chapter 4.1

Question 1

Phospholipids from a bilayer and this is important for the following reasons

Answer 1

• 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 layer point into the centre of the cell membrane, repelled by the water on both sides.

Question 2

Functions of the phospholipids bilayer

Answer 2

• allows lipid-soluble substances to enter and leave the cell
• prevents water-soluble substances entering and leaving the cell
• make the membrane flexible and self sealing (this is important as it allows endocytosis and exocytosis

Question 3

Proteins are embedded in the cell membrane in two main way. And what are their jobs?

Answer 3

1. Proteins that span the bilayer from one side to the other side. These are called Integral membrane proteins, also called intrinsic proteins (they are in the cell membrane)
   Their job is to:
   - form protein channels which form water filled tubes to allow water soluble ions to diffuse across the membrane.
   -or are carrier proteins that bind to ions or molecules like glucose and amino acids and then change shape in order to move these molecules across the membrane
2. Ones that’s do not extend across the bilayer. These are called extrinsic/ peripheral proteins.
   They act either as:
   -act as mechanical support to the membrane
   -or, in conjunction with the glycolipids (part of a glycoproteins), as cell receptors for molecules like hormones

Question 4

Functions of proteins

Answer 4

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

Question 5

Functions of the cholesterol

Answer 5

• reduce lateral movement of other molecules including phospholipids as cholesterol pull together the fatty acid tails of the phospholipid molecules but without making the membrane as a whole too rigid.
• make the membrane less fluid at higher temperatures
• prevents leakages of water and dissolved ions from cells as cholesterol molecules are very hydrophobic it is very hydrophobic (and prevent water entering across the cell membrane – use of a water pore instead)
• it keeps the cell from bursting and being stable at normal body temperature
• add strength
• Cholesterol molecules are very hydrophobic and therefore play an important role in preventing loss of water and dissolved ions from the cell.

Question 6

What are glycolipid made up of

Answer 6

These are made up of a carbohydrate covalently bonded with a lipid. 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. human ABO blood system operates as a result of glycolipids on the cell-surface membrane

Question 7

Functions of glycolipids

Answer 7

• act as recognition sites
• help to maintain the stability of the membrane
• help cells to attach to one another so form tissues.

Question 8

Function of glycoproteins

Answer 8

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

Question 9

What molecules cannot diffuse across freely across the cell-surface membrane

Answer 9

• not soluble in lipids and therefore cannot pass through the phospholipid layer
• too large to pass through the protein channels in the membrane.
• of the same charge as the charge on the protein channels and so, even if they are small enough to pass through, they are repelled
• electrically charges (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.
  -hydrophilic molecules cannot pass through the bilayer of the membrane
  Bicing tubing is used in the lab to represent the cell-membranes permeability

Question 10

FUNCTIONS OF MEMBRANES WITHIN CELLS:

Answer 10

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

Question 11

FLUID-MOSAIC MODEL OF THE CELL-SURFACE MEMBRANE:

Answer 11

The arrangement of various molecules make up the structure of the cell-surface membrane. This model is known as the fluid-mosaic model for the following reasons:

• fluid: because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape.
• mosaic: because their proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.