Chapter 4

Question 1

if Phospholipids are mixed with water

Answer 1

-form spheres with the hydrophilic phosphate heads facing out towards the water and the hydrophobic fatty acid tails facing in towards each other-ball-like structures called micelles -Sheet-like structures called bilayers

Question 2

fluid mosaic model meaning

Answer 2

the currently accepted model of membrane structure, proposed by singer and Nicolson in 1972, in which protein molecules are free to move about a fluid bilayer of phospholipid molecules

Question 3

The fluid mosaic model describes cell membranes as ‘fluid’ because:

Answer 3

-The phospholipids and proteins can move around via diffusion -The phospholipids mainly move sideways, within their own layers -The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position

Question 4

The fluid mosaic model describes cell membranes as ‘mosaics’ because:

Answer 4

The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

Question 5

The phospholipid bilayers that make up cell membranes also contain proteins

Answer 5

-The proteins can either be intrinsic (or integral) or extrinsic (peripheral) -Intrinsic proteins are embedded in the membrane with their arrangement determined by their hydrophilic and hydrophobic regions -Extrinsic proteins are found on the outer or inner surface of the membrane

Question 6

Phospholipid bilayers can form compartments

Answer 6

– the bilayer forming the cell surface membrane establishing the boundary of each cell Internally, membrane-bound compartments formed from phospholipid bilayers provide the basic structure of organelles, allowing for specialization of process within the cell

Question 7

If phospholipids are spread over the surface of water they form

Answer 7

-a single layer with the hydrophilic phosphate heads in the water and the hydrophobic fatty acid tails sticking up away from the water -This is called a phospholipid monolayer

Question 8

Phospholipids structurally contain two distinct regions

Answer 8

-a polar head and two nonpolar tails -The phosphate head of a phospholipid is polar (hydrophilic) and therefore soluble in water -The fatty acid tail of a phospholipid is nonpolar (hydrophobic) and therefore insoluble in water

Question 9

The fluid mosaic model also helps to explain

Answer 9

-Passive and active movement between cells and their surroundings -Cell-to-cell interactions -Cell signalling

Question 10

Cellular membranes are formed from

Answer 10

a bilayer of phospholipids which is roughly 7nm wide and therefore just visible under an electron microscope at very high magnifications

Question 11

The Fluid Mosaic Model of Membranes

Answer 11

Membranes are vital structures found in all cells The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment, and intracellular membranes form compartments within the cell such as the nucleus, mitochondria and RER

Question 12

Cell membranes contain several different types of molecules

Answer 12

Three types of lipid: -Phospholipids -Cholesterol -Glycolipids (also containing carbohydrates)

Question 13

Two types of proteins

Answer 13

-Glycoproteins (also containing carbohydrates) -Other proteins (eg. transport proteins)

Question 14

Components of the cell surface membrane: Cholesterol

Answer 14

-Cholesterol molecules also have hydrophobic tails and hydrophilic heads -Fit between phospholipid molecules and orientated the same way (head out, tail in) -Are absent in prokaryotes membranes

Question 15

Components of the cell surface membrane: Glycolipids:

Answer 15

-These are lipids with carbohydrate chains attached -These carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)

Question 16

Components of the cell surface membrane: Proteins

Answer 16

-The proteins embedded within the membrane are known as intrinsic proteins (or integral proteins) -They can be located in the inner or outer phospholipid monolayer -Most commonly, they span the entire membrane – these are known as transmembrane proteins -Proteins can also be found on the inner or outer surface of the membrane, these are known as extrinsic proteins (or peripheral proteins)

Question 17

transmembrane proteins examples

Answer 17

Transport proteins are an example of transmembrane proteins as they cross the whole membrane

Question 18

The Cell Surface Membrane

Answer 18

-Phospholipids -Cholesterol -Glycolipids & glycoproteins -Proteins

Question 19

Functions of phospholipids

Answer 19

~Form the basic structure of the membrane (phospholipid bilayer) ~The tails form a hydrophobic core comprising the innermost part of both the outer and inner layer of the membrane ~Act as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane) ~This ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in ~Can be chemically modified to act as signalling molecules by: -Moving within the bilayer to activate other molecules (eg. enzymes) -Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

Question 20

Functions of Cholesterol

Answer 20

-Increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures (allowing cells to survive at lower temperatures) -This occurs because cholesterol stops the phospholipid tails packing too closely together Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid -Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together -They also contribute to the impermeabilty of the membrane to ions Increases mechanical strength and stability of membranes (without it membranes would break down and cells burst)

Question 21

Functions of Glycolipids & glycoproteins

Answer 21

-Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules -This allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface -There are three main receptor types: signalling receptors for hormones and neurotransmitters receptors involved in endocytosis receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell -Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)

Question 22

Functions of proteins

Answer 22

-Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane. -There are two types: ~channel (pore) proteins ~carrier proteins -Each transport protein is specific to a particular ion or molecule -Transport proteins allow the cell to control which substances enter or leave

Question 23

Membranes become less fluid when there is:

Answer 23

-An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains -A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed

Question 24

Membranes become more fluid when there is:

Answer 24

-An increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are less intermolecular forces -At higher temperatures, the molecules have more energy and therefore move more freely, which increasing membrane fluidity

Question 25

cell signalling

Answer 25

is the process by which messages are sent to cells -Cell signalling is very important as it allows multicellular organisms to control / coordinate their bodies and respond to their environments -Cell signalling pathways coordinate the activities of cells, even if they are large distances apart within the organism

Question 26

The basic stages of a cell signalling pathway are:

Answer 26

Question 27

Transmission of messages in cell signalling pathway requires

Answer 27

crossing barriers such as cell surface membranes -Cell surface membranes are therefore very important in signalling pathways as the membrane controls which molecules (including cell signalling molecules) can move between the internal and external environments of the cell -Signalling molecules are usually very small for easy transport across cell membranes -Typically in cell signalling pathways, signalling molecules need to cross or interact with cell membranes

Question 28

Ligands

Answer 28

Signalling molecules are often called ligands

Question 29

Ligands are involved in the following stages of a cell signalling pathway:

Answer 29

-Ligands are secreted from a cell (the sending cell) into the extracellular space -The ligands are then transported through the extracellular space to the target cell -The ligands bind to surface receptors (specific to that ligand) on the target cell —These receptors are formed from glycolipids and glycoproteins -The message carried by the ligand is relayed through a chain of chemical messengers inside the cell, triggering a response