Physiology
Basic Cellular
Which of the following is characterised by passive carrier-mediated transport down a chemical concentration gradient across a cell membrane:
Answer:
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.Cell Membrane Transport
Physiology / Basic Cellular / Cell Structure and Function
Last Updated: 3rd October 2021
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](https://mrcemsuccess.com/wp-content/uploads/2017/04/Diffusion.png)
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.
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 |
