Cellular Communication

Question 1

What is the main barrier to communication between cells?

Answer 1

The plasma membrane — a semi-permeable lipid bilayer that separates the inside of the cell from the outside environment.

Question 2

What is the plasma membrane mainly composed of?

Answer 2

Phospholipids and proteins.

Question 3

What are the two key parts of a phospholipid molecule?

Answer 3

A hydrophilic (polar) head and two hydrophobic (nonpolar) fatty acid tails.

Question 4

Why do phospholipids form a bilayer in water?

Answer 4

Because the hydrophilic heads face water, while hydrophobic tails avoid water, causing a self-assembling bilayer.

Question 5

What property describes a molecule that has both hydrophilic and hydrophobic parts?

Answer 5

Amphipathic.

Question 6

What is signal transduction?

Answer 6

The process by which a signal is transmitted through the cell membrane and converted into a specific cellular response.

Question 7

Why can’t most signaling molecules easily cross the plasma membrane?

Answer 7

Because they are often large or charged, and the membrane is selectively permeable.

Question 8

Approximately how many cells make up the human body?

Answer 8

Between 30 and 50 trillion.

Question 9

Why is cell communication essential in multicellular organisms?

Answer 9

To allow cell differentiation, development, coordination, and tissue function.

Question 10

What does the plasma membrane do besides acting as a barrier?

Answer 10

It regulates transport, receives signals, and supports communication between cells.

Question 11

What kind of image shows the close proximity of two cell membranes?

Answer 11

Electron micrograph.

Question 12

Why did multicellular organisms take so long to evolve?

Answer 12

Because unicellular organisms lacked communication mechanisms to coordinate development.

Question 13

What is the function of proteins in the plasma membrane?

Answer 13

They act as receptors, channels, transporters, and signal transducers.

Question 14

What are the two parts of a phospholipid molecule?

Answer 14

A polar, hydrophilic (water-loving) head and a non-polar, hydrophobic (water-fearing) tail.

Question 15

How do saturated and unsaturated fatty acid tails differ structurally?

Answer 15

Saturated tails are straight; unsaturated tails have kinks due to double bonds.

Question 16

Why do cold-adapted animals have more unsaturated fatty acids in their membranes?

Answer 16

Unsaturated fatty acids prevent tight packing, maintaining membrane fluidity in cold environments.

Question 17

What are the four major phospholipids in mammalian cells?

Answer 17

Phosphatidylethanolamine, phosphatidylserine, phosphatidylcholine, and sphingomyelin.

Question 18

How are phospholipids asymmetrically distributed in the plasma membrane?

Answer 18

Phosphatidylcholine and sphingomyelin are on the outer layer; phosphatidylethanolamine and phosphatidylserine are on the inner layer.

Question 19

What happens to phosphatidylserine during apoptosis?

Answer 19

It flips from the inner to the outer membrane, signaling cell death.

Question 20

What percentage of the animal cell membrane mass is made of lipids?

Answer 20

About 50%.

Question 21

What are the three major classes of membrane lipids?

Answer 21

Phospholipids, glycolipids, and cholesterol.

Question 22

What is the role of glycolipids in the membrane?

Answer 22

They contain sugar groups that help with cell identity and recognition.

Question 23

How does cholesterol affect membrane fluidity?

Answer 23

It buffers membrane fluidity—making it more fluid in cold temperatures and more stable in heat.

Question 24

What is the fluid mosaic model?

Answer 24

A model describing the dynamic and flexible structure of the plasma membrane with lipids and proteins moving laterally.

Question 25

What is a key feature of the fluid mosaic membrane?

Answer 25

Lipids and proteins can move within the membrane, allowing dynamic changes and cell movement.

Question 26

What is the function of inositol phospholipids in the membrane?

Answer 26

They reside on the cytosolic side and play roles in cell signaling.

Question 27

What are three main types of cell communication mechanisms discussed?

Answer 27

Gap junctions, cell surface signaling molecules, and chemical messengers.

Question 28

What are examples of chemical messenger types based on structure?

Answer 28

Gases (e.g. nitric oxide), eicosanoids, purines (e.g. ATP), amines, peptides/proteins, steroids, and retinoids.

Question 29

What is the simplest way for a signal molecule to enter a cell?

Answer 29

Passive diffusion through the plasma membrane.

Question 30

What kind of molecules can diffuse through the membrane?

Answer 30

Small, non-polar molecules like oxygen.

Question 31

What types of molecules can pass directly through the plasma membrane, and why?

Answer 31

Lipophilic molecules like steroid hormones (e.g., testosterone) and gases like nitric oxide (NO) can pass through because they are small and nonpolar. Steroid hormones are derived from cholesterol, making them membrane-permeable.

Question 32

What makes nitric oxide (NO) an important signaling molecule?

Answer 32

It's a short-lived gas that relaxes smooth muscle cells and dilates blood vessels. It was recognized in 1992 as "Molecule of the Year" for its role in signaling.

Question 33

How do signaling molecules that cannot cross the membrane affect the cell?

Answer 33

They bind to receptors on the cell surface, which then activate intracellular signaling pathways (signal transduction) that relay the signal inside the cell, leading to changes like altered gene expression or cell behavior.

Question 34

What is an example of cell-cell communication in simple organisms?

Answer 34

The slime mold (social amoeba) lives as individual cells, but under starvation, they aggregate into a multicellular structure, form a slug, and eventually a fruiting body to release spores. This is triggered by communication between cells.

Question 35

What signaling molecule and receptor are involved in slime mold aggregation?

Answer 35

he signaling molecule is cyclic AMP (cAMP), which binds to a G-protein coupled receptor (GPCR) on the cell surface, initiating signal transduction inside the cell.

Question 36

What are G-protein coupled receptors (GPCRs) and how do they work?

Answer 36

GPCRs are membrane receptors with seven transmembrane domains. When a ligand like cAMP binds, they activate internal G-proteins that trigger a cascade of intracellular events.

Question 37

What are the three main classes of membrane-bound cell surface receptors?

Answer 37

G-protein coupled receptors (GPCRs) – activate G-proteins for signaling. Ion channel-linked receptors – open to allow ions through when a signal binds. Enzyme-linked receptors – often involve kinase activity that triggers phosphorylation cascades.

Question 38

What are the three main classes of membrane-bound receptors, and where are they often found?

Answer 38

G-protein coupled receptors (GPCRs) – activate G-proteins to start signal cascades. Ion channel-linked receptors – found especially in neurons; gated channels that allow ions to pass. Enzyme-linked receptors – ligand binding causes receptor dimerization and activates catalytic domains.

Question 39

What happens during GPCR signaling involving phosphatidylinositol?

Answer 39

When a signal binds to a GPCR, it activates phospholipase C, which cleaves phosphatidylinositol (a membrane lipid) into two signaling molecules that help trigger intracellular responses.

Question 40

Why are intracellular signaling pathways complex and multi-step?

Answer 40

They allow amplification of a signal—one ligand binding can activate thousands of molecules, and phosphorylation cascades help fine-tune the response through multiple steps.

Question 41

What is signal amplification in cell signaling?

Answer 41

A single extracellular molecule (like a hormone) binds a receptor, triggering a cascade (e.g. cAMP production, kinase activation) that greatly increases the number of active molecules inside the cell.

Question 42

Why is signal specificity important in cell communication?

Answer 42

Some messages (like hormones) need to reach the whole body, while others (like local developmental signals) should only affect nearby cells. Communication must be targeted and appropriate to the context.

Question 43

What is contact-dependent signaling and where is it important?

Answer 43

Occurs when signaling molecules and receptors are membrane-bound and require direct contact between cells. Important in development, e.g., lateral inhibition in fruit flies, where only one cell becomes a neuron due to Notch-Delta signaling.

Question 44

What are gap junctions and their function in communication?

Answer 44

Gap junctions are direct tunnels between adjacent cells allowing ions and small molecules (like cAMP) to flow. Used for coordination and synchronization, e.g., in zebrafish pigment cells controlled by connexin proteins.

Question 45

What is paracrine signaling and how does it differ from autocrine?

Answer 45

Paracrine: signals are secreted and act on nearby cells. Autocrine: signals are secreted and received by the same cell. Example: Histamine in inflammation (paracrine).

Question 46

How does synaptic signaling work in neurons?

Answer 46

An electrical signal travels down the axon, triggering release of neurotransmitters (e.g., acetylcholine) at the synapse. This chemical signal is received by a nearby target cell—fast and precise communication.

Question 47

What is endocrine signaling and how is it different from synaptic signaling? Answer:

Answer 47

Endocrine signaling uses hormones released into the bloodstream to reach distant cells—slower but widespread. Synaptic signaling is fast and highly localized (e.g., neuron to muscle).

Question 48

What are fast vs. slow cellular responses to signaling?

Answer 48

Fast: Alters existing protein function (e.g., phosphorylation). Slow: Changes gene expression (transcription & translation) to make new proteins.