transport across membranes Flashcards

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1
Q

Describe the basic structure of the cell membrane.

A

Phospholipid bilayer. Contains binding sites (receptors) for chemical recognition. Flexible due to FA’s but can rupture if over stretched. The hydrophobic inner portion of the membrane provides insulation.

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2
Q

What are the 2 classes of membrane proteins?

A

Integral and peripheral. Integral cannot be removed without disrupting the membrane. Peripheral can be removed without major disruption.

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3
Q

What can integral membrane proteins act as?

A

Carriers, ion channels, enzymes and receptors.

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4
Q

What are peripheral membrane proteins important for? Give 1 example.

A

Cell shape and motility. Contact IMP’s in cytoplasmic surface and have enzymatic function. Example: dystrophin - peripheral membrane protein of skeletal muscle. In DMD, muscles degenerate until they fail.

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5
Q

Are membrane carbohydrates intracellular or extracellular?

A

All extracellular.

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6
Q

What is the protein content of myelin, plasma membranes and mitochondrial membranes?

A

Myelin: low, 18%. Plasma membrane: 50%. Mitochondrial: 75% as it is involved in energy transduction.

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7
Q

What is the permeability co-efficient?

A

A measure of the ease at which a molecule can pass through a given membrane.

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8
Q

Define osmolarity.

A

Total number of solute particles.

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9
Q

What does an isosmotic solution contain?

A

An isosmotic soultion contains the same number of solute particles as plasma (ECF). Fewer - hyposmotic. Greater - hyperosmotic.

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10
Q

What determines the tonicity of a solution?

A

The number of non-penetrating solute particles. In the ECF this is Na+. In the ICF this is K+. Isotonic? Same as plasma. Fewer? Hypotonic. Greater? Hypertonic.

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11
Q

What 3 characteristics must a molecule have to diffuse through the lipid bilayer?

A

Small, lipophilic and not charged. O, N, CO2 and urea diffuse easily across.

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12
Q

Why do ions go through the lipid bilayer a lot quicker than they should?

A

They go through protein channels. These provide an aqueous route for ions to get across the membrane.

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13
Q

How does H2O pass through the lipid membrane?

A

Through aquaporins which are ubiquitous.

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14
Q

What are the 2 types of ion channel?

A

Voltage-gated (Na+ channels) or ligand-gated (ACh).

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15
Q

When is electrochemical equilibrium reached?

A

When chemical and electrical gradients are in balance.

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16
Q

What is a carrier-mediated transport system?

A

A molecule, such as glucose, binds to a receptor. The receptor undergoes a conformational change which exposes the binding site on the other side of the membrane. Glucose diffuses through.

17
Q

What are the 2 types of carrier-mediated transport system?

A

Facilitated diffusion (glucose) and active transport (Na+/K+ ATPase).

18
Q

If a molecule cannot cross the cell membrane, what happens when its concentration changes?

A

Osmotic flux - water moves to even out the concentrations.

19
Q

What is the osmolarity of normal human plasma?

A

285 mOsmoles/L (300).

20
Q

What is the safest fluid?

A

NaCl - isotonic saline.

21
Q

What happens in vivo in patients with uraemia?

A

Non-penetrating NaCl in the ECF prevents water following urea into cells and so haemolysis does not occur. Plasma is hyperosmotic but isotonic. In vitro, cells suspended in a solution of urea will burst as urea is not a non-penetrating solute.

22
Q

Which factors favour diffusion through the membrane?

A

A large surface area, high permeability and high concentration gradient.