The Building Blocks of Life

In 1873, two scientists named Schleiden and Schwann were just putting the finishing touches to their Cell Theory. This theory stated that all organisms consist of one or more cells, and that the cell is the basic structural unit for all organisms. In discussions with each other, these ground-breaking scientists realised that although cells have lots of differences, they all have the same basic structure.

It is important to realise that most cells do not look like the general plant and animal cells that are illustrated in most science text books. There are over 200 types of cell in the human body and most look quite different from the general animal cell below. Each type of cell is a different shape, a different size, and a different job to do. Plants too show this variation. It is ironic that the first cells I teach my students to visualize under the microscope—onion cells—are not typical plant cells as they lack chloroplasts.

What Are the Differences between Animal and Plant Cells?

A typical Animal Cell

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Comparison of Plant and Animal Cells

Cell Organelle Functions

When you look at an animal or plant cell under a microscope, the most obvious feature you will see is the large, dark nucleus. If you have a really good microscope and an excellent preparation, you may be able to make out chloroplasts and vacuoles in plants. These are all examples of organelles. Most organelles are found in both plant and animal cells, and have the same functions in each type of cell. This is an example of division of labour, where each type of organelle has a specific role within the cell, all working together to ensure the survival of the cell.

Shared Organelles:

• Nucleus - the command centre of the cell. This houses nearly all the genetic material of the cell. Within the nucleus is a spherical structure called the nucleolus which makes RNA and ribosomes.
• Cell Membrane - the border guard of the cell - this envelope surrounds the cell and controls which substances can move in and out of the cell. Known as a fluid-mosaic membrane as it moves like a fluid and is studded with glycoproteins and glycoplipds giving it a mosaic-like appearance.
• Mitochondria - the power station of the cell. These sausage shaped organelles convert glucose to ATP during respiration. ATP is the energy currency of the cell, used in almost all energy-requiring cellular processes.
• Lysosomes - the cell's recycling plants. These spherical sacs contain powerful digestive enzymes. Their role is to break down materials.
• Ribosomes - the factories of the cell. These tiny organelles are made up of two subunits and are the site of protein synthesis.
• Endoplasmic Reticulum (ER) - made of a series of flattened, membrane-bound sacs called cisternae. ER comes in two forms: Rough ER transports proteins that were made on the attached ribosomes; Smooth ER is involved in lipid (fat) production.
• Golgi Apparatus/Body - the post office of the cell. These flattened sacs look a little like a pile of pitta bread; the Golgi receives proteins from the ER, modifies and packages them for transportation. From here the packaged proteins are either sent to other parts of the cell, or sent to the cell membrane for secretion (think international exports).

Plant-only Organelles:

• Chloroplasts - the solar panels for a plant. These double-membraned organelles are the site of photosynthesis in plant cells. Light energy is used to convert carbon dioxide (from the air) and water into carbohydrate molecules such as glucose.
• Cell Wall - the defensive walls of the plant. They are made of cellulose and provide structural rigidity for the plant.
• Vacuole - acts like the bladder inside a football. The large vacuole is filled with water and various solutes. The pressure of the fluid inside this organelle ensures the cell wall is held rigid (turgid). Ifa plant isn't watered for a few days, water is lost from the vacuole and each cell becomes less turgid and more flaccid. This is why a plant wilts.
• Amyloplasts - the plant's food surplus warehouse. Extra glucose is converted into insoluble starch grains and stored in amyloplasts for use when times are harder.
• Plasmodesmata - secret tunnels in the cell wall that allow neighbouring plant cells to communicate.

Animal-only Organelles:

• Centrioles -The mechanical winch of the animal cell, these small tubes of protein fibres are involved in moving chromosomes during cell division.

The Cell Song

Differentiation and Stem Cells

There are over 200 cell types in your body, each with it's own characteristic size, shape and function. The cells are adapted to do their job - a process that happens before you are born. This is a one-way process; a skin cell cannot spontaneously change into a red blood cell or a neuron.

There is a group of cells, however, that can change into many different types of cell - stem cells. Stem cells are cells that haven't become adapted to do a particular job. They come in several types:

• Multipotent: can change into several cell types. Haematopoietic cells are blood cells that can develop into many different types of blood cell, but cannot turn into any other cell type
• Pluripotent: these stem cells can give rise to any adult cell type. Scientists can force 'normal' cells to become pluripotent (induced pluripotent stem cells or iPS) through gene manipulation
• Totipotent: these stem cells can give rise to all the adult cells and extra-embryonic tissues such as the placenta. In humans, cells are only totipotent up to day 4 (around 16-cell stage) of foetal development.

Below are a series of micrographs showing how different cells are specialised to do their job.

Adaptation in Plants and Animals

Nerve cells in the Hippocampus region of the brain

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Functions and Structures of Animal Cells

What Are Tissues and Organs?

A cell may be the basic structural unit of life, but there are higher orders of structure in multicellular organisms.

1. A Cell is the building block of life.
2. A Tissue is a group of similar cells that carry out the same function.
3. An Organ is a group of tissues that work together to carry out a particular function.
4. An Organ System is a collection of organs that work together for a particular function.

Using these definitions we can see that skin is an organ - being made up of a number of different tissues including nervous tissue, muscle tissue, skin tissue, vascular tissue (blood vessels) and fat tissue. It also becomes clear that a blood transfusion is actually a tissue transplant, as blood contains several different cells all working together for a common purpose:

• Red blood cells
• White Blood cells
• Platelets
• Plasma

A good model for how the body works is a school. The teachers, cleaners, lab technicians, office staff, management and teaching assistants do different jobs. They all work together to run the school - if one group stopped working, the school would not function.

Interesting Facts About Cells

• A single nerve cell in a giraffe can be over two metres long
• Conditions like cancer are caused when cells divide too often.
• The biggest cell in the world is the Ostrich Egg
• There are more bacterial cells in the human body than human cells

Characteristics of Living Things

Cells Quiz

For each question, choose the best answer. The answer key is below.

1. Which organelle is the largest?
   • Nucleus
   • Mitochondria
   • Skin
   • Golgi Apparatus
2. Which organelle is unique to animals?
   • Nucleus
   • Chloroplast
   • Centriole
   • Plasmodesmata
3. How many types of stem cells are there?
   • 1
   • 2
   • 3
   • 4
4. How many cell types are there in the human body?
   • 220
   • 200
   • 180
   • 170
5. What is a tissue?
   • Collection of cells working together
   • Collection of organelles working together
   • Control centre of a cell

Answer Key

1. Nucleus
2. Centriole
3. 3
4. 200
5. Collection of cells working together

Cells Summary

1. There are two types of cell - eukaryotes have a Nucleus, prokaryotes do not.
2. Cells are filled with smaller structures called organelles - each organelle has a specific job.
3. Animal and Plant cells are identified according to their organelles - chloroplasts, cellulose cell walls and vacuoles are unique to plants.
4. There are around 200 different types of cell in the body, each with a different job. The different cell types have adaptations to help them do their job.
5. Division of Labour is where different parts perform specialised functions, each contributing to the functioning of the whole.
6. Stem cells are a group of cell that have not fully differentiated and so can develop into two or more cell types. There are three broad categories of stem cells.
7. A group of similar cells that carry out the same function are called tissues.
8. A group of tissues that carry out the same function are called organs.
9. There are 7 life processes supported by various organs or organ systems. These can be remembered using MRS. GREN.

Comments

bernard on November 18, 2019:

its very much enriching, and clear for understanding of the basic concepts...

Omoniyi olajumoke on July 23, 2019:

Thank you so much

Random Person you don't know on September 14, 2018:

Hello! i thought this was very useful for my science! i want to let you know follow your dreams and passion, whatever it might be!

Wow it is very useful on September 08, 2018:

Good

Phan'Davy on April 04, 2018:

Very helpful...

good site

Student on December 04, 2017:

Mitochondria is the powerhouse of the cell

nutbuster6000 on December 04, 2017:

knowledge is power

mms on November 06, 2017:

i googled a question and this website came up but it didnt give me answers to the question i asked so not helpfulll

mjd on November 06, 2017:

sorry don't like the song but good try... it wasn't helpful because it's still too complex and repetitive.

probably best not to try again!

maddison on November 06, 2017:

i dont like the song at all!!!!!!!!!!!!!!!!!!!

Eshal Z on October 25, 2017:

I don't really like Science but, somewhat I like it now.

gia on October 25, 2017:

i am not really interested in science but still it's a bit fun

Grace 3.0 on September 29, 2017:

I liked it for my science class

Itsme on September 28, 2017:

I was made to read and listen to this for science

Oooh on September 28, 2017:

I’m forced to read this for science :,)

Grace on April 04, 2015:

Grace yarzue

feri on October 29, 2014:

great explanation but i can't clear my doubt my doubt is what is the difference between plant and animal cell?option are (a).growth (b)respiration (c)nutrition (d)locomotion the answer is nutrition can anyone clear this doubt.

Dr. John Anderson from Australia on Planet Water on January 09, 2014:

Very informative. Great hub.

citlaly gabriel on September 16, 2013:

6th grade islearning about. Plant cell and animal cell it is super interesting :-*

citlaly :-) :- on September 16, 2013:

This thing is cool

James! on October 30, 2012:

I would like to ask where are more cell types, in plants or in animals? And where are more genes?

rrr on October 16, 2012:

sweet

Tina on October 04, 2012:

Love your song!!!!!

Melanie from Midwest, USA on June 02, 2012:

Perhaps.

Rhys Baker (author) from Peterborough, UK on June 02, 2012:

I can only find reference to (on PubMed) vacuolation during apoptosis or cellular injury, not during normal cellular function. This suggests that vacuoles are mainly created during abberations in cellular function?

This is amazing if animal cells also create vacuoles. I'm still rather skeptical as it seems these are identical to peroxisomes in structure and function as they lack a tonoplast. Also small circular organelles used for temporary storage sound more like vesicles than vacuoles. Perhaps this is merely a semantic point.

If not, then the UK education system is lying to its students. Every revision site and every syllabus (including the National Curricula) state that animal cells lack vacuoles. Perhaps it is due to the recent nature of this finding?

Melanie from Midwest, USA on June 01, 2012:

I couldn't find a link that didn't require a paid login (I have a handle, so I could take a screenshot for you), but this article, "Autophagy promotes survival of retinal ganglion cells after optic nerve axotomy in mice" make mention of vacuoles in animal cells.

http://micro.magnet.fsu.edu/cells/plants/vacuole.h...

This page talks about vacuoles in plant cells but states that they are present in animal cells as well.

Unfortunately, I can't refer you to many scientific papers. The database I use requires a paid subscription.

If you do a quick search on Jstor, I'm sure something will pop up. :)

Rhys Baker (author) from Peterborough, UK on June 01, 2012:

The reason I ask is that this must be a recent development as it took no part in my Cell Biology degree or in any of my teaching syllabi. If I have been wrong for the last two years I would like to correct a few of my presentations.

I have only been able to find reference to this in non-academic websites

I am aware that lysosomes and peroxisomes (small circular organelles that appear similar in structure to small vacuoles and have similar uses) exist in both plants and animals. Obviously membrane packaging by the Golgi for exocytosis throws up similar structures under light microscopy.

Rhys Baker (author) from Peterborough, UK on June 01, 2012:

That's fascinating - have you a paper that states this?

Melanie from Midwest, USA on June 01, 2012:

Animal cells actually do contain vacuoles (not all animal cells do, though), they just have a minimal role in comparison to vacuoles in plant or fungi cells.

Rhys Baker (author) from Peterborough, UK on June 01, 2012:

@Marcy: Thanks for the pinterest share - I love it when people think my writing is good enough not only to read themselves but to be passed on to others. It is the highest form of praise. I'm glad you found it useful and interesting

Rhys Baker (author) from Peterborough, UK on June 01, 2012:

@Sun Pen: Long time no see! I'm glad you enjoyed my hub. This topic can get complicated and convoluted very quickly; I'm glad I explained it simply enough without patronising people.

Marcy Goodfleisch from Planet Earth on May 24, 2012:

I keep coming back to the neat information here! Shared on Pinterest; I know so many people with kids who would love this one!

Sun Pen 50 from Srilanka on May 21, 2012:

Lot of useful and interesting information. Explained well and in a simple way. Thanks.

Rhys Baker (author) from Peterborough, UK on May 20, 2012:

@Marcy: I was toying with the idea of putting in a video of me drawing the two types of cells and talking the viewer through each one. My camera wasn't up to the task however!

I'm glad you found this interesting

Rhys Baker (author) from Peterborough, UK on May 20, 2012:

I'm glad you found it fun and that you liked the illustrations - most of the work on this hub was searching for appropriate open use diagrams

Rhys Baker (author) from Peterborough, UK on May 20, 2012:

@Michael: O dear...if your head is spinning I shudder to think what I am doing to my A-level students! I'm glad you learnt something. I did try to pitch this slightly below degree level. This hub covers basic cell topics from Yr 7(11yrs old) up to AS level (17years old) in my biology lessons.

Rhys Baker (author) from Peterborough, UK on May 20, 2012:

@Mmargie: Don't fret about the quiz :) I'm so pleased that my hub has taught you 80% of the content - there is some reasonably complicated content here. I always try to use songs in my lessons where possible. They stick in your head where lectures and practicals may not. Thanks for your comment!

Rhys Baker (author) from Peterborough, UK on May 20, 2012:

@JKenny: Thanks for the comment. I do try to get the information across in as basic and clear a way without patronising or skipping over important details. From your comment it seems that I have achieved this

James Kenny from Birmingham, England on May 19, 2012:

Wow! What a wonderfully written and presented. You display the information in such a clear way. It takes me back to the textbooks I had in school. Great job. Voted up etc.

Mmargie1966 from Gainesville, GA on May 19, 2012:

GREAT HUB! I did miss one on the quiz, but I knew NONE of this (really remember none) before I read it. I really appreciated the tables and loved the song.

Michael J Rapp from United States on May 19, 2012:

Wow! This takes me back to Anatomy and Physiology class in college. I always learn a thing or two from your Hubs, even if they leave my head spinning just a bit. Very thorough and well done!

Rebecca Mealey from Northeastern Georgia, USA on May 19, 2012:

Great little biology lesson. You made it fun and informative.Nicely illustrated.

Marcy Goodfleisch from Planet Earth on May 19, 2012:

What an informative hub! My kids would have loved having this resource when they were in school. Thanks for the terrific details, the tables and graphics here - great job!

Voted up and up!

whonunuwho from United States on May 19, 2012:

A great hub and extremely interesting. I find it satisfying to realize that plant and animal organisms depend upon each other in order for all to sustain life. That is one misunderstanding that many big business types fail to comprehend and can be witnessed in the extreme waste and poor conservation habits of many. Only a few profit from the wasteful tactics that generate their money in lumber, fishing and waste disposal of big factories world wide. The biological sanctuary in which we are all a part, is now suffering greatly and is in need of many adjustments. The planet will make these adjustments on its own, if we do not step up to the plate ourselves.