Biological Membranes Flashcards
Fluid Mosaic Model
Accounts for the presence of lipids, protein, and carbohydrates in a
dynamic, semisolid plasma membrane that surrounds cells
Phospholipid
Bilayer:
Each phospholipid has a hydrophilic head and hydrophobic
tail. They are arranged so the heads are facing outward
and the tails make up the inside of the membrane.
Proteins are embedded in the bilayer
Lipid Rafts
Lipids move freely in the plane of the membrane and can
assemble into lipid rafts
Flippases:
Specific membrane proteins that maintain the bidirectional
transport of lipids between the layers of the phospholipid
bilayer in cells
How do proteins and carbohydrates move within the membrane?
May also move within the membrane, but are slowed by
their relatively large size
True/False: Triacylglycerols are found in low levels in the membrane.
True
How does temperature effect cholesterol membrane fluidity
Low temperature = increased fluidity
High temperature = decreased fluidity
How does temperature effect lipid fluidity in the membrane?
Higher temperature causes and increase in fluidity
Lower temperature causes a decrease in fluidity
Transmembrane
Proteins:
A type of integral protein that spans the entire
membrane. They are often glycoproteins.
Embedded Proteins
Are most likely part of a catalytic complex or
involved in cellular communication
Membrane-Associated
Proteins:
May act as recognition molecules or enzymes
Ligands
: Extracellular ligands can bind to membrane
receptors, which function as channels or as
enzymes in second messenger pathways
Gap Junctions
Allow for rapid exchange of ions and other small
molecules between adjacent cells
Tight Junctions
Prevent solutes from leaking into the space
between cells via a paracellular route, but do not
provide intercellular transport
Desmosomes:
Desmosomes bind adjacent cells by anchoring to
their cytoskeletons
Hemidesmosomes
are similar,
but their main function is to attach epithelial cells
to underlying structures
What does glycogenesis do?
Produces glycogen
What two main enzymes are used in glycognesis?
Branching enzyme and glycogen synthase
Where does glycogenesis occur?
Liver and muscle cells
Where is glycogen stored?
Liver
What type of bonds does glycogen synthase create?
Creates alpha-1,4 glycosidic bonds between glucose.
What type of bonds do branching enzymes make?
Creates branches with alpha 1-6 glycosidic bonds
What is the function of Glycogenolysis?
Breakdown of glycogen
What are the two main enzymes involved in glycogenolysis?
Glycogen phosphorylase and debranching enzyme
How does glycogen phosphorylase work?
Removes single glucose 1-phosphate molecules by
breaking a-1,4 glycosidic bonds
Where is glycogen phosphorylase activated? Why is it activated?
In the liver, it is
activated by glucagon to prevent low blood sugar. In exercising skeletal muscle, it is activated by epinephrine and AMP to provide glucose for the muscle itself.
What does the debranching enzyme do?
Moves a block of oligoglucose from the branch and connects it to the chain using an a-1,4 glycosidic bond. It also removes the branchpoint, which is connected via an a-1,6 glycosidic bond, releasing a
free glucose molecule
Where does gluconeogenesis occur?
Occurs in both the cytoplasm and mitochondria, predominantly in the liver
with a small contribution from the kidneys.
What does gluconeogenesis do?
Most gluconeogenesis is simply
the reverse of glycolysis, using the same enzymes. The 3 irreversible steps
of glycolysis must be bypassed by different enzymes
What does pyruvate carboxylase do and in which biochemical pathway is it found?
Occurs in gluconeogenesis and it converts pyruvate to oxaloacetate which is converted to PEP by PEPCK. Together these two enzymes bypass pyruvate kinase.
What activates pyruvate carboxylase?
Acetyl-coA
What activates phosphoenolpyruvate carboxykinase? (PEPCK)
Glycogen and cortisol
Fructose-1,6 bisphosphatase converts _____ to ____
Fructose 1,6 - Bisphosphate to fructose-6-phosphate, bypassing PFK-1
What is the rate limiting step of gluconeogenesis?
Fructose-1,6- bisphosphatase
What inhibits Fructose-1,6-bisphosphatase?
AMP and insulin
Where is GLUT-2 found? What are its effects on Km?
Found in liver (for glucose storage) and pancreatic b-islet
cells (as part of the glucose sensor). Has Km
Where is GLUT-4 found? What stimulates GLUT-4? What does it do to the km?
Found in adipose tissue and muscle. Stimulated by insulin. Decreases Km
Formula glycolysis
Glucose + 2NAD+ + 2ADP + 2P –> 2Pyruvate + 2ATP + 2NADH + 2H+
List the three irreversible enzymes used in Glycolysis
Glucokinase/hexokinase
PFK-1
Pyruvate Kinase
Glucokinase:
Converts glucose to glucose 6-phosphate in the
pancreatic b-islet cells as part of the glucose
sensor.
Hexokinase
Converts glucose to glucose 6-phosphate in in
peripheral tissues. Inhibited by its product G 6-P
Phosphofructokinase-1
PFK-1. Phosphorylates fructose 6-phospate to
fructose 1,6-bisphosphate in the rate-limiting
step. Activated by AMP and fructose 2,6-
bisphosphate. Inhibited by ATP and citrate
What activates PFK1
AMP and Fructose 2,6-bisphosphate
What inhibits PFK-1
ATP and citrate
Phosphofructokinase-2
PFK-2. Produces fructose 2,6-bisphosphate that activates PFK-1. It is activated by insulin; inhibited by glucagon.
Glyceraldehyde-3-phosphate
dehydrogenase:
Produces NADH, which can feed into the electron transport chain.
Pyruvate Kinase:
Perform substrate-level phosphorylation, placing
an inorganic phosphate onto ADP to form ATP
What happens to the NADH produced in glycolysis when O2 is present?
Perform substrate-level phosphorylation, placing
an inorganic phosphate onto ADP to form ATP
What happens to the NADH produced in glycolysis if O2 or mitochondrial are absent?
If O2 or mitochondria are absent, the
NADH produced in glycolysis is oxidized by cytoplasmic lactate
dehydrogenase.
What does pyruvate dehydrogenase do?
A complex of enzymes that convert pyruvate to Acetyl-CoA right before the
citric acid cycle.
What stimulates pyruvate dehydrogenase?
Insulin
What inhibits pyruvate dehydrogenase?
Acetyl-coA
Where does the pentose phosphate pathway occur?
Also known as the hexose monophosphate (HMP) shunt, it occurs in the
cytoplasm of most cells.
What are the reactants and products of the pentose phosphate pathway?
Reactant: Glucose-6-phosphate
Product: NADPH, sugars for biosynthesis, and glycolysis intermediates
What is the rate limiting step enzyme for the pentose phosphate pathway? What activates it and inhibits it?
Glucose-6-phosphate dehydrogenase (G6PD), which is activated by NADP+ and insulin and inhibited by NADPH
Galactose
Comes from lactose in milk. Trapped in the cell by galactokinase, and converted to 1-phosphate via galactose-1-phosphate uridyltransferase and an epimerase
Fructose
Comes from honey, fruit, and sucrose. Trapped in the cell by
fructokinase, then cleaved by aldolase B to form glyceraldehyde
and DHAP.
