Physiology
Basic Cellular
Regarding the cell membrane, which of the following statements is INCORRECT:
Answer:
Membrane lipids comprise a hydrophilic head and two hydrophobic fatty acid tails and are arranged in a bilayer such that the hydrophobic tails face inwards, with the hydrophilic heads facing externally. This means that lipid-soluble substances such as cholesterol incorporate into the membrane, whilst molecules with both hydrophobic and hydrophilic domains such as proteins can be tethered part in and part out of the membrane. The membrane is selectively permeable; lipid-soluble molecules such as O2 and CO2, and small molecules such as water and urea can readily pass through the lipid bilayer, but larger molecules such as glucose, and polar molecules such as ions cannot, and their transport is mediated by transporter and ion channel membrane proteins.Cell Membrane Structure
Physiology / Basic Cellular / Cell Structure and Function
Last Updated: 29th July 2024
Human cells are enclosed by a plasma membrane composed of a phospholipid bilayer with embedded proteins which provide signalling, transport and structural functions. such as ion channels, receptors and enzymes.
Cell Membrane Structure. (Image modified by FRCEM Success. Original by CNX OpenStax [CC BY 4.0 , via Wikimedia Commons)
Bilayer Structure
Membrane lipids comprise a hydrophilic head and two hydrophobic fatty acid tails and are arranged in a bilayer such that the hydrophobic tails face inwards, with the hydrophilic heads facing externally. This means that lipid-soluble substances such as cholesterol incorporate into the membrane, whilst molecules with both hydrophobic and hydrophilic domains such as proteins can be tethered part in and part out of the membrane.
The membrane is selectively permeable; lipid-soluble molecules such as O2 and CO2, and small molecules such as water and urea can readily pass through the lipid bilayer, but larger molecules such as glucose, and polar molecules such as ions cannot, and their transport is mediated by transporter and ion channel membrane proteins.
Phospholipid Bilayer. (Image by OpenStax [CC BY 4.0 , via Wikimedia Commons)
Glycocalyx
Most cells are also covered by a thin gel-like layer called the glycocalyx, containing glycoproteins and carbohydrate chains extending from the lipid membrane, which protects the membrane and plays a role in cell-cell interaction.
Membrane Proteins
Peripheral membrane proteins associated with cell signalling include enzymes bound to the inner surface such as phospholipases, which produce arachidonic acid, and adenylyl cyclase which generates the second messenger cyclic adenosine monophosphate (cAMP). cAMP activates protein kinase enzymes to initiate numerous changes in cell function by phosphorylating membrane and intracellular proteins.
Transmembrane proteins penetrate the entire thickness of the bilayer, and include ion channel proteins and receptors such as G-protein-coupled receptors (GPCRs). On binding the appropriate molecule, GPCRs activate specific membrane-associated GTP-binding proteins, which cleave guanosine triphosphate (GTP) to guanosine diphosphate (GDP), and depending on the type, activate or inhibit other membrane bound signalling enzymes such as adenylyl cyclase.
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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 |
