Biology: cell structure, function, and homeostasis (grade 9) · lesson

Organelles have jobs

💡 Each organelle does a job, so a cell has more of the parts it uses most.

The nucleus stores DNA, the instructions for the cell. Ribosomes build proteins. Mitochondria break down sugar to release usable energy, stored as ATP. Lysosomes break down worn-out parts, and the Golgi apparatus packages proteins.

Plant cells also have chloroplasts, which use light energy to make sugar, and a stiff cell wall. Animal cells have neither. Chloroplasts are found where there is light, such as in leaves, not in roots.

A cell's parts match its work. A heart muscle cell beats all day, so it is packed with mitochondria. A cell that makes lots of protein has many ribosomes.

🧠 Let's work it out

Ask: what does this cell do all day?
Then ask which organelle does that job.
A cell that uses lots of energy needs lots of mitochondria.
A cell that gets no light has no use for chloroplasts.

What do you think comes next?

organelle: a small part inside a cell that does one job
ATP: the molecule cells use as ready-to-spend energy

🔗 Khan Academy: From cells to organisms · Khan Academy (free) ↗

From gene to protein

💡 A gene's base sequence sets the order of amino acids, and that order sets a protein's shape and job.

A gene is a stretch of DNA. In the nucleus, the gene is copied into a messenger molecule called mRNA. The mRNA travels to a ribosome, which reads its bases three at a time. Each group of three picks one amino acid.

The ribosome links the amino acids in that order. The chain then folds into a shape, and the shape decides what the protein can do. Many proteins are built on ribosomes on the rough ER, packaged by the Golgi apparatus, and shipped out of the cell in small sacs called vesicles.

If one DNA base changes, one group of three changes. That can swap one amino acid. The protein may fold the wrong way and stop working.

🧠 Let's work it out

Trace the path in order: DNA in the nucleus → mRNA copy → ribosome builds the chain → Golgi packages → vesicle ships it out.
Each step needs the one before it.
A protein must exist before it can be packaged.

What do you think comes next?

mRNA: a copy of a gene that carries the code to a ribosome
amino acid: one of the small building blocks joined to make a protein

🔗 PhET: Gene Expression Essentials · PhET Interactive Simulations (free) ↗

The cell membrane and osmosis

💡 The membrane controls what crosses it, and water moves toward the side with more dissolved material.

Every cell, plant or animal, is wrapped in a thin cell membrane. It lets some substances in and keeps others out. Plant cells also have a cell wall outside the membrane, but animal cells do not.

Water can cross the membrane on its own. This movement is called osmosis. Water moves from where there is more water to where there is less, which is the side with more dissolved salt or sugar.

Put a red blood cell in very salty water and water leaves the cell, so it shrinks. Put it in pure water and water rushes in, so it swells.

🧠 Let's work it out

Compare the two sides.
Where is the water more crowded by salt?
Water moves toward that side.
If that side is outside the cell, water leaves and the cell shrinks.

What do you think comes next?

osmosis: water moving across a membrane toward the side with more dissolved material

Levels of organization

💡 Cells build tissues, tissues build organs, and organs work together as systems.

A tissue is a group of similar cells doing one job, like muscle tissue. An organ, like the heart, is made of several tissues working together.

An organ system is a team of organs. The heart, blood vessels, and blood make up the circulatory system. Each level is built from the level below it.

🧠 Let's work it out

To order levels, ask what each one is made of.
A tissue is made of cells, so cells come first.
An organ system is made of organs, so it comes last.

What do you think comes next?

tissue: a group of similar cells that do the same job
organ system: a group of organs that work together

Homeostasis and feedback

💡 In negative feedback, the body's response undoes a change and brings a level back to normal.

Your body keeps things like temperature, blood sugar, and calcium near a set level. This steady state is called homeostasis.

Most control works by negative feedback. A change is sensed, and the response pushes the other way. When blood sugar rises, insulin helps cells take in sugar, and the level falls. As it reaches normal, the response slows down. Positive feedback is rare: it pushes a change even further.

When you are too hot, you sweat and blood vessels near the skin widen, so heat escapes. When you are too cold, you shiver and those vessels narrow to keep heat inside.

🧠 Let's work it out

Look at the data in three parts: before, during, and after.
Heart rate 70 at rest, 140 while running, 75 five minutes later.
It rose to meet a need and then came back.

What do you think comes next?

homeostasis: keeping the body's conditions steady
negative feedback: a response that reverses a change