Biology Chapter 2

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Cell surface membrane

The cell membrane on the outside of the cell controls the movement of materials into and out of the cells. It is composed of phospholipid molecules with embedded proteins

Phospholipids are polar molecules with water-loving (hydrophilic) heads and water-repelling (hydrophobic) tails. This feature causes the molecules to orientate themselves in a double layer or bilayer. The heads point outwards and the tails are on the inside. Water soluble molecules and ions are unable to pass through the phospholipid layer. These molecules have to move through protein channels

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Cellulose

The main substance in a plant cell wall is cellulose. Cellulose is a polysaccharide, and a polymer made of beta glucose molecules joined together by glyosidic bonds. They are long and straight and they form fibres that are very strong. This is what gives plant cell walls their strength and resistance to being stretched.

Parallel rows of beta glucose are held together by hydrogen bonds. Small bundles of cellulose molecules bond to make thin fibres called microfibrils. These form bigger and stronger fibres, which are criss-crossed. This is what makes the walls resistant to stretching

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Centrioles

The centrosome is made up of two structures called centrioles. These centrioles lie at right angles to each other. Each centriole consists of 9 triples of microtubules

Centrioles separate and move to opposite sides of the nucleus before nuclear division. This only occurs in animal cells because there are no centrioles in a plant cell, and in the animal cells the centrioles help organise the microtubules to form the spindle

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Endoplasmic reticulum

This consists of a network of folded single membranes forming interconnecting sheets, tubes or sacs. It originates on the outer membrane of the nucleus. It may remain attached to this. The cytoplasm of metabolically active cells is packed with endoplasmic reticulum

There are two distinct types of ER:

  • Rough ER has ribosomes attached to its outer surface. Vesicles form at the margins of the ER and becomes pinched off. This is the site of protein synthesis and packaging
  • Smooth ER has no ribosomes, and it is the site of synthesis of substances like lipids which are needed in the cell. In muscles, a form of smooth ER stores calcium ions
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Eukaryotes

They are organisms with cells containing a nucleus and other membrane bound organelles such as mitochondria and lysosomes. The origin of the word is “true nucleus”

These type of organisms tend to be more complex than prokaryotes. Membrane bound organelles have their own functions. This division of labour/ sharing of the roles means that the cell can carry out many different reactions at the same time. For example mitochondria can partake in aerobic respiration, and lysosomes are able o break down worn out cellular components

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Fluid mosaic model

The cell membrane is so tiny that it can’t be seen in detail, even under a microscope. Therefore biologist have given an proposal for a model to explain the structure. This describes how the molecules of the membrane are arranged

The main features of the model are:

  • A bilayer of phospholipids molecules
  • Various proteins molecules floating in the phospholipid bilayer

They are arranged in a mosaic pattern, and aren’t bonded together so are able to move freely around and change positions giving the impression of a liquid. The membrane is stabilised by small cholesterol molecules

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Golgi Apparatus

The Golgi apparatus consists of a stack of flattened membranes sacs. One side of the stack if formed by the fusion of membranes of vesicles from the rough ER. At the opposite side of the stack, vesicles are pinched off the margins of the top sacs

It is prominent in metabolically active cells such as secretory cells. It is also the site of specific biomolecules synthesis, such as hormones, enzymes or polysaccharide macromolecules. These are packed into the vesicles and punched off, and in animal cells the Golgi apparatus also creates lysosomes

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Lysosomes

They are small spherical vesicles bounded by a single membrane. They contain a concentrated mixture of hydrolytic enzymes. These elements break down substrates particularly during digestion. They are produced in the Golgi apparatus or by the rough ER.

Lysosomes are involved in the breakdown of the contents of imported food vacuoles. In phagocytes, lysosomes are responsible for engulfing harmful bacteria. They fuse and digest and broken-down organelles in the cytoplasm. When an organism dies, the lysosomes of the cells escape and cause self-digestion

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Magnification

Means making an object look bigger. A photograph of an enlarged image can be used to calculate the real size pf the object if the magnification is known

actual length= length in photograph/magnification

e.g cell measures 30mm in a photograph enlarged 10 000 times.

0.003= 30/10 000

 

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Microtubles

Microtubules are straight, unbranched hollow cylinders, and are only 25nm wide. They are usually only seen in the cytoplasm of eukaryotic cells and are made of a globular protein called tubulin. They are build up and broken down as the cells require. They form the centrosomes found in animal cells next to the nucleus

The spindle fibres which appear during the nuclear division are microtubules. They are responsible for the movement of chromosomes to the poles of the cell during mitosis and meiosis. The cilia and flagella of motile cells contain microtubules

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Mitochondria

They are rod-shaped organelles, relatively large. They are found in all cells and present in large numbers in cells that are metabolically very active.

They are the site of aerobic respiration and each mitochondria has a double membrane. The outer membrane is a smooth boundary, while the inner membrane is infolded to form cristae. These cristae walls contain proteins which are the enzymes responsible for some of the stages of aerobic respiration. The interior of the mitochondria is called the matrix. This is an aqueous solution of metabolites and enzymes

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Nucleus

The largest organelle in the eukaryotic cell. It is also surrounded by a double membrane which contains pores. The pores are so numerous that they make up about 33% of the nuclear membrane surface area. This suggests that communication between the nucleus and the cytoplasm is very important

The nucleus contains the genetic material of the cell, and also the coded instructions for the production of proteins. These proteins give the cell their characteristics, and the chromosomes are the tread-like strands of DNA which carry the genetic code

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Organelles

An organelle is a tiny structure which is found in the cytoplasm of a cell. Most of these are invisible when viewed with a light microscope. With the higher resolution of the electron microscope, their structure can be seen clearly.

Most organelles are membrane covered structures and form little compartments within the cell. They contain a unique environment within which they carry out a specific function.

For example, chloroplasts contain light harvesting pigments and the enzymes which carry out the reactions of photosynthesis

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Prokaryotes

The word literally means “before the nucleus”. Bacteria are an example of this. They don’t contain a nucleus or any type of membrane bound organelle like mitochondria or chloroplasts. They also have a cell wall made of murine. Some of motile and have flagella

They have a single loop of DNA which lies in the cytoplasm. They don’t have individual chromosomes. Some prokaryotes like the chlorea bacterium also have additional DNA in the form of tiny rings called plasmids. Plasmids vary in number and can be transferred between bacterial cells

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Resolution

When an object is magnified, it looks bigger. If the magnification is further increased then the image is enlarged but becomes blurred. In order to improve the sharpness of the image, a better microscope is required. The resolution is the ability to distinguish between two objects that are close together

Light waves will limit the resolution pf an optical microscope, and therefore it is impossible to resolve two objects that are closer than half the wavelength of light used to view them. An electron microscope, which uses a beam of electrons rather than light, has greatly improved resolving power.

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Ribosomes

This is the site of protein synthesis. They are minute structures composed of 2 sub-units. Unlike other organelles, they don’t have membranes as part of their structure. They are present in both eukaryotic and prokaryotic cells, but the smaller ribosomes are found in the prokaryotic cells

Each ribosome is constructed of proteins and the nucleic acid RNA. Ribosomes are found in vast numbers in cells that carry out protein synthesis. Some are also found attached to the rough endoplasmic reticulum.

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Transmission electron microscope (TEM)

This microscope can magnify objects up to 100 000 times. It is able to resolve objects that are very close together, and enable scientists to view the structures within cells that are invisible under the light microscope

They make use of a series of magnetic lenses which focus a beam of electrons through the specimen. Then the image is produced on a screen

The electron microscope has a vacuum inside so all the water has to be removed from the specimen, this means that only dead material can be viewed with a TEM

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