Eukaryotic Animal Cell Structure: A Visual Guide
Structure of Animal Cells
The first topic I teach my students on my A-level (16-18yrs old) Biology course is the structure of an animal cell. This is a topic that students have been introduced to early in their school career, but much of the detail has been left out for simplicity sake.
Ask any GCSE student what a cell is made of and they will confidently respond:
"Cytoplasm, Cell Membrane and Nucleus"
This is correct, but leaves out most of the complexity of Eukaryotic Cells. The cell is the fundamental unit of life, and so all students of biology must have an in-depth understanding of the structure and function of the Eukaryotic Cell.
Structure of Eukaryotic Animal CellClick thumbnail to view full-size
- Eukaryotic - a cell type that has a nucleus and other membrane-bound organelles
- Prokaryotic - a cell type that has no nucleus or membrane bound organelles (e.g. Bacteria)
- Organelle - a specialised part of a cell with a particular function (e.g. mitochondria)
What are the Functions of an Animal Cell?
There are over 200 different cell types in the human body, each with a very specific job. We are only alive because each of our cells has a specialised job to do. Our cells are alive and so do everything that we do, namely:
Just as organisms are made up of cells, cells are made up of organelles. There are many different organelles that co-operate to keep the cell working.
Division of Labour - Inner Life of a Cell
What is Division of Labour?
Cells break tasks up into parts, each completed by a different organelle. This is known as division of labour. An example of division of labour is the journey of a protein from nucleus to the cell membrane.
The Plasma Membrane is like a border guard - it controls what goes in and out of the cell. Division of labour requires the careful separation of conditions depending on the function of a cell, or even of a part of a cell. Imagine the chaos that would ensure if you tried to cook a three course dinner all in one big bowl! Different chemical reactions are kept separate by membranes. The grandest of these is the plasma membrane. This surrounds the cell and is known as a 'fluid mosaic membrane.'
- Fluid - the components are constantly moving, much like the surface of a sea
- Mosaic - the membrane is studded with proteins, carbohydrates, pores, channels and other molecules such as cholesterol.
- Phospholipid bilayer - the membrane is a double layer with the hydrophobic tails (water-hating) facing away from the water of the cell and the hydrophillic (water-loving) heads facing the water.
The cytoplasm is a soup made of water, amino acids and enzymes - all wrapped up in the cell membrane. It contains and supports all of the organelles of the cell. The cytoplasm is like the shop floor of a factory where many chemical reactions of the cell occur.
Diagram of the Nucleus
Structure of the Nucleus
The defining feature of eukaryotes! The nucleus is the single largest organelle in the animal cell. The nucleus serves as a storage and replication facility for DNA - the blueprint of life. There is nearly 6 feet of DNA in each human cell, which is carefully structured into chemical called chromatin.
The nucleus itself can be further divided into:
- Nuclear envelope - a double membrane that surrounds the nucleus for most of the cell cycle. This dissolves during prophase to allow for separation of chromatids. The envelope is like a club bouncer - it closely controls what enters and leaves the nucleus.
- Nuclear Pores - a series of 'holes' in the nuclear envelope that allow the free passage of RNA out of the nucleus, as well as nucleotides, phosphates and certain sugars into the nucleus. As a hole, any molecule smaller than the hole can freely move through it
- Nucleolus - a subsection of the nucleus that makes ribosomes (the protein building machinery of the cell). Appears as a dark spot in the nucleus and disappears during mitosis.
Endoplasmic Reticulum Photo
These look like a series of flattened tubes and sacs, usually starting from the nuclear envelope. Annoyingly, the ER is perhaps one of the most fluid structures in the cell. The amounts of each of the three types (rough, smooth, and sarcoplasmic) can readily change to meet the needs of the cell, and it doesn't look exactly the same each time.The function of the reticulum depends on its type, although all three are used to make, modify, repackage or redirect molecules in the cell. I think of the ER as a series of factories with an on-site delivery wing:
- Rough ER - Looks rough as it is studded with Ribosomes. The RER helps make proteins and modify pre-made proteins. This could include adding new groups, or helping the protein to fold into its 3-D (tertiary) structure.
- Smooth ER - Mainly used to make fats and lipids. Lipids are used for detoxifying poisons and providing energy. This means you find lots of SER in the liver and brain - two organs that need lots of energy!
- Sarcoplasmic Reticulum: Found almost exclusively in muscle cells, the SR is used to store calcium ions. When cells release calcium they make an electrical signal that causes a muscle cell to contract. When problems occur with the SR, it can cause unwanted contraction of muscles - particularly dangerous in the heart!
Ribosomes are factories for churning out proteins - they are the site of protein synthesis. Ribosomes are made of about 60% RNA and are mainly found embedded in Endoplasmic Reticulum. You can find some floating in the cytoplasm, but they are very difficult to identify on an electron micrograph.
Diagram of a Mitochondrion
Think of the mitochondria as a power station for the cell. They provide energy, in the form of ATP, to be used for all the activities of the cell. Cells that need a lot of energy will have a lot of mitochondria packed into them. This includes cells in the the brain, liver and muscles.
Lysosomes and Peroxisomes
All cells need to get rid of waste or contain biohazard spills. These are the roles fulfilled by the lysosomes and peroxisomes. The lysosomes contain digestive enzymes that are used to break down used, damaged or useless molecules. When cells need to self destruct they make extra lysosomes to help them literally 'eat' themselves.
Peroxisomes are used to protect the cells from the toxic by-products of many of its reactions. These include chemicals such as free radicals and hydrogen peroxide.
The Golgi Body is like the post office of the cell. The Golgi adds small molecules (that act as stamps) onto lipids, carbohydrates and proteins, packages them up and sends them to the parts of the cell where they are needed. The vesicles that bud from the trans face of the Golgi can fuse with the plasma membrane in a process called exocytosis that allows biomolecules produced in the cell to leave the cell. Cells involved in excretion (such as the mucous membrane in your nose and throat) have lots of Golgi
Functions of Organelles
Storage for DNA, controls cell activities
Rough Endoplasmic Reticulum
Helps fold and modify proteins made by ribosomes
Factory production line
Assembles proteins from amino acids
Smooth Endoplasmic Reticulum
Make lipids (fats) in the cell
Supply the cell with energy (ATP)
Processes, packages and transports proteins
Contain digestive enzymes
Site of chemical reactions in cell
Fibres that move things around the cell
Proteins used to separate chromosomes during cell division
Animal Cell Rap
How to Learn the Parts of an Animal Cell
Learning the structure and functions of the different organelles of an animal cell is a basic requirement of many biology courses. This may seem like a daunting task, particularly if you are just stepping up from the more simplistic cell structures taught at earlier points in your biology career.
Use the checklist below to improve your knowledge of the Eukaryotic Animal Cell:
- I can name the organelles of an idealised cell, using a keyword list;
- I can name the organelles of an idealised cell, from memory;
- I can identify organelles on a scanning electron micrograph;
- I can draw my own labelled diagram of a cell from memory;
- I can explain the functions of each organelle in one or two sentences;
- I can use a metaphor/analogy to explain the functions of each organelle to a non-expert audience.
- I can outline an example of division of labour, such as secreting a hormone.
Animal Cell ImagesClick thumbnail to view full-size
Functions of Organelles Quizview quiz statistics
Animal Cell Resources
A collection of visually stunning animations exploring the cell, from the same people that brought you "The Inner Life of a Cell" Take a look - definitely recommended and will leave you thinking about your cells a little differently
- Molecular Expressions Cell Biology: Animal Cell Structure
An in depth exploration of all aspects of Animal Cell Structure. A simply amazing resource. Highly Recommended
- Structures of Eukaryotic Cells and Their Functions
A lovely tabular summary of the structures in Eukaryotic cells, including whether present or absent in plant or animal cells