The importance of membranes between different types of cells

Question 1

topics

Answer 1

• photosynthesis - chloroplast and thylakoids
• pacinian corpuscle
• digestion - na+
• resting potentials

Question 2

photosynthesis: The importance of membranes between different types of cells

Answer 2

• Membranes play a significant role during the light dependant reaction of photosynthesis which occurs in the thylakoid of the chloroplast.
• The thylakoid membrane contains the chlorophyll pigment, ATP synthase and an electron transport chain embedded inside of it. - The thylakoid membrane provides a large surface area for the electron transport chain which is used during chemiosmosis.
• Energy in the excited electrons from photoionisation is used to pump H+ ions from the stroma to the thylakoids across a diffusion gradient, the H+ ions diffuse back into the stroma via ATP synthase and the energy from this movement is used to synthesise ATP via ADP + Pi → ADP.
• Thus the electron transport chain is embedded in the thylakoid membrane is significant as it ensures that H+ ions cannot diffuse through the phospholipid bilayer in the membrane, and can only diffuse via the ATP synthase channels to produce ATP.
• This ATP is then used to reduce glycerate 3 phosphate into triose phosphate during the Calvin cycle.
• Triose phosphate is then used to synthesise glucose which is a main component of cellulose in plant cell walls.
• If the thylakoid membrane does not prevent the diffusion of H+ ions across the phospholipid bilayer, no glucose is produced the plant cell walls would be unsupported due to lack of cellulose and unable to withstand turgor pressure, turning them flaccid.
• Therefore the transfer of energy via ATP in the plant is essential in photosynthesis to synthesise glucose used to support plant cell walls.

Question 3

receptors: The importance of membranes between different types of cells

Answer 3

• The pacinian corpuscle membrane is significant in generating action potentials in response to pressure.
• Once the pacinian corpuscle detects pressure, the pressure deforms the membrane causing its embedded stretch mediated Na+ ion channels to open.
• If the influx of Na+ ions raises membrane to threshold potential, the cell becomes depolarised producing a generator potential which can then send an action potential along the sensory neurone.
• If the membrane of the pacinian corpuscle would fail at having embeded stretch mediated Na+ ion channels, an action potential would not be generated and thus information regarding pressure and vibration would not be sent to the brain - and no pain would be felt.
• Pain is important as it can activate our instincts of survival - for example a correctly functional pacinian corpuscle would make us aware if we have stepped on glass by releasing action potentials via the sensory neurones to make us feel pain and so reflexively we would take our foot away from the glass.
• Whereas, with a faulty pacinian corpuscle we would not be aware of stepping on glass due to the failure of an action potential firing along the sensory neurone, and so we would continue to walk in glass which could potentially decrease our survival.
• Therefore, the membrane of a pacinian corpuscle is important in releasing action potentials to activate our instinctive responses to protect ourselves from external harm.

Question 4

digestion: The importance of membranes between different types of cells

Answer 4

• The plasma membranes are selectively permeable due to hydrophilic phosphate heads and hydrophobic fatty acid tails, which allows them to control the movement of substances in and out a cell.
• The membranes lining the small intestine are selectively permeable to substances to maximise the reabsorption of nutrients from digestion, the membrane consists of microvilli which increase the surface area for transport protein for the co-transport of glucose.
• Firstly, Na+ ions are actively transported out of the epithilial cell into the blood reducing the sodium ion concentration in the epithiliam cell.
• Na+ ions can then diffuse from the lumen down their concentration gradient into the epithiliam cell, Na+ ions and glucose attatch to the co-transporter protein and are transported into the epithiliam cell (glucose is transported against the concentration gradient).
• This allows for the maintance of a constant blood glucose concentration as glucose is then absorbed into the capillaries via facilitated diffusion.
• It is crucical that animals maintain a constant blood glucose concentration as glucose is a respiratory substrate that helps to release the energy (ATP) required by respiring cells via aerobic respiration.
• A failure in the response of glucagon can lead to hypoglycaemia which is a symptom of diabetes, it involves fatigue and dizziness.
• In conclusion, it is important that animals respond to changes in blood glucose concentration in order to prevent illnesses such as diabetes.

Question 5

nerves: The importance of membranes between different types of cells

Answer 5

• The cell membrane on a post-synaptic cell is also signifcant in maintaining the resting potential of a neurone via the sodium-potassium pump.
• The post synaptic cell membrane contains voltage controlled carrier protein for the diffusion of Na+ and K+ ions.
• Carrier protein actively transport 3 sodium ions out of the axon for every 2 potassium ions that are actively transported in, against the concentration gradient via active transport.
• The movement of ions via the sodium-potassium pump establishes an electrochemical gradient where there is a larger concentration of positive ions outside the axon than there is inside the axon.
• Energy is transferred to the sodium potassium pump via the hydrolysis of ATP as active transport requires energy.
• Failure to maintain the resting potential would cause the cell to become depolarised, resulting in faulty action potentials. If the neurone has prelonged depolarisation due to the failure of a sodium-potassium pump an epileptic seizure can occur.
• Therefore it is important that the resting potential is maintained by the voltage controlled carrier protein in the membranes of neurones to avoid prelonged depolarisation that can cause epilepsy.