There are fundamental differences between plant cells and animal cells. The structure of a palisade cell from a leaf as seen with an optical microscope. The appearance, ultrastructure and function of • cell wall • chloroplasts. Candidates should be able to apply their knowledge of these and other eukaryotic features in explaining adaptations of other plant cells.

Plant cells and animal cells share some of the same organelles, but there are some that differ. Cell walls and chloroplasts are examples of things that plant cells have which animal cells don't. Bother can have vacuoles but plants almost always have large central ones and animals rarely have them, and when they do they are small and scattered. Another difference is that plants store glucose as starch and animals store it as glycogen.

A palisade cell is traps sunlight with chlorophyll, it is found in the leaf.

cell-specialisation-jesse.wikispaces

Cell wall
This is needed in a plant cell to offer structural support, stop cells bursting and provide the symplastic pathway for water movement. They are made up of polysaccharides, like cellulose, and have a middle lamella which holds adjacent cells together.

sigmaaldrich

Chloroplasts
These are needed in plant cells as they need the suns energy to carry out photosynthesis.
They are made up of three parts:

• The chloroplast envelope- double plasma membrane to control the movement of substances in and out of the cell.
• The grana- stacks of disks (thylakoids) which contain chlorophyll and have a large surface area for the first stage of photosynthesis. Tubular extensions can link them.
• The stoma- matrix with the enzymes needed for the second stage of photosynthesis
• DNA and ribosomes to manufacture proteins for photosynthesis

artinaid

Root hair cells adaptations

• Carrier proteins for active transport
• Lots of mitochondria to produce ATP for active transport
• Long thin shape to increase surface area for diffusion
• Large vacuole containing a high proportion of ions to decrease the water potential and encourage osmosis into the cell

Xylem vessel adaptations

• Thick walls to cope with the negative pressure of transpiration
• Thickening happens in a spiral so the plant is still flexible
• They are hollow and elongated so that water can move up them
• They are dead so that water does not need to diffuse through anything and can go quickly through
• Have a substance called lignin in their walls to offer strength and make it water proof (so water doesn't move out by osmosis)

Principles of cell fractionation and ultracentrifugation as used to separate cell components.

Cell fractionation is splitting cells up into its organelles.

1. The tissue is chopped up and up into a ice cold, isotonic, buffer solution.
2. This is then put in a blender to break open the cells which is called 'homogenisation'.
3. The 'homogenate' is then filtered to get rid of debris like connective tissue.
4. The mixture is spun on a centrifuge, the densest organelle will collect at the bottom.
5. The separated bit at the bottom, the 'pellet' is left in the tube when the homogenate on top which is called the supernatant is poured off into a new tube.
6. This new tube is span again to collect the next densest organelle- this is repeated to collect the desired organelles, with the speed increasing each time.

In step one the liquid is cold to slow down enzymes (that might have been freed from lysosomes) so that they don't digest the organelles. It is isotonic to maintain a normal water potential thereby preventing organelles from bursting with water! Buffer solution maintains the PH so that it is appropriate for the organelles.

There are rules on how fast and long you have to spin the centrifuge to get the desired organelle relating to the order of density. From most dense to least the order of these key organelles goes: nucleus; mitochondria; lysosomes; ribosomes.

The principles and limitations of transmission and scanning electron microscopes.

Transmission electron microscopes (TEM) shine a beam of electrons through a slice of stained specimen, some electrons are absorbed and others travel through; the pattern makes a photomicrograph on a fluorescent screen which records a magnified image of the specimen. The electron beam is manipulated by electromagnets:

Scanning electron microscopes (SEM) shin a beam of electrons on the specimen, but instead of going through they scatter off the surface- the electrons are collected and the pattern amplified to give a 3D image of the specimens surface. SEMs have a very complex system but this diagram shows some of the basic components:

                                                   TEM                SEM

Maximum resolution:                   1nm                  10nm

Maximum magnification:              250000x           100000x

Image:                                        2D                    3D

In both case the specimen must be dead because it is done in a vacuum. If there was air present the electrons would reflect off of it not the specimen.

TEM requires a complex staining process, and for the specimen to be cut up into extremely small pieces so the electrons can get through. Often TEM show up random artefacts (which are areas on the image that don't really exist) which are a result of the way the specimen was prepared.

The difference between magnification and resolution.

Electron microscopes can magnify objects many times to a good resolution.

Magnification is how many times the image has been enlarged.

Resolution is how clear the picture is. If resolution is 1nm it means that if two objects are one nanometer or more apart the microscope will show them as two different objects; if they are any closer than that they will look like one object.

Waves are only affected by objects that are bigger than half of their length: so big wavelength will only get diffracted by a relatively big object in the way- think of the sea with massive waves- if you put a twig in front of it the water would not be disturbed, you would have to put a tree in front of it to make any difference.

A small wavelength will be diffracted by tiny objects in the way: so the ripples from a raindrop would be affected by a twig. Electrons have a very small wavelength and so are effected by tiny objects- this means that more detail shows up in an electron microscope where the waves have been disturbed than it does on a light microscope (with a bigger wavelength.)

Candidates should be able to apply their knowledge of organelles in explaining adaptations of other eukaryotic cells.

A cell will have organelles to suit its job. The ultrastructure of a cell is just the detail inside it- so its types and amounts of organelles.

Here are some examples of where having more of an organelle fits a cells function:

• Mitochondria produce energy (ATP) so are beneficial in muscle cells which need energy for movement and epithelial cells which need energy for active transport.
• Endoplasmic reticulum produces protein (among other things) and so is needed in large amounts for cells that secrete enzymes like the epithelial cells and liver cells
• The same is true for ribosomes which will increase with ER as it covers the surface
• Golgi apparatus enables a cell to egest products like enzymes so would again be helpful in secretory cells; it also produces lysosomes which are needed a lot in phagocytic cells like white blood cells
• Microvilli are helpful in cells that need to absorb things as they increase the surface area, hence why there are so many in the epithelial cells

There can also be adaptations in the organelles- for example in a cell that needs a lot of energy the mitochondria may have more cistae so there is more respiration can happen.

The appearance, ultrastructure and function of the nucleus.

Nucleus:

• Nuclear envelope
• Double membrane
• Controls the movement of substances in and out of the nucleus
• Can have ribosomes on the surface
• Can be continuous with (joined/next to) endoplasmic reticulum
• Contains chromatin
• DNA for the cell
• Form chromosomes take when the cell is not dividing
• Nucleolus
• Makes rRNA (coding for ribosomes)
• Assembles ribosomes
• Makes mRNA (coding for proteins) and controls protein synthesis
• Nucleoplasam
• Viscous liquid
• Molecules and organelles of the nucleus suspended in it
• Holds the structure of the nucleus

thelogos.co.uk

Not to scale

The appearance, ultrastructure and function of microvilli.

Microvilli are tiny protrusions that help to increase surface area. Epithelial cells in the small intestine need have these in order to increase the rate of diffusion: there is a bigger surface area so more things can diffuse at the same time.

They are also thin walled to decrease the diffusion distance and so increase the rate of diffusion. This is key in the small intestine so that all the useful molecules from food eaten can be absorbed by the cells before the food moves on.

Under a light microscope the microvilli in the intestines are so tightly packed they look like a bush and are referred to as a 'bush border'.

course1.winoa.edu

Not to scale

The appearance, ultrastructure and function of Golgi apparatus.

The Golgi apparatus receives proteins in vesicles (packages) from the endoplasmic reticulum: it modifies them (like adding carbohydrate) and packages them in a new vesicle which is labelled (so it can travel to the right place). The Golgi apparatus can also transport, modify and store lipids.

Mostly the vesicles are going to the cell surface membrane to release their content outside the cell: an example of this in practice is enzymes being secrete in the pancreas to digest food.

The Golgi apparatus is made up of flattened sacs called cisternae. To make a vesicle a bit of the cisternae breaks off into a circular shaped package- this will contain either proteins, carbohydrates or lipids.

Not to scale

Not to scale

The appearance, ultrastructure and function of endoplasmic reticulum.

Endoplasmic reticulum (ER) provides a surface for the synthesis of different molecules needed in a cell. It can also store and transport these materials.

The molecules involved depend on the type of ER:

• Rough endoplasmic reticulum (RER) makes and transport proteins and glycoproteins; it has ribosomes on its surface because they are involved in protein synthesis.
• Smooth endoplasmic reticulum (SER) stores, transports and synthesises lipids and carbohydrates.

ER is sometimes continuous with (attached to) the nucleus of a cell.

Not to scale

ER is made up of 'flattened sacs' called cisternae surrounded by a membrane. Both types are complex structures but SER looks more tubular compared to the 'liney' RER.

Not to scale

The appearance, ultrastructure and function of ribosomes.

Riosomes play a role in protein synthesis. mRNA (coding for proteins) is read by them and used to assemble amino acids.

They can be found in the cytoplasm or on the surface of rough endoplasmic reticulum.

They consist of two sub-units (a large and a small) that are not bound by a membrane.

cartage.org

The appearance, ultrastructure and function of lysosomes.

Lysosomes are sacs containing digestive enzymes. They are created by the golgi apparatus.

Not to scale

The enzymes in the lysosomes have many important functions

• Digest material that phagocytic cells engulf: commonly this would mean digesting a pathogen that had been engulfed by a white blood cell.
• Digest old organelles (if left to build up they can be toxic)
• Digest old cells (autolysis)
• Release enzymes outside the cell so they can digest things externally (exocytosis)
• Digesting food that is delivered to the cell into useful components

When a lysosome has digested something, including waste products like old organelles, and useful chemicals are absorbed into the cytoplasm to be used again by the cell.

The appearance, ultrastructure and function of mitochondria.

Mitochondrion are the organelles where aerobic respiration takes place.

The stages of respiration that take place there require enzymes, so there is DNA that allows the cell to manufacture its own proteins (enzymes.) This DNA is held in the matrix; a material that also contains proteins and lipids.

Not to scale

A mitochondria has a double membrane: the two both control the movement of substances in and out of the cell. The inner membrane provides the surface area for the reactions in respiration: it is folded many times into cristae to create a large surface area. This area is covered in the enzymes needed for respiration.

Not to scale

The appearance, ultrastructure and function of the cell surface membrane.

The cell surface membrane is an example of a plasma membrane (click for more detail on plasma membranes) and has several jobs to perform:

• Separates the cell content from the surrounding environment
• Controls the movement of substances into and out of the cell
• Allows for the conditions inside the cell to differ from those outside

  

  Not to scale

The structure of an epithelial cell from the small intestine as seen with an optical microscope.

Epithelial cells make up a lot of tissue in the human body- including that of the small intestine. They are eukaryotic cells (nucleus and a variety of organelles) which specialise in absorption and secretion.

With a light (optical) microscope the following can be seen:

Extra reading: http://www.bio.davidson.edu/people/kabernd/BerndCV/Lab/EpithelialInfoWeb/