Plasma membrane

• Eukaryotic cells


  • Able to be larger than prokaryotic cells because of their internal membrane system
  • Far more complex than prokaryotic cells
  • Genetic material is organized into chromosomes and enclosed in a nuclear envelope
  • Contained within a Plasma Membrane
  • Several specialized organelles including numerous internal membranes together termed the endomembrane system
  • Complex cytoskeleton
• Plant cells


  • Have specialised structures including chloroplasts, a vacuole and a cell wall
• Chloroplasts


  • Capture light energy and convert it into chemical energy
  • This process occurs in the stacks of thylakoid sacs called grana
• Plant cell vacuole


  Stores various chemicals and plays a role in cell growth
• Plant cell wall

  • Maintains the cell shape and prevents mechanical damage
  • Composed of cellulose fibres embedded in a protein/polysaccharide matrix consisting mainly of hemicellulose and pectin
• Nucleus

  • The largest and most easily recognized organelle in the cell
  • Contains most of the genes which control the cell (some genes are present in the mitochondria and chloroplasts)
  • The nuclear membrane encloses the nucleus separating it from the cytoplasm
  • The nuclear membrane is a double membrane and contains nuclear pores about 100 nm in diameter
  • DNA is organized (along with proteins called histones) into a material called chromatin
  • The nucleolus is where the components of ribosomes are manufactured
  • During cell division the chromatin condenses into chromosomes (Humans have 46)
  • Messenger RNA is synthesized inside the nucleus from a DNA template and released into the cytoplasm via the nuclear pores where it controls protein synthesis
• Plasma membrane

  • Defines and contains the cell, separating the cell from its external environment
  • Controls the entry of nutrients and the exit of waste products
  • Maintains the electrolyte balance in the cell
  • Acts as sensor to external signals
  • Assemblies of lipids and protein molecules held together mainly by non-covalent interactions (fluid mosaic model)
  • The lipid bilayer provides the basic structure of the membrane and serves as an impermeable barrier to most water-soluble molecules
  • The protein molecules are dissolved in the lipid bilayer and carry out most of the specialised functions of the membrane
• Lipid bilayer

  • Membrane lipids are Amphipathic molecules which will spontaneously form bilayers in an aqueous environment
  • If damaged the lipid bilayer is able to repair itself
  • Lipids constitute about half of the mass of biological membranes
  • The three major types of lipids in cell membranes are phospholipids, cholesterol, and glycolipids
  • Phospholipids are the most common and typically have a polar hydrophilic head and two hydrophobic carbon tails
• Lipid membrane fluidity


  • Individual lipid molecules are able to freely diffuse within lipid bilayers
  • Lipid molecules only rarely move from one side of the lipid bilayer to the other
  • Lipid molecules often exchange places with adjacent lipid molecules over 1000,000 times a second
  • Lipid molecules also rotate rapidly about their axis
  • The fluidity of a particular membrane depends upon its lipid composition
  • The high level of cholesterol in eukaryotic membranes increases membrane stability by interacting with phospholipid molecules
• Cholesterol


  • The steroid cholesterol is a vital component of animal cells
  • At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids and provides stability
  • Without cholesterol the plasma membrane cannot function properly
• Membrane proteins


  • The polypeptide chain of membrane proteins often crosses the lipid bilayer several times, these are called trans membrane proteins
  • Transmembrane proteins are amphipathic they have hydrophobic regions and hydrophilic regions
  • Peripheral membrane proteins are only associated with the membrane by non-covalent linkages and are easily dislodged
  • The shape and distribution of membrane proteins can be observed using freeze fracture electron microscopy
  • Membrane proteins can move about in the lipid bilayer as demonstrated by antibody experiments
• Functions of membrane proteins


  • Transport proteins e.g. the sodium/potassium ATPase pump which pumps 3 sodium ions out of the cell and 2 potassium ions into the cell
  • Receptor sites: the exterior region of a transmembrane protein may act as a receptor for a chemical messenger such as a hormone or growth factor
  • Structural roles: Membrane proteins called Integrins allow the cell to attach to the extracellular matrix
  • Cell junctions: Tight junctions are present between some cell types. They act to separate the apical and basal membranes which have different functions
• Cystic fibrosis

  • Caused by a defective chloride ion channel (a transmembrane protein) - due to a deletion of phenylalanine
  • The failure of the chloride channel results in a build-up of viscous mucus within the lungs making the individual prone to infections
• Membrane carbohydrates

  • Carbohydrates can account for up to 10% of the membranes mass
  • The carbohydrate in the plasma membrane is nearly all on the outer surface
  • A glycocalyx consisting of a thin layer of carbohydrate is present on the outside of the plasma membrane of most cells
  • Cell surface carbohydrates are known to be important in cell-cell and cell to extracellular matrix recognition
  • The ABO blood types are determined by carbohydrates on the surface of red blood cells
  • Membrane glycoproteins are also involved in infection mechanisms, e.g. with HIV
• HIV infection

  • HIV must bind to the immune cell surface protein CD4 and a "co-receptor" CCR5 to infect a cell
  • HIV cannot enter the cells of resistant individuals that lack normal CCR5
• Endocytosis

  1. Phagocytosis - Uptake of insoluble material

  2. Pinocytosis - Cells pinch their PM to take up extracellular fluid in small vesicles

  3. Receptor-mediated endocytosis - Binding of macromolecules to specific cell surface receptors which triggers endocytosis
• Exocytosis

  1. Involves fusion of vesicles from the interior of the cell with the plasma membrane

  2. The vesicles contents are then expelled into the surrounding medium

  3. Important in the secretion of numerous proteins including hormones, and extracellular structural proteins such as collagen and fluids such as mucus
• The sidedness of the plasma membrane
• Plant cell

  A) chloroplast

  B) plasmodesmata

  C) cell wall

  D) plasma membrane

  E) perioxisome

  F) cytoskeleton

  G) microfilament

  H) intermediate filament

  I) microtubules

  J) tonoplast

  K) central vacuole

  L) centrosome

  M) RER

  
• Endosymbiosis theory

  mitochondria -> aerobic bacteria

  chloroplasts -> cyanobacterium
• Freeze-fracture supported the fluid mosaic model.

  Freeze- fracture is specialised preparation technique that spilts a membrane along the middle of the phospholipid bilayer
• Role of plasma membrane:

  • controls entry of nutrient and exit of waste
  • maintains electrolytes balance
  • acts as a sensor to external signals
• Lateral movement of phospholipids is frequent

  flip-flop is rare
• functions of membrane proteins:

  • transport
  • enzymatic activaty
  • signal transduction
  • intercellular joining
  • cell-cell recognition
  • attachment to cytoskeleton and extracellular matrixs
• receptor-mediated endocytosis:

  Macromolecules (transferrin) become concentrated in endocytic pits

  Endocytic pits are coated with a bristle-like protein called clathrin

  Clathrin polymerises around the vesicle forming a cage like structure