Cell Membranes and Permeability

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Questions and Answers

What is the primary function of cell membranes regarding structural integrity?

To facilitate enzymatic reactions.

To maintain the cell's structural integrity. (correct)

To maximize energy production.

To allow rapid diffusion of all substances.

How would you describe a membrane that restricts the passage of all substances?

Selectively permeable.

Semi-permeable.

Impermeable. (correct)

Freely permeable.

What mechanism allows certain polar molecules to cross the cell membrane?

Random diffusion.

Osmosis.

Transport proteins or channels. (correct)

Active transport.

What describes the passive transport mechanism in cell membranes?

Depends on random thermal motion of molecules. Signup and view all the answers

What is the role of enzymes in the plasma membrane?

Serve as receptors and facilitate biochemical reactions. Signup and view all the answers

Which type of molecules can easily diffuse across the phospholipid bilayer?

Small non-polar molecules Signup and view all the answers

What happens to the rate of net diffusion when concentrations reach equilibrium?

It stops Signup and view all the answers

Which mechanism requires no ATP for movement across the membrane?

Facilitated diffusion Signup and view all the answers

At what distance does oxygen diffusion take approximately 1.7 seconds?

100 μm Signup and view all the answers

What characterizes facilitated diffusion compared to simple diffusion?

It involves protein channels or carriers Signup and view all the answers

Which scenario indicates a substance diffusing from greater to lesser concentration?

Oxygen entering a cell Signup and view all the answers

What is the correct relationship between diffusion time and distance for proteins compared to glucose?

Glucose diffuses faster than proteins at all distances. Signup and view all the answers

What is the primary function of carrier proteins in membrane transport?

To bind and change shape to transport specific molecules Signup and view all the answers

How does facilitated diffusion differ from simple diffusion?

It is mediated by membrane proteins and is highly selective Signup and view all the answers

What distinguishes ligand-gated channels from other types of ion channels?

They only open when a specific molecule binds to them Signup and view all the answers

What type of channel is always open regardless of external conditions?

Constitutively open (passive) channels Signup and view all the answers

What role does phosphorylation play in the function of phosphorylation-gated channels?

It functions as a signal for the channel to close or open Signup and view all the answers

Which factor primarily influences the selectivity of ion channels?

The charge of the ions combined with size Signup and view all the answers

How does acute regulation of facilitated diffusion differ from chronic regulation?

Acute regulation occurs much more quickly than chronic regulation Signup and view all the answers

How do electrical forces contribute to ion transport across the membrane?

They influence the net flux of ions through the selective channels. Signup and view all the answers

What is the primary function of the pump described in the content?

To maintain high K+ concentration and low Na+ concentration inside the cell Signup and view all the answers

What characterizes secondary active transport?

It uses an electrochemical gradient to move other molecules against their gradients. Signup and view all the answers

Which of the following best describes exocytosis?

The fusion of intracellular vesicles with the plasma membrane to release contents outside. Signup and view all the answers

What distinguishes phagocytosis from pinocytosis?

Phagocytosis involves solid materials, while pinocytosis involves liquids. Signup and view all the answers

What role do kinesins and dyneins play in vesicular trafficking?

They carry loaded vesicles along microtubules. Signup and view all the answers

What is the primary function of K+ leakage channels in a cell?

To allow K+ ions to leak out of the cell Signup and view all the answers

What outcome occurs when the electrical gradient opposing K+ leakage balances the concentration gradient favoring K+ leakage?

The net flux of K+ ions becomes zero Signup and view all the answers

Which statement accurately describes the Na+/K+ pump?

It requires ATP to transport ions against their electrochemical gradients Signup and view all the answers

What creates the cell membrane potential in an excitable cell?

The separation of charges across the cell membrane Signup and view all the answers

How is the net movement of ions across the membrane determined?

By the electrochemical gradient, concentration gradient, and electrical forces Signup and view all the answers

Which of the following ions is primarily concentrated outside the cell?

Na+ Signup and view all the answers

What is the effect of the intracellular negativity on K+ ion movement?

It opposes the movement of K+ ions out of the cell Signup and view all the answers

What type of transport is the Na+/K+ pump an example of?

Active transport Signup and view all the answers

Study Notes

Cell Membranes

Cell membranes maintain the structural integrity of the cell and define its boundaries.
They act as selective barriers, regulating the transport of substances in and out of the cell.
They control the cell's internal environment and contain enzymes and receptors.
Comprised of about 50% protein and 50 lipid molecules per protein.

Membrane Permeability

The permeability of the cell membrane determines which substances can enter or leave the cell.
Freely permeable membranes allow any substance to cross.
Impermeable membranes prevent any substance from crossing.
Cell membranes are selectively permeable, allowing the passage of some materials while restricting others.
The lipid bilayer structure generally restricts the passage of polar molecules.
Cells can insert specific transport proteins or channels into the membrane to permit the passage of certain polar molecules.
Variations in the organization and identity of membrane lipids and proteins determine the permeabilities of different cells and organelles.

Membrane Transport Mechanisms

Passive transport does not require ATP.
Active transport requires ATP.

Diffusion

All molecules exhibit random thermal motion, resulting in a random walk.
Diffusion is only effective for transport over short distances.
The rate of diffusion depends on the size of the molecule.
Net diffusion occurs from an area of greater concentration to an area of lesser concentration.
Net diffusion stops when concentrations reach equilibrium.

Net Diffusion of Molecules Across a Cell Membrane

The rate of net diffusion is described by Fick's Law.
Factors influencing diffusion rate:
- Diffusion coefficient (dependent on molecule size)
- Surface area of the membrane
- Concentration difference across the membrane
- Thickness of the membrane

Simple Diffusion

Water, O2, CO2, and lipid-soluble molecules like fatty acids and steroids can easily diffuse across the phospholipid bilayer.

Facilitated Diffusion

Hydrophilic molecules (K+, Na+, Ca2+, amino acids, monosaccharides) cannot pass directly through the phospholipid bilayer.
They require protein channels or carrier proteins to cross the membrane.
Facilitated diffusion occurs when hydrophilic molecules are aided through the membrane down their concentration gradient.
Important metabolites are often acted upon immediately upon entering the cell, keeping intracellular concentration low and facilitating further uptake.

Carrier Proteins

Carrier proteins have binding sites for the transported molecule.
Binding causes a conformational change in the protein, releasing the molecule on the other side of the membrane.

Channels

Channels form permanent water-filled pores spanning the membrane.
They have a selectivity filter that allows only specific ions to pass through.
Selectivity is based on size and charge of the ion.

Classification of Channels by Channel Gating

Constitutively open (or passive) channels: almost always open (e.g., water channels, K+ leak channels, Na+ leak channels).
Ligand-gated channels: open upon binding of a specific molecule (the ligand).
Phosphorylation-gated channels: open or closed depending on the phosphorylation state of the channel protein.
Voltage-gated channels: open or closed in response to changes in membrane potential.
Mechanically-gated channels: open or closed in response to stretching or pulling forces (found in sensory receptors).

Electrical Forces and Membrane Potential

Cells use energy to concentrate K+ inside the cell and Na+ outside the cell.
K+ leakage channels allow K+ to leak out of the cell down its concentration gradient.
This leakage creates a separation of charges across the membrane, resulting in the cell membrane potential.
All cells have a membrane potential.
The electrical gradient opposes the movement of K+ ions out of the cell, balancing the chemical gradient at equilibrium.
Ions diffuse according to their electrochemical gradient, which combines the concentration gradient and the electrical gradient.

Active Transport

Active transport pumps molecules against their electrochemical gradient using energy.
Primary active transporters (or pumps): directly use ATP to transport molecules against their gradients.
Secondary active transport: utilizes the energy stored in an electrochemical gradient (like the Na+ gradient) to move other molecules against their electrochemical gradients. Transport of the two molecules is coupled.

Example: Na+/K+ Pump

Uses the energy released by ATP hydrolysis to move K+ and Na+ across the plasma membrane against their electrochemical gradients.
Maintains high intracellular K+ concentration and low intracellular Na+ concentration.
For every ATP, 3 Na+ ions are transported out of the cell and 2 K+ ions are transported in.

Example: Transepithelial Transport of Glucose in the Intestine

Uses secondary active transport with the Na+ gradient to move glucose across the intestinal epithelium.

Vesicular Transport (or Bulk Transport)

Exocytosis: moves substances out of the cell.
Endocytosis: moves substances into the cell.
Transcytosis: moves substances across a cell layer.
Substance (or Vesicular) Trafficking: moves substances from one area of the cell to another.
All require energy.

Vesicular Transport: Exocytosis

Intracellular vesicles fuse with the plasma membrane, spilling the contents of the vesicle out of the cell.
Requires Ca2+.

Vesicular Transport: Endocytosis

Infoldings in the plasma membrane import materials into the cell.
Types of endocytosis:
- Phagocytosis: cell eating (engulfing large particles).
- Pinocytosis: cell drinking (engulfing fluid).
- Receptor-mediated endocytosis: specific molecules bind to receptors on the plasma membrane, triggering endocytosis.

Phagocytosis

Uses pseudopods to engulf large particles.
Important for immune cells to engulf bacteria and debris.

Vesicular Trafficking

Motor molecules called kinesins and dyneins carry loaded vesicles along microtubules.
This transport allows for efficient movement of substances within the cell.

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Description

Explore the fundamental concepts of cell membranes, their structure, and their role in maintaining cellular integrity. This quiz covers the selectivity of membranes and the mechanisms that regulate the transport of substances in and out of cells. Test your knowledge on membrane permeability and the lipid bilayer's functions.