Cells from Cells

Cell Reproduction

• Process by which new cells are formed – Recall the difference between cell reproduction and the reproduction of a multicellular organism • # of “parents” is different • Body cells & single-celled organisms divide to produce 2 new daughter cells • Sexual reproduction = 2 parents mate and offspring receive ½ of their genes from each

This karyotype shows the 46 chromosomes present in the nucleus of every cell in the body of a male human.

Cell Division

When you cut your skin, blood flows to the area until a scab forms. This scab restores the skin’s continuity, preventing bacteria from entering the body. Then the skin cells underneath can undergo cell division to produce new cells that fill in the gap. Once the skin layer is restored, the scab falls off.

• • • Process by which a parent cell divides into two daughter cells Main reproductive process in single-celled

organisms

Process by which a fertilized egg becomes a multicellular organism with millions of cells

The Cell Membrane and Diffusion

• • Cells also divide when they grow too large to perform normal functions – cell membrane has key role here – e.g. digestion (nutrients into cell/waste out) Cell membrane acts as a barrier – Most materials pass through via diffusion – Water crosses through via osmosis

Diffusion is the movement of molecules from areas of high concentration to areas of low concentration.

Moving from High Concentrations to Low Concentrations

• The cell membrane is permeable to certain substances • Materials the cell needs (e.g. oxygen) diffuse across the membrane from outside the cell (higher concentration) to the inside (lower concentration) • Cell membrane is considered

selectively permeable

because not all materials can cross it • Water also moves from areas of high to low concentration

Some examples

• • A chicken egg as a cell...

Wilted lettuce?

We’re all glorified Amoebas!

• •

What do I mean by that?

Think about how you and the amoeba: (a) get food, (b) get water, (c) exchange gases, & (d) remove wastes.

Notice anything similar?

Growing Cells & Limiting Cell Size

• • • Surface of a cell must be big enough to allow for oxygen and nutrients to pass through Cells use nutrients and produce more organelles and more cytosol  get bigger Cell then needs more supplies/waste removal • • • Every cell needs enough S.A. to meet its volume needs Large organisms have less S.A. per unit of volume than small organisms  When a cell reaches a certain size, it must divide to produce smaller cells

If an amoeba were as big as a human, critical substances, such as oxygen, would take years to get through the cell’s cytoplasm to reach the centre of the cell. This would be far too long. In the meantime, the nucleus and other organelles would not receive the nutrients they need to function.

Questions...

1. Describe in your own words how substances cross the cell membrane.

2. When a cell divides to produce daughter cells, how similar are the daughters to the parent?

3. Why do the cells of multicellular organisms divide?

4. Do you have to worry about seeing a headline like “Giant Paramecium Threatens City”? Explain your answer.