Movement Across Membranes Flashcards
(31 cards)
What is the purpose of a membrane?
Maintains the internal environment
Selectively permeable
What do impermeable substances need for membrane transport?
Transport proteins
Energy
What are channel proteins?
Narrow aqueous pore
Selective to size and charge
Passive
May be gated (voltage or ligand)
Usually ions (e.g. Na+, K+) or water (aquaporins)
What are carrier proteins?
Specific binding site
Undergoes a conformational change
Different types of carrier proteins
Active (pumps) or passive
What are the different types of carrier proteins?
Uniport – single substance
Symport – two substances in the same direction
Antiport – two substances in the opposite direction
What are driving forces?
Forces that drive substances into/out of cells
What are 3 main driving forces?
Chemical
Electrical
Electrochemical
What are driving forces based on?
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 are chemical driving forces?
•Based on concentration differences across the membrane
•All substances have a concentration gradient
•Force directly proportional to the concentration gradient
What are electrical driving forces?
•Also known as membrane potential
•Based on the distribution of charges across the membrane
•Only charged substances e.g. Na+, K+
•Force depends on size of membrane potential and charge of the ion
What are electrochemical driving forces?
•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 main types of membrane transport?
Passive
Active
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 upregulated by insulin
In the absence of insulin, glucose cannot enter the cell
Insulin signals the cell to insert GLUT 4 transporters into the membrane, allowing glucose to enter cell
What happens when GLUT4 goes wrong?
•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
•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
What is primary active transport?
Directly uses a source of energy, commonly ATP
Common example is Na+/K+-ATPase:
What happens when Na+/K+-ATPase goes wrong?
•ATP7B protein is a Cu2+-ATPase present in the liver that transports copper into bile
•Wilson’s disease
•Rare disorder
•Mutations in ATP7B gene
•Results in deposition of copper in the liver and other tissues e.g. brain, eyes
•Symptoms include liver disease, tremor,
Kayser-Fleischer rings,
What is secondary active transport?
•Transport of a substance against its gradient COUPLED to the transport of an ion (usually Na+ or H+), which moves down its gradient
•Uses energy from the generation of the ions electrochemical gradient (usually by primary active transport)
Give an example of secondary active transport?
•Example is the Na+/glucose cotransporter proteins (SGLT):
•Present in intestinal lumen and renal tubules
•Transports glucose from low to high concentration
•Na+/K+-ATPase generates a sodium gradient to enable
co-transport of sodium and
glucose
What happens when Na+/glucose cotransporter proteins (SGLT) goes wrong?
•Glucose-Galactose Malabsorption:
•Very rare disorder
•Mutations in SGLT1
•Less functional SGLT1 - inability to transport glucose and galactose, resulting in their malabsorption
•Symptoms include severe, chronic diarrhoea, dehydration, failure to thriv
What is cellular signalling?
•Communication between cells takes place via signalling molecules e.g. hormones, neurotransmitters and growth factors
•Signalling molecules bind to receptors:
•Intracellular – e.g. steroid hormones
•Cell-surface – e.g. peptide hormones
•Second messengers e.g. cAMP, IP3, DAG, Ca2+ - amplification
•Affect gene expression in the nucleus either directly or through signalling cascades
What happens when cellular signalling goes wrong?
•G proteins integral part of G-protein- coupled receptors on cell membrane surfaces
•Cholera:
•Vibrio cholerae bacteria produce the cholera toxin
•This crosses the cell membrane
•Modifies a subunit of the G protein
•Results in increased second messenger (cAMP) levels
•This stimulates several transporters in the cell membrane of intestinal cells
•Results in massive secretion of ions and water into the gut
•Leads to severe diarrhoea and dehydration that can be fatal
What are the other types of membrane transport?
Endocytosis
Exocytosis
