Chapter 2.5- Biological Membranes Flashcards
What are the ways in which substances can pass through cell membranes?
- some very small molecules simply diffuse through the cell membrane, in between its structural molecules
- some substances dissolve in the lipid layer and pass through
- other substances pass through special protein channels or are carried by carrier proteins
Why are cell membranes described as being partially permeable?
not all type of molecules can pass though the memrbane
What is meant by permeability?
the ability of a structure to allow molecules to pass through
What is the plasma membrane?
cell surface membrane
Whats the role of membranes at the surface of cells?
- separate the cell’s components from its external environment (in humans each cells environment is the tissue fluid or cells surrounding it
- regulate transport of materials into and out of the cell
- contain enzymes involved in specific metabolic pathways
- has antigens, so that the organisms immune system can recognise the cell as being ‘self’ and not attack it
- release chemicals that signal to other cells
- site of chemical reactions
- contain receptors for such chemical signals, and so is a site for cell communication or signalling; hormones and drugs may bind to membrane-bound receptors
Whats the role of membranes within cells?
- separate the organelle contents from the cell cytoplasm, so that each organelle is a discrete entity and able to perform its function
- mitochondria have folded inner membranes, called cristae. These give a large surface area for some of he reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur
- inner-membranes of chloroplasts, called thylakoid membranes, house chlorophyll. On these membranes some of the reactions of photosynthesis occur.
- digestive enzymes on the plasma membrane of epithelial cells that line the small intestine, and these enzymes catalyse some of the final stages in the breakdown of certain types of sugars
What is the fluid mosaic model?
theory of cell membrane structure with proteins embedded in a sea of phospholipids.
How are the phospholipids arranged in the plasma membrane?
- phospholipid bilayer (double layer) with proteins floating in it, making up a mosaic pattern
- hydrophilic heads of each phospolipid are in contact with the watery exterior or watery interior (cytoplasm). —-The hydrophilic tail regions are in the centre of the membrane, away from the water
What are the various proteins that are found on the plasma membrane?
- some proteins have pores and act as CHANEEL PROTEINS to allow ions, which have an electrical charge and are surrounded by water molecules, to pass through
- some proteins are CARRIER PROTEINS, and by changing their shape, carry specific molecules across the membrane
- other proteins may be attached to carrier proteins and functions as enzymes, antigens or receptor sites for complimentary-shaped signalling molecules such as hormones
What type of proteins are channel and carrier proteins?
integral proteins
What is the function of cholesterol in the plasma membrane?
- regulates the fluidity of the membrane
- maintains mechanical stability
- resists the effects of temperature changes on the structure of the membrane
What are glycolipids?
a carbohydrate chain attached to a lipid
What are glycoproteins?
protein with a chain of carbohydrate molecules attached
What do the glycoproteins and glycolipids make up?
glycocalyx
What are the channel proteins which carry ions across the plasma membrane lined with?
-water-filled channel is lined with hydrophilic amino acids
What is the glycocalyx?
very hydrophilic carbohydrate molecules which are located on the outside of the membrane and attract water with dissolved solutes, helping the cell interact with its watery environment and obtain dissolved substances
What is the total thickness of the plasma membrane?
between 5 and 10 nm
How is the plasma membrane in neurones (nerve cells) differentiated and specialised?
- protein channels and carriers in the plasma membrane covering the long axon allow entry and exit of ions to bring about the conduction of electrical impulses along their length
- they have a myelin sheath (Schwann cells) formed by flattened cells wrapped around them several times, giving several layers of cell membrane. The membrane forming the myelin sheath is about 20% protein and 76% lipid
What is the function of glycoprotein and glycolipid molecules?
- antigens or receptor molecules
- can bind with extracellular signalling molecules such as hormones
- attach to water molecules (to stabilise membrane / cell)
- cell adhesion / to hold cells together (in a tissue)
- communication between cells
How is the plasma membrane in mitochondria differentiated and specialised?
-inner-membrane contains 76% protein and 24% lipid. This is because their inner-membranes contain many electron carriers that are made of protein, and hydrogen in channels associated with ATP synthase enzymes
What is diffusion?
movement of molecules from an area of high concentration of that molecule to an area of low concentration; it may or may not be across a membrane;it does not involve metabolic energy (ATP)
Why are phospholipids described as being amphipathic molecules?
they have a dual nature in that one end of the phospholipids (the phosphate group) is hydrophilic (water-loving and polar)
whilst the other end of the phospholipid (the fatty acid chains) is hydrophobic and non-polar.
Why is diffusion described as a passive process?
doesn’t use ATP and only uses kinetic energy
When diffusion is taking place, where do the molecules move?
they move down their concentration gradient
What molecules can pass through cell membranes by simple diffusion?
oxygen, carbon dioxide, steroids, the fat-soluble vitamins(A,D,E,K), glycerol, alcohols and ammonia.
What molecules can easily diffuse across the plasma membrane and what molecules need assistance from channel proteins?
Lipid soluble (hydrophobic) molecules easily pass through the membrane by diffusion whilst hydrophilic substances cannot diffuse through; instead they cross the membrane via water-filled pores or channels in intrinsic proteins.
What are intrinsic proteins?
proteins that span the entire width of the membrane
What are the name of the specific water channel proteins that allow water to diffuse across the membrane?
- aquaporins
- water is a polar molecule and insoluble in lipids so requires channels to cross the membrane
How does temperature affect the rate of simple diffusion?
-as temperature increases, molecules have more kinetic energy, so their rate of diffusion will increase.
How does diffusion distance affect the rate of simple diffusion?
the thicker the membrane across which molecules have to diffuse, the slower the rate of diffusion
How does surface area affect the rate of simple diffusion?
more diffusion can take place across a larger surface area. Cells specialised for absorption have extensions to their cell surface membranes, called microvilli. These increase the surface area
How does the size of the diffusing molecule affect the rate of simple diffusion?
smaller ions or molecules diffuse more rapidly than larger molecules
How does the concentration gradient affect the rate of simple diffusion?
steeper the gradient (the more molecules there are on the side of the membrane compared with the other side), the faster the diffusion to the side where there are fewer molecules, down the gradient.
What is meant by facilitated diffusion?
movement of molecules from an area of high concentration of that molecule to an area of low concentration, across a partially permeable membrane via protein channels or carriers;it does NOT involve metabolic energy (ATP)
What are the steps of facilitated diffusion of large molecules through a carrier protein?
- molecule binds to carrier protein molecules on the cell surface
- The protein changes shape (known as a conformational change), in doing so the
molecule is transported through to the inside of the cell - The molecule detaches from the transporter protein and the protein reverts to its original
shape
How do small polar molecules diffuse across the membrane via facilitated diffusion?
they diffuse through water-filled protein channels (pores) embedded in the membrane.
What substances are diffused across the membrane via facilitated diffusion?
Used to transport large and polar molecules: • Glucose • Fructose • Non-fat-soluble vitamins • urea • ions
What is osmosis?
passage of water molecules down their water potential gradient, across a partially permeable membrane
What is a solution?
solute dissolved in a solvent
What is meant by water potential?
measure of the tendency of water molecules to diffuse from one region to another
What is water potential measured in?
kilopascal (kPa)
What is the water potential of water?
0kPa
What happens when solutes are added to water and how does this affect the water potential?
- solutes form bonds with the water molecules, meaning there are fewer free water molecules and the solution gets more concentrated
- water potential reduces (-kPa)
If two aqueous solution are separated by a partially permeable membrane in what way does water molecules diffuse?
-water molecules will move from the solution with the higher water potential to the solution with the lower water potential
What happens when a animal cell is placed in a solution which has a higher water potential?
- the water molecules move by osmosis, down the water potential gradient, across then plasma membrane, into the cell
- in animal cells if a lot of water enters the cell, then the cell will swell and burst as the plasma membrane breaks. This is called cytolysis
What happens to plant cells which are put in a solution which has a higher water potential?
- the rigid and strong cellulose cell wall will prevent bursting.
- The cell will swell up to a certain size when its contents are pushed up against the cell wall, which will resist any further swelling.
- the swollen cell is described as being turgid. turgidity of plant cells helps support plants, especially those that are not woody.
What happens when animal cells are placed in a solution with a lower water potential?
- water leaves the cells by osmosis, across the partially permeable plasma membrane
- animal cell shrivels and described as being crenated
What happens when plant cells are placed in a solution with a lower water potential?
- cytoplasm shrinks and the membrane pulls away from the cellulose cell wall.
- the cells are described as being plasmolysed.
What are plant tissues with plasmolysed cells described as?
flaccid
What does passive mean?
describes processes that do not require metabolic energy (from ATP)
What is active transport?
the movement of substances against their concentration gradient (from low to high concentration of that substance) across a cell membrane, using ATP and protein carriers
What do carrier proteins do in active transport?
- membranes have specific regions, or sites, that combine reversibly with only certain solute molecules or ions
- they have a region that binds to and allows the hydrolysis of a molecule of ATP, to release energy, and in this way they act as enzymes
- this energy helps the carrier protein change its conformation, and in doing so it carries the ion from one side of the cell membrane to the other
How do guard cells use carrier proteins in active transport?
- active transport of potassium ions from surrounding cells into the guard cells.
- this influx of ions lowers the water potential in the guard cells, so that water enters from surrounding cells, by osmosis. As the guard cells swell, their tips bulge and this opens the stoma between them
What is bulk transport?
transports large molecules and particles that are too big to pass directly through the plasma membrane, in or out. This process requires energy from ATP
What is endocytosis?
- bulk transport where large proteins are brought into a cell
- segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle.
What is phagocytosis?
- ‘eating by cells’ and refers to this intake of solid matter
- if cells ingest liquids by endocytosis, this is called Pino(endo) cytosis
- phagocytic cell, e.g. amoeba, approaches a bacterium
- cell extends to surround the bacterium
- bacterium is now enclosed within a phagocytic vesicle, a phagosome
What is exocytosis?
- bulk transport where large molecules may be exported out of cells
- a vesicle containing them is moved towards and then fuses with the plasma membrane
What are the steps in exocytosis?
- a membrane-bound vesicle, counting the substance to be secreted, is moved towards the cell surface membrane
- The cell surface membrane and the membrane of the vesicle fuse together
- The fused sites open, releasing the contents of the secretory vesicle
What requires energy (ATP) in exocytosis?
-a molecule of ATP is hydrolysed for every step that a motor protein takes along the cytoskeleton thread, as it drags along the vesicle
How does a drop in temperature affect membrane structure and permeability?
- saturated fatty acids become compressed
- unsaturated fatty acids become compressed and the kinks in their tales push adjacent phospholipid molecules away. This maintains the membranes fluidity
- the amount of fluidity within the membrane depends on the proportion of unsaturated and saturated fatty acids.
How does cholesterol act as a buffer when the temperature is low?
- prevents a reduction in the membranes fluidity
- prevents the phospholipid molecules from packing together too closely, because cholesterol molecules are in between groups of phospholipid molecules
At lower temperatures, what are the two things that make your membrane more fluid?
- temperature
- unsaturated fatty acids
How does an increase in temperature affect the plasma membrane?
- the membrane has more kinetic energy so is moving around more.
- becomes more fluid
- becomes more permeable
- proteins may move around
How does an increase in temperature affect cell signalling and phagocytosis?
- some proteins are arranged in certain ways. allowing them to move away disrupts the arrangement they are in so will negatively effect cell signalling
- phagocytosis is easier as the membrane is softer
How does cholesterol acts as a buffer in higher temperatures?
- dual role in temperature change
- it makes membrane less fluid/resists changes in temperature
How does an increase in temperature affect proteins on plasma membranes?
- atoms within their large molecules vibrate, and this breaks the hydrogen bonds and ionic bonds that hold the tertiary structure together-they unfold
- tertiary structure changes and cannot change back again when they cool-they are denatured
- cytoskeleton threads underneath the plasma membrane become denatured. They are holding the membrane in place and giving the membrane structure, This will cause the membrane to begin to fall apart and become more permeable
What is the effect of solvents on phospholipids?
organic solvents like acetone and ethanol will damage the cell membranes as they dissolve lipids
How would u carry out a experiment to see the effect of temperature on beetroot cell membranes?
- Bore pieces of beetroot (must all have the same surface area)
- wash to remove all pigment released by boring/cutting
- put each in specified quantity of water in a test tube
- run many water baths at different temperatures (0,20,40,60 degrees)
- place in each test tube in different water baths for a period of time
- remove the beetroot and hold the test tube against a white background
- use a calorimeter to measure light intensity and record in a table
How would you carry out a experiment to see the effect of chemicals (solvents) on beetroot cell membranes?
- add different volumes of either acetone or ethanol into the test tube containing the beetroot
- leave for a specified time period
- remove the beetroot and hold the test tube against a white background
- use a calorimeter to measure light intensity and record in a table
What are two examples of processes that use active transport?
- guard cells ATP made by chloroplasts provides energy to actively transport potassium ions from surrounding cells into the guard cells. This influx of ions lowers the water potential in the guard cells, so that water enters from the surrounding cells, by osmosis. As the guard cell swells, their tips bulge and this opens the stoma between them
- sodium/potassium ion(s).neurone
What are intrinsic and extrinsic proteins on the plasma membrane?
- intrinsics proteins are embedded within the membrane (INTEGRAL PROTEINS)
- extrinsic proteins are entirely outside of the membrane bound to it by weak intermolecular forces (PERIPHERAL PROTEINS)
