Membranes

• All cells are surrounded by a cell membrane, also known as the plasma membrane, with the basic structure being the same in them all.
• The cell surface membrane creates an enclosed space, separating the internal cell environment from the external environment, and intracellular membranes form compartments within the cell, this is seen with the nucleus and mitochondria.
• The cell surface membrane separates the internal cell environment from the external environment.
• Intracellular membranes form compartments within the cell
• What are some examples of intracellular membranes?

  Mitochondria, nucleus and RER
• Plasma membranes are partially permeable and can control which substances can enter and leave the cell
• Membranes act as barriers that can create separate environments by keeping the components of a cell isolated from the external environment and compartmentalising cellular functions by separating the inside of organelles from the cytoplasm.
• How do membranes create separate environments?

  By keeping the components of a cell isolated from the external environment and compartmentalising cellular functions by separating the inside of organelles from the cytoplasm
• Membranes control the movement of substances in and out of the cell/organelle it is surrounding.
• What is an example of membrane permeability?

  The cell membrane is selectively permeable to polar molecules, and a transport protein is required for larger molecules
• Substances can cross membranes by diffusion, osmosis and active transport
• What organelles are surrounded by a single membrane?

  Endoplasmic reticulum, golgi, lysosomes, vesicles and vacuoles
• What organelles are surrounded by a double membrane?

  Mitochondria and the nucleus
• The arrangement of the phospholipid bilayer, integrated proteins, glycoproteins and glycolipids is commonly referred to as the fluid-mosaic model.
• What is the membrane model called?

  Fluid mosaic model
• Why are membranes considered ‘fluid’?

  Due to the constant movement of the phospholipids and proteins within the structure
• The phospholipids in a membrane usually move sideways, within their own layers and the many types of proteins throughout the bilayer moving about within it.
• Why are membranes considered ’mosaic’?

  Proteins are scattered throughout the bilayer like tiles in a mosaic
• The fluid mosaic model helps to explain the passive and active movement between cells and their surroundings, cell-to-cell interactions and cell signalling
• What are some things the fluid mosaic model explains?

  • The passive and active movement between cells and their surroundings
  • Cell-to-cell interactions
  • Cell signalling
• The fluidity of the plasma membrane is advantageous as it allows greater flexibility to the cell and motion of the membrane components required for transport.
• Why is the fluidity of the plasma membrane advantageous?

  It allows greater flexibility to the cell and motion of the membrane components required for transport
• What are some of the structures within the plasma membrane?

  Phospholipids, cholesterol, proteins, glycoproteins and glycolipids
• What is a phospholipid?

  A molecule consisting of glycerol, two fatty acid groups and a phosphate group
• Phospholipids arrange themselves into a bilayer which ranges from 5-10nm in thickness, and is therefore just visible under an electron microscope at very high magnifications
• What is the thickness of a phospholipid bilayer?

  5-10nm
• Due to the size of phospholipid bilayers, they are just visible under an electron microscope at very high magnifications.
• What type of microscopes are phospholipid bilayers visible under?

  Electron microscopes, at very high magnification
• The phospholipid heads are polar, soluble and hydrophilic
• The phospholipid tails are non-polar, insoluble and hydrophobic
• The hydrophilic phospholipid heads face outwards and the hydrophobic fatty acid tails face inwards when forming a phospholipid layer.
• What directions do phospholipid heads face?

  Outwards
• What directions do phospholipid tails face?

  Inwards
• Phospholipids are amphipathic moelcules as they are made up of both polar (hydrophillic) and non-polar (hydrophobic) sections.
• What are amphipathic molecules?

  Molecules with both hydrophilic and hydrophobic regions.
• Why are phospholipids amphipathic?

  They contain both hydrophilic and hydrophobic regions
• What happens is phospholipids are spread over the surface of water?

  They form a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water, this is known as a phospholipid monolayer
• What happens if phospholipids are shaken or mixed with water?

  Become micelles
• If phospholipids are mixed or shaken with water, they form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid facing inwards, this is called a micelle.
• What is a sphere of phospholipids known as?

  Micelle