Lecture 8 Review

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Questions and Answers

What is a key property of membranes that makes them essential in cell biology?

Membranes consist only of proteins.

Membranes are rigid structures.

Membranes are impermeable to all substances.

Membranes can act as selective barriers. (correct)

What allows lipids to assemble into a bilayer membrane?

Their electrical charge.

Their amphiphilic chemical properties. (correct)

Their ability to form hydrogen bonds.

Their high molecular weight.

Which class of molecules is typically most permeable through lipid bilayers?

Charged molecules

Ions

Small nonpolar molecules (correct)

Large polar molecules

In which type of cells are membrane-bound organelles found?

Eukaryotic cells Signup and view all the answers

What is the thickness of a typical membrane?

5 nm Signup and view all the answers

What is the primary function of the plasma membrane in relation to water-soluble molecules?

Blocks the passage of most water-soluble molecules Signup and view all the answers

Which statement accurately describes the composition of the plasma membrane?

It is composed of 50% lipids and 50% proteins by mass Signup and view all the answers

What characterizes membrane lipids, specifically phospholipids?

They are amphipathic Signup and view all the answers

What is the charge of most lipids in cell membranes?

They have a net negative charge Signup and view all the answers

Which of the following roles is NOT a function of the plasma membrane?

Generates energy for the cell Signup and view all the answers

What forms the basic structure of cell membranes?

Phospholipid bilayers Signup and view all the answers

What is the primary factor affecting membrane fluidity?

Membrane composition Signup and view all the answers

How does the presence of cholesterol affect membrane permeability?

Makes membranes stiffer and less permeable Signup and view all the answers

Which term describes the lateral movement of lipids within a membrane?

Fluidity Signup and view all the answers

What is a major function of transmembrane proteins?

Transporting substances across the membrane Signup and view all the answers

What role do scramblases play in membrane biology?

Redistributing lipids to generate membrane asymmetry Signup and view all the answers

What happens to membrane fluidity with an increase in saturated lipids?

Fluidity decreases sharply Signup and view all the answers

Which of the following describes the asymmetry of the plasma membrane?

Different distribution of lipids between the two leaflets Signup and view all the answers

What is the primary consequence of having shorter fatty acid tails in membrane lipids?

Increased membrane fluidity Signup and view all the answers

Which method assesses the mobility of proteins within membranes?

Fluorescence Recovery After Photobleaching (FRAP) Signup and view all the answers

Peripheral membrane proteins interact with the membrane via which method?

Binding to the hydrophilic surface of the membrane Signup and view all the answers

What effect does increasing the ratio of unsaturated to saturated lipids have on membrane properties?

Increases membrane fluidity Signup and view all the answers

How is phospholipid asymmetry generated in the cell membrane?

Through specific enzyme action like flippases and scramblases Signup and view all the answers

Study Notes

Membrane Structure

Membranes are crucial in cell biology. They define the boundaries and compartments within cells
Lipids form bilayers, enabling membrane assembly. The chemical properties of lipids allow them to self-assemble into a bilayer structure.
Lipid bilayers exhibit varying permeabilities to different molecules, as dictated by properties of the molecules themselves.
Animations can visually demonstrate diffusion rates in GFP-tagged membrane proteins using FRAP (Fluorescence Recovery After Photobleaching).
FRAP is a technique for analyzing protein diffusion within cell membranes.
Eukaryotic cells have membrane-bound organelles, like mitochondria, peroxisomes, Golgi apparatus, lysosomes, and the endoplasmic reticulum, in addition to the plasma membrane.

Membrane Components

Membranes are composed mainly of proteins and lipids, roughly 50% each, by mass.
Membrane lipids are "amphipathic", having both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails
Phospholipids are the most abundant lipid type in membranes, having a phosphate group in the hydrophilic head.
Phospholipids assemble into a bilayer in water due to the hydrophobic effect.
The membrane bilayer is asymmetric, with different types of phospholipids and proteins concentrated in each half of the bilayer.

Membrane Characteristics

The fluid mosaic model describes the dynamic arrangement of proteins and lipids in membranes, enabling lateral diffusion
Membranes function as selective barriers, regulating passage of substances.
Phospholipid bilayers prevent water-soluble molecules from easily passing through. Small hydrophobic molecules and some small polar molecules can pass more readily.
Membrane fluidity is influenced by factors like temperature, fatty acid tail length, the ratio of saturated to unsaturated lipids, and cholesterol content.
Higher temperatures generally correspond to higher fluidity, and higher cholesterol content generally corresponds to lower fluidity.

Membrane Proteins

Membrane proteins have various functions, including transport, signaling, and enzymatic activity
Integral proteins span the membrane, often having hydrophobic regions embedded in the lipid bilayer. Other integral proteins can be attached to the monolayers or linked by lipids.
Peripheral proteins are loosely associated with the membrane surface. They may bind directly to the hydrophilic parts of the bilayer.
Integral proteins may cross the membrane as α-helices, and these proteins' hydrophobic amino acid side chains interact with hydrophobic lipid tails. Multiple α-helices can form hydrophilic pores within the membrane, allowing movement of polar or charged molecules.
Protein diffusion within membranes can be restricted by interactions with the cell cortex, extracellular matrix, or other proteins on adjacent cells. Specific proteins can help facilitate the diffusion restriction.

Measurement of Protein Mobility

FRAP (Fluorescence Recovery After Photobleaching) is a technique to determine protein mobility within the membrane. FRAP experiments typically involve tagging membrane proteins with a fluorescent protein like GFP, followed by photobleaching a portion of the membrane, and then observing how fluorescence is restored over time.

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Description

Explore the essential roles of membranes in cell biology, including their composition and the assembly of lipid bilayers. This quiz covers the diffusion rates of proteins in membranes using techniques like FRAP, as well as the significance of organelles in eukaryotic cells.