Biology AQA A Level Microscopes, Cell Structure and Mitosis



Resolution: is how detailed an object is and the minimum distance apart that two objects can be in order for them to appear as two separate objects.

Light Microscopes 

use light to form an image- light microscopes have very long wavelengths of light and therefore harder to distinguish between two objects smaller than 0.2micrometers apart. 
--> they have a lower resolving power in comparison to electron microscopes. 
Samples normally have to be dead and VERY thin- also have to be stained. 

Electron Microscopes

uses a beam of electorns, these have shorter wavlengths and therefore a higher resolving power. 
Transmission Electorn Microscope and the Scanning Electron Microscope. 

Main limitations: must be in a vacuum, complex staining process and for TEM, specimen mus be EXTREMELY THIN. SEM, can use thick specimen, get a 3D image but has a lower resolving power than the TEM. 

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Magnification: how many times bigger the image is in comparison to the actual object.

Increasing magnification does not necessarily increase the resolution, it increases the image size. 

How to calculate magnification: 

mag.= image size/real size of object

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Cell Fractionation

This is the process where cells are broken up and the different organelles are separated out.

Before cell frac. can begin, the tissue is placed in a COLD, BUFFERED, ISOTONIC SOLUTION.
--> COLD: reduces enzyme activity that may break down organelles
--> BUFFERED: so that pH does not fluxuate- any change in pH can alter the structure of the organelle and affect the functioning of hte enzymes.
--> ISOTONIC: prevent organelles from bursting as a result of osmotic gain. 

cells broken up by a homogeniser- this releases the organelles from the cells. The resultant fluid is known as the homogenate. 
Homogenate is then filtered to remove debris and complete cells. 

process by which the fragments in the filtered homogenate are separated.
homogenate spun slowly in a centrifuge- first pellet= heaviest organelles, e.g. nuclei- supernatent (liquid at the top) drained and spun again at higher speeds- second pellete= chloroplasts or mitochondria. (and so on) 

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Cell Structure pt 1

-contains genes, controls cells activities
-attatched to the nuclear envelope (double membrane that surrounds the nucleus) which is continuous with the endoplasmic reticulum (ER).
-has nuclear pores (which allow the passage of large molecules)
-nucleolus (small spherical regin within the nucleoplasm that manufactures ribosomal RNA and assembles ribosomes).
Functions: acts as a control centre of the cell and retains genetic material in the form of DNA and chromosomes. 

double membrane around organelle, the inner two membranes are folded to form cristae (this provides a very large surface area for respiration). 
-mitochondria is the site of aerobic respiration which produce a lot of the energy-carrier molecule ATP. 

Epithelial cells need a lot of mitochondria so that they can produce enough energy to carry out active transport. 

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Cell Structure pt 2

organelles that carry out photosynthesis
- have a chloroplast envelope which is highly selective of what comes in and out.
-grana which are stacks of disk-like structures called thylakoids (where chlorophyll is located!)
Adapted to carry out photosynthesis because:
- Granal membrane- very large surface area for the attatchment of chlorophyll
- fluid of stroma consists of enzymes needed to synthesis sugars.
- chloroplasts contain both DNA and ribosomes so can manufacture proteins. 

Endoplasmic Reticulum
continues with outer nucleus membrane
-cisternae is enclosed in a network of tubules found in membranes. 
Two types: Rough Endoplasmic Reticulum- contains many ribosomes- provides a large surface area for synthesis of proteins. 
Smooth Endoplasmic Reticulum-no ribosomes- synthesis, store and transport lipids and carbohydrates. 

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Cell Structure pt 3

Golgi apparatus
- a stack of membranes that make up cisternae with small rounded hollow structures called vesicles.
-proteins and lipids produced by ER pass through the golgi app. 
it add carbohydrates to proteins, produces enzymes and transports lipids. 
- hydrolyse materials ingested by phagocytic cells
-relases enzymes to the outside of the cell (exocyrosis) in order to destroy materials around it
-digest worn out organelles and completely break down cells after they have died. 
Ribosomes-site of protein synthesis.
80s- eukaryotic cells 
70s- prokaryotc cells
Cell Wall- 
consists of cellulose, provides mechanical strength so prevents the cell from bursting due to pressure gained from osmotic gain of water.
fluid-filled spaces with sap- bounded by a single membrane (tonoplast). 

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Eukaryotic and Prokaryotic Cells

Comparison between eukaryotic and prokaryotic cells: 

Eukaryotic -
distinct nucleus, DNA associated with proteins, no plasmids and DNA is linear, membrane-bounded organelles are present, ribosomes are larger, no capsule.

no true nucleus, DNA not associated with proteins, some DNA in the form plasimds, no membrane bounded organelles, ribosomes are smaller, cell wall made of murien, may have a capsule or flagellum. 

Viruses are acellular, non-living particles.
-they contain nucleic acid such as DNA or RNA but only multiply inside living host cells. 

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Mitosis pt 1

mitosis produces two daugter cells that have the same number of chromosomes as the parent cell.

meiosis produces four daughter cell, each with half the number of chromosomes of the parent cell.

the division of a cell that results in each of the daughter cells having an exact copy of the DNA of the parent cell. 

Interphase- period of considerable cellular activity that includes a very important event- replication of DNA. Two copies of DNA after replication remain joined at a place called the centomere. 

Prophase- chromosomes first become visible, nucleolus disappears and nuclear envelope breaks down. 


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Mitosis pt 2

Chromosomes align in the centre of the cell, along the equator. Spindle fibres attactch to centromere. 

Centromere divides in two and spindle fibres pull each individual sister chromatid to opposite poles of the cell. 

spindle fibres desintegrate and nucleous as well as nuclear envelop reforms, cell mambrane also begins to form. 

cytoplam divides forming two cells.

importance of mitosis: for growth, repair and reprodution. 

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Binary Fission and the Replication of Viruses

Prokaryotic cells divide by a process called binary fission.
-circular DNA molecules replicate as well as plasmids and both copies attach to the cell membrane
-cell membrane begins to grow between the two DNa molecules and begin to pinch inwards- dividing the cytoplasm into two
-a new cell wall then form between the two DNA molecules- this then forms two identical daughter cells. 

Viruses are non-living and so cannot undergo cell division.

attach to their host cell using attachment proteins--inject their nucleic acid into the host cell-- genetic information provides instructions for the host cell's metabolic processes to produce viral components-- these are then assembled into new viruses cells-- the cell then ruptures and releases the virus. 

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