Transport Across Membranes

• What separates the contents of a living cell from the external environment?

  The cell-surface membrane
• What are the different methods by which substances move across membranes?

  Simple diffusion, Facilitated diffusion, Osmosis, Active transport, Co-transport
• How would you determine a reliable mitotic index (MI) from tissue observed with an optical microscope?

  • Count cells in mitosis in a field of view
  • Divide this by the total number of cells in the field of view
  • Repeat many/at least 5 times, or select fields of view at random
• What is the basic structure of all cell membranes?

  A phospholipid bilayer
• Which components make up the fluid-mosaic model of membrane structure?

  Phospholipids, proteins, glycoproteins, glycolipids, and cholesterol
• What role does cholesterol play in cell membranes?

  Cholesterol restricts the movement of other molecules in the membrane
• What are the key components of a phospholipid bilayer in cell-surface membranes?

  • Phospholipid bilayer
  • Membrane proteins
  • Cholesterol
  • Carbohydrate groups
  • Glycoproteins
• What are the various components present within a cell membrane?

  • Carrier protein or co-transporter
  • Fibrous protein e.g. receptors
  • Aquaporin
  • Ion channel
  • Glycoproteins
  • Glycolipids
  • Extrinsic protein
  • Carbohydrate protein
  • Cholesterol
  • Signalling molecule e.g. a hormone
  • Phospholipid
  • Cytoskeleton of fibrous proteins
• What are some functions of glycoproteins in a cell surface membrane?

  Cell adhesion, cell recognition, acting as receptors, and antigens recognized by lymphocytes
• How do glycoproteins contribute to cell adhesion?

  They help cells join together to form tight junctions in certain tissues
• What chemical signals do glycoproteins act as receptors for?

  Peptide hormones and neurotransmitters
• What is the role of glycoproteins as antigens?

  They allow recognition of self and non-self cells by lymphocytes
• What are glycolipids and what are their functions?

  • Lipids with carbohydrate chains attached by a glycosidic bond
  • Antigens for self and non-self cell recognition by the immune system
  • Involved in cell adhesion for tissue formation
  • Can be receptors
• What is a receptor protein in the context of cell membranes?

  • Seven transmembrane protein crossing the membrane 7 times
  • N-terminus (NH₂ end) outside the cell
  • C-terminus (COOH end) inside the cell
• What are the main functions of the cytoskeleton in eukaryotic cells?

  • Supports the shape of the cell
  • Anchors organelles to fixed locations
• What method is used to visualise the cytoskeleton?

  Fluorescent staining
• What are the three types of cytoskeletal fibres present in eukaryotic cells?

  Each type is formed from a different kind of subunit
• What are the different modes of movement across membranes?

  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
  • Active transport
  • Co-transport
• What are the two types of proteins involved in facilitated diffusion?

  Carrier proteins and channel proteins
• What role do carrier proteins play in active transport?

  They carry substances against their concentration gradients
• How is osmosis defined in terms of water potential?

  Osmosis is the movement of water from a region of higher water potential to a region of lower water potential
• Why is ATP hydrolysis important for active transport?

  It provides the energy required to move substances against their concentration gradients
• What is simple diffusion?

  • Passive movement of small, non-polar molecules through the phospholipid bilayer
  • Examples include oxygen, carbon dioxide, and steroid hormones
• What defines passive transport processes across cell membranes?

  • The cell does not use energy from respiration
  • Processes: diffusion, facilitated diffusion, osmosis
• What is active transport?

  The active movement of substances against their concentration gradients using energy
• How does facilitated diffusion differ from simple diffusion?

  • Simple diffusion involves movement directly through the phospholipid bilayer
  • Facilitated diffusion requires carrier proteins for glucose transport
  • Facilitated diffusion requires channel proteins for ion transport
• What is diffusion?

  The net movement of particles down a concentration gradient from higher to lower concentration
• Is diffusion an active or passive transport process?

  Passive
• How does diffusion occur at the plasma membrane?

  • Particles move randomly due to kinetic energy
  • Collide with each other and bounce off in different directions
  • Gradually spread out throughout the available space
• What type of molecules can easily slip through the phospholipid bilayer during diffusion?

  Small, non-polar molecules such as oxygen and carbon dioxide
• Why is the concentration of oxygen inside a cell low during aerobic respiration?

  Because aerobic respiration constantly uses up oxygen
• What is simple diffusion and what molecules typically move by this method?

  • Movement of small, non-polar molecules across membranes
  • Molecules include oxygen, carbon dioxide, and steroid hormones
• What factors affect the rate of diffusion?

  • Greater concentration gradient
  • Larger membrane surface area
  • Increase in temperature
  • Shorter diffusion path (thinner membranes, more channels or carrier proteins)
• What is Fick’s law in the context of diffusion rates?

  The rate of diffusion is proportional to surface area multiplied by the concentration difference divided by the diffusion path length
• How do increasing surface area and concentration gradient affect diffusion rates according to Fick’s law?

  Increasing surface area or concentration gradient will increase diffusion rates
• How does increasing the diffusion path length affect diffusion rates?

  Increasing the distance decreases diffusion rates
• What is the general relationship between molecule size and polarity and their ability to diffuse across a membrane?

  Smaller and less polar molecules diffuse easier and faster
• How do the tails of phospholipid molecules in a cell membrane affect diffusion?

  The hydrophobic tails act as a barrier to polar or charged particles
• What are the characteristics of molecules that diffuse quickly across a membrane?

  • Small size
  • Non-polar nature
  • Examples: oxygen, carbon dioxide
• Which type of molecules face a barrier when trying to diffuse across a phospholipid bilayer?

  Polar or charged particles