The cell is the smallest living thing in the human organism, it is the functional and structural unit of all living things and all living structures in the human body are made of cells.
There are a huge variety of different types of cells in the human body, which contains several billion cells, with hundreds of cell-specific functions. Cells vary in shape (round, flat, long and thin, short and thick) and size, e.g. small granule cells of the cerebellum in the brain (4 micrometers), up to the huge oocytes (eggs) produced in the female reproductive organs (100 micrometers) and function.
A labelled diagram of a human cell is shown below. Each of the labelled structures will then be discussed individually.
The cytoplasm is the jelly-like substance, that primarily consists of water (approx. 90%) and fills the majority of the cell. In addition to water, the cytoplasm also contains dissolved nutrients and waste products.
Its main function is to hold together the organelles which make up the cytoplasm. It also nourishes the cell by supplying it with salts and sugars and provides a medium for metabolic reactions to occur.
The main functions of the cytoplasm are:
- It provides mechanical support to the cell and keep the shape of the cell, which is achieved by exerting pressure against the cell membrane (turgor pressure).
- It acts a storage area for small carbohydrate, lipid and protein molecules.
- It nourishes the cell by supplying it with salts and sugars and provides a medium for metabolic reactions to occur in.
- It is the site of most cellular activities including metabolism and cell division.
- It contains ribosomes which are essential for protein synthesis.
The nucleus is the largest organelle in the cell and contains most of the cell’s genetic material. The nucleus is enclosed by two lipid membranes, called the nuclear envelope, that separate the nucleus and its contents from the cytoplasm. There are tiny holes called nuclear pores situated within the nuclear envelope that assist in the regulation of the exchange of materials such as proteins and RNA, between the nucleus and the cytoplasm.
The nucleus also contains the nucleolus, a smaller structure that lacks a surrounding membrane and occupies approximately 25% of the volume of the nucleus. The primary function of the nucleolus is to transcribe ribosomal RNA (rRNA) and combine it with proteins to form incomplete ribosomes. rRNA is important for the construction of ribosomes, which are the site of protein translation.
Other functions of the nucleus include:
- The control gene expression and facilitate the replication of DNA during the cell cycle.
- The production of messenger RNA (mRNA) which encodes for enzymes and, therefore, assists with the control of the metabolic functions of cells.
- The control of the structure of the cell via the transcription of DNA which encodes for structural proteins.
Mitochondria are membrane-bound organelles that are responsible for the production of the cell’s supply of chemical energy. This is achieved by using molecular oxygen to utilise sugar and small fatty acid molecules to generate adenosine triphosphate (ATP). This process is known as oxidative phosphorylation and requires an enzyme called ATP synthase. ATP acts as an energy-carrying molecule and releases the energy in situations when it is required to fuel cellular processes.
Mitochondria have two phospholipid bilayers, an outer membrane, and an inner membrane. The inner membrane is intricately folded inwards to form numerous layers called cristae. The cristae contain specialised membrane proteins that enable the mitochondria to synthesise ATP. Between the two membranes lies the intermembrane space, which stores large proteins that are required for cellular respiration. Within the inner membrane is the perimitochondrial space, which contains a jelly-like matrix. This matrix contains a large quantity of ATP synthase.
A labelled diagram of a mitochondrion is shown below:
The endoplasmic reticulum (ER) is the site of protein assembly by the ribosomes. The ER is situated in the cytoplasm and consists of a double membrane formed into a network of hollow tubes, flattened sheets, and round sacs, which together are knowns as the cisternae. The ER is connected to the nuclear envelope. There are two types of endoplasmic reticulum: smooth and rough ER.
Smooth ER does not have any ribosomes attached to it, which gives it the ‘smooth’ appearance. The smooth ER is involved in the synthesis of lipids, including oils, phospholipids and steroids. It is also responsible for metabolism of carbohydrates, regulation of calcium concentration and detoxification of drugs.
Rough ER is covered with ribosomes, which gives it the ‘rough’ appearance. The rough ER is responsible for protein synthesis and plays a role in membrane production. The folds present in the membrane increase the surface area, allowing more ribosomes to cover the ER, resulting in greater protein production.
A 3D representation of the ER is shown below:
Ribosomes are composed of RNA and protein and play an important role in protein synthesis. They are found in the cytoplasm and can occur alone, in groups or attached to the endoplasmic reticulum, forming the rough ER (see above).
Ribosomes bind messenger RNA (mRNA) to form a structure known as a polyribosome. They then use the information contained within the mRNA to build a protein with a specific amino acid sequence in a process is called translation.
The Golgi apparatus
The Golgi apparatus is responsible for the post-translational processing of newly made proteins. It consists of a stack of flat membrane-bound sacs called cisternae and is found in close proximity to the nucleus and endoplasmic reticulum.
Proteins are transported from the rough ER to the Golgi apparatus, where they are modified and packaged into vesicles. The Golgi apparatus then transfers these proteins to another location within the cell where they are required. For this reason, the Golgi apparatus is sometimes referred to as the ‘post office’ of the cell.
Vesicles and lysosomes
Vesicles are small, membrane-bound spherical sacs which facilitate the metabolism, transport and storage of molecules. Vesicles are produced in the both the Golgi apparatus and the endoplasmic reticulum. They can also be made from parts of the cell membrane. Vesicles can be classified according to their contents and function, for example, transport vesicles are responsible for the transport of molecules within the cell.
Lysosomes are formed by the Golgi apparatus or the endoplasmic reticulum and contain powerful enzymes that can digest cell structures and food molecules such as carbohydrates and proteins. Lysosomes are abundant in animal cells that ingest food through food vacuoles. When a cell dies, the lysosome releases its enzymes and digests the cell.
Next: Cell Structure and Function Part 2– The Cell Membrane
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