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S3 Cell Biology – Summary Notes
Cells
All living things are made up of cells.
They are the basic building blocks of life – some organisms are made of only one and others, like humans, are made of trillions.
Different types of cells contain different things.
Animal Cells
There are three main structures in an animal cell.
The nucleus – this controls all the cell’s activities
The cell membrane – this controls what substances enter and exit the cell
The cytoplasm – the site of all the chemical reactions in a cell
Plant Cells
Plant cells are very important for life on earth!
Like animal cells, they are tiny and must be viewed under a microscope.
There are some similarities and some differences between animal and plant cells.
Like animal cells, plant cells contain a nucleus, cytoplasm and a cell membrane.
They have 3 additional structures:
- Cell wall
- Chloroplasts
- Vacuole
These all have a specific function that is vital for the survival of the plant.
The cell wall supports and strengthens the cell, giving it a rigid shape.
The cell wall is made of a special sugar called cellulose which humans cannot digest!
Chloroplasts are disc shaped structures found in the cytoplasm.
Chloroplasts contain a green pigment that traps light for photosynthesis.
The vacuole is a compartment in plant cells containing cell sap (water, salts, sugar).
The vacuole helps maintain water balance and pressure inside the cell.
Calculating Cell Size
Cells are too small to be measured in millimetres (mm).
Instead we measure them in micrometres (μm).
Cancer and Cell Division
Cancer happens when DNA inside a cell becomes damaged
This causes uncontrolled cell division.
The cells grow into a lump called a tumour
Some cells can break off and move through the blood, allowing the cancer to spread.
Cancer risk factors include:
Smoking Diet Body weight Sun Exposure Alcohol consumption Genes Lifestyle Age
DNA
Chromosomes are found in the nucleus of the cell
These are very long molecules of tightly coiled DNA.
Humans have 46 chromosomes (23 matching pairs). One pair comes from your mum and one pair comes from your dad.
DNA stands for Deoxyribonucleic Acid
It is a special molecule that carries all of our genetic information.
This a small section of DNA that are found on a chromosome
There are lots of genes on a single chromosome.
A gene carries the instructions to make a protein.
DNA structure
DNA is made up of two parts, a backbone and bases.
It contains two strands which wind around each other, and we call this shape a ‘double helix’.
DNA bases
There are 4 different DNA bases – A, T, C and G.
A always pairs with T and G always pairs with C
DNA profiling
DNA profiling is used in solving crimes, in paternity cases and to assess health risks.
This is only possible as everyone has unique DNA, unless you have an identical twin.
The UK has something called the ‘National DNA database’ where suspects’ DNA samples are stored, even if that person is then found innocent.
Inheritance
We have two copies of every gene.
One copy comes from our mother the other comes from our father.
Alleles are different forms of the same gene (e.g. blue and brown for the eye colour gene).
Rennet is an enzyme used when making cheese.
It works on milk, speeding up the clotting process (making lumps) and produces curds and whey.
Enzymes can either break down substrates into smaller molecules or build them up into bigger ones.
Enzymes are specific , as one enzyme can only speed up one reaction.
Enzymes are not changed by the reactions they speed up, meaning they can be used over and over again in cells.
Microorganisms
These are tiny living organisms that can only be seen using a microscope.
The two most common types of microorganisms used in industry are bacteria and fungi.
Structure of a Bacterial Cell
Unlike other cells, bacteria do not have a nucleus.
Instead they have one large loop of DNA.
In industry, bacteria are used to turn milk into yoghurt and cheese.
Bacteria convert the sugar in the milk ( lactose ) to lactic acid. This clots (thickens) the milk.
Fungal cells
Fungal cells do have a nucleus.
They also contain a vacuole for storing water.
Microorganisms in Industry
Microorganisms have many properties which make them a good choice for use in industry:
- Rapid growth
- Diverse use of food sources
- Wide range of products produced
- Microorganisms are used to produce bread, beer, wine, yoghurt and cheese.
They can also be used to make fuel (biofuel)
Yeast
Yeast are single-celled fungus that are used in the baking and brewing industries.
Yeast break down sugar to produce carbon dioxide gas and ethanol (alcohol) in a process called fermentation.
Fermentation word equation:
Yeast is used to make bread, wine and beer!
Photosynthesis
Animals have to EAT food to gain energy
When you trace any food back to the start, there is always a green plant.
Plants get their energy from sunlight!
They use this energy to build a small sugar called glucose, and store this as a larger sugar called starch.
