WEEK 3 (Plasma membrane)

Question 1

Describe the structure of the phospholipid

Answer 1

• Hydrophilic polar head group
• Hydrophobic non-polar fatty acid chains (one tail is saturated and is straight & one tail is unsaturated and is bent)

Question 2

Why is cholesterol in membranes important?

Answer 2

The cholesterol in the membranes hold the phospholipid tails together which compacts them and stabilises the membrane by restricting their movements and modulating the fluidity of all membrane components

Question 3

Define ‘Passive transport’

Answer 3

Movement of substances down a concentration gradient due to the kinetic energy of the substance; no cellular energy is required (this continues until equilibrium is reached)

Question 4

Define ‘Simple diffusion’

Answer 4

Unassisted net movement of small, non-polar substances down their concentration gradient across a selectively permeable membrane

Question 5

Define ‘Facilitated diffusion’

Answer 5

Movement of ions and small, polar molecules down their concentration gradient assisted across a selectively permeable membrane by a transport protein

Question 6

Define ‘Channel-mediated diffusion’

Answer 6

Movement of ion down its concentration gradient through a protein channel

Question 7

Define ‘Carrier-mediated diffusion’

Answer 7

Movement of small, polar molecule down its concentration gradient by a carrier protein

Question 8

Define ‘Osmosis’

Answer 8

Diffusion of water from a region of low osmolality to a region of high osmolality across a selectively permeable membrane

The direction is determined by relative solute concentrations and continues until equilibrium is reached

Question 9

What are the functions of plasma membrane?

Answer 9

• Physical barrier that establishes a flexible boundary, protects cellular contents and supports cell structure
• Selective permeability regulates entry & exit of ions, nutrients and waste molecules
• Electrochemical gradients establishes and maintains an electrical charge difference across membrane
• Contains receptors to recognise and respond to molecular signals

Question 10

Define ‘Active Transport’

Answer 10

Transport of ions or small molecules across the membrane against a concentration gradient by transmembrane proteins pumps requiring an expenditure of cellular energy

Question 11

Define ‘Primary Active Transport’

Answer 11

Movement of a substance up its concentration gradient; powered directly by ATP

Question 12

Define ‘Secondary Active Transport’

Answer 12

Movement of a substance up its concentration gradient and is powered by harnessing the movement of a second substance down its concentration gradient

Question 13

Define ‘Symport’

Answer 13

Movement of a substance up its concentration gradient in the same direction as Na+

Question 14

Define ‘Antiport’

Answer 14

Movement of a substance up its concentration gradient in the opposite direction from Na+

Question 15

What is ‘Vesicular transport’?

Answer 15

When a vesicle formed or lost as material is brought into a cell or released from a cell

Question 16

What is ‘Exocytosis’

Answer 16

Bulk movement of substance out of the cell by fusion of secretory vesicles with the plasma membrane

Question 17

___________ is the bulk movement of substances into the cell by vesicles forming at the plasma membrane

Answer 17

Endocytosis

Question 18

Define ‘Phagocytosis’

Answer 18

A type of endocytosis in which vesicles are formed as particulate materials external to the cell are engulfed by pseudopodia

Question 19

What is ‘Pinocytosis’?

Answer 19

A type of endocytosis in which vesicles are formed as interstitial fluid is taken up by the cell

Question 20

_________________ is a type of endocytosis in which plasma membrane receptors first bind specific substance; receptor and bound substance then are taken up by the cell

Answer 20

Receptor-mediated endocytosis

Question 21

Describe what happens with Channel-linked receptors

Answer 21

A ligand attaches to the protein channel which facilitates the channel opening and allows the ions to flow through

Question 22

Describe what happens with enzymatic receptors

Answer 22

A ligand attaches to the receptor which is attached to a protein which is activated and phosphorylates other enzymes leading to activation

Question 23

Describe what happens with G-protein-coupled receptors

Answer 23

1. A ligand binds to a receptor, causing a conformational change to activate receptor
2. G protein binds to activated receptor
3. GTP binds to G protein causing G-protein activation. Activated G protein leaves the receptor and it attaches to and activates an effector protein (an ion channel or an enzyme)
4. Activated effector protein makes secondary messenger available within the cell which leads to protein kinase enzyme activation
5. Active protein kinase enzyme phosphorylates other enzymes

Question 24

What are ‘Integrins’?

Answer 24

Plasma membrane proteins that are linked to both the cytoskeleton and ECM components and allow continuous exchange of influences in both directions between the cytoplasm and material in the ECM

Question 25

What is the mechanism of the fixative Osmium tetroxide?

Answer 25

Osmium binds to the polar heads of the phospholipids and the oligosaccharide chains producing the two dark outer lines that enclose the light band of osmium-free fatty acids seen using TEMs

Question 26

Higher concentrations of which substances reduce lipid fluidity?

Answer 26

Cholesterol & Saturated fatty acids

Question 27

What is the name of the model that best illustrates the plasma membrane?

Answer 27

Fluid mosaic model

Question 28

What is the difference between ‘Integral proteins’ and ‘Transmembrane proteins’?

Answer 28

Integral proteins are firmly embedded in the lipid layers and Transmembrane proteins completely span the bilayer

Question 29

What are the three types of endocytosis?

Answer 29

• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis

Question 30

Describe what occurs in Phagocytosis

Answer 30

When a particle (e.g pathogen or dead cells) becomes bound to the surface, the plasmalemma and cytoplasm surrounds it and forms a PHAGOSOME which merges with a lysosome for degradation of its contents

Question 31

Describe what happens in Pinocytosis

Answer 31

Smaller pieces of the cell membrane fuse and entrap extracellular fluid and its dissolved contents forming PINOCYTOTIC VESICLES which either fuse with lysosomes or fuse with the opposite cell surface membrane where they release their contents (transfer of substances is called TRANSCYTOSIS)

Question 32

Describe what happens in Receptor-mediated endocytosis

Answer 32

Receptors on substances (e.g low-density lipoproteins and protein hormones) bind to receptors in special membrane regions that invaginate and pinch off internally as vesicles that fuse with the endosomal compartment. In some endosomes, the membranes of the late endoscopes have ATP driven H+ pumps that activate the hydrolytic enzymes of lysosomes; in other endosomes, the ligands are separated from their receptors to from separated endoscopes and receptors are recycled; in other endosomes, transcytosis occurs

Question 33

The formation and fate of vesicles in receptor-mediated endocytosis also depends on what type of proteins?

Answer 33

Specific peripheral proteins on the cytoplasmic side of the membrane

Question 34

What is the major polypeptide in the cytoplasmic surface of the coated pits?

Answer 34

Clathrin

Question 35

Vesicle trafficking through the endosomal compartment is directed by what?

Answer 35

Peripheral membrane G-proteins called RAB PROTEINS and small GTPases that bind with guanine nucleotides and associated proteins

Question 36

What is the endosomal compartment?

Answer 36

A dynamic collection in the peripheral cytoplasm of membranous tubules and vacuoles

Question 37

What are the two types of exocytosis and what are they?

Answer 37

• Constitutive secretion - used for products released from cells continuously as soon as synthesis is complete
• Regulated secretion - occurs in response to signals coming to the cells

Question 38

What is the process of membrane movement and recycling called?

Answer 38

Membrane trafficking

Question 39

What happens in paracrine signalling?

Answer 39

The chemical ligand diffuses in extracellular fluid but is rapidly metabolised so its effect is only local on target cells near its source

Question 40

What happens in autocrine signalling?

Answer 40

Signals binds receptors on the same cells that produced the messenger molecule

Question 41

What happens in juxtacrine signalling?

Answer 41

Signalling molecules are cell membrane-bound proteins which bind surface receptors of the target cell when the two cells make direct physical contact