Eukaryotic cell structures

In a nutshell

Cells are the fundamental structural units that make up an organism. Within every cell there are organelles that carry out specific functions and each cell type has a different internal structure that suits its function. This is known as the ultrastructure of the cell.

Cellular structures

Eukaryotic cell

1. Chromosome, 2. Microtubule, 3. Nucleoplasm, 4. Nuclear membrane, 5. Smooth endoplasmic reticulum, 6. Rough endoplasmic reticulum. 7. Nucleolus, 8. Ribosomes, 9. Golgi apparatus, 10. Centrioles, 11. Plasma membrane, 12. Lysosome, 13. Mitochondrion, 14. Cytoplasm

Membrane-bound organelles

Eukaryotic cells contain membrane-bound organelles. These are internal structures that contain a membrane. Only eukaryotic cells have membrane-bound organelles, prokaryotic cells do not. The membrane surrounding the organelles keeps them separate from the rest of the cell. This forms discrete compartments to carry out different functions. Below are the membrane-bound organelles found in eukaryotic cells.

Organelle	Structure	Function
Nucleus	The nucleus has a double membrane called the nuclear envelope. There are holes in the nuclear envelope known as nuclear pores. Inside the nucleus is the nucleolus which contains the RNA and does not have a membrane. Note: Chromatin is the genetic material. Chromatin consists of DNA wound around histone proteins.	The nuclear envelope acts as a barrier to separate the nucleus from the rest of the cell. The nuclear pores allow large substances, such as mRNA to leave the nucleus and others to enter the nucleus. Ribosomes are made in the nucleolus and the chromosomes contain the organism’s genes. When the cell is preparing to divide, the chromatin will condense and coil into chromosomes. These make up the organism’s genome.
Rough endoplasmic reticulum (RER)	A system of membrane-bound, flattened sacs called cisternae, with ribosomes attached. They are continuous with the nuclear membrane.	The RER functions as the intracellular transport system of the cell. Channels of cisternae form that transport substances around the cell. The presence of ribosomes on the surface of the RER provides a large surface area for protein synthesis. After the amino acids have been made into proteins, they will travel through the membrane and into the cisternae where they are transported to the Golgi apparatus for modification.
Smooth endoplasmic reticulum (SER)	A system of interconnected tube-like structures (cisternae) with no attached ribosomes. They are continuous with the nuclear membrane.	The SER is involved in lipid synthesis. It contains enzymes that catalyse the synthesis of cholesterol, the synthesis of phospholipids and the synthesis of steroid hormones. Example There are lots of SER in the testes which make the steroid hormone testosterone.
Golgi apparatus	Stacks of flattened, membrane-bound sacs formed by fusion of vesicles from the endoplasmic reticulum.	The main function of the Golgi apparatus is protein modification. The proteins are packaged into vesicles that are pinched off and released. Examples Sugar molecules are added to proteins to make glycoproteins. Lipids are added to proteins to make lipoproteins. Proteins are folded into their 3D structures.
Mitochondria (Singular=mitochondrion)	Oval shaped organelles with a double membrane. Between the two membranes is space filled with fluid. The inner membrane is folded to form cristae. Inside a mitochondrion is a matrix which is also filled with fluid.	The mitochondria are involved in aerobic respiration which produces ATP.
Chloroplasts	Large double membrane organelles found in plant and algal cells. The inner membrane is continuous with the thylakoids. The thylakoids are stacks of flattened membrane sacs. Each stack is called a granum. The plural of granum is grana. Inside the chloroplasts is a fluid-filled matrix called the stroma. Inside the chloroplast there is also loops of DNA and starch grains.	This is where photosynthesis happens. The grana is the site where light energy is used to make ATP and the stroma is where the carbohydrates are made during photosynthesis. Chloroplasts are found in the highest abundance in leaf cells.
Vacuole	The tonoplast is the name given to the membrane surrounding the vacuole. The vacuole contains fluid.	Plant cells have a large permanent vacuole which is filled with water and solutes. The presence of the vacuole maintains the turgidity of the cell as the vacuole pushes against the cell wall when it is full.
Lysosomes	Small, single membrane bound organelles that are formed by the Golgi apparatus. They contain digestive enzymes called hydrolytic enzymes. Example Phagocytic cells like neutrophils and macrophages contain lots of lysosomes to digest invading pathogens.	Lysosomes keep the enzymes separated from the cellular contents. They can engulf foreign matter and digest it. They can also engulf old organelles for the cell to reuse.
Cilia and undulipodia	Protrusions from the cell that are surrounded by the cell surface membrane. The cilia contain microtubules and they are formed from centrioles.	Cilia line the epithelial cells that line the airways. They beat and move mucus to the back of the throat to be swallowed. Most cells have a cilium that acts as a signalling receptor to allow the cell to detect the environment. An undulipodium is a longer cilium and the only human cell to have one is a spermatozoon.

Non membrane-bound organelles

Eukaryotic cells also contain non-membrane bound organelles. These are internal structures that do not contain a membrane. Below are the non-membrane bound organelles found in eukaryotic cells.

Organelle	Structure	Function
Ribosomes ( $80S$ )	Very small spherical organelles made of ribosomal RNA (rRNA) and protein. They are made in the nucleolus and they are made as two subunits that pass out through the nuclear envelope and combine in the cytoplasm.	Ribosomes are used in protein synthesis and are found on the RER.
Centrioles	Small hollow cylinders made of microtubules. They are formed from two microtubules that are at right angles to one another. The microtubules are made from tubulin.	The centrioles form the spindle for cell division. The centrioles are also involved in the formation of the cilia and undulipodia. Plant cells usually don’t have centrioles, but some unicellular algae do.
Cytoskeleton	The cytoskeleton is a network of protein structures within the cytoplasm. The cytoskeleton is made up of: rod-like microfilaments ( $7\ nm$ ) made from actin, intermediate filaments ( $10\ nm$ ), straight microtubules made from tubulin ( $18-30\ nm$ ) and cytoskeletal motor proteins (myosins, kinesins and dyneins) that act as molecular motors and hydrolyse ATP to provide energy.	Microfilaments provide support and help maintain the cell’s shape. Microtubules also offer structural support and form a track to allow organelles to move through the cytoplasm. Intermediate filaments anchor the nucleus in the cytoplasm and extend between some cells to facilitate cell-cell signalling.
Cellulose cell wall	The plant cell wall is made from cellulose fibres and it surrounds the plasma membrane.	The cell wall prevents the plant cells bursting when they are turgid. It helps to maintain the shape of the cell and provides strength and support to both the plant cell and the plant. The cell wall is permeable to allow solutions to pass through. *Note:* Fungi have a cell wall but it is made from chitin not cellulose.

Cellular organisation

Multicellular organisms are made up of many cells that work together. The cells follow a specific level of organisation to allow the proper functioning of the multicellular organism. The organisation is detailed in the table below.

Level	Description	Example(s)
Tissues	Tissues are group of similar cells that work together to form a particular function.	In the human body there are four main tissue types; epithelial tissue, connective tissue, muscle tissue and nervous tissue.
Organs	An organ is made up of several tissues that work together to carry out a particular function.	The leaf is an organ formed of vascular tissue, epithelial tissue and mesophyll tissue.
Organ systems	An organ system is made up of lots of organs working together to carry out large-scale functions.	The digestive system is an organ system made up of the stomach, pancreas and intestines.