Biology OCR Gateway A - B1
Cell-Level Systems
B1.1
Cells
Eukaryotes are made from complex cells - they are called eukaryotic cells (e.g animals and plants)
Prokaryotes are made from smaller and simpler cells - they are called prokaryotic cells (e.g bacteria)
In both animal and plant cells…
● Nucleus: Contains DNA (genetic material) in the form of chromosomes that controls the
cell’s activities.
● Cytoplasm: Where most of the chemical reactions happen.
● Mitochondria: Site of cellular respiration and contains the enzymes needed for the
reactions involved.
● Cell membrane: controls what goes in and out by providing a barrier. They contain
receptor molecules for cell communication
But only in plant cells…
● Cell wall: made of cellulose and provides support
● Chloroplasts: Where photosynthesis occurs. Contains
a green substance called chlorophyll.
In prokaryotic cells such as bacteria…
● Chromosomal DNA: One long strand of DNA floating freely in the cytoplasm
● Plasmids: They are extra loops of DNA that aren’t part of the chromosome.
● Cell Membrane: Controls what goes in and out of the cell and is also supported by a cell wall.
Microscopy
Microscopes are used to magnify images, they increase the resolution which allows us to see more detail. They are used in a variety of research
projects especially in the study of microscopic things such as cells.
There are two types of microscopes: light microscopes and electron microscopes.
Light Microscopes:
● Used to see things like nuclei and chloroplasts
● Uses light and magnifiers to see the image through a lense
Electron Microscopes:
● Allows further smaller things to be seen in more detail such as the mitochondria.
● Works by using electrons to ‘print’ a magnified image of the cell or other things.
● They can also see plasmids and viruses
● An example includes TEMs (Transmission Electron Microscopes) - higher magnification and resolution but not portable and expensive.
Furthermore, they are complicated to use as preparations are more difficult.
Use of Light Microscopes
The parts of the microscope:
● Eyepiece lens: looked through to see the image and also magnified it.
● Objective lense: magnifies the image, there are usually three different objective lenses
● Stage: where the slide is placed, supports it.
● Clip: holds the slide in place.
● Handle: To carry the microscope with.
● Lamp: shines the light.
● Coarse adjustment knob: adjusts the stage up and down
● Fine adjustment knob: adjusts the fine focus on the viewing image - used when viewing
at all magnifications especially at higher ones.
Preparing the specimen:
, It is very important that the specimen is prepared before the investigation otherwise it might not be visible.
1. Use a thin slice of specimen.
2. Put it on a clean slide and use a pipette to put one drop of water on it to secure it in place.
3. Add a drop of stain if needed - especially if the specimen is transparent or colourless. It will make it easier to see the specimen.
Different stains are used to highlight different structures or tissues.
4. Place a coverslip, ensure that there are no air bubbles so press down gently.
How to use a microscope:
1. Clip the slide onto the stage
2. Select the lowest-power objective lens.
3. Use the coarse adjustment knob to move the stage up just below the objective lens
4. Look down at the eyepiece and use the fine adjustment knob to adjust its focus.
5. Repeat the steps but with a higher-power objective lens making sure you put down the stage first to avoid the lens from making
contact on the coverslip of the slide.
Calculation Magnification
Total magnification = eyepiece lens magnification x objective lens magnification
Or
Magnification = image size / real size
B1.2
DNA
DNA contains all of an organism’s genetic material. It contains a genetic code that determines the characteristics of a living organism.
Chromosomes are long molecules of coiled up DNA. The DNA is divided up into short sections called genes.
DNA Structure
DNA is a double helix or in other words a double-stranded spiral. Each of the DNA strands is made up of lots of nucleotides joined together in
a long chain which makes DNA a polymer.
Each nucleotide contains a small molecule called “base”. DNA has four different bases which are:
● A - Adenine
● T- Thymine
● C- Cytosine
● G- Guanine
Each base forms cross-links to a base on the other strand. This keeps the two strands tightly wound together. A always pairs up with T and C is
always with G. They are complementary bases that only pair up with their specific complementary base.
Nucleotides
● Each DNA nucleotide has the same sugar and a phosphate group. The base on each nucleotide is
the only part of the molecule that varies.
● The base is attached to the sugar.
DNA is a polymer
● Polymers are large, complex molecules composed of long chains of monomers joined together.
● Monomers are small, basic molecular units.
● DNA is a polymer made up of nucleotide monomers.
Cell-Level Systems
B1.1
Cells
Eukaryotes are made from complex cells - they are called eukaryotic cells (e.g animals and plants)
Prokaryotes are made from smaller and simpler cells - they are called prokaryotic cells (e.g bacteria)
In both animal and plant cells…
● Nucleus: Contains DNA (genetic material) in the form of chromosomes that controls the
cell’s activities.
● Cytoplasm: Where most of the chemical reactions happen.
● Mitochondria: Site of cellular respiration and contains the enzymes needed for the
reactions involved.
● Cell membrane: controls what goes in and out by providing a barrier. They contain
receptor molecules for cell communication
But only in plant cells…
● Cell wall: made of cellulose and provides support
● Chloroplasts: Where photosynthesis occurs. Contains
a green substance called chlorophyll.
In prokaryotic cells such as bacteria…
● Chromosomal DNA: One long strand of DNA floating freely in the cytoplasm
● Plasmids: They are extra loops of DNA that aren’t part of the chromosome.
● Cell Membrane: Controls what goes in and out of the cell and is also supported by a cell wall.
Microscopy
Microscopes are used to magnify images, they increase the resolution which allows us to see more detail. They are used in a variety of research
projects especially in the study of microscopic things such as cells.
There are two types of microscopes: light microscopes and electron microscopes.
Light Microscopes:
● Used to see things like nuclei and chloroplasts
● Uses light and magnifiers to see the image through a lense
Electron Microscopes:
● Allows further smaller things to be seen in more detail such as the mitochondria.
● Works by using electrons to ‘print’ a magnified image of the cell or other things.
● They can also see plasmids and viruses
● An example includes TEMs (Transmission Electron Microscopes) - higher magnification and resolution but not portable and expensive.
Furthermore, they are complicated to use as preparations are more difficult.
Use of Light Microscopes
The parts of the microscope:
● Eyepiece lens: looked through to see the image and also magnified it.
● Objective lense: magnifies the image, there are usually three different objective lenses
● Stage: where the slide is placed, supports it.
● Clip: holds the slide in place.
● Handle: To carry the microscope with.
● Lamp: shines the light.
● Coarse adjustment knob: adjusts the stage up and down
● Fine adjustment knob: adjusts the fine focus on the viewing image - used when viewing
at all magnifications especially at higher ones.
Preparing the specimen:
, It is very important that the specimen is prepared before the investigation otherwise it might not be visible.
1. Use a thin slice of specimen.
2. Put it on a clean slide and use a pipette to put one drop of water on it to secure it in place.
3. Add a drop of stain if needed - especially if the specimen is transparent or colourless. It will make it easier to see the specimen.
Different stains are used to highlight different structures or tissues.
4. Place a coverslip, ensure that there are no air bubbles so press down gently.
How to use a microscope:
1. Clip the slide onto the stage
2. Select the lowest-power objective lens.
3. Use the coarse adjustment knob to move the stage up just below the objective lens
4. Look down at the eyepiece and use the fine adjustment knob to adjust its focus.
5. Repeat the steps but with a higher-power objective lens making sure you put down the stage first to avoid the lens from making
contact on the coverslip of the slide.
Calculation Magnification
Total magnification = eyepiece lens magnification x objective lens magnification
Or
Magnification = image size / real size
B1.2
DNA
DNA contains all of an organism’s genetic material. It contains a genetic code that determines the characteristics of a living organism.
Chromosomes are long molecules of coiled up DNA. The DNA is divided up into short sections called genes.
DNA Structure
DNA is a double helix or in other words a double-stranded spiral. Each of the DNA strands is made up of lots of nucleotides joined together in
a long chain which makes DNA a polymer.
Each nucleotide contains a small molecule called “base”. DNA has four different bases which are:
● A - Adenine
● T- Thymine
● C- Cytosine
● G- Guanine
Each base forms cross-links to a base on the other strand. This keeps the two strands tightly wound together. A always pairs up with T and C is
always with G. They are complementary bases that only pair up with their specific complementary base.
Nucleotides
● Each DNA nucleotide has the same sugar and a phosphate group. The base on each nucleotide is
the only part of the molecule that varies.
● The base is attached to the sugar.
DNA is a polymer
● Polymers are large, complex molecules composed of long chains of monomers joined together.
● Monomers are small, basic molecular units.
● DNA is a polymer made up of nucleotide monomers.
