Where are the "Photos"?

I have an iPhone 4S. Few days ago, when I was syncing my phone on iTunes, I was surprised because it says that I have over than 2000 photos and 100 videos. But when I checked my Gallery, there was just 300 photos and 35 videos. where do you think the other photos and videos are? It made me so confused because it consumed my memory about 5 GB. And I did not sync any photos or videos from PC. I hope you can help me. Thank you. and Sorry for my English