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Cell Biology
Cell Structure
The basic structural unit of all living organisms is the cell. Some organisms are made up of only one cell (unicellular) whereas some organisms are made up of many cells (multicellular). The cell is made up of sub-cellular structures which carry out specific functions that keep the cell and the whole organism alive. The following notes look at the important sub-cellular components of the cell and the two different categories of cells.
Comparison of size of different organisms
Viruses
Most viruses vary from about 20 nm to 500 nm in diameter. Some viruses can be larger than this such as ebolavirus which has a diameter of 80 nm but is 970 nm in length. The picture on the left is coronavirus which has a diameter of about 80 - 90 nm.
Bacteria
Most bacteria are about 0.2 - 2 micrometers in diameter but can be as large 10 micrometers. The picture on the left shows E. Coli bacteria that can be found in the gut. These have a diameter of 0.5 micrometer and are 2 micrometers long. They are described as having a rod shape.
Animal cell
Most animal cells are 10 - 20 micrometers in diameter. The animal cell on the left shows the ultrastructure which can only be observed using an electron microscope. The ultrastructure shows the organelles inside the cell. The egg cell has the biggest diameter of 0.1 mm. The longest neuron can be about 1 m long. The typical range of sizes of animal and plant cells is 10 - 100 micrometers in diameter.
Eukaryotes
Plant and animals are described as Eukaryotes because they have eukaryotic cells. The cells of Eukaryotes have a membrane bound nucleus whereas prokaryotic cells do not have a nucleus. Additionally Eukaryotic cells have more sub-cellular structures such as mitochondria. The diagram below shows what can be seen when viewing an animal cell using an optical microscope.
Animal cell
Cell membrane
All cells have a cell membrane that encloses everything inside the cell. This controls what passes in and out of the cell as only certain molecules can pass throught the cell membrane. It is therefore described as partially permeable.
Nucleus
The nucleus is a clearly visible structure that is usually spherical in shape. It contains DNA in the form of chromosomes and controls the functioning of the cell. Important functions that the nucleus is responsible for are cell division and making proteins (protein synthesis).
Cytoplasm
The cytoplasm is a thick jelly like substance that contains small structures which can usually not be seen in detail using a light microscope. The cytoplasm is where chemical reactions occur and different reactions can occur in different sub-structures called organelles. Respiration is a chemical reaction that releases energy which all cells require. This process occurs in organelles called mitochondria that have a rod like shape.
Plant cell
Plant cells have everything that animal cells have but in addition to this they have the following:
Cell wall
This is made of a material called cellulose which is a polysaccharide that forms fibres called fibrils arranged in a criss-cross pattern making the cell wall very strong. Because in a plant cell the water pushes against the membrane at high pressure the strength of the cell wall prevents the cell becoming damaged. The main function of the cell wall is that it strengthens the cell allowing it to become turgid. This is why plant cells have a regular shape unlike the irregular shape of animal cells.
Permanent vacuole
This is a large structure where chemicals are stored and it plays an important role in making the cell turgid.
Chloroplast
Lens shaped structure that is the site of photosynthesis and contains and contains the green pigment chlorophyll. There are lots of these organelles in the cytoplasm of cells where photosynthesis occurs such as palisade mesophyll or spongy mesophyll cells.
Exercise 1
1. Explain the following terms:
a. Unicellular
b. Multicellular
2. State 3 common features of both animal and plant cells.
3. State 3 additional features found in plant cells but not in animal cells.
4. What is the function of the following:
a. Cell membrane
b. Cytoplasm
c. Nucleus
Prokaryotes
Prokaryotes are organisms that have less cell complexity and lack a true nucleus. They have less sub-cellular structures compared to Eukaryotes. Bacteria are a good example of prokaryotes. They have circular DNA that makes up their genome but this is not a true nucleus as it lacks a membrane.
Bacteria: A typical bacterial cell
All bacteria have a cell wall which is made of a material different to that of a plant cell wall.
• They do not have a membrane bound nucleus and are called prokaryotes
• They do have DNA in the form of a loop called a nucleoid
• They also have rings of DNA called plasmids
• Pili are long thin structures that allow bacteria to attach to tissue and cause infection
• Some bacteria are motile (can move) and will have a flagellum or flagella which is a whip like structure that rotates
• Ribosomes are structures involved in protein synthesis which are smaller in bacteria compared to ribosomes of eukaryotes
Smaller than animal or plant cells (eukaryotes) having typical diameter of 0.2 – 2 μm.
Some bacteria have a capsule which is a layer of slime outside the cell wall that protects them from drying out or prevents them being digested by the host’s digestive system.
Exercise 2
1. State two differences between a prokaryotic and eukaryotic cell.
2. State the function of the following:
a) plasmid
b) ribosome
c) flagella/flagellum
Specialised cells
Cells become specialised to carry out a particular function. The structure of a cell may relate to its function in a tissue, organ or organ system.
Specialised animal cells
Red blood cells
The function of red blood cells is to carry oxygen from the lungs to the tissues.
The biconcave shape provides a large area that increases the rate of diffusion. Red blood cells do not have a nucleus and this allows more oxygen to be carried by the cells.
Other specialised cells include:
- muscle cells that contain filaments that allow them to contract
- nerve cells or neurons that are long and are able to generate a resting potential to send electrical signals for communication
- sperm cells are able to move and fertilise an egg
Specialised plant cells:
Root hair cells have a long structure which increases the area for absorption of water and nutrients from the soil.
Transport in cells
Diffusion
The term 'particle' refers to atoms, molecules or ions. The particles in solids are held together by forces (bonds) and are therefore not free to move. They can however vibrate in fixed positions but will not be able to spread out. The particles in a liquid or gas are able to move freely and any material in either of these states is described as a fluid. The particles in fluids are able to move and spread out. A good example of diffusion is if someone sprays air freshener in one end of the room then soon anybody at the other end will be able to smell the fragrance. This is because the particles have moved from one end to the other. This is described as diffusion and the definition is as follows:
Diffusion is the movement of particles from an area of high concentration to an area of low concentration down a concentration gradient.
Diffusion in living organisms
Oxygen diffuses from the alveoli (air sacs) in the lungs into the blood across the alveolar epithelium and capillary wall. This is because the concentration of oxygen in the alveoli is higher compared to that in the blood so the oxygen moves down the concentration gradient.
Carbon dioxide diffuses into the leaf of a plant from the air surrounding the leaf. This is because the concentration of carbon dioxide outside the leaf is higher compared to the concentration inside the leaf. Carbon dioxide molecules therefore move into the leaf where it is continually being used up, due to photosynthesis, and will remain at a lower concentration. This maintains a concentration gradient between the inside and outside of the leaf, ensuring carbon dioxide constantly moves in.
Factors affecting the rate of diffusion
Concentration
The greater the difference in concentration (concentration gradient) between two different areas across which diffusion occurs the greater the rate of diffusion.
Note: Diffusion can only occur if a concentration gradient exists.
Temperature
An increase in temperature will increase the rate of diffusion because the particles have more energy and move faster at a higher temperature compared to a lower temperature.
Surface area of membrane
The greater the surface area of a membrane the more particles can move across in a given period of time. Increasing the surface area will therefore increase the rate of diffusion.
A unicellular (single celled) organism is small but has a large surface to volume ratio so can rely completely on diffusion for important processes such as gaseous exchange. However, larger multicellular organisms require specialised systems to increase the rate at which gases are transported. Examples being lungs of the respiratory system and the circulatory system that physically pumps many important substances around the body in blood.
Exercise 3
1. State the meaning of the word diffusion.
2. Explain how the following factors affect the rate of diffusion:
a) concentration
b) temperature
c) surface area
3. Why do multicellular organisms require specialised organ systems for transport of substances?
4. Provide two examples of diffusion in living organisms and include:
a) the name of the particles/gas
b) the direction in which the named particles/gas move
c) the reason why they move in the direction stated
Microscopy
There are two types of microscopy:
- light microscopy
- electron microscopy
Light microscopy
In light microscopy microscopes use light to observe the object being viewed. The source of light can be natural using a swivel mirror to direct the light or an electrical light source using a bulb. One of the main advantages of the light microscope is that live cells/organisms can be viewed in order to better study the processes occurring in cells. The magnification is, however, limited to x1500 and the resolution for two objects is 200 nm. The resolution or resolving power is the distance below which two objects will not be seen as two separate objects. Below this minimum distance a blurred image be produced. The resolving power is based on the wavelength of the energy/particle used to view the object which is usually half the wavelength.
Electron microscopy
Electron microscopy uses electrons instead of light to view the object so the specimen has to be placed in a vacuum to prevent collision of electrons with other particles allowing the electrons to move in a straight line. The specimen has to be prepared into extremely thin slices since in one form of electron microscopy the electrons pass through the specimen. This is called transmission electron microscopy (TEM). Another type of electron microscopy is called scanning electron microscopy where electrons a reflected from the surface providing a profile of the object's surface. Unfortunately due to the preparation of the specimen inclusions may appear which are artifacts that would not normally be present but occur due to the preparation process. Because a vacuum is required in an electron microscope the specimen under observation are not alive so processes such as mitosis cannot be observed.
However, electron microscopy does allow ultrastructure to be observed and this is crucial to the understanding of how the cell functions as it allows structure of organelles such as mitochondria and rough endoplasmic reticulum to be observed. This is because the magnification achieved using an electron microscope (x500,000) and resolving power (2 nm) is much higher compared to a light microscope. This is possible because electrons have a much shorter wavelength than light.
Magnification
The following formula can be used to calculate magnification, size of image or size of real object:
Some questions may require use of standard index form (see section on standard index form in Maths). The following example shows how the answer can be given in standard index form if required.
Example 2
The following triangle rule (IAM) can be use to perform calculations and is based on the formula previously given.
Replacing the terminology: size of image by image size (I), size of real object by actual size (A) and magnification (M) gives the triangle rule shown above.
If the actual size of a red blood cell is is 7.5 μm and it has been magnified by x1500 then calculate the image size giving the answer in m and standard index form correct to 2 significant figures.
7.5 μm
Answer
Image size = magnification x actual size
= 1500 x 7.5 μm
= 11250 μm
Dividing by 1000,000 to convert to m gives:
11250/1000,000
= 0.011250 m
Converting to standard index form correct to 2 significant figures gives:
Exercise 4
1. State the magnification that can be achieved by:
a) A light microscope
b) An electron microscope
2. State one advantage and one disadvantage of the following:
a) Light microscopy
b) Electron microscopy
3. Why does an electron microscope provide better resolution than a light microscope?
Transport in cells
Diffusion
ffusion occurs because of the constant random motion of matter that was observed by Robert Brown in 1827. Robert Brown was a Scottish Botanist who first observed this property of matter in pollen suspended on water. Whilst looking at the pollen through a microscope he noticed them moving in a random manner. He initially thought that the living pollen were somehow related to the motion but carried out further observations on inanimate particles of glass, granite and smoke that gave similar results.
Pollen grains on anthers
This constant random motion of particles was described as Brownian motion and in the 1860's theoretical physicists became interested in finding an explanation. It was not until 1905 when the German physicist Albert Einstein published a paper which explained that the random movement of the molecules of water were making the larger particles suspended in it to move as they collided with the pollen. His paper went further to describe that particle size, viscosity of fluid and temperature affected the speed of the particle suspended in a fluid. Brownian motion helped to reinforce the idea of matter being composed of atoms.
Diffusion can be described as the movement of particles from an area of high concentration to an area of low concentration until equally distributed.
Diffusion is a passive process which means that does not require energy and occurs along a diffusion gradient due to the difference in concentration of the two areas.
Examples of diffusion
In the leaf of plants
Upper epidermis
Palisade cells
Spongy mesophyll cells
Lower epidermis
The diagram above shows the cross section of a leaf
Since there is a higher concentration of CO 2 outside the leaf compared to inside the leaf it will diffuse into the airspace inside the leaf.
Osmosis
Osmosis is like diffusion but is the diffusion of water and not any other particle. It is an important process as many biological systems rely on osmosis. Osmoregulation is the process by which the human body regulates the amount of water in blood to prevent adverse osmotic effects that would occur if body water was not controlled.
Osmosis always takes place across a partially permeable membrane. It requires a difference in concentration between two solutions separated by a partially permeable membrane.
The direction that the water molecules move will always be in the direction of the more concentrated solution.
Definition of osmosis
Osmosis is the diffusion of water from an area of low solute concentration to an area of high solute concentration through a partially permeable membrane.
As shown in the diagram above water molecules tend to stick to solute molecules so the water molecules are less mobile. Another important point is that the solute molecules are too large to pass through the partially permeable membrane. Only water can pass through the gaps in the partially permeable membrane. As water molecules can move more freely on the low solute concentration side of the membrane then water will move from right to left in the diagram above. When describing osmosis in terms of water concentration it has to be reworded as follows:
Osmosis is the movement of water from an area of high water concentration to an area of low water concentration through a partially permeable membrane.
Note: High water concentration means low solute concentration so ensure that you understand the meaning of both to correctly define osmosis or interpret scenarios given in questions.