TRANSPORT ACROSS MEMBRANES - diffusion ,osmosis ,active transport + co-transport

Question 1

Describe the fluid mosaic model of membranes?

Answer 1

Fluid: phospholipid bilayer in which induvidual phospholipids can move = membrane has flexible shape.
Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol and glycolipids in membranes

Answer 2

Cholestrol:steroid molecule in some plasma membranes; connects phospholipids bybinding to the the hydrophobic tail + reduces fluidity to make bilayer more stable.
Cholestrol helps maintain the shape of animal cells which dont have cell walls.

Glycolipids: cell signaling and cell recognition.

Question 3

Explain the functions of extrinsic and intrinsic proteins in membranes.

Answer 3

Extrinsic protein/peripheral protein :
- binding sites /receptors eg for hormones.
- antigens (glycoproteins)
- binds cells together
- involved in cell signalling
- provides mechanical suport

Intrinsic protein/ integral protein:
→ election carriers (respiration/ photosynthesis)
→ channel proteins (facilitated diffusion/ active transport)
- carrier proteins

Question 4

Explain the function of membrane within cells?

Answer 4

• provide internal transport system
• selectively permeable to regulate passage of molecules into/out of organelles.
• provide reaction surface
  -isolate organelles from cytoplasm for specific metabolic reactions.

Question 5

Explain the functions of the cell-surface membrane?

Answer 5

• isolates cytoplasm from extracellular environment
  -selectively permeable to regulate transport of substances
• involved in cell signalling/cell recognition.

Question 6

Name and explain 3 factors that affect membrane permeability.

Answer 6

TEMPERATURE - high temperature denatures membrane proteins/ phospholipid molecules have more kinetic energy and move further apart. This increases the permeability of the membrane.
Below 0° ice crystals can form ,piercing the membrane ,increasing permeability.

pH - changes in tertiary structure of membrane proteins

SOLVENT - non polar solvents can insert themselves into phospholipid bilayer.for example alcohol or acetone.
Ethanol can form hydrogen bonds with a phospholipid molecule and push the phospholipid increasing the movement therefore the membrane structure is disrupted.
Solvents Can denature proteins by disrupting bonds.

Question 7

Outline how colorimetry could be used to investigate membrane permeability.

Answer 7

1. Use plant tissue with soluble pigment in vacuole. Tonoplast and cell surface membrane is disrupted - this increases permeability and pigment diffuses into the solution.
   2 . Select colorimeter filter with complementary colour
2. Use distilled water to set colorimeter to 0. Measure absorbent / % transmissionvalue of solution
   4.high absorbance/ low transmission = more pigment in solution

Question 8

define osmosis

Answer 8

Water diffuses across partially permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established.

Question 9

what is water potential?

Answer 9

• pressure created by water molecules measured in kPa
• water potential of pure water at 25° and 100 kPa = 0
• more solute = water potential more negative

Question 10

How does osmosis affect plant and animal cells?

Answer 10

osmosis INTO cell:
- plant : protoplast swells = cell becomes turgid
- animal = lysis

osmosis OUT of cell:
- plant : protoplast shrinks = cell flaccid
- animal: crenation

Question 11

define simple diffusion

Answer 11

• passive process requires no energy from ATP hydrolysis
• net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration ( i.e. down a concentration gradient )

Question 12

define facilitated diffusion

Answer 12

passive process down a concentration gradient
occurs at specific points on the plasma membrane where there are special protein molecules. occurs at protein channel and carrier proteins

Question 13

Explain how channel and carrier proteins work?

Answer 13

CHANNEL: hydrophilic channels bind to specific ions = one side of the protein closes + the other opens. Waster soluble ions pass through the membrane. Charged ions can diffuse through the membrane.

CARRIER: binds to complementary molecule = conformational change releases molecule on other side of membrane ; in facilitated diffusion, passive process; in active transport, requires energy from ATP hydrolysis.

Question 14

name 5 factors that affect the rate of diffusion

Answer 14

• temperature
• diffusion distance
• surface area
• size of molecule
• difference in concentration (how steep the concentration gradient is)

Question 15

How are cells adapted to maximise the rate of transport across their membranes?

Answer 15

• many channel and carrier proteins
• folded membrane increases surface area

Question 16

Explain the difference between the shape of a graph of concentration for simple vs facilitated diffusion?

Answer 16

SIMPLE DIFFUSION - straight diagonal line, rate of diffusion increases proportionally as concentration increases

FACILITATED DIFFUSION - straight diagonal line, later levels off when all channel/carrier proteins are saturated.

Question 17

define active transport?

Answer 17

Active process where ATP hydrolysis releases phosphate group that binds to the carrier proteins causing it to change shape.

Specific carrier proteins transports molecules/ions from area of low concentration to an area of high concentration. (against concentration gradient).

Question 18

compare and contrast active transport with facilitated diffusion?

Answer 18

• both may involve carrier proteins
• active transport requires energy from ATP hydrolysis whereas facilitated diffusion is a passive process.
• facilitated diffusion may also involve channel proteins.

Question 19

define co-transport?

Answer 19

Movement of substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient.

Question 20

Explain how co-transport is involved in the absorption of glucose in the small intestine.

Answer 20

Glucose molecules move down the concentration gradient by facilitated diffusion in to get epithelium cells lining the ileum. This takes place with the help of carrier proteins

Glucose molecules then move into the bloodstream and are carried away. However facilitated diffusion cannot absorb all of the glucose. Glucose is also absorbed by active transport. Using ATP, the sodium – potassium pump actively transports sodium ions out of the epithelial cells and into the bloodstream. At the same time, it actively transports potassium ions into the epithelial cells.

There is now a low concentration of sodium ions in the epithelial cells and a high concentration of sodium ions in the lumen of the ileum.
Sodium ions can diffuse through the sodium-glucose transporter down the concentration gradient into the epithelial cell. The sodium-glucose co-transporter also transports glucose molecules into the cell so glucose molecules are being transported against their concentration gradient. Glucose is being actively transported into the cell so glucose is absorbed at a fast rate.

Question 21

What are some adaptations of the epithelial cells?

Answer 21

• epithelial cells have a lot of mitochondria, which provides that ATP needed for the sodium-potassium pump.
  -the membrane of the epithelial cell is folded into large number of microvilli which increases the surface area of the membrane, providing more space for the membrane protein needed for facilitated diffusion and active transport.
  -glucose molecules absorbed are rapidly carried away by the bloodstream, which means there is a steep concentration gradient between the epithelial cells and the blood capillaries so facilitated diffusion of glucose takes place rapidly.

Question 22

why do non-polar molecules diffuse quicker?

Answer 22

Polar molecules are repelled by lipids and will not go across the bilayer membrane.

Question 23

How is ATP and carrier proteins used in active transport?

Answer 23

Certain molecules combine to the receptor site of the carrier proteins. eg Na+
- ATP will bind to the carrier protein on he inside of the membrane and is hydrolysed into ADP and pi.
-ATP and pi causes the protein to change shape and open towards the inside of the membrane. This causes the molecule to be released on the other side of the membrane.
-the pie molecule is then released from the protein, resulting in the carrier protein reverting to its original shape.