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Biology - Cells - The Fundamental Units of life

Every living thing has cells. Cells are sacs of fluid surrounded by membranes, inside the fluid float chemicals and organelles, which are structures inside the cell, which are used during metabolic processes. Each cell is capable of converting fuel to useable energy, this means that cells not only make up living things, they are living things. Cells are found in all plants, animals and bacteria. Many of the basic structures found inside all types of cells, as well as the way those structures work, are very similar, so the cell is said to be the fundamental unit of life.

The most important characteristic of a cell is that it can reproduce by dividing. If cells did not reproduce living things would not continue to live. Cell division is the process by which cells duplicate and replace themselves.

Viruses are similar to bacteria, but they are not truly living organisms because they lack one crucial characteristic; they cannot grow and divide by themselves. In this respect they are more parasites in that they need to take over the cells of a host to reproduce.

Increasingly more complex organisms are made up of increasingly more complex groups of cells, and the organisms survive based on products that the cells make. As scientists develop new ways of studying cells and groups of cells in more depth, they will certainly discover that there is more to be learned.

Examining eukaryotes

Cells fall into two major categories; eukaryotes and prokaryotes. Prokaryotes are cellular organisms that do not have a ‘true’ nucleus. A nucleus is the control centre of a cell, this nucleus contains the genetic information, which controls the way we develop, and is associated with other organelles that function in the production of amino acids and proteins based on what the genetic information dictates. Prokaryotes have some genetic material, but it is not as well organized as it is in eukaryotes, but still prokaryotes are able to reproduce. Eukaryotes are organisms that contain chromosomes, eukaryotes have the following characteristics:

They have a nucleus that stores their genetic information

Animal cells have an organelle called a mitochondria that effectively combines oxygen and food to convert energy to a useable form

Plant cells have chloroplasts, which use energy from sunlight to create food for the plant

Eukaryotic cells have internal membranes which create compartments inside the cells that have different functions

Plant cells have a cell membrane and a cell wall, which is rigid; animal cells only have a cell membrane, which is soft

The cytoskeleton, which reinforces the cytoplasm of the cell, controls cellular movements.

Cells and Organelles

You have organs that are made up of cells, the organ systems perform certain functions within you, cells have organelles that perform certain functions in the cell. Although it takes many millions of cells to create a human, each cell functions on its own. An organelle in one cell does not do the work for another cell, this means that each cell metabolizes individually.

The Plasma Membrane

The fluid inside a cell (intracellular fluid) is called plasma or cytoplasm (cyto means cell). The membrane holding the fluid in the cell is known as the plasma membrane or sometimes called the cell membrane. The cells themselves are floating in a fluid called a matrix; this matrix is insoluble, which means substances do not dissolve in its fluid. The matrix is just simply supporting the cells. The fluid that squeezes between each cell is called the extracellular fluid. The job of the plasma membrane is to separate the chemical reactions occurring inside the cell, from the chemicals that are floating in the extracellular fluid.

If the plasma membrane did not separate the intracellular fluid from the extracellular fluid, waste products that are excreted from the processes inside the cell, to the outside fluid could flow back inside, causing damage to the cell.

The Fluid Mosaic Model

The plasma membrane has a bilayer of phospholipids (fats with phosphorous attached) which at body temperature are liquid. Each phospholipid has a head that is attached to water (hydrophilic – water loving) and a tail that repels water (hydrophobic – water fearing). Both layers of the plasma membrane have the hydrophilic heads pointing towards the outside, and the hydrophobic tails from the inside of the bilayer. Because cells reside in a watery solution, and they contain a watery solution inside them, the plasma membrane forms a circle around each cell so that the hydrophilic heads are in contact with the fluid, and the hydrophobic tails are protected on the inside.

Proteins and substances, such as cholesterol, become embedded in the bilayer, giving the membrane a mosaic look. Because the plasma membrane has the consistency of vegetable oil at body temperature, the proteins and other substances are able to move across it. That’s why the plasma membrane is described using the fluid mosaic model.

The molecules that are embedded in the membrane also serve a purpose. For example, the cholesterol that is stuck in there makes the membrane more stable and prevents it from solidifying when your body temperature is low. Carbohydrate chains attach to the outer surface of the plasma membrane on each cell. These carbohydrates are specific to every person, and they supply characteristics, such as your blood types.

Transport through the Plasma membrane

Some substances need to move from the extracellular fluid to inside the cell; some substances need to move from inside the cell to the extracellular fluid. The exchanges take place in the plasma membrane.

Some of the proteins that are stuck in the plasma membrane help to form channels in the membrane, through which, some substances, such as hormones or ions, are allowed to pass through. They are recognised by a protein molecule (a receptor) within the cell membrane, or they attach to a carrier molecules, which is allowed through the channels. Because only certain substances can pass through the membrane, it is said to be selectively permeable.

Permeability describes the ease with which substances can pass through a border such as a cell membrane. Permeable means that most substances can easily pass through the membrane. Impermeable means that substances cannot pass through the membrane. Selectively or semi permeable means that only certain substances are able to pass through the membrane.

Transporting substances across the plasma membrane can require the cell to use some of its energy to help move the substance across the border. If energy is used, the transport is called active. If molecules can pass through the plasma membrane without using the cells energy supply, the molecules are using passive transport.

Active Transport

Sometimes the molecules are too big to pass through the plasma membrane, or dissolve in the fluid so that they can be filtered through the membrane. In cases like this, cells have to use active transport to help get molecules in or out of the cell.

Passive Transport

A membrane can allow molecules to be passively transported through it in three ways:

Diffusion ~ sometimes organisms need to move from an area where they are highly concentrated to one area where they are less concentrated. This form of transport is much easier than moving molecules from a low to a high concentration. To go from a high to a low concentration, the molecules need only diffuse across the membrane, separating the areas of concentration.

Osmosis ~ this term is used when talking about water molecules diffusing across a membrane. Osmosis is basically the diffusion of water and works in a similar way to diffusion. However, with osmosis, the concentration of substances in the water is taken into consideration. If a solution is isotonic, that means the concentrations of the solutes and solvent are equal on both sides of the membrane. If one solution is hypotonic, there is a lower concentration of solutes (and more solvents) in it when compared to another solution. If a solution is hypertonic, there is a higher concentration of substances in it (and less solvent) when compared to another solution.

Filtration ~ the last form of passive transport is used most often in the capillaries, which are so thin that diffusion easily takes place through them. The pressure at which blood flows through the capillaries is enough force to push water and small solutes that have dissolved in the water right through the capillary membrane. So basically, the capillary membrane acts as filter paper, allowing fluid to surround the body’s cells keeping large molecules from getting into the tissue fluid.

The Nucleus

Every cell of every living thing has a nucleus, and every nucleus contains genetic material. The genetic material directs the production of proteins that make the entire organism function; the nucleus makes the cell function.

In the nucleus of cells that are not currently dividing, clumps of thread-like genetic material called chromatin appear. However, just before a cell divides the chromatin bunches up into chromosomes, which contain DNA (deoxyribonucleic acid).

The DNA has two strands, each of which has sequences of nitrogenous bases that form the genetic code. The code, which is derived from the nucleotide bases in the genes on strands of DNA, is interpreted by a ribonucleic acid (RNA) molecule called messenger RNA (mRNA). The mRNA uses the information from the genetic code to create amino acids in the cell. The amino acids are then taken by transfer DNA (tRNA) to an organelle called a ribosome, where the final proteins are made.

Every cell of every eukaryote has a nucleus which contains genetic material. In cells that are not currently dividing chromatin can be seen. Chromatin refers to all of the DNA in the cell and its accompanying proteins. Chromatin cannot easily be seen prior to cell division, at which point the chromatin bunches up into chromosomes.

The DNA of eukaryotes is double stranded. Each strand of DNA has sequences of nitrogenous bases that form the genetic code. The code is then interpreted, and then a RNA molecule called mRNA is produced from the DNA template. The mRNA uses information from the genetic code for certain amino acids in the cell, which are then taken by tRNA to a ribosome where the final proteins are made. This code is read and converted to a messenger that is carried to the cytoplasm, where it is translated to produce a protein.

Proteins either contribute to the structure of the cell, or they contribute to the function of the cell, meaning they are used as enzymes in metabolic processes. Either way, it is the genetic material inside the nucleus that ultimately controls the structure and function of each and every cell in all eukaryote organisms.

Each nucleus has a round mass inside it called a nucleolus, which produces the third type of RNA molecule – ribosomal RNA (rRNA). rRNA helps to make ribosomes, which get transferred from the nucleus to the cytoplasm to help in making proteins.

Surrounding each nucleus is a double layer formed from proteins and lipids that separates the nucleus from the cytoplasm. This two-layered structure is called the nuclear envelop or nuclear membrane.

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) looks like folded-up sheets resembling a piece of coral. The ER is a series of canals that connects the nucleus to the cytoplasm of the cell. The part of the ER that is dotted with ribosomes is called rough endoplasmic reticulum; the part of the ER that has no ribosomes is called smooth endoplasmic reticulum. Ribosomes on the rough ER serve as the place for synthesis of proteins that are directed by the genes to be put together in the ER.

The smooth ER contains transport vesicles that shuttle cellular products from cytoplasm to organelle, from organelle to organelle, or from organelle to plasma membrane. In addition to protein synthesis, the ER is involved in the metabolism of lipids.

The main function of ER is to make and transport proteins. The ER is essentially the ‘womb’ for new protein chains. Protein synthesis, or production, begins in the nucleus, with the mRNA molecule carrying the genetic information as to what amino acids should be produced. The tRNA molecules carrying the genetic information as to what amino acids should be produced. The tRNA molecules bring the amino acids from the cytoplasm to the ribosomes, which are produced by rRNA. At the ribosomes, the amino acids are joined together to form a protein, and the protein is stored in the ER until it can be moved to the Golgi apparatus.

The Golgi Apparatus

The Italian scientist, Camillo Golgi, discovered the Golgi apparatus. The Golgi apparatus is very close to the ER, it looks like a maze with water droplets splashing off of it. The ‘water droplets’ are transport vesicles bringing material from the ER to the Golgi apparatus.

Inside the Golgi apparatus, products produced by the cell, such as hormones or enzymes, are packed for export to other organelles or to the outside of the cell. The Golgi apparatus surrounds the product to be secreted with a sac called a vesicle. The vesicle finds its way to the plasma membrane, where certain proteins allow a channel to be produced so that the products inside the vesicle can be secreted to the outside of the cells. Once outside, the products can enter the bloodstream and be transported through the body to where they are needed.

Lysosomes

Lysosomes are special vesicles formed by the Golgi apparatus to ‘clean up’ the cell. They contain digestive enzymes, which are used to break down products that may be harmful to the cell and ‘spit’ them back out into the extracellular fluid.

Lysosomes also remove dead organelles by surrounding the dead organelle, breaking down the proteins of the dead organelle, and releasing them to reconstruct a new organelle. Because the Lysosomes acts upon its own cell the process is known as autodigestion.

Peroxisomes

Peroxisomes are little sacs of enzymes produced by the smooth ER, to help protect the cell from toxic products. The peroxisomes break down hydrogen peroxide, because too much could kill you. Hydrogen peroxide is normally produced in some metabolic reactions, so small amounts are inside you. Hydrogen peroxide becomes harmful to cells if too much accumulates, so the cells peroxisomes are continually breaking down this substance.

The chemical formula for hydrogen peroxide is H 2O 2, very similar to the chemical formula of water (H 2O). Peroxisomes turn hydrogen peroxide into water and an extra oxygen molecule, both of which are always needed in the body, and can be used by any cell.

The Mitochondria

The ER supplies the products, the Golgi apparatus distributes the products, and the mitochondria supply the energy for all processes to take place.

The mitochondria convert fuel to useable energy. When food is digested into its smallest molecules and nutrients, and air is taken in (inspired), the smallest molecules and nutrients cross into the bloodstream. These molecules and nutrients include glucose and oxygen.

If there is more fuel digested than necessary for your body to function, the excess fuel gets stored for when it is needed, as fat.

Inside an organism the amount of energy a cell uses is measured in molecules of adenosine triphosphate (ATP). It is the mitochondria that produce the ATP and to produce it, mitochondria use products of glucose metabolism as fuel.

Mitochondria act like furnaces when they convert glucose into ATP; they use oxygen and give off carbon dioxide, and water. Because the process uses oxygen, it is said to be aerobic. This chemical process of respiration occurs in every cell, so it is called aerobic cellular respiration.

Aerobic cellular respiration can be diagrammed like this, with each step breaking down the products in the step preceding it:

Food + Air

Carbohydrates + oxygen and nitrogen

Glucose + oxygen (final products of digestion and inhalation)

ATP + carbon dioxide and water

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