Biological Membranes and Phospholipids Unit 2

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Questions and Answers

What are biological membranes?

Biological membranes are fluid, flexible, and dynamic.

What are the key questions explored regarding phospholipids and biological membranes?

How do phospholipid molecules assemble into biological membranes? What are phospholipids made of? Do all substances pass through these membranes with equal ease? Does the structure of a membrane play a role in the movement of substances across it?

Explain how the structure of a phospholipid molecule contributes to the formation of a phospholipid bilayer.

Phospholipids have a hydrophilic head, which is attracted to water, and a hydrophobic tail, which repels water. In an aqueous environment, phospholipids spontaneously arrange themselves into a bilayer with the hydrophilic heads facing the water and the hydrophobic tails facing inward, forming a barrier between the internal and external environments of the cell.

The hydrophobic head of a phospholipid molecule dissolves readily in water.

False Signup and view all the answers

What is the role of the smooth endoplasmic reticulum in the synthesis of phospholipids?

The smooth endoplasmic reticulum synthesizes phospholipids and transports them as vesicles to the cell membrane. Signup and view all the answers

Which of the following are examples of polar compounds?

glucose Signup and view all the answers

Describe the arrangement of phospholipids in a lipid bilayer.

In a lipid bilayer, phospholipids arrange themselves with their hydrophilic heads facing both the aqueous environment outside and inside the cell, while their hydrophobic tails are tucked away in the interior of the membrane, forming a barrier between the two. Signup and view all the answers

Why are molecules that have both hydrophilic and hydrophobic ends called amphipathic?

Amphipathic molecules have both hydrophilic and hydrophobic ends, allowing them to interact with both polar and non-polar environments. Signup and view all the answers

What are the two main consequences of the amphipathic nature of the phospholipid?

They form a bilayer in water, creating a suitable barrier. Signup and view all the answers

Explain why phospholipids form bilayers in water, referencing the hydrophilic phosphate heads and hydrophobic hydrocarbon tails.

Phospholipids have a hydrophilic phosphate head and two hydrophobic hydrocarbon tails. In an aqueous environment, the hydrophilic heads are attracted to water and position themselves on either side of the bilayer, while the hydrophobic tails face each other within the membrane, minimizing their exposure to the water. Signup and view all the answers

The ______ of the membrane allows for formation of vesicles.

fluidity Signup and view all the answers

Describe the fluid mosaic model of the membrane structure.

The fluid mosaic model depicts the cell membrane as a dynamic structure composed of a phospholipid bilayer embedded with proteins. The phospholipid bilayer provides a fluid medium for the proteins to move laterally within the membrane, while the proteins act as a mosaic, performing various functions. Signup and view all the answers

Why is the plasma membrane described as a fluid structure?

All of the above Signup and view all the answers

Some membrane proteins can move freely within the membrane, while others are tethered to the cytoskeleton and cannot move far.

True Signup and view all the answers

What are the two main types of proteins that make up the mosaic aspect of the cell membrane?

The two main types of proteins are integral proteins and peripheral proteins. Signup and view all the answers

Describe the difference between integral and peripheral proteins in terms of their location and function.

Integral proteins are embedded within the cell membrane, spanning the entire structure, while peripheral proteins are attached to the outer surface of the membrane. Integral proteins often act as channels for transport, enzymes, carriers, or receptors, while peripheral proteins serve as scaffolding, receptors, or transport shuttles. Signup and view all the answers

Explain why the protein content of membranes varies.

The protein content of membranes varies depending on the specific function of the membrane. For example, the myelin sheath around neurons has a relatively low protein content compared to the plasma membrane around cells, which has a higher protein content. Signup and view all the answers

What are glycoproteins?

Glycoproteins are proteins with carbohydrates attached to their non-protein component. Signup and view all the answers

Describe the structure of a glycoprotein and its location in the cell.

A glycoprotein is made up of a protein component embedded within the cell membrane, with a carbohydrate component projecting out into the cell's exterior environment. Signup and view all the answers

What are the key functions of glycoproteins?

Glycoproteins play crucial roles in cell recognition by the immune system, hormone receptor interactions, and cell adhesion. Signup and view all the answers

What are cell adhesion proteins?

Cell adhesion proteins are proteins that anchor the cell membrane to structures inside the cell, such as the cytoskeleton, proteins outside the cell, and other cells. Signup and view all the answers

What is a ligand?

A ligand is a molecule that binds to a receptor, triggering a specific cellular response. Signup and view all the answers

Describe the function of the insulin receptor.

The insulin receptor binds to insulin, a hormone released by the pancreas when blood sugar levels are high. This binding triggers a cascade of events that causes the cell to open glucose transport proteins, allowing glucose to enter the cell from the blood and lower blood sugar levels. Signup and view all the answers

What is the function of cell recognition proteins?

Cell recognition proteins serve as identification tags on the surface of cells, allowing the immune system to distinguish between self and non-self cells. Signup and view all the answers

Explain the role of Major Histocompatibility Complex (MHC) proteins in cell recognition.

MHC proteins are found on the surface of cells and act as markers that identify cells as belonging to a particular individual. These MHC proteins interact with immune cells to determine which cells belong to the body and which cells are foreign. Signup and view all the answers

Which of the following is NOT a function of glycoproteins?

Energy production Signup and view all the answers

What are glycolipids?

Glycolipids are molecules composed of carbohydrates linked to lipids. Signup and view all the answers

How do glycolipids contribute to the structure and function of the cell membrane?

Glycolipids are located within the cell membrane, with their carbohydrate component projecting out into the extracellular environment. This structure enables them to play roles in cell recognition, cell adhesion, and signal transduction, contributing to the cell's overall function. Signup and view all the answers

Why are glycolipids considered important for cell recognition?

The carbohydrate chains associated with glycolipids have specific shapes that can be recognized by the immune system, allowing cells to be identified as self. Signup and view all the answers

What is a semipermeable membrane?

A semipermeable membrane allows the passage of certain small solutes while being freely permeable to the solvent, typically water. Signup and view all the answers

What is a selectively permeable membrane?

A selectively permeable membrane allows the passage of specific molecules or particles, while preventing the passage of others. Signup and view all the answers

What are the four main mechanisms for molecule movement across the cell membrane?

Molecules can move across the cell membrane through simple diffusion, facilitated diffusion, osmosis, and active transport. Signup and view all the answers

What is simple diffusion?

Simple diffusion is the passive, net movement of particles from areas of high concentration to areas of low concentration. Signup and view all the answers

Which of the following correctly describes facilitated diffusion?

A and E Signup and view all the answers

What is osmosis?

Osmosis is the movement of solvent molecules, typically water, across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. Signup and view all the answers

What is active transport?

Active transport is the movement of molecules across a membrane against their concentration gradient, requiring energy in the form of ATP, typically provided by protein pumps. Signup and view all the answers

Describe the process of active transport using protein pumps.

Protein pumps are integral membrane proteins that use energy from ATP to move molecules against their concentration gradients. The molecule binds to the protein pump, ATP is hydrolyzed, the pump changes shape, and the molecule is released on the other side of the membrane. Signup and view all the answers

How does a channel protein facilitate diffusion?

Channel proteins are integral proteins that form a passageway for molecules to move across the membrane down their concentration gradient. They do not require energy as they facilitate passive diffusion. Signup and view all the answers

Study Notes

Unit 2 Transport

Biological membranes are fluid, flexible, and dynamic
Key questions include: How do phospholipids assemble into biological membranes? What are phospholipids made of? Do all substances pass through membranes with equal ease? Does the membrane structure affect substance movement?

Lipids and Phospholipids

Lipids are one of the four biological molecules found in organisms.
Phospholipids make up cell membranes.
Phospholipids are essentially triglycerides in which a fatty acid has been replaced by a phosphate group.
Phospholipids have a hydrophilic head and hydrophobic tails.
The hydrophilic head dissolves in water.
The hydrophobic tails do not dissolve in water.

Formation of Phospholipid Bilayers

The hydrophobic and hydrophilic regions of phospholipids cause them to spontaneously form a double layer (phospholipid bilayer) in water.
The hydrophobic tails are sandwiched between two layers of hydrophilic heads.
The heads of phospholipids face the water, while the tails interact with each other.
This structure forms a barrier, making the membrane selectively permeable.
Phospholipids are synthesized in the smooth endoplasmic reticulum and transported to the cell membrane.

Amphipathic Nature of Phospholipids

Amphipathic molecules have both hydrophilic and hydrophobic regions.
This causes the bilayer formation in water.
The attractions between the hydrophobic tails and hydrophilic heads make the bilayer stable.
This stability and flexibility are key in allowing for cell shape variability and functions like endocytosis.

Consequences of Amphipathic Nature

Phospholipids form a bilayer in water serving as a barrier.
Internal attractions between hydrophobic tails and hydrophilic heads make the bilayer even more stable.

Exam Question - Phospholipid Bilayer

Phospholipids form bilayers in water due to their amphipathic nature. The hydrophilic heads interact with water, while the hydrophobic tails face each other, away from water creating a barrier.
Drawing a phospholipid bilayer should show a two-layer structure with phosphate heads (circle) facing outward towards the water and hydrophobic tails (squiggly lines) facing inward.

Lipid Bilayers as Barriers (Page 9)

Cell membranes have water on both sides (cytoplasm on the inside, extracellular fluid on the outside).
Phospholipids arrange themselves to form a bilayer, orientating hydrophilic heads to face the water and hydrophobic tails towards each other.
The hydrophobic tails face inwards, away from the water, creating a barrier.

Lipid Bilayers as the Basis of Cell Membranes

Cell membranes are flexible and supportive.
Cellular membranes (plasma and organelle membranes) have a similar structure.
Membranes hold cell contents together.

Fluid Mosaic Model of Membrane Structure

Membranes are fluid mosaics of lipids and proteins.
Proteins within the membrane can move.
The bilayer portion is the fluid component.
A collection of embedded proteins creates a mosaic.
Membranes are dynamic and ever-changing.
The phospholipid bilayer is flexible and permits changes in cellular shape and allows for lateral movement of lipids and some proteins.

Membrane Fluidity

Phospholipid bilayers are flexible due to individual phospholipid molecules being not bonded and proteins drifting more slowly.
Membrane lipids and some proteins move within the membrane.
Tethered membrane proteins are anchored to the cytoskeleton and cannot move far.

Fluid Mosaic Model: Proteins (Page 20)

Integral proteins span the membrane and are permanently embedded; examples are transmembrane proteins.
Peripheral proteins are not embedded, are attached to one surface, and attach only temporarily; they might attach to integral proteins.
Carbohydrates are variably present.

Integral and Peripheral Proteins in Membranes (page 24, 26, 27, 28)

Membranes have varying protein content due to function differences.
Integral proteins contain hydrophobic amino acids that are embedded in the inner hydrophobic tails of the lipid bilayer; some are transmembrane and extend across both layers.
Peripheral proteins are attached to one surface, often as scaffolding, that can hold shape and be receptors.

Glycoproteins and Glycolipids

Glycoproteins are proteins with carbohydrates; often embedded in the cell membrane, they project towards the extracellular environment.
Some functionalities include cell recognition by the immune system, cell adhesion, hormone receptors, and markers that identify self cells.
Glycolipids are composed of carbohydrates and lipids; the hydrocarbon chains naturally fit into the core of the cell membrane.

Cell Adhesion Proteins

These proteins attach to cell membranes and the inner/outer cytoskeleton, and also other cells.
Cell adhesion proteins can be integral or peripheral within the membrane

Cell Recognition Protein Example (MHC)

Major Histocompatibility Complex (MHC) proteins are specific to an individual.
These proteins help identify cell structures that belong to the body versus foreign cells to the immune system

Movement of Water Molecules by Osmosis and Aquaporins

Osmosis involves water moving from low to high solute concentrations through a semi-permeable membrane
Aquaporins are integral proteins that act like pores speeding to facilitate water movement.
A cell submerged in water will see water molecules pass from outside the cell (low solute concentration) to inside the cell (high solute concentration).

Selectivity in Membrane Permeability

Semi-permeable membranes allow small molecules and solvents to pass freely.
Selectively permeable membranes control the types of particles to pass through.
Membrane characteristics depend on molecule size and hydrophobicity/hydrophilicity.
Biological membranes serve as barriers based on their hydrophobic core and hydrophilic exterior.

Simple Diffusion

Simple diffusion is a passive movement of solute particles from high to low concentration across a selectively permeable membrane.
Oxygen and small, non-polar molecules can readily diffuse this way.
The rate of simple diffusion is not affected by transport proteins.

Facilitated Diffusion

Facilitated diffusion is a passive transport mechanism that uses transport proteins to move molecules/ions along a concentration gradient to help move molecules that cannot directly cross the membrane.
It doesn't require ATP.
The rate is dependent on transport protein numbers,.

Primary Active Transport

Primary active transport moves molecules against their concentration gradient using ATP energy and protein pumps.
Integral protein pumps use ATP hydrolysis to move molecules across the membrane.

Active Transport (Page 60)

Active Transport moves molecules across a membrane from a region of low concentration to a higher concentration using energy that the cell produces (requires ATP energy). It is actively done by the cell to maintain essential functions.

Protein Pumps

Protein pumps use energy (i.e. from ATP) to move molecules against their concentration gradient.
The energy from ATP changes the shape of a protein pump enabling it to carry the molecule through the membrane.

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Description

Explore the fascinating world of biological membranes and phospholipids in this quiz. Learn about the structure, composition, and functions of phospholipids, and how they form essential cell membranes. Test your understanding of how substance movement is influenced by membrane structure.