Module 2: Section 1 - Cell Structure

Plasma Membrane - regulates the movement of substances into and out of the cell, has receptor molecules to allow it to respond to chemicals such as hormones

Cell Wall - supports plant cells

Nucleus - controls the cells acitivities (by controlling transcription of DNA), nuclear pores allow movement of molecules between the nucleus and the cytoplasm

Nucleolus - makes ribosomes

Lysosome - contains digestive enzymes which can be used to digest invading cells or break down worn out components of the cell, enzymes kept separate by the surrounding membrane

Ribosome - site of protein synthesis

Rough endoplasmic reticulum - contains ribosomes, transports proteins made at the 'free' ribosomes

Smooth endoplasmic reticulum - site of lipid synthesis

Vesicle - transports substances out of the cell (via the plasma membrane), some formed by the Golgi or the endoplasmic reticulum, others formed at cell surface

Golgi apparatus - processes and packages lipids and proteins, makes lysosomes

Mitochondrion - site of aerobic respiration (ATP production)

Chloroplast - site of photosynthesis

Centriole - involved with the separation of chromosomes during cell division

Cilia - movement of cell or substances along the cell surface

Flagellum - movement of cell

Cytoskeleton - network of protein threads running through the cytoplasm

In eukaryotic cells these are arranged as microfilaments and microtubules

Four main functions:

1. Supporting the organelles and keeping them in position

2. Strengthen the cell and maintain its shape

3. Movement of materials within the cell eg. the movement of chromoesomes during cell division depends on the contraction of microtubules in the spindle

4. Cell movement (cilia and flagella)

1. mRNA is produced in the nucleus, then leaves the nucleus through a nuclear pore and travels to the ribosome where the protein is synthesised

2. The protein travels to the Golgi apparatus in vesicles where it is modified

2. Protein is then transported to the cell membrane also in vesicles

4. The vesicle fuses to the cell membrane and the protease is released by exocytosis.

Prokaryotic cells – cells lack membrane organelles such as a nucleus e.g. bacteria

Eukaryotic cells – cells contain manyorganelles some of which are bound by a membrane e.g. animals, plants and fungi

Similarities:

• Both have plasma membranes

• Both have a  cytoplasm (but cytoplasm is less well developed in prokaryotes)

• Both have ribosomes

• Both have DNA and RNA

Prokaryotic cells – cells lack membrane organelles such as a nucleus e.g. bacteria

Eukaryotic cells – cells contain manyorganelles some of which are bound by a membrane e.g. animals, plants and fungi

Differences:

• Prokaryotes are smaller (<2μm) than eurkaryotes (10 - 100 μm)
• DNA cirlcular in prokaryotes, linear in eukaryotes
• Prokaryotes don't have a nucleus (DNA free in cytoplasm), eukaryotes do
• Cell wall in prokaryotes = peptidoglycan, cell wall in eukaryotes = cellulose (plants), chitin (fungi) or none (animals)
• Flagella in eukaryotes made of flagellin, arranged in a helix. Flagella in eukaryotes made of microtubule proteins arranged in a '9 + 2' formation
• Eukaryotes have larger ribosomes

Magnification - the number of times larger the image is compared to the object

= image size/object size

Resolution - the ability to distinguish between two separate points

Light microscopes

Advantages - specimen can be viewed in colour and in real time 

Disadvantages  - low magnification (x1 500) and resolution (0.2 μm), difficult to see the organelles of the cell]

Transmission electron microscope - use electromagments to focus a beam of electrons, which is transmitted through the specimen. Denser parts of the specimen absorb more electrons = darker

Advantages - high magnification (x 1 000 000) and resolution (0.0002 μm) 

Disadvantages  - specimen must be dead (fixed and in vaccum), expensive, requires a lot of skill

Scanning electron microscope - scans a beam of electrons across the specimen. This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form a 3D image

Advantages - high magnification (x 500 000) and resolution (0.002 μm) - less than TEM

Disadvantages - same as TEM

Advantages of staining specimens to be viewed under a microscope - makes it easier to see the specimen, enables different organelles to be identified

Stains for light microscope:

• Acetic Orcein stains DNA dark red

• Gentian Violet stains bacterial cell walls

• Eosin stains cytoplasm

• Sudan red stains lipids

• Iodide stains cellulose yellow, and starch blue/black

Stains for electron microscopes:

• Objects dropped in a solution of heavy metals (eg. lead) - scatter the electrons, creating contrast

1. Take a thin slice of your specimen and place it onto a clean microscope slide

2. Add a drop of water or a drop of stain onto the specimen

3. Take a clean cover slip and lower it slowly onto the specimen, taking care to avoid air bubbles