B2.1 membrane and membrane transport

Question 1

What are the primary functions of cell membranes?

Answer 1

1- protect the cell from harmful factors
2- maintain beneficial substances inside
3- regulate what enters and exits the cell
4- control substance movement
5- contain receptors for monitoring surroundings
6- channels for molecule transport
7- carriers for homeostasis
8- structures for cell communication within and outside the organism.

Question 2

What was proposed by Seymour J. Singer and Garth L. Nicolson in 1972 regarding cell membrane structure?

Answer 2

In 1972, Singer and Nicolson proposed the fluid mosaic model, suggesting that proteins are inserted into the phospholipid layer and do not form a separate layer on the phospholipid bilayer surfaces. They described the membrane as a mosaic of proteins floating in a fluid bilayer of phospholipids.

Question 3

What evidence was used to revise the Singer-Nicolson model of cellular membranes?

Answer 3

Evidence from electron microscopy and studies of cell behavior in different environments and solutions, aided by the ability to culture cells in laboratories, contributed to the revision of the Singer-Nicolson model.

Question 4

What is the currently accepted model of cellular membranes?

Answer 4

The currently accepted model is the fluid mosaic model, which describes all cellular membranes, including plasma membranes and organelle membranes, as flexible structures made up of various molecules, allowing them to function correctly.

Question 5

What components form the backbone of the cell membrane?

Answer 5

The backbone of the cell membrane comprises glycerol, two fatty acids, and a phosphate group linked to a polar organic alcohol.

Question 6

Describe the water solubility of phospholipids.

Answer 6

Phospholipids have a hydrophilic (water-loving) side due to the phosphate group, and a hydrophobic (water-fearing) side because of the fatty acids

Question 7

What term describes molecules with both hydrophilic and hydrophobic regions?

Answer 7

Molecules possessing both hydrophilic and hydrophobic regions are termed amphipathic.

Question 8

What causes phospholipids to form a bilayer structure?

Answer 8

Phospholipids naturally align as a bilayer due to the attraction between their hydrophobic regions and their hydrophilic regions to the surrounding water in the cytoplasm or extracellular fluid.

Question 9

What contributes to the fluidity of the cell membrane?

Answer 9

The relatively weak attraction between the hydrophobic fatty acid “tails” allows the cell membrane to remain fluid or flexible, enabling animal cells to have variable shapes and facilitating processes like endocytosis.

Question 10

What maintains the structure of the cell membrane?

Answer 10

The overall structure of the membrane is maintained by the relationship between its chemical makeup and the chemical properties of water.

Question 11

Why is it challenging for large molecules to move through the phospholipid bilayer?

Answer 11

The tight packing of molecules in the bilayer makes it difficult for large molecules to pass through it.

Question 12

What makes it hard for hydrophilic molecules like ions to move through the cell membrane?

Answer 12

Hydrophilic molecules struggle to pass through the membrane due to the hydrophobic region in the middle of the phospholipid bilayer.

Question 13

How does the bilayer act as a barrier for molecules?

Answer 13

The phospholipid bilayer forms an effective barrier between the inside and outside of the cell, allowing the cell to control the passage of large and polar molecules.

Question 14

What is diffusion in the context of membrane transport?

Answer 14

Diffusion involves particles moving from areas of higher concentration to lower concentration, often across a membrane in living systems.

Question 15

How does oxygen move into and carbon dioxide move out of cells through diffusion?

Answer 15

Oxygen diffuses into cells due to a lower concentration inside compared to outside, while carbon dioxide diffuses out because its concentration is higher inside the cell.

Question 16

What enables the easy diffusion of oxygen and carbon dioxide through the cell membrane?

Answer 16

Oxygen and carbon dioxide, being small and uncharged molecules, can move easily between phospholipid molecules of the membrane, facilitating their diffusion.

Question 17

What contributes to the diverse functions of cellular membranes?

Answer 17

Proteins embedded in the fluid matrix of the phospholipid bilayer create diversity in membrane function, forming a mosaic or tile-like effect characteristic of cellular membranes.

Question 18

What are the two major types of proteins found in cellular membranes?

Answer 18

Integral proteins and peripheral proteins are the two major types of proteins found in cellular membranes.

Question 19

Describe the structure and placement of integral proteins in the membrane.

Answer 19

Integral proteins exhibit amphipathic character, with hydrophobic regions within the phospholipid backbone and hydrophilic regions exposed to the water molecules on either side of the membrane.

Question 20

How do peripheral proteins differ from integral proteins in their interaction with the membrane?

Answer 20

Peripheral proteins do not penetrate the hydrophobic region but remain bound to the membrane’s surface, found on both the inner and outer sides. They are often anchored to integral proteins.

Question 21

What defines hormone-binding proteins and their function?

Answer 21

Hormone-binding proteins possess specific shapes that match particular hormones. Their attachment triggers a change in protein shape, relaying messages to the cell interior.

Question 22

How do enzymatic proteins function within cellular membranes?

Answer 22

Enzymatic proteins are found on membrane surfaces and often organize into pathways, catalyzing metabolic reactions within the cell.

Question 23

What role do cell adhesion proteins play, and what types of junctions do they form?

Answer 23

Cell adhesion proteins create connections between cells, forming junctions such as gap junctions and tight junctions. They often carry attached carbohydrate molecules for cell recognition.

Question 24

Describe the function of cell-to-cell communication proteins.

Answer 24

Proteins of this type span membranes, creating passageways for substance transportation between adjacent cells.

Question 25

How do channel-forming proteins contribute to cellular functions?

Answer 25

Channel-forming proteins form passageways through membranes, facilitating the transport of substances across the membrane.

Question 26

What is the role of pump proteins in cellular processes, and what energy source do they require?

Answer 26

Pump proteins are involved in active transport, requiring energy in the form of ATP to shuttle substances across the membrane.

Question 27

What are the two general types of cellular transport?

Answer 27

Passive transport and active transport

Question 28

How does passive transport differ from active transport in terms of energy requirement?

Answer 28

Passive transport does not require cellular energy (ATP), while active transport demands ATP for its processes.

Question 29

Describe the movement involved in passive transport.

Answer 29

Passive transport involves movement from high to low concentration along a concentration gradient, driven by the kinetic energy of molecules until equilibrium is achieved.

Question 30

What distinguishes active transport from passive transport regarding concentration gradient and equilibrium?

Answer 30

Active transport moves substances against the concentration gradient, requiring energy, and does not lead to equilibrium.

Question 31

What defines osmosis in cellular transport?

Answer 31

Osmosis is a passive transport process involving the movement of water across a partially permeable membrane, driven by a concentration gradient of solutes.

Question 32

How does a hypertonic solution differ from a hypotonic solution in the context of osmosis?

Answer 32

A hypertonic solution has a higher solute concentration compared to a hypotonic solution, causing water to move from hypotonic to hypertonic regions across the membrane.

Question 33

What happens when isotonic solutions are separated by a partially permeable membrane?

Answer 33

In isotonic solutions separated by a partially permeable membrane, no net movement of water occurs as equilibrium is achieved.

Question 34

How does the cell membrane facilitate the movement of water molecules through osmosis?

Answer 34

The cell membrane, impermeable to many solute molecules, permits the movement of water through specialized protein channels called aquaporins, enabling water to move across the membrane.

Question 35

Explain the movement of water molecules in osmosis concerning concentration gradients.

Answer 35

Water molecules move passively across the membrane due to a higher concentration gradient to a lower concentration, facilitated by aquaporin channels, resulting in a net movement towards the lower concentration area.

Question 36

What are the two types of integral proteins involved in facilitated diffusion?

Answer 36

Carrier proteins and channel proteins are the two types of integral proteins involved in facilitated diffusion.

Question 37

How do carrier proteins function in facilitated diffusion?

Answer 37

Carrier proteins change shape to transport specific substances across the membrane, working along or against concentration gradients. They transport both water-soluble and insoluble molecules.

Question 38

How do channel proteins differ from carrier proteins in function and structure?

Answer 38

Channel proteins have pores allowing molecules of appropriate size and charge to pass through. They don't change shape like carrier proteins, opening and closing based on chemical or mechanical signals. Channel proteins specifically transport water-soluble molecules and are selective for the ions they carry.

Question 39

What contributes to the selective permeability of cell membranes?

Answer 39

The presence of channel proteins and carrier proteins makes cell membranes selectively permeable, allowing specific ions through at certain times.

Question 40

What factors influence the rate of facilitated diffusion?

Answer 40

The rate of facilitated diffusion depends on factors such as the concentration gradient across the membrane, the number of active carrier proteins, the openness of channel proteins, and the types of substances being transported.

Question 41

What is the energy source required for active transport, and what does it often involve?

Answer 41

Active transport requires ATP for energy and often moves substances against their concentration gradients.

Question 42

How does active transport help in maintaining different concentrations inside and outside the cell?

Answer 42

Active transport allows the cell to maintain different concentrations of molecules inside compared to outside by moving substances against concentration gradients.

Question 43

What enables active transport to occur at the cellular membrane level?

Answer 43

Highly selective proteins in the membrane bind with specific substances, facilitating active transport processes.

Question 44

How do different protein carriers involved in active transport differ in their functions?

Answer 44

Various protein carriers involved in active transport differ in their mechanisms of moving substances across the membrane.

Question 45

What is the sodium-potassium pump, and what type of transport does it demonstrate?

Answer 45

The sodium-potassium pump is an example of active transport that uses ATP to move ions against their concentration gradients.

Question 46

Why is the sodium-potassium pump crucial in nerve cells (neurons)?

Answer 46

The pump is crucial in neurons to ensure animals can appropriately respond to environmental stimuli by maintaining proper ion concentrations.

Question 47

What factors determine the ease of passive substance movement across a membrane?

Answer 47

The size and charge of substances influence their ability to passively cross a membrane. Small, non-polar substances move easily, while polar or large molecules face difficulty in crossing.

Question 48

Provide examples of substances that move easily and those that struggle to passively cross membranes.

Answer 48

Small, non-polar gases like oxygen, carbon dioxide, and nitrogen move easily, while ions (like chloride, potassium, sodium) and large molecules (such as glucose and sucrose) face difficulty in passive membrane crossing.

Question 49

Why is diffusion of small, simple molecules not selective?

Answer 49

Diffusion of small, simple molecules is not selective because they move freely without much resistance across the membrane.

Question 50

What are glycolipids and glycoproteins, and where are they found in cell membranes?

Answer 50

Glycolipids are membrane phospholipids with attached carbohydrate chains, while glycoproteins are cell membrane proteins with carbohydrate chains. These chains are exclusively found on the extracellular side of the cell membrane.

Question 51

What roles do glycoproteins and glycolipids play in cellular functions?

Answer 51

Glycoproteins and glycolipids aid in cell identification and cell adhesion, facilitating cells sticking to each other due to the diverse sequences and structures of their carbohydrate chains.

Question 52

What role does cholesterol play in cell membranes?

Answer 52

Cholesterol in animal cell membranes influences membrane fluidity by interacting with the fatty acid tails of the phospholipid bilayer, allowing effective functioning across a wider temperature range.

Question 53

How does cholesterol affect membrane fluidity?

Answer 53

Cholesterol modulates membrane fluidity, ensuring optimal function by interacting with the hydrophobic tails of phospholipids, particularly at different temperatures.

Question 54

What distinguishes the role of cholesterol in animal cell membranes from that in plant cell membranes?

Answer 54

Plant cells lack cholesterol and instead rely on saturated or unsaturated fatty acids to maintain proper membrane fluidity, whereas cholesterol aids in this function in animal cells.

Question 55

What is the similarity between the consistency of membranes and olive oil?

Answer 55

Membranes are akin to olive oil in their consistency, needing fluidity for proper functionality, which is influenced by the presence and interaction of cholesterol in animal cell membranes.

Question 56

How do phospholipids contribute to the stability of the lipid bilayer?

Answer 56

Phospholipids form stable bilayers due to the arrangement of their hydrophilic polar heads facing aqueous environments and hydrophobic tails forming the inner layer away from water, stabilized by hydrogen bonding with water molecules.

Question 57

What allows for the fluidity of the cell membrane?

Answer 57

Weak hydrogen bonding of the polar heads with water and the relatively free movement of individual phospholipids and unanchored proteins facilitate the fluidity of the cell membrane.

Question 58

How do unsaturated fatty acids differ from saturated fatty acids in terms of membrane behavior?

Answer 58

Unsaturated fatty acids, with double bonds causing kinks in their structure, have lower melting points and provide membrane fluidity, ideal for cooler temperatures. Saturated fatty acids, being straighter, pack tightly, increasing membrane density and strength at higher temperatures.

Question 59

What effect do temperature changes have on the composition of fatty acids in the cell membrane?

Answer 59

In cooler temperatures, cell membrane fatty acids have more unsaturated bonds, while higher temperatures favor mostly saturated bonds, influencing membrane fluidity and strength accordingly.

Question 60

Why might the exterior plasma membrane of cells have a different fatty acid composition compared to interior cell membranes?

Answer 60

The exterior membrane differs due to less exposure, while the interior is more protected. This difference in exposure influences the fatty acid composition of the membranes.

Question 61

How do individual cells like bacteria handle temperature changes concerning their membranes?

Answer 61

Individual cells, such as bacteria, vulnerable to temperature fluctuations, use mechanisms to maintain membrane fluidity, including enzymes like fatty acid desaturases that increase double bonds in fatty acid tails.

Question 62

What role do fatty acid desaturases play in bacterial membranes?

Answer 62

Fatty acid desaturases are enzymes present in bacterial membranes. They accelerate reactions that increase the number of double bonds in fatty acid tails, aiding in maintaining membrane fluidity under variable temperature conditions.

Question 63

What is the role of cholesterol in membrane fluidity?

Answer 63

Cholesterol molecules, closely associated with fatty acid tails, stabilize membranes at higher temperatures and maintain flexibility at lower temperatures, impacting plasma membrane fluidity.

Question 64

How does the presence of cholesterol differ in plasma membrane and endoplasmic reticulum (ER) membranes?

Answer 64

There are more cholesterol molecules in the plasma membrane compared to the endoplasmic reticulum (ER) membrane due to the plasma membrane being exposed to more temperature extremes.

Question 65

Why do plant cells have little to no cholesterol in their cell membranes?

Answer 65

Plant cells possess cell walls that stabilize their plasma membranes, eliminating the need for cholesterol to maintain membrane stability.

Question 66

What are endocytosis and exocytosis, and how do they operate?

Answer 66

Endocytosis brings macromolecules into the cell through plasma membrane pinching and vesicle formation, while exocytosis releases molecules by vesicle fusion with the plasma membrane.

Question 67

How does the fluidity of the plasma membrane relate to endocytosis and exocytosis?

Answer 67

Both processes depend on the fluid nature of the plasma membrane, allowing membrane shape changes and vesicle formation or fusion.

Question 68

Explain the steps involved in protein exocytosis.

Answer 68

1. Proteins produced in the rough ER enter the ER lumen and are packed into vesicles. 2. Vesicles carrying proteins fuse with the cis side of the Golgi apparatus. 3. Proteins undergo modifications as they move through the Golgi and exit in vesicles from the trans face. 4. Vesicles with modified proteins fuse with the plasma membrane, leading to the release of contents from the cell.

Question 69

What are gated ion channels, and how do they operate?

Answer 69

Gated ion channels are specialized channels in cell membranes that open or close due to chemical and electrical stimuli. They control the electrical potential across membranes, crucially in neurons and muscles.

Question 70

Explain the function of nicotinic acetylcholine receptors.

Answer 70

Nicotinic acetylcholine receptors are neurotransmitter-gated ion channels that bind to acetylcholine. Upon binding, these receptors open, allowing positive ions like Na+, K+, and Ca2+ to pass through, altering membrane potential to generate nerve impulses or muscle movements.

Question 71

How do neurotransmitters function in nerve-muscle junctions?

Answer 71

Neurotransmitters like acetylcholine, acting at nerve-muscle junctions, bind to receptors like nicotinic acetylcholine receptors, triggering the movement of positive ions and facilitating muscle movement through changes in membrane potential.

Question 72

What characterizes myasthenia gravis, an autoimmune disorder?

Answer 72

Myasthenia gravis involves the production of antibodies against nicotinic acetylcholine receptors, which bind to these receptors, reducing the body's response to acetylcholine. This results in incomplete muscle movements and responses.

Question 73

What are voltage-gated channel proteins, and how do they function?

Answer 73

voltage-gated channel proteins open in response to changes in membrane polarity. Examples include Na+ and K+ channels. They open and close in response to electrical stimuli, allowing rapid movement of specific ions across the membrane for brief periods.

Question 74

Describe the sequence of events involving sodium and potassium channels during membrane depolarization and repolarization.

Answer 74

Sodium channels open first during depolarization, allowing sodium ions to move from outside to inside the neuron, which depolarizes the membrane. They quickly close. Potassium channels open more slowly, letting potassium ions move from inside to outside, restoring the membrane's normal potential in a process called repolarization.

Question 75

What is the sodium-potassium pump, and how does it function?

Answer 75

The sodium-potassium pump is an active transporter that uses ATP to move ions against their concentration gradient. It maintains higher potassium concentrations inside cells and higher sodium concentrations outside. This creates a membrane potential, crucial for nerve impulses. The pump cycles involve binding sodium ions, ATP hydrolysis, shape changes, and potassium ion release.

Question 76

Explain the stages involved in the sodium-potassium pump mechanism.

Answer 76

1. Pump protein binds ATP and three intracellular sodium ions. 2. ATP hydrolysis phosphorylates the pump, changing its shape and releasing sodium ions. 3. Shape change increases affinity for potassium ions. 4. Extracellular potassium ions bind, releasing the phosphate. 5. Phosphate release restores the original shape, releasing potassium ions into the cell. The carrier is ready for another cycle.

Question 77

How does the sodium-potassium pump impact the membrane potential?

Answer 77

The pump maintains the membrane potential by exchanging three intracellular sodium ions for two extracellular potassium ions, creating a charge differential. This process is vital for nerve impulse generation and cell function.

Question 78

What is indirect active transport, and what is a common example of this process?

Answer 78

Indirect active transport uses energy released from the movement of one molecule down a concentration gradient to transport another molecule against the gradient. An example is the transport of glucose into intestinal cells, where the movement of sodium ions provides the energy for glucose uptake.

Question 79

Explain the steps involved in sodium-dependent glucose transport.

Answer 79

1. Sodium ions, more concentrated outside cells, and glucose molecules bind to a specific transport protein. 2. Sodium ions move down their gradient into the cell, providing energy to the carrier. 3. The captured energy is utilized to transport glucose into the cell through the same carrier protein.

Question 80

What are SGLT1 and SGLT2, and what roles do they play in transport?

Answer 80

SGLT1 and SGLT2 are sodium-dependent glucose transporters. SGLT1 operates in the intestines, while SGLT2 functions in kidney nephrons. Both use energy from sodium transport to move glucose, with SGLT2 primarily responsible for glucose reabsorption in the kidneys, reducing glucose excretion in urine.

Question 81

Why are cell connections important in multicellular organisms?

Answer 81

Cell connections in multicellular organisms, facilitated by cell-adhesion molecules (CAMs), enable coordinated behavior, structural integrity, and crucial functions like binding skin and muscle cells for effective performance.

Question 82

What roles do desmosomes play in cell connections?

Answer 82

Desmosomes contribute to forming resilient yet flexible cell sheets in organs like the heart, stomach, and bladder. They enable tissues to withstand stretching by holding adjacent cells together.

Question 83

How do plasmodesmata function in plant cells?

Answer 83

Plasmodesmata are tubes connecting the cytoplasm of neighboring plant cells. These structures allow the exchange of substances, particularly water and small solutes, facilitating intercellular communication and material transport.