New Unit starts today! Today’s Essential Question: Why do cells divide? Chapter 10.1

Quick Investigation: In areas of rapid growth, do cells divide quickly or grow extremely large? Compare the cells in two regions of an onion root: Root Cap - area of older, protective cells Apical Meristem - area of rapid growth In your class NB, copy the question, then create a table to sketch and describe your observations

What did you find out? Conclusion: Organisms grow by _______________________. Areas of Onion Root tips Root CapMeristem Sketch of cells Description of cells

Quick Investigation 2: What limits an organisms growth? Try making the largest “organism” you can by blowing: One big bubble Many small bubbles Is growth limited by the SIZE of each cell or the NUMBER of cells?

Why can’t cells grow extremely large? (for homework you will investigate the size of cells)

1. DNA “ overload ” When a cell gets too big, there is not enough DNA to send instructions and control cell activities. Library analogy: Could Needham library supply books for our whole state?

2. Transport is too slow! If a cell gets too big, nutrients and wastes cannot easily diffuse in and out of the whole cell. Materials cannot get to the center (nucleus) of the cell

Cell Division: Problem Solved! When a cell gets too big, the DNA signals the cell to divide. How big is “too big”? Surface Area to Volume Ratio

Definitions Ratio: Surface Area : Volume:

Definitions Ratio - a comparison of two numbers using division 2:1 Surface Area : L x W x # of sides Volume: L x W x H

Calculating Surface Area to Volume Ratio

Too big? Time to divide! Bacteria- Process called ______________ Bacteria Eukaryotic Cells- Process called ________ Eukaryotic Cells

The “Life Cycle” of Cells How does a single cell (zygote) quickly grow into a human baby with trillions of cells? Why can’t some adult cells such as nerve cells divide after injury while other cell types such as skin type divide daily?

Why do cells divide? 1.Growth and Development (Stem cells - we will tackle these separately!) 2.Replacement 3.Repair

Sample Cell lifespans Lining of esophagus- 2-3 days Lining of small intestine- 1-2 days Red blood cell- 120 days max White blood cell- 10hrs - decades Skeletal muscle- many years Nerve cells- many decades

Next time: HOW do cells divide? What do you remember about mitosis?