Biology Chapter 7: Membrane Structure & Function

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Questions and Answers

Name the four major large molecules of all living things.

Lipids, Proteins, Carbohydrates, Nucleic Acids

Explain what is meant when we say a molecule is amphipathic.

It means the molecule has a hydrophobic and a hydrophilic region.

Describe the Davson-Danielli model of membrane structure. What were 2 problems with it?

A phospholipid bilayer between 2 layers of protein. Problem 1: Generalization that all membranes of the cells were identical. Problem 2: Protein placement was wrong.

Who proposed the fluid mosaic model? When? Describe.

S.J. Singer and G. Nicholson proposed it in 1972. This model proposes that membrane proteins are dispersed, individually inserted into the phospholipid bilayer with their hydrophilic regions protruding. Signup and view all the answers

What is membrane fluidity? Describe movements seen in the fluid.

It means that membranes are not locked in place; they can move because of weak bonds they experience. The movements are lateral. Signup and view all the answers

Describe how decreasing temperature affects membrane fluidity.

The phospholipids pack together, and the membrane solidifies. Signup and view all the answers

Describe how phospholipids with unsaturated hydrocarbon chains affect membrane fluidity.

They remain fluid to a lower temperature. Signup and view all the answers

Describe how cholesterol affects membrane fluidity.

It makes membranes less fluid at higher temperatures. Signup and view all the answers

Describe integral proteins.

They penetrate the hydrophobic core of the lipid bilayer. Signup and view all the answers

Describe peripheral proteins.

They are not embedded in the lipid bilayer but are appendages loosely bound to the surface of the membrane. Signup and view all the answers

What are the major functions of transport proteins?

A protein can provide a hydrophilic channel across the membrane or it can move substances from one side to the other by changing shape. Signup and view all the answers

What is the major function of enzymatic activity in membrane proteins?

A protein in the membrane can be an enzyme with its active site; several of the enzymes can form a team that carries out steps of a metabolic pathway. Signup and view all the answers

What is signal transduction in membrane proteins?

A membrane protein may have a binding site with a shape for a chemical messenger; external messengers can cause a shape change in the messenger protein. Signup and view all the answers

What is cell-cell recognition?

Some glycoproteins serve as identification tags recognized by membrane proteins of other cells. Signup and view all the answers

What is intercellular joining?

Membrane proteins of adjacent cells may hook together at junctions. Signup and view all the answers

What is attachment to the cytoskeleton and ECM?

Microfilaments or other parts of the cytoskeleton may be noncovalently bound to membrane proteins; proteins bound to ECM molecules can coordinate cellular changes. Signup and view all the answers

Distinguish between glycolipids and glycoproteins.

A glycolipid is a lipid attached to carbohydrates, while glycoproteins are proteins attached to carbohydrates. Signup and view all the answers

Distinguish between channel proteins and carrier proteins.

Channel proteins provide a channel that certain molecules or ions can use; carrier proteins change shapes to allow passengers to pass through. Signup and view all the answers

Are transport proteins specific?

True Signup and view all the answers

What are aquaporins?

A channel protein in the plasma membrane of cells that specifically facilitates osmosis. Signup and view all the answers

How does CO2 cross the membrane?

Dissolve in the lipid bilayer and cross with the help of membrane proteins. Signup and view all the answers

How does glucose cross the membrane?

Uses carrier proteins, holds onto their passengers, and changes shape to pass through the membrane. Signup and view all the answers

How does H+ cross the membrane?

Pass through a hydrophilic channel. Signup and view all the answers

How does O2 cross the membrane?

Dissolve in the lipid bilayer and cross with the help of membrane proteins. Signup and view all the answers

How does H2O cross the membrane?

Water molecules pass through aquaporins at the rate of about 3 billion a second. Signup and view all the answers

Define diffusion.

The movement of molecules so that they spread evenly in the available space. Signup and view all the answers

Define concentration gradient.

The region along which the density of a chemical substance increases or decreases. Signup and view all the answers

Define passive transport.

The diffusion of a substance across a biological membrane. Signup and view all the answers

Define osmosis.

Passive transport of water. Signup and view all the answers

Define isotonic.

Referring to a solution that has no effect on water. Signup and view all the answers

Define hypotonic.

Referring to a solution that will make the cell lose water. Signup and view all the answers

Define hypertonic.

Referring to a solution that will make the cell gain water. Signup and view all the answers

Define turgid.

Swollen or distended. Signup and view all the answers

Define flaccid.

Limp. Signup and view all the answers

Define plasmolysis.

When the cytoplasm shrivels and the plasma membrane pulls away from the cell wall, occurring when the cells lose too much water. Signup and view all the answers

What is facilitated diffusion?

It is the spontaneous passage of molecules or ions through a biological membrane with the assistance of specific proteins. It is passive transport. Signup and view all the answers

Why don't plant cells burst in hypertonic solutions?

True Signup and view all the answers

What is active transport?

It is the movement of substances across a cell membrane with a release of energy. It goes against the concentration gradient. Carrier proteins are involved. ATP supplies the power for this process. Signup and view all the answers

What is membrane potential?

It is the voltage across a membrane. The outside of the membrane is positive; the inside is negative. Signup and view all the answers

What are the two forces that drive diffusion of ions across the membrane?

Electrical and chemical forces. The combination of them is called the electrochemical gradient. Signup and view all the answers

What is cotransport?

It is the coupling of the downhill diffusion of one substance with the uphill transport of another against its concentration gradient. Signup and view all the answers

Define endocytosis.

Cellular uptake of biological molecules and particulate matter via formation of new vesicles. Signup and view all the answers

Define phagocytosis.

A type of endocytosis where large molecules are taken up by the cell. Signup and view all the answers

Define pinocytosis.

A type of endocytosis where the cell ingests extracellular fluid and its dissolved solutes. Signup and view all the answers

Define exocytosis.

The opposite of endocytosis. Signup and view all the answers

Define receptor-mediated endocytosis.

Movement of specific molecules into a cell by the inward budding of membranous vesicles. Signup and view all the answers

What is a ligand?

It is a molecule that binds specifically to another molecule, usually a large one. Signup and view all the answers

Study Notes

Major Biomolecules

Four major molecules in all living organisms: Lipids, Proteins, Carbohydrates, Nucleic Acids.

Amphipathic Molecules

Molecules with both hydrophobic (water-repellent) and hydrophilic (water-attracting) regions.

Davson-Danielli Model

Proposed structure: phospholipid bilayer sandwiched between two protein layers.
Problems:
- Oversimplified all cell membranes as identical.
- Incorrect assumptions about protein placement within the membrane.

Fluid Mosaic Model

Proposed by S.J. Singer and G. Nicolson in 1972.
Describes membrane with proteins dispersed individually in the phospholipid bilayer, allowing for fluid movement.

Membrane Fluidity

Membranes are flexible due to weak bonds, allowing lateral movement of components.

Effects of Temperature on Membrane Fluidity

Decreasing temperature leads to phospholipids packing closely, solidifying the membrane.
Phospholipids with unsaturated hydrocarbon chains maintain fluidity at lower temperatures.
Cholesterol reduces membrane fluidity at higher temperatures.

Integral vs. Peripheral Proteins

Integral proteins penetrate the hydrophobic core of the lipid bilayer.
Peripheral proteins are loosely attached to the membrane surface.

Functions of Membrane Proteins

Transport: Create hydrophilic channels or change shape to move substances across.
Enzymatic Activity: Act as enzymes in metabolic pathways.
Signal Transduction: Bind to external chemical messengers, triggering shape changes.
Cell-Cell Recognition: Glycoproteins act as identification tags for cells.
Intercellular Joining: Proteins connect adjacent cells at junctions.
Attachment to Cytoskeleton and ECM: Link cellular structures to membrane proteins or ECM components.

Glycolipids vs. Glycoproteins

Glycolipids are lipids with carbohydrate attachments; glycoproteins are proteins with carbohydrate attachments.

Channel Proteins vs. Carrier Proteins

Channel proteins provide openings for specific molecules; carrier proteins change shape to transport substances.

Specificity of Transport Proteins

Transport proteins are specific for the substances they transport.

Aquaporins

Channel proteins that facilitate water transport across cell membranes at high rates.

Crossing Membranes

CO2 and O2 dissolve in the lipid bilayer and rely on membrane proteins.
Glucose uses carrier proteins that change shape to pass through.
H+ ions require passage via hydrophilic channels.
Water molecules predominantly pass through aquaporins.

Key Transport Concepts

Diffusion: Movement of molecules to evenly distribute in available space.
Concentration Gradient: Variation in chemical substance density across a region.
Passive Transport: Movement across a membrane without energy expenditure.
Osmosis: Passive transport of water across a membrane.

Tonicity Definitions

Isotonic: Solutions that do not affect cell water content.
Hypotonic: Solutions that cause cells to lose water.
Hypertonic: Solutions that lead to water gain in cells.

Cell States

Turgid: Swollen state of a cell due to water intake.
Flaccid: Limp state of a cell when it loses water.
Plasmolysis: Cellular shrinkage and membrane detachment due to excessive water loss.

Facilitated Diffusion

Movement of molecules or ions through a membrane with the aid of specific proteins, requiring no energy.

Plant Cells in Hypertonic Solutions

Cell walls prevent bursting even in hypertonic environments.

Active Transport

Movement of substances against their concentration gradient, requiring energy (ATP) and involving carrier proteins.

Membrane Potential

Voltage difference across a membrane, with the outside positively charged and inside negatively charged.

Forces Driving Ion Diffusion

Electrical and chemical forces create the electrochemical gradient that influences ion movement.

Cotransport

Coupling of the downhill transport of one substance with the uphill transport of another against its concentration gradient.

Endocytosis Types

Endocytosis: Uptake of molecules through vesicle formation.
Phagocytosis: Uptake of large molecules or particles.
Pinocytosis: Ingestion of extracellular fluid and solutes.

Exocytosis

The process of exporting materials out of the cell, the reverse of endocytosis.

Receptor-Mediated Endocytosis

Specific uptake of molecules into a cell via vesicular inward budding, triggered by ligand binding.

Ligands

Molecules that specifically bind to larger molecules, often involved in signaling and uptake processes.

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Test your knowledge on the key concepts of membrane structure and function with this flashcard quiz from Biology Chapter 7. Learn about the major molecules of life, the characteristics of amphipathic molecules, and the Davson-Danielli model of membrane structure. Perfect for students preparing for exams!