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What are eukaryotic cells?

Eukaryotic cells are complex and include all animals and plant cells.
Eukaryotes are organisms that are made up of eukaryotic cells.


What are prokaryotic cells?

Prokaryotic cells are smaller and simpler, e.g. bacteria.
A prokaryote is a prokaryotic cell (it's a single-celled organism).


What are the components of a animal cell?

Nucleus - contains genetic material that controls the activities of the cell.
Cytoplasm - gel-like substance where most of the chemical reactions happen. It contains enzymes that control these chemical reactions.
Cell membrane - holds the cell together and controls what goes in and out.
Mitochondria - these are where most of the reactions for aerobic respiration take place. Respiration transfers energy that the cell needs to work.
Ribosomes - these are where proteins are maid in the cell.


What are additional components of a plant cell?

Rigid cell wall - made of cellulose. It supports the cell and strengthens it.
Permanent vacuole - contains cell sap, a weak solution of sugar and salts.
Chloroplast - these are where photosynthesis occurs, which makes food for the plant. They contain a green substance called chlorophyll, which absorbs the light needed for photosynthesis.


What are the components of a bacteria cell?

Cell membrane
Cell wall
A single circular stand of DNA - that floats freely in the cytoplasm.
Plasmids - small rings of DNA.


What are the different types of microscopes?

Light Microscopes:
Use light and lenses to form an image of a specimen and magnify it. They let us see individual cells and large sub-cellular structures, like nuclei.

Electron Microscopes:
Use electrons instead of light to form an image. They have a much higher magnification than light microscopes.
They also have a higher resolution.
Electron microscopes let us see much smaller things in more detail, like the internal structures of mitochondria and chloroplasts. The even let us see tinier things like ribosomes and plasmids.


What is the equation for magnification?

Magnification (e.g.x100) = Image size / Real size


How do you prepare a microscope slide for onion cells?

1) Add a drop of water to the middle of a clean side.
2) Cut up an onion and separate it out into layers. Use tweezers to peel off some epidermal tissue from the bottom of one of the layers.
3) Using tweezers, place the epidermal tissue into the water on the slide.
4) Add a drop of iodine solution. Iodine solution is a stain. Stains are used to highlight objects in a cell by adding colour to them.
5) Place a cover slip on top. To do this, stand the cover slip upright on the slide, next to the water droplet. Then carefully tilt and lower it so it covers the specimen. Try not to get any air bubbles under there - they'll obstruct your view of the specimen.


How do you use a light microscope to look at the slide?

1) Clip the slide you've prepared onto the stage.
2) Select the lowest-powered objective lens.
3) Use the coarse adjustment knob to move the stage up to just below the objective lens.
4) Look down the eyepiece. Use the coarse adjustment knob to move the stage downwards until the image is roughly in focus.
5) Adjust the focus with the fine adjustment knob, until you get a clear image of what's one the slide.
6) If you need to see the slide with greater magnification, swap to a higher-powered objective lens and refocus.


How should you draw observation drawings?

A pencil with a sharp point.
Clear unbroken lines.
Takes up at least half the space.
No colouring or shading.
Sub-cellular structures drawn to proportion.
Title and magnification.
Label important features using straight, uncrossed lines.


What is differentiation?

The process by which a cell change to become specialised for its job.
As cells change, they develop different sub-cellular structures and turn into different types of cells. This allows them to carry out specific functions.


What are examples of specialised cells?

Sperm cells are specialised for reproduction:
Long tail and streamlined head to help swim.
Lots of mitochondria to provide energy.
Carries enzymes in head to digest through egg membrane.

Nerve cells are specialised for rapid signalling:
The cells are very long to core move distance.
They have branched connections to form a network throughout the body.

Muscle cells are specialised for contraction:
They are long so they have space to contract.
Lots of mitochondria to generate the energy needed for contraction.

Root hair cells are specialised for absorbing water and minerals:
Have a large surface area for absorbing water and mineral ions.
Many hairs to maximise of finding water and mineral ions.

Phloem and Xylem cells are specialised for transporting substances:
Xylem cells is hollow in the centre and phloem cells have very few sub-cellular structures, so that stuff can flow through them.


What are chromosomes?

Most cells in your body have a nucleus. The nucleus contains genetic material in the form of chromosomes.
Chromosomes are coiled up lengths of DNA molecules.
Each chromosome carries a large number of genes. Different genes control the development of different characteristics, e.g. hair colour.
Most people have 23 pairs of chromosomes.


What is the process of mitosis?

1) In a cell that's not dividing, the DNA is all spread out in long strings.
2) Before it divides, the cell has to grow and increase the amount of sub-cellular structures such as mitochondria and ribosomes.
3) It then duplicates its DNA - so there's one copy for each new cell. The DNA is copied and forms X-shaped chromosomes. Each 'arm' of the chromosome is an each duplicate of the other.
4) The chromosomes line up at the centre of the cell and cell fibres pull them apart. The two arms of each chromosome go to opposite ends of the cell.
5) Membrane form around each of the sets of chromosomes. These become the nuclei of the two new cells - the nucleus has divided.
6) Lastly, the cytoplasm and cell membrane divide.

Two new daughter cells are created they are identical to the parent cell.


What is binary fission?

1) The circular DNA and plasmid(s) replicate.
2) The cell get bigger and the circular DNA strands move to opposite 'poles' (ends) of the cell.
3) The cytoplasm begins to divide and new cell walls begin to form.
4) The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA, but can have a variable number of copies of the plasmid(s).


What is the process to grow bacteria?

1) Bacteria are cultured in a "culture medium".
2) The culture medium used can be a nutrient broth solution or solid agar jelly.
3) Bacteria grown on agar 'plates' will form visible colonies on the surface of the jelly, or will spread out to give and even covering of bacteria.
To make an agar plate, hot agar jelly is poured into a shallow round plastic dishes called Petri dishes.
When the jelly's cooled and set, inoculating loops can be used to transfer micro-organisms to the culture medium. Alternatively, a sterile dropping pipette and spreader can be used to get an even covering of bacteria.
The micro-organisms can then multiply.


How can you investigate the effects of antibiotics on bacterial growth?

1) Place paper discs soaked in different types of antibiotics on an agar plate that has an even covering of bacteria. Leave some space between the discs.
2) The antibiotic should diffuse into the agar jelly. Antibiotic-resistant bacteria will continue to grow on the agar around the paper discs, but non-resistant strains will die. A clear area will be left where the bacteria has died called the inhibition zone.
3) Make sure you use a control. This is a paper disc that has not been soaked in an antibiotic. Instead, soak it in sterile water. To check if it is the antibiotic causing the effect rather than the paper.
4) Leave the plate for 48 hrs at 25oC.
5) The more effective the antibiotic, the larger the inhibition zone.


How do you sterilise culturing equipment?

1) The Petri dishes and culture medium must be sterilised before use (heating to a high temperature), to kill any unwanted micro-organisms.
2) If an inoculating loop is used to transfer the bacteria to the culture medium, it should be sterilised first by passing it through a hot flame.
3) After transferring the bacteria, the lid of the Petri dish should be lightly taped on - to stop micro-organisms from the air getting in.
4) The Petri dish should be stored upside down - to stop condensation falling onto the agar surface.


What are stem cells?

They are undifferentiated cells that can divid to produce lots more undifferentiated cells, and can differentiate into different types of cells.

Stem cells can be found in early human embryos.

Adult stem cells can be found in bone marrow, unlike embryonic stem cells they can't turn into any cell type, only cells like red blood cells.


How can stem cells be used to cure diseases?

Medicine already uses adult stem cells to replace faulty blood cells in patients.
Embryonic stem cells could also used to replace faulty cells, as well as make insulin-producing cells for people with diabetes, and nerve cells for people paralyzed by spinal injuries.
Can be used for therapeutic cloning to make the same genetic information as the patient.


What are the ethical pros and cons of stem cells?

Mostly uses unwanted embryos from fertility clinics, which would have probably been destroyed anyway.
Suffering is more important than the right of embryos.

Killing a potential human life.
Could concentrate on finding other sources of stem cells.


How are stem cells used in plants?

Stem cells are found in the meristems (parts of the plant where growth occurs), which can differentiate into any type of plant cell.
Them stem cells can be used to produce clones quickly and cheaply.


What is diffusion?

The spreading out of particles from an area of higher concentration to an area of lower concentration.

Diffusion can be found in the lungs as oxygen moves from a high concentration in the alveoli to a low concentration in the blood.


What is osmosis?

The movement of water molecules across a partially permeable membrane from a region of higher concentration to a region of lower water concentration.


What is active transport?

Active transport allows for plants to absorb minerals from a very dilute solution, against a concentration gradient. This is essential for its growth, but active transport needs energy from respiration.

Active transport also happens in humans, for example in taking glucose from the gut, and from the kidney tubules.


Why do organisms need exchange surfaces?

In a single-celled organisms, gases and dissolved substance can diffuse directly into the cell across the cell membrane. It's because they have a large surface area compared to their volume of the cell.

Multicellular organisms have a smaller surface area compared to their volume - not enough substance can diffuse from their outside surface to supply their entire volume. This means they need some sort of exchange surface for efficient diffusion. The exchange surface structures have to allow enough of the necessary substances to pass through.


How are exchange surfaces adapted to be efficient?

They have a thin membrane, so substances only have a short distance to diffuse.
They have a large surface area so lots of a substance can diffuse at once.
Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly.
Gas exchange surfaces in animals (e.g. alveoli) are often ventilated too - air moves in and out.


What is the job of the lungs and how are alveoli specialised for diffusion?

The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it.
To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place.

An enormous surface area.
A moist lining for dissolved gases.
Very thin walls.
A good blood supply.


What are villi?

The inside of the small intestine is covered in millions and millions of these tiny little projections called villi.
They increases the surface area in a big way so that digested food is absorbed much more quickly into the blood.

A single layer of surface cells.
A very good blood supply to assist quick absorption.


How are leaves structured for diffusion of gases?

Carbon dioxide diffused into the air spaces within the leaf, then it diffuses into the cells where photosynthesis happens.
The underneath of the leave is the exchange surface. It is covered in tiny little hole called stomata, in which oxygen and water vapour diffuse out of.
The size of stomata are controlled by guard cells, which close if the plant is losing water faster than it can replenish.
The flattened shape of the leaf increases the area of this exchange surface so that it's more effective.
The walls of the cells inside the lead form another exchange surface. The air spaces inside the leaf increases the area of this surface so there's more chance of carbon dioxide getting into the cells.