1. Cell Structure

What you need to know...

• • Cell ultrastructure and functions to include:

    • cell walls

    • mitochondria

    • chloroplasts

    • cell membrane

    • vacuole

    • nucleus

    • ribosomes

    • plasmids

  • using examples from typical plant, animal, fungi and bacteria cells.

Source: SQA

Notes

All living things are made of cells. The cell is the basic unit of life. But, what is a cell made of? Lots of things. Every cell consists of an intricate system of different structures which all work together to allow the cell to function. You will already know some of these structures and what they do, but in this topic we're going to take this much further.

We'll use the 2D cut-through cell diagrams you're used to to help explain where these structures are, what they look like and what they do. However, don't forget that cells exist in 3D and not only that, their structures move!

Animal Cells

Animal cells have many different structures depending on their function. However, first we'll consider what the typical structures of most animal cells are. You already know that animal cells consist of a cell membrane, nucleus and a fluid cytoplasm. In this course you need to learn more about the functions of the cell membrane and the nucleus. You also need to learn about two other organelles which are found in the cytoplasm of animal cells.

A typical animal cell has the common structures shown in the diagram above. These include...

Cytoplasm: The cytoplasm is the liquid part of the cell. It consists mainly of water and has many different substances dissolved in it. Many of the cell's chemical reactions occur in the cytoplasm.

Cell membrane: The cell membrane contains the contents of the cell and provides a barrier to control what enters and leaves the cell. The cell membrane is often described as "selectively permeable" as it allows some but not all substances across (permeable) and can choose which substances can pass across (selective). We'll learn more about this in the transport topic.

Nucleus: The nucleus controls everything which takes place in the cell. It does this as it is the site of the cell's DNA. DNA contains the genetic code which is translated into proteins. All of the chemical reactions which take place in cells are controlled by these proteins. You'll learn more about all of this in the DNA and Enzymes topics.

Mitochondria: Mitochondria are the power houses of animal, plant and fungal cells. They are found in the cytoplasm and the majority of the respiration chemical reactions take place in the mitochondria, which releases chemical energy from food molecules. Obviously, we'll discuss this in more detail in the Respiration topic.

Ribosomes: Ribosomes are tiny structures which are also found in the cytoplasm. Ribosomes are the sites of protein production in cells. We'll discuss this in more detail in the DNA & Protein Production topic.

Although the diagram above shows the typical structures of an animal cell, very few animal cells would actually look anything like this. Animal cells are specialised for their functions. Look at the following diagrams of different animal cells...why do they have different structures?

Red Blood Cell: Biconcave shape provides a large surface area to absorb oxygen. Also mature cells have no nucleus to increase the volume of the oxygen-binding haemoglobin protein molecules.

Nerve Cell: Long, thin shape to transmit nerve impulses. High concentration of mitochondria to provide energy for nerve impulse transmission.

Small Intestine Epithelial Cell: Large surface area of membrane lining the gut to absorb the products of digestion. High concentration of mitochondria to provide the energy required for active transport.

Check out this BBC video clip on animal cell types and structure, and have a look on sciencephoto.com for more images of animal cells.

Plant Cells

As you know, plant cells have many of the same structures as animal cells. However, they have other structures for you to learn about as you can see from the diagram below. Plant cells have a cytoplasm, cell membrane and nucleus which all perform the same functions as animal cells. Many people think that plant cells do not contain mitochondria, but of course they do! Mitochondria are needed to release energy from sugar, plant cells need this energy to function just as animal cells. The following diagram shows the structures of a typical plant cell.

You already know what the functions of the structures which are also found in animal cells, but what are the functions of the structures which are found only in plant cells?

Cell Wall: Plant cell membranes are surrounded by a wall which is made of cellulose fibres. Plant cell walls provide structure to the cell, and to the plant. The cell wall allows the cell to fill with water without bursting. Plant cell walls are fully permeable.

Chloroplasts: As well as mitochondria, plant cells also contain chloroplasts. The chloroplast is the site of Photosynthesis in the cell. So, this is where energy from light is used to produce sugar from carbon dioxide and water. We'll discuss this in more detail in the Photosynthesis topic.

Vacuole: Plant cells have a large central vacuole which fills with fluid, or sap, which helps provide structure to the cell and the plant.

As with animal cells, the diagram of the plant cell above is a generalised diagram to show the structures. Plant cells can be varied also depending on their function. The diagram below shows the variety of cells in a leaf. What differences can you see in the cells? How do these relate to their function?

Upper & Lower Epidermis Cells: Layers of epidermis cells are found at the top and the bottom of the leaf. These contain and protect the leaf and therefore contain relatively few chloroplasts.

Palisade Mesophyll Cells: The palisade mesophyll cells are found in the top half of the leaf. Obviously, sunlight will primarily be hitting the upper surface of the leaf. The palisade cells are therefore packed with chloroplasts and are long, thin and tightly packed to absorb as much of the light energy as possible for photosynthesis.

Spongy Mesophyll Cells: The spongy mesophyll is in the lower half of the leaf. There will be less light here, so the cells are less tightly packed. Carbon dioxide enters through the lower surface of the leaf in daylight and is crucial for photosynthesis. The arrangement of the cells in the spongy mesophyll provides a large surface area to absorb the carbon dioxide and allow the excess oxygen to diffuse out.

Guard Cells: The lower surface of the leaf has little holes in it called stomata to allow gases to exchange. Each stoma is surrounded by two guard cells. Most plants close their stomata at night when they don't need carbon dioxide, as there is no light for photosynthesis, to prevent water loss. The guard cells have adaptations to open and close the stomata.

If you're finding plant cells interesting, you could find out much more from Crash Course on YouTube.

Fungal Cells

Fungal cells are similar to plant and animal cells in that they have a nucleus, cell membrane, cytoplasm and mitochondria. Like plant cells, fungal cells have a cell wall but they aren't made of cellulose, they're made of chitin instead.

Bacterial Cells

Bacteria cells are very different from animal, plant or fungal cells. They don't have organelles such as nuclei, mitochondria or chloroplasts. Although they do have ribosomes and a cell wall, these are both different in structure to the ribosomes and cell walls in the cells above. Bacteria cells do have a cytoplasm and cell membrane though. One of the key structures of a bacteria cell you need to know about is the plasmid.

Plasmids: Plasmids are small circular sections of DNA which bacteria cells have in their cytoplasm in addition to their large circular chromosome. Plasmids can be replicated rapidly and can transfer between bacterial cells easily. You'll learn more about how we make use of these plasmids in the genetic engineering topic.

You can begin to learn a little more about bacterial plasmids in this YouTube video.