L1 - structure and function of membranes

• Membranes are structures that cover the surface of every cell, and surround most organelles within cells
• Membrane functions:

  -keeping all cellular components inside the cell

  -allowing selected molecules to move in and out of the cell

  -isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately

  -a site for biochemical reactions and cells to receive hormones

  -allowing a cell to change shape
• Compartmentalisation is vital for the cell’s metabolism which includes different incompatible reactions
• Membranes within the cell are not referred to as surface membranes, but as plasma membranes
• In the 1950’s the plasma membrane was shown to be a double layer when viewed using an electron microscope:

  This supported an earlier theory that membranes were composed of a lipid bilayer

  -in 1972, Singer and Nicholson proposed a model in which proteins occupy different positions within the membrane

  -this is known as the fluid mosaic model
• Membrane structure:

  -cell surface membranes contain various proteins and lipids

  -the components of the plasma membrane play an important role in the functions of the membrane and the cell or organelle they are part of
• Membranes are formed from:

  -phospholipid bilayer (45%)

  -proteins (45%)

  -cholesterol, glycolipids, and glycoproteins (10%)

  -this can vary between cell types
• Phospholipid bilayer

  
• Phospholipids are very similar to triglyceride lipids, except one of the fatty acid molecules is replaced by a phosphate molecule

  
• Phospholipids are made up of two parts:

  -hydrophilic heads (water liking)

  -hydrophobic tails (water fearing)

  
• Hydrophilic phospholipid heads:

  -attracted to the water

  -called polar

  -dissolves easily
• Hydrophobic phospholipid tails:

  -not attracted to the water

  -called non polar

  -insoluble
• Phospholipids in aqueous environments:

  -when exposed to water, phospholipids form either a micelle (a circle) or a bilayer

  -the phospholipid bilayer is the basic structure of membranes

  -in each structure, the hydrophilic heads face the water and the hydrophobic tails point inwards away from the water

  -this behaviour is key to the role that phospholipids play in membranes
• Phospholipid bilayer:

  -cells normally exist in aqueous environments, and the inside of organelles are often aqueous too

  -this structure allows the hydrophilic heads, which form the inner and outer surface of a membrane to interact with the water, whilst sandwiching the fatty acid tails to be form a hydrophobic core inside the membrane

  
• Membrane proteins:

  -have important roles in the various functions of membranes

  -there are two types; intrinsic and extrinsic
• Intrinsic (integral) proteins:

  -transmembrane proteins embedded through both layers of the membrane
• Extrinsic proteins:

  -peripheral proteins present in one side of the membrane. They can be present in either layer
• These proteins can have several functions:

  -carrier proteins

  -channel proteins

  -structural

  -receptors

  -enzymes
• Many intrinsic proteins are carrier or channel proteins. They transport ions, sugars, amino acids that cannot enter the cell by diffusion alone but are essential to cellular functions
• Intrinsic proteins can also be structural, holding the cell membrane together
• Other intrinsic proteins act as receptors for hormones and neurotransmitters or enzymes for catalysing reactions
• Glycoproteins:

  -intrinsic proteins embedded in the cell surface membrane with attached carbohydrate chains of varying shapes and lengths

  -play a role in cell adhesion (cells joining together) and receptors for chemical signals
• Glycolipids:

  -lipids with attached carbohydrate chains

  -are cell markers (antigens)
• Cholesterol:

  -a lipid with one hydrophilic end and a hydrophobic end

  -has the molecular formula of C27H46O

  -positioned between the phospholipids where the hydrophilic end associates with the phospholipid head and the hydrophobic tails pulling them together

  -regulates fluidity and stability of the membrane without causing them to be too rigid

  -cholesterol stops the phospholipids getting too close to one another and crystallising
• Phospholipid function:

  -form the main part of the membrane as two rows with hydrophilic heads pointing out forming a bilayer

  
• Proteins function:

  -either intrinsic or extrinsic. may be structural, anchors, transporters, receptors or enzymes

  
• Cholesterol function:

  -this sits in the phospholipid bilayer and gives the membrane mechanical stability. it acts as a plug to reduce the escape and entry of polar molecules

  
• Glycoprotein function:

  -some proteins have short branching carbohydrate chains that look like antennae. These chains can vary and give cells specific markers. This allows cells to recognise each other to allow them to work together. It allows the immune system to recognise ‘foreign’ markers and remove invaders

  
• Glycolipid function:

  -these are lipids that have branching chains of carbohydrates attached to them. They are involved in cell to cell recognition and as receptor sites for chemical signals

  
• Cell signalling:

  -glycoproteins and glycolipids on cell surface act as receptors and markers (specific shapes which match other molecules)

  -the carb and protein molecule has a specific shape which will only fit to complementary molecules

  -one cell will release molecules (hormones) and they will fix to receptors on specific target cells

  -the hormone will travel throughout the body in the blood

  -the hormone will only affect target cells because they have receptors which are the right shape
• Hormones are protein molecules with a specific shape. They are released from endocrine glands