Membrane; Membrane Transport;Organelles;Compartmentalisation

• Structure of Phospholipids

  • Phosphate heads: Polar - Hydrophilic
  • Lipid (Fatty acids chains)Tails: Non-polar - Hydrophobic
  • Classified as "Amphipathic" - Both loving & hating
  • Arranged in a bilayer - Extracellular & Intracellular
  • Bilayer held together by weak hydrophobic interactions between tails
  • Amphipathic layers restrict the passage of many substances

  
• • Proteins of different sizes, shapes and functions and in different positions
  • Cholesterol between hydrocarbon tails 
  • Glycoproteins and glycolipids linked to lipids or proteins

  
• Categories of Membrane Proteins: Integral Proteins

  • Embedded in the hydrocarbon chains - Partly hydrophobic
  • May extend across the membrane with hydrophilic parts
• Categories of Membrane Proteins:

  • Not embedded in the membrane - Hydrophilic
  • Most attached to integral proteins
  • May have a single hydrocarbon chain attached

  
• Categories of Membrane Proteins

  • The protein content of cells depends on the cell’s function(s).
  • The protein content is higher in cells that are more active e.g. chloroplasts and mitochondria have a protein content of approximately 75 %
  • Skeletal muscles, liver, brain, pancreas, artery - the largest proportion of protein molecules
• • Peripheral proteins are hydrophilic - do not have hydrophobic regions
  • Interact with the hydrophilic regions of the integral proteins & hydrophilic heads
  • Membrane proteins are unevenly oriented across the lipid bilayer

  
• Functions of membrane proteins:

  • Receptors: Hormone & Neurotransmitters binding sites - chemical signals
  • Immobilised enzymes where the active site is outside the cell
  • Cell adhesion
  • Recognition: Communication between cells e.g. at synapses
  • Passive transport channels for facilitated diffusion
  • Pumps for active transport

  
• Transmembrane proteins span the entire width of the phospholipid bilayer.

  
• Selectivity in membrane permeability:

  Passive Transport & Active transport

  
• Facilitated Diffusion

  • the movement of the molecules is down the concentration gradient
  • the movement is assisted (facilitated) by transport proteins.
• Facilitated diffusion - Channel Proteins:

  • Passage for polar molecules
  • Sodium, Potassium, Calcium
  • Selective due to the binding sites of the hydrophilic amino acids, being highly ion-specific
  • size of the pore - size filter
  • open or close in response to specific stimuli:
  • changes in voltage, voltage-gated channels
  • binding of small molecules to channel protein or ligand-gated channels

  
• Facilitated diffusion: Carrier Proteins

  • Protein binds to solute molecules and undergoes conformational change
  • then transferred to the other side of the membrane.
  • Highly specific - GLUT or glucose transporter, carrier protein helps transport of glucose into the red blood cell down its concentration gradient

  
• Active transport: Pump Proteins

  • Ions or molecules moving against the concentration gradient; needs pumping + energy
  • lower concentration to region of high concentration
  • requires ATP
  • pump sodium ions out of neurons and pumping potassium ions in (root air cells and small intestines)
  • remove secretory or waste materials from the cell into the extracellular fluid
  • Maintain right concentration of ions in cell. Helps red blood cells maintain their internal sodium and potassium levels

  
• Direct active transport:

  • Substances moved through the protein pump using atp

  Indirect active transport

  • Substance moved by facilitated diffusion through a carrier protein with a substance that had been moved by active transport to create a gradient

  
• The fluid mosaic model of membrane structure was proposed by Singer and Nicolson in 1972.

  • lipid bilayer is fluid
  • nature of fatty acids in the phsopholipid molecules
  • the amount of cholesterol
  • the protein embedded in the fluid bilayer - mosaic
• Glycolipids and glycoproteins

  • consist of small amount of carbohydrates
  • glycolipids - lipids, glycoproteins - proteins
  • found in extracellular
  • Functions:
  • Cell recognition
  • Cell adhesion
  • Cell signalling

  
• parts of the phospholipid bilayer

  
• Free ribosomes produce proteins for use mainly within the cell

  Attached ribosomes synthesise proteins mainly meant for secretion or for use in lysosomes.

  All ribosomes are made up of proteins and tRNA and not mRNA. Eukaryotic free ribosomes are of the 80S type
• The large, permanent vacuole of a plant cell is capable of breaking down macromolecules as needed - Act as a lysosome for plant cell
• The complete set of proteins that an organism can make is termed the organism's proteome
• Purpose of the double membrane in the mitochondrion:

  • Separate the intermembrane space from the cytoplasm
  • Protects mitochondrial DNA
• In the chloroplast

  The thylakoid membrane has a large surface, for more light-dependent reactions to occur.

  hence more photosystem and chlorophyll - increases light absorption.

  Light-independent reaction = Calvin Cycle
• Significance of the seperation of the nucleus and the cytoplasm in the eukaryotic cells

  • allows better regulation of gene expression
• Function of the photosystems I and II in the thylakoid membranes of the chloroplast:

  • Absorb sunlight
  • Uses the energy from the sunlight; used to power the transfer of electrons from water to carbon dioxide
  • Electrons transfer generates glucose

  
• ​Free ribosomes are located in the cytoplasm

  • Produces proteins for the cell

  Bound ribosomes are attached to the endoplasmic reticulum.

  • Produces proteins to be transported out of the cell
• What is the role of clathrin in the formation of vesicles?

  To help the vesicle form more efficiently
• Carbon dioxide is a waste product of aerobic respiration, water is a by-product and energy is produced for use in the body as ATP.
• • Cholesterol is found in the membranes of animal cells
  • Type of lipid called 'steroids'
  • Allows membrane to be fluid but not to permeable
  • Prevents phospholipids crystallising at low temperatures - forms into solids
  • Prevents being too fluid at high temperatures - may allow substances pass through easily
  • Helps membrane form concave shape during endocytosis
• Positions of Cholesterol:

  • Between phospholipids
  • Found between hydrocarbon tails
  • Attaches to phosphate heads
• Diffusion movement of molecules from high concentration to low concentration, membrane or no membrane

  
  

  Osmosis - movement of water molecules from high water concentration to low water concentration across a selectively permeable membrane

  
  

  Simple diffusion - movement of substances from high concentration to low concentration through the phospholipids
• Facilitated diffusion - movement of substances from high concentration to low concentration through channel proteins

  Active transport - movement of substances from low concentration to high concentration through a membrane using energy (ATP) using pump proteins
• • Facilitated diffusion and active transport allow selective permeability
  • Simple diffusion is not selective - some substances can pass through
  • Substances passing through by simple diffusion has to regulate concentration by other mechanism - breathing rate to control oxygen concentration
• Glycoproteins and glycolipids are short carbohydrate chains attached to proteins on the extracellular side of the plasma membrane

  • Called the glycocalyx
  • Role in cell-cell recognition and binding of cells
  • Glycocalyx is hydrophilic so it attracts water to the cell’s surface
• Phospholipids can have saturated and unsaturated fatty acids:

  Saturated fatty acids have higher melting points - making membranes stronger at higher temperatures.

  Unsaturated fatty acids have lower melting points - so membranes are fluid and flexible at temperatures where cells are found.
• Adaptations in fatty acids:

  • Cold-blooded animals and hibernating animals have a higher content of unsaturated fatty acids to maintain membrane fluidity at lower temperatures
  • Cold-blooded animals eat rich nutrients with higher content of unsaturated fatty acids
• • Cholesterol regulates membrane fluidity
  • Cholesterol stabilises membranes at higher temperatures
  • Cholesterol prevents stiffening of membranes at lower temperatures
• Pump protein is a permeable protein that uses ATP in order to transport molecules across the membrane. To move across the concentration gradient.
• main function of the thylakoid membranes in photosynthesis - to increase surface area for photosynthesis
• The space between the inner membrane and thylakoid membrane is known as the thylakoid lumen, and contains many enzymes and substrates required for light-dependent reactions, such as the photosystems which help to produce ATP. 
• The cristae increase the surface area of the inner membrane, which allows for more electron transport chains and ATP generation. The more cristae a mitochondrion has, the more efficiently it can produce ATP through cellular respiration.