Movement across Membranes

Question 1

Movement of small, non-polar substances:

Answer 1

Diffuse through phospholipid bilayer

{{Exception: water (polar) but so abundant that it diffuses in}}

Question 2

Movement of large substances across membrane:

Answer 2

Using transport / carrier proteins
Endocytosis/ pinocytosis/ phagocytosis
Protein pump (not channel proteins)

Question 3

Movement of polar substances across membrane:

Answer 3

Through pore/ channel proteins
Using transport/ carrier proteins
Active transport/ use of ATP

Question 4

Diffusion:

Answer 4

Concentration gradient
Diffusion distance (inverse square law): epithelial cells
Diffusion area: villi/ microvilli
Pores in cell membrane
Size/ nature of diffusing molecule

Question 5

Facilitated diffusion:

Channel proteins

Answer 5

Channel proteins: water filled so allow polar ions

Channel open/ close to signals like change in voltage or binding of another molecule

Question 6

Facillitated diffusion:

carrier proteins

Answer 6

Carrier proteins bind molecules to them n then change shape as a result of binding in such a way that molecules are released inside membrane

Question 7

Does facilitated diffusion require energy?

Answer 7

No. It’s passive in all cases and material is moves down conc gradient.

Question 8

Active transport:

Answer 8

Energy required to move against conc gradient (ATP)
Carrier protein spanning membrane
Molecules bind to carrier protein causing it to change shape - active configuration
(ATP attaches to protein membrane inside cell)
‘Active’ configuration no longer bonds to molecules so molecules released inside
ATP -> ADP + P
Release of molecules: protein reverts to binding configuration to take up more molecules from outside

Question 9

Diffusion:

Answer 9

Concentration gradient
Diffusion distance (inverse square law): epithelial cells
Diffusion area: villi/ microvilli
Pores in cell membrane
Size/ nature of diffusing molecule

Question 10

Facilitated diffusion:

Channel proteins

Answer 10

Channel proteins: water filled so allow polar ions

Channel open/ close to signals like change in voltage or binding of another molecule

Question 11

Facillitated diffusion:

carrier proteins

Answer 11

Carrier proteins bind molecules to them n then change shape as a result of binding in such a way that molecules are released inside membrane

Question 12

Does facilitated diffusion require energy?

Answer 12

No. It’s passive in all cases and material is moves down conc gradient.

Question 13

Active transport:

Answer 13

Energy required to move against conc gradient (ATP)
Carrier protein spanning membrane
Molecules bind to carrier protein causing it to change shape - active configuration
(ATP attaches to protein membrane inside cell)
‘Active’ configuration no longer bonds to molecules so molecules released inside
ATP -> ADP + P
Release of molecules: protein reverts to binding configuration to take up more molecules from outside

Question 14

Components of a cell surface membrane. (3mks)

Answer 14

phospholipids; proteins [extrinsic/ intrinsic/ pore]; glycoproteins; cholesterol; glycolipids;

Question 15

Explain what is meant by the term active transport. [june11 2bi]

Answer 15

(movement of substances) against/ up, concentration gradient or from low to high concentration; using, ATP/ (metabolic) energy; using a, transport/ carrier, protein;

Question 16

State two examples of active transport in cells. Name the substance that is transported and the cell involved: [june11 2bii]

Answer 16

1) (mineral) ions/ salts/ named e.g, (into) root hair (cell); [e.g. nitrates, phosphates, calcium ions, magnesium ions etc.]
2) hydrogen ions (out of) companion cells; [ref to loading of sucrose into, phloem cell/ companion cell; ref to uptake of glucose by cells lining, (small) intestine/ nephron/ PCT]
3) (mineral) ions/ salts/ named e.g, (across) endodermis;
sucrose out of sieve tube at sink;
[substance// cell// direction;
Na+/ K+// neurone// K+ in Na+ out;
Na+/ K+ // named cell/ Ion pump to drive cotransport;
K+// guard cell (to open stomata)// in;
Na+// cell of loop of Henle// out;
Ca2+// muscle cell// into sarcoplasmic reticulum;
Ca2+// presynaptic knob// out;
H+// in cell, respiring (aerobically)/ photosynthesising//
for chemiosmosis;
named ion(s)// cells lining DCT// in/ out;

Question 17

Mechanism of release of enzymes into the gut: [june11 2c]

Answer 17

bulk transport

Question 18

Mechanism of a plant cell taking up water: [june11 2c]

Answer 18

osmosis

Question 19

Mechanism of calcium ions entering a nerve cell down a concentration gradient: [june11 2c]

Answer 19

facilitated diffusion

Question 20

Mechanism of oxygen entering a red blood cell: [june11 2c]

Answer 20

diffusion

Question 21

Function of a cholesterol: [june09 2ai]

Answer 21

stabilise the membrane OR maintain/ affect / control,
fluidity OR reduces permeability to, polar / charged,
particles [reduces / affects, lateral movement of
phospholipids ]

Question 22

Function of a (glyco)protein: [june09 2ai]

Answer 22

allow communication across membrane OR allow, polar/ charged, particles to pass through membrane [cell recognition / receptor site / cell signalling
/ cell attachment ]

Question 23

Function of a phospholipid (bilayer): [june09 2ai]

Answer 23

to act as a barrier (to, polar/ charged, particles)/ select what enters or leaves cell [acts as) selectively permeable or partially permeable membrane; [allows small/ fat soluble molecules to pass through]

Question 24

Explain what is meant by the term cell signalling. [june09 2bi]

Answer 24

communication between cells;
cell, recognition / identification;
cells work together / coordination between action of different cells;
to trigger, response / reaction ( inside the cell);
[e.g. action of hormone / cytokines ]

Question 25

Explain how a glycoprotein can act as a receptor. [june09 2bii]

Answer 25

(receptor) specific shape; [tertiary structure]
complementary to (shape of), trigger/ named trigger/ 
communicating [fits/ idea of lock & key in correct context]; molecule; (trigger) binds / attaches to receptor;

Question 26

The structure of cell membranes can be described as ‘proteins floating in a sea of lipids’. This membrane structure allows certain substances to pass through freely whereas other substances cannot.
State the term used to describe a membrane through which some substances can pass freely but others cannot. [jan13 3a]

Answer 26

partially/ selectively/ differentially, permeable

Question 27

The model of cell membrane structure is called the ..1..
model. Phospholipid bilayers with specific membrane proteins account for the ability of the membrane to allow both passive and …2… transport mechanisms. Ions and most polar molecules are insoluble in the phospholipid bilayer. However, the bilayer allows diffusion of most non-polar molecules such as …3…. . Protein channels, which may be gated, and …4… proteins enable the cell to control the movement of most polar substances. [jan13 3b]

Answer 27

1 fluid mosaic;
2 active;
3 fats/ lipids/ oils/ cholesterol/ oxygen/ carbon dioxide/
(named) steroid hormones/ fat soluble vitamins; [O2 and CO2; Vitamin A / D / E / K]
4 carrier/ (co)transport(er);

Question 28

Explain how cell surface membranes contribute to the process of cell signalling. [4mks] [jan13cii]

Answer 28

1 release of signal molecule by, exocytosis / secretion OR described ; [hormone / messenger (molecule) / named hormone for signal throughout]
2 idea that: proteins / glycoproteins / glycolipids, act as / have, receptors OR described ; [eg ‘place for signal molecules to bind’ or ‘binding site’
for ‘receptor’]
3 idea that: receptor / signal, is specific ;
4 idea that: shape of receptor and signal are complementary ;
5 idea that: attachment of signal molecule causes change (inside cell / on cell surface) ; [e.g. cause release of cAMP; e.g. hormones trigger a reaction in the cell]
6 cell surface membrane allows entry of some signal
molecules; [diffusion (in context of steroid hormones)]