Week 6 plasma membrane: composition & functions

Question 1

Cell membranes main function

Answer 1

-Boundary that separates the cell from its surroundings -> found in all cells
-Provides structural integrity to the cell and protects its contents
-Exhibits selective permeability - allowing only specific molecules to pass through
-It plays a key role in interacting with other cells (glycoproteins) & cell signalling
Internal membranes of organelles allow compartmentalisation (for a specific composition -> specific
reactions/functions)

Question 2

Lipid bilayer

Answer 2

Phospholipid Bilayer - the basic structural framework of the membrane
Lipids are arranged in two closely apposed sheets, forming a lipid bilayer, in which proteins are embedded and some carbohydrates
This creates a semi-permeable barrier, isolating the intracellular content

Question 3

Fluid mosaic model

Answer 3

The fluid mosaic model - describes the dynamic and complex structure of the bilayer
MOSAIC:
-Phospholipids
-Glycolipids
-Sterols (cholesterol in mammalian cells)
-Various proteins
-Glycoproteins
FLUID:
Dynamic, fluid and flexible
->Phospholipids and proteins move laterally within the layer

Question 4

Phospholipids

Answer 4

Phospholipids (75% lipids of the membrane) and are arranged in two-layer sheets
Phospholipids are amphipathic, containing hydrophobic and hydrophilic regions
-Hydrophilic heads
-Hydrophobic tails

Question 5

Hydrophilic heads

Answer 5

Outwards
A phosphate group and a glycerol
->Polar, allowing interacting with water

Question 6

Hydrophobic tails

Answer 6

Nonpolar, repelling water and
interacting with other lipid tails
->Self-Assembly into the bilayer to shield water - no energy needed

Question 7

Membrane fluidity

Answer 7

Phospholipid fluidity can lead to:
-Rapid lateral diffusion within the plane (107 times/sec)
-Spin in place (rotations – up to 500/sec.)
-Flexion (contraction movement)
-Very rarely, Flip-flop (side-to-side movement)
Membrane proteins :
-Usually, no movements -> are anchored to the cytoskeleton
-Only some proteins can slightly and slowly move driven by the motor proteins

Question 8

Cholesterol

Answer 8

-20% of membrane lipids
Stabilise the membrane  affecting its fluidity and stiffness
->Improve stability
->Reduce permeability - down molecule passage

Question 9

Glycolipids

Answer 9

-5%
->Lipids containing sugars
Sugar groups (hydrophilic) face outwards -> asymmetry

Question 10

Membrane proteins

Answer 10

-Integral proteins
-Peripheral proteins

Question 11

Integral proteins (embedded)

Answer 11

Trans-membrane proteins spanning the membrane
->Integral proteins facing only one side
-Amphipathic nature:
->Hydrophobic amino acids lie in the bilayer
->Hydrophilic regions facing the aqueous environment

Question 12

Peripheral proteins

Answer 12

-Temporary attached by interacting with membrane lipids or proteins

Question 13

Functions of membrane proteins

Answer 13

Major functions of membrane proteins;
-Transport of molecules
-Enzymatic activity
-As receptors in cell signalling
-Structural support (linked to cytoskeleton)

Question 14

Glycoproteins

Answer 14

With sugar groups facing outwards (Glycosylation occurs in RE or Golgi)
-Protection from mechanical damage
-Cell-to-cell communication

Question 15

Transport across a cell membrane

Answer 15

Membranes are selectively permeable to control molecules / ions passage
Two types: Passive and Active Transport

Question 16

Passive transport

Answer 16

-Movement of molecules/ions across membranes without energy expenditure
-Movement DOWN the concentration (or electrical) gradient;
->From areas with high concentration to area with a low concentration of the molecule, until the equilibrium is reached
Three categories: diffusion, osmosis and facilitated diffusion

Question 17

Simple diffusion

Answer 17

Movement of small, nonpolar
molecules* that can diffuse
directly through the lipid bilayer
->No energy cost
->Down their concentration gradient
Small and non-polar -> O2, CO2, steroid hormones
Small uncharged polar molecules too, such as urea and ethanol
Each solute diffuses down its concentration gradient (independent to others)
Many molecules are too large or polar to directly diffuse the membrane

Question 18

Osmosis

Answer 18

Movement of water across a cell
membrane from an area with higher water potential (lower solute conc.) to an area with lower water potential (higher solute conc.)
Water potential is inversely proportional to the solute’s concentration
->As solute concentration increases, water potential decreases
->When the solutes are unable to across, osmosis balances solute concentrations, to achieve equilibrium

Question 19

Osmotic tonicity

Answer 19

Tonicity -> the surrounding solution can affect the volume / pressure of a cell by altering their water content, through osmosis

Question 20

Isotonic solution

Answer 20

Solute concentration is equal inside and outside the cell
-No net water movement

Question 21

Hypertonic solution

Answer 21

Solute concentration is higher outside than inside the cell
-Water moves out of the cell -> cell shrinkage

Question 22

Hypotonic solution

Answer 22

Solute concentration is lower outside than inside the cell
-Water moves into the cell -> cell swelling

Question 23

Effect of osmotic pressure on cells

Answer 23

Abnormal tonicity can damage cells
Shrivelled -> cell loses water (hypertonic)
Normal -> No net water movement (isotonic)
Lysed -> cell gains water (hypotonic)

Question 24

Intravenous fluid

Answer 24

Intravenous fluids must match the bodies osmotic balance
To prevent cell shrinkage or
swelling -> major adverse effects
Isotonic only

Question 25

Facilitated diffusion

Answer 25

The passive movement of molecules down a concentration gradient (or electric potential) via a transport protein (transmembrane proteins): -Protein channel -Protein carrier/transporters They are; ->Highly specific (glucose vs. fructose) ->Reversible ->No energy (ATP) cost Which molecules; -Specific ions (e.g. Cl-) -Hydrophilic small molecules (e.g. glucose) -Water facilitated diffusion (via Aquaporins)

Question 26

Facilitated diffusion - protein ion channels

Answer 26

Form a hydrophilic “tube / corridor” across the membrane through which specific molecules / ions diffuse -Specific -No molecule-channel interaction -Rapid rate -Can be gated -> open or close in response to signals (e.g. voltage changes in neurons)

Question 27

Protein carriers / transporters

Answer 27

-Uniporter -> one molecule/ion interaction at a time 1) The solute interacts with the protein carrier 2) Triggering a change in shape that translocate the solute across the membrane -Specificity -Slower rate than protein channels Other protein carrier transports (symporters and antiporters) are active transport

Question 28

Active transport

Answer 28

-Moves substances against their concentration or electrochemical gradients -Requires energy, usually in the form of ATP -Performed by specific membrane proteins or vesicles -To maintain concentration gradients

Question 29

Protein transporter – active transport

Answer 29

-Carriers that shift more than 1 molecule type at a time, against their concentration gradient -Specific and based on binding with the molecule Symporters -> moving 2 distinct molecules in the same direction across the cell membrane ->Antiporter - moving 2 molecules in opposite directions across the membrane

Question 30

Na+-K+ pump

Answer 30

Na+-K+ pump uses the energy of one ATP to pump 3 Na+ out and 2 K+ in

Question 31

Vesicle transport

Answer 31

Bulk transport requires active vesicle transport (endomembranous system) Two main types according the direction: -Exocytosis -Endocytosis

Question 32

Exocytosis

Answer 32

From inside to outside ->Transport vesicles (from Golgi) fuse with the plasma membrane, and release their contents

Question 33

Endocytosis

Answer 33

Uptake of macromolecules and large particles (from outside to inside) ->Substances are brought into the cell by engulfing them in a membrane-bound vesicle

Question 34

Types of endocytosis

Answer 34

3 types of endocytosis: ->Phagocytosis (“cellular eating”) ->Pinocytosis (“cellular drinking”) ->Receptor-mediated endocytosis

Question 35

Summary

Answer 35