1. Cells, Tissues & Organs

What you need to know...
  • Specialisation of cells, in animals and plants, leads to the formation of a variety of tissues and organs.

Source: SQA

In the previous unit we explored the world of the cell in detail, in this unit we zoom out and look at how cells work together in multicellular organisms. One of the advantages of multicellular organisms is that their cells can become specialised to perform particular functions. In this topic we'll learn about the way in which specialised cells come together to form tissues and organs in animals and plants. 

An organ in a multicellular organism is a collection of tissues which are grouped in order to carry out a particular function. Each tissue in an organ consists of similar specialised cells. The general relationship between organs, tissues and cells is represented in the following diagram.

Every organ you can think of in an animal body, including your own, consists of tissues made up of specialised cells. One organ which we'll use as an example here is the heart. 

The heart is obviously an organ with a particular function - pumping blood around the body. We'll come back to the heart in a later topic. As an organ then, the heart is made up of many tissues. I've shown three such tissues in the diagram above.

Muscle Tissue: Muscle tissue consists of muscle cells which are specialised in such a way so that they can contract which allows movement. The heart consists of a large amount of muscle tissue which contracts to provide the force to pump the blood out.

Nerve Tissue: In order to ensure the heart muscle tissues contracts at the right frequency, your heart also contains nerve tissue which consists of nerve cells. Nerve cells are long and thin and can transmit impulses over long distances very quickly. We'll come back to nerve cells later in this unit.

Blood Tissue: A functioning heart obviously contains a large amount of blood tissue - both in the chambers to be pumped, but also in the blood vessels supplying the heart muscle tissue with the oxygen and glucose it needs in order to respire to produce the ATP energy it needs to contract. Blood tissue contains red blood cells which are specialised to absorb, carry and release oxygen. We'll revisit blood later in this unit.

Plants have cells, tissues and organs also. One example of a plant organ which is a leaf. A leaf is a collection of tissues which carry out the reactions of photosynthesis. We've mentioned the specialised cells found in a leaf in Unit 1, but here's the same diagram labelled with the tissues instead.

Epidermis Tissue: Epidermis tissue is made up of epidermis cells. A leaf has two layers of epidermis tissue: upper and lower. Epidermis tissues contain and protect the leaf and therefore the cells are long and thin and do not contain many chloroplasts.

Palisade Mesophyll Tissue: The palisade mesophyll tissue is where the majority of photosynthesis occurs in the leaf. It is the uppermost of the two mesophyll tissues in order to absorb the majority of the light energy as it hits the leaf. It is made up of palisade mesophyll cells which have a large number of chloroplasts, are packed tightly together and are tall and thin in order to absorb as much light energy as possible.

Spongy Mesophyll Tissue: Some photosynthesis occurs in the spongy mesophyll cells, so they do contain chloroplasts, but there is much less than in the palisade tissue above. The spongy mesophyll tissue provides a large surface area for the diffusion of the gases involved in photosynthesis into and out of the leaf. They therefore have a large amount of air spaces between the cells to allow this diffusion to occur.

You can view many more specialised cell types in lots of detail in this diagram. There's a video clip on cells, tissues and organs on the BBC website, and this is a great teacher made video explaining cells, tissues and organs in quite a bit of detail.