Movement across membranes Flashcards
What does membrane permeability do?
- Maintains the internal environment
- Selectively permeable
- Gases diffuse very easily
What might impermeable substances need to transport across the membrane?
- Transport proteins
- Energy
- E.g. ions will need help
What type of channels are there within the membrane?
- Channels
- Narrow aqueous pore
- Selective (size, charge)
- Passive
- May be gated (voltage or ligand)
- Usually ions (e.g. Na+) or water (aquaporins)
What type of carriers are there within the membrane?
- Specific binding site
-Carrier undergoes a conformational change
Different types: - Uniport – single substance
- Symport – two substances in the same direction
- Antiport – two substances in the opposite direction
Active (pumps) or passive
What are the three main forces that drive substances in/out of cells?
- Chemical
- Electrical
- Electrochemical
What are driving forces based on?
The presence of a gradient
- Substances either move with the gradient (high to low) or can move against the gradient (low to high) with help
What is the chemical driving force?
- Based on concentration differences across the membrane
- All substances have a concentration gradient
- Force directly proportional to the concentration gradient
What is the electrical driving force?
- Also known as membrane potential
- Based on the distribution of charges across the membrane
- Only charged substances e.g. Na+, K+ will set up a small potential difference across the membrane
- Force depends on size of membrane potential and charge of the ion
What is the electrochemical driving force?
- Combines the chemical and electrical forces
- Net direction is equal to the sum of chemical and electrical forces
- Only charged substances e.g. Na+, K+
What are the two types of membrane transport?
Passive:
- simple diffusion
- facilitated diffusion
Active:
- Primary
- secondary (uses movement of an ion that is being actively tansported)
What is passive transport?
- Does not require an input of energy
- Substance moves down its gradient (high to low)
Two types: - Simple diffusion e.g. gases
- Facilitated diffusion - mediated by proteins (channel or carrier
Give an example of passive transport?
GLUT4 carrier protein:
- Expressed in skeletal muscle and adipose tissue
- Glucose uptake by facilitated diffusion
- Expression up-regulated by insulin
So in the absence of insulin, glucose can not enter the cell.
If insulin is present it signals the cell to insert GLUT 4 transporters into the membrane, allowing glucose to enter the cell
What can happen if we have too much glucose during pregnancy?
Principal fetal nutrient
- Fetal gluconeogenic enzymes inactivated
- Low arterial PO2
From maternal circulation
Transports 0.6 mmol/min/g placental tissue (~0.11g glc/min/g)
Carrier system saturates at ~20 mmol/L (Glut-1; ~3.6g/L)
- Fetal glucose levels directly related to mother’s
- No mechanism to limit uptake below saturation point
- Excess glucose can cause significant fetal harm
What is GLUT 1 Deficiency syndrome?
- GLUT1 present in many cells, including the brain, where it transports glucose across the blood-brain barrier via facilitative diffusion
- GLUT1 Deficiency Syndrome:
- Very rare disorder and there is mutations in gene that encodes GLUT1
- Less functional GLUT1 - reduces the amount of glucose available to brain cells
- Symptoms include: seizures, microcephaly, developmental delay
What is active transport?
- Requires an input of energy
- Substance moves against its gradient (low to high)
Two types: - Primary
- Secondary
