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Cells and Systems 6-8 / Week 03 / Tuesday
2/6
Week 03 · Inside a Cell

Tuesday

Boundaries and instructions
// A cell is a system of parts
⏱ about 20 min

Tuesday: Boundaries and Instructions

Raven holds the sealed bag from yesterday over the sink. "If this were a real membrane, what would it let through?"

"Water," Rocket says. "Food. And it would push waste out." He pauses. "But not everything. It has to choose."

Nova dims her light to a thin beam on the bag. "Scientists say the membrane regulates what enters and exits," she says.

"Like a door with a guard," Raven says. "What do you notice about the nucleus in the picture?"

Rocket looks at the diagram pinned by the sill. "It has its own wrapping. A membrane inside a membrane."

"To protect the instructions," Raven says. Nova hums. "DNA is the recipe book. The nucleus is where it is kept."

The membrane: a selective boundary

The National Human Genome Research Institute describes the cell membrane as a lipid bilayer that is semipermeable.

Semipermeable means some things pass through and others do not. The membrane regulates the transport of materials entering and exiting the cell.

It brings nutrients in and moves wastes out. It also gives the cell some structural support and protection.

In the Next Generation Science Standards, the cell membrane forms the boundary that controls what enters and leaves the cell.

Food and water outside the cell
The membrane lets nutrients in
Mitochondria convert energy from food
The cell grows and does its job
The membrane moves wastes out
THE MEMBRANE AT WORK
  • Read the question.
  • Tap your answer.
What does semipermeable mean?
Which two things does the membrane move, according to the glossary?
In the bag model, which object is the membrane?

The nucleus: where the instructions are kept

The nucleus is a membrane-enclosed organelle that contains the chromosomes, which carry the DNA.

DNA is the cell's hereditary material. The nucleus is often the most prominent organelle when you look at a picture of a cell.

Around the nucleus is a membrane called the nuclear envelope. It protects the DNA and separates the nucleus from the rest of the cell.

Pores in the nuclear envelope let certain molecules pass in and out. Even the nucleus has a selective boundary.

The nucleus sends directions to the cell to grow, mature, divide or die. That is why it is called the command center.

The nucleus houses ____, the cell's hereditary material.
The membrane around the nucleus is called the nuclear ____.
StatementTrue or false?
The cell membrane lets everything through.?
The nucleus contains the chromosomes.?
The nuclear envelope separates the nucleus from the rest of the cell.?
Wastes stay inside the cell forever.?
WHY THIS EXERCISEBoth the cell and the nucleus have boundaries that choose what passes. That is how a system stays organized.
A NUTRIENT'S JOURNEY
  • ?A nutrient is outside the cell
  • ?It passes through the cell membrane
  • ?It floats in the cytoplasm
  • ?A mitochondrion converts energy from it
WHY THIS EXERCISETracing one nutrient shows how the parts of the cell depend on each other.
Try it
Put a sealed bag of water in a bowl. Add a few drops of food coloring to the bowl. Does the color get inside the bag?
A plastic bag is not semipermeable. Write one way a real membrane is different from your bag.
Check the seed jar. Add water if the towel is drying.
Try this
Find a door, a gate or a window screen in your home. Which things does it let through, and which does it stop?
Write one sentence comparing it to a semipermeable membrane.
Draw a cell with its membrane. Use arrows to show a nutrient going in and a waste going out. Draw the nucleus with its own envelope.

Strong thinking. Tomorrow is Windowsill Lab: you build the bag-of-water cell with beads and string.

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