4.1 - structure of the cell surface membrane

Question 1

What are the membranes around and within cells known as

Answer 1

Plasma membranes

Question 2

What is the cell-surface membrane

Answer 2

The name given to: the membrane that surrounds the cell, forming a boundary between the cell cytoplasm and the environment

Question 3

What are the 2 basic functions of the cell-surface membrane

Answer 3

• allows different conditions to be established inside and outside the cell
• controls movement of substances in and out the cell

Question 4

What are the different molecules that form the structure of the cell-surface membrane

Answer 4

• Phospholipids
• proteins
• cholesterol
• glycolipids
• glycoproteins

Question 5

Describe how the phospholipids form the phospholipid bilayer

Answer 5

• The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides
• The hydrophobic tails of both phospholipid layers point to the centre of the cell-surface membrane repelled by water on both sides

Question 6

What type of material moves through the membrane via the phospholipid portion, and how does this relate to the functions of phospholipids in the membrane

Answer 6

• lipid-soluble material
  FUNCTIONS
  —> allow lipid-soluble substances to enter and leave the cell
  —> prevent water-soluble substances entering and leaving the cell
  —> make the membrane flexible and self-sealing

Question 7

How are proteins embedded in the phospholipid bilayer

Answer 7

Proteins are dispersed throughout the membrane: there’s 2 types
- Extrinsic/peripheral proteins
- Intrinsic/integral proteins

Question 8

What are extrinsic proteins

Answer 8

• occur in the surface of the bilayer and never extend completely across it
• Either: give mechanical support to membrane, or act as cell receptors for peptide hormones (role in cell recognition)

Question 9

What are intrinsic proteins

Answer 9

• completely span the phospholipid bilayer from one side to the other
• act as: protein channels = form water-filled tubes to allow water-soluble ions to diffuse across the membrane
• act as: carrier proteins = bind to ions or molecules (like glucose and amino acids), then change shape in order to move these molecules across the membrane

Question 10

What are the functions of the proteins in the membrane

Answer 10

• structural support
• act as channels, transporting water-soluble substances across the membrane
• allow active transport across the membrane through carrier proteins
• form cell-surface receptors for identifying cells
• help cells adhere together
• act as receptors, e.g. for hormones

Question 11

Where do cholesterol molecules occur

Answer 11

• present in all cell membranes (except bacterial)
• fit between the phospholipids = bind to the hydrophobic tails of phospholipids = causing them to pack closer together = restricting movement of phospholipids = making membranes less fluid and more rigid

Question 12

What are the functions of cholesterol molecules in the membrane

Answer 12

• add strength to the membranes, maintaining the shape (important for cells that aren’t supported by other cells, e.g. red blood cells)
• very hydrophobic = play role in preventing loss of water and dissolved ions from the cell
• fits between the phospholipids = bind to the hydrophobic tails of phospholipids = causing them to pack closer together = restricting movement of phospholipids = making membranes less fluid and more rigid —> reduce lateral movement
• make the membrane less fluid at high temps

Question 13

How do the phospholipids molecules form a bilayer

Answer 13

They automatically arrange themselves into a bilayer

Question 14

Why can’t water-soluble substances pass through the phospholipid bilayer

Answer 14

• the centre of the bilayer is where hydrophobic tails meets = centre is hydrophobic
• act as as a barrier to these dissolved substances

Question 15

What are glycolipids made up of

Answer 15

A carbohydrate covalently bonded with a lipid

Question 16

Describes the structure of glycolipids

Answer 16

• the carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals
• e.g. the human ABO blood system operates as a result of glycolipids on the cell-surface membrane

Question 17

What are the functions of glycolipids in the membrane

Answer 17

• act as recognition sites
• help maintain the stability of the membrane
• help cells to attach to one another and so form tissues (cell adhesion)

Question 18

What are glycoproteins

Answer 18

• Carbohydrate chains are attached to many extrinsic proteins on the outer surface of cell membrane
• act as cell-surface receptors, more specifically for hormones and neurotransmitters

Question 19

What are the functions of glycoproteins in the membrane

Answer 19

• act as recognition sites
• help cells to attach to one another and so form tissues
• allows cells to recognise each one another, e.g. lymphocytes can recognise an organism’s own cells

Question 20

What is the functions of membranes within cells

Answer 20

• control entry and exit of materials in discrete organelles such as mitochondria and chloroplasts
• separate organelles from cytoplasm so that specific metabolic reaction can take place within them
• provide an internal transport system, e.g. endoplasmic reticulum
• isolate enzymes that might damage the cell, e.g. lysosomes
• provide surfaces on which reactions can occur, e.g. protein synthesis using ribosomes on rough endoplasmic reticulum

Question 21

Why might most molecules not be able to freely diffuse across the cell-surface membrane

Answer 21

• they’re not soluble in lipids: therefore can’t pass through the phospholipid layer
• too large to pass through the channels in the membrane
• they’re the same charge as the charge on the protein channels = even if they’re small enough, they’re repelled anyway
• they’re electrically charged (polar) = have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer

Question 22

What is the fluid-mosaic model

Answer 22

The model that shows the arrangement of all the molecules that create the cell-surface membrane

Question 23

Why is it called the Fluid-Mosaic model

Answer 23

• fluid: individual phospholipid molecules can move relative to one another = gives membrane a flexible structure that’s constantly changing
• mosaic: the proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way that the stones/tiles in a mosaic do

Question 24

What’s the function of the phospholipid bilayer

Answer 24

• allow movement of lipid soluble molecules, e.g. steroid hormones
• prevent water soluble molecules leaving the cell
• make membrane flexible and self-sealing
• forms a barrier to dissolved substances

Question 25

What’s the function of the fatty acid tails

Answer 25

• forms (water) impermeable barrier to water-soluble substances, and allows non-polar molecules to pass through
• allows cell to maintain different conc. either side
• makes membrane self-sealing/able to fuse other membranes/flexible

Question 26

Fill in the blanks

Answer 26

Question 27

Sketch a CSM diagram

Answer 27

Question 28

Describe what the phospholipid bilayer is formed from

Answer 28

• the molecules automatically arrange themselves into a bilayer
• phospholipid molecules have a hydrophilic ‘head’ that points out towards the water either side of the membrane
• and a hydrophilic ‘tail’ that points inwards. This makes the centre hydrophobic = so doesn’t allow water-soluble substances (like ions) through it - acting like a barrier

Question 29

What is cholesterol and where is it

Answer 29

• A type of lipid
• present in all cell membranes (except bacterial cell membranes)