[svpam_ms]

Biology A-level: Cells and Organelles

Source taken from:http://www.s-cool.co.uk/alevel/biolo...-to-cells.html

Microscopes - magnification and resolution

The cell is the basic unit of an organism and consists of a jelly-like material surrounded by a cell membrane.

It can be seen with a light microscope (LM) but many of the structures within a cell - organelles - can only be seen clearly with an electron microscope (EM). That is partly because an EM has a greater magnifying power (ability to enlarge something).


However, increasing only magnification has its limits because at some point magnification reveals nothing more - the details only look bigger and vaguer. This is because if 2 objects are a distance of less than half the wavelength of light apart, they cannot be distinguished as separate by a LM. Any object less than half the wavelength of light in size will not be seen at all by a LM.

Using electrons instead of light means that the illumination has a much shorter wavelength than light. This is good because minute detail can be detected. We say that an EM has a bigger resolving power (bigger resolution) than an LM.

Prokaryotic and eukaryotic cells

There are 2 basic cell types:

Prokaryotic: bacteria and cyanobacteria (which used to be called blue-green algae).

Eukaryotic: all other cells, such as protoctista, fungi, plant and animal cells.

Eukaryotic cells, i.e. animal and plant cells.





Prokaryotic cells - i.e. bacteria and cyanobacteria.




Prokaryotic Eukaryotic
Features Plant Animal
Size(diameter) 0.5 - 5 µm 40 µm 15 µm
Cell wall Yes (contains peptidoglycan) Yes (contains cellulose) No
Genetic Material DNA is naked. A single circular molecule DNA linear, associated with histones (proteins), in a nucleus, surrounded by a nuclear envelope.
Ribosomes 70S ribosomes (smaller) 80S ribosomes (larger)
ER, Golgi apparatus No Yes
Mitochondria No(respiration occurs on an infolding of the cell membrane called the mesosome.) Yes
Chloroplasts No Yes No

[svpam_ms]

Home
GCSE
A-level
Art
Biology
Business Studies AS & A2 Level
Chemistry
Economics
English Literature
French
Geography
History
Maths
Physics
Psychology
Sociology
Articles
Teacher support
Take a break
Careers
A-level
Biology A-level: Cells and Organelles
Organelles
print page
Bookmark and Share With Your Friends!
Much of what you will need to know applies to the structure of eukaryotic cells. They are characterised by having membrane-bound organelles.

Cytosol and Endoplasmic Reticulum (ER)

Cytoplasm refers to the jelly-like material with organelles in it.

If the organelles were removed, the soluble part that would be left is called the cytosol. It consists mainly of water with dissolved substances such as amino acids in it.

Also present in the cytosol are larger proteins and enzymes used in reactions within the cell. Running through the cytosol is endoplasmic reticulum (ER), a system of flattened cavities lined by a thin membrane. It is the site of the synthesis of many substances in the cell and so provides a compartmentalised area in which this takes place. The cavities also function as a transporting system whereby substances can move through them from one part of the cell to another.

There are 2 types of ER - rough (RER) and smooth (SER). SER obviously looks as though it has a smooth surface. It is where lipids and steroids are made so you would expect there to be a lot of SER in liver cells where lipid is metabolised.

RER looks rough on the surface because it is studded with very small organelles called ribosomes. Ribosomes are made of RNA and protein and are the site of protein synthesis (see DNA and Genetic Code).

There may be free ribosomes in the cytoplasm as well, which also are the site of protein synthesis. The proteins (which include enzymes) that are synthesised then move into the cavities of the RER to be transported.

Golgi apparatus

The Golgi apparatus is a series of flattened layers of plate-like membranes.

The proteins that are made by the RER for export from the cell are pinched off at the end of the cavity of the RER, so that a layer of membrane surrounds them. The whole structure is called a vesicle. This vesicle will move through the cytosol and fuse with the membrane of the Golgi apparatus.

In the cavity of the Golgi apparatus, the vessel proteins are modified for export - for example, by having a carbohydrate added to the protein. At the end of a Golgi cavity, the secretory product is pinched off so that the vesicle containing the substance can move through the cytosol to the cell surface membrane.

The vesicle will fuse with this membrane and so release the secretory product. If the vesicle contains digestive enzymes, it is called a lysosome. Lysosomes may be used inside the cell during endocytosis, or to break-down old, redundant organelles.


Mitochondria

A typical cell may contain 1,000 mitochondria, though some will contain many more. Generally, they are sausage-shaped organelles whose walls consist of 2 membranes.

The inner membrane is folded inwards to form projections called cristae. Inside this is the matrix.

Most of the reactions for aerobic respiration take place in the mitochondria so it is an incredibly important organelle.

During respiration, ATP is produced, which is used to provide energy for the cells' reactions. Most of the ATP is produced on the inner mitochondrial membrane.It is highly folded so there is maximum surface area available.



Cell wall and chloroplasts

These are only found in plant cells.

Chloroplasts will be discussed in photosynthesis - but, like the mitochondria - they have an envelope of two membranes making up the outer "wall".

They have pairs of membranes called thylakoids arranged in stacks, each stack being called a granum. Connecting different grana together are inter-granal thylakoids. Surrounding the internal membranes, inside the envelope is the stroma.

The reactions of photosynthesis take place in the membranes and stroma of the chloroplast.



The cell wall is rigid and made of cellulose fibres running through a mixture of other polysaccharides (more complex sugars) such as pectins and hemicelluloses.

The sticky middle lamella that holds next-door cells together is made of calcium pectate and magnesium pectate.

In young cells, the cellulose fibrils of the primary cell wall run parallel to each other. In older cells, a secondary cell wall may be laid down where the fibres are all parallel to each other, but at a different angle to those of the primary cell wall.

The cell wall is fully permeable unless a substance called lignin is deposited in the cellulose layers. Lignin makes the cell wall very strong and resistant to strain but it also makes it impermeable. If all the gaps between the fibres are filled in, the wall becomes completely impermeable and the cell will die.

Nucleus

The nucleus is separated from the surrounding cytoplasm by the double membrane around it, the nuclear envelope. This regulates the flow of substances into and out of the nucleus.

At some points around the nucleus, the 2 membranes fuse to create nuclear pores - these are channels through which substances can move. The outer of the 2 membranes is continuous with the ER.

Within the nuclear envelope is the nucleoplasm. In this are suspended thread-like chromosomes (for chromosome structure see DNA and Genetic Code).

Another structure within the nucleus is the nucleolus. The RNA, which will be made into ribosomes, is synthesised in the nucleolus.

Other organelles

Vacuole: a fluid-filled space in the cytoplasm surrounded by a membrane called the tonoplast. It contains a solution of sugars and salts called the cell sap.

Microtubules: hollow rod-like structures with walls of tubulin protein. They provide the structural support of cells and can aid transport through the cell.

Microfilaments: rod-like structures made of contractile protein. Again, like microtubules, provide support and aid movement.

Centrioles: a pair of short hollow cylinders, usually found near the nucleus of an animal cell. They are involved in the formation of spindle fibres used in mitosis (see Reproduction and Cell Cycle Learn-it).

Cilia: hollow tubes extending outside some cells. They move fluid, which is outside the cell - for example, ciliated cells lining the respiratory tract move mucus, away from the lungs.

Flagella: similar to cilia, though longer. Used in the movement of the whole cell. The only structure like this in humans is the tails of the sperm.


Investigating the function of cell organelles

To obtain reliable information about the activity of an organelle, it is necessary to isolate it and test it individually.

First the cells are broken open or cell fractionation occurs to produce a homogenate or suspension. This is done using a blender with the cells in an isotonic, cold solution. Because the solution is isotonic, the organelles neither gain, or loose water by osmosis and as it is cold, the action of enzymes, which might damage the organelles, is prevented.

Differential centrifugation of the suspension is then carried out. A tube containing the suspension is spun in a centrifuge at a speed, which causes the heaviest organelles to be thrown to the bottom, forming a sediment. The other lighter organelles remain floating in the clear supernatant fluid above the sediment.

The sediment may be removed and the activity of the heaviest organelles such as the nucleus, determined. The supernatant may then be spun at a faster speed so that lighter organelles like the mitochondria sediment out.

[svpam_ms]

Both the cell surface membrane and the membranes surrounding certain organelles have the same basic structure. Much of the membrane is made up of a 'sea' of phospholipids with protein molecules 'floating' in between the phospholipids. Some of these proteins span the whole width of the membrane.

Because the membrane is fluid, and because of the mosaic arrangement of the protein molecules, the structure of the membrane is called the fluid mosaic model.


The phospholipids are arranged in two layers (a bilayer). The phosphate heads are polar molecules and so are water-soluble. The lipid tails are non-polar and therefore are not water-soluble.

This means that the phospholipids are arranged with the heads in contact with the cytoplasm or extra-cellular fluid, both of which are watery environments. The tails are protected from this, by being as far from the cytoplasm and extra-cellular fluid as possible.

The proteins in the membrane, line pores in the lipid bilayer. The polar groups of the protein molecules mean that substances that would not be able to penetrate the lipid bilayer, (because they are insoluble in lipid), can still move from one side of the membrane to the other.

There are also short polysaccharide chains that are attached to the outer surface of the membrane. Most of these carbohydrates are attached to proteins and are called 'Glycoproteins'. They may help in the recognition of, and interaction with, other cells. They may also play a part in the recognition of hormones and foreign molecules.

Cholesterol is also present in the membrane. It maintains the fluidity and increases the stability of the membrane. Without cholesterol the membrane would easily split apart.

Functions of a membrane it's:

Selectively permeable barrier.

Structural, keeping the cell contents together.

Allows communication with other cells.

Allows recognition of other external substances.

Allows mobility in some organisms, e.g. amoeba.

The site of various chemical reactions.

[svpam_ms]

It is important that the cell is supplied with all the substances it needs (e.g. oxygen) and that waste substances (e.g. carbon dioxide), or substances for export, leave the cell. There are various processes by which this can happen...

Diffusion

This is the process that is used in oxygen entering a cell, and carbon dioxide leaving.

These molecules will move from where they are at a high concentration to where they are at a lower concentration. i.e. they diffuse down a concentration gradient.

The blood system in humans continually brings more oxygen to the cell and takes carbon dioxide away. This maintains a high concentration gradient.

Since the movement is always down the concentration gradient, it requires no energy. The small molecules pass from one side of the membrane to the other by moving between the lipid molecules.


Fick's Law

Fick's law is used to measure the rate of diffusion. It states that:


(The symbol α means 'proportional to')

The larger the area and difference in concentration and the thinner the surface, the quicker the rate.

So, for example, in the lung the surface area is made very large by the presence of many alveoli. The difference in concentration is maintained by breathing, which brings in air with a high oxygen concentration and removes the air with a high carbon dioxide concentration and by a good blood supply. The capillaries surrounding the alveoli take away the oxygenated blood and replace it with blood with a high carbon dioxide concentration. The walls of the alveoli are only one cell thick, so the surface across which diffusion occurs is thin and the rate is high.

In plants, a good example would be root hair cells. They have a very large surface area due to the drawing out of the cytoplasm to produce a very fine root hair. Water continues to enter the root by osmosis because there is a high concentration of mineral salts in the cells and the water is moved up the plant by the xylem. Water only has to penetrate one cell in order to enter the plant and so again the rate of diffusion is high.

Osmosis

This is a special case of diffusion in which we are concerned only with the movement of water.


If two solutions are separated by a semi-permeable membrane, which only allows certain sized molecules through (as in a plasma membrane), there will be a net (overall) movement of the water molecules, from the less concentrated solution (the one with more water molecules), to the solution which is more concentrated (has more solute molecules). This is because as in ordinary diffusion the molecules move to even-out any difference in concentration.

However, because of the semi-permeable membrane, which does not allow the larger solute molecules to cross, only the water molecules can move. The water molecules will continue to cross the semi-permeable membrane until an equilibrium is reached, where the two solutions are of equal concentration.

Water potential

This is a measure of the tendency of water molecules to move from one place to another. The symbol used for water potential is the Greek letter psi, Ψ

Water always moves from a region of higher water potential to one of lower water potential, or down the concentration gradient. So we can define osmosis as the movement of water molecules from a region of higher water potential to a region of lower water potential through a semi-permeable membrane.

Solute potential and pressure potential

The water potential of a cell is dependent upon the combination of its solute and pressure potentials. The water potential of pure water is zero and since adding solutes lowers water potential, they make the water potential less than zero, i.e. negative. The more solute, the more negative the water potential becomes. The amount that the solute molecules lower the water potential is called the solute potential. It always has a negative value and is given the symbol, Ψs

Pressure also has a role to play in determining water potential. The greater the pressure inside a cell, the greater the tendency will be for water to leave it. This contribution to water potential is called the pressure potential. It always has a positive value because it increase water potential and is given the symbol Ψp

Osmosis in animal and plant cells

If the water potential surrounding an animal cell is higher than that of the cell, it will gain water, swell and burst. If the surrounding solution's water potential is lower than that of the cell, it will loose water and shrivel up. This is why it is so important to maintain a constant water potential inside the bodies of animals.

In animal cells:

Water potential = Solute potential

Or:

Ψ = Ψs


Pressure potential is important in plant cells because they are surrounded by a cell wall which, is strong and rigid. When water enters a plant cell, its volume increases and the living part of the cell presses on the cell wall. The cell wall gives very little and so pressure starts to build up inside the cell. This has the tendency to stop more water entering the cell and also stops the cell from bursting. When a plant cell is fully inflated with water, it is called turgid.

So in plant cells the equation used to calculate the water potential of a cell is therefore:

Water potential = Solute potential + Pressure potential

Or:

Ψ = Ψs + Ψp

If a plant cell is placed in a solution with a lower water potential, it will loose water. The living part of the cell or protoplast will shrink and pull away from the cell wall. At this point the pressure potential is zero and so the water potential of the cell is equal to its solute potential. This process is called plasmolysis and the cell is said to be plasmolysed. The point at which the protoplast is just about to pull away from the cell wall is called incipient plasmolysis.


Facilitated diffusion

If charged particles or large molecules are to move across the membrane, another process needs to be found, as they are less soluble (or even insoluble) in lipid. They move through protein-lined pores.

Channel proteins

These line a water-filled pore in the membrane so water-soluble molecules can easily pass through.

Different channels allow different substances to pass through (the channels are selective). Some channels are gated (they will only open when appropriately stimulated).


Carrier proteins

In this case, the substance actually combines with a protein and is carried from one side of the membrane to the other. (The exact details of this process remain unclear.) These proteins are specific for a particular substance.

In both these cases, substances are moving down the concentration gradient so no energy is required.


Active transport

Sometimes substances need to be moved from where they are at a lower concentration to where they are at a higher concentration - against the concentration gradient. This allows cells to take up essential molecules even when they are at a low concentration outside.

Because molecules are moved against the concentration gradient, it requires energy.

It is thought that active transport uses carrier proteins similar to those involved in facilitated diffusion.


Endocytosis and exocytosis

If very large molecules or groups of molecules need to enter or exit a cell, they do so using vesicles.

The material to be transported out of the cell is surrounded by membrane. The vesicle will fuse with the cell surface membrane and the contents leave. This is called exocytosis (see Golgi apparatus earlier).


Materials entering the cell can do so when the plasma membrane invaginates to surround the material. The membrane seals off to form a vesicle, which can then move into the cell. This is endocytosis.


If the material is fluid, minute vesicles are formed. This type of endocytosis is called pinocytosis.

If the material is relatively large, and is digested by enzymes after fusion of the vesicle with a lysosome, it is called phagocytosis. This occurs in white blood cells that ingest bacteria and other foreign bodies.

[svpam_ms]

Some organisms do exist as single cells - for example, Amoeba, but many organisms are multicellular and consist of from hundreds to billions of cells. The functions of the organism are divided up amoungst the groups of cells, which become specialised for particular roles. Specialised cells show division of labour by being grouped into tissues.

A tissue is defined as a collection of cells, together with any extracellular secretion, that is specialised to perform one or more particular function. Tissues may contain only one type of cell, or several types.

Some examples of tissues...

Epithelial tissues are animal tissues and form sheets covering surfaces. Two tissues that you need to know about are squamous and ciliated epithelia. Both are one cell thick and so are called simple epithelia. The cells rest on a basement membrane which, is a network of collagen and glycoproteins that is secreted by cells underneath the epithelial tissue.

Squamous epithelia

In this tissue, the cells are of one type and are smooth, flat and very thin. They are packed closely together like tiles on a roof and provide a low friction surface over which fluids can move. It is found lining the cheeks, inside blood vessels, lining the chambers of the heart and forms the alveoli in the lungs.


Ciliated epithelia

This tissue is made up of cells with cilia and so is often found in areas where it is needed to transport something - for example, lining the oviducts and bronchioles of the lungs. Sometimes the cells are shaped like cubes and the tissue is called cuboidal ciliated epithelia. If the cells are tall and narrow, it is referred to as columnar ciliated epithelia.


Xylem and phloem

These two plant tissues differ from the above examples in that they are made up of more than one cell type. Xylem has the dual function of support of the plant and transport of water and dissolved mineral salts. It is made up of vessel elements, tracheids, fibres and parenchyma cells. Phloem tissue is responsible for translocation which is the transport of soluble organic substances - for example, sugar. The substances travel along sieve elements but other types of cells are also present, the companion cells, parenchyma cells and fibres.

Palisade mesophyll

This tissue is found in the leaf and is made up of one type of cell. The cells are tall and thin and are tightly packed together. Their function is to harness the light energy required for photosynthesis and so each cell is packed with chloroplasts.

Organs

An organ is part of the body which, forms a structurally and functionally separate unit and is made up of more than one type of tissue. Examples of plant organs are leaves, roots and stems. Examples of animal organs are the liver, brain, heart and kidney. Organs may be organised into groups with particular functions and are then called systems - for example, the digestive system.