Cell Membrane

Question 1

Structure and function:

(Reminder from ‘The Cell’ Flashcard deck)

Answer 1

Function: Regulates materials entering and exiting the cell.

Structure: Two layers of phospholipids, proteins

Question 2

Structure of the Cell membrane

What is the model of the cell membrane called? Cell membrane definition (relating to structure)

Answer 2

Model is called the fluid mosaic model

The cell membrane is a fluid combination of lipids (phospholipids and cholesterol), protein and carbohydrates

(Diagram of the fluid mosaic model of the cell membrane)

Question 3

Phospholipids

What is the chemical structure of a phospholipid? (The 2 regions that make up a phospholipid)

State if these 2 regions are hydrophilic or hydrophobic and what molecules make up these regions

Answer 3

Polar (charged) phosphate head
- Hydrophilic (attracted to water)
- Consists of Choline, Phosphate and Glycerol

Long non polar lipid tails
- Hydrophobic (Repelled by water)
- Fatty acids (2)

(Diagrams of Phospholipid on OneNote)

Question 4

When out in water what will phospholipids do and why?

Answer 4

When out in water, phospholipids will self-organise to keep their heads ‘wet’ and their tails ‘dry’.

Question 5

What are the 3 formations that the phospholipids will form when in water?

Describe the size of 2 of the formations.

Answer 5

Micelle (size from 2-20nm)
Liposome (size varies based on type)
Lipid Bilayer sheet

(Diagram on OneNote)

Question 6

The phospholipid bilayer is one way that the tail can be removed from the water.

Can phospholipid molecules flow past each other laterally or vertically in the bilayer?

Answer 6

Phospholipid molecules can flow past each other laterally but cannot move vertically.

(laterally = sideways)

Question 7

Cholesterol

Describe the structure of cholesterol (3 aspects)

What is the function of cholesterol?

Answer 7

Structure:
- Polar head group
- Steroid ring structure
- Non-polar hydrocarbon tail

Function:
It makes the phospholipids pack more tightly and regulates the fluidity and flexibility of the membrane.

(Diagram on OneNote)

(Analogy – People wearing fluffy jumpers -> can slip past each other. People wearing Velcro suits)

Question 8

What are the two types of protein in the cell membrane?

Answer 8

Integral proteins - permanently embedded (extended across 2 lipid layers)

Peripheral proteins - penetrate just one surface

Question 9

Membrane proteins are diverse in terms of structure and function.

What are the different functions of membrane proteins

Name examples of proteins for each function

Answer 9

Transport: Protein channels (facilitated) and protein pumps (active)

Receptors: Peptide based hormones (insulin)

Anchorage: Cytoskeleton attachments and extracellular matrix

Cell recognition: Antigens

Intracellular joinings: Tight junctions and plasmodesmata

Enzymatic activity: Metabolic pathways (eg ETC)- Electron transport chain (electron carries)

Question 10

Movement of materials across the plasma membrane

Name the processes for the movement of large molecules across the plasma membrane

Name the processes for the movement of small molecules across plasma membrane

Answer 10

Movement of large molecules
Vesiculation
- Endocytosis (movement in)
- Exocytosis (movement out)

Movement of small molecules
- Simple diffusion
- Facilitated diffusion (Passive Transport)
- Active Transport

Question 11

Movement of large molecules

What is vesiculation?

Answer 11

The process of forming vesicles or the presence of vesicles

Question 12

What are the 2 types of endocytosis?

Explain what they involve

Answer 12

Pinocytosis
- Process by which fluids and dissolved substances are taken into cells (using vesicles)
- “cellular drinking”

Phagocytosis
- Process by which cells uptake or engulf solid particles (using vesicles)
- “cellular eating”
e.g. white blood cells engulfing foreign bacteria

Question 13

What are the 2 types of exocytosis?

The 2 different ways it can be

Answer 13

Exocytosis may be…
- Constitutive (continuous)
- Regulated (triggered by receptors)

Question 14

Simple diffusion

What is diffusion? (A Level definition)

Answer 14

It is the passive movement of a molecule or ion down a concentration gradient, from a region of higher concentration to a region of lower concentration.
It is passive transport (transport not requiring ATP provided by the cell)

Protein-free bilayers are impermeable to ions but freely permeable to water

Question 15

Facilitated diffusion

What is facilitated diffusion? (A level definition)

Answer 15

It is the passive movement of polar molecules or charged ions down a concentration gradient across a membrane, by channel or carrier proteins in the membrane. It does not require ATP (passive)

Question 16

Is the rate of solute movement in Facilitated diffusion faster than simple diffusion?
What is facilitated diffusion susceptible to?

Answer 16

Rate of solute movement is much faster than simple diffusion, but the end result is the same

It is susceptible to competitive and non-competitive inhibition

Question 17

Facilitated diffusion exhibits saturation kinetics
(Diagram on OneNote)

Facilitated diffusion is selective (i.e. specific binding sites)

Question 18

What are the two types of membrane transport proteins?

Answer 18

Carriers and channels
(Diagram on OneNote)

Question 19

Describe the structure of channel proteins (in simple terms)

What passes through channel proteins?

What do carrier proteins bind to?

How do carrier proteins allow this to cross the membrane?

What is the rate at which these particles pass through channel proteins and carrier proteins?

Answer 19

Channel proteins form water-filled pores and usually allow ions to pass thro’ (i.e. 10^(6) - 10^(7) ions s-1)

Carrier proteins bind a specific solute and undergo conformational change (100x slower 10^(4) molecules s-1)

(Look at A Level ‘Transport across the membrane’ pink flashcards to learn more about these 2)

Question 20

Active Transport

What is active transport?

How is it different to facilitated diffusion?

What kind of molecules can be transported?

What is it active transport mediated by?

Answer 20

It is the transport of ions and molecules against the concentration gradient

It is different to facilitated diffusion in that it moves solutes against the concentration gradient

Charged particles are transported (ions and polar molecules) - insoluble in lipid

Active transport is mediated by carrier proteins coupled to an energy source

Question 21

What are the 2 types of active transport mechanisms?

Answer 21

Mechanism of Active Transport may be symporter or antiporter

Symporter- an integral membrane protein involved in the transport of two (or more) different molecules across the cell membrane in the same direction.

Antiporter- an integral membrane protein involved in the transport of two (or more) different molecules across the cell membrane in different/opposite directions.

(Look at diagram on OneNote)

e.g. Intracellular pH is controlled by Na+/H+ antiporter

Question 22

Two main families of transport proteins use ATP hydrolysis to pump molecules across membranes, what are they?

Answer 22

P class transporters (ion pumps)
These couple phosphorylation and conformational change to pump ions across membranes (phosphate may come from the hydrolysed ATP?)

ABC Transporters
Have different structure and mechanism

Question 23

Clinical Relevance: some drugs exert their effect by blocking membrane transporter proteins

Example 1 Digitalis

Digitalis is an ancient herbal remedy for the treatment of heart disease
It is derived from the dried leaves of the poisonous foxglove plant Digitalis purpurea
It contains a potent mixture of several compounds, some of which act on heart muscle cells by specifically blocking ion transport across the plasma membrane
Nowadays medicines derived from digitalis are used to treat congestive heart failure

Question 24

Clinical Relevance: drugs may be INEFFECTIVE because Multidrug (MDR) transporters pump drugs out of cells

Example 2
When MDR is over-expressed in cancer cells, it can make them simultaneously resistant to a variety of chemically unrelated cytotoxic drugs

Example 3
When MDR is over-expressed in Plasmodium falciparum (the protozoan parasite which causes malaria), the parasites develop resistance to anti-malarial drugs such as chloroquine