CHAPTER-4

Question 1

What Are Phospholipids?

Answer 1

Molecules that make up cell membranes.

Have a hydrophilic (water-loving) head and hydrophobic (water-hating) tails.

Create membrane-bound compartments (like cells & organelles).

Act as a barrier, keeping unwanted substances out.

Question 2

Phospholipids Structure in Water

Answer 2

a) Monolayer on water.

b) Micelles (heads out, tails in).

c) Bilayers (double layer → cell membranes).

d) Bilayers form enclosed compartments (cells/organelles).

Question 3

Liposomes (Artificial Phospholipid Bags)

Answer 3

Artificial Phospholipid Bags

Used to deliver drugs (e.g., cancer treatments) by fusing with target cells.

Also used in cosmetics (e.g., aloe vera, vitamins) and food supplements for better absorption.

Question 4

Phospholipid Bilayer Structure

Answer 4

Double layer of phospholipids:

Heads (hydrophilic) face outward → interact with water.

Tails (hydrophobic) face inward → repel water.

Molecules move freely within their layer (like a liquid).

Appears as two dark lines (hydrophilic heads) with a pale middle (hydrophobic tails).

Thickness: ~7 nm.

Question 5

The Fluid Mosaic Model (Singer & Nicolson, 1972)

Answer 5

Fluid:

Phospholipids & proteins move around (like olive oil).

Phospholipids mainly slide sideways in their layer.

Mosaic:

Proteins are scattered randomly, creating a “tile-like” pattern.

Question 6

Membrane Fluidity

Answer 6

Unsaturated tails (bent) → increase fluidity (loosely packed).

Saturated tails (straight) → decrease fluidity (tightly packed).

Longer tails = less fluid.

Cold temps make membranes stiffer, but some organisms (e.g., bacteria) adapt by adding more unsaturated fats.

Question 7

Membrane Proteins

Answer 7

Intrinsic (Integral) Proteins

Embedded in the membrane (some span across it = transmembrane proteins).

Have hydrophobic regions (stick to fatty tails) and hydrophilic regions (face water).

Extrinsic (Peripheral) Proteins

Sit on the surface (inner or outer) of the membrane.

Often attached to intrinsic proteins or phospholipids.

Question 9

Key Takeaways

Answer 9

The membrane is dynamic (not rigid) and selectively permeable.

Proteins “float” in a sea of phospholipids, with carbs & cholesterol adding stability.

Fluidity depends on:

Fatty acid type (saturated vs. unsaturated).

Temperature.

Cholesterol content.

Question 10

components of cell
membranes

Answer 10

lipid-
phospholipids, cholesterol and glycolipids.
There are also
proteins and glycoproteins.

Question 11

Phospholipids: Structure and Dual Roles in Cells

Answer 11

Form the phospholipid bilayer:

Polar (hydrophilic) heads face outward (interact with water).

Non-polar (hydrophobic) tails face inward (block water-soluble molecules).

Function:

Creates a semi-permeable barrier that:

Blocks polar molecules (e.g., sugars, ions, amino acids).

Allows small non-polar molecules (e.g., O₂, CO₂) to pass freely.

Signaling Role
Modified phospholipids act as cellular messengers:

Question 12

Cholesterol: The Membrane’s Multitasker

Answer 12

Amphipathic molecule:

Small hydrophilic head (interacts with phospholipid heads).

Rigid hydrophobic tail (wedges between phospholipid tails).

Abundance: Makes up ~50% of animal cell membranes (absent in prokaryotes; rare in plants).

• Cold temps: Prevents tight packing → keeps membrane flexible.
• Hot temps: Restrains phospholipid movement → prevents over-fluiditiy.

Question 13

1. Glycolipids & Glycoproteins

Answer 13

Glycolipids: Lipid + Carbohydrate (mainly in plant cell glycocalyx).

Glycoproteins: Protein + Carbohydrate (mainly in animal cell glycocalyx).

Glycocalyx: Sugary outer coating formed by these molecules (“cellular antennae”).

Question 14

Key Functions of Membrane Components

Answer 14

Signaling Receptors:

Bind hormones/neurotransmitters (e.g., glucagon receptors in liver cells).

Trigger internal reactions when activated.

Endocytosis Receptors:

Bind molecules for cellular uptake (e.g., cholesterol via LDL receptors).

Cell Adhesion Receptors:

Help cells stick together in tissues (e.g., immune cell interactions).

2. Cell Recognition & Antigens
   Act as unique ID tags (e.g., ABO blood group antigens).

Carbohydrate differences determine cell type (like national flags).

3. Transport Proteins
   Channel Proteins: Hydrophilic pores for ions (e.g., Na⁺ channels).

Carrier Proteins: Change shape to move molecules (e.g., glucose transporters).

Specificity: Each transports only certain substances.

4. Enzymes
   Catalyze reactions (e.g., digest disaccharides in the gut).
5. Cytoskeleton Anchors
   Maintain cell shape and enable movement (e.g., attach to actin filaments).

Question 15

Cell Signaling

Answer 15

Definition: The process of cells communicating by sending and receiving molecular messages.

Reception:

A signaling molecule (e.g., hormone) binds to a receptor on the cell membrane.

Transduction:

The signal is converted into a cellular response (e.g., via second messengers like cAMP).

Response:

The cell acts (e.g., releases enzymes, changes gene expression).