Topic 1 : Cells
1.4 Membrane transport
Passive and active transport
- Passive transport (no energy required) — Movement occurs along a concentration gradient
- Active transport (energy required) — The substance is moved against a concentration gradient
Passive transport: diffusion and osmosis
⇒ Diffusion is the passive net movement of particles from a region of high concentration to a region of low concentration. All living things need to take in nutrients to live and to get rid of waste
- Take in to the cells: Oxygen, Nutrients (food), Water etc.
- Remove from the cells:Carbon dioxide, Urea etc.
⇒ Facilitated Diffusion (passive) – Specific ions and other particles that cannot move through the phospholipid bilayer sometimes move across
protein channels.
- A particular type of diffusion involving a membrane with specific carrier proteins that is capable of combining with the substance to aid its movement.
- Molecules cannot pass the membrane due to its size or charge
- Each protein channel structure allows only one specific molecule to pass through the channel. For example, magnesium ions pass through a channel
protein specific to magnesium ions.
⇒ Example of facilitated diffusion: Osmosis: The movement of water molecules from an area of high water potential to an area of low water potential through a partially permeable membrane.
- Hypertonic solution – Is a solution with a higher osmolarity (higher solute concentration) then the other solution. If cells are placed into a hypertonic
solution, water will leave the cell causing the cytoplasm’s volume to shrink and thereby forming indentations in the cell membrane, leading to weak
structure. The cell in this case is plasmolysed - Hypotonic solution – Is a solution with a lower osmolarity (lower solute concentration) then the other solution. If cells are placed in a hypotonic solution,
the water will rush into the cell causing them to swell and possibly burst, leading to strong structure. The cell in this case is turgid. - Both of the above solutions would damage cells, therefore isotonic solutions are used (same osmolarity as inside the cell)
- Isotonic solution: A solution that has the same salt concentration as cells and blood. The cell in this case is flaccid
- In medical procedures, isotonic solutions are commonly used as: eye drops; packing donating organs, fluid introduction to blood system
Potassium channels
Type of passive transport | Description of membrane |
Simple diffusion | passive net movementof particles from a region of high concentration to a region of low concentration |
Facilitated diffusion | Specific ions and other particles that cannot move through the phospholipid bilayer sometimes move across protein channels |
Osmosis | Only water moves through the membrane using aquaporins, which are proteins with specialized channels for water movement |
Active transport
⇒ Active transport is movement of molecules through a cell membrane from a region of low concentration to a region of high concentration against the concentration gradient using ATP (energy)
- Many different protein pumps are used for active transport. Each pump only transports a particular substance; therefore cells can control what is absorbed and what is expelled.
- Pumps work in a specific direction; substances enter only on one side and exit through the other side.
- Substances enter the pump from the side with a lower concentration.
- Energy from ATP is used to change the conformational shape of the pump.
- The specific particle is released on the side with a higher concentration and the pump returns to its original shape.
The sodium-potassium pump
⇒ The sodium–potassium pump follows a repeating cycle of steps that result in three sodium ions being pumped out of the axon (of neurons) and two potassium ions being pumped in. Each time the pump goes round this cycle it uses one ATP
Endocytosis and exocytosis
⇒ They are all thanks to the fluidity of the plasma membrane
⇒ Endocytosis: The taking in of external substances by an inward pouching of the plasma membrane, forming a vesicle
- Plasma membrane is pinched as a result of the membrane changing shape.
• External material (i.e. Fluid droplets) are engulfed and enclosed by the membrane.
• A vesicle is formed that contains the enclosed particles or fluid droplets, now moves into the cytoplasm.
• The plasma membrane easily reattaches at the ends that were pinched because of the fluidity of the membrane.
• Vesicles that move through the cytoplasm are broken down and dissolve into the cytoplasm.
• Endocytosis is future categorized into phagocytosis (solid excretion); pinocytosis (liquid excretion); receptor-mediated endocytosis (using
receptors)
⇒ Exocytosis: The release of substances from a cell (secretion) when a vesicle joins with the cell plasma membrane.
- After a vesicle created by the rough ER enters the Golgi apparatus, it is again modified, and another vesicle is budded from the end of the Golgi
apparatus, which moves towards the cell membrane. - This vesicle migrates to the plasma membrane and fuses with the membrane, releasing the protein outside the cell through a process called exocytosis.
- The fluidity of the hydrophilic and hydrophobic properties of the phospholipids and the fluidity of the membrane allows the phospholipids from the vesicle
to combine to the plasma membrane to form a new membrane that includes the phospholipids from the vesicle. - Exocytosis can happen continuously (constitutive secretion e.g. saliva) or response to a signal (regulated secretion e.g. insulin)