Topic 2.5 Biological Membranes

Question 1

Explain the basic membrane structure.

Answer 1

-formed by a phospholipid bilayer
-hydrophilic phosphate heads for the inner and outer surface of membrane
-fatty acid tails are sandwiched between to form a hydrophobic core inside membrane

Question 2

Explain the fluid mosaic model.

Answer 2

The fluid mosaic model describes the structure of biological membranes as flexible, dynamic arrangement of phospholipids, proteins, and carbohydrates. The term ‘fluid’ refers to the mobility of the phospholipids and proteins within the bilayers, while ‘mosaic’ highlights the scattered distribution of proteins embedded in or associated with the membrane.

Question 3

List all components of the membrane.

Answer 3

• phospholipids
• cholesterol
• glycolipids
• proteins
• glycoproteins

Question 4

Give the role of phospholipids

Answer 4

• form basic structure of the bilayer, provide semi-permeable barrier
• hydrophilic heads face outwards, whereas hydrophobic tails face inwards preventing water-soluble substances from crossing freely
• allows diffusion of small, non-polar molecules

Question 5

Explain the role of cholesterol.

Answer 5

• embedded between phospholipids in animal cell membranes
• regulates fluidity by preventing membranes from becoming too rigid or too fluid
• provides mechanical stability
• reduces membrane permeability to water-soluble molecules

Question 6

Explain the role of Glycolipids

Answer 6

-Lipids with carbohydrate chains attached
- Acts as antigens in cell recognition
- contributes to cell signalling and adhesion between cells.

Question 7

Explain the role of channel proteins

Answer 7

-facilitate passive transport of ions and molecules

Question 8

Explain the role of enzymes

Answer 8

• catalyse specific reactions at the membrane surface

Question 9

Explain the role of carrier proteins

Answer 9

• involved in active transport and facilitated diffusion

Question 10

Explain the role of receptors.

Answer 10

• bind to cell signalling molecules, triggering intracellular responses

Question 11

Explain the role of glycoproteins.

Answer 11

• glycoproteins are proteins with carbohydrate chains attached
• function as receptors for hormones, neurotransmitter, and drugs
• facilitate cell signalling and recognition
• also aid in cell adhesion.

Question 12

Explain how membranes act as partially permeable membranes between the cell and its environment.

Answer 12

The cell surface membrane (plasma membrane) regulates the entry and exit of substances maintaining homeostasis. It selectively allows certain molecules to enter while preventing harmful substances or waste products, such as toxins or excess ions, from entering or leaving.

Question 13

Explain how membranes act as partially permeable membranes between organelles and the cytoplasm.

Answer 13

Compartmentalisation: membranes create distinct environments within organelles, such as lysosomes and mitochondria, necessary for specific metabolic reactions.

For example, the nuclear membrane separates the DNA and processes like transcription from the cytoplasm, protecting genetic material.

Question 14

Explain how membranes act as a partially permeable membrane within organelles

Answer 14

Sub-compartmentalisation within organelles enables specialised functions.

For example, in mitochondria, the inner membrane forms cristae, which house the electron transport chain for ATP synthesis.

Another example is in chloroplasts, the thylakoid membranes provide sites for photosynthetic reactions.

Question 15

Explain how membranes house enzymes for respiration

Answer 15

Respiration: Inner mitochondrial membrane contains enzymes for the electron transport chain and ATP synthase for oxidative phosphorylation.

Question 16

Explain how membranes house enzymes during photosynthesis.

Answer 16

Photosynthesis: Thylakoid membranes in chloroplasts host photosystems and electron carriers.

Question 17

Explain how membranes house digestive enzymes.

Answer 17

Digestive Enzymes: Membranes of cells lining the gut carry enzymes like maltase for digestion.

Question 18

Explain the role of membranes detecting cells for cell signalling.

Answer 18

Membrane receptors (glycoproteins or glycolipids) bind to signalling molecules (e.g., hormones, neurotransmitters).

For example, insulin binds to its receptors on liver cells to trigger glucose uptake.

Question 19

Explain the role of membranes during signal transaction for cell signalling.

Answer 19

Binding of signalling molecules triggers intracellular responses via a cascade of reactions.

For example, binding of adrenaline to its receptors activates a secondary messenger system.

Question 20

Explain the role of membranes coordinating responses for cell signalling

Answer 20

Signals allow cells to coordinate activities, such as immune responses or growth regulation.

Question 21

Explain what membrane-bound receptors are.

Answer 21

Membrane-bound receptors are proteins or glycoproteins located on the cell surface that bind specifically to hormones, drugs, or other signalling molecules. They have specific shapes complementary to the hormone or drug, ensuring only the correct molecule binds.

Question 22

What is Hormone binding?

Answer 22

Hormone binding: hormones act as signalling molecules, binding to specific receptors to trigger a response inside the cell.

Question 23

What is drug binding?

Answer 23

Drug binding: drugs mimic or block natural signalling molecules by binding to these receptors.

Question 24

How do moderate temperatures affect membrane structure and permeability?

Answer 24

Membranes are fluid and semi-permeable, which is ideal for normal functioning.

Increased temperature provides phospholipids with more kinetic energy, slightly increasing permeability.

Question 25

How do low temperatures affect membrane structure and permeability?

Answer 25

Phospholipids have less kinetic energy, so they pack more closely together, making the membrane less fluid and more rigid. At very low temperatures, ice crystals can form, puncturing the membrane and increasing permeability when the cell thaws.

Question 26

How do high temperatures affect membrane structure and permeability

Answer 26

Phospholipids gain significant kinetic energy, making the membrane more fluid and less stable. Proteins in the membrane can denature, disrupting their function (e.g., loss of channel/carrier protein activity) As a result, the membrane becomes leaky, allowing substances to pass through uncontrollably.

Question 27

How does water as a solvent affect membrane structure and permeability.

Answer 27

Maintains membrane integrity because phospholipids naturally arrange themselves into a bilayer in an aqueous environment.

Question 28

How do organic solvents affect membrane structure and permeability.

Answer 28

These disrupt the phospholipid bilayer by dissolving lipids, breaking down the membrane structure. Ethanol, for instance makes the membrane more permeable, allowing substances to leak in or out. Higher concentrations of solvents cause greater disruption, leading to cell damage or death.

Question 29

Explain the effect of solvent concentration on membrane structure and permeability

Answer 29

Low concentrations of alcohol can destabilise the membrane slightly increasing permeability without completely destroying it. High concentrations can dissolve the membrane entirely, leading to a totals loss of function.

Question 30

What is Simple diffusion?

Answer 30

Simple diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration through a phospholipid bilayer down a concentration gradient, this is a passive process and does not require ATP. Only small, non-polar molecules and lipid soluble substances can diffuse freely.

Question 31

What is Facilitated diffusion?

Answer 31

Facilitates diffusion is the net movement of larger or charged particles down a concentration gradient through membrane proteins either channel or carrier protein). This is a passive process, so it does not require ATP

Question 32

What is Active transport?

Answer 32

Active transport is the movement of molecules or ions against a concentration gradient (from low to high concentration) through carrier proteins, using energy from ATP. ATP provides energy to change the shape of the carrier protein, enabling the movement of substances.

Question 33

What is Endocytosis?

Answer 33

Endocytosis is the process by which the cell takes in large particles or liquids by engulfing them in a vesicle. It requires ATP for vesicle formation.

Question 34

Explain the two types of Endocytosis.

Answer 34

- Phagocytosis: "Cell eating" (e.g., engulfing large particles like bacteria by white blood cells). - Pinocytosis: "Cell drinking" (e.g., uptake of extracellular fluids).

Question 35

What is Exocytosis?

Answer 35

Exocytosis is the process by which vesicles containing substances fuse with the cell membrane to release their contents outside the cell. It requires ATP for vesicle movement and membrane fusion. An example of this would be the secretion of enzymes, hormones (e.g., insulin), or neurotransmitters.

Question 36

What is Osmosis?

Answer 36

Osmosis is the movement of water molecules from a region of higher (less negative) water potential to a region of lower (more negative) water potential across a partially-permeable membrane, down the water potential gradient. Occurs through the phospholipid bilayer or aquaporins. This is a passive process so it does not require ATP.

Question 37

How do different water potentials affect animal cells

Answer 37

Hypotonic solution (higher water potential outside): - Water moves into the cell by osmosis. - Cell swells and may burst (lysis) due to lack of a rigid cell wall. Isotonic solution (equal water potential): - No net movement of water; the cell maintains its shape. Hypertonic solution (lower water potential outside): - Water moves out of the cell by osmosis. - Cell shrinks (crenation).

Question 38

How do different water potentials effect plant cells?

Answer 38

Hypotonic solution (higher water potential outside): - Water moves into the cell by osmosis. - Vacuole enlarges, and the cell becomes turgid (swollen but supported by the cell wall). Isotonic solution (equal water potential): - No net movement of water; the cell becomes flaccid (limp). Hypertonic solution (lower water potential outside): - Water moves out of the cell by osmosis. - Cytoplasm and vacuole shrink, and the cell undergoes plasmolysis (membrane pulls away from the wall).

Question 39

List all the factors that effect the rate of diffuison

Answer 39

- SA:V ratio - concentration gradient - temperature - molecule size - medium of diffusion

Question 40

Explain how surface area to volume ratio affects diffusion.

Answer 40

Smaller cells or structures with a higher SA:V ratio facilitate more efficient diffusion. This is because a larger surface area relative to volume allows more substances to diffuse per unit time.

Question 41

Explain how temperature affects the rate of diffusion

Answer 41

Higher temperatures provide molecules with more kinetic energy, leading to increased movement and, consequently, a higher rate of diffusion.

Question 42

Explain how Concentration gradient affect the rate of diffusion

Answer 42

A steeper concentration gradient (a larger difference in concentration between two regions) increases the rate of diffusion. Molecules move more rapidly from areas of high concentration to areas of low concentration when the gradient is steep.

Question 43

Explain how molecule size affects the rate of diffusion

Answer 43

Smaller molecules diffuse more rapidly than larger ones due to less resistance as they move through the medium.

Question 44

Explain how medium of diffusion affects the rate of diffusion

Answer 44

The nature of the medium (e.g., solid, liquid, gas) affects diffusion rates. In general, diffusion occurs fastest in gases, slower in liquids, and slowest in solids due to differences in particle spacing and movement.