Biology

Plant and animal cells (eukaryotic cells) have a cell membrane, cytoplasm and

genetic material enclosed in a nucleus.

Bacterial cells (prokaryotic cells) are much smaller in comparison. They have

cytoplasm and a cell membrane surrounded by a cell wall. The genetic material

is not enclosed in a nucleus. It is a single DNA loop and there may be one or

more small rings of DNA called plasmids.

Most animal cells have the following parts:

• a nucleus • cytoplasm • a cell membrane • mitochondria • ribosomes.

In addition to the parts found in animal cells, plant cells often have:

• chloroplasts • a permanent vacuole filled with cell sap.

Plant and algal cells also have a cell wall made of cellulose, which strengthens

the cell.

The membrane around the contents of a cell that control what moves in and out

of the cell.

The water-based gel in which organelles of all living cells are suspended and most

of the chemical reactions of life take place.

Organelle found in many living cells containing the genetic information surrounded

by the nuclear membrane.

The site of aerobic cellular respiration in a cell.

The site of protein synthesis in a cell.

The organelles in which photosynthesis takes place.

Space in the cytoplasm filled with cell sap.

The rigid structure around the plant and algal cells, it is made of cellulose & strengthens the cell.

The complex carbohydrate that makes up the plant and algal cell walls and gives them strength.

1. Use a dropping pipette to put one drop of water onto a microscope slide.

2. Separate one of the thin layers of the onion.

3. Peel off a thin layer of epidermal tissue from the inner surface.

4. Use forceps to put this thin layer on to the drop of water that you have placed on the microscope slide.

5. Make sure that the layer of onion cells is flat on the slide.

6. Put two drops of iodine solution onto the onion tissue.

7. Carefully lower a coverslip onto the slide. Do this by placing one edge of the coverslip on the slide and then using a mounted needle to lower the other edge onto the slide.

8. Use a piece of filter paper to soak up any liquid from around the edge of the coverslip.

9. Put the slide on the microscope stage

0. Turn the nosepiece to the lowest power objective lens.

1. Looking from the side (not through the eyepiece) turn the coarse adjustment knob so that the end of the objective lens is almost touching the slide.

2. Now looking through the eyepiece, turn the coarse adjustment knob in the direction to increase the distance between the objective lens and the slide. Do this until the cells come into focus.

3. Now rotate the nosepiece to use a higher power objective lens.

4. Slightly rotate the fine adjustment knob to bring the cells into a clear focus and use the lowpower objective (×40 magnification) to look at the cells.

5. When you have found some cells, switch to a higher power (×100 or ×400 magnification).

6. In the space below make a clear, labelled drawing of some of these cells. Make sure that you draw and label any component parts of the cell.

7. Use an eyepiece graticule to measure the length of one of the epidermal cells that you have drawn. Remember to include the units.

8. Now measure the same cell in your drawing.

9. Calculate the magnification of your drawing, using the formula: magnification = length of drawing of cell/actual length of cell.

The head of the sperm contains the genetic material for fertilisation. The acrosome in the head contains enzymes so that the sperm can penetrate an egg. The middle piece is packed with mitochondria to release energy needed to swim and fertilise the egg. The tail enables the sperm to swim.

The nerve cell is extended, so that nerves can run to and from different parts of the body to the central nervous system. The cell has extensions and branches, so that it can communicate with other nerve cells, muscles and glands. The nerve cell is covered with a fatty sheath, which insulates the nerve cell and speeds up the nerve impulse.

Muscle cells contain filaments of protein that slide over each other to cause muscle contraction. The arrangement of these filaments causes the banded appearance of heart muscle and skeletal muscle. They contain many well-developed mitochondria to provide the energy for muscle contraction. In skeletal muscle, the cells merge so that the muscle fibres contract in unison.

The root hair cell has a large surface area to provide contact with soil water. It has thin walls so as not to restrict the movement of water.

There are no top and bottom walls between xylem vessels, so there is a continuous column of water running through them. Their walls become thickened and woody. They therefore support the plant.

Dissolved sugars and amino acids can be transported both up and down the stem.  Companion cells, adjacent to the sieve tubes provide energy required to transport substances in the phloem.

When cells express specific genes that characterise a certain type of cell we say that a cell has become differentiated. Once a cell becomes differentiated it only expresses the genes that produce the proteins characteristic for that type of cell. Differentiated cells are important in a multicellular organism because they are able to perform a specialised function in the body. However, specialisation comes at a cost. The cost is that the differentiated cells often lose the ability to make new copies of themselves. Multicellular organisms must therefore retain some unspecialised cells that can replenish cells when needed. These unspecialised cells are called stem cells.

Most types of animal cell differentiate at an early stage. Many types of plant

cells retain the ability to differentiate throughout life.

In mature animals, cell division is mainly restricted to repair and replacement. 

We need microscopes to study most cells. Microscopes are used to produce magnified images. There are two main types of microscope: Light microscopes are used to study living cells and for regular use when relatively low magnification and resolution is enough.  Electron microscopes provide higher magnifications and higher resolution images but cannot be used to view living cells

Diffusion is the movement of a substance from an area of high concentration to an area of low concentration.
Diffusion happens in liquids and gases because their particles move randomly from place to place.
Diffusion is an important process for living things; it is how substances move in and out of cells.

In living things, substances move in and out of cells by diffusion. For example:
Respiration produces waste carbon dioxide, causing the amount of carbon dioxide to increase in the cell. Eventually, the carbon dioxide concentration in the cell is higher than that in the surrounding blood. The carbon dioxide then diffuses out through the cell membrane and into the blood.

Different factors effect diffusion, such as: the difference in concentration the concentration of a substance is defined as the number of solute molecules that can be found within a given volume. Volumes of high concentration gradient have a large difference in the concentration of molecules over a unit length. A large difference in concentration leads to a greater probability of molecular collisions over the region and therefore increases the rate of diffusion. Generally, the greater the concentration gradient, the greater the rate of diffusion. The temperature as temperature increases the average kinetic energy of particles increases. Greater kinetic energies lead to increased velocities. The increased velocity means that there is a greater chance of collisions between particles, resulting in an increased rate of diffusion. Generally, the rate of diffusion increases with temperature, and the surface area of the membrane.

Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane.

It is a rigid structure that provides a plant cell with support and keeps non-woody plants upright.

Active transport is the process by which dissolved molecules move across a cell membrane from a lower to a higher concentration. In active transport, particles move against the concentration gradient - and therefore require an input of energy from the cell.

In humans, active transport takes place during the digestion of food in the small intestine.

When the cell increases in size, the volume increases faster than the surface area, because volume is cubed where surface area is squared. When there is more volume and less surface area, diffusion takes longer and is less effective.

For a cube, the equation for surface area is S=6*L*L, where L is the length of a side. Similarly, the volume of a cube is V =L*L*L. So for a cube, the ratio of surface area to volume is given by the ratio of these equations: S/V = 6/L.