Biology: Unit F211: Cells, Exchange and Transport Module 1: Cells

1.1.1 – Cell Structure

When looking at a cell under a microscope, its ultra structure is revealed. Ultra structure is the detail inside of the cell. There are many organelles with specific functions a – this is the division of labour; all of the organelles working together, each contributing to the cell’s survival. All of the organelles below are found in all Eukaryotic cells (plants and animals). Eukaryotic cells have a true nucleus surrounded by a nuclear envelope.

The nucleus houses nearly all of the cell’s genetic material and has 4 components: the Nucleolous is the tightly wrapped DNA making ribosomes and RNA. It is the dense, spherical structure at the centre of the nucleus. Chromatin is the diffuse DNA in use in the cell at the time. It is made up of DNA and proteins. The nuclear envelope protects the DNA from mutation. The nuclear pores are gaps in the envelope which allow mRNA to leave the nucleus, permitting communication between the nucleus and the rest of cell.

The endoplasmic reticulum is made up of flat membrane bound sacs called cisternae, which are continuous with the outer nuclear membrane. Rough endoplasmic reticulum is studded with ribosomes for making proteins, and the RER acts as a method of storage and transportation of the proteins that have been synthesised on the attached ribosomes. Smooth endoplasmic reticulum is involved in the synthesis of lipids, used to make hormones.

The golgi apparatus is a stack of membrane bound sacs. The golgi receives proteins from the rough endoplasmic reticulum and modifies them, perhaps adding sugar, folds them into enzymes, and sorts them according to their destination. Enzymes are pinched off into secretary vesicles which can then travel to the plasma membrane to be secreted outside of the cell, or to other organelles inside the cell itself.

The mitochondria have two membranes separated by a fluid filled space. The inner membrane is highly folded to form christae. Mitochondria are the site when adenosine triphosphate (ATP) is produced during aerobic respiration.

The ribosomes are tiny organelles found in both eukaryotic and prokaryotic cells. They are the site of protein synthesis – an assembly line which reads coded instructions (mRNA) from the nucleus to join amino acids to create proteins.

The lysosome is a spherical sac surrounded by a membrane which contains digestive enzymes. Anything in the cell which needs to be destroyed, such as invading bacteria, misfolded proteins or worn out ribosomes, are sent to the lysosome to be destroyed. Broken up products are reused and recycled to make new compounds and organelles.

The centrioles are small protein microtubules which are found next to the nucleus of animal cells. They form the spindle fibres which move the chromosomes during mitosis cell division.

The chloroplast is found in most plant cells, and is the site of photosynthesis. Stacks of membrane sacs called thylakoids make up piles of granum. Chlorophyll molecules are present on the thylakoid membranes so that light energy can be used to form carbohydrate molecules from water and carbon dioxide.

The cell wall is found in only plant eukaryotic cells. They are made from cellulose (a carbohydrate polymer made from glucose sub-units). The cellulose forms a sieve like network of strands that make the wall strong. The cell wall is held rigid by the pressure of the fluid inside the cell – turgor pressure – supporting the cell, and so supporting the whole plant.

The cytoskeleton is the network of protein fibres found within cells. These protein tubes are called microtubules, and are

The plasma membrane is selectively permeable. It is a phospholipid bilayer interspersed with proteins. It separates the cell from the outside environment, gives physical protection and allows import and export of selected chemicals. It prevents the cell from bursting, or lysis.

about 25nm in diameter. The cytoskeleton has 2 functions; to give particular shape and structure (the shape of a red blood cell for example) and to enable organelles to move around in the cell. Chromosomes are also moved by the cytoskeleton during mitosis. The proteins which move organelles or chromosomes are called protein motors. They pull the organelle around using the network of microtubules in the cytoskeleton.

The flagella (undulipodia) are almost identical in structure to the cilia, but there are often fewer of them, and they are longer. They are extensions of the cytoskeleton and plasma membrane, and are used for the propulsion of the cell itself, for example, the tail of the sperm cell. The microtubules use energy form ATP in order to move.

The cilia are extensions of the plasma membrane and cytoskeleton. They waft in one direction. They are found on ciliated epithelial cells in the trachea, and in the oviduct. They sweep mucus away from the lungs and move the egg out of the oviduct and towards the uterus.

Eukaryotic Cells are between 20 – 40 µm in length.

True nucleus with a nuclear envelope Mitochondria,

Endoplasmic Reticulum, Golgi,

Lysosome, Vacuole, Cilia,

Flagella, Chloroplasts

Large Ribosomes – 30 nmComplex cytoskeleton made from protein microtubules

DNA wrapped in

histone proteins forming

chromosomes.

Prokaryotic Cells have a different structure to eukaryotes. Bacteria are an example of a prokaryote. They do not have a true nucleus with nuclear envelope. DNA mutates easily because it is not protected by a nuclear envelope or proteins.

Microscopy

Magnification: The degree to which the size of an image is larger than the object itself.

Resolution: The degree to which it is possible to distinguish two points that are very close together as separate.

Light Microscopes

Maximum magnification: x1500 Maximum resolution : 200 nm

Advantages:

Small, portable, cheap, living cells can be viewed, real colour images

Preparing a specimen – Staining

Small ribosomes – 20nm

Cell wall made from peptidoglycan (protein and sugar)

DNA nucleoid – Not bound into chromosomes by histone proteins- “naked” Not bound by a nuclear envelope- “free”. May also contain a plasmid loop of DNA which is useful for genetic engineering.

Pilli – for bacterial conjugation. Tubs allow bacteria to exchange plasmid DNA.

Infolds of plasma membrane are the site of respiration, there are no mitochondria. Mesosomes help cell division.

Capsule – a jelly like coating which sticks to things and protects the cell from destruction.1-5 micrometres (µm) long

Disadvantages:

Low resolution, low magnification

Coloured stains are chemicals that bind to chemicals or specific organelles in the specimen. This allows them to be seen under the microscope and enables us to highlight particular organelles:

Light Green – Plant cell Walls Aceitic Orcein – DNA red Iodine – very dark blue starch grains Methylene Blue – Dark blue nucleus, light blue cytoplasm

Electron Microscopes

Electron microscopes use electrons rather than visible light as the radiation source.

Transmission Electron Microscopes

500,000 x Magnification High resolution 2D black and White image Works in a similar way to a light microscope – passes a

beam of electrons through a thinly cut specimen. You must prepare the sample by dehydrating the specimen (replacing water with ethanol), coating in heavy metals, mounting on a copper grid and placing it in a vacuum within the microscope.

Electron Microscopes use electromagnetic lenses and electron guns with computer detectors and screens.

Scanning Electron Microscope

100,000 x magnification High resolution 3D black and white image Scanning Electron Microscopes work by bouncing

electrons off the surface of an object to build up a 3D image.

Advantages Disadvantages High resolutionHigh MagnificationScanning created 3D image

Large, Expensive, Immovable Must be trained to use themSpecimens must be dead, and coated in very expensive metalsArtefacts (things created or destroyed during preparation which do not give an accurate portrayal of the thing in real life) are created.