# Cells and Their Organelles 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.

Cells are visible in the epithelium tissue of the mammalian gut (*left*) and onion root hair tip (*right*)
Image sources (from left to right):
Berkshire Community College Bioscience Image Library - Epithelial Tissues: Mucous Glands in Simple Columnar Epithelium, CC0, https://commons.wikimedia.org/w/index.php?curid=70159512
Johnathan Gutierrez, CC BY-SA 4.0, https://creativecommons.org/licenses/by-sa/4.0

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**.

Some types of mammalian cells
Image sources (from left to right):
UC Regents Davis campus - http://brainmaps.org, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=22012513
Electron Microscopy Facility at The National Cancer Institute at Frederick (NCI-Frederick) - [1], Public Domain, https://commons.wikimedia.org/w/index.php?curid=407197
CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=600755
SubtleGuest at English Wikipedia, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=19688924

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.

Bacterial (prokaryotic) cell on the left and animal (eukaryotic) cell on the right.
Image source: Biorender.com

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.

Organelle	Function
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.

Plant cell organelle	Function
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).

Bacterial cell
Image source: Biorender.com

Bacterial cell organelle	Function
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

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