Cells and Their Organelles
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When using a microscope to magnify a section of tissue from any organism, you will observe that it is composed of units known as cells. The cell is a basic structural and functional unit of an organism.
Multicellular organisms, such as animals and plants, exhibit a complex organisation. For instance, the human body is comprised of approximately 30 trillion cells. In contrast, some organisms, like amoebae, consist of a single cell. Cells come in various shapes and sizes, each equipped with specialised functions that contribute to the overall functioning of living systems. Different functions of a cell can be assigned to specialised compartments called organelles.
Cells can be categorised as eukaryotic or prokaryotic. Two large groups of organisms, bacteria and archaea have prokaryotic cells, while all other organisms have eukaryotic cells. Take a look at the diagram below and try to identify two major similarities and one difference between prokaryotic and eukaryotic cells.
All cells share common characteristics, one of the most notable being the presence of a plasma membrane and ribosomes. The cell membrane serves to control what goes in and out of the cell. The ribosomes perform the synthesis of proteins.
The major difference is the presence of a nucleus and other membrane-enclosed organelles in eukaryotic cells. Membranes separate such organelles from the rest of the cell in order to maintain conditions (e.g. pH or concentration of some molecules) optimised to perform particular tasks. A jelly-like substance surrounding the organelles is called cytosol.
The functions of major organelles that can be found in most eukaryotic cells are described in the table below.
Nucleus
Stores most of the cell’s genetic material
Mitochondrion
The site of aerobic respiration, the process that produces adenosine triphosphate (ATP) – a molecule that cells use as a major source of energy
Ribosome
Makes proteins
Rough endoplasmic reticulum
Is covered by ribosomes. Transports proteins to other parts of the cell
Smooth endoplasmic reticulum
Makes lipids, stores calcium ions, breaks toxins
Golgi apparatus
Modifies proteins and packages them into vesicles
Lysosome
Digests molecules
Cytoskeleton
A network of protein filaments that defines the cell shape, enables the cell movement and intracellular transport
Plant cells have some organelles distinguishing them from animal cells.
Permanent vacuole
Contains the cell sap which stores degradative enzymes, waste products and inorganic ions
Chloroplast
Glucose is synthesised here in course of photosynthesis
Cell wall
A rigid structure surrounding the cell which protects the cell and defines its shape
Prokaryotic cells have no nucleus and other membrane-bound organelles. Many prokaryotes are surrounded by two membranes (gram-negative bacteria).
Nucleoid
A region where the bacterial chromosome is located
Plasmid
Small circular DNA that often carries genes for antibiotic resistance
Flagellum
An organelle responsible for movement
The video below provides a summary of the topic and some additional details:
