Cells OCR Biology AS

OCR Biology AS

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Sub-Cellular Organelles - Cells

Nucleus: The largest organelle, found in eukaryotic cells; it is surrounded by a nuclear envelope and contains the chromosomes/DNA.

Nucleolus: Dense, spherical structure inside the nucleus. Makes ribosomes and RNA which pass into the cytoplasm and are used for protein synthesis. 

Ribosomes: Very small organelles found in both prokaryotic and eukaryotic cells, where protein synthesis takes place. Bound to rough endoplasmic reticulum and found in the cytoplasm. 

Mitochondria: Organelles found in most eukaryotic cells, in which respiration takes place. They are surrounded in an envelope of which the inner membrane is folded to form cristae. Oxegen combines with glucose to create energy (ATP).

Lymosomes: Spherical sacs with a single membrane. Contain digestive enzymes that are used to break down bacterium that enter the cell via endocytosis. 

Chloroplasts: Only found in plant cells. Have two membranes and contribute in photosynthesis. 

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Sub-Cellular Organelles 2 - Cells

Plasma Membrane: Surrounds the outside of both eukaryotic cells and prokaryotic cells. Made up of a phospholipid bilayer with hydrophilic heads and hydrophobic tails. Controls the movement of various substances in and out of the cell. 

Centrioles: Small tubes of protein fibres, a pair of them are found next to the nucleus in animal cells. In mitosis and cell division, the centrioles divide and migrate to the poles of the nucleus. Forms the spindle which moves chromosomes during cell division.

Flagella and Cilia: Hair-like extensions projecting from the surface of the cell. Move by ATP. Example: make sperm cells swim.

Golgi Apparatus: Stack of membrane-bound flattened sacs, modifies proteins recieved from the rough endoplasmic reticulum before exporting them from the cell in vesicles. 

Smooth Endoplasmic Reticulum: Involved in the making of lipids. 

Rough Endoplasmic Reticulum: Transports proteins made by the attached ribosomes. 

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Production and Secretion of Proteins - Cells

1. The gene containing the instructions for the production of the hormones is copied on to a peice of mRNA.

2. mRNA leaves the nucleus through a nuclear pore. 

3. mRNA attaches to a ribosome on the rough endoplasmic reticulum. 

4. Ribosome reads the instruction to assemble the protein. 

5. Molecules are 'pinched off' in vesicles from the ribosome and travel towards the golgi apparatus. 

6. Vesicle fuses with the golgi apparatus. 

7. Golgi apparatus processes and packages the vesicles, ready for release. 

8. The molecules are 'pinched off' in vesicles from the golgi apparatus and travel towards the plasma membrane. 

9. Vesicles fuse with the plasma membrane, and it opens to release molecules outside. 

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The Cytoskeleton - Cells

Function of the Cytoskeleton:

  • Strengthening the cell and maintaining it's shape. 
  • To help with the transport and movement of organelles.
  • Cell movement, eg Flagella and Cilia. 

The cytoplasm contains a network of two different kinds of protein fibres which help maintain the cell's stability by providing an internal framework. These are: 

Microfilaments: Small solid strands made of actin, 7nm in diameter.

Microtubules: Protein cylinders made of tubulin molecules, 25nm in diameter.  

Microtubules do not move, but they provide an anchor to move along. Flagella and Cilia are made from a cylinder containing 9 microtubules.

Flagella move with the aid of the protein Dynein. When a molecule of Dynein 'swivels', it pulls one microtubule past the next, causing the cilium to bend. Cilia move in time with each other to create a 'wave'. 

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Prokaryotic (Bacteria) Cells - Cells

Prokaryotic cells do not have a nucleus. They are known as bacteria and are much smaller than eukaryotic cells.

Prokaryotic cells have:

  • One membrane.
  • No membrane-bound organelles.
  • Cell wall is made of peptidolycan instead of cellulose (stops the cell from bursting). 
  • Their ribosomes are smaller than those found in eukaryotic cells (30nm in diameter).
  • Circular DNA. 
  • DNA is not surrounded by a membrane. 
  • ATP production takes place in specialised infolded regions of the plasma membrane
  • Some have flagella, but have a different structure compared to eukaryotic cells.
  • No cytoskeleton.
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Ultrastructure of Plant Cells - Cells

Plant Cells:

  • Have a cell wall, which is outside of the plasma membrane and is made of cellulose
  • Plant cells also contain a vacuole. This contains the cell stability and making the cell turgid (unable to take in any more water), increases the pressure inside of the cell. This in turn helps support the plant. 
  • The membrane of the vacuole is called the tonoplast. 

In a typical beetroot cell, betalain is kept inside of the vacuole. If the beetroot cell is heated, the tonoplast and cell membrane will begin to disintegrate and the proteins inside of the cell will denature. 

The cellulose cell wall is fully permeable. 

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Microscopes - Cells

Magnification: The number of times greater that an image is than the actual object. 

Resolution: The ability to distinguish btween two objects very close together; the greater the resolution, the greater the detail. 

Light Microscope Magnification - 1500x    LMR: 200nm

Advantage of using LM: Used to view a variety of samples, including living organisms or sections of larger plants and animals. Also relatively inexpensive.

Transmission Electron Microscope Magnification - 500,000x    TEMR: 0.1nm

Advantages of TEM: Much higher resolution than LM. 

Disadvantages of TEM: Expensive to buy, samples have to be placed in a vacuum as electrons are deflected by particles in the air, requires skill to prepare samples. 

Scanning Electron Microscope Magnification: 100,000x (as produces a 3D image).

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