Edexcel AS Biology - Unit 2

Observing Cells

Light Microscope

Advantages: Can be used to observe living tissue, cheap so available in schools, easily transported.

Disadvantages: Limited powers of resolution and magnification, preparing the tissue can produce artefacts in the tissue being examined.

Electron Microscope

Advantages: Huge power of magnification and resolution.

Disadvantages: Specimens examined in a vacuum (can't look at living material), very expensive due to needing to be kept at a constant temperature and pressure.

Prokaryotic Cells

• Have no nucleus 'pro' = before 'karyon' = nucleus
• Ancient cells, always unicelluar e.g. bacteria
• Genetic material coiled to for a loop - nucleoid
• Sometimes additional genetic material - plasmids
• Cytoplasm contains enzymes, ribosomes and food-storage granules
• Respiration takes place on mesosomes

Eukaryotic Cells (Part 1)

• Most living organisms made up of eukaryotic cells
• Contain membrane-bound organelles e.g. nucleus, mitochondria, chloroplasts

The typical animal cell: Surrounded by cell membrane, inside is jelly-like cytoplasm with a nucleus - the two together are known as the protoplasm.

Membranes: important both within the cell and as external cell boundary

The nucleus: normally largest organelle in the cell, spherical in shape, surrounded by a double membrane (with holes/pores). Contains nucelic acids and proteins (DNA & RNA). When cell is not dividing DNA bonded to protein to form Chromatin. Nucleus contains at least one extra dense area of pure DNA and protein known as nuclelous.

Vacuoles: not permanent, lost and formed as required, e.g. white blood cells engulf pathogens in vacuole. Contractile vacuoles important in fresh water animals.

Eukaryotic Cells (Part 2)

Centrioles: pair of centrioles normally present in each cell, each is made up of 9 tubules, important in cell division as it forms the spindle to move chromosomes.

Cytoskeleton: microfilaments and microtubules form 3D web-like structure inside cells to fill cytoplasm. Gives the cytoplasm structure and holds organelles in place.

Mitochondria:

• tiny rod-like structures in cytoplasm
• Act as 'powerhouses', respiration occurs to form ATP
• Cells that require lots of energy have lots of mitochondria (muscle)
• Surrounded by outer and inner membrane, inner membrane folded to form cristae surrounded by fluid matrix
• Contain their own genetic information, divide under control of nucleus