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 opposite. Each type of cell is a different shape, a different size and has a different job to do. Plants too show this variation – it is ironic that the first cells I teach my students to visualise under the microscope – onion cells – are not typical plant cells as they lack chloroplasts.
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.
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.
Interesting Facts About CellsA 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
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.
A Cell is the building block of life.
A Tissue is a group of similar cells that carry out the same function.
An Organ is a group of tissues that work together to carry out a particular function.
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.
Cells SummaryThere are two types of cell – eukaryotes have a Nucleus, prokaryotes do not.
Cells are filled with smaller structures called organelles – each organelle has a specific job.
Animal and Plant cells are identified according to their organelles – chloroplasts, cellulose cell walls and vacuoles are unique to plants.
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.
Division of Labour is where different parts perform specialised functions, each contributing to the functioning of the whole.
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.
A group of similar cells that carry out the same function are called tissues.
A group of tissues that carry out the same function are called organs.
There are 7 life processes supported by various organs or organ systems. These can be remembered using MRS. GREN.
, What are the Differences Between Animal and Plant Cells? www.ozeldersin.com bitirme tezi,ödev,proje dönem ödevi