Membranes Flashcards
Describe the structure of a cell membrane. (5)
membrane is a 7 nm thick phospholipid bilayer with protein molecules
spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the
layers. Hence the structure is described as a fluid mosaic – molecules flow freely, there are many types of molecules
Phospholipid bilayers are …
a barrier to most water soluble substances because the interior of the membrane is hydrophobic
Cholesterol is needed for …
membrane fluidity and stability
What do transport proteins do?
they transport molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape
Example of Proteins acting as enzymes ?
In the cell surface membranes of microvilli in the gut
What 2 things do Glycolipids and glycoproteins do?
- form receptors – for example, for hormones or neurotransmitters
- form antigens, which are cell recognition markers
The cell surface membrane …
controls exchange between the cell and its environment.
Where do some chemical reactions take place on membranes?
Inside cell organelles, as in photosynthesis and respiration.
Diffusion is …
the net movement of molecules or ions from a region of their higher concentration to one
of lower concentration. Oxygen, carbon dioxide and water cross membranes by diffusion through the
phospholipid bilayer.
What is facilitated diffusion?
Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins, down the conc. gradient
Describe how substances move across cell-surface membrane via facilitated diffusion
- Carrier/Channel proteins
- Passive bc it doesn’t require ATP
- Proteins complimentary to substance
- Substances move down conc. gradient
Osmosis is …
Water moves from regions of higher water potential to regions of lower water potential. When this takes place through a partially permeable membrane such as the cell surface membrane.
Water Potential = ψ = ψp + pψs
Water Potential (ψ) = Pressure Potential (ψp) + Solute Potential (ψs)
What is the process of Plasmolysis
In dilute solutions, animal cells burst as water moves into the cytoplasm from the solution. In dilute
solutions, a plant cell does not burst, because the cell wall provides resistance to prevent it expanding.
The pressure that builds up is the pressure potential. A plant cell in this state is turgid. In concentrated
solutions, animal cells shrink, while in plant cells the protoplast shrinks away from the cell wall.
Active Transport is where …
- Some ions and molecules move across membranes
- Against the concentration gradient
- only involves CARRIER PROTEINS
- ATP to provide energy
Exocytosis and Endocytosis involve (occurs due to availability of mem to fuse and pinch off) …
the formation of vacuoles to move larger quantities of materials respectively out of, or into, cells by bulk transport. There are two types of endocytosis (in), namely phagocytosis (cell eating) and pinocytosis (cell drinking).
process of Mitochondria
- Molecule binds to complimentary site on carrier proteins
- ATP is Hydrolyzed to ADP & Carrier proteins changes tertiary structure
- Molecules transported across mem
- One way flow as Molecule can only fit into carrier proteins on one side of membrane
Why measure rate of uptake in /ug per million cells per hour?
- Allows direct comparison
- Easy to manipulate
- Diff no. of cells
What happens to carrier proteins in Facilitated diffusion?
Changes the tertiary structure, so the binding of the substrate e.g. sugar, changes the tertiary structure of carrier protein allowing molecules to enter the cell
Water Potential
Pure water has a water potential (ψ) of zero. Adding solute reduces the water potential by an amount known as the solute potential (ψs), which has a negative value. Adding pressure to a solution
increases the water potential by an amount known as the pressure potential (ψp), which has a positive
value.
What is the movement of Glucose in the blood?
- Glucose moves in the blood via carrier proteins using facilitated diffusion
- Na+/k+ ATPase pump (embedded in the basal mem - bottom of mem facing towards capillaries not lumen) pumps out 3Na+ ions in exchange for 2K+ ions (Active trans)
- Maintains low conc. grad of Na+ inside the cell
What happens in Exocytosis
vesicles move towards cell surf mem + fuse + release content outside the cell
Scientists have investigated the effects of competitive and non-competitive inhibitors of the enzyme maltase.
Describe competitive and non-competitive inhibition of an enzyme. (5)
- Inhibitors reduce binding of enzyme to substrate / prevent formation of ES complex;
(Competitive inhibition), - Inhibitor similar shape (idea) to substrate;
- (binds) in to active site (of enzyme);
- (Inhibition) can be overcome by more substrate;
(Non-competitive inhibition), - Inhibitor binds to site on enzyme other than active site;
- Prevents formation of active site / changes (shape of) active site;
- Cannot be overcome by adding more substrate;
Sodium ions from salt (sodium chloride) are absorbed by cells lining the gut. Some
of these cells have membranes with a carrier protein called NHE3.
NHE3 actively transports one sodium ion into the cell in exchange for one proton
(hydrogen ion) out of the cell.
Use your knowledge of transport across cell membranes to suggest how NHE3
does this.
[3 marks]
Co-transport, uses hydrolysis of ATP. Sodium ions move/diffuse into the cell ALONG their concentration gradient (i.e. from high to low concentration) and in doing so drive H+ ions out of the cell into the lumen AGAINST their concentration gradient. Sodium ion and proton bind to the protein. The NHE3 transporter is a carrier protein, not a channel. Carrier proteins transport ions across membranes by changing their shape.
facilitated diffusion from Lumen of Small Intestine, Epithelial Cell, Blood Capillary.
- G & Na+ ions are co - transported from the lumen to the epithelium sodium to lower his conc. grad. Glucose moves against its conc. grad
- Na+ ions actively transported from epithelium through carrier proteins
- K+ ions pumped into Epithelium from blood to conc. grad of Na+ is maintained therefore G is absorbed into the blood.
- G transported from epithelium into blood via facilitated diffusion via channel proteins. G= transported to various bodily tissues for respiration
Explain why using a stain makes the appearance of the cell - surface membrane as two dark lines
- Membrane has a phospholipid bilayer
- The stain binds the phospholipid head
- On inside and outside of membrane
Water Potential of Potato tissue
- Water potential of solution is more negative then that of the potato tissue
- Tissue looses water by osmosis
Describe how you would use the student’s processed results to find the water potential of the potato tissue
- Plot a graph with conc. on the x-axis and % change in mass on y-axis
- Find conc. where curve crosses the x-axis and where the % change is 0
- Use external resource to find Water Potential of sucrose conc. where curve crosses x-axis
How to calculate the mean rate of uptake ?
Rate = change / time
! Read units properly !
