BCE0005: Unknown identifier: 'spriteRenderer'.

 #pragma strict
 
 var diamond : Texture;
 var square : Texture;
 var triangle : Texture;
 var circle : Texture;
 var start : Texture;
 private var number : int = Random.Range(1,4);
 
 function Start () {
 spriteRenderer.Sprite = start;
 }
 
 function Update () {
 if (number == 1) {
 spriteRenderer.Sprite = diamond;
 }
 else if (number == 2) {
 spriteRenderer.Sprite = square;
 }
 else if (number == 3) {
 spriteRenderer.Sprite = triangle;
 }
 else {
 spriteRenderer.Sprite = circle;
 }
 
 }

I don't get what I got wrong here. It said BCE0005: Unknown identifier: 'spriteRenderer'. Anyone help please? Thanks