Membrane Structure and Function

Question 1

What are membranes?

Answer 1

membranes cover the surface of every cell, and also surround most organelles within cells. They have a number of functions, such as:

• keeping all cellular components inside he cell
• allowing selected molecules to move in an out of the cell
• isolating organelles from the rest of the cytoplasm, allowing cellular processes to occur separately
• a site for biochemical reactions
• allowing a cell to change shape

Question 2

Plasma Membrane

Answer 2

• is the boundary that separates the living cell from its nonliving surroundings
• selective permeable
• fluid mosaic of lipids and proteins
• lipid bilayer
• contains embedded proteins

Question 3

Phospholipids

Answer 3

• are the most abundant lipid in the plasma membrane
• are amphipathic, containing both hydrophobic (heads) hand hydrophilic (tails)
• head composed of phosphate group attached to one carbon of glycerol is hydrophilic
• two fatty acid tails are hydrophobic

Question 4

Daveson - Danielli Model

Answer 4

-proteins are not embedded on top

Question 5

Falsification of Daveson-Danielli Model

Answer 5

• freeze fracture
• proteins vary in size
• proteins are mostly hydrophobic

Question 6

Singer and Nicholson

Answer 6

-proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer of the plasma membrane

Question 7

Cholesterol

Answer 7

helps with fluidity

• disrupts the regular packing of the hydrocarbon tails of phospholipid molecules to prevent crystallizing
• restricts molecular motion
• help membranes to curve in a concave shape

Question 8

Phospholipid Bilayer

Answer 8

hydrophilic heads and hydrophobic tails

Question 9

Fluid Mosaic Model

Answer 9

• a membrane is a fluid structure with a “mosaic” of various proteins embedded in it when viewed from the top
• phospholipids can move laterally a small amount and can “flex” their tails
• membrane proteins also move side to side or laterally making the membrane fluid

Question 10

Freeze Fracture

Answer 10

studies of the plasma membrane supporting the fluid mosaic model of membrane structure

a cell is frozen and fractured with a knife. The fracture plane often follows the hydrophobic interior of a membrane, splitting the phospholipid bilayer into two separated layers. The membrane proteins go wholly with one of the layers.

Question 11

The Fluidity of Membranes

Answer 11

• phospholipids in the membrane can move within the bilayer two ways
• the type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane
• the steroid cholesterol has different effects on membrane fluidity at different temperature

Question 12

Membrane Proteins and their Functions

Answer 12

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

Question 13

Types of Membrane Proteins

Answer 13

Integral proteins
-penetrate the hydrophobic core of the lipid bilayer
-are often transmembrane proteins, completely spanning in the membrane
Peripheral Proteins
-are appending loosely bound to the surface of the membrane

Question 14

Six Major Functions of Membrane Proteins

Answer 14

Transport
Enzymatic Activity
Signal transduction
cell-cell recognition- ability to distinguish one type of neighboring cell from another
intercellular joining
attachment to the cytoskeleton and extracellular matrix

Question 15

Synthesis and Sidedness of Membranes

Answer 15

• membranes have distinct inside and outside faces
• this affects the movement of proteins synthesized in the endomembrane system
• membrane proteins and lipids are made in the ER and Golgi Apparatus

Question 16

Membrane Permeability

Answer 16

• membrane structure results in selective permeability

- a cell must exchange materials with its surrounding, a process controlled by the plasma membrane

Question 17

Permeability of the Lipid Bilayer

Answer 17

Hydrophobic molecules
-are lipid soluble and can pass through the membrane rapidly
Polar Molecules
-Do NOT cross the membrane rapidly

Question 18

Transport Proteins

Answer 18

Allow passage of hydrophilic substances across the membrane

Question 19

Passive Transport

Answer 19

• is diffusion of a substance across a membrane with no energy investment
• Diffusion of water is known as Osmosis

Question 20

Simple Diffusion

Answer 20

• the tendency for molecules of any substance to spread out evenly into the available space
• move from high to low concentration
• down the concentration gradient

Question 21

Effects of Osmosis on Water Balance

Answer 21

• Osmosis is the movement of water across a semipermeable membrane
• is affected by the concentration gradient of dissolved substances called the solution’s tonicity

Question 22

Water Balance of Cells without Walls

Answer 22

• Tonicity is the ability of a solution to cause a cell to gain or lose water
• has a great impact on cells without walls

Question 23

Isotonic Solutions

Answer 23

• concentration of solutes is the same as it is inside the cell
• there will be no net movement of water

Question 24

Hypertonic Solutions

Answer 24

• concentration of solutes if greater than it is inside the cell
• the cell will lose water (plasmolysis)

Question 25

Hypotonic Solutions

Answer 25

- concentration of solutes is less than it is inside the cell - the cell will gain water

Question 26

Water Balance of Cells with Walls

Answer 26

Turgor Pressure- is the pressure of water inside a plant cell pushing outward against the cell membrane

Question 27

Facilitated Diffusion

Answer 27

- is a type of passive transport aided by proteins - transport proteins speed the movement of molecules across the plasma membrane - channel proteins provide corridors that allow a specific molecule or ion to cross the membrane - carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane

Question 28

Active Transport

Answer 28

- uses energy to move solutes against their concentration gradients - requires energy, usually in the form of ATP - the sodium-potassium pump

Question 29

Contransport

Answer 29

- occurs when active transport of a specific solute indirectly drives the active transport of another solute - involves transport by a membrane protein - driven by a concentration gradient

Question 30

Bulk Transport

Answer 30

- across the plasma membrane occurs by exocytosis and endocytosis - large proteins cross the membrane by different mechanisms

Question 31

Exocytosis and Endocytosis

Answer 31

Exo- transport vesicles migrate to the plasma membrane, fuse with it, and release their contents End- the cell takes in macromolecules by forming new vesicles from the plasma membrane