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Physiology

Basic Cellular

Question 163 of 180

Regarding transport across a cell membrane, which of the following statements is CORRECT:

Answer:

Diffusion is the passive movement of ions across a cell membrane down their electrochemical or concentration gradient through ion channels. Ion channels can be voltage-gated (regulated according to the potential difference across the cell membrane) or ligand-gated (regulated by the presence of a specific signal molecule). Facilitated diffusion is the process of spontaneous passive transport of molecules or ions down their concentration gradient across a cell membrane via specific transmembrane transporter (carrier) proteins. The energy required for conformational changes in the transporter protein is provided by the concentration gradient rather than by metabolic activity. In secondary active transport there is no direct coupling of ATP but the initial Na+ electrochemical gradient that drives the secondary active transport is set up by a process that requires metabolic energy. Examples include the sodium/calcium exchanger, or the sodium/glucose symporter.

Proteins provide several routes for the movement of materials across membranes:

  • Large pores, constructed of several protein subunits that allow the bulk flow of water, ions and sometimes larger molecules
  • Transporter (carrier) proteins, some of which use direct or indirect metabolic energy to move molecules against electrochemical gradients
  • Ion channels, specialised to allow the passage of particular ion species across the membrane under defined conditions

Diffusion

Diffusion is the passive movement of ions across a cell membrane down their electrochemical or concentration gradient through ion channels.

Ion channels are transmembrane proteins which provide a charged, hydrophilic pore through which ions can move across the lipid bilayer. Ion channels are selective for particular ions and their pores may be opened or closed; in this way ion channels confer upon the cell the ability to closely control the movement of ions across the membrane.

The transition between an open and closed ion channel state is called gating, and is brought about by a change in the conformation of the protein subunits that opens or closes the ion-permeable pore. Ion channels can be voltage-gated (regulated according to the potential difference across the cell membrane) or ligand-gated (regulated by the presence of a specific signal molecule).

Facilitated Diffusion

Facilitated diffusion is the process of spontaneous passive transport of molecules or ions down their concentration gradient across a cell membrane via specific transmembrane transporter (carrier) proteins. The energy required for conformational changes in the transporter protein is provided by the concentration gradient rather than by metabolic activity.

By LadyofHats Mariana Ruiz Villarreal [Public domain], via Wikimedia Commons

Diffusion Across Cell Membrane. (Image by LadyofHats Mariana Ruiz Villarreal [Public domain], via Wikimedia Commons)

Active Transport

Primary active transport uses chemical energy in the form of ATP to pump ions against their electrochemical gradient. The Na+/K+ -ATPase antiporter pump uses metabolic energy to move 3 Na+ ions out of the cell for every 2 K+ ions in, against their respective electrochemical gradients. This allows the cell to maintain a high concentration of K+ ions and a low concentration of Na+  ions intracellularly.

By Clod94 via Wikimedia Commons

Primary Active Transport. (Image by Clod94 via Wikimedia Commons)

Secondary active transport relies on an electrochemical gradient (usually the Na+ electrochemical gradient) created by primary active transport to pump another ion (or molecule) against its electrochemical or concentration gradient. There is no direct coupling of ATP but the initial Na+ electrochemical gradient that drives the secondary active transport is set up by a process that requires metabolic energy. Examples include the sodium/calcium exchanger, or the sodium/glucose symporter.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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