1.2 Ultrastructure of cells

ESSENTIAL LEARNING:

  1. Electron microscope (and its advantages and significance – NoS)
  2. Prokaryotic cell structure
  3. Eukaryotic cell structure
  4. Cell organelles
  5. Identification and drawing cells from photomicrographs

THE SYLLABUS (highlighted are the more important points for learning and application)

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ELECTRON MICROSCOPES

The advent and development of the electron microscope in the last century enabled profound, new insights into cell biology. The ‘new’ microscopes use beams of electrons in much the same way that optical microscopes use light to magnify an image. More significant are the improved powers of resolution of an electron microscope – up to 1 nm. This makes it possible to distinguish the smallest contents of cells.

Electron microscopes have their disadvantages, not least cost and bulk. Another interesting conundrum is associated with the fact that electron microscopes cannot be used to view living material and it is necessary to treat biological specimens before viewing. So perhaps what we ‘see’ with an electron microscope isn’t what is actually there? It is not impossible that we view ‘artefacts’ rather than reality. Scientists recognise this and develop models for what they see. Supreme amongst these models is the Fluid Mosaic Model for Cell Membranes – see Unit 1.3.

These pages are scanned from BIOLOGY IN CONTEXT for Cambridge International A Level, by Glenn and Susan Toole

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SIMPLE CLASSIFICATION OF LIVING  THINGS

Living things are classified according to their similarities and differences. There is considerable argument about classification systems. Recent ideas propose 3 DOMAINS – Archaea, Bacteria and Eukaryota.

three-domain-classification

But for now, let’s stay with the simple FIVE KINGDOM CLASSIFICATION

five-kingdom-classificationThe 5 Kingdom Classification differentiates the MONERA (Bacteria) from the other four Kingdoms. This is largely the result of the Monera, despite their vast number of different species and types, all being PROKARYOTES – having simple cell structures without compartamentalised organelles. EUKARYOTES – the other 4 Kingdoms (Protista, Plants, Animals and Fungi) – all have compartamentalised cell structures. Moreover it is suggested that Eukaryotic cells evolved from Prokaryotic cells through some sort of symbiotic, mutualistic associations – the ENDOSYMBIOSIS THEORY.


ENDOSYMBIOSIS THEORY

The proposal, which is robust and plausible, suggests that bacteria with specific functions ‘invaded’ other host bacteria without those functions, and the two species then managed to survive together, dependent one on another. The method of ‘invasion’ may well have been some sort of endocytosis, where the host bacteria formed a membrane vesicle around the ‘invader’ in order to drag it into the cytoplasm. Hence membrane-bound organelles. Neat!

endosymbiosis


PROKARYOTES

These pages are scanned from BIOLOGY IN CONTEXT for Cambridge International A Level, by Glenn and Susan Toole

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This how the bacteria E. coli appears, through an electron microscope:

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And this is the sort of question which might be faced in an IB exam, where the requirements are to recognise, identify and draw a prokaryotic cell. The idea of scale is also often incorporated.

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BINARY FISSION OF BACTERIA

The syllabus also refers to BINARY FISSION – the method of division of Prokaryotes. This is NOT a form of Mitosis, because there is no prior replication of the DNA into sister chromatids. It can occur very rapidly indeed, exponentially (2-4-8-16-32, etc), which explains why small numbers of bacteria can quickly become huge and dangerous populations.

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EUKARYOTES

These pages are scanned from BIOLOGY IN CONTEXT for Cambridge International A Level, by Glenn and Susan Toole

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Identification and interpretation of organelles – drawings

1. Exocrine gland cells from the pancreas

From: Biology (for the IB Diploma) Second Edition by C.J. Clegg

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2.Palisade mesophyll cells of the leaf

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PAST PAPER QUESTIONS

Google link: https://drive.google.com/file/d/0Byq5ornnlm2geHYxZFRrSU1JdGs/view?usp=sharing

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