Module 2 - Biological Membrane Flashcards
Describe the structure of phospholipid bilayer?
- Structure a phospholipid consists of glycerol attached to 3 fatty acids and 1 phosphate group.
- Fatty acids are hydrophobic, making them hydrophobic tail
- The phosphate group is highly charged, making them polar resulting in hydrophilic head.
- So a phosphate group contains both a hydrophobic region (tail) and hydrophilic region (head).
- In an aqueous solution, phospholipids arrange themselves in bilayers.
- the hydrophilic head facing outwards, toward the water molecules, interacting with them
- the hydrophobic tails facing inwards, away from the water molecules, due to its non polar region
Describe the roles of the membranes at the surface of cells?
- Cell signalling and communication
- Controls what enters and exits substances (selectively permeable)
- Involved in cell recognition like by the immune system
- May contain receptors like hormones or neurotransmitter
Describe the roles of the membrane within the cells?
- Compartmentalisation; separates organelles from the cytoplasm
- Controls what enters/leaves the organelles
- Provides surface for reactions to take place
Explain how the structure of the plasma membrane allows it to carry out its functions. (5 marks)
The plasma membrane is composed with phospholipid bilayer, with hydrophobic tails facing inwards and hydrophilic heads facing outwards. This structure forms a selectively permeable barrier, where non-polar molecules like O2 and CO2 diffuse freely across the membrane while restricting the passage of polar or charged molecules. Proteins embedded within the membrane serve various roles such as channel proteins for passive transport and carrier proteins for active transport using ATP. These proteins help to maintain the movement of ions and larger molecules across the membrane. Glycoproteins and glycolipids are involved in cell signalling and cell recognition, as the cell responds to external signals. Cholesterol molecules are scattered with the phospholipid bilayer, providing the membrane stability and fluidity, ensuring the membrane is flexible while maintaining its integrity. (The fluid mosaic structure also allows flexibility and movement, which is important for processes like endocytosis and exocytosis.)
A student states that membranes are only found on the surface of cells. Evaluate this statement? (4 marks)
This statement is incorrect because the membranes are also found inside the cell, in organelles such as nuclear envelope and mitochondria. These membranes are important for compartmentalisation and they also create an environment specific reactions, which increases the efficiency of the processes.
Explain how cell surface membranes contribute to cell signalling? (3 marks)
The cell surface membrane contains glycoproteins and Glycolipids act as receptor proteins that can detect specific signal molecules like hormones. The receptor proteins have a specific shape complementary to the signal molecule, allowing it to bind. This binding triggers a response inside the cell, such as activating enzymes or gene expression changes.
The membranes inside the mitochondria are highly folded.
Suggest two advantages of this? (2 marks)
Increased surface area for enzyme carriers and increases production of ATP during aerobic respiration.
What is meant by cell signalling?
Communications between cells
List the components of cell surface membrane?
Glycolipids
Glycoproteins
Cholesterol
Proteins
Phospholipids
Describe the routes that water molecules take through the cell surface membrane. (2 marks)
Water molecules can cross the cell surface membrane in two routes; either by simple diffusion across the phospholipid bilayer or through protein channels called aquaporins. Both processes rely on osmosis.
Explain why plant cells do not burst when they are left in pure water? (2 marks)
Plant cell walls do not burst due to the presence of rigid cell wall and turgor pressure which limits the uptake of water.
List how substances move across cell surface membranes by different transport mechanisms?
Simple diffusion
Facilitated diffusion
Active transport
Osmosis
Endocytosis and exocytosis
Describe and explain how substances move across cell surface membranes by different transport mechanisms? (6 marks)
- Small non-polar moves across the membrane by simple diffusion, from high conc. gradient to low conc. gradient, directly through phospholipid bilayer.
- Polar or charged molecules move across the membrane by facilitated diffusion, which involves carrier proteins and channel proteins. It doesn’t require energy from ATP to move substances, as it’s a passive process.
- Active transport involves carrier proteins and does require energy from ATP to move substance against the conc. gradient.
- Osmosis is the diffusion of water molecules across a partially permeable membrane, from a region of high water potential to a region of low water potential.
- Endocytosis and exocytosis are bulk transport processes that move large molecules like proteins into and out of the cell using vesicles and it does require energy from ATP as its a active process.
Describe how you would carry out an investigation to determine the effect of temperature on membrane permeability in beetroot cells? (6 marks)
- Using a cork borer, cut an equal-sized of beetroots in 5 pieces and rinse them to remove excess pigment.
- Place each beetroot in 5 test tubes, with the same volume of distilled water in it.
- Place each test tube in a water bath, set at a different temp. (0°C, 20°C, 40°C, 60°C, 80°C), for same period of time, ensuring all beetroot pieces are submerged.
- After incubation, remove the beetroot pieces from each test tube and measure the absorbance of coloured solution using a colorimenter.
- Higher abundance indicates more pigments has leaked out, meaning greater permeability.
- This results can be plotted in a graph of temperature VS abundance.
Describe the fluid mosaic model of membrane structure? (4 marks)
Membranes are composed with phospholipid bilayers, with hydrophobic tails facing inwards and hydrophilic heads facing outwards. Proteins are embedded within the phospholipids bilayer, some scattered within the membrane (intrinsic) and some on the surface (extrinsic). It’s ‘fluid’ because phospholipids are constantly moving and its ‘mosaic’ because proteins scattered within the phospholipids.
Explain the roles of the following components in the cell membrane:
(a) Phospholipids
(b) Cholesterol
(c) Glycoproteins
(6 marks)
a) Phospholipids:
- Forms the basic structure of the phospholipid bilayer
- Provides a barrier to most water soluble substances, allowing selective permeability.
b) Cholesterol
- Fits between the phospholipids, stabilising the membrane
- Maintains fluidity, especially at low temp.
c) Glycoproteins
- Involved in cell signalling and cell recognition
- Can act as a receptors for hormones pr neurotransmitter
Describe how membrane proteins are involved in the functions of cell membranes? (4 marks)
- Channel proteins allows passive transport like facilitated diffusion for polar molecules or ions.
- Carrier proteins allows active transport using ATP.
- Glycoproteins and glycolipids act as receptors for hormones or signalling molecules.
- Other membrane proteins function as enzymes, catalysing the reactions within the membrane or on its surface.
Explain how the structure of the plasma membrane allows it to be both stable and flexible? (3 marks)
- Phospholipid bilayer allows flexibility as phospholipids can move laterally.
- Cholesterol adds stability by interacting with fatty acid tails and reducing movement.
- The mosaic arrangement of proteins provides support and dynamic function.
State and explain two ways cholesterol affects the properties of the cell membrane? (4 marks)
- Reduce the fluidity by binding with the phospholipid tails making it more tightly packed.
- Increases the membrane stability, especially at varying temp. preventing the membrane becoming too fluid or too rigid.
Describe the structure and function of glycoproteins and glycolipids in the plasma membrane? (6 marks)
- For glycoproteins, proteins are attached to the carbohydrates chain.
- For glycolipids, lipids are attached to the carbohydrates chain.
- Both are involved in cell recognition.
- Both act as receptors in cell signalling.
- Both are found on the outer surface of the membrane.
- They also help with cell adhesion, enabling tissue formation.
How does the fluid mosaic model explain the ability of membranes to self heal after minor damage? (3 marks)
Phospholipid bilayers are not too rigidly fixed, allowing them to move laterally. The bilayer naturally reseals itself, due to the hydrophobic effect, where the tail is facing inwards avoiding the water molecules. The fluid nature allows the rearrangements of components to cover small tears or gaps.
Compare the roles of intrinsic and extrinsic proteins in the plasma membrane? (4 marks)
- Intrinsic proteins are scattered throughout the membrane, while extrinsic proteins are on the surface.
- Intrinsic proteins act as channels or carrier proteins to facilitate molecules across the membrane.
- Some intrinsic proteins like glycolipids or glycoproteins act as receptors or enzymes.
- Extrinsic proteins can be involved in cell signalling or act as enzymes.
Some bacteria living in cold environments have membranes that contain more unsaturated fatty acids than bacteria in warmer climates.
Explain how this adaptation helps the bacteria survive in cold environments. (6 marks)
- Bacteria use more unsaturated fatty acids in cold climates.
- These fatty acids have kinks in their hydrophobic tails
- These kinks prevent phospholipids from being tightly packed together.
- This maintains the fluidity of the membrane.
- Membrane must remain partially fluid to carry out protein functions.
- Kinks ensure the transport of substances continues efficiently.
- Also allows membrane to remain semi-permeable and functional in cold conditions.
Researchers disrupted cholesterol in red blood cell membranes and found that the cells became more fragile and ruptured easily.
Using your knowledge of membrane structure, explain why this occurred? (5 marks)
- Cholesterol provides stability to the membrane.
- Without cholesterol, phospholipids move too freely, making the membrane too fluid.
- This reduces the membrane’s mechanical strength.
- RBC can experience pressure and deformation of blood vessels.
- Fragile membrane can rupture easily due to stress without cholesterol.
