Cell Biology: Membrane Structure and Function

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Questions and Answers

What is the primary structural unit of the plasma membrane?

Glycoproteins

Carbohydrates

Phospholipids (correct)

Proteins

What percentage of the plasma membrane is composed of carbohydrates?

5-10%

2-10% (correct)

1-2%

10-20%

Which component of the plasma membrane is described as amphipathic?

Glycolipids

Proteins

Glycoproteins

Phospholipids (correct)

What is the primary role of proteins embedded in the plasma membrane?

To perform specific functions Signup and view all the answers

What best describes the fluid mosaic model of the plasma membrane?

A dynamic structure with free movement of components Signup and view all the answers

What is one of the primary functions of a biological membrane?

Defines the cell boundary and separates the interior from the exterior Signup and view all the answers

Which component is NOT primarily associated with the structural integrity of a membrane?

Nucleic acids Signup and view all the answers

How does cholesterol contribute to membrane fluidity?

It stabilizes the membrane by reducing its flexibility Signup and view all the answers

What best describes the fluid mosaic model of the cell membrane?

A dynamic and flexible system with proteins embedded in a lipid bilayer Signup and view all the answers

Which type of lipid is specifically mentioned as affecting membrane function?

Phospholipids Signup and view all the answers

Which of the following is a function of membrane proteins?

Facilitate enzymatic reactions and transport Signup and view all the answers

What concept describes the uneven distribution of components in a biological membrane?

Asymmetry Signup and view all the answers

What term describes regions of membrane with concentrated lipids and proteins that function differently from the surrounding membrane?

Lipid rafts Signup and view all the answers

What is the transition or melting temperature (Tm) of a lipid bilayer?

The temperature at which it transitions to a more fluid-like state Signup and view all the answers

Which membrane contains a higher percentage of proteins?

Inner mitochondrial membrane at 70% Signup and view all the answers

Which type of membrane proteins are characterized by having peptide bonds and polarity?

Transmembrane proteins Signup and view all the answers

What describes the structure of β barrel proteins?

They are stiff and involved in forming pores. Signup and view all the answers

Which proteins are classified as lipid-anchored?

Proteins covalently bonded to lipids Signup and view all the answers

What determines the classification of transmembrane proteins?

The number of hydrophobic amino acids they have Signup and view all the answers

Which of the following describes the function of membrane proteins?

Serve diverse roles including transport and receptors Signup and view all the answers

How are hydropathy plots used in the study of membrane proteins?

To show the distribution of hydrophobic and hydrophilic amino acids in a sequence Signup and view all the answers

What is the primary function of the glycocalyx surrounding the cell?

To aid in cell signaling and recognition Signup and view all the answers

How are oligosaccharides primarily attached to proteins in the membrane?

Covalent bonds Signup and view all the answers

Which of the following statements about blood group antigens is true?

Blood group O has no antigens and can receive all blood types Signup and view all the answers

What role do integrins play in relation to neutrophils?

They facilitate attachment and adhesion to endothelial cell walls Signup and view all the answers

What type of damage does the glycocalyx protect the cell from?

Mechanical and chemical damage Signup and view all the answers

Which statement accurately describes the asymmetric distribution of carbohydrates in membranes?

Carbohydrates are only found on the extracellular face of membranes Signup and view all the answers

What is the significance of the ABO blood group antigens?

They are involved in immune recognition and can trigger antibody responses Signup and view all the answers

What is one major component of the extracellular matrix (ECM)?

Oligosaccharides Signup and view all the answers

What is the primary model that describes the membrane structure as a 2D fluid?

Fluid mosaic model Signup and view all the answers

Which of the following movements is characteristic of lipids in the fluid mosaic model?

Rotational/spin Signup and view all the answers

What kind of diffusion do larger proteins experience compared to lipids?

Slower diffusion Signup and view all the answers

What is a rare process associated with lipid mobility in membranes?

Flip-flop Signup and view all the answers

Which of the following restricts protein mobility in membranes?

Interactions with external macromolecules Signup and view all the answers

Which domain is limited to the lateral diffusion of proteins due to tight junctions?

Apical domain Signup and view all the answers

What is one factor that contributes to the restriction of lipid mobility?

Association with proteins Signup and view all the answers

Which of the following best describes the interaction of proteins with cytoskeleton structures?

Limits lateral diffusion Signup and view all the answers

Which molecules can diffuse through a lipid bilayer?

Small non-polar molecules Signup and view all the answers

What is the primary energy source for primary active transport?

ATP hydrolysis Signup and view all the answers

Passive transport requires energy input.

False Signup and view all the answers

In facilitated diffusion, molecules require a ______ for transport.

membrane protein Signup and view all the answers

What is the role of sodium channels during an action potential?

They open in response to depolarization, allowing Na+ influx. Signup and view all the answers

Match the following transport mechanisms with their descriptions:

Passive Transport = No energy required, down concentration gradient Active Transport = Requires energy, against concentration gradient Facilitated Diffusion = Transport via membrane proteins Ion Channels = Allow passage of ions through hydrophilic pores Signup and view all the answers

Which of the following types of channels can be opened or closed in response to changes in membrane potential?

Voltage-gated channels Signup and view all the answers

The Na+/K+ pump maintains the resting membrane potential by transporting out and into the cell.

Na+, K+ Signup and view all the answers

What initiates the opening of ligand-gated ion channels?

Binding of a ligand Signup and view all the answers

Secondary active transport involves direct use of ATP.

False Signup and view all the answers

What is the purpose of the calcium influx at the nerve terminal?

Exocytosis of neurotransmitters Signup and view all the answers

What type of molecules can diffuse through a lipid bilayer?

Small, non-polar molecules and slightly polar molecules like water. Signup and view all the answers

What is the role of the concentration gradient?

It determines the direction of movement of non-charged molecules. Signup and view all the answers

Which transport mechanism requires energy?

Active transport Signup and view all the answers

What is facilitated diffusion?

Transport of molecules across a membrane via protein channels or carriers without energy input. Signup and view all the answers

What determines the specificity of a transporter protein?

Amino acid sequence Signup and view all the answers

Primary active transport uses ATP hydrolysis to maintain ionic gradients.

True Signup and view all the answers

What is the role of ion channels in action potentials?

They facilitate the rapid influx or efflux of ions, changing the membrane potential. Signup and view all the answers

What type of transport does the Na+/K+ pump demonstrate?

Primary active transport Signup and view all the answers

What happens at the postsynaptic cell during neurotransmission?

Neurotransmitters bind to ligand-gated ion channels, leading to depolarization. Signup and view all the answers

The movement of ions through __________ channels is gated and dependent on voltage changes.

voltage-gated Signup and view all the answers

Match the following transport methods with their descriptions:

Primary Active Transport = Uses ATP to move substances against their gradient Secondary Active Transport = Utilizes ion gradients to transport other substances Facilitated Diffusion = Passive transport via protein channels Simple Diffusion = Movement of molecules without membrane proteins Signup and view all the answers

Study Notes

The Membrane: Structure and Function

Cell boundary - defines a cell and separates interior from exterior
Selective barrier to molecules - impermeable to hydrophilic molecules
Boundary of intracellular compartments
Membranes are dynamic and fluid, allowing for cell function and communication.
Phospholipids make up the majority of the lipid component of the membrane.
Phospholipids create a bilayer - impermeable barrier to most molecules in the environment.
The fluid mosaic model describes the membrane as a 2D fluid.

Composition

Lipids account for approximately 50% of the cell membrane
Proteins account for approximately 50% of the cell membrane
Carbohydrates are integral to the membrane, accounting for approximately 2-10% of the membrane structure

Lipids in Membrane Structure

Lipids create a bilayer - impermeable barrier
The bilayer is fluid and allows for diffusion of molecules
The transition temperature (Tm) of a bilayer determines it's transition from rigid to fluid
Bilayer has a distinct inner and outer leaflet, each leaflet is composed of a different percentage of lipids.

Proteins in the membrane

Proteins are embedded in the membrane and are associated with specific functions
Variable protein content between different cell types
Membrane proteins are frequently linked to carbohydrates
Common Functions of Membrane Proteins:
- Transporters
- Receptors
- Ion channels
- Enzymes
- Anchors
Transmembrane proteins - can be single-pass or multi-pass
- ⍺ helix: single-pass or multi–pass
  - Flexible
- β barrel: porins
  - Stiff
Hydropathy plots: Show the distribution of hydrophobic amino acids along a peptide sequence
Classification of membrane proteins:
- Transmembrane Proteins (Integral membrane proteins):
  - α-helix (20-30 hydrophobic aa) or β-sheets
  - Single-pass
  - Multi-pass
- Lipid-Anchored Proteins:
  - Covalent bond to lipid
  - Cytosolic
  - Extracellular
- Membrane-Associated Proteins (Peripheral membrane proteins):
  - Cytosolic
  - Extracellular

Carbohydrates in the Membrane:

Only found in the extracellular face of membranes
Integral components to membrane - synthesized by the cell
Oligosaccharides (15 sugar monomers):
- Covalently attached to protein
- Found on the core of the protein membrane
Main component of the extracellular matrix (ECM)

Glycocalyx

The Glycocalyx is a carbohydrate coat that surrounds the cell.
Functions of the Glycocalyx:
- Cell-cell and cell-ECM adhesion
- Protection - mechanical, chemical, pathogen damage
- Microenvironments
- Signaling
- Recognition:
  - ABO blood group antigens
  - Major histocompatibility complex (MHC)

Membrane Fluidity

Membranes are dynamic and fluid.
Movements that lipids and proteins make within the membrane:
- Lateral Diffusion (proteins and lipids):
  - Lipids: 1-10 μm/second
  - Proteins: 0.1-1 μm/second
- Rotational/spin (protein and lipids)
- Flexion (lipids only)
- Flip-flop (lipids only)
  - Spontaneously - rare
  - Enzymatic activity - flippases

Restrictions to Lipid Mobility

Lipids associated with proteins
Lipid rich and protein rich domains

Restrictions to Protein Mobility

Tight junctions: lateral diffusion limited to membrane domains
- Apical domain
- Basolateral domain
Protein aggregation: Proteins can cluster together
Interation with external complexes: (focal adhesions - ECM)
Interation with internal complexes: (cytoskeleton - cortex)
- Cytoskeleton fibers
- Integrins

Membrane Faces

Membranes form closed compartments
Internal face is known as the cytosol
External face is known as the exterior

Selective Permeable Membrane

Lipids can diffuse through a phospholipid membrane
Larger molecules and ions need to be transported by specific proteins
The difference in concentration gradient between the outside of the cell and inside, as well as the charge of the molecule, determines the rate and the direction of transport.

Passive Transport

Simple diffusion: Molecules pass through the membrane without the help of any protein, down their concentration gradient.
Facilitated diffusion: Molecules pass through the membrane with the help of a protein.
- Channels: These proteins form hydrophilic pores, allowing molecules to pass through.
- Transporters/Carriers: These proteins bind to specific molecules, leading to a conformational change that allows transport of the molecule.
- Saturation: The rate of transport reaches a maximum when all protein binding sites are occupied.

Active Transport

Primary Active Transport: Molecules are moved against their concentration gradient, requiring energy, obtained directly from ATP hydrolysis.
- P-type pump:
  - Phosphate group is added to the protein to drive transport
  - Examples: Na+/K+ pump, Ca2+ pump
- **V-type: **
  - Not phosphorylated
  - Primarily pumps H+
  - Examples: Found in vacuoles, endosomes, lysosomes
- F-type:
  - Function as ATP synthase or a proton pump depending on the concentration gradient
  - Examples: Found in bacteria, mitochondria, thylakoid membrane
- ABC:
  - Transmembrane proteins with ATP binding domains
  - Examples: Sugar, amino acid and peptide transporters
Secondary Active Transport:
- Molecules are moved against their concentration gradients, but the energy required is obtained from an electrochemical gradient, typically created by primary active transport.
- Symport: Two molecules move in the same direction across the membrane.
  - Example: Glucose and Na+ are transported together
- Antiport: Two molecules move in opposite directions across the membrane.
  - Examples: Na+/Ca2+ and Na+/H+ antiporters
- Cotransport: The movement of one molecule is coupled to movement of another.
  - This is a general term used to describe symport or antiport.

Action Potentials and Synapses

Membrane Potential:
- The difference in electrical charge across the membrane, created by the distribution of ions inside and outside the cell.
- K+ Leak channels are important for maintaining the resting membrane potential, typically around -60mV.
Action Potential:
- A rapid change in the membrane potential, triggered by a depolarization of the membrane.
- Voltage-gated channels, primarily for Na+ and K+, play a crucial role in controlling the action potential.
- Depolarization: The membrane potential becomes less negative, moving closer to 0 mV.
  - Caused by the influx of positive ions, often Na+
- Repolarization: The membrane potential returns to its negative resting value.
  - Caused by the efflux of K+
Synapse:
- The junction between two neurons, or between a neuron and another cell.
  - Electrical signals are converted into chemical signals at the synapse.
  - Neurotransmitters are released from vesicles in the presynaptic neuron. These bind to specific receptors on the postsynaptic neuron.
Postsynaptic Cell:
- Neurotransmitter binding to receptors triggers a conformational change.
- This change can open ligand-gated ion channels on the post-synaptic cell, ultimately converting the chemical signal back into an electrical signal.
Neuromuscular Junction:
- The synapse between a motor neuron and a muscle fiber.
- Acetylcholine released from the motor neuron binds to receptors on the muscle fiber, triggering an action potential.
- This leads to a rise in intracellular Ca2+ levels, ultimately causing muscle contraction.

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Description

Explore the fascinating structure and function of cell membranes in this quiz. Learn about the components that make up the membrane, including lipids, proteins, and carbohydrates, and how they contribute to its selectivity and fluidity. Test your understanding of the fluid mosaic model and the dynamics of membrane behavior.