Cell Organelles and Their Functions

HSC Biology | Free Study Notes

This topic helps you understand how different parts of a cell work together to keep it alive, and why some cells are able to carry out specialised jobs more effectively than others.

In this lesson

• what organelles are

• the functions of the nucleus, mitochondria and ribosomes

• the roles of chloroplasts, vacuoles and lysosomes

• which organelles are found in plant cells, animal cells, or both

• how organelles help cells carry out life processes

What are organelles?

Organelles are specialised structures inside cells that carry out particular functions.

You can think of them as parts of a working system. Each organelle has a job, and together they help the cell survive, grow and function efficiently.

Most organelles you study in this topic are found in eukaryotic cells.

The nucleus

The nucleus is the control centre of the cell.

Function of the nucleus

• contains the cell’s genetic material, DNA

• controls cell activities

• provides instructions for making proteins

Why the nucleus matters

Because DNA is stored in the nucleus, it helps control what the cell does and which proteins it makes. This is important for growth, repair and specialised cell function.

Mitochondria

Mitochondria are the site of aerobic respiration in eukaryotic cells.

Function of mitochondria

• release energy from glucose

• produce ATP, which the cell uses as an energy source

Why mitochondria matter

Cells need energy for active transport, growth, repair and chemical reactions. Cells with high energy demands usually contain many mitochondria.

Examples include muscle cells.

Ribosomes

Ribosomes are the site of protein synthesis.

Function of ribosomes

• make proteins by joining amino acids together

Why ribosomes matter

Proteins are needed for:

• enzymes

• structural support

• transport

• cell signalling

Ribosomes are especially important because all cells need proteins. Unlike most of the other organelles on this page, ribosomes are found in both prokaryotic and eukaryotic cells.

Chloroplasts

Chloroplasts are organelles found in plant cells and some protists.

Function of chloroplasts

• carry out photosynthesis

• contain chlorophyll to absorb light energy

Why chloroplasts matter

Chloroplasts allow plants to make glucose from carbon dioxide and water using light energy. This is why they are essential in autotrophic organisms.

Animal cells do not contain chloroplasts.

Vacuoles

Vacuoles are membrane-bound sacs used for storage.

Function of vacuoles

• store water

• store dissolved substances such as salts or sugars

• help maintain internal pressure in plant cells

Why vacuoles matter

In plant cells, the large central vacuole helps keep the cell firm by maintaining turgor pressure. This supports the plant’s structure.

Animal cells may have small vacuoles, but they do not usually have one large central vacuole like plant cells.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes.

Function of lysosomes

• break down waste materials

• digest worn-out cell parts

• destroy substances taken into the cell

Why lysosomes matter

Lysosomes help keep the cell clean and efficient by breaking down materials that are no longer needed.

They are more commonly discussed in animal cells at this level.

Which organelles are found in which cells?

Organelles in plant and animal cells

Organelle	Plant cells	Animal cells
Nucleus	Yes	Yes
Mitochondria	Yes	Yes
Ribosomes	Yes	Yes
Chloroplasts	Yes	No
Vacuoles	Yes, usually one large central vacuole	Sometimes small vacuoles
Lysosomes	Less commonly emphasised	Yes

How organelles work together

Organelles do not work alone. They support one another.

For example:

• the nucleus contains the instructions for making proteins

• ribosomes build those proteins

• mitochondria provide energy for cell activities

• lysosomes break down waste

• vacuoles store important substances

• chloroplasts make glucose in plant cells, which can later be used in respiration

This coordination is one reason eukaryotic cells are more complex than prokaryotic cells.

Worked example

Exam-style question

Explain why muscle cells contain many mitochondria.

Worked answer

Muscle cells need large amounts of energy for contraction. Mitochondria carry out aerobic respiration and produce ATP, so having many mitochondria allows muscle cells to release more energy.

Why this works

This answer:

• names the organelle

• links its function to the needs of the cell

• uses cause and effect clearly

Common mistakes

• Saying mitochondria “make food”. Chloroplasts make glucose in photosynthesis, mitochondria release energy from glucose.

• Saying ribosomes are only found in eukaryotic cells. They are found in both prokaryotic and eukaryotic cells.

• Mixing up the nucleus and nucleolus. At this level, focus on the nucleus as the part that contains DNA and controls cell activities.

• Saying all cells have chloroplasts. Only plant cells and some protists do.

• Confusing vacuoles with lysosomes. Vacuoles mainly store substances, while lysosomes digest materials.

Quick quiz

1. Which organelle contains DNA and controls cell activities?

2. What is the main function of mitochondria?

3. Which organelle is the site of protein synthesis?

4. Which organelle carries out photosynthesis?

5. What is one function of a vacuole?

HSC Biology Home