Cells: Membranes And Membrane Transport

Question 1

What type of bonds are non-polar hydrophobic molecules unable to form?

Answer 1

Hydrogen

Question 2

What are the three components that membranes are composed of?

Answer 2

• Lipids: phospholipids, glycolipids, and sterols
• Proteins
• Carbohydrates in the form of glycoproteins and glycolipids

Question 3

Describe the structure of a phospholipid molecule

Answer 3

A backbone (glycerol) + 3-C alcohol

Attached to backbone:
- negatively charged phosphate head= polar and easily form H bonds with water

• 2 non-polar fatty acid tails forming hydrophilic tails

Question 4

What in a result of the amphipathic nature of phospholipids?

Answer 4

In aqueous environment spontaneously organise themselves so that hydrophobic tails point in wards and hydrophilic heads point outward, creating a shield.

This results in the formation of 2 layers.

Question 5

What is the overarching function of lipid bilayers?

Answer 5

Separate the internal environment of a cell (or organelle) from the external

Regulate the movement of substances in and out

Question 6

Describe the permeability of a lipid bilayer

Answer 6

Depends one size and hydrophilic/hydrophobic nature:

• non-polar, lipid soluble easily pass through (e.g. steroids)== no ions (Na+)
• large uncharged polar (e.g. glucose) hydrophilic so are repelled
• small polar (water, ethanol) and non polar (O2 and CO2) mostly permeablep

Question 7

What molecules simply diffuse across a membrane?

Answer 7

Non-polar molecules—> O2 and CO2 in gas exchange

Very small polar (water, ethanol)

Question 8

What are the two types of membrane proteins?

Answer 8

Integral

Peripheral

Question 9

What are integral proteins?

Answer 9

• embedded in membrane layer
• difficult to isolate
• amphipathic—> interact w/ phobic exterior and philic interior== embedded

Question 10

What are peripheral proteins?

Answer 10

• hydrophilic
• found on surface and only interact w phili regions of integral proteins
• easier to remove

Question 11

What are the 5 functions of membrane proteins?

Answer 11

1. Transport proteins movement in and out cell (channel and carrier proteins)
2. Recognition cell recognition, immune system: self from non-self cells
3. Receptors for chemical signals &binding sites for hormones and neurotransmitters—> trigger intracellular reactions
4. Enzymes show enzymatic activity & catalyse reactions
5. Adhesion help in cell adhesion to other cells/environment & help cell motility

Question 12

What are aquaporins?

Answer 12

• Integral protein w/ 4 identical water channels
• Tetrameric protein of 4 monomeric subunit—> each subunit a has a water channel
• lined w/ specific hydrophilic side chains== passage of water but not ions
• Bidirectional depending on conc. gradient

Question 13

What is the function of an aquaporin?

Answer 13

Rapid movement of water in and out of cell by forming hydrophilic channels

Volume of water that needs to be transported determines amount of aquaporins ( e.g. in kidney there are many)

Question 14

What are the key characteristics of facilitated diffusion?

Answer 14

• Movement of molecules down a conc. gradient
• Movement is assisted by transport proteins (carrier&channel)

Question 15

What are channel proteins?

Answer 15

Transmembrane proteins that assemble to form channels for the passage of polar molecules

Question 16

What is an example of a type of channel protein?

Answer 16

Ion channels:

Tiny pores that act as pathways only for ions—> highly selective so different channels for different ions

Question 17

What is the selectivity of ion channels due to?

Answer 17

Size of pore acting as a filter

Binding sites of hydrophilic amino acid side chains highly ion specific

Question 18

Why are ion channels in facilitated diffusion highly specific?

Answer 18

Open and close in response to specific stimuli:

• changes in voltage
• binding of small molecules to channel proteins or ligand-gated channels
  -pressure (other mechanical forces)

When gates open, ions pass through pore & down con. Gradient

Question 19

How does facilitated diffusion work with carrier proteins?

Answer 19

1. Binds to solute molecules
2. Undergoes conformational change, transferring molecule to other side of membrane
3. Sites are highly specific for transported solutes

Example: GLUT (glucose transporter carrier protein) transports glucose into red blood cell down conc. gradient

Question 20

Why is active transport exergonic?

Answer 20

Releases energy requiring breakdown of ATP

Question 21

What are the 3 things active transport helps with?

Answer 21

1. Take up of essential nutrience (glucose from intestine lumen to epithelial cells lining small intestine)
2. Remove secretory waste materials from inside cell to extracellular fluid
3. Maintain right ion conc. in cells (e.g. sodium and potassium levels in RBCs)

Question 22

What are the two main types of active transport?

Answer 22

Direct: energy released by breakdown of ATP used directly to transport molecules across membrane

Indirect: movement of one solute down conc. gradient drives movement of second solute against con. Gradient

Question 23

What results in selective permeability?

Answer 23

Facilitated diffusion: gated ion channels, carrier proteins
and active transport

Question 24

Describe the structure of a glycolipid

Answer 24

Covalent bonding of carbohydrates to lipids

Amphipathic (restricted to external surface)

Carb group polar, lipid non-polar (embedded in layer)

Question 25

What is the general use of a glycolipid?

Answer 25

Membrane stability as form H bonds w/ water molecules surrounding cell

Question 26

What are glycoproteins?

Answer 26

Covalent bonding of short carb chains (oligosaccherides) to protein molecules Carb groups of molecule protrude into extracellular environment

Question 27

What are the 4 functions of glycolipids and glycoproteins?

Answer 27

1. **Cell recognition** act as ‘markers’ to help cells recognise each other (help immune system recognise itself) 2. **Cell adhesion** help attach and bind to other cells to form *tissues* 3. **Cell signalling**act as receptors for enzymes and other molecules for cell signalling (e.g. receiving/transmitting chemical signals) 4. **Glycocalyx**

Question 28

What is the glycocalyx

Answer 28

Sticky layer formed by carb groups of the glycolipids/proteins that protrude from cell surface. Helps protect cell surface in addition to signalling, adhesion, etc)

Question 29

What are the 2 key statements of the fluid mosaic model?

Answer 29

1. Lipid bilayer is fluid: depends on nature of fatty acids in phospholipid molecule & amount of cholesterol 2. The proteins (integral &peripheral) are embedded in the fluid layer which resembled]s a mosaic

Question 30

How do fatty acid chains vary?

Answer 30

1. Number of carbon atoms (16-20) (increase number=higher melting point) 2. Saturation

Question 31

How does membrane fluidity change depending on fatty acid saturation?

Answer 31

Saturated: higher melting points=provide stability Unsaturated: lower melting points =ensure fluidity

Question 32

How to saturated fatty acids provide stability?

Answer 32

No kinks so chains lay parallel to each other. Fit together snugly to make membrane dense and rigid.

Question 33

What happens to phospholipid molecules when temperature drops? Why are the kinks in unsaturated fatty acids important then?

Answer 33

Low temperatures=molecules come closer together. Makes membrane more **gel-like**, *decreasing fluidity*. Kinks important as prevent molecules from joining too closely & so maintain membrane fluidity. *this is why saturated matte acids solidify more easily **cold blooded organisms adapt to cold by increasing proportion of unsaturated fatty acids (hibernation) thus regulating fluidity**

Question 34

Describe cholesterol

Answer 34

Amphipathic steroid. Hydrophilic and hydrophilic sides interact correspondingly to phospholipid regions, holding phospholipid molecules together.

Question 35

How are cholesterol molecules inserted into the lipid bilayer?

Answer 35

1. At **low** temperatures ‘inserted cholesterol prevents fatty acid chains from fitting closely together—>prevent freezing 2. At **high** temperatures cholesterol stabilises membrane—>reduces fluidity 3. Decreases permeability of membrane to ions and molecules

Question 36

How are large amounts of materials transported through cell?

Answer 36

Endocytosis and Exocytosis Both require energy==active transport

Question 37

What are the 3 stages of phagocytosis?

Answer 37

**Pagocytosis**: ingestion of large *solid* particles: [seen in WBC&uncellular organisms e.g. amoeba] 1. WBC: Projections of cell called **pseudopodia** surrounds foreign particles 2. They meet and engulf resulting in a vesicle called **phagosome** 3. Phagosome fuses with **lysosome** to digest particle and release nutrience

Question 38

What is pinocytosis?

Answer 38

A form on endocytosis where fluid is ingested by cell to form small vesicles

Question 39

What is exocytosis?

Answer 39

Material enclosed in vesicles Vesicles move to plasma membrane and fine with it They then discharge its contents into exterior *lipids and proteins are added*

Question 40

What is an example of exocytosis?

Answer 40

Glycolipids produced by endoplasmic reticulum and modified by Golgi apparatus. Vesicles released by Golgi apparatus fuse w plasma membrane discharging its contents outside. Waste is also excreted through exocytosis.

Question 41

What are the 3 classifications of ion channels?

Answer 41

Voltage-gated Ligand-gated Mechanically-gated (respond to mechanical cues e.g. vibrations)