Organelles are specialised structures in eukaryotic cells. Understanding the different types of organelles and their roles helps us comprehend how cells operate and maintain life processes. Use this resource to learn more.
Organelles perform distinct functions needed for cell survival and efficiency. They are held within the cytoplasm ("cyto" meaning "cell" and "plasm" meaning "something molded or created"), which is a gel-like filling the inside of the cell. The cytoplasm helps move materials within the cell and maintain the cell's shape.
The cytoplasm also contains the cytoskeleton, a network of protein fibres that helps organise the contents of a cell.
There are many types of organelles, with some that are unique to each type of cell. Let's explore the organelles of animal and plant cells. Plant (left) and animal (right) cells, by Tomáš Kebert & umimeto.org via Wikimedia Commons, licensed under CC BY-SA 4.0
Nucleus
The nucleus, present in both animal and plant cells, is the control centre of the cell. It manages gene expression, cell growth and replication, and houses the cell's genetic material in the form of deoxyribonucleic acid, or DNA. DNA is organised into chromatin, a complex made of DNA and proteins.
Within the nucleus is the nucleolus, a dense area where ribosomal ribonucleic acid (rRNA) is found. The nucleolus provides the blueprints for making and assembling ribosomes.
A double membrane called the nuclear envelope surrounds the nucleus. It has pores (holes) that regulate the movement of molecules in and out of the nucleus.
Just like the cell has cytoplasm, the nucleus contains a similar gel-like substance called nucleoplasm.
An interactive three-dimensional model of the cell nucleus with features labelled.
Nucleus model:
Nuclear envelope: The outermost reddish layer surrounding the nucleus.
Membrane that encloses the nucleus and protects the DNA inside.
Nuclear pore: Small openings in the nuclear envelope, depicted as lighter spots. Channels that allow the passage of specific molecules into the nucleus, such as nucleic acid.
Nucleoplasm: The dense, brownish material within the nucleus. Gel-like substance to provide structure and further protection.
Chromatin: Fibres present around the nucleolus, depicted as orange lines. These fibres are coiled and condensed to form chromosomes.
Nucleolus: A distinct, darker central structure. Here, RNA is synthesised and assembled so it can be transported to other regions of the cell.
Located near the nucleus, and only in animal cells, is a structure called a centrosome. Centrosomes are important for cell division. They duplicate before a cell starts to divide and move to opposite ends of the cell. This will be explained more in The cell cycle.
Mitochondria
The mitochondria are known as the powerhouse of the cell – this might sound familiar! They produce energy through cellular respiration, which involves converting glucose and oxygen into adenosine triphosphate (ATP), the energy the cell needs to function.
Mitochondria have a double membrane, with the inner membrane folded into structures called cristae. The high level of folding provides more surface area for respiration, allowing the mitochondria to generate more ATP. The space inside the cristae is called the matrix.
Mitochondria are special because they contain their own DNA. This allows them to replicate independently within the cell and adapt to increased energy needs, such as when a muscle cell is working harder during exercise.
An interactive three-dimensional model of a mitochondrion with features labelled.
Mitochondrion model:
Outer membrane: The outermost smooth blue layer surrounding the entire structure.
Inner membrane: The folded orange membrane located within the blue outer membrane.
Cristae: The infoldings of the inner membrane, shown as orange extensions into the green interior.
Mitochondrial DNA (mtDNA): Shown as small, circular, dark structures within the green matrix.
Matrix: The innermost green area of the structure.
Matrix (function): Also within the green matrix area, related to its function.
Ribosomes: Small blue dots within the green matrix area.
Intermembrane space: The space shown between the blue outer membrane and the orange inner membrane.
Intermembrane space full of H+ Ions: Highlighted with a concentration of ions within the intermembrane space.
ATP synthase: Located in the orange inner membrane as part of the electron transport chain complex.
Ribosomes
Ribosomes are the cell's protein factories. These tiny structures read the genetic instructions from messenger RNA (mRNA) and assemble proteins from amino acids. Proteins are crucial for many cell functions, including growth and repair.
Ribosomes are the only organelles that are not membrane-bound. They can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum.
You will delve into the structure of ribosomes and learn more about protein synthesis later.
Endoplasmic reticulum
The endoplasmic reticulum ("endoplasmic" meaning "within the cell" and "reticulum" meaning "small net") is a network of membranes that play a key role in the synthesis of proteins and lipids. It is often shortened to "ER".
It also helps transport materials within the cell and ensures that cells function smoothly.
There are two forms of ER:
Rough ER has ribosomes attached and helps fold and modify new proteins. It prepares them for transport within or out of the cell via the Golgi apparatus. Because it processes proteins, rough ER is typically located near the nucleus.
Rough endoplasmic reticulum, by SMART-Servier Medical Art via Wikimedia Commons, licensed under CC BY-SA 3.0
Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification. It detoxifies by helping drugs and toxins dissolve better in water so they can be cleared from the body. Often, smooth ER is located near the cell membrane.
Smooth endoplasmic reticulum, by SMART-Servier Medical Art via Wikimedia Commons, licensed under CC BY-SA 3.0
Golgi apparatus
Like the endoplasmic reticulum, the Golgi apparaus modifies, sorts and packages proteins and lipids for transport. It consists of flattened, stacked membranes called cisternae.
Once proteins and lipids have been processed within the lumen (channel) of a cisterna, they pinch off into small vesicles which are transported to their destinations inside or outside the cell.
An interactive three-dimensional model of the Golgi apparatus with features labelled.
Golgi apparatus model:
Trans face: Side of the Golgi apparatus that sends vesicles off to other parts of the cell. It is the side farthest from the ER.
Secretory vesicles: Vesicles that transport proteins such as hormones.
Cisternae: Flattened membrane vesicles that can be found in the ER and the Golgi apparatus. They help package and modify certain proteins.
Transport vesicles: Vesicles that deliver proteins from the endoplasmic reticulum to the Golgi apparatus.
Cis face: The side nearest to the ER. Receives transport vesicles from it.
In plant cells, the Golgi apparatus helps make components of the cell wall.
You may see the Golgi apparatus also being referred to as the Golgi body or Golgi complex; they all mean the same thing.
Vacuoles
Vacuoles are storage organelles found in both plant and animal cells, but they serve different roles and vary in size.
In plant cells, there is a central vacuole that is large and takes up most of the cell space. It stores water, nutrients and waste products to help maintain the structure and pressure inside the cell. The pressure control is important because it helps the plant stand upright.
In animal cells, the vacuoles are smaller and there are more of them. They store nutrients and waste products. Vacuoles in animal cells are less important for maintaining cell shape.
Cell membrane
The cell membrane (or plasma membrane) acts as a protective barrier for cells. It is made up of a flexible phospholipid bilayer with embedded proteins and other molecules. The cell membrane helps the cell communicate with its surroundings, and controls what moves in and out of the cell.
We will look at the cell membrane in more detail later.
Lysosomes
Lysosomes ("lys" meaning "loosening" and "some" meaning "body") are the waste disposal system of animal cells. They contain digestive enzymes that break down (or lyse) excess or old cell parts, food particles and invading viruses or bacteria.
They help cells stay healthy by recycling cellular components and keeping the cell clean.
Plant cells don't need lysosomes because their central vacuole performs this function.
Cell wall
Plant cells contain a cell wall, a rigid outer layer that provides structural support and protection. It also regulates interactions with the environment.
A plant cell wall is mainly made up of cellulose, a strong carbohydrate that helps the cell stay firm and stable so that it can grow. Cell walls are attached to the plasma membrane and help maintain the rectangular shape of plant cells. Since animal cells lack a cell wall, their shape is much more variable due to their flexible plasma membrane.
Chloroplasts
Plant cells also produce their own energy through photosynthesis. This process converts carbon dioxide, water and energy from sunlight into oxygen and glucose.
Photosynthesis occurs in chloroplasts ("khloros" meaning "green" and "plastos" meaning "formed"). Chloroplasts contain a green pigment called chlorophyll which captures light energy.
They have membranes called thylakoids ("thylakos" meaning "sac"), which form stacks known as grana. The thylakoids are where the light-dependent reactions of photosynthesis occur. The gel-like fluid surrounding the grana is called stroma.
