B3 - Living and Growing (OCR Gateway Science B)

Animal cell structures:

• NUCLEUS - contains DNA in the form of chromosomes
• CELL MEMBRANE - holds the cell together and controls what goes in and out.
• RIBOSOMES - where proteins are synthesised.
• CYTOPLASM - gel-like substance where most of the cell's chemical reactions happen.
• MITOCHONDRIA - where most of the reactions involved in respiration take place. Respiration provides energy for cell processes. Cells that needs lots of energy contain any mitochondria, eg: liver cells - which carry out lots of energy-demanding metabolic reactions and muscle cells - which need energy to contract (and cause movement)

Plant cell structures:

• NUCLEUS
• CYTOPLASM
• CHLOROPLASTS -  where photosynthesis happens
• CELL WALL - made of cellulose, supports the cell
• VACUOLE - a relatively large structure that contains cell sap, a weak solution of sugar and salts
• CELL MEMBRANE

Bacterial cells are a bit different:

Bacterial cells are smaller and simpler than plant and animal cells.

• CELL MEMBRANE
• CYTOPLASM
• CELL WALL
• Bacteria don't have chloroplasts or mitochondria.
• Bacterial cells don't have a 'true' nucleus - instead they have a singe circular strand of DNA that floats freely in the cytoplasm.

Chromosomes are made of DNA:

• Chromosomes are long molecules of coiled up DNA. The DNA is divided up into short sections called genes.
• DNA is a double helix (a double-stranded spiral). Each of the two DNA strands is made up of lots of small groups called 'nucleotides'.
• Each nucleotide contains a small molecule called a 'base'. DNA has just four different bases.
• You only need to know the four bases by their first initials - A, C, G and T.
• Each base forms cross links to a base on the other strand. This keeps the two DNA strands tightly wound together.
• A always pairs up with T, and C always pairs up with G. This is called complementary bass-pairing.

Watson and Crick were the first to model DNA:

• Scientists struggled for decades to work out the structure of DNA.
• Francis Crick and James Watson were the first scientists to build a model of DNA - they did it in 1953.
• They used data from other scientists to help them understand the structure of the molecule, eg:

• X-ray data showing that DNA is a double helix formed from tow chains wound tightly together.
• Other data showing that the bases occurered in pairs.
• By putting this information together they were able to build a model showing what DNA looks like.
• Don't forget, new discoveries like Watson and Crick's aren't widely accepted straight away. Other scientists need to repeat the work first to make sure the results are reliable.

DNA can replicate itself:

• DNA copies itself everytime a cell divides, so that each new cell still has the full amount of DNA.
• In order to copy itself, the DNA double helix first 'unzips' to form two single strands.
• New nucleotides (floating freely in the nucleus) then join on using complementary base-pairing (A with T and C with G). This makes an exact copy of the DNA on the other strand. 
• The result is two double-stranded molecules of DNA that are identical to the original molecule of DNA.

Proteins are made by reading the code in DNA:

• DNA controls the production of proteins (protein synthesis) in a cell.
• A section of DNA that codes for a particular protein is called a gene.
• Proteins are made up of chains of molecules called amino acids. Each different protein has its own particular number and order of amino acids.
• This gives each protein a different shape, which means each protein can have a different function.
• It's the order of the bases in a gene that decides the order of amino acids in a protein.
• Each amino acid is coded for by a sequence of three bases in a gene
• The amino acids are joined together to make proetins, following the order of the bases in the gene.
• Each gene contains a different sequence of bases - which is what allows it to code for a unique protein.

mRNA carries the code to the ribsomes:

• Proteins are made in the cell cytoplasm by tiny structures called ribosomes.
• To makes protein, ribsomes use the code in the DNA. DNA is found in the cell nucleus and can't move out of it because it's really big. So the cell needs to get the code from the DNA to the ribsome.

• This is done using a molecule called mRNA - which is made by copying the code from DNA.
• The mRNA acts as a messenger between the DNA and the ribosome - it carries the code between the two.

DNA controls a cell by controlling protein production:

• The protein produced in a cell affect how it functions. Some of them determine cell structure, others (like enzymes) control cell reactions.
• Different types of cell have different functions because they make different proteins.
• They only make certain proteins because only some of the full set of genes is used in any one cell. Some genes are 'switched off', which means the proteins they code for aren't produced.
• The genes that are switched on determine the function of the cell, eg: in a muscle cell only the gene for that code for muscle cell proteins are switched on. Genes that code for proteins specific to bone, nerve or skin cells are all switched off. This allows the muscle cell to function as a muscle cell.

Proteins have many different functions:

There are hundreds of different proteins and they all have different functions. You need to know four examples:

• ENZYMES - see below
• CARRIER MOLECULES - used to transport smaller molecules, eg: haemoglobin (found in red blood cells) binds to oxygen molecules and transports them around the body.
• HORMONES - used to carry messages around the body, eg: insulin is a hormone released into the blood by the pancreas to regulate the blood sugar level.
• STRUCTURAL PROTEINS - are physically strong, eg: collagen is a structural protein that strengthens connective tissues (like ligaments and catilage).

Enzymes control cell reactions:

• Cells have thousands of different chemical reactions going on inside them all the time - like respiration, photosynthesis and protein synthesis.
• These reactions need to be carefully controlled - to get the right amounts of substances and keep the organism working properly.

• You can usually make a reaction happen more quickly by raising the temperature. This would speed up the useful reactions but also the unwanted ones too which isn't good. There's also a limit to how far you can raise the temperature inside a living creature befores its cells start getting damaged.
• So living things produce ENZYMES, which act as BIOLOGICAL CATALYSTS. A catalyst is a substance that speeds up a reaction, without being changed or used up in the reaction itself.
• Enzymes reduce the need for high temperatures and we only have enzymes to speed up the useful chemical reactions in the body.
• Every biological reaction has its own enzyme designed especially for it.
• Each enzyme is coded for by a different gene and has a unique shape which it needs to do its job.

Enzymes are very specific:

• Chemical reactions usually involve things either being split apart or joined together.
• The substrate is the molecule changed in the reaction.
• Every enzyme has an active site - the part where it joins on to its substrate to catalyse the raeaction.

• Enzymes are really picky - they usually only work with one substrate. This can also be phrased as: enzymes have a high specificity for their substrate.
• This is because, for the enzyme to work, the substrate has to fit into the active site. If the substrate's shape doesn't match the active site's shape, then the reaction won't be catalysed. This is called the 'lock and key' mechanism, because the substrate fits into the enzyme just like a key fits into a lock.

Enzymes like it warm but not too hot:

• Changing the temperature changes the rate of an enzyme-catalysed reaction.
• Like with any reaction, a higher temperature increases the rate at first. This is because more heat means the enzymes and substrate particles have more energy. This makes the enzymes and the substrate particles move about more, so they're more likely to meet up and react - they have a higher collision rate.
• Low temperatures have the opposite effect - there's a lower collision rate and so a slower reaction.
• If it gets too hot, some of the bonds holding the enzyme together will break.
• This makes the enzyme lose its shape - its active site doesn't fit the shape of the substrate any more, this means it can't catalyse the reaction and the reaction stops - the enzyme can't function.
• The enzyme is now said to be denatured. Its change in shape is irreversable.
• Each enzyme has its own optimum temperature when the reaction goes fastest. This is the temperature just before it gets too hot and starts to denature. The optimum temperature for the most important human enzymes is about 37*c - the same temperature as our bodies.

Enzymes like the right pH too:

• The pH also has an effecton enzymes.
• If the pH is too high or too low, it interferes with the bonds holding the enzyme together. This changes the shape of the active site and denatures the enzyme.
• All enzymes have an optimum pH that they work best at. It's often neutral pH 7 but not always. For example, pepsin is an enzyme used to break down proteins in the stomach. It works best at pH 2, which means it's well suited to the acidic conditions in stomaches.

Q10 values show how rate of reaction changes with temperature:

• The Q10 value for a reaction shows how much the rate changes when the temperature is raised by 10*c
• You can calculate it by using this equation: Q10 = rate at higher temp / rate at lower temp

Gene mutations are changes to genes:

• A mutation is a change in the DNA base sequence.
• If a mutation occurs within a gene, it could stop the production of the protein the gene normally codes for - or it might mean a different protein is produced instead.

Most mutations are harmful:

• Producing the wrong protein or no protein at all can be a bit of a disaster - especially if the protein is an important enzyme or something.
• If a mutation occurs in reproductive cells, then the offspring might develop abnormally or die at an early stage of their development.
• If a mutations occurs in body cells, the mutant cells can sometimes start to multiply in an uncontrolled way and invade other parts of the body. This is cancer.

Some mutations are beneficial, some have no effect:

• Occasionally, a different protein might be produced after a mutation that actually benefits the organism - the new protein is an improvement on the one it was supposed to be.
  

• This gives the organism a survival advantage over the rest of the population. It passes on the mutated gene to its offspring, and they survive better too, so soon the mutation becomes common in the population.
• This is natural selection and evolution at work. A good example is a mutation in a bacteria that is resistant to antibiotics, so the mutant gene lives on, creating a resistant 'strain' of bacteria.
• Some mutations aren't harmful or helpful they - they don't change the protein being coded for, so they have no effect on the organism.

Radiation and certain chemicals cause mutations:

Mutations can happen spontaneously - when a chromosome doesn't quite copy itself properly. However, the chance of a mutation is increased if your exposed to:

• a) Ionising radiation, including X-rays and ultraviolet light, together with radiation from radioactive substances. For each of these examples, the greater the dose of radiation, the greater the dose of radiation, the greater the chance of mutation.
• b) Certain chemicals which are known to cause mutations. Such as chemicals are called mutagens. If the mutations produce cancer then the chemicals are often called carcinogens. Cigarette smoke contains chemical mutagens (or carcinogens).

Being multicellular has some some important advantages:

There's nothing wrong with single-celled organisms - they're pretty successful. Bacteria, for example, aren't in danger of extinction any time soon. But there are some big advantages in being multicellular and so some organisms have evolved that way. Here are some advantages:

• Being multicellular means you can be bigger. This is great because it means you can travel further, get your nutrients in a variety of different ways, fewer things can eat or squash you etc.
• Being multicellular allows for cell differentation. Instead of being just one cell that has to do everything, you can have different types of cells that do different jobs. Your cells can be specially adapted for their particular jobs, eg: carrying oxygen in the blood, reacting to light in the eyes.
• This means multicellular organisms can be more complex - they can have specialised organs, different shapes and behaviour - and so can be adapted specifically to their particular environment.

However, being multicellular means that an organism also has to have specialised organ systems, including:

• A system to communicate between different cells, eg: a nervous system.
• A system to supply cells with the nutrients they need, eg: circulatory system.
• A system that controls the exchange of substances with the environment, eg: respiratory system.

Mitosis makes new cells for growth and repair:

Mitosis is when a cell reproduces itself by splitting to form two identical offspring.

This happens in the body when you want identical cells - eg: when you want to grow and you need lots of the same type of cell or when you need to replace worn-out cells and repair tissues. The important thing to understand in mitosis is what happens to the DNA.

• 1) Before mitosis starts, the DNA in the cell is replicated.
• 2) Then at the beginning of mitosis, the DNA coils into double-armed chromosomes. These arms are exact copies of each other - they contain exactly the same DNA.
• 3) The chromosomes line up at the centre of the cell, and then divide as cell fibres pull them apart. The two arms of each chromosomes go to opposite poles (ends) of one cell. Membranes form around each of these two different sets of chromosomes.

• A system to communicate between different cells, eg: a nervous system.
• A system to supply cells with the nutrients they need, eg: circulatory system.
• A system that controls the exchange of substances with the environment, eg: respiratory system.

Mitosis makes new cells for growth and repair:

Mitosis is when a cell reproduces itself by splitting to form two identical offspring.

This happens in the body when you want identical cells - eg: when you want to grow and you need lots of the same type of cell or when you need to replace worn-out cells and repair tissues. The important thing to understand in mitosis is what happens to the DNA.

• 1) Before mitosis starts, the DNA in the cell is replicated.
• 2) Then at the beginning of mitosis, the DNA coils into double-armed chromosomes. These arms are exact copies of each other - they contain exactly the same DNA.
• 3) The chromosomes line up at the centre of the cell, and then divide as cell fibres pull them apart. The two arms of each chromosomes go to opposite poles (ends) of one cell. Membranes form around each of these two different sets of chromosomes.

• 4) The cytoplasm divides, and you get two new cells containing exactly the same genetic material.
• 5) And that's mitosis. You've ended up with two new cells that are genetically identical to each other. Before these can divide again, the DNA has to replicate itself to give each chromosome two arms again.