Cells

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  • Created by: Laelle
  • Created on: 01-04-16 15:04

Methods of studying cells

Cells can be viewed under microscopes, not by the visible eye.
Microscopes are instruments that provide a magnified image of an object.

Light microscopes use light rays which have a relatively long wavelength, where as electron microscopes that use electron beams have a shorter wavelength.

Magnification: how many times bigger the image produced is compared to the object under the microscope.

Resolution= the minimum distance apart two obejcts can be in order to appear as separate items
Due to having a shorter wavelength than light microscopes, electron microscopes have a resolving power of just 0.1nm and light microscopes 0.2um.
A greater resolution means greater clarity and detail of the image provided.

Increasing magnification increases the size of an object- revealing more detail, but not its resolution.
Past the resolving power limit of a microscope the object will appear bigger, but also more blurred.

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Cell fractionation- part 1

Cell fractionation is the process of breaking down a cell and isolating its various organelles, in order to study the structure and function of organelles.

Before Cell fractionation takes place, the tissue is placed in a solution that is:

IOSOTONIC: to prevent the cells from bursting or shrinking due to water gain or loss by osmosis

BUFFERED: to prevent pH fluctuation that may affect the cells structure or function of enzymes

COLD: to reduce enzyme activity that may break down the cell

The first stage is HOMOGENATION:
The cells are placed in homogeniser (blender) that breaks them down and releases organelles. The homogenate is then filtered to remove any complete cells and large pieces of debris.

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Cell fractionation- part 2

The second stage is ULTRACENTRIFUGATION

1) the filtrate is placed in a tube and spun in a centrifuge at a low speed; this creates as centrifugal force

2) the heaviest organelle- the nuclei, is forced to the bottom of the tube, forming a thin sediment/pellet of nuclei.

3) the supernatant (fluid at the top of tube) is then transferred to another tube, isolating the sediment of nuclei. It is spun at a higher speed, again creating a centrifugal force.

4) the next heaviest organelle- the mitochondria is forced to the bottom of the tube forming a thin sediment/pellet

Etc. The next heaviest organelle removed by a higher speed.
Centrifugal speed (x gravity)
Nuclei: 1000 revolutions min-1, mitochondria: 3500, lysosomes: 16500

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Eukaryotic cells structure

  • make up eukaryotic/multi-cellular organisms
  • distinct nucleus (bound by a nuclear envelope)
  • larger (than prokaryotic cells)
  • larger ribosomes (80s)
  • membrane bound organelles

Animal, plant, fungi + algae cells

Animal cells
-cell-surface membrane
-nucleus
-ER (rough + smooth)
-ribosomes
-golgi apparatus
-lysosomes

Plant cells
(All of the above) & cellulose cell wall, vacuole, chloroplasts

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Nucleus

STRUCTURE:

  • Nuclear envelope, a double membrane that surrounds the organelle. The outer membrane is continuous with rough ER. It controls the entry and exit of material and contains any reactions within
  • Nucleoplasm, granular, jelly-like material that makes up the bulk of the nucleus
  • Nuclear pores: approximately 3000 occur on the nuclear envelope. Allows the passage of large molecules like mRNA out of the nucleus
  • Chromosomes: protein-bound, linear DNA
  • Nucleolus: small spherical region within the nucleoplasm. Manufactures rRNA and assembles ribosomes

FUNCTIONS: controls the cells activities through the production of mRNA and tRNA, hence protein synthesis. Retains genetic information in the form of chromosomes and DNA. Manufacturing of rRNA and ribosomes.

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Mitochondria

STRUCTURE: double membrane surrounds the organelle, the inner membrane forms folded extensions called cristae.

  • Cristae: provides large surface area for the attachment of enzymes and other proteins involved in aerobic respiration.
  • Matrix: the remainder of the organelle, it contains enzymes for respiration and lipids, proteins, ribosomes and DNA to control the production of their own proteins.

FUNCTION:

The site of aerobic respiration and production of ATP.
Large amounts of mitochondria and cristae are contained in cells like muscle and epithelial that line the ileum, as they have a high level of metabolic activity and require lots of energy.

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Chloroplast

A small, flat structure that occur in plant and algal cells.

STRUCTURE:

Chloroplast envelope: surrounds the organelle, and is highly selective of what goes in and out

Thylakoids membranes and stacked to form structures called grand. The grana joined to adjacent grana stacks by lamellae. The first stage of photosynthesis occurs in the grana- light absorption, as they contain chlorophyll.

The stroma is the fluid-filled matrix that makes up the rest of the chloroplast. The second stage of photosynthesis- the synthesis of sugars, occurs here. It also contains starch grains.

FUNCTION: carry out photosynthesis

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Endoplasmic reticulum

3D system of sheet-like membranes enclosing a A network of tubules and cistern are (flattened sacs). Spread through out the cytoplasm. It's continuous with the he outer nuclear membrane

Rough endoplasmic reticulum:

  • surface covered in ribosomes

It provides a large surface area for the attachment of ribosomes (for the synthesis of proteins + glycoproteins)

Also provides a pathway for the transport of materials, especially proteins throughout the cell

Smooth endoplasmic reticulum:

  • no ribosomes on its surface & more tubular
    It synthesises, stores and transports lipids and carbohydrates
    Cells like liver, secretory + epithelial cells have large ER as they manufacture and store large quantities of carbohydrates, proteins and lipids.
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Mitosis

Cell division that occurs in eukaryotes (multicellular organisms)
Produces daughter cells, genetically identical to the parent cell (exact copy of DNA)
For growth of animals and plants, and repair of damaged tissue

Prior to cell division, INTERPHASE occurs.

  • The cell prepares to divide by unraveling and replicating to double its genetic content.
  • Organelles also replicate and the ATP content increases to provide energy for cell division.
  • The chromosomes remain joined at the centromere.
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Prophase

The chromosomes become visible, they condense getting fatter and thicker.

Animal cells contain centrioles (tiny bundles of protein). They move to opposite ends (poles) of the cell and form spindle fibres from each centriole that span the cell from pole to pole.

Plant cells lack centrioles, but form spindle apparatus (collection of spindle fibres)

The nucleolus and nuclear envelope disintegrate, leaving the chromosomes free-floating in the cytoplasm.

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Metaphase + Anaphase

METAPHASE:

The chromosomes line up at the equator of the cell, and become attached to the spindle fibres by their centromeres.

ANAPHASE:

The centromeres divide into 2, the chromosomes separating into sister chromatids. The spindle fibres contract and the sister chromatids are pulled to opposite poles of the cell.
They appear V-shaped.
The mitochondria provides the energy needed for this by congregating around the spindle fibres.

If the cells are treated with chemicals that destroy the spindle, chromosomes won't reach the poles but remain at the equator of the cell

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Telophase & cytokinesis

The chromosomes reach their poles and uncoil becoming longer and thinner again. The spindle fibres disintegrate & then nucleolus and nuclear envelope re-forms around the chromosomes. There is now 2 nuclei.

In Cytokinesis, the cytoplasm divides.
There are now 2 daughter cells genetically identical to the parent cell.

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Importance of DNA

H

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Binary fission- prokaryotes cell division

The original cell divides into 2 genetically identical daughter cells.

1) the circular DNA molecule replicates and each of the 2 copies attached the cell-surface membrane

2) the plasmid also replicates

3) the cell-surface membrane begins to grown between the 2 molecules and pinches inwards dividing the cytoplasm.

4) two daughter cells are formed form the original cell and are genetically identical. The 2 cells each contain a copy of the circular DNA and a variable number of copies of the plasmid.

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