Mitochondria, Plastids and the Nucleus: Quick Revision

A brief note on Mitochondria, Plastids and the Nucleus. Read it before a test. Try each question at the end before you open its answer.

Mitochondria and plastids

• Mitochondrion: two membranes; the inner one folds into cristae; outer compartment and matrix. It makes ATP by aerobic respiration.
• Plastids: chloroplasts (chlorophyll), chromoplasts (carotene, xanthophylls), leucoplasts (stores).
• Leucoplasts: amyloplasts (starch), elaioplasts (oils and fats), aleuroplasts (proteins).
• Chloroplast: two membranes, stroma, thylakoids stacked in grana, joined by stroma lamellae. Chlorophyll is in the thylakoids.
• Both have their own circular DNA and 70S ribosomes, and both divide.

Ribosomes and movement

• Ribosomes: RNA and protein, no membrane. 80S = 60S + 40S in the eukaryotic cytoplasm, 70S = 50S + 30S elsewhere.
• Cytoskeleton: microtubules, microfilaments, intermediate filaments.
• Cilia and flagella: a 9 + 2 axoneme, growing from a basal body.
• Centriole: a cartwheel of nine triplets round a hub. Centrioles form basal bodies and spindle fibres.

The nucleus

• Nuclear envelope: two membranes, a perinuclear space and pores for two-way traffic.
• Nucleolus: no membrane, the site of rRNA synthesis.
• Chromatin: DNA, histones, non-histone proteins and RNA. It forms chromosomes in division.
• Centromere: middle (metacentric), a little off (sub-metacentric), near one end (acrocentric), at the end (telocentric).
• Microbodies: small membrane-bound vesicles of enzymes.

A worked example

Name the organelle: it has two membranes, its own DNA, and stacks of flat sacs inside.

1. Two membranes and its own DNA: a mitochondrion, a chloroplast or the nucleus.
2. Stacks of flat sacs: these are grana of thylakoids.
3. Organelle: a chloroplast.

Practice with new numbers

A program to try

The program lists six organelles with the membranes round them and whether they have their own DNA.

Mistakes to avoid

• Putting cristae on the outer membrane. They are folds of the inner one.
• Giving mitochondria 80S ribosomes. They have 70S ribosomes.
• Giving a centriole a central pair. It has nine triplets and no central pair.
• Saying an acrocentric chromosome has no short arm. That is a telocentric one.

Check yourself

1. Which plastids are colourless, and what do they do?

2. What are the subunits of a 70S ribosome?

3. What does the nucleolus make?

4. Where is the centromere of a metacentric chromosome?