Cell Theory

 

• All living things are made up of one or more cells

• All cells arise from other cells

• Cells are the basic unit of life and have the ability to reproduce, acquire energy, grow and respond to their environment.

• Cells can only grow so big and then will either stay that size or divide into two. The reason they can only grow to a certain size is that as the cell gets larger the volume inside gets bigger more quickly than the surface area. As the volume inside the cell gets bigger more nutrients are required to stay alive, however, the surface area is now not large enough to allow enough nutrients to enter the cell, or allow the waste products to leave the cell fast enough. The cell would, therefore, die if it continued to get larger.

• All living things whatever their size start from a single living cell

 

Eukaryotic cells

 

Eukaryotic cells contain membrane-bound organelles, such as the nucleus, mitochondria, endoplasmic reticulum, ribosomes, and Golgi apparatus. Examples of eukaryotic cells include animal and plant cells and come in many sizes, shape and vary in specialisation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Image by Mediran (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons)

 

All living cells are surrounded by a fluid called tissue fluid. This is essential for the survival of the cell. The cell (plasma) membrane surrounds the cell and separates the fluid inside the cell (cytoplasm) from the tissue fluid. 

 

Cytoplasm is a thick translucent elastic fluid inside the cell that contains the organelles and a series of microtubules that form a cytoskeleton. The cytoskeleton provides support and shape to the cell. 

The cytoplasm consists of about 80% water and 20% proteins. It is in here that molecules such as enzymes break down waste and also aid in metabolic activity.

 

Cell Membrane

Structure

The cell membrane is made of a bilayer (double layer) of phospholipid molecules, embedded with protein molecules. The phospholipids that make up this double layer of fat consist of a hydrophilic (water-loving) head and a hydrophobic (water-hating) fatty acid tail.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Image by OpenStax [CC BY 4.0 (http://creativecommons.org/licenses/by/4.0)], via Wikimedia Commons)

 

Function

The cell or plasma membrane provides a boundary for the cell, separating the fluid inside the cell (cytoplasm) from the tissue fluid. It also gives the cell its shape.

 

The cell membrane is a semipermeable structure meaning it allows some substances to enter or leave the cell. The membrane needs to allow certain nutrients and molecules to enter the cell fast enough to keep it alive e.g. oxygen, glucose and remove waste products so as not to kill it e.g. carbon dioxide.

 

Substances move across the cell membrane by a process called diffusion. Diffusion is the movement of particles down a concentration gradient from an area of high concentration to an area of low concentration.

 

 

 

 

 

 

 

 

 

 

 

 

(Image by JrPol - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=4586487)

 

The rate of diffusion will be influenced by a number of factors including: the difference in concentration i.e. the greater the difference the faster the rate of diffusion, the size of the molecule i.e. the smaller the molecule the faster the rate of diffusion and the temperature i.e the higher the temperature the faster the rate of diffusion.

 

Osmosis is the term given to the diffusion of water molecules across the cell membrane. If too much water diffuses into the cell it will swell and burst. If not enough water diffuses into the cell it will shrink and collapse. Both will cause death to the cell.

 

 

Cell Membrane Proteins

 

There are many proteins embedded in the cell membrane. These proteins have various functions.

 

 

 

 

 

 

 

 

 

 

 

 

Protein Channels

Some proteins form a channel through the cell membrane. These channels allow for the passage of ions (electrically charged atoms) to pass in and out of the cell. These channels can open and close and are specific to the ion passing through e.g. potassium (K+), sodium (Na+), calcium (Ca+).

 

These ions, known as electrolytes in medical terms, determine the electrical properties of our body. Any problems with the flow of these ions can affect the electrical activity of the heart and nervous system.

 

Transporter (carrier) proteins

Some molecules such as glucose are too big to cross the membrane and need help. These larger molecules can attach to special proteins and these will transport them across the membrane. This is called facilitated diffusion.

 

Medically this is important in the control of blood sugar levels. Insulin is a hormone that activates the protein to carry glucose into the cell. In diabetes, there is a lack of insulin and therefore the glucose can't get into the cell causing the blood sugar level to rise.

 

Receptor sites

Some proteins on the outside of the cell membrane act as receptor sites for hormones to attach to. Hormones are chemical messengers that tell the cell what to do. These sites are specific to certain hormones and vary depending on the type of cell e.g. certain receptors on the cells of the female reproductive system only allow the hormone oestrogen to attach.

 

Medically drugs can also attach to receptor sites. Drugs are therefore developed to target certain cells. e.g. receptor sites on heart cells. This is how the drug generally knows where to go. However, sometimes the drugs attach to receptor sites of other cells not intended for and this is what causes side effects.

 

Recognition sites

Some proteins on the outer surface of the membrane allow the cells of the immune system (white blood cells) to recognise the body's own cells from foreign cells. These recognition sites are unique to the individual. White blood cells (WBC) roam around the body looking for foreign cells. If there are any cells in the body that don't have the correct recognition site e.g. bacteria, viruses, the WBC's will destroy them.

 

Medically this is why organ transplants can be rejected by the body and why it is important to get the closest match. All transplant patients will still, however, need to be put on medication to dampen down the immune system to stop it attacking the donated organ. However, this can result in problems of increased risk of infection because the immune system is weakened.

 

Some individuals own WBC's attack their own healthy cells. This is called an autoimmune disease e.g. diabetes type 1. There is usually a genetic factor which causes either problem with the WBC or lack of recognition sites on the cells.

 

Organelles

 

An organelle is one of the numerous structures with specific functions, suspended in the cytoplasm of a eukaryotic cell. The main organelles are:

 

• Nucleus

• Endoplasmic Reticulum

• Golgi bodies

• Lysosomes

• Mitochondria

 

Nucleus

The nucleus is a large structure in the centre of the cell. It is surrounded by a membrane (similar to the cell membrane) but in addition, it has many little pores that allow the transport of substances in and out of the nucleus.

 

Inside the nucleus is a jelly-like fluid (similar to cytoplasm), called nucleoplasm. It is made up of water, proteins, and DNA.

 

Inside the nucleus is also a smaller structure called the nucleolus (there can be more than one). The nucleolus contains RNA which is responsible for the manufacture of ribosomes. Ribosomes are responsible for the manufacture of proteins including enzymes and hormones.

 

The function of the nucleus is to store the cell's hereditary material (DNA) and coordinates the cell's activities. When not dividing “resting” the genetic material appears as a threadlike mass (chromatin) when dividing (reproducing) the chromatin forms chromosomes which hold our DNA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Image by Mariana Ruiz LadyofHats [Public domain], via Wikimedia Commons)

 

Endoplasmic Reticulum (ER)

The ER is a network of tubes that connect the membrane of the nucleus to the cell membrane. They act as a circulatory system inside the cell for the transport of molecules from one part of the cell to another.

 

The rough ER contains ribosomes and is responsible for the synthesis of proteins. The smooth ER does not contain ribosomes and is responsible for the synthesis of lipids (fats) and steroid hormones.

 

Many of the proteins and some of the lipids are transferred to the Golgi apparatus. Other proteins, that form digestive enzymes, are transferred to the lysosomes. Many of the lipids synthesised in the smooth ER diffuse through the cytoplasm and then through the cell membrane.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Image by Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=28223972)

 

Golgi Apparatus

The Golgi apparatus (complex) is usually located close to the ER. It consists of a stack of membranous sacs.

 

The function of the Golgi apparatus is to store, modify and package the substances made in the ER ready for transport to other areas in the cell or for transport outside the cell. This includes enzymes and hormones. This is where carbohydrates can be joined to the proteins to form glycoproteins. Glycoproteins are what form the recognition sites on the cell membrane.

 

Lysosomes are also formed in the Golgi apparatus.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Image by Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=28223976)

 

 

Lysosomes

Lysosomes contain digestive enzymes. Generally, they fuse (join) to food vacuoles (large storage sacs) where they release their enzymes to break down molecules. If lysosomes were to rupture inside the cell the enzymes released would destroy the living cell.

 

Lysosomes, therefore, play a role in the genetically programmed process of self-destruction. This is important in the embryonic development of the fingers and toes and the shrinking of the uterus during menopause.

 

If the cell is infected with a virus, lysosomes will rupture destroying the cell and the virus in an aim to preventing the spread of the virus.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(Image by lumoreno - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17380475)

 

Mitochondria

Mitochondria are organelles that are virtually cells within a cell and can divide using their own DNA. They are responsible for the release of energy and are found in large quantities in cells where there is a high energy demand e.g. sperm.

 

Cellular respiration involves a series of chemical reactions that occur in the mitochondria whereby glucose is broken down into carbon dioxide, water, and energy (Krebs cycle). Energy is stored until required in ATP (adenosine triphosphate). 

Recent research also suggests they play an important role in when a cell will die and are also thought to influence which eggs in a woman should be released during ovulation and which should be destroyed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Image by Mariana Ruiz Villarreal LadyofHats