2.1

• The main structure of the plasma membrane are a huge number of phospholipids. They are draw using a circle to represent the ‘head’ and two ‘tails’ coming off of the head.

  The head is composed of a glycerol molecule  and an organic alcohol attached to a phosphate group. There are two tails, composed of fatty acid (hydrocarbon) chains. HEnce the name phospholipid. The head and tail have different properties.
• Phospholipid's head is attracted to water - we call this being hydrophilic. The tails are repelled - we call this hydrophobic. When a molecule has both hydrophobic and hydrophilic properties we refer to it as being amphipathic

  When any phospholipids are grouped together the naturally form two layers, with the hydrophilic heads on the exterior, attracted to fluid regions, and the hydrophobic tails point inwards away from the fluid. This forms a phospholipid bilayer

  The fatty acids ‘tails’ do attract each other very strongly, which allows the plasma membrane to be flexible and fluid
• Phospholipids

  
• The plasma membrane is more complex than just a phospholipid bilayer, there are many other components with different functions.
• Cholesterol is found only in animal cell membranes, not plant cell membranes (they have additional fatty acids instead). They play a role in the membrane’s fluidity, allowing the membrane to function at a wider range of temperatures.
• There are also many types of proteins throughout the phospholipid bilayer. These proteins can be integral (transmembrane/ reach across the whole bilayer) or they can be peripheral. THere can also be glycoproteins which are attached to peripheral proteins and are involved in recognition of cells, communication and immune responses. It’s these proteins embedded within the layer and able to move through out it that gives this particular model it’s name - the fluid mosaic model. 
• Cholesterol’s function is to maintain integrity and mechanical stability of the membrane structure. It is also amphipathic (just like the phospholipids.

  The phospholipid bilayer is in constant movement, and cholesterol interacts with the fatty acid tails, embedding in between them.

  Controls fluidity of the membrane, more cholesterol make it less fluid.

  Controls permeability of small molecules, more cholesterol decreases the permeability.

  THey can group together to form lipid rafts, which anchor peripheral proteins and hep secure them to the membrane.
• Cholesterol

  
• • Junctions – Serve to connect and join two cells together, where two proteins can hook together and join temporarily or permanently. There are known as junctions.
• • Enzymes – These can be on the interior or exterior of the cell, ad are often grouped together in sequence to catalyse metabolic reactions, forming what’s known as metabolic pathways 
• • Transport – Responsible for facilitated diffusion and active transport (we will learn about next lesson). These integral proteins are also amphipathic, with the hydrophilic midsection and hydrophobic exterior.
• • Recognition – May function as markers for cellular identification, often through glycoproteins or carbohydrate molecules.
• • Anchorage – Attachment points for cytoskeleton and extracellular matrix 
• Transduction – Function as receptors for peptide hormones, with specific shapes for specific proteins. This causes a change in shape of the molecule which relays a message - signal transduction.
• Proteins Variants

  
• 1. List the four major components of the Plasma membrane. Phospholipid bilayer, glycoproteins, proteins (integral and peripheral) and cholesterol
  2. Outline the difference between integral and peripheral proteins.Peripheral proteins are attached to the surface of the membrane where are integral proteins completed penetrate the lipid bilayers and facilitate the movement of substances
• 1. Phospholipids are described as amphipathic molecules.a) what does this mean? A molecule that contains both hydrophobic and hydrophilic regions what other structures in the membrane are also amphipathic? Cholesterol and integral proteins
  2. Outline why we need cholesterol in our plasma membranes. Cholesterol helps to regulate the fluidity of the plasma membrane, and helps to keep its structure in a wider range of temperatures. It helps to anchor proteins by forming lipid rafts and helps to control the permeability of the membrane.
• Plasma Membrane

  
• History of FLuid Mosaic Model: 1920's Gorter and Grendel

  • The Gorter and Grendel model showed that the phospholipids in the membrane of cells were arranged into a bilayer
  • Evidence for this model:
  • The number of phospholipids extracted from red blood cell membranes was double the area of the plasma membrane if it was arranged as a monolayer
  • Problems with this model:
  • Their model did not explain the location of proteins or how molecules that were insoluble in lipids moved into and out of the cell
• 1930's Davson and Danielli

  • Davson and Danielli's model of the membrane suggested that the proteins were arranged in layers above and below the phospholipid bilayer
  • Evidence for this model:
  • Membranes were effective at controlling the movement of substances in and out of cells
  • Electron micrographs showed the membrane had two dark lines with a lighter band between. In electron micrographs, proteins appear darker than phospholipids
• Problems with the 1930's Davson and Danielli model of the membrane

  • Freeze-etched electron micrographs of the centre of the membrane showed globular structures scattered throughout
  • Improvements in technology used to analyse the proteins in the membranes showed that proteins were globular, varied in size and had parts that were hydrophobic
  • These problems suggested it was unlikely that the proteins would form continuous layers
• The History of the Fluid Mosaic Model

  
• 1970's Singer and Nicolson

  • Singer and Nicolson proposed the fluid mosaic model which stated that membranes were fluid and that the globular proteins were both peripheral and integral (with some crossing both membranes) and dispersed throughout the membrane
• 1970's Singer and Nicolson model

  • Evidence for this model:
  • Analysis of freeze-etched electron micrographs showed proteins extending into the centre of membranes
  • Biochemical analysis of the plasma membrane components
  • The use of coloured fluorescent markers of antibodies. Antibodies were tagged with red and green fluorescent markers. These antibodies were bound to membrane proteins on different cells. Forty minutes after these cells were fused together the markers were seen to have mixed throughout the fused cells membrane showing that membrane proteins are free to move within the layer
• Future models of the membrane

  • With further developments in technology more is still to be discovered about the plasma membrane and so the model we use to represent it continues to evolve
  • e.g. the presence of the cellular cytoskeleton on the inside and the extracellular matrix on the outside makes the membrane less fluid than suggested by the fluid mosaic model
• There are 2 different types of Passive Transport: Simple Diffusion does not require transport proteins and requires no energy. Facilitated Diffusion requires transport proteins and requires no energy. They both go with the concentration gradient.
• Adenosine Triphosphate (ATP)

  
• Active transport requires energy o move materials across the plasma membrane against the material’s concentration gradient. There are two types of active transport:

  Primary active transport sources energy from the breakdown of ATP. THese can be uniport, meaning they only transfer one material in one direction (still against the material’s concentration gradient). They can also be cotransport where two materials are moved, both materials move against their own concentration gradient. Primary active transport utilises protein pumps which always use energy (in the form of ATP).
• Active transport requires energy o move materials across the plasma membrane against the material’s concentration gradient. There are two types of active transport:

  Secondary active transport couples with the transport of materials along their concentration gradients. An antiport moves materials in opposite directions, where as a symport moves both materials in the same direction. Secondary active transport utilises carrier proteins which use concentration gradients of a different material to move a different material against its concentration gradient. 
• Protein Channels in Active Transport

  
• The Sodium Potassium Pump

  
• An example of primary active transport, in the cotransport protein is the sodium-potassium pump. You would find many sodium-potassium pumps in neurones as the movement of sodium and potassium ions in and out of the cell help to generate electrical impulses.
• First Part of the Sodium Potassium Pump: In theis cotransport, integral proteins, 3 sodium ions are moved out of the cell in exchange for 2 potassium ions moving into the cell. This requires an ATP molecule to provide energy for this exchange.

  The integral protein begins open in the intracellular space, and closed off to the extracellular space. This allows sodium ions inside the cell to bind into the protein. THere are three bonding sites for three sodium ions.

  The hydrolysis (break down) of ATP produces ADP and Pi, where Pi will bond to the integral protein.
• The second part of the SOdium Potassium Pump: The pump will undergo a conformational change ( alteration in protein shape - we will learn more about this in the biological molecules unit) and translocate sodium across the membrane.

  THe conformational change reveals two binding sites for potassium on the extracelllar surface.

  When two potassium ions bind, this causes the inorganic posphate to be released, converting the pump bac to its orginal conformation.

  The potassium ion are then tranlocated into the cell, completing the exchange.
• What is the primary function of the sodium-potassium pump in animal cells? The primary function of the sodium-potassium pump is to pump sodium ions (Na+) out of the cell and potassium ions (K+) into the cell, against their respective concentration gradients. This creates an electrical impulse in neurones
• Explain what active transport is and how it differs from passive transport. Active transport requires the expenditure of energy (usually in the form of ATP) to move molecules or ions against their concentration gradient, from an area of lower concentration to an area of higher concentration. This is in contrast to passive transport, which does not require energy and involves the movement of substances from an area of higher concentration to an area of lower concentration, following the concentration gradient.
• 1. How many sodium ions (Na+) are actively transported out of the cell, and how many potassium ions (K+) are actively transported into the cell by one cycle of the sodium-potassium pump? One cycle of the sodium-potassium pump actively transports three sodium ions (Na+) out of the cell and two potassium ions (K+) into the cell.
• 1. What is the energy source that powers the sodium-potassium pump, allowing it to perform active transport? The energy source for the sodium-potassium pump is adenosine triphosphate (ATP). ATP is hydrolyzed during the pump's operation to provide the energy needed to transport ions against their concentration gradients.
• Explain why the sodium-potassium pump is considered an example of primary active transport. The sodium-potassium pump is considered an example of primary active transport because it directly uses energy derived from the hydrolysis of ATP to transport ions against their concentration gradients. It does not rely on the energy from a pre-established concentration gradient of another molecule.
• 1. What characterizes gram-positive bacteria, and how does their cell wall structure differ from gram-negative bacteria? Gram-positive bacteria have a thick layer of peptidoglycan in their cell walls, making them stain blue/purple with crystal violet dye. This contrasts with gram-negative bacteria, which have a thin layer of peptidoglycan and an outer membrane, preventing the crystal violet dye from staining the peptidoglycan.