Questions and Answers
Cholesterol within the phospholipid bilayer increases the lateral movement of molecules.
False
What are the two main types of proteins found in the plasma membrane?
Integral and peripheral proteins
The hydrophilic heads of phospholipids face _____ while the hydrophobic tails face _____
outward; inward
Which type of protein is responsible for transporting water-soluble ions across the plasma membrane?
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The plasma membrane is permeable to all types of molecules.
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Match the following components of the plasma membrane with their functions:
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What prevents excessive movement of molecules in high temperatures within the plasma membrane?
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Which of the following molecules can easily diffuse across the plasma membrane?
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Facilitated diffusion requires ATP to transport molecules across the membrane.
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What is the primary driving force behind simple diffusion?
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In a hypertonic solution, water will move _____ the cell.
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What occurs to animal cells when placed in a hypotonic solution?
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Match each term with its corresponding description:
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What is osmosis?
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Active transport can move molecules from high to low concentration.
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What effect does cholesterol have on membrane stability?
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E.coli uses cholesterol to maintain its cell shape.
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What mechanism allows LDL to enter liver cells?
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The cell wall of prokaryotes like E.coli is primarily made of _____
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Match the following cell types with their characteristics:
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Which transport mechanism requires energy to move substances against a concentration gradient?
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Cholesterol molecules in the cell membrane increase its fluidity at high temperatures.
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What is the term for water movement through a selectively permeable membrane?
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Carrier proteins are used in both _____ transport and _____ transport mechanisms.
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Match the type of transport with its characteristic:
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Which of the following is a characteristic of the fluid mosaic model?
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In co-transport, both molecules move in the same direction across the membrane.
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Name one adaptation of cell membranes that enhances transport efficiency.
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Study Notes
Plasma Membrane Overview
- Plasma membranes encompass cell membranes and membranes surrounding organelles like mitochondria.
- Described by the fluid mosaic model, indicating a mixture of various components (mosaic) and movement of molecules (fluid).
Components of the Plasma Membrane
- Phospholipids: Form a bilayer with hydrophilic heads facing outward and hydrophobic tails inward, creating a barrier.
- Cholesterol: Embedded within the phospholipid bilayer; restricts lateral movement of molecules, contributing to membrane rigidity.
- Proteins: Can be integral (spanning the entire membrane) or peripheral (attached to one side). Integral proteins include channel and carrier proteins.
Phospholipid Structure
- Hydrophilic Head: Contains a negatively charged phosphate group and glycerol; attracts water.
- Hydrophobic Tails: Composed of fatty acid chains (saturated and unsaturated); repel water but attract other lipids.
Role of Cholesterol
- Maintains membrane fluidity by preventing excessive movement in high temperatures.
- Essential for preventing dehydration or membrane rupture due to fluidity changes in extreme temperatures.
Protein Functions in Membranes
- Peripheral Proteins: Provide mechanical support and connect proteins to lipids or carbohydrates (e.g., glycoproteins and glycolipids).
- Glycoproteins: Involved in cell recognition and signaling; consist of proteins connected to carbohydrates.
- Channel Proteins: Facilitate transport of water-soluble ions (e.g., sodium) across the membrane.
- Carrier Proteins: Bind larger molecules (e.g., glucose, amino acids); undergo conformational changes to transport these molecules across the membrane.
Selective Permeability
- The plasma membrane is selectively permeable, allowing only certain molecules to diffuse through.
- Lipid-soluble molecules and small nonpolar molecules (e.g., oxygen, carbon dioxide) can easily diffuse across the phospholipid bilayer.
- Larger or polar substances (e.g., sodium ions, glucose) cannot pass through by simple diffusion due to size or polarity.
Plasma Membrane Overview
- Plasma membranes surround all cells and organelles, such as mitochondria, establishing a protective barrier.
- The fluid mosaic model describes the plasma membrane structure, emphasizing the mixture of components and the dynamic movement of molecules.
Components of the Plasma Membrane
- Phospholipids: Arrange in a bilayer with hydrophilic (water-attracting) heads on the outside and hydrophobic (water-repelling) tails on the inside, forming a selective barrier.
- Cholesterol: Integrates within the phospholipid bilayer, modulating fluidity and providing structural integrity against excessive movement.
- Proteins: Include integral proteins that span the membrane and peripheral proteins located on the membrane surface. Integral proteins encompass channel and carrier types.
Phospholipid Structure
- Hydrophilic Head: Comprises a negatively charged phosphate group and glycerol; these heads face aqueous environments, attracting water.
- Hydrophobic Tails: Made up of fatty acid chains (both saturated and unsaturated) that repel water while attracting other lipids.
Role of Cholesterol
- Cholesterol stabilizes membrane fluidity, reducing excessive movements in high temperatures, which prevents potential membrane damage.
- It plays a critical role in maintaining membrane integrity and function under extreme temperature variations.
Protein Functions in Membranes
- Peripheral Proteins: Offer structural support and facilitate links between proteins and lipids or carbohydrates, including glycoproteins and glycolipids.
- Glycoproteins: Serve as recognition and signaling molecules, created by the attachment of carbohydrates to proteins.
- Channel Proteins: Allow selective transport of water-soluble ions, like sodium, through the membrane.
- Carrier Proteins: Transport larger molecules, such as glucose and amino acids, by binding and undergoing structural changes to move substances across the membrane.
Selective Permeability
- The plasma membrane functions as a selective barrier, regulating the passage of substances.
- Lipid-soluble and small nonpolar molecules (e.g., oxygen, carbon dioxide) readily diffuse through the bilayer due to their affinity for the hydrophobic environment.
- Larger or polar molecules (e.g., sodium ions, glucose) cannot diffuse simply, as their size or polarity restricts passage through the membrane.
Transport Across Membranes
- Plasma membranes are structured as a phospholipid bilayer, facilitating the passage of lipid-soluble molecules and small uncharged molecules like carbon dioxide (CO2), oxygen (O2), and water (H2O).
- Large polar molecules, such as glucose, are unable to diffuse through the bilayer unless aided by specific transport mechanisms.
Simple Diffusion
- Simple diffusion involves the movement of molecules from areas of high concentration to low concentration until equilibrium is reached.
- This process is passive; it does not require ATP as molecules naturally possess kinetic energy that enables their movement.
- Simple diffusion occurs only in liquids and gases; solids cannot diffuse through membranes.
Facilitated Diffusion
- Facilitated diffusion is a passive transport process that allows ions and polar molecules to move across membranes using specific proteins.
- It employs protein channels for transporting water-soluble ions and carrier proteins for larger molecules.
- Protein channels selectively open in response to specific ions, ensuring that only designated substances can pass.
- Carrier proteins change their shape upon binding with corresponding molecules, assisting their transport across the membrane.
Osmosis
- Osmosis refers to the movement of water from an area of higher water potential to an area of lower water potential via a partially permeable membrane.
- Water potential is a measure of the pressure exerted by water molecules, with pure water exhibiting a maximum water potential of zero.
- The presence of solutes lowers water potential, making it more negative as solute concentration increases.
Isotonic, Hypotonic, and Hypertonic Solutions
- Isotonic solutions have equal water potential inside and outside the cell, resulting in no net movement of water.
- Hypotonic solutions contain a higher water potential (less negative) compared to the cell, leading to water entering the cell which can cause lysis in animal cells.
- Hypertonic solutions possess a lower water potential (more negative) than the cell, causing water to exit the cell, which leads to cell shrinkage.
Effects on Cells
- In hypotonic solutions, animal cells risk bursting (lysis) and plant cells become turgid; plant cell walls provide structural support.
- Hypertonic solutions lead to water loss in both animal and plant cells, resulting in shrinkage.
Active Transport
- Active transport moves molecules from low to high concentration against the concentration gradient, a process that requires energy input.
- This transport mechanism relies on ATP and specific carrier proteins known as pumps.
- Carrier proteins selectively bind to target molecules; the hydrolysis of ATP supplies energy for a shape change in the protein, facilitating transport across the membrane.
- During active transport, ATP is converted into ADP and inorganic phosphate (Pi); the release of Pi allows the carrier protein to revert to its original shape, enabling continued transport.
Cell Surface Membrane Structure
- Mainly consists of a phospholipid bilayer, allowing selective permeability and flexibility.
- Phospholipids feature hydrophilic heads that interact with water and hydrophobic tails that avoid it, facilitating lipid solubility.
- Contains glycoproteins and glycolipids that play roles in cell recognition and signaling.
- Cholesterol molecules embedded within the bilayer enhance membrane stability and limit molecular movement during temperature changes.
- The fluid mosaic model illustrates the membrane's dynamic structure with various embedded proteins and lipids.
Types of Transport Mechanisms
-
Simple Diffusion:
- Small, nonpolar, or lipid-soluble molecules migrate from higher to lower concentrations directly through the lipid bilayer.
-
Facilitated Diffusion:
- Larger polar or water-soluble molecules pass through using channel proteins for direct passage or carrier proteins that change shape to transport specific substances (e.g., sugars, amino acids).
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Active Transport:
- Involves the movement of molecules from low to high concentration using energy derived from ATP hydrolysis and only utilizes carrier proteins.
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Co-Transport:
- Two molecules transported simultaneously; one moves down its gradient while the other moves against it. This mechanism is crucial for glucose absorption in the intestines, where sodium ions are co-transported with glucose.
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Osmosis:
- Water molecules move across a selectively permeable membrane from an area of high water potential to low water potential, influencing cell volume and pressure.
Adaptations of Cell Membranes
- Increased surface area through folded membranes improves transport efficiency, enhancing various cellular functions.
- A higher density of carrier and channel proteins facilitates the transport of larger or polar molecules across the membrane.
Cholesterol and Membrane Stability
- Cholesterol serves to stabilize membranes by reducing movement and maintaining structural integrity.
- Red blood cells, lacking external support from other cells, require higher cholesterol levels to maintain their shape.
Significance of the Cell Wall in Prokaryotes
- E. coli and other prokaryotes rely on a rigid cell wall made of peptidoglycan to maintain shape and structural integrity without cholesterol.
LDL Transport Mechanism
- Low-Density Lipoproteins (LDL) attach to specific receptors on liver cell membranes and enter the cell via a channel protein, enabling lipid-soluble molecules to enter through receptor-mediated endocytosis.
Exam Preparation Tips
- Understand various transport mechanisms: know when, how, and why each is used.
- Recognize the significance of cell membrane adaptations in different cell types for optimal function.
- Practice application-based questions to interpret experimental data regarding cell transport dynamics.
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