Chap 4C - Cell membrane

Describe the hydrophilic and hydrophobic interactions in the cell membrane

Hydrophilic interactions	Hydrophobic interactions
Between :  Hydrophilic portions of proteins and phosphate groups of phospholipids with the aqueous medium of the cytoplasm and outside of the cell Carbohydrate antennae (hydrophilic) and the aqueous medium outside of the cell	Between :  Hydrophobic portions of proteins and the hydrocarbon tails of phospholipids

Explain why the cell membrane is called fluid mosaic

• ‘Fluid’ refers to the fact that individual phospholipids and protein molecules are in continuous motion and are able to move laterally within the bilayer [1 mark]

• ‘Mosaic’ describes the diverse and different types of proteins present in a random arrangement scattered among the phospholipid molecules [1 mark] 

• Membrane held together mainly be hydrophobic interactions between hydrocarbon tails [1 mark]

• Unsaturated fatty acid tails of phospholipids have kinks, keeping molecules from packing too closely together which enhances membrane fluidity [1 mark]

Describe the 2 experiments as evidence for the fluid mosaic model

1. Cell-fusion experiment 

• Where a human cell and mouse cell is fuse

• Within an hour, the proteins from the two species become completely intermingled in the membrane of the hybrid cell -> revealed the fluid nature of the cell membrane

2. Freeze-fracture experiment 

• Where the cell membrane is frozen and split along the middle of the bilayer -> revealed the mosaic nature of the cell membrane

Describe the overall structure of the cell membrane

1. Phospholipids : form a bilayer structure because of their amphipathic nature (polar heads with hydrophobic tails)

2. Proteins : scattered in a ‘patchwork’ arrangement within the bilayer, can either be loosely attached to the phosphate heads of the bilayer or be embedded into the lipid bilayer

3. Cell membranes are asymmetric 

• The external layer have more proteins as some of these proteins serve as receptors for the attachment of signal molecule

• The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary between different types of cells

Describe the structure and arrangement of phospholipids

• Phosphate group is negatively-charged at oxygen atoms (hydrophilic)

• Hydrocarbon chains are non-polar (hydrophobic)

• Phospholipids (in aq environment) are arranged in bilayer due to amphipathic nature

  • Phosphate heads interact with water and face the aq environment

  • Hydrocarbon chains are sandwiched by 2 phosphate head layers, form the hydrophobic core

Describe arrangement of phospholipids: 

• In aqueous environments, the phospholipids in cell membranes align with one another such that their (hydrophobic) non-polar fatty acid tails are buried inside the two layers formed by the (hydrophilic) polar phosphate heads [1 mark]

This produces a sheet composed of two parallel, molecular layers to form the phospholipid bilayer [1 mark]

Describe the functions of phospholipids

1. Separates the cell contents from the external environment, and allows for compartmentalisation within the cell where specialised biochemical reactions occur
   

2. Regulate membrane fluidity and hence membrane permeability through : 

1. The degree of saturation of fatty acids

• Saturated fatty acids can be closely packed together -> more interactions between tails -> membrane to be more rigid and less fluid

• Unsaturated fatty acids have spaces between them -> prevents the phospholipid molecules from packing close together -> weaker intermolecular interactions between chains -> increasing membrane fluidity

2. The length of fatty acid chains

• As length of hydrocarbon chain increase, fluidity decreases as there is stronger intermolecular interactions between hydrocarbon chains/tails 
  

3. Constitutes the basic bilayer structure of the membrane

• Orientation of polar phosphate heads and non-polar hydrocarbon tails ensures stability of membrane as the polar heads interact with the aqueous environment (via hydrogen bonds)

4. It allows fat-soluble substances and small molecules (CO2, O2) to move across easily but act as a barrier to most water-soluble molecules and ions (due to the hydrophobic core of the bilayer)

Describe the structure of cholesterol

• Amphipathic

• It fits between the phospholipids molecules with the hydroxyl group interacting with the hydrophilic heads of the phospholipids and the hydrophobic portion of the molecule fitting between the fatty acid hydrocarbon chains

Describe the functions of cholesterol

1. To regulate membrane fluidity according to the environment

• Act as a buffer resisting changes in temperature 

1. At relatively high temperatures (38C)

• Cholesterol makes membrane less fluid -> hindering the mobility of hydrocarbon tails of phospholipids

2. At lower temperatures (5C)

• Cholesterol disrupts close packing of phospholipids -> break up interaction between tails

• More cholesterol -> more fluidity 

• Lowers the temperature required for the membrane to solidify

• This is important as the fluidity of the membrane influences many of its functions

• A completely frozen and therefore rigid membrane will be impermeable to even lipid- soluble, non-polar substances.

• Membrane components (proteins) may not be able to perform their functions if a membrane is not fluid enough.

• Fluidity is important in facilitating self-sealing (during endocytosis and exocytosis)

2. For mechanical stability of the membrane as membranes without cholesterol tend to break easily
   

3. It reduces uncontrolled leakage, by diffusion, of certain polar molecules and ions through the membrane so that these can be diverted to the proper channels where their movement can be properly regulated

• Eg. Myelin sheath in nerve cells depends on cholesterol to prevent leakage of ions -> slow down nervous transmission

Describe types of proteins in cell membrane

1. Peripheral or extrinsic proteins 

• Loosely attached at the polar surfaces of the phospholipids or proteins

• Can be found on the interior or exterior of cells or to exposed parts of integral proteins 

• They contain charged and polar amino acid residues, which are bound to the charged portion of lipids by ionic bond and hydrogen bonds

2. Integral or intrinsic proteins 

• Proteins that are embedded in the lipid bilayer, either partly or completely span the membrane

• Proteins that completely span the membrane are transmembrane proteins

• They contain hydrophobic and hydrophilic amino acid residues

• The hydrophobic amino acid residues interact with the fatty acid chains of the phospholipids, while the hydrophilic amino acid residues face the aqueous medium or interact with other hydrophilic amino acids

• These hydrophobic and hydrophilic interactions hold the protein in place

Describe the function of proteins in the cell membrane

1. Transport proteins (only for transmembrane proteins)

• Allow water-soluble ions, glucose, amino acids and proteins to be transported into or out of the cells as these molecules cannot diffuse through the hydrophobic core of the cell membrane

• Channel/carrier : facilitated diffusion of hydrophilic molecules across the membrane

• Pump – active transport of hydrophilic molecules across the membrane

2. Energy transducer

• Electron carriers and ATP synthase allow the transport of electrons and protons respectively during respiration (in mitochondria) and photosynthesis (in chloroplasts) for ATP synthesis

3. Cell surface receptor 

• Act as a receptor , binding with chemicals  allowing cells to respond to external stimuli

4. Enzymes 

5. Structural support 

• Some integral proteins attached to the cytoskeleton or extracellular matrix to give cell membrane a stronger framework

• Microfilaments are fibers located throughout the cytoplasm of cells functioning primarily in maintaining the structural integrity of a cell

Describe glycoproteins

• Forms the glycocalyx – carbohydrate-rich layer outside plasma membrane 

Describe carbo in the cell membrane

Short oligosaccharides (<15 sugar units) that are found on the external surface. As short carbohydrate chains (oligosaccharides) attached to proteins forming glycoproteins and lipids forming glycolipids Normally only present on the extracellular face

Describe the function of carbo in the cell membrane

(In the form of glycoproteins and glycolipids) 

1. Orientation of the membrane structures

• Carbohydrates are highly hydrophilic and help to orientate the glycoproteins and glycolipids to face the exteriors

• Also forms hydrogen bonds with water and thereby stabilising the membrane structure

2. Cell Communication

• Involved in a variety of physiological phenomena like recognition of the same cell type for immune responses 

3. Structural relationships (Cell to cell adhesion)

• Act as 'glue' for adhesion of cells to neighbouring cells for tissue formation

Describe functions of cell membrane within the cell

1. Form compartments (compartmentalisation) within the cell (formation of organelles) (Advantages on the top) 

2. Membranes are required for the formation of transport vesicles during intracellular transport

Describe the functions of cell membrane on the surface of the cell

1. Separates cell contents from external environment -> cell exist as an entity separate from environment, maintain constant internal environment and allows cell to function independently of environment 

2. Cell-cell recognition 

• Glycoproteins and glycolipids are unique in different tissues of an individual and between different people -> allow cells to recognise each other and recognise presence of foreign proteins on membrane which triggers immune response

3. Receptor site

• Have binding site that only specific hormones/signal molecules can recognise and bind to so the cell can respond to chemical messengers 

4. Selective barrier

• Regulate the passage of substances into and out of cells -> allows passage of enough oxygen, nutrients and wastes to meet the metabolic needs of the entire cell

5. Attachment to the cytoskeleton and extracellular matrix

• Membrane proteins can be attached to the cytoskeleton (e.g. microfilaments) on the inside of the cell, and fibres of the extracellular matrix (collagen fibres interwoven with carbohydrate-containing protein molecules called proteoglycans) on the outside of the cell.

• This gives the cell membrane a stronger framework, thus providing structural support and helping to maintain cell shape

6. Cell-cell adhesion 

• Membrane proteins of adjacent cells may be joined together in various kinds of junctions -> maintains structural relationships with neighbouring cells

A) Tight junctions

• Plasma membranes of neighboring cells are tightly pressed against each other, forming a continuous seal around the cell.

• This prevents extracellular fluids from moving across a layer of epithelial cells.

B) Gap junctions

• Consist of membrane proteins that surround a pore through which ions, sugar, amino acids and other small molecules may pass.

• This provides a cytoplasmic channel from one cell to an adjacent cell, allowing for cellular communication.

C) Desmosomes

• Function like rivets, fastening cells together into strong sheets

• Desmosomes attach muscle cells to each other in a muscle. Some ‘muscle tears’ involves the rupture of desmosomes