Unicellular Amoeba organism

Unicellular organisms

Unicellular organisms are those which are made of only one cell. The main types of unicellular organisms are archaea (Archaebacteria), bacteria (Eubacteria) and protozoa (Protista). Unicellular organisms can be either prokaryotic (archaea and bacteria) or eukaryotic (protozoa, some algae, and fungi).

In the image below we can see examples of Protist (Paramecium and Amoeba), Bacteria and Fungi (Yeast).

Unicellular organisms can still have specialised features like a flagellum

A unicellular organism is only classified as such if it carries out all metabolic process that sustain life within 1 cell. A multicellular organism requires other cells to survive.

Therefore we can say that in unicellular organisms, the division of tasks that sustain life is done only at an organelle level and as a result is very inefficient. For example a single celled organism that photosynthesises is only doing so in it's chloroplasts, a multi-celled organism is photosynthesising in all the chloroplasts of all the cells that contain them, so the former produces small amounts of glucose compared to the latter.

As the one cell has to perform all the functions necessary for life, they contain simple but effective structures/organelles to perform these processes. As a result, most unicellular organisms have 'generalised' cells.

Perhaps the most important part of a unicellular organism is its cellular membrane as it is constantly exposed to the outside environment. The membrane acts to exclude unwanted substances and to bring in essential nutrients.

Thinking back to when we discussed how the SA/V ratio of cells influences their size, it is understandable why unicellular organisms are very small in size, the maximise this exchange of essential nutrients. Small size means that cell requirements and wastes can move easily by diffusion, osmosis and active transport. As a result of this, unicellular organisms are often aquatic or live in very damp areas.

The contents/structures of a unicellular organism will vary based on which kingdom it belongs to but the two features that all unicellular organisms contain is genetic material and ribosomes (which produce essential proteins) regardless of if they are eukaryotic or prokaryotic. In the diagram below the ribosomes are represented by the little blue circles in both the eukaryote and the prokaryote.

Colonial biofilm organisms

Colonial organisms

Colonial organism are a group of single-celled organisms which form a colony in which the cells are physically connected and depend on each other, they can be eukaryotic or prokaryotic.

Colonial organisms are thought to have been a stepping stone in evolution from unicellular to multicellular organisms. These growth formations are advantageous as physical proximity (closeness) allows them to effectively distribute nutrients.

It is important to understand that a colonial organism relies on the other organisms for survival. This is not like bees or ants that live together in colonies which could live on their own, but choose to group together to make life easier.

If a single cell is taken from the colony it will be able to survive, just less efficiently.

Colonial organisms have cells in organised structures. These structures work together to meet the needs of other cells in organism. Many types of colonial organisms have cells that secrete an external framework or supporting structure. This means that some appear large but their cells are in close contact with their surroundings.

Cells in colonial organisms have become more specialised than they would if they lived alone. Many colonial organisms are aquatic and stationary (sessile) and rely on filter feeding or a symbiotic relationship with other organisms for nutrition.

Some examples of colonial organisms are corals, sponges and some prokaryotic cyanobacteria that form biofilms. A biofilm (seen below) is a microbial colony containing multiple species that are growing together in a structure. This type of growth is useful for the microbes as it allows the group to work together to survive much more efficiently than an individual would on its own. Biofilms can be resistant to drugs due to the presence of a protective matrix and minimise the effects of environmental changes.

Biofilm is multiple different organisms working together

Colonial organisms that are the same species are much more common. For example, Volvox (seen below) is a colonial organism that is made up of a hollow sphere that contains 500-60,000 algae cells which are connect by strands of cytoplasm. This allows them to swim in a co-ordinated fashion. Each colony is only one cell thick, has a red 'eyespot' that detects light. Each cell in the colony contains chloroplasts (and so photosynthesises). Even though Volvox shows some specialisation, it lacks tissues and organs and is therefore still classed as unicellular.

Multicellular organisms are composed of multiple types of specialised tissues

Multicellular organisms

A multicellular organisms are organisms which are made up of more than one cell that each carry out a specific function based on their specialisation. Similar cells are grouped together and perform their functions together.

All animals and plants, some protists, fungi and algae multicellular. Multicellular organisms are dependent on all the cells in their system for survival. Each type of cell within an organism will perform a specific function and the cells will work together in a complementary manner to improve the organisms overall health.

If a single cell is removed from a multicellular organism it will not survive.

Furthermore, as a result of making each process highly efficient, multicellular organisms are able to live for a considerably longer amount of time than unicellular organisms.

Multicellular organisms are made up of highly specialised and interdependent cells that are organised into tissues, organs and organ systems. We will look into this more in a future lesson.

In multicellular organisms, the division of labour (that is at an organelle level in unicellular organisms) is at a cellular level or at an even higher level in more complex organisms such as at a tissue or organ level. Because large groups of cells are dedicated to a certain function this function is performed in bulk and so is very efficient.

You can think of a multicellular organism like two or more people living together. If one person does all the house cleaning this lets the other person use their time to do the cooking and the gardening. In a multicellular organism because one cell is working to gain nutrients the other cell is able to work on removing wastes from the body.

For example, the human body has highly specialised cells called nerve cells that pass electrochemical signals around the body, it also has epithelial cells which absorb nutrients in the gut.

The relative sizes of common microscopic organisms

As a result of working together, the majority of cells in a multicellular organism are not exposed to the outside environment. Instead, specialised cells are arranged around the organism to protect the others. For example in the image below we can see specialised cells on the very tip of a plant root called a root cap. These cells are designed to protect the delicate new cells that are being produced in the mitotic zone which is the primary growth area of a root.

The root cap is designed to protect the delicate growth area of the root called the mitotic zone

Stem cells have the ability to differentiate into any specialised cell in the body

Stem cells

When organisms begin development as embryos, all our cells are embryonic stem cells.

• Stem cell: An un-differentiated cell. Found in high numbers in early embryos. Each cell as the potential to become differentiated.

As the embryo continues to grow and cells divide, these stem cells begin to differentiate, so that the cells become certain types of cells which perform specific functions. The changing of a stem cell into a type of cell is differentiation and the final form they take is specialisation. This is important because a neuron in the brain has very different abilities to a skin cell, but each are important in their different ways to the overall functioning of an organism.

• Cell differentiation: The process by which a less specialised cell changes to become a specialised type of cell. For example, the process of a blood stem cell differentiating into a red blood cell.

• Cell specialisation: The specific function which a cell has, determined by their physiology and cellular structures.

Tissue: A group of cells which work together to perform a function. For example, muscle cells work together in muscle tissues to produce motion of the body.

Tissues are collections of specialised cells, which work cohesively to perform a set function. They are specialised because their cell organelles are specifically designed to undertake certain tasks in the body. It is important that cells are specialised, so that a bunch of different complex tissues can be forms.

There are four main types of tissue for plants and animals:

Tissues are composed of specialised cells

Organs are made up of multiple types of tissue

Organs

Organ: A structure which is composed of a number of tissues which work together to perform a shared function.

Organs are groups of tissues arranged in an order to perform a large-scale function. Organ structure is influenced by the function it must perform.

For example, the stomach digest food, so it has a large internal cavity containing acids, enzymes, and bacteria which break food down. The cavity is surrounded by epithelial cells, so that nutrients can absorbed. Additionally, the walls of the stomach contain muscle cells, which are able to contract and influence movement. This helps the organ to physically break the food down.

Some examples:

• The lungs which contain smooth muscle tissue, epithelial tissue, bronchioles, alveoli, lymphatic tissue and blood tissues.

• The heart which contains cardiac muscle tissue, connective tissues, blood tissues and nervous tissue.

• The kidneys which contain connective tissue, adipose (fat) tissue, blood tissues, lymphatic tissue and renal tissue.

• The roots in plants which are made of vascular tissue, epidermal tissues (dermal tissue) and parenchyma tissues.

• The stem in plants is made of epidermis (dermal tissue), vascular, and some ground tissue.

• The leaf in plants is made up of epidermis (dermal tissue), ground tissue, vascular tissue (veins in leaf).

The image above shows the different tissues that make up the digestive tract.

Organ systems are composed of multiple organs

Organ systems

Organ System: A group of related organs which work cohesively together to perform an aspect of bodily function.

The image below shows an example of an organ system in the body, in this case the digestive system.

The digestive system is involved in the break down of nutrients