B1

Light Microscopes (1600s)

• Use a beam of light to form an image of an object
• Can magnify up to around x2,000 and resolution of 20nm
• Cheaper than electron microscopes
• Can be used anywhere
• Can magnify live specimens

Electron Microscopes (1930s)

• Can see subcellular structures
• Use a beam of electrons to form an image
• Can have a magnification of up to x2,000,000
• Larger than light microscopes and much more expensive
• Have to be kept in pressure, humidity and temperature controlled rooms

Transmission Electron Microscopes (TEM) - high resolution (0.2nm) and magnification 2D images

Scanning Electron Microscope (SEM) - 3D images with lower magnification and resolution (10nm)

Magnification - The magnification is how enlarged an image is.

Magnification - Size of Image / Size of Real Object

Resolution - The resolution is the ability to distinguish between 2 separate points, so how detailed the image is.

The average animal cell is 10-30um in length.

 All animal cells contain:

• A Nucleus - The nucleus controls the activities of the cell and is surrounded by a nuclear membrane. It containes the DNA on trhe genes on the chromosomes that give the cells its necessary instructions to build new protiens for making new cwlls and organisms. The average diameter is 10um.
• Cytoplasm - The cytoplasm is the gel in which organelles are suspended and chemical reactions, such as respiration occur.
• A Cell Membrane - This controls the passage of substances in and out of the cell.
• Mitochondria - These are the site of aerobic respiration and are around 1um in diameter.
• Ribosomes - The ribosomes are where protien synthesis takes place which makes the protiens that are needed in the cell.

Plant cells range from 10-100um in length.

All plant cells  contain:

• A Nucleus - The nucleus controls the activities of the cell and is surrounded by a nuclear membrane. It containes the DNA on trhe genes on the chromosomes that give the cells its necessary instructions to build new protiens for making new cwlls and organisms. The average diameter is 10um.
• Cytoplasm - The cytoplasm is the gel in which organelles are suspended and chemical reactions, such as respiration occur.
• A Cell Membrane - This controls the passage of substances in and out of the cell.
• Mitochondria - These are the site of aerobic respiration and are around 1um in diameter.
• Ribosomes - The ribosomes are where protien synthesis takes place which makes the protiens that are needed in the cell.
• A Cell Wall - The cell wall is made from cellulose and helps to strengthen and support the cell.
• Chloroplasts - These contain chlorophyll and are able to absorb light so that a plant can produce glucose through photosynthesis.
• A Permanent Vacuole - These are a space in the cytoplasm that are filled with cell sap and keep trhe cells rigid.

Eukaryotic Cells

Eukaryotic cells al have a cell membrane, cytoplasm and a nucleus that contains genetic material. All animals, plants, fungi, protisoa are eukaryotes (made up of eukaryotic cells).

Prokaryotic Cells

Prokaryotes are single celled living organisms that are 02.-2um in length. Prokaryotic cells have cytoplasm and a cell membrane which is surrounded by a cell wall, however, unlike plant cells, the cell wll is not made of cellulose. In prokaryotic cels, the genetic material is not stored in the nucleus, but in a single loop of DNA that is suspended in the cytoplasm. They may also conain smaller rings of DNA called plasmids which contain information on specific features, such as antibiotic resistance. An example of a prokaryote is bacteria that can also have a flagella / flagellum that enable it to move around; some also have a protective slime capsule.

Cells can be specialised to carry out a particular job. As an organism develops, its cells differentiate to become different type sof specialised cells. Animal cells tend to differentiate atr an early stage, where as plant cells have the ability to specialise throughtout life. Some specialised cells work individually, but others are adapted to work as part of a tissue.

Cells can be specialised to increase their surface area (for osmosis, active transport and diffusion), allow them to move efficiently, enable them to respire more, have the ability to photosynthesise and to transport minerals, gases and water to where they are needed.

Diffusion is the spreading out of gas particles of solute particles. The net (overall) movement of the particles is from an area of high concentration to an area of low concentration. The random movement of the particles causes them to bump into each other and spread out.

 If there is a large difference in concentration between the area of high concentration and the area of low concentration, the rate of diffusion is much faster than if there was a smaller distance. The difference between the two areas of concentration is called the concentration gradient; the steeper the gradient, the faster the rate of diffusion.

Net Movement = Particles Moving In - Particles Moving Out

As the temperature increases, the particles gain more energy, making them move faster. This increases the rate of diffusion as the particles bump into each other more frequently.

Dissolved substances and gases move in and out of animal cells across the cell membrane; some of these include: oxygen, carbon dioxide, glucose and urea which is the waste product of the break down of amino acids in the liver. Gas exchange also happens by diffusion.

Cells are adapted to have a larger surface area to increase the rate of diffusion.

Osmosis happens across a partially permeable membrane which only allows water to move in and out of them. Water molecules cluster around a solute when it is dissolved, so there are less 'free' molecules which can diffuse to other areas. Water Potential is the concentration of free water molecules. Water diffuses by osmosis from a region of high water potential to a region of low water potential.

Osmosis also has a concentration gradient, like diffusion, where the steeper the gradient, the faster the rate of osmosis. If the concentration of solutes is the same inside and outside the cell, the solution is isotonic to the cell. If the concentration of the solutes outide the cell are higher than the concentration inside the cell, then the solution is hypertonic to the cell.When the concentration is higher inside than outside the cell, the solution is hypotonic to the cell.

In animal cells, the cytoplasm should be isotonic to the surrounding fluid, however, if the fluid becomes hypotonic, water moves into the cell by osmosis, causing it to swell up and sometimes even burst. If the solution is hypertonic to the cell, it can shrivel up as the water exits the cell across the membrane.Osmosis moves into plants, filling the vacuole and making them turgid (rigid) which helps to support the plant. If the plant loses water, the plant begins to wilt (becomes flaccid) and if too much water is lost the vacuole and cytoplasm can shrink and pull away from the cell wall (become plasmolysed. Sometimes, the plant can not recover if its cells are plasmolysed, causing the plant to die.

Active transport occurs when substances have to be moved across a semi permeable membrane against the concentration gradient, meaning that the substances move from an area of low concentration to an area of high concentration. Active transport requires energy to do this which comes from respiration. This allows cells to absorb ions from very dilute solutions and transpot ions from one place to another across the cell membrane.

Active transport is used in the root hairs cells because the soil has a lower concentration of mineral ions than inside the cell. Eventhough it is against the concentration gradient, the cells are able to absorb these ions as a result of active transport. Another example of active transport, is the absorption of glucose from the gut which is foten against a very steep concentration gradient.

 Some cells which have to undertake a lot of active transport are adapted so that they have lots of mitochondria, which are the site of respiration, allowing them to have a lot of energy.

The larger the object, the lower the surface area to volume ratio. If an object has a higher surface area to volume area, then the rate of exchange (osmosis, diffusion, active transport) is quicker as there is more membrane for the exchange to take place across.

 In very large organisms, the small ratio can mean that substances are not able to reach every cell and waste can not be removed fast enough. Therefore, surfaces within the organisms have been adapted so that the rate of exchange increases. These adaptions include: having a large surface area, having a thin membrane so that the exchange only happens over a short distance, efficient blood supply in animals that removes waste and helps to maintain a steep concentration gradient and animals being ventilated which maintains a steep concentration gradient ofr gas exchange.