1. Membrane Structure and Function
Chapter 7
Membrane Structure and Function

2. You should be able to:
Define the following terms: amphipathic molecules, aquaporins, diffusion
Explain how membrane fluidity is influenced by temperature and membrane composition
Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions
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3. Explain how transport proteins facilitate diffusion
Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps
Explain how large molecules are transported across a cell membrane
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4. Overview: Life at the Edge
The plasma membrane is the boundary that separates the living cell from its surroundings
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
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5. Fig. 7-1
Figure 7.1 How do cell membrane proteins help regulate chemical traffic?

6. Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins
Phospholipids are the most abundant lipid in the plasma membrane
Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files.
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7. Membrane Models: Scientific Inquiry
Membranes have been chemically analyzed and found to be made of proteins and lipids
Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer
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8. WATER Hydrophilic head Hydrophobic tail WATER Fig. 7-2
Figure 7.2 Phospholipid bilayer (cross section)
Hydrophobic
tail
WATER

9. In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins
Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions
In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
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10. Phospholipid bilayer Hydrophobic regions of protein Hydrophilic
Fig. 7-3
Phospholipid
bilayer
Figure 7.3 The fluid mosaic model for membranes
Hydrophobic regions
of protein
Hydrophilic
regions of protein

11. The Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids, and some proteins, drift laterally
Rarely does a molecule flip-flop transversely across the membrane
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12. (a) Movement of phospholipids
Fig. 7-5
Lateral movement
(~107 times per second)
Flip-flop
(~ once per month)
(a) Movement of phospholipids
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
(b) Membrane fluidity
Figure 7.5 The fluidity of membranes
Cholesterol
(c) Cholesterol within the animal cell membrane

13. (a) Movement of phospholipids
Fig. 7-5a
Lateral movement
(107 times per second)
Flip-flop
( once per month)
Figure 7.5a The fluidity of membranes
(a) Movement of phospholipids

14. As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about as fluid as salad oil
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15. Unsaturated hydrocarbon tails with kinks Saturated hydro- carbon tails
Fig. 7-5b
Fluid
Viscous
Unsaturated hydrocarbon
tails with kinks
Saturated hydro-
carbon tails
Figure 7.5b The fluidity of membranes
(b) Membrane fluidity

16. The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
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17. (c) Cholesterol within the animal cell membrane
Fig. 7-5c
Cholesterol
Figure 7.5c The fluidity of membranes
(c) Cholesterol within the animal cell membrane

18. Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
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19. Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE

20. Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core
Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
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21. EXTRACELLULAR N-terminus SIDE C-terminus CYTOPLASMIC SIDE  Helix
Fig. 7-8
EXTRACELLULAR
SIDE
N-terminus
Figure 7.8 The structure of a transmembrane protein
C-terminus
CYTOPLASMIC
SIDE
 Helix

22. Six major functions of membrane proteins:
Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)
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23. (b) Enzymatic activity (c) Signal transduction
Fig. 7-9ac
Signaling molecule
Enzymes
Receptor
Figure 7.9a–c Some functions of membrane proteins
ATP
Signal transduction
(a) Transport
(b) Enzymatic activity
(c) Signal transduction

24. (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to
Fig. 7-9df
Glyco-
protein
Figure 7.9d–f Some functions of membrane proteins
(d) Cell-cell recognition
(e) Intercellular joining
(f) Attachment to
the cytoskeleton
and extracellular
matrix (ECM)

25. The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
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26. Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
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27. ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2
Fig. 7-10 ER 1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus 2
Vesicle
Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane 3
Plasma membrane:
Cytoplasmic face 4
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane glycolipid

28. Concept 7.2: Membrane structure results in selective permeability
A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
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29. The Permeability of the Lipid Bilayer
Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
Polar molecules, such as sugars, do not cross the membrane easily
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30. Transport Proteins
Transport proteins allow passage of hydrophilic substances across the membrane
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called aquaporins facilitate the passage of water
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31. A transport protein is specific for the substance it moves
Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
A transport protein is specific for the substance it moves
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32. Concept 7.3: Passive transport is diffusion of a substance across a membrane with no energy investment
Diffusion is the tendency for molecules to spread out evenly into the available space
Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction
At dynamic equilibrium, as many molecules cross one way as cross in the other direction
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33. Membrane (cross section)
Fig. 7-11
Molecules of dye
Membrane (cross section)
WATER
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute
Figure 7.11 The diffusion of solutes across a membrane
Net diffusion
Net diffusion
Equilibrium
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

34. Membrane (cross section)
Fig. 7-11a
Molecules of dye
Membrane (cross section)
WATER
Figure 7.11a The diffusion of solutes across a membrane
Net diffusion
Net diffusion
Equilibrium
(a) Diffusion of one solute

35. Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
No work must be done to move substances down the concentration gradient
The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
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36. (b) Diffusion of two solutes
Fig. 7-11b
Net diffusion
Net diffusion
Equilibrium
Figure 7.11b The diffusion of solutes across a membrane
Net diffusion
Net diffusion
Equilibrium
(b) Diffusion of two solutes

37. Effects of Osmosis on Water Balance
Osmosis is the diffusion of water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
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38. Higher concentration Lower Same concentration concentration of sugar
Fig. 7-12
Lower
concentration
of solute (sugar)
Higher concentration
of sugar
Same concentration
of sugar
H2O
Selectively
permeable
membrane
Figure 7.12 Osmosis
Osmosis

39. Water Balance of Cells Without Walls
Tonicity is the ability of a solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water
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40. cell cell Hypotonic solution Isotonic solution Hypertonic solution H2O
Fig. 7-13
Hypotonic solution
Isotonic solution
Hypertonic solution
H2O
H2O
H2O
H2O
(a) Animal
cell
Lysed
Normal
Shriveled
H2O
H2O
H2O
H2O
Figure 7.13 The water balance of living cells
(b) Plant
cell
Turgid (normal)
Flaccid
Plasmolyzed

41. Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments
The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
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42. (a) A contractile vacuole fills with fluid that enters from
Fig. 7-14
50 µm
Filling vacuole
(a) A contractile vacuole fills with fluid that enters from
a system of canals radiating throughout the cytoplasm.
Contracting vacuole
Figure 7.14 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation
(b) When full, the vacuole and canals contract, expelling
fluid from the cell.

43. Water Balance of Cells with Walls
Cell walls help maintain water balance
A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm)
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
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44. In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
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45. Facilitated Diffusion: Passive Transport Aided by Proteins
In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
Channel proteins include
Aquaporins, for facilitated diffusion of water
Ion channels that open or close in response to a stimulus (gated channels)
For the Cell Biology Video Water Movement through an Aquaporin, go to Animation and Video Files.
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46. Channel protein Solute (a) A channel protein Solute Carrier protein
Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein
Figure 7.15 Two types of transport proteins that carry out facilitated diffusion
Solute
Carrier protein
(b) A carrier protein

47. Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
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48. Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria
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49. Concept 7.4: Active transport uses energy to move solutes against their gradients
Facilitated diffusion is still passive because the solute moves down its concentration gradient
Some transport proteins, however, can move solutes against their concentration gradients
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50. The Need for Energy in Active Transport
Active transport moves substances against their concentration gradient
Active transport requires energy, usually in the form of ATP
Active transport is performed by specific proteins embedded in the membranes
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51. The sodium-potassium pump is one type of active transport system
Active transport allows cells to maintain concentration gradients that differ from their surroundings
The sodium-potassium pump is one type of active transport system
For the Cell Biology Video Na+/K+ATPase Cycle, go to Animation and Video Files.
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52. Cytoplasmic Na+ binds to
Fig
EXTRACELLULAR
FLUID
[Na+] high
[K+] low
Na+
Na+
[Na+] low
Na+
Figure 7.16, 1 The sodium-potassium pump: a specific case of active transport
CYTOPLASM
[K+] high
Cytoplasmic Na+ binds to
the sodium-potassium pump. 1

53. Na+ binding stimulates
Fig
Na+
Na+
Na+
ATP P
Figure 7.16, 2 The sodium-potassium pump: a specific case of active transport
ADP
Na+ binding stimulates
phosphorylation by ATP. 2

54. Phosphorylation causes the protein to change its
Fig
Na+
Na+
Na+
Figure 7.16, 3 The sodium-potassium pump: a specific case of active transport P
Phosphorylation causes
the protein to change its
shape. Na+ is expelled to
the outside. 3

55. extracellular side and triggers release of the phosphate group.
Fig K+ K+ P
Figure 7.16, 4 The sodium-potassium pump: a specific case of active transport P
K+ binds on the
extracellular side and
triggers release of the
phosphate group. 4

56. restores the protein’s original shape.
Fig K+ K+
Figure 7.16, 5 The sodium-potassium pump: a specific case of active transport
Loss of the phosphate
restores the protein’s original
shape. 5

57. K+ is released, and the cycle repeats. 6 K+ K+ Fig. 7-16-6
Figure 7.16, 6 The sodium-potassium pump: a specific case of active transport K+
K+ is released, and the
cycle repeats. 6

58. 1 2 3 6 5 4 EXTRACELLULAR FLUID [Na+] high Na+ [K+] low Na+ Na+ Na+
Fig
EXTRACELLULAR
FLUID
[Na+] high
Na+
[K+] low
Na+
Na+
Na+
Na+
Na+
Na+
Na+
[Na+] low
ATP P
Na+ P
CYTOPLASM
[K+] high
ADP 1 2 3 K+
Figure 7.16, 1–6 The sodium-potassium pump: a specific case of active transport K+ K+ K+ K+ P K+ P 6 5 4

59. Facilitated diffusion
Fig. 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
Figure 7.17 Review: passive and active transport

60. How Ion Pumps Maintain Membrane Potential
Membrane potential is the voltage difference across a membrane
Voltage is created by differences in the distribution of positive and negative ions
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61. Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:
A chemical force (the ion’s concentration gradient)
An electrical force (the effect of the membrane potential on the ion’s movement)
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62. An electrogenic pump is a transport protein that generates voltage across a membrane
The sodium-potassium pump is the major electrogenic pump of animal cells
The main electrogenic pump of plants, fungi, and bacteria is a proton pump
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63. – EXTRACELLULAR FLUID + ATP – + H+ H+ Proton pump H+ – + H+ H+ H+ – +
Fig. 7-18 –
EXTRACELLULAR
FLUID +
ATP – + H+ H+
Proton pump H+ – + H+ H+ H+
Figure 7.18 An electrogenic pump – +
CYTOPLASM H+ – +

64. Cotransport: Coupled Transport by a Membrane Protein
Cotransport occurs when active transport of a solute indirectly drives transport of another solute
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
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65. – + H+ ATP H+ – + H+ H+ – + H+ H+ – + H+ H+ – + – + Diffusion of H+
Fig. 7-19 – + H+
ATP H+ – +
Proton pump H+ H+ – + H+ H+ – + H+
Diffusion
of H+
Sucrose-H+
cotransporter
Figure 7.19 Cotransport: active transport driven by a concentration gradient H+ –
Sucrose + – +
Sucrose

66. Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
Bulk transport requires energy
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67. Exocytosis
In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export their products
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68. Endocytosis
In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving different proteins
There are three types of endocytosis:
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis
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69. In phagocytosis a cell engulfs a particle in a vacuole
The vacuole fuses with a lysosome to digest the particle
For the Cell Biology Video Phagocytosis in Action, go to Animation and Video Files.
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70. In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
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71. PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Fig. 7-20b
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM)
Vesicle
Figure 7.20 Endocytosis in animal cells—pinocytosis

72. In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation
A ligand is any molecule that binds specifically to a receptor site of another molecule
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73. Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit
Fig. 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptor-
mediated
endocytosis
(TEMs)
Coat
protein
Figure 7.20 Endocytosis in animal cells—receptor-mediated endocytosis
Plasma
membrane
0.25 µm

74. Facilitated diffusion
Fig. 7-UN1
Passive transport:
Facilitated diffusion
Channel
protein
Carrier
protein

75. Fig. 7-UN2
Active transport:
ATP

76. Environment: 0.01 M sucrose “Cell” 0.01 M glucose 0.01 M fructose
Fig. 7-UN3
Environment:
0.01 M sucrose
0.01 M glucose
0.01 M fructose
“Cell”
0.03 M sucrose
0.02 M glucose

77. An animal cell membrane will be more fluid at room temperature if it contains
a) more cholesterol.
b) longer chain fatty acids.
c) more cis-unsaturated and polyunsaturated fatty acids.
d) more trans-unsaturated fatty acids.
e) any of the above
Answer: c
This question relates to Concept 7.1.

78. Osmosis If a marine algal cell is suddenly transferred from seawater to freshwater, the algal cell will initially
a) lose water and decrease in volume.
b) stay the same: neither absorb nor lose water.
c) absorb water and increase in volume.
Answer: c
This question relates to Concept 7.3.

79. Transport Kinetics: Passive Diffusion Which of the curves below illustrates uptake of nitrous oxide into the cell, if nitrous oxide diffuses passively across the membrane? A B C
Answer: c
This question relates to Concept 7.3. The X-axis is the difference in concentration, expressed as the concentration outside the cell minus the concentration inside the cell. A negative value means that the concentration is higher inside the cell than outside the cell.

80. Transport Kinetics: Passive Diffusion Which of the curves below illustrates uptake of lactose into the cell via a passive transporter? A B C
Answer: b
This question relates to Concept 7.3. The X-axis is the difference in concentration, expressed as the concentration outside the cell minus the concentration inside the cell. A negative value means that the concentration is higher inside the cell than outside the cell. Facilitated diffusion shows saturation kinetics, as the rate becomes limited by the number of transporter proteins in the membrane at saturating concentrations of solute.

81. Transport Kinetics: Passive Diffusion Which of the curves below illustrates uptake of phosphate into the cell by an active transport system? A B C
Answer: a
This question relates to Concept 7.4. The X-axis is the difference in concentration, expressed as the concentration outside the cell minus the concentration inside the cell. A negative value means that the concentration is higher inside the cell than outside the cell. Active transport will work against a concentration gradient (negative values of the X-axis in the figure above) and show saturation kinetics.