CH 7 Membrane Structure Lecture

Question 1

What is the main structural component of the cell membrane (plasma membrane)?

Answer 1

The phospholipid bilayer, composed mostly of phospholipids, cholesterol, and proteins.

Explanation: Phospholipids form the core structure, with cholesterol and proteins embedded or attached, contributing to function and stability.

Question 2

What does it mean for phospholipids to be amphipathic?

Answer 2

Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.

Explanation: This dual nature allows the bilayer to exist in aqueous environments, with heads facing water and tails shielded inside.

Question 3

Why doesn’t the cell membrane dissolve in water?

Answer 3

Its amphipathic phospholipids have hydrophobic tails that avoid water, forming a stable bilayer in aqueous environments.

Explanation: The hydrophobic interior prevents dissolution, a key survival feature for cells.

Question 4

What does “fluidity” mean in the fluid mosaic model?

Answer 4

Lipids and proteins can move laterally within the membrane layer.

Explanation: This lateral movement allows flexibility and dynamic function, like transport and signaling.

Question 5

What creates the “mosaic” in the fluid mosaic model?

Answer 5

Various components—lipids, proteins, and carbohydrates—scattered throughout the membrane.

Explanation: The diverse mix of molecules forms a patchwork, contributing to structure and function.

Question 6

Are all membrane components equally fluid?

Answer 6

No, lipids move rapidly and randomly, while proteins move more slowly, often directed by cytoskeletal proteins.

Explanation: Trick tests the distinction in movement rates and control mechanisms.

Question 7

What does semipermeable mean for the cell membrane?

Answer 7

Some substances pass easily, others do not, based on size, charge (+, -), and hydrophilic/hydrophobic nature.

Explanation: This selective permeability controls what enters or exits the cell.

Question 8

Which molecules pass the membrane most easily?

Answer 8

Small, nonpolar molecules (e.g., O₂, CO₂) pass easily through the hydrophobic core.

Explanation: Their properties align with the bilayer’s nonpolar interior.

Question 9

Why do polar molecules struggle to cross the membrane?

Answer 9

Their charge is repelled by the hydrophobic tails, requiring protein assistance to pass.

Explanation: Highlights the barrier posed by the bilayer’s structure.

Question 10

What is “sidedness” in the context of the membrane?

Answer 10

The inside and outside of the membrane have distinct functions, with different protein types, concentrations, and cholesterol content.

Explanation: This asymmetry supports specialized roles on each side.

Question 11

How do cold temperatures affect membrane fluidity?

Answer 11

Cold slows lipid movement, decreasing fluidity and risking gelling (solidification).

Explanation: Reduced kinetic energy tightens lipid packing.

Question 12

How do unsaturated fats maintain fluidity in cold temperatures?

Answer 12

Unsaturated fats have cis double bonds, creating kinks that prevent tight packing and gelling.
Explanation: Kinks keep the membrane fluid by disrupting alignment.

Question 13

How do saturated fats help at warm temperatures?

Answer 13

Saturated fats lack double bonds, allowing tighter packing to prevent excessive fluidity and disintegration.
Explanation: Tight packing stabilizes the membrane in heat.

Question 14

What is cholesterol’s role in membrane fluidity?

Answer 14

Cholesterol buffers fluidity—prevents gelling in cold by spacing lipids and limits excess movement in heat.

Explanation: Its wedge shape maintains stability across temperature changes.

Question 15

Do phospholipids switch between saturated and unsaturated forms in response to temperature?

Answer 15

No, cells adjust the proportion of unsaturated vs. saturated phospholipids, not individual molecules switching.

—> Fatty acid tails will switch from saturated to unsaturated

Explanation: Trick tests if you catch the biosynthesis nuance over a chemical change.

Question 16

What are the three key functions of membrane proteins?

Answer 16

Attachment (to cytoskeleton or external structures),
Transport (moving substances), and
Enzymatic activity (catalyzing reactions, chemical reactions).

Explanation: These roles support cell structure, balance, and metabolism.

Question 17

What defines an integral protein?

Answer 17

Integral proteins span the entire membrane, extending from the hydrophilic to hydrophobic layers (e.g., transmembrane proteins).

Explanation: Their depth enables functions like transport across the bilayer.

Question 18

How do peripheral proteins differ from integral proteins?

Answer 18

Peripheral proteins are loosely attached to hydrophilic layers (surface or near cytoplasm), smaller, and don’t span the membrane.

Explanation: Their surface position limits them to support roles.

Question 19

Can a single membrane protein have multiple roles?

Answer 19

es, some proteins can attach, transport, and catalyze, though many specialize in one role.

Explanation: Trick tests awareness of multifunctional proteins.

Question 20

What is the main function of membrane carbohydrates?

Answer 20

Cell recognition and signaling, helping the immune system distinguish self from non-self and identify cell types.

Explanation: They act as markers for cellular identity.

Question 21

What are glycolipids and glycoproteins?

Answer 21

Glycolipids are carbohydrates bound to lipids; glycoproteins are carbohydrates bound to proteins.

Explanation: Both are structures for recognition on the membrane surface.

Question 22

How does size affect membrane permeability?

Answer 22

Smaller molecules pass more easily through the bilayer than larger ones.

Explanation: Size limits diffusion through the hydrophobic core.

Question 23

Why do nonpolar molecules cross the membrane more easily than polar ones?

Answer 23

Nonpolar molecules align with the hydrophobic tails, while polar molecules are repelled, needing protein help.
Explanation: Polarity dictates interaction with the bilayer.

Question 24

What defines passive transport?

Answer 24

Moves substances down their concentration gradient (high to low) without ATP, sometimes using proteins.

Explanation: Relies on natural diffusion, not energy expenditure.

Question 25

What defines active transport?

Answer 25

Moves substances against their gradient (low to high) using ATP and carrier proteins. Explanation: Energy-driven to maintain cellular gradients.

Question 26

How do channel proteins function?

Answer 26

Channel proteins form tunnel-like passages for specific molecules (e.g., aquaporins for water) to cross the membrane. Explanation: They provide direct routes, often for ions or water.

Question 27

How do carrier proteins work?

Answer 27

Carrier proteins bind a substance, change shape to enclose it, and release it on the other side (e.g., glucose transporters). Explanation: Their shape change facilitates transport.

Question 28

What are aquaporins?

Answer 28

Channel proteins that speed up water transport across the membrane, faster than natural diffusion. Explanation: Essential for efficient water movement.

Question 29

What is diffusion?

Answer 29

Movement of molecules from high to low concentration to reach equilibrium, no ATP needed. Explanation: A basic passive transport process.

Question 30

What is osmosis?

Answer 30

Diffusion of water across a semipermeable membrane from high water (low solute) to low water (high solute) concentration. Explanation: Focuses solely on water movement.

Question 31

What happens to a cell in a hypotonic solution?

Answer 31

Water enters, causing the cell to swell and potentially burst (lysis). Explanation: Low external solute drives water influx.

Question 32

What is an isotonic solution’s effect on a cell?

Answer 32

Water moves in and out equally, maintaining normal cell shape. Explanation: Balanced solute concentrations prevent net movement.

Question 33

What happens in a hypertonic solution?

Answer 33

Water exits, causing the cell to shrivel (crenation). Explanation: High external solute pulls water out.

Question 34

What is facilitated diffusion?

Answer 34

Passive transport using proteins to move molecules (e.g., glucose) down their gradient, no ATP required. Explanation: Assists molecules that can’t diffuse alone.

Question 35

How does the sodium-potassium pump work?

Answer 35

Uses ATP to move 3 Na⁺ out and 2 K⁺ into the cell against their gradients, maintaining electrochemical balance. Explanation: Critical for nerve function and cell stability.

Question 36

What is phagocytosis?

Answer 36

“Cell eating”—the cell engulfs solid particles (e.g., bacteria) into a vesicle via the plasma membrane. Explanation: A type of endocytosis for large solids.

Question 37

What is pinocytosis?

Answer 37

“Cell drinking”—the cell takes in extracellular fluid and dissolved substances via vesicles. Explanation: Another endocytosis form, for liquids.

Question 38

What is receptor-mediated endocytosis?

Answer 38

Selective endocytosis where receptors bind specific molecules, forming a vesicle to bring them in. Explanation: Ensures targeted uptake.

Question 39

Describe the process of exocytosis.

Answer 39

A vesicle fuses with the plasma membrane, releasing its contents (e.g., hormones) outside the cell. Explanation: Moves substances out, opposite of endocytosis.

Question 40

Is bulk transport always active?

Answer 40

No, endocytosis and exocytosis often require ATP (active), but receptor-mediated endocytosis can be passive if driven by ligand binding alone. Explanation: Trick tests energy nuance—most cases are active, but exceptions exist.

Question 41

1. Which statement best distinguishes the primary role of the cell membrane from the endomembrane system? A) The cell membrane synthesizes proteins, while the endomembrane system regulates transport across the cell. B) The cell membrane acts as a selective barrier, while the endomembrane system synthesizes and processes macromolecules. C) Both primarily focus on ATP production for cellular energy. D) The cell membrane stores nutrients, while the endomembrane system maintains fluidity.

Answer 41

B) The cell membrane acts as a selective barrier, while the endomembrane system synthesizes and processes macromolecules. Explanation: The cell membrane (plasma membrane) controls what enters/exits via its semipermeable phospholipid bilayer, while the endomembrane system (e.g., Rough ER, Golgi) synthesizes (proteins, lipids) and modifies macromolecules for transport or secretion.

Question 42

How do phospholipids contribute differently to the cell membrane versus the endomembrane system? A) In the cell membrane, they form a bilayer; in the endomembrane system, they are only in vesicles. B) In the cell membrane, they are amphipathic and fluid; in the endomembrane system, they form organelle membranes with similar properties. C) In the cell membrane, they are static; in the endomembrane system, they enable vesicle fusion. D) In the cell membrane, they lack cholesterol; in the endomembrane system, cholesterol is abundant.

Answer 42

B) In the cell membrane, they are amphipathic and fluid; in the endomembrane system, they form organelle membranes with similar properties. Explanation: Phospholipids in both form bilayers with hydrophilic heads and hydrophobic tails, providing fluidity. In the cell membrane, they regulate permeability; in the endomembrane system (e.g., ER, Golgi), they enable vesicle formation and organelle structure continuity.

Question 43

Which component is unique to the cell membrane and NOT found in the endomembrane system? A) Cholesterol B) Integral proteins C) Peripheral proteins D) None—the cell membrane is part of the endomembrane system, so they share all components.

Answer 43

A) Cholesterol Explanation: Trick lies in specificity—cholesterol stabilizes the cell membrane’s fluidity but is less prominent in internal endomembrane organelles (e.g., ER, Golgi), where it’s minimal or absent. Integral and peripheral proteins are in both. The cell membrane is part of the endomembrane system, but cholesterol’s role is unique to it.

Question 44

Which process involves both the cell membrane and the endomembrane system working together? A)Osmosis across the Rough ER B) Exocytosis of a protein C) Diffusion of glucose through aquaporins D) Synthesis of lipids in the cell membrane

Answer 44

B) Exocytosis of a protein Explanation: Exocytosis requires the endomembrane system (Rough ER synthesizes protein, Golgi modifies it, vesicles transport it) and the cell membrane (vesicle fuses to release protein outside). Osmosis and diffusion are membrane-specific, and lipid synthesis occurs in the Smooth ER, not the cell membrane.

Question 45

Compare how membrane fluidity is maintained in the cell membrane versus the endomembrane system.

Answer 45

In the cell membrane, fluidity is maintained by unsaturated fats (kinks from cis double bonds prevent gelling in cold) and cholesterol (buffers temperature changes). Proteins move laterally, guided by the cytoskeleton. In the endomembrane system (e.g., ER, Golgi), fluidity is similar due to phospholipid bilayers, but cholesterol is less critical, and fluidity supports vesicle budding/fusion (e.g., from Golgi to cell membrane). Both rely on amphipathic phospholipids, but the cell membrane’s external role emphasizes cholesterol’s buffering. Explanation: Highlights shared bilayer properties and distinct regulatory factors.

Question 46

How do transport proteins differ in function between the cell membrane and the endomembrane system?

Answer 46

In the cell membrane, transport proteins (e.g., channel proteins like aquaporins, carrier proteins like glucose transporters) move substances (water, ions, glucose) across the bilayer for cellular uptake or release, often passively or actively (e.g., Na⁺/K⁺ pump). In the endomembrane system, transport proteins in vesicle membranes (e.g., from Rough ER to Golgi) shuttle macromolecules internally between organelles or to the cell membrane for secretion, typically via vesicle fusion, not direct diffusion. The cell membrane regulates external exchange; the endomembrane system manages internal trafficking.

Question 47

Why might a glycoprotein synthesized by the endomembrane system NOT function properly if placed directly in the cell membrane without modification?

Answer 47

A glycoprotein from the endomembrane system (e.g., Rough ER) needs Golgi modification (e.g., carbohydrate addition) to function as a recognition marker in the cell membrane. Without this, it might lack the correct sugar groups, impairing its role in cell signaling or immune recognition. The trick is assuming synthesis alone is enough—modification is key.

Question 48

True or False: The cell membrane’s semipermeability relies on the same vesicle transport mechanisms as the endomembrane system.

Answer 48

False Explanation: The cell membrane’s semipermeability depends on its phospholipid bilayer and transport proteins (e.g., channels, carriers) for diffusion or active transport. The endomembrane system uses vesicles for bulk transport (e.g., exocytosis), not to establish permeability, which is a bilayer property.

Question 49

True/False: Both the cell membrane and the endomembrane system use the sodium-potassium pump to maintain their functions.

Answer 49

The sodium-potassium pump is specific to the cell membrane, maintaining electrochemical gradients (3 Na⁺ out, 2 K⁺ in) using ATP. The endomembrane system (e.g., ER, Golgi) doesn’t use this pump; its transport involves vesicle movement, not ion gradients. Trick tests if you apply a cell membrane-specific mechanism too broadly.

Question 50

Most proteins found in the membrane are also

Answer 50

amphipathic (hydrophillic and hydrophobic regions) -allows them to exisit -ensures they do not compromise the integrity of the membrane by being the wrong chemical composition

Question 51

In the Fluid Mosaic Model proteins

Answer 51

Move more slowly and are often directed by the cytoskeleton