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Cell organelles are specialised structures within cells that come in various shapes and sizes. They serve a crucial purpose because we must compartmentalise all the operations within the cell. A membrane must surround the systems within a cell that performs different functions; hence, organelles are entirely membrane-bound. They also distinguish one function from another. We will look in detail at the cell organelles and their functions.
Types of Cell Organelles
The cytoplasm and the nucleus are the two major sections of a cell. A nuclear envelope surrounds the nucleus, which contains the genetic code in the format of a chromosome. The cytoplasm is a liquid network surrounding the nucleus, separated by the cell’s outer membrane. Organelles are tiny structures in the cytoplasm that perform functions that keep the cell in a state of homeostasis. They are involved in various functions, including energy synthesis, protein and secretion formation, toxin destruction and reacting to external signals.
Cell organelles can be divided into three categories:
General cell organelles: Cell membrane, mitochondrion, rough endoplasmic reticulum, smooth endoplasmic reticulum, lysosome, plastid, ribosomes, Golgi bodies, nucleus and cytoskeleton are a few examples present in both animal and plant cells at all times.
Temporal cell organelles: Chromosome, centrosome, endosome and autophagosome are only found at specific stages of the cell’s life cycle.
Cell organelles specific to cell type: Chloroplast, central vacuole and cell wall are present in plant cells but not in animal cells.
Structure of Cell Organelles
Cell membrane
The cell membrane is common for both plants and animals. It varies in shape for different types of animal cells.
Cell wall
Plants have an additional cell wall that helps keep the cell’s structure intact. The cell wall is made up of cellulose and hemicellulose. It is rigid, semi-permeable and non-living. It has the following parts:
- Primary cell wall: It is the first cell wall to be formed and is situated closest to the inside of the cell. It is made up of cellulose, so it can stretch to accommodate growth.
- Middle lamella: It is the outermost layer that holds the neighbouring cells together. It is primarily made of pectins.
- Secondary cell wall: It is formed inside the primary wall after the cell has fully grown. It is the thickest layer of a cell and provides its characteristic shape.
Cytoplasm
The cytoplasm is the colourless matrix inside the cell wall. It houses everything other than the nucleus. It is 80 percent water, with the remaining matter composed of nutrients.
Mitochondria
These are made of double membranes. The outer membrane is a smooth case, and the inner membrane is folded into finger-like projections. Inside the membranes is a mesh-like structure that contains enzymes required for metabolic processes. Single or double-stranded DNA is also present as a component of mitochondria.
Nucleus
Three main structures constitute the nucleus.
● Nuclear envelope: It is much the same as a cell membrane and keeps the nuclear matter and nucleolus inside the nucleus. It lets proteins and RNA move in and out of the nucleus.
● Chromatin material: This is made up of compared DNA strands.
● Nucleolus: It is like a nucleus in the structure; that is, it is spherical. But it does not have a membrane. So the organelle without a cell membrane is a nucleolus.
Endoplasmic reticulum (ER)
The endoplasmic reticulum is present near the nucleus. On the basis of the attachment of ribosomes on the surface of the endoplasmic reticulum, these are categorised into two categories, the rough ER and the smooth ER. They are in three forms – vesicles, tubules and cisternae. ER is responsible for the formation of proteins (rough ER), lipids and steroids (smooth ER).
Golgi apparatus
The structure of the Golgi apparatus is made up of
● Cisternae: Parallel bundles of flat sac-like structures; these are the smallest part of the Golgi apparatus.
● Tubules: These are tubular structures growing from the cisternae.
● Vesicles: They are spherical bodies performing specific functions.
Lysozyme
They are mostly spherical or granular in structure but can be irregular in shape. Lysozymes are surrounded by a membrane that encases their enzymes.
Conclusion
Each organelle has a specific function, and if one malfunctions or goes out of line, the cell will most likely stop working. As an organism evolves, some organelles may become obsolete or be replaced by others, in which case the organelles may naturally disappear. The function of cells will be impacted. However, if mitochondria or other critical organelles such as the nucleus are missing, the cell would not be able to survive.
