B2.1 Membrane & membrane transport

Structure and function of phospholipids in a membrane.

• Phospholipids form the basic structure of cell membranes, which are formed from Phospholipid bilayers

• The plasma membrane forms the border between a cell and its environment, 

• In Eukaryotes the plasma membrane can divide the cytoplasm in compartments.

• A bilayer of phospholipids and other substances forms a continuous sheet that controls the passage of substances.

• The phospholipid molecules have a polar and a non-polar region.

• Polar, a hydrophilic end containing phosphate group bonded to glycerol;

•  and a non-polar, hydrophobic, lipophilic end containing fatty acids.

• As phospholipids have a hydrophobic and hydrophilic part they are known as

 AMPHIPATHIC;

• When phospholipids are placed in water the hydrophilic phosphate heads orient towards the water and the hydrophobic hydrocarbon tails orient away from the water. This forms a phospholipid monolayer

• The phospholipid bilayer has two regions - a hydrophobic core and a hydrophilic outer layer

• The hydrophobic regions are attracted to each other and the hydrophilic regions are attracted to water in the cytoplasm or the extracellular fluid.

• As the interior of the bilayer is hydrophobic, non-polar, lipid-soluble molecules like steroids can easily pass through the lipid bilayer.

• The properties which allow the bilayer to form a barrier are:

• Large molecules cannot pass through the barrier as the hydrophobic region is tightly packed and has low permeability to larger molecules

• Uncharged polar molecules ( glucose)and ions ( Na+) cannot pass through the hydrophobic hydrocarbon chains which make the tails of the phospholipid structure.

• Small, uncharged molecules can readily pass through the lipid bilayer. Thus, polar molecules like water and ethanol or non-polar molecules like oxygen and carbon dioxide can easily enter or leave cells.

IBDP Q: Explain how the properties of phospholipids help to maintain the structure of cell membranes. (7 marks)

Ans: 

a. phospholipid consisting of a head and two tails;
b. head is glycerol and phosphate;
c. tails are fatty acid chains;
d. head hydrophilic and tails hydrophobic;
e. hydrophilic heads are attracted to or soluble in water;
f. hydrophobic tails not attracted to water but attracted to each other;
g. These properties of phospholipids lead to the formation of the double layer in water;

h. Phospholipids are unusual because part of the molecule is hydrophilic and part is hydrophobic. Substances with this property are described as AMPHIPATHIC;
i. There is stability in the double layer because the heads on the outer edge are attracted to water and the tails are attracted to each other in the middle;

j. The phospholipids can move in the fluid state as there are bends in the phospholipid tail which prevents close packing hence there are weak bonds between the phospholipid tails;
k. The phospholipids can move hence the phospholipid bilayer is in a fluid or a flexible state, this is because of the attraction of non-polar tails to each other;

l. This fluidity allows membranes to change shape e.g. vesicles to form or fuse with membrane and also allows cells to divide;
m. non-polar amino acid side chains are attracted to hydrophobic tails;

B2.1.4 Integral and peripheral proteins in membranes.

• Membrane proteins can be:

1. Integral Proteins or Intrinsic Proteins 

• These are partially hydrophobic, they are amphipathic.

• The hydrophobic part is embedded in the phospholipid bilayer.

• They may fit in one of the two phospholipid layers or extend across both.

• Many integral proteins are transmembrane proteins, with hydrophilic parts projecting through the regions of phosphate heads on either side.

• Many integral proteins are carrier molecules or channels to transport substances like ions, sugars and amino acids.

    B. Peripheral Proteins or Extrinsic Proteins 

• These are hydrophilic proteins

• They are attached to either the surface of integral proteins or to the plasma membrane via a hydrocarbon chain.

• Some peripheral proteins are receptors for hormones and neurotransmitters or enzymes for catalyzing reactions

Functions of membrane proteins

• Membrane proteins can have different functions:

  • Hormone binding sites (also called hormone receptors), for example the insulin receptor;

  • Immobilised enzymes with active sites exposed on the surface of the membrane, for example in the small intestine;

  • Cell adhesion to form tight junctions between groups of cells in tissues and organs;

  • Cell-to-cell recognition, 

Glycoproteins act as cell markers, or antigens, for cell-to-cell recognition

• Electron carriers are arranged in chains in the membrane so that electrons can pass from one carrier to another, thus helping in electron transport;

• Channels for passive transport to allow hydrophilic particles across by facilitated diffusion;

• Pumps for active transport use ATP to move particles across the membrane.

B2.1.9 Structure and function of glycoproteins and glycolipids 

1. When cell membrane lipids have carbohydrate chains attached on the extracellular side, they are known as glycolipids.

2. Glycoproteins are cell membrane proteins that have chains of carbohydrates attached to them.

3. Carbohydrates are only found on the exterior, extracellular side of the cell membrane.

Functions of glycoproteins and glycolipids

1. Cell recognition: Glycolipids and glycoproteins play an important role in cell recognition. They act as ‘markers’ on the cell surface and help cells of the body recognise each other. They also help cells of the immune system to recognise foreign cells.

2. Cell adhesion: Both glycolipids and glycoproteins help cells to attach and bind to other cells to form tissues. Cell-adhesion molecules or CAMs are cell-surface glycoproteins that play an important role in cell adhesion.

3. Cell signalling: They act as receptors for enzymes and other molecules helping in cell signalling, i.e. receiving and transmitting chemical signals.

B2.1.10 Fluid mosaic model of membrane structure 

Fluid mosaic model of membrane structure

• The model of membrane accepted today is based on the Singer and Nicolson fluid mosaic model

• All membranes wherever they occur have the same basic structure;

• They are 7-10 nm thick;

• They are made of a bilayer of phospholipids, proteins, cholesterol and short carbohydrate chains attached to proteins (glycoproteins).

• Peripheral proteins are attached to the inner or outer surface.

• Integral proteins are embedded in the phospholipid bilayer, in some cases with parts protruding on one or both sides.

• The fluid mosaic model also helps to explain:

A. Passive and active movement between cells and their surroundings

B. Cell-to-cell interactions

C. Cell signalling

Why is the cell membrane called the fluid mosaic model?

• Called ‘mosaic’ because the proteins are likened to the tiles in a mosaic.

• The whole structure is flexible ’fluid’ because the phospholipids molecules are free to move laterally in each of the two layers of the bilayer, so the proteins can also move and proteins can float into a position anywhere in the membrane. This gives the model its name fluid mosaic model.

B2.1.3 Simple diffusion across membranes

• Simple diffusion is a type of membrane transport that involves particles passing directly between the phospholipids in the plasma membrane

• Diffusion is the net movement, as a result of the random motion of molecules or ions, of a substance from a region of higher concentration to a region of lower concentration( down the concentration gradient)

• The random movement is caused by the kinetic energy of the molecules or ions

• If diffusion takes place for a long enough period, molecules eventually reach equilibrium, where they are evenly distributed on either side of a membrane.

• The centre is hydrophobic so charged ions cannot diffuse through.

• Polar molecules which have partial positive and negative charges over their surface can diffuse through at low rates

• Small, uncharged particles, non polar can pass between the lipid molecules e.g.: oxygen, and carbon dioxide.

The rate at which substances diffuse across a membrane depends on several factors:

1. 'Steepness' of the concentration gradient

The greater the difference in concentration across a membrane,the higher the rate of diffusion

2. Temperature

The higher the temperature the higher the rate of diffusion

3. Surface area

The greater the surface area the higher the rate of diffusion

4. Properties of the molecules or ions

(i)Large molecules diffuse more slowly as they require more energy to move

(ii)Uncharged molecules, e.g.oxygen, diffuse faster as they move directly across the phospholipid bilayer

(iii)Non-polar molecules diffuse more quickly as they are soluble in the non-polar phospholipid bilayer. 

(iv)Although polar molecules cannot easily pass through the hydrophobic part of the membrane, smaller polar molecules (e.g. urea) can diffuse at low rates and polar molecules with partial positive and negative charges can diffuse  at low rates.

B2.1.5 Movement of water molecules across membranes by osmosis and the role of aquaporins.

• Osmosis is the passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, across a partially permeable membrane.

• Osmosis is the diffusion of water.

• A dilute solution has a high concentration of water molecules and a concentrated

• the solution has a low concentration of water molecules.

• Osmosis can also be described as the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.

• Attractions between the solute particles and water molecules are the reason for water moving to regions with a higher solute concentration.

• Osmosis can happen in all cells, because water molecules despite being hydrophilic, are small enough to pass through the phospholipid bilayer.

• Some cells have water channels called aquaporins which are integral proteins, this increases membrane permeability to water. E.g. kidney cells that reabsorb water and root hair cells that absorb water from the soil.

• At its narrowest point, the channel in the aquaporin is only slightly wider than water molecules, which therefore pass through in a single pile.

• Positive charges in the aquaporin prevent H ions( protons) from passing through
  
  

B2.1.6 Channel proteins for facilitated diffusion.

• During facilitated diffusion, the net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient.

• Some particles move across membranes by facilitated diffusion.

• Some substances are large, polar and charged molecules and cannot pass between the phospholipids and require channel proteins to move them through the membrane.

• Channel proteins carry molecules which are polar, charged and larger, Eg: glucose, and amino acids.

• Channel proteins are specific for certain substances.

• allowing specific ions to diffuse through when channels are open but not when they are closed. 

• Movement is passive (no energy is required).

• It is the movement of particles from a region of high concentration to a region of low concentration of that particle.

• There are sodium and potassium channel proteins in the membranes of neurons that open and close, depending on the voltage across the membrane. 

• They are voltage-gated channels and are used to transmit nerve impulses.

B2.1.7 Pump proteins or carrier proteins for active transport

• Active transport is the movement of substances across a membrane using energy from ATP.

• Active transport can move substances against the concentration gradient: From a region of lower concentration to an area of higher concentration.

• Protein pumps (globular proteins) in the membrane are used for active transport.

• The protein pump is specific for the molecule to be transported.

• Protein pumps change shape as they transport molecules, this change in shape requires energy.

• The substance to be transported attaches to a binding site, causing a shape change in the carrier protein.

• Initially, the binding site of the carrier protein is open to one side of the membrane

• When the carrier protein switches shape it opens to the other side of the membrane.

• Differences between channel & pump proteins :
  

B2.1.8 Selectivity in Membrane Permeability

• Selective permeability is the ability of the membrane to differentiate between different types of molecules, only allowing some molecules through while blocking others.

• Facilitated diffusion and active transport are mechanisms that allow cell membranes to be selectively permeable

• because channel proteins and pump proteins are specific to particular particles.

• However, simple diffusion is not selective and depends only on the size and polarity of particles.

• Simple diffusion provides no ability for membranes to be selective with regard to

small, polar molecules.

• Simple diffusion does allow for selective permeability with regard to large or polar molecules