Biology Chapter: Plasma Membrane Structure

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Questions and Answers

What is the effect of unsaturated fatty acids on membrane fluidity?

They decrease membrane fluidity.

They make membranes more rigid.

They increase membrane fluidity. (correct)

They have no impact on fluidity.

How does cholesterol at low concentrations affect the membrane?

It increases the membrane's overall fluidity. (correct)

It stabilizes the membrane's structure.

It decreases the fluidity of the membrane.

It makes the membrane more rigid.

What is a consequence of a significant increase in atmospheric pressure on membrane fluidity?

It improves membrane fluidity.

It has no effect on diffusion.

It hinders diffusion and decreases fluidity. (correct)

It enhances the movement of phospholipids.

What impact do shorter hydrocarbon chains have on membrane packing?

They stack poorly and allow for more movement. Signup and view all the answers

What role does cholesterol play at high temperatures in the membrane?

It increases fluidity. Signup and view all the answers

What is considered an isotonic solution for red blood cells in humans?

0.9 weight percent salt Signup and view all the answers

What primarily determines the selectivity of ion channels?

Pore size and charge on the inner wall Signup and view all the answers

Which type of ion channels is always open?

Passive or leakage channels Signup and view all the answers

What mechanism triggers the opening and closing of gated ion channels?

Specific stimuli Signup and view all the answers

How does membrane potential affect electrical activity in cells?

It influences the opening of ion channels. Signup and view all the answers

What type of channels open in response to binding a specific neurotransmitter?

Chemically-gated channels Signup and view all the answers

What is the role of active transport in the context of ion concentration?

It maintains ion concentrations far from equilibrium. Signup and view all the answers

Which ion channels open in response to physical deformation?

Mechanically-gated channels Signup and view all the answers

What structural characteristic allows phospholipids to form cell membranes?

They are amphipathic, having both hydrophilic heads and hydrophobic tails. Signup and view all the answers

What is the primary difference between carrier proteins and channel proteins in membrane transport?

Carrier proteins require a conformation change, while channel proteins do not. Signup and view all the answers

How do coupled transporters function in active transport?

One molecule moves against its gradient while another moves down its gradient. Signup and view all the answers

What occurs during the process of osmosis?

Water moves from low solute concentration to high solute concentration. Signup and view all the answers

Which of the following correctly defines an isotonic solution?

A solution with equal solute concentration inside and outside the cell. Signup and view all the answers

What energy source do ATP-driven pumps utilize to transport solutes?

Chemical energy from hydrolysis of ATP Signup and view all the answers

In endocytosis, how does the cell transport substances?

By engulfing substances in a vesicle that fuses with the membrane. Signup and view all the answers

What type of molecules can generally pass through a membrane by simple diffusion?

Fat-soluble and small uncharged molecules. Signup and view all the answers

What is the main role of membrane transport proteins?

To facilitate the movement of all types of solutes across the membrane. Signup and view all the answers

Which mechanism requires energy when transporting molecules across the cell membrane?

Active transport Signup and view all the answers

How does the concentration of K+ ions typically differ inside and outside of a cell?

K+ is higher inside the cell than outside. Signup and view all the answers

What happens to a cell placed in a hypertonic solution?

The cell shrinks due to water loss. Signup and view all the answers

Which of the following statements about facilitated diffusion is true?

It is a type of passive transport using specific membrane proteins. Signup and view all the answers

What is the main driving force behind osmosis?

Osmotic pressure from solute concentration differences Signup and view all the answers

What is the role of the Na+-K+ pump in animal cells?

To maintain a gradient of Na+ and K+ across the cell membrane. Signup and view all the answers

Study Notes

The Plasma Membrane: A Fluid Mosaic Model

Acts as the boundary between the cell and its surroundings.
Composed of a phospholipid bilayer.
Phospholipids are amphipathic, meaning they have both hydrophilic heads and hydrophobic tails.
The membrane is approximately 8 nanometers thick and is a dynamic structure.

Phospholipids: The Building Blocks of the Membrane

Composed of four structural elements: glycerol, two fatty acids, and a head group.
The hydrophobic fatty acid tails face inward, while the hydrophilic head groups face outward, interacting with the aqueous environment.

Membrane Transport Proteins: Facilitating Movement

Many molecules need to move across the membrane, both into and out of the cell.
Most molecules cannot pass through the membrane on their own and require the assistance of transport proteins.
These proteins act as "private passageways" for specific substances.
Different cells have different sets of membrane proteins, depending on their specific functions.

Movement of Small Molecules: Concentration Gradients and Permeability

Maintaining a balance of solutes on both sides of the membrane is crucial for cell function.
The concentration of ions varies significantly on each side (e.g., Na+ and Cl- are higher outside the cell, while K+ is higher inside).
The cell must balance the number of positive and negative charges both inside and outside.
Impermeable membranes prevent ions and hydrophilic molecules from easily passing through the hydrophobic membrane.
Only small, hydrophobic molecules can freely cross the membrane.

Carrier Proteins: Binding and Transport

One type of transport protein that moves solutes across the membrane by binding them on one side and transporting them to the other.
This process requires a conformational change in the carrier protein.

Channel Proteins: Creating Hydrophilic Pores

Another type of transport protein that forms small, hydrophilic pores in the membrane allowing solutes to pass through.
Movement through these channels occurs via diffusion.
They are also called ion channels when only ions move through them.

Carrier vs. Channel Proteins: A Comparison

Channels allow solutes to pass through if they have the right size and charge, acting like a "trapdoor".
Carriers require the solute to fit into a specific binding site, acting like a "turnstile".
This specificity makes carriers similar to enzymes and their substrates.

Mechanisms of Transport: Passive vs. Active

Molecules move from a higher to a lower concentration, following their concentration gradient, when a pathway exists.
This process does not require energy and is called passive transport or facilitated diffusion.
Moving molecules against their concentration gradient requires energy; this is called active transport.
Active transport requires the harnessing of an energy source by the carrier protein.

Active Transport: Using Energy to Move Molecules

Three main methods for transporting solutes against their electrochemical gradient:
- Coupled transporters: One molecule moves down its gradient, providing energy for another molecule to move up its gradient.
- ATP-driven pumps: Coupled to the hydrolysis of ATP, using the released energy to move molecules.
- Light-driven pumps: Uses light as an energy source like bacteriorhodopsin.

Exocytosis and Endocytosis: Transporting Large Molecules

These processes are used to transport large molecules across membranes.
Exocytosis is used to export bulky molecules from the cell.
Endocytosis is used to import useful substances into the cell.
Both processes involve packaging the material to be transported into a vesicle that fuses with the membrane.

Osmosis: The Movement of Water

Water moves from a region of low solute concentration (high water concentration) to a region of high solute concentration (low water concentration).
This movement is driven by osmotic pressure, which is caused by the difference in water pressure.

Definitions: Solution, Solute, and Solvent

Solution: A mixture of dissolved molecules in a liquid.
Solute: The substance that is dissolved.
Solvent: The liquid in which the solute is dissolved.

Osmotic Solutions: Tonicity

Tonicity refers to the relative concentration of solutes in a solution compared to another solution, usually a cell.
Isotonic: Equal solute concentration on both sides of the membrane.
Hypotonic: Lower solute concentration outside the cell; water rushes into the cell, potentially causing it to burst.
Hypertonic: Higher solute concentration outside the cell; water rushes out of the cell, causing it to shrivel.

Osmosis: Effects on Cells and Organisms

Animal cells maintain normal cell structure with the Na+-K+ pump, which prevents water from entering the cell.
Plant cells have cell walls, which create turgor pressure due to osmosis and active transport of ions into the cell, keeping leaves and stems upright.
Protozoans have special water-collecting vacuoles to remove excess water.

Channel Proteins: Creating Selective Openings

Channel proteins create hydrophilic openings in the membrane that allow small, water-soluble molecules to pass through.
Gap junctions and porins create very large openings.
Ion channels are highly specific in terms of pore size and the charge of the molecule being moved.
They primarily move Na+, K+, Cl-, and Ca2+.

Ion Channel Characteristics

Ion selectivity: Channels allow specific ions to pass through, while blocking others, based on pore size and charge.
Gating: The ability to regulate the movement of ions, allowing for control of ion concentrations within the cell.
- Channels are gated, meaning they can be open or closed.
- Specific stimuli, like neurotransmitters or changes in voltage, trigger the change in shape and opening or closing of the channel.

Membrane Potential: The Basis of Electrical Activity

The membrane potential is the difference in electrical charge across the cell membrane.
Active transport creates a difference in ion concentration across the membrane.
When ion channels open, ions rush in or out because of the concentration gradient, changing the voltage across the membrane.
This change in voltage triggers the opening or closing of other ion channels, enabling electrical activity to move across the membrane.

Diversity of Ion Channels

Ion channels vary in:
- Ion selectivity: Which ions they allow to pass through.
- Gating: The conditions that influence their opening and closing.

Types of Membrane Ion Channels

Passive, or leakage, channels: Always open.
Chemically (or ligand)-gated channels: Open upon binding with a specific neurotransmitter.
Voltage-gated channels: Open and close in response to changes in the membrane potential.
Mechanically-gated channels: Open and close in response to physical deformation of the membrane.

Membrane Fluidity: Influencing Factors

The fluidity of the membrane is sensitive to:
- Type of phospholipids present:
  - Unsaturated fatty acids: More fluid due to kinks in the tails, caused by double bonds, which stack poorly.
  - Shorter chains: Stack poorly, allowing for more movement.
- Cholesterol:
  - High concentration: Makes the membrane more rigid.
  - Low concentration: Makes the membrane more fluid and mobile.
- Temperature:
  - Higher temperature: Increases molecular motion, increasing fluidity.
- Atmospheric pressure:
  - Significant changes in pressure hinder diffusion and decrease fluidity.

Membrane Fluidity and Viscousity

Unsaturated hydrocarbon tails with kinks are more fluid.
Saturated hydrocarbon tails pack more tightly, making the membrane more viscous.

Cholesterol's Role in Membrane Fluidity

Acts as a buffer, adjusting fluidity at different temperatures:
- High temperature: Loosens the membrane.
- Low temperature: Stiffens the membrane.

Membrane Components: Forming Large Complexes

Some membrane components can form large molecular complexes, forming junctions that have limited mobility:
- Cell-cell junctions: Connect cells together.
- Cell-matrix junctions: Connect cells to the extracellular matrix.

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Description

Explore the intricacies of the plasma membrane and its role as a fluid mosaic model. This quiz covers the composition of phospholipids, the structure's dynamic nature, and the importance of membrane transport proteins in cellular movement. Test your understanding of the essential concepts in cell biology!