Inside a cell a poisonous substance, H₂O₂, keeps forming. One small sac breaks it into water and oxygen. Another sac turns a seed's fat into sugar to keep the seedling alive. And in a big plant cell, most of the inside is one large empty-looking space. Telling these three apart is the job of this note.
1. The peroxisome
Peroxisomes occur in almost every cell, and there are more of them in animal kidney and liver cells. A peroxisome forms by outpocketing of smooth ER: a bulge pushes out from the ER and becomes a small sac.
It has a single envelope. It measures 0.2-1.7 μm across and looks granular inside. Another name for it is microsome. Christian de Duve discovered it in 1967.
The main enzyme of the peroxisome is catalase. It breaks the poisonous H₂O₂ down: 2H₂O₂ → 2H₂O + O₂. So the poison cannot pile up in the cell.
It has three more jobs. It controls the oxygen concentration in the cell. It helps regenerate NAD. And it helps break down the nitrogen bases of DNA and RNA and reuse them.
2. The glyoxisome
Glyoxisomes sit in the fat-storing cells of seeds, and they too have an envelope.
Put a fat-storing seed in the soil and let the days go by.
germinationdrag the handle
Fat into sugar
The two bars show the idea, not measured amounts.
When the seed germinates they break the lipid down into usable sugar. The reason is simple: the seedling cannot yet make food by photosynthesis. Until it can, it grows on this sugar. Put the steps in order.
which firstput the steps in order
From seed to seedling
It starts inside the seed: what is stored there?
Yes. Then?
Not yet. Ask which step leads to which.
Right order. The sugar keeps the seedling going until it can make its own food by photosynthesis.
3. The vacuole
The empty-looking space you see in the cytoplasm is the cell vacuole. In an immature cell there are many small vacuoles. As a plant cell matures they merge into one large vacuole.
The thin protoplasmic membrane around the vacuole is the tonoplast. The fluid inside is the cell sap. It holds water, inorganic salts, organic acids, sugars, proteins, fatty compounds and pigments.
The vacuole does four jobs: it holds the cell sap, holds waste, keeps the cell's pH steady and keeps its water pressure steady. Exam questions call this balancing the cell's pH and water.
Let a young plant cell grow up and watch its vacuoles.
growing updrag the handle
Many small, then one large
4. Match the terms, name the organelle
First match five pairs of terms. Each chip belongs under one heading.
match themchip, then box
Match the terms
Pick up a chip, then drop it in the box it belongs with.
Right. Next one.
Not yet. Ask which organelle or part this goes with.
All matched. Catalase is in the peroxisome, lipid to sugar is the glyoxisome, the vacuole's membrane is the tonoplast and its inner fluid is the cell sap.
Now the three organelles side by side. Say which one each of the eight clues belongs to.
sort themtap a chip, then pick a bin
Which organelle?
Pick up a clue, then drop it in a bin.
Right. Next one.
Not yet. Is this a job, a name, or the fluid inside?
All in place. The peroxisome has catalase, comes from smooth ER, is abundant in kidney and liver, and was found by de Duve. The glyoxisome turns seed lipid into sugar. The vacuole has the tonoplast and the cell sap, and keeps pH and water pressure.
5. Check yourself
Three questions in the exam's style. Start with the first.
check yourselfpick one answer
Question 1
The cytoplasmic organelles are:
i. Plastid ii. Glyoxysome iii. Cell wall
Right. i and ii are right: plastids and glyoxysomes are cytoplasmic organelles.
i and iii: the cell wall is not an organelle.
ii and iii: the cell wall is not an organelle.
Not all three: the cell wall is a non-living covering, not an organelle.
Take the second one slowly.
check yourselfpick one answer
Question 2
What is the thin membrane around the vacuole inside the cytoplasm called?
Protoplasm is the living matter of the cell, not the vacuole membrane.
The cell wall is the outer non-living layer of the cell.
Nucleoplasm is the fluid inside the nucleus.
Right. The thin membrane of protoplasm that surrounds the vacuole is called the tonoplast.
The last question.
check yourselfpick one answer
Question 3
Where is the tonoplast found?
The tonoplast is not in the plastid.
Not in the chromoplast.
Right. The tonoplast is the thin membrane surrounding the cell vacuole.
The cell sap is the fluid inside the vacuole; the membrane is around it.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhere are peroxisomes abundant and how do they form?
AnswerAbundant in animal kidney and liver cells; formed by outpocketing of smooth ER.
QuestionWho discovered the peroxisome and when?
AnswerChristian de Duve, in 1967.
QuestionWhat is the main enzyme of the peroxisome and its reaction?