transport across cell membranes

• Why is the phospholipid bilayer called the fluid-mosaic model?

  • This bilayer is ‘fluid’ because the phospholipids are constantly moving.
  • Proteins are scattered through the bilayer, like tiles in a mosaic.
• What are the properties of the phospholipid bilayer?

  • Phospholipids have hydrophobic head and a hydrophilic tail. 
  • Molecules automatically arrange themselves into a bilayer — heads face out towards the water on either side of the membrane
  • Small, non-polar substances (CO2, H2O, O2) can diffuse through the membrane and lipid soluble substances (hormones)
  • Centre of bilayer is hydrophobic so water soluble/polar substances (sodium ions) and large molecules (glucose) cannot diffuse through
• What are the properties of cholesterol in a phospholipid bilayer?

  • Cholesterol provides strength and rigidity to the membrane by reducing lateral movement of phospholipids, making the membrane less fluid
  • Prevents membranes from freezing, fracturing and becoming too rigid. 
  • At higher temps it reduces membrane fluidity - preventing water and dissolved ions leaking out of the cell
  • Is hydrophobic, so it helps prevent water soluble substances diffusing across the membrane.
  • Are absent in prokaryotes membranes
• What are intrinsic and extrinsic proteins?

  Extrinsic proteins are protein carriers or channel proteins involved in the transport of molecules.

  Intrinsic proteins provide mechanical support, and are connected to proteins or lipids to make glycoproteins
• What are glycolipids/glycoproteins?

  Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (extrinsically), which enables them to act as receptor molecules. This allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface.
• What do glycolipids and glycoproteins do?

  • Act as antigens (e.g., blood group antigens) to help the immune system recognise self vs non-self cells.
  • Act as receptors for hormones and neurotransmitters (e.g., insulin receptors).
  • Allow cells to respond to specific molecules.
  • The hydrophilic carbohydrate chains attract water and provide stability.
• How does the cell membrane behave at 0°C?

  • Phospholipids have little energy and pack closely, making the membrane rigid.
  • Channel/carrier proteins denature, increasing permeability of the membrane
  • Ice crystals may form and pierce the membrane, increasing permeability when it thaws.
• How does the cell membrane behave at 0-45°C?

  The membrane is partially permeable.

  Phospholipids have more kinetic energy and become more fluid, allowing normal diffusion and transport.
• How does the cell membrane behave above 45°C

  • Phospholipids gain too much kinetic energy, making the membrane highly fluid and unstable.
  • Proteins denature, leading to gaps in the membrane and increased permeability.
  • Substances, including vital ions and molecules, may leak out, disrupting cell function.
• What is diffusion?

  • Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration until equilibrium is reached. 
  • No energy/ATP is required.
  • For molecules to diffuse across the cell membrane they must be lipid soluble/non polar/small.
• What is simple diffusion?

  When molecules diffuse directly through a cell membrane, it’s also known as simple diffusion.
• What factors affect the rate of diffusion?

  • The concentration gradient — A steeper concentration gradient increases the rate of diffusion.
  • The thickness of the exchange surface — the thinner the exchange surface (shorter the distance the particles have to travel), the faster the rate of diffusion. 
  • The surface area — the larger the surface area, the faster the rate of diffusion.

  The temperature - Higher temp increases kinetic energy of molecules, making them move faster and increasing the rate of diffusion
• What is facilitated diffusion?

  Large molecules, ions or polar molecules diffuse through carrier proteins or channel proteins in the cell membrane

  • Facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration. 
  • No energy/ATP is required.

  There are two types of protein involved — carrier proteins and channel proteins. 
• How do carrier proteins work?

  Carrier proteins move large molecules across the membrane, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.

  • First, a large molecule attaches to a carrier protein in the membrane.
  • Then, the protein changes shape.  
  • This releases the molecule on the opposite side of the membrane 
• How do channel proteins work?

  • Channel proteins form pores in the membrane for charged particles to diffuse through (down the concentration gradient). 
  • Forms tubes filled with water enabling water soluble ions to pass through the membrane - however it is selective and only opens to certain ions
  • Different channel proteins facilitate the diffusion of different charged particles/ions.
• What are the factors affecting the rate of facilitated diffusion?

  • The concentration gradient — the higher the concentration gradient, the faster the rate of facilitated diffusion. 
  • The number of channel or carrier proteins — More proteins = faster rate, as more molecules can pass through at once. If all proteins are in use, increasing concentration won’t increase the rate further.
  • Temperature — Higher temperature increases the kinetic energy of molecules, making diffusion faster. However, if the temperature gets too high, proteins may denature, reducing the rate.
• What is an isotonic, hypertonic and hypotonic?

  If two solutions have the same water potential they’re said to be isotonic.

  Solutions with a higher water potential compared with the inside of the cell are called hypotonic.

  Solutions with a lower water potential than the cell are called hypertonic.
• What happens if a cell is placed in a hypotonic and hypertonic solution?

  • If a cell is placed in a hypotonic solution (has a higher water potential) it will swell as water moves into it by osmosis.
  • If a cell is placed in a hypertonic solution (has a lower water potential) it may shrink as water moves out of it by osmosis.
• What happens if a cell is placed in an isotonic solution?

  • Cells in an isotonic solution won’t lose or gain any water — there’s no net movement of water molecules because there’s no difference in water potential between the cell and the solution. 
• What is osmosis?

  • Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potential. 
• What is water potential?

  • Water potential is the potential of water molecules to diffuse out of or into a solution.
  • Pure water has a water potential of zero. Adding solutes to pure water lowers its water potential — so the water potential of any solution is always negative. The more negative the water potential, the stronger the concentration of solutes in the solution.
• How do plant cells behave in a hypotonic solution?

  • Water enters cell through osmosis
  • Volume of cell increases
  • Cell wall able to withstand increased pressure so cell doesn’t burst
  • Pressure increases till cell is turgid
• How do animal cells behave in a hypotonic solution?

  • Water enters cell through osmosis
  • Volume of cell increases
  • No cell wall to withstand increased pressure
  • Cell membrane stretched too far and cell bursts
• How do plant cells behave in a hypertonic soluiton?

  • Water leaves cell through osmosis
  • Volume of cell decreases
  • Protoplast shrinks/pulls away from cell wall
  • Cell is plasmolysed 
• How do animal cells behave in a hypertonic solution?

  • Water leaves cell through osmosis
  • Volume of cell decreases
  • Cell shrinks/shrivels up
• What factors affect the rate of osmosis?

  • The water potential gradient — the higher the water potential gradient, the faster the rate of osmosis. As osmosis takes place, the difference in water potential on either side of the membrane decreases, so the rate of osmosis levels off over time. 
  • The thickness of the exchange surface — the thinner the exchange surface, the faster the rate of osmosis. 
  • The surface area of the exchange surface —
  • Temperature — Higher temperature = Faster osmosis (up to a point). More kinetic energy → Water molecules move faster, increasing the rate of osmosis.
• What is active transport?

  • Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient (low to a high concentration). Carrier proteins and co-transporters are involved in active transport.
  • Requires ATP
• What do carrier proteins do in active transport?

  • Certain molecules bind to receptor sites on carrier protein. 
  • ATP binds to the protein on the inside of the membrane and is hydrolysed into ADP and Pi, causing protein to change shape and open towards inside of the membrane. 
  • This causes the molecule at the start to be released into the other side of the membrane. 
• What factors affect the rate of active transport?

  • The speed of individual carrier proteins — the faster they work, the faster the rate of active transport. 
  • The number of carrier proteins present — the more proteins there are, the faster the rate of active transport.  
  • The rate of respiration in the cell and the availability of ATP — More ATP = Higher rate (since active transport requires energy). ATP is produced in respiration; factors affecting respiration (e.g., oxygen, glucose) will also affect active transport.
• What are co-transporters?

  • Co-transporters are a type of carrier protein. They bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentration gradient. 
• How does co-transport work in the absorption of glucose?

  Glucose is absorbed into the bloodstream in the small intestine. In the mammalian ileum (the final part of a mammal’s small intestine) the concentration of glucose is too low for glucose to diffuse out into the blood. So glucose is absorbed from the lumen (middle) of the ileum by co-transport.