Cell Structure and Function

• Cell theory:

  • All organisms are composed of one or more cells. ​
  • The cell is the basic unit of life in all living things.​
  • All cells are created by pre-existing cells.
• Living things can be made unicellular or multicellular
• Prokaryotic cells  are organisms whose cells lack a nucleus and other organelles. Two distinct groups: the bacteria and the archaea. Most prokaryotes are small, single-celled organisms that have a relatively simple structure.
• Eukaryotic cells are organisms that contain a nucleus and membrane-bound organelles. Examples of eukaryotic cells are plants, animals, protists, fungi. Eukaryotes may be either single-celled or multicellular.
• Prokaryotic cells are simpler and smaller ​

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  Both cells have membranes, cytoplasm and DNA​

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  The DNA is freely floating in the cytoplasm, or is found as a small circular molecule known as plasmid
• Eukaryotic cells are complex ​

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  Both cells have membranes, cytoplasm and DNA​

  Both cells have membranes, cytoplasm and DNA​

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  Membrane—bound organelles such as mitochondria or chloroplast ​

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  The DNA is bound to protein known as histones and together they form a complex known as chromatin 
• Plasma Membrane:

  Structure :

  • This is found around the outside of the cell. ​
  • It can also be called the cell surface membrane. ​
  • Consists of phospholipid bilayer together with proteins and other components.​

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  Function​:

  • The plasma membrane controls entry and exit of substances into and out of the cell.  
• Nucleus:

  Structure ​

  • Largest organelle – 10-20μm diameter​
  • Surrounded by the nuclear envelope – double membrane that has many gaps or pores ​
  • The membrane pores allow mRNA to pass out of the nucleus​

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  Function​

  • Controls cellular activity​
  • Houses the DNA - chromatin and associated proteins called histones​
  • The chromatin is coiled and super-coiled to form the chromosomes
• Mitochondria:

  The mitochondrion is an energy-generating organelle. The site of aerobic respiration and therefore the site of adenosine triphosphate ATP production.

  Mitochondrion is surrounded by two membranes. The inner layer folds inwards to form the cristae. The cristae project into a liquid called the matrix.

  The inner membrane is coated in enzymes, which catalyze the reactions of aerobic respiration to produce ATP.
• Aerobic respiration if the process where glucose is reacted with oxygen to produce carbon dioxidee and water. As this reaction is exothermic, the energy transferred from this reaction is used to produce ATP. Almost all processes in the cell that require energy obtain it from ATP.  
• Ribosomes:

  • The smallest of the organelles and the site of protein synthesis. ​
  • Ribosomes use the information coded in an mRNA molecule to assemble the correct order of amino acids in the protein. ​
  • Some ribosomes float freely in the cytoplasm, whereas others are attached to the rough endoplasmic membrane. 
• Endoplasmic reticulum:

  • Membrane bound flattened sacs/cavities/channels running through the cytoplasm​
  • Used to store and transport substances – proteins and lipids​
  • Many folds in the membrane give a large surface area
• Rough ER is often liked to the nuclear membrane

  Proteins that will be released from the cell or incorporated into the plasma membrane are made on these attached ribosomes and then folded and transported in the RER to the Golgi apparatus.
• Smooth ER (no ribosomes)

  Responsible for synthesising, storing and transporting lipids and some carbohydrates. 
• Golgi body:

  • Stack of flattened sacs known as cisternae (singular is cisterna)​
  • Each cisterna is surrounded single membrane, filled with fluid​
  • Receives vesicles with protein from the RER. Golgi modifies proteins that have transported from the RER, e.g. by adding carbohydrates to them​
  • Chemically processed – often carbohydrate added to form glycoproteins​
  • modified proteins then transported by Golgi vesicles that form when ends of the cisternae are pinched off​
  • vesicles can form vesicles, carrying their contents to the plasma membrane where they can be released to the outside of the cell.
• Golgi Apparatus:

  Golgi body forms from vesicles and produces vesicles​

  Proteins are modified before leaving cell​

  e.g. glyco - proteins
• Lysosome:

  • The cell’s recycling facility ​
  • Single membrane – 0.5μm​
  • Formed as buds that break away from the Golgi Apparatus​
  • No structures inside, only hydrolytic (digestive) enzymes​
  • Digest and recycle material from worn out organelles​
  • Digest bacteria, virus particles, cell fragments. They are taken into the cell by a process of phagocytosis​
  • It is important that these enzymes are kept separate from the rest of the cytoplasm because of damage they can do.
• Centrioles:

  Centrioles are structures made of tubular protein called tubulin. ​

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  They are involved in the formation of the spindle in mitosis as well as formation of cilia and flagella. ​

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  They are present in many types of plant cells. 
• Chloroplast :

  • Chloroplast are enclosed by an envelope (double membrane) and contain membranes called thylakoids arranged in stacks called grana (singular granum)
  • The chloroplast is the site of photosynthesis, the process whereby plants and algae use light to make complex organic molecules from carbon dioxide and water. ​

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  • The first stage of photosynthesis occurs in the thylakoid membranes which contain chlorophyll and other pigments  that absorb light energy as well as proteins involved in the production of ATP. ​
  • The second stage takes place in the stroma. 
• Cell wall:

  The plant cell wall consists mainly of the carbohydrate cellulose. ​

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  It provides strength and rigidity for protection and support. If plant cells take up too much water, the cell wall prevents them from bursting. If animal cells take up too much water they burst. 
• Vacuoles:

  • Permanent vacuoles only exist in plant cells. Animal cells can contain temporary vacuoles but they are not common features.
  • Cell vacuoles are fluid-filled sacs surrounded by a single membrane. Their size varies; in some cells the vacuole almost fills the cell. ​
  • The fluid is a dilute solution of molecules and ions. ​
  • The vacuole can be used to maintain the water balance of the cell, store mineral salts, amino acids, sugars and waste products. 
  • The vacuole is important in keeping the cell firm. When the vacuole is full of sap the cell is said to be turgid.