Cell Membrane Transport Mechanisms

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Questions and Answers

What is the first step of the signal transduction pathway in cellular communication?

Reception (correct)

Response

Degradation

Transduction

Which molecule is important for coated pit formation during receptor-mediated endocytosis?

Cytosol

Clathrin (correct)

LDL

Cholesterol

What is the likely consequence of a mutation in the LDL receptor based on the context provided?

Normal cholesterol levels

Increased cholesterol uptake

High cholesterol levels in blood (correct)

Decreased cholesterol synthesis

During receptor-mediated endocytosis, what occurs after substrates attach to membrane receptors?

Membrane pockets invaginate Signup and view all the answers

What triggers apoptosis in normal cellular processes?

Signal transduction Signup and view all the answers

What is the role of surface receptors in signal transduction?

Surface receptors recognize and bind signal molecules Signup and view all the answers

How do protein kinases contribute to cellular response pathways?

By transferring phosphate groups from ATP to target proteins Signup and view all the answers

What is the function of protein phosphatases in cellular signaling?

To remove phosphate groups from target proteins Signup and view all the answers

Why is amplification important in signal transduction pathways?

It allows a strong internal response from few signal molecules Signup and view all the answers

Which statement best describes the effect of signal molecule binding to a surface receptor?

It induces a structural change in the receptor Signup and view all the answers

Which component is primarily responsible for the fluidity of the lipid bilayer?

Phospholipids Signup and view all the answers

How does temperature affect membrane fluidity?

Low temperatures can pack phospholipids closely together, decreasing fluidity Signup and view all the answers

What is a key function of desaturases in membranes?

Produce unsaturated fatty acids during synthesis Signup and view all the answers

Which statement about membrane asymmetry is true?

The two halves of the membrane bilayer have distinct proteins Signup and view all the answers

What role do sterols play in membrane fluidity?

Decrease fluidity at high temperatures and increase it at low temperatures Signup and view all the answers

Which factor influences the fluidity of a lipid bilayer?

Composition of lipid molecules Signup and view all the answers

What would likely happen if the fatty acid composition of a membrane were to become predominantly saturated?

Decreased fluidity of the membrane Signup and view all the answers

What characteristic is true of the fluid mosaic model?

Proteins are embedded and float freely within a fluid lipid bilayer Signup and view all the answers

What is the primary function of antiport transport mechanisms?

To use energy from one ion's movement to transport another molecule in the opposite direction Signup and view all the answers

Which of the following processes involves moving large molecules or particles into a cell?

Bulk-phase endocytosis Signup and view all the answers

What defect is associated with cystic fibrosis according to the findings of Lap-Chee Tsui?

Defective cystic fibrosis transregulator protein (CFTR) Signup and view all the answers

What happens during exocytosis?

Secretory vesicles release their contents outside the cell Signup and view all the answers

In endocytosis, what is the role of the plasma membrane?

It encloses materials from outside the cell. Signup and view all the answers

Which type of endocytosis is characterized by the uptake of extracellular fluid along with solutes?

Pinocytosis Signup and view all the answers

What occurs to secretory vesicles after they fuse with the plasma membrane during exocytosis?

They release their contents into the cell exterior. Signup and view all the answers

How does the defective gene associated with cystic fibrosis affect protein structure?

It leads to a three nucleotide deletion causing improper folding. Signup and view all the answers

What is the primary characteristic of integral membrane proteins?

They are embedded within the phospholipid bilayer. Signup and view all the answers

Which membrane transport mechanism does NOT require energy?

Facilitated diffusion Signup and view all the answers

What is a key distinction of facilitated diffusion compared to simple diffusion?

It follows concentration gradients but depends on membrane proteins. Signup and view all the answers

Which type of proteins are mainly responsible for facilitated diffusion?

Channel and carrier proteins Signup and view all the answers

What happens to facilitated diffusion at high concentrations of the transported substance?

It becomes saturation limited. Signup and view all the answers

Which two factors significantly influence the rate of diffusion?

Concentration gradient and charge of molecules Signup and view all the answers

What type of molecules move rapidly through simple diffusion?

Small uncharged molecules Signup and view all the answers

Which of the following statements about passive transport is true?

It relies on the process of diffusion. Signup and view all the answers

Study Notes

Antiport

Moves ions through a membrane channel in one direction, providing energy for active transport of another molecule in the opposite direction.

Secondary Active Transport

A type of active transport that uses the energy from the movement of one molecule down its concentration gradient to drive the movement of another molecule against its concentration gradient.

Transport Mechanisms

Cellular processes that allow molecules and other substances to move across cell membranes.

Lap-Chee Tsui

Discovered the defective gene that causes cystic fibrosis.
The gene codes for the cystic fibrosis transmembrane conductance regulator protein (CFTR).
CFTR is a channel protein that allows chloride ions to pass across the plasma membrane.
A three nucleotide deletion in the gene leads to the loss of phenylalanine.
This causes improper folding and structure of the protein, leading to improper function.

Exocytosis and Endocytosis

Exocytosis releases molecules to the outside of the cell by means of secretory vesicles.
Endocytosis brings materials into cells in endocytic vesicles.

Transporting Larger Substances

Exocytosis and endocytosis move large molecules and particles in and out of cells.
Both processes require energy.

Exocytosis

Secretory vesicles carry secreted materials.
They move through the cytoplasm and contact the plasma membrane.
The vesicle membrane fuses with the plasma membrane, releasing contents to the cell exterior.
The process replenishes the plasma membrane.

Endocytosis

Encloses materials outside the cell in the plasma membrane.
Pockets inward and forms an endocytic vesicle on the cytoplasmic side.
There are two main forms:
- Bulk-phase (pinocytosis): Engulfs fluids along with dissolved solutes.
- Receptor-mediated endocytosis: Binds specific molecules to receptors on the cell surface, triggering the formation of vesicles that bring the molecules into the cell.

An Overview of the Structure of Membranes

The fluid mosaic model describes membranes.
Membranes consist of a fluid lipid bilayer in which proteins are embedded and float freely.
Membranes are not static.
Membranes are asymmetric and possess key functions.

Functions of the Membrane

Protection
Allow selective exchange of molecules between the cell and its environment.

Fluid Mosaic Model

The membrane is a fluid structure that allows the movement of proteins.

Membrane Asymmetry

Membranes are asymmetrical, meaning the membrane proteins of one half of the bilayer are structurally and functionally distinct from the other half.

Fluid Mosaic Model is Supported by Experimental Evidence: Membranes are Fluid

Proteins move freely in the membrane, allowing for movement of molecules.

Fluid Mosaic Model is Supported by Experimental Evidence: Membrane Asymmetry

Membranes have specialized regions, meaning they are asymmetrical.

The Lipid Fabric of a Membrane

Phospholipids are the dominant lipids in membranes.
Membrane fluidity is determined by the composition of the phospholipids and temperature.
Organisms can adjust fatty acid composition to regulate membrane fluidity.

Phospholipid Bilayers

Phospholipids are amphipathic, meaning they have both hydrophilic and hydrophobic regions, which contribute to the formation of the bilayer.

Maintaining Proper Fluidity

The fluidity of the lipid bilayer is dependent on how densely individual lipid molecules can pack together.
Fluidity is influenced by two major factors:
- The composition of the lipid molecules.
- Temperature.

1. Composition

Saturated fatty acids pack tightly together, making the membrane less fluid.
Unsaturated fatty acids have double bonds, which create kinks in the tails, making them less dense and the membrane more fluid.

2. Temperature

If the temperature drops low enough phospholipid molecules become closely packed and the membrane forms a highly viscous semisolid gel.
Membrane fluidity is also related to the degree to which membrane lipids are unsaturated.
Most membrane systems have a mixed population of saturated and unsaturated fatty acids.

Adjusting Fatty Acid Composition

Proper fluidity is maintained by adjusting fatty acid composition.
Desaturases are enzymes that produce unsaturated fatty acids during fatty acid synthesis.
Membrane fluidity is regulated by the regulation of desaturases.

Desaturases

At higher temperatures, they decrease activity, which leads to increases in saturated fats, and decreases in membrane fluidity.

Sterols

Sterols (ex. cholesterol) are lipids that can regulate membrane fluidity.
At high temperatures, they decrease fluidity, and at low temperatures, they increase fluidity.
Their presence increases membrane stability.

Membrane Proteins

Membrane proteins perform key functions including:
- Transport
- Enzymatic activity
- Signal transduction
- Attachment/recognition

Two Structural Categories of Membrane Proteins

Integral membrane proteins: Embed into the phospholipid bilayer.
Peripheral membrane proteins: Located on the surface of the membrane.

1. Integral Membrane Proteins

Embed into the phophpolipid bilayer.
Composed of predominantly nonpolar amino acids, often coiled into alpha helices.
They are called transmembrane proteins if they span the membrane.

2. Peripheral Membrane Proteins

Located on the surface of the membrane.
Do not interact with the hydrophobic core.
Held together by noncovalent bonds.
Most are on the cytoplasmic side of the membrane.
Made up of a mixture of polar and nonpolar amino acids.

Passive Membrane Transport

Passive transport does not require the cell to expend chemical energy (ex. ATP).

Passive Transport and Diffusion

Passive transport is driven by diffusion.
Diffusion: Net movement of a substance from a region of higher to lower concentration.
The rate of diffusion depends on the concentration difference or concentration gradient.
There are two types:
- Simple diffusion
- Facilitated diffusion

Simple Diffusion

The passive transport of substances across the lipid portion of the membrane with concentration gradients.
Small uncharged molecules move rapidly.
Large or charged molecules may be strongly impeded from crossing membranes.

2 Factors That Influence Diffusion: Size and Charge of Molecules

The size and charge of molecules determine how easily they can pass through the phospholipid bilayer.
Small, non-polar molecules pass through easily, while large, polar molecules and charged molecules need help from membrane proteins.

Facilitated Diffusion

Passive transport of substances at rates higher than predicted from their lipid solubility.
Depends on membrane proteins.
Follows concentration gradients.
Is specific for certain substances.
Becomes saturated at high concentrations of the transported substance.

Transport Proteins Carry Out Facilitated Diffusion

Integral membrane proteins can act as transport proteins.
There are two types:
- Channel proteins: Transport water and ions, such as K+ and Na+.
- Carrier proteins: Specific for single solutes, such as glucose.

Osmosis: The Passive Diffusion of Water

The movement of water across a semi-permeable membrane from a region of higher to lower water concentration.

Endocytosis: Pinocytosis

A form of endocytosis that brings fluids into cells in vesicles.

Receptor-Mediated Endocytosis

A form of endocytosis that targets specific molecules for uptake.

Familial Hypercholesterolemia

A genetic disorder that causes high cholesterol in the blood.
Defective receptor-mediated endocytosis of low-density lipoprotein (LDL) leads to the accumulation of cholesterol in the blood.

Phagocytosis

A form of endocytosis that engulfs large particles, such as bacteria or cell debris, into vesicles.
The vesicle fuses with a lysosome, where the engulfed material is broken down.

The Cellular Internet

All cells communicate with each other to regulate their activities.

It’s all about communication.

Cells communicate with each other through signaling pathways, which transmit information from the outside of the cell to the inside.

Signal Transduction Pathway

A signal on a cell’s surface is converted into a specific cellular response.

Why is communication so important in multicellular organisms?

Communication between cells enables complex organisms to function as a coordinated unit.

Intercellular Chemical Messengers

A cell can synthesize a specific molecule that acts as a signaling molecule to affect the activity of the target cell.

Signal Transduction

Cells process a signal in three steps:
- Reception: The signal molecule binds to a receptor protein on the cell’s surface.
- Transduction: The signal is converted into a form that can trigger a specific cellular response.
- Response: The cell performs a specific action in response to the signal.

Surface Receptors

Surface receptors are integral membrane proteins that recognize and bind signals.
Binding of a signal molecule induces a molecular change in the receptor, activating its cytoplasmic end.
This activation transmits the signal inside the cell.

Response of Surface Receptor

The binding of a signal molecule changes the shape of the receptor, switching it from an inactive to an active form, which can then trigger a cascade of events inside the cell.

Cellular Response Pathways

The binding of a signal molecule to a surface receptor triggers a cellular response without entering the cell.
The signal is relayed inside the cell by protein kinases.
Protein kinases are enzymes that transfer a phosphate group from ATP to particular target proteins.
This phosphorylation stimulates or inhibits the activity of target proteins, producing the cellular response.

How do we balance cellular response pathways?

Protein phosphatases reverse the response by removing phosphate groups from target proteins, turning off the signal transduction pathway.

Protein Kinase Cascade

A series of protein kinases that phosphorylate each other, amplifying the signal.

Cellular Response Pathways: Amplification

The signal transduction pathway amplifies the signal as it proceeds.
For example, each activated enzyme activates many other molecules in the next step of the pathway.
This amplification ensures that even a small number of signal molecules can induce a large cellular response.

Putting it into perspective

What is the purpose of membranes? Membranes serve as barriers and selectively control the movement of molecules into and out of cells. They also provide structural support and facilitate communication between cells.
How are they designed? Membranes are composed of a phospholipid bilayer with embedded proteins. The phospholipids form a barrier to the movement of polar molecules, while proteins provide pathways for the movement of specific molecules.
What are their mechanisms for selection? Membranes use passive transport mechanisms, such as simple diffusion and facilitated diffusion, and active transport to move molecules across their surface.
Why do cells need to communicate? Cells need to communicate with each other to coordinate their activities and ensure the proper functioning of the organism.
What are the three main steps of communication? The three main steps of communication are reception, transduction, and response. Reception involves the binding of a signal molecule to a receptor protein on the cell's surface. Transduction converts the signal into a form that can trigger a cellular response. Response is the specific action taken by the cell in response to the signal.

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Description

This quiz covers key concepts in cellular transport mechanisms, including antiport, secondary active transport, and cystic fibrosis genetics. Test your knowledge on how molecules move across cell membranes and the role of specific proteins like CFTR. Make sure to understand both exocytosis and endocytosis as essential processes.