Monthly Archives: February 2009

Using potato and beetroot to study cell structure and function

This experiment showed how potato discs placed in concentrated sucrose solution overnight lost mass, due to the movement of water out of the tissue into the surrounding environment. The potato in distilled water gained mass, due to the movement of water from the environment into the cells. This diffusion of water through a semi-permeable membrane (the plasma membrane) is called ‘osmosis’. Learn about the three forms of passive transport (diffusion, facilitated diffusion and osmosis) and active transport (including ion pumps,  cotransport and endocytosis) at this site: Northland College Biology.

Next week we will start Chapter 3 and on Wednesday, do an experiment with cylinders of beetroot. We will be testing the temperature tolerance limits of beetroot cell membranes.

Would you call this Osmosis?

Image Source

I found this image on Flickr – do you think they know the scientific meaning of osmosis – “movement of water through a semi-permeable membrane”?

Today we weighed our potato discs that had been placed in distilled water and concentrated sucrose solution overnight. Which samples gained weight and which lost weight? You can try this simple osmosis interactive to check your understanding of what happens to cells in hypotonic, isotonic and hypertonic solutions. Here is another great interactive animation of osmosis and passive transport in living cells.

This site has links to some excellent Biology interactives, from simple cell transport animations and mitosis to DNA replication, photosynthesis and meiosis.

Our edible cell models

This is the animal cell model made by a group of students in year 11 Biology – each of the lollies represents a different cell organelle. Can you find the ribosomes, nucleus, smooth and rough endoplasmic reticulum, golgi bodies, vacuole and mitochondria? Below is the model of a plant cell – note the cell wall, chloroplasts and large vacuole.

Paramecium, up close and personal!

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This beautiful image is of a Paramecium (a common unicellular organism found in freshwater environments), using enhanced phase-contrast microscopy. It is an example of a protozoan – that is, a ‘first animal’. If you look closely, you can see the cilia (tiny, hair-like projections) that propel the organism through the water and the phospholipid bilayer (cell membrane) that is the barrier between the external and internal environment of the cell. In the top right of the image you can see water being ejected from the contractile vacuole, which helps to maintain the correct water balance inside the cell (osmoregulation). Water will naturally diffuse into the cell, as the cell’s contents have a higher concentration of salts and nutrients. The contractile vacuole is a specialized structure that collects this excess water and expels it from the cell, to prevent the cell membrane from rupturing.

Animal and Plant cells

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This image clearly shows the three features that differentiate plant cells from animal cells. Plant cells have:

  1. A rigid cell wall, made up of cellulose, a complex carbohydrate molecule.
  2. A large, fluid filled vacuole (animal cells usually have much smaller vacuoles)
  3. Chloroplasts, containing chlorophyll, which give plants their green colour and facilitate the process of photosynthesis.

Light microscopy and electron microscopy

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This beautiful fluorescent image is from the isolated cells of the heart of a chicken embryo. The red cells are cardiomyocytes and the green cells are fibroblasts. Biologists use different types of microscopes for different purposes:

  • A simple light microscope (one lens) use visible light, that passes through a specimen, to view tissue samples and larger single celled organisms (eg. Paramecium and Amoeba). Specimens can be stained to improve visibility.
  • Compound Light Microscopes (with at least two separate lenses) can increase the magnification by multiplying the eyepiece and objective lenses. The image above was probably taken with a Flourescence compound microscope, which uses ultra violet light to reveal compounds that have been stained with fluorescent dyes.
  • Electron Microscopes (such as the Transmission electron microscope and the Scanning electron microscope) greatly increase the magnification by using a beam of electrons instead of visible light to illuminate specimens.

Prokaryotic and Eukaryotic cells

Image Source

This diagram clearly shows the difference between prokaryotic cells (such as bacteria) and eukaryotic cells (such as those from an animal). Notice that the prokaryotic cell does not have membrane-bound organelles, such as a clearly defined nucleus, mitochondria or golgi apparatus. The eukaryotic cell has specific organelles, bound by membranes. More good diagrams here. Description of cellular organelles here.

Scientists believe that eukaryotic cells evolved from a symbiotic relationship between prokaryotic cells about 2 billion years ago. See an short animation of how this may have happened and read more about this process here.

The Cell Theory

Robert Hooke (1635-1703) was the first person to describe and name the ‘cells’ that make up living organisms. He drew the empty plant cells, shown above, from thin sections of cork. Anton van Leeuwenhoek (1632-1723) used a simple microscope to examine the tiny ‘animicules’ scraped from his teeth.

The cell theory states that:

1. All living things consist of cells or their products

2. Cells are the basic functional unit of life

3. New cells are produced from existing cells.