell wall; this is called plasmolysis and the cell is now said to be plasmolysed. If the membrane is torn away from the wall at every possible point, complete plasmolysis has occurred. This is irreversible and results in cell death.
the new spaces fill with the outside hypertonic solution, since the cell wall is fully permeable.
88
label
89
how to remember hypo vs hypertonic
Hyp**O** = higher water potential **O**utside
Hyp**E**r = higher water potential in c**E**ll
