All cell functions are limited by their size as they must house the organelles essential for life: DNA, protein molecules and internal structures. Even these smallest cells must also have enough surface area to respirate, obtain nutrients and dispose of waste materials.
Prokaryotic cells evolved far earlier than eukaryotic cells, and as such present day prokaryotes have a different cellular anatomy. The most common type of prokaryotes are Bacteria and Archaea, and their key features are as follows:
- Organelles in the cytosol are not membrane-bound.
- Chromosomes are grouped in the nucleoid with no nuclear membrane.
- Prokaryotes are much smaller cells.
Eukaryotes including animals, fungi, plants and protists also have defining characteristics.
- All organelles are membrane-bound.
- Chromosomes are found in a membrane-bound nucleus.
Regardless of which category a cell falls into, all their organelles are organised into four basic functional groups: support, hydrolysis, energy and manufacturing.
The plasma membrane (PM) forms the boundary for the cell. In animal cells, the plasma membrane is the second outermost player; however, in plant cells, this membrane is reinforced by the cell wall. The most important characteristics of the PM is its selective permeability. Composed of a phospholipids, proteins and carbohydrates, the PM regulates transportation across the cell. Non-polar substances such as oxygen gas and carbon dioxide can easily permeate the thin layer. Other substances require more active transportation involving the embedded proteins.
The cytoplasm contains cytosol and suspends all the organelles. In addition, it gives the cell its shape.
The cytoskeleton reinforces the shape of the cell and helps move the cell. It consists of a network of fibres that run through the cytoplasm. The three fibres that make up the cytoskeleton include microfilaments, intermediate filaments and microtubules.
- Microfilaments: composed of actin; the smallest; when paired with myosin, is involved in movement
- Intermediate filaments: medium sized; permanent cellular fixtures; help maintain the shape of the cell and the placement of organelles
- Microtubules: largest; made of tubulin; shape and support the cell; serve as tracks for the movement of molecules in the cell; separate chromosomes during cellular division
Mircovilli are tiny projections that help increase the cells surface area.
The extracellular matrix (ECM) of animal cells wraps around the plasma membrane, and is composed of glycoproteins (like collagen) secreted by the cell. The ECM helps strengthen tissues and transmit external stimuli into the cell that tun genes on and off to modify activity.
Centrosomes are regions in the cell where microtubules initiate. In animal cells, the centrosomes contain a pair of centrioles.
Found only in plant cells, the cell wall forms the outermost layer. Made up of polysaccharides (including cellulose) and proteins, it is much tougher and less pliable than the plasma membrane. However, it helps plants stand upright in the correct shape.
Also only found in plant cells, plasmodesmata are channels that run through the cell wall. They help connect the cytoplasms of adjacent cells.
Flagellum and cilia are locomotive organelles found in some animal cells. They are hair-like projections composed of membrane inclosed microtubules. Examples of animal cells with flagella and cilia: sperm (flagella), cells in the digestive tract (cilia), cells in the trachea (cilia).
Lysosomes are digestive organelles that hydrolyse (breakdown with water) macromolecules like proteins, carbohydrates and nucleic acids. Hydrolysis breaks down these macromolecules into monomers that are then released and recycled. A cell is threatened by a burst lysosome as they are very acidic.
Vacuoles are only found in plant cells and take up a large deal of space in the cell. Their prominent role is storage and breakdown of waste products and hydrolysis of macromolecules. Older cells will contain larger vacuoles, suggesting that vacuoles play an important role in plant growth. Some plant cells can be 80% vacuole. Tonoplasts encompass the vacuoles.
Peroxisomes are organelles with a variety of metabolic functions including hydrolysis of macromolecules (mainly large fatty acid chains). The broken down fatty acids fuel mitochondria. Another role of peroxisomes is to detoxify alcohol. A waste product produced here is hydrogen peroxide, which is toxic to the body. Fortunately, peroxisomes also contain catalase, which breaks down the acid into water or another neutral compound.
Mitochondria are the cite of cellular respiration and generation of ATP. Mitochondria are unique that they have a double-membrane called with infolds called cristea and also because they have their own DNA called mDNA. Almost mDNA is exclusively inherited through our mother because male mDNA is usually destroyed during conception. In addition to its own DNA, mitochondria also contain enzymes and ribosomes. All of these additional components are found in the matrix and catalyse respiration reactions to produce ATP.
Chloroplasts, found only in plants and algae, are the site of photosynthesis.
The nucleus contains the majority of the cell’s DNA, and is the cite of messenger RNA (mRNA) synthesis. mRNA contains the code for protein synthesis outside of the nucleus. Surrounding the nucleus is the nuclear envelope, which contains nuclear pores that allows mRNA to leave the nucleus. The complex of DNA is called chromatin. Whenever a cell prepares for cell division, fibres of chromatin coil up into chromosomes.
Ribosomes are the final location for mRNA and the site of protein synthesis. Ribosomes come in various sizes and locations. They can be free floating in the cytoplasm or attached to the rough endoplasmic reticulum (ER). Free floating ribosomes are responsible for synthesising proteins that are used in the cell. Ribosomes attached to the rough ER synthesise proteins for export or use in the cell.
The endoplasmic reticulum comes in two forms: smooth and rough. The ER makes up the largest component of the cell and consists of a network of membranes and sacs. The inner portion of the ER is called the cisternal space. The smooth ER synthesises lipids, metabolises carbohydrates and detoxifies drugs and poisons. As proteins are synthesised by the ribosomes attached to the rough ER, the polypeptide chains travel across the membrane and into the cisternal space. Here, the polypeptides are concentrated to be packaged into transport vesicles and shipped to the Golgi apparatus.
The Golgi apparatus modifies incoming transport vesicles and then repackages them and ships them out again. The Golgi apparatus has polarity, a cis and trans faces. The cis face receives and the trans face ships.
Tight junctions are sites were two neighbouring animal cells are fused so tightly that they are water-tight.
Desmosomes fasted animal cells together with intermediate filaments. This anchoring also gives them the name anchoring junctions. These types of IC junctions are common in tissues subject to stress such as muscles and skin.
Finally, gap junctions provide channels between animal cells for the movement of ions, sugars and other small molecules. Gap junctions are found in embryonic and heart tissue.