Cell Structure: Grade 9 Understanding for iGCSE Biology 2.2 2.3 2.4
All living organisms are made from cells. Indeed the cellular nature of life is one of the universal features shared by all life on earth. Some organisms are made from just one cell (unicellular organisms) while at some point around 1 billion years ago, cells starting clumping together and specialising to form multicellular organisms such as animals and plants.
What do all cells have in common?
All cells are surrounded by a cell membrane. The cell membrane is made from a mixture of proteins and a type of lipid called a phospholipid. The cell membrane serves many functions but perhaps the most significant is acting as a partially permeable barrier that can control which molecules can enter and leave the cell.
Inside the cell membrane there is a watery solution of chemicals called the cytoplasm. The cytoplasm is the site of many metabolic reactions in the cell because many enzymes are dissolved in the cytoplasm. The cytoplasm also contains many tiny nano machines for assembling proteins called ribosomes.
And that is about it for things all cells have in common. Prokaryote cells (bacteria) have a very different cell structure with no organelles but in this section you need to understand the simplified structure of two eukaryote cells: a typical animal (on the left below) and a typical plant cell (on the right).
Both animal and plant cells have a nucleus. This is the largest organelle and contains the DNA which is the genetic material. The DNA is found in long thread-like structures called chromosomes. The nucleus controls the division of the cell and also the various functions of the cell by regulating which proteins get made.
Animal and Plant cells both contain mitochondria which are the organelles associated with aerobic respiration. Mitochondria are recognisable in the cytoplasm of the cell as sausage-shaped organelles with a folded inner membrane (see diagram above).
Structures found only in Plant cells
1) All plant cells have a thick rigid cell wall made of the carbohydrate cellulose. The cell wall allows plant cells to become turgid since when the cell takes in water by osmosis, the rigid cell wall prevents the cell from bursting. The cell wall also acts as a transport pathway across plant tissues and can provide a barrier to some pathogens.
2) All plant cells have a large permanent central sap vacuole. This organelle is bounded by a membrane called the tonoplast and in many plant cells takes up the majority of the volume of the cell.
The sap vacuole provides a compartment in the cell into which excretory molecules can be moved to stop them poisoning the cytoplasm. It also plays a role in the water balance of plant cells since because of all the solute dissolved in it, the cell sap has a low water potential. This helps draw in water by osmosis from the cytoplasm and hence from outside the cell across the cell membrane.
3) Many but not all plant cells contain chloroplasts. These are organelles associated with the process of photosynthesis. Chloroplasts can be recognised in a light microscope image as small, green structures in the cell. The green pigment comes from the chlorophyll molecules that trap energy from sunlight. In an electron micrograph, chloroplasts are distinguished due to their stacks of membrane discs called grana.
Differences between plant and animal cells
PMG Biology 
Hi there, I’m currently just skimming through your post and they are extremely helpful. One thing I’m confused about is that I was doing a past paper which asked to draw a cell diagram, however in the mark scheme it didn’t show mitochondria and ribosome in the answers? Do you know any reason for this? I’ve just done my AS biology papers and I’m looking to improve my IGCSE biology grade just before I apply to university.
Thanks.
I don’t really know the answer to your question. Ribosomes were not required for the old EdExcel IGCSE specification but are in the 9-1 one that is being examined for the first time this summer. Without seeing the question, I can’t think why these two cell structures were not given credit…. Sorry I can’t be of more help.
Thanks for the prompt answer.
The question is in the 2012 January paper, question 7(e)
“(e) Potato tissue is made from plant cells.
Draw and label a plant cell in the space below”
Click to access January%202012%20QP%20-%20Paper%201B%20Edexcel%20Biology%20IGCSE.pdf
It’s quite odd as I’m looking at the old specification I did and parts of the book includes the ribosomes whilst some parts forget to label it.
So my main concern would be that if I labelled ribosomes and mitochondria, would I lose marks?
Sir why is there chloroplasts in cells is it something to do with the plant photosynthesysing?
Yes that’s right. Chloroplasts are the organelle in which photosynthesis occurs.
sir how many types of staines can we using to stain a nuclear of a cell? is it only methane blue and iodine solution.
There are hundreds of different cell stains that have been invented. We use iodine solution and methylene blue for simple cell preparations as they are cheap and non-toxic.
I was wondering if the cells we had were similar to perhaps the cells of other organisms. For example our muscle tissue is made up of the same cells that make up a Gorillas muscle tissue
Great question. Our muscle tissue would be almost identical in structure to all mammalian muscle tissue. So yes at a cellular level we are very similar to other organisms.
Have cells evolved over time
Yes – cells have been around for around 3 billion years and the cells we see now are very different to earlier specimens. For example, mitochondria and chloroplasts are descended from free living bacteria that about 2 billion years ago were engulfed by the cell membrane of a bacterial cell.
Are iodine and methylene blue the only stains we use to view structures under a microscope
You may use two different stains later in the course; one to look at the DNA in the nucleus of cells, one to stain bacteria. There are hundreds of different cell stains in existence.
If we can’t see mitochondria, how do we know it’s there?
Well you can see them clearly with an electron microscope. They are visible with a light microscope if they are stained correctly. Mitochondria are usually about 5 micrometers in length.
Sir,
Are humans the most complicated organisms in terms of structure? If not what is?
Difficult question to answer, Toby….. Human structure is very similar to most mammals so I guess we are no more complicated in structure than other primates. Our brain is more complicated than other animals and that is what has allowed us to develop the “higher functions” that distinguish us from other animals.
What is the difference between the outer and inner membrane?
They have slightly different functions in respiration but structure is similar. The inner membrane has lots of infoldings (called cristae) to increase the surface area and the last stage of aerobic respiration happens on this membrane. Don’t worry too much about the details here…..