Chapter 5: Membrane Structure and Function

What is the freeze-fracture method?

Freezes then splits membrane so that upper and lower layers separate; protein remain intact and go with one layer or the other

What are Integral proteins?

Embedded proteins in the membrane

Peripheral proteins

Proteins that only occur on the cytoplasmic side of the membrane

Which aspect of an integral protein allows it to associate with the nonpolar core of the membrane?

Its hydrophobic core region

What is the only type of cell that have an extracellular matrix?

Animal

What does the extracellular matrix contain?

Various protein fibers and very large complex carbohydrate molecules

Are membranes rigid?

No, they are flexible

Which model is used to describe the plasma membrane?

The fluid mosaic model

Why are cells pliable?

The phospholipid bilayer is fluid

The content of which substance is responsible for the fluidity of a cell membrane?

Lipid

What happens to the phosopholipid bilayer as the concentration of unsaturated fatty acid residues increase?

The bilayer becomes more fluid

Why is it rare for phospholipid molecules to flip flop from one monolayer to the other?

This would require the hydrophilic head to move through the hydrophobic center of the membrane

State the importance of cholesterol molecules in the plasma membrane

It prevents the plasma membrane from becoming too fluid at higher temperatures and too solid at lower temperatures

How do cholesterol molecules prevent the plasma membrane from becoming too fluid at high temperatures?

It stiffens the membrane and makes it less fluid

How do cholesterol molecules prevent the plasma membrane from becoming too solid (frozen) at low temperatures?

By not allowing contact between certain phospholipid tails

Why are plasma membranes considered a mosaic?

Due to presence of many proteins (number + kinds of proteins varies)

What are glycolipids?

Phospholipids that have carbohydrate chains attached

What are glycoproteins?

Proteins that have carbohydrate chains attached

Why are membranes said to be asymmetrical?

Since carbohydrate chains only occur on the outside surface and peripheral proteins occur on one surface or the other

What is the glycocalyx?

The "sugar coat" that are given to the cell by the carbohydrate chains attached to proteins in animal cells

Describe the function of the glycocalyx

Protects the cell, cell-to-cell adhesion / recognition, repetition of signaling molecules

Describe the function of channel proteins

Form channels to allow substances to pass molecules through the membrane

Describe the function of carrier proteins

Receive substance and change its shape (allows substances to move across membrane)

Describe the function of cell recognition proteins

Helps body recognize when its being invaded by pathogens so that an immune response can occur

Describe the function of receptor proteins

Have shape that allows only specific molecules to bind to it (binding of molecule causes protein to change its shape and bring cellular response)

Describe the function of enzymatic proteins

Carry out metabolic reactions directly

Describe the function of junction proteins

Involved in forming various types of junctions between animal cells

What does it mean when the membrane is selectively permeable?

The membrane only allows certain substances into the cell while keeping others out

Why do polar molecules require energy to drive their transport and crossing across a membrane?

They are chemically incompatible with the center of the membrane

Which substances can freely cross a membrane?

Small, noncharged molecules (CO2, O2, glycerol, alcohol)

How are molecules freely able to cross the membrane?

They are able to slip between the hydrophilic heads of the phospholipids and pass through the hydrophobic tails of membrane (they are similarly nonpolar)

What do molecules do when they follow their concentration gradient?

Molecules move from an area of high concentration to low concentration

What are aquaporins?

Channel proteins that allow water to cross a membrane more quickly than expected

How do aquaporins prevent cell membranes from bursting from environmental pressure changes?

They equalize water pressure differences between their interior and exterior environments

How do ions and polar molecules move as quickly as necessary across the membrane?

They are assisted by carrier proteins

In what way are carrier proteins specific for the substances they transport across the plasma membrane?

The carrier protein must recognize particular shapes of molecules and must combine with an ion/molecule before changing its shape and transporting them across the membrane

Vesicle formation is reserved for the movement of which molecules?

Macromolecules or larger (such as a virus)

What is diffusion?

The movement of molecules from a higher to lower concentration (down their concentration gradient) until equilibrium is achieved and molecules are distributed equally

Is there any net movement between a solute and solvent after reaching equilibrium?

No, they continue to move about but there is no net movement

What are the factors that affect the rate of diffusion?

Temperature, pressure, electrical currents, and molecular size

As temperature increases, does the rate of diffusion increase or decrease?

Increases

What is osmosis?

Diffusion of water across a selectively permeable membrane

What does a greater osmotic pressure indicate?

The more likely it is that water will diffuse in that direction

Describe an isotonic solution

Solute concentration and water concentration both inside and outside the cell are equal (no net gain or loss of water)

Describe a hypotonic solution

Causes cells to swell or burst due to intake of water; a solution with a lower concentration of solute than inside the cell; low solute concentration and higher water concentration

How do you refer to a hypotonic solution in animal cells?

Lysis

How do you refer to a hypotonic solution in plant cells?

Turgid

Why don't plant cells burst in hypotonic solutions?

The cell wall prevents it from bursting

Describe a hypertonic solution

Causes cells to shrink/shrivel due to loss of water; high percentage of solute and low percentage of water

How do you refer to a hypertonic solution in animal cells?

Crenation

How do you refer to a hypertonic solution in plant cells?

Plasmolysis

Why don't diffusion and facilitated transport require energy?

Because the molecules are moving down their concentration gradient

What happens during active transport?

Molecules or ions move through the plasma membrane and accumulate either inside or outside the cell

What is required for active transport?

Carrier proteins and expenditure of energy (ATP)

Why are proteins called pumps?

Proteins use energy to move a substance against its concentration gradient as water pumps use energy to move water against the force of gravity

How are proteins, polysaccharides, and nucleic acids transported in or out of the cell?

By vesicles

What happens during phagocytosis?

When the material taken in by endocytosis is larger

What happens during pinocytosis?

Vesicles form around a liquid or very small particles

Describe receptor-mediated endocytosis

Receptor proteins recognize compatible molecules and bring them into the cell

What is a protective extracellular matrix?

A meshwork of proteins and polysaccharides in close association with the cell that produced them