Topic 2A - Cell Structure and Division

- Prokaryotic organisms are prokaryotic cells (single celled organisms)

- Eukaryotic organisms are made up of eukaryotic cells.

- Eukaryotic cells are complex and include animal and plant cells, as well as cells in algae and fungi.

- Prokaryotic cells are smaller and simpler, e.g. Bacteria.

- Both types of cells contain organelles. Organelles are parts of the cells and each organelle has a specific function.

- Algal cells are a lot like plant cells - they have the same organelles, including a cell wall and chloroplasts.

- Fungal cells are a lot like plant cells but with two key differences:

• Their cell wall is made of chitin, not cellulose.
• They don't have chloroplasts becasue they don't photosynthesis.

- Algae carry out photosynthesis, like plants, but can be single-celled or mulitcellular.

- Fungi include mushrooms and yeast.

Description

- Membrane found on the surface of animal cells and inside the cell wall of other cells.

- Made of mainly proteins and lipids.

Function

- Regulates the movement of substances in and out of the cell.

- Receptor molecules allow it to respond to chemicals like hormones.

Description

- Large organelle surrounded by a nuclear envelope (double membrane), which contains many pores.

- Nucleus contains chromosomes which are made from protein-bound linear DNA and one or more structures called a nucleolus.

Function

- Controls the cells activities (by controlling the transcription of DNA).

- DNA contains instructions to make proteins.

- The pores allow substances (e.g. RNA) to move between the nucleus and cytoplasm.

- Nucleolus makes ribosomes.

Description

- Oval-shaped with a double membrane.

- Inner membrane is folded to form structures called cristae. Inside is the matrix, which contains enzymes involved in respiration.

Function

- Site of aerobic respiration, where ATP is produced.

- Found in large numbers in cells that are very active and require a lot of energy.

Description

- Small, flattened structure found in plant and algal cells.

- Surrounded by double membrane and has membranes inside called thylakoid membranes.

- Thylakoid membranes are stacked up in parts of the chloroplast to form grana.

- Grana are linked together by lamellae - thin, flat pieces of thylakoid membrane.

Function

- Site where photosynthesis takes place.

- Some parts of photsynthesis happen in the grana and other parts happen in the stroma (thick fluid found in chloroplasts).

Description

- Group of fluid-filled, membrane-bound flattened sacs.

- Vesicles are often seen at the edge of the sacs.

Function

- Processes and packages new lipids and proteins.

- Makes lysosomes.

Description

- Small fluid-filled sac in the cytoplasm.

- Surrounded by a membrane and produced by the Golgi apparatus.

Function

- Stores lipids and proteins made by the Golgi apparatus.

- Transports lipids and proteins out of the cell via the cell-surface membrane.

Description

- Round organelle surrounded by a membrane.

- No clear internal structure.

- Type of Golgi vesicle.

Function

- Contains digestive enzymes called lysozymes - kept separate from the cytoplasm by the surrounding membrane.

- Used to digest invading cells or break down worn out components of the cell.

Description

- Very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum.

- Made of proteins and RNA.

- Not surrounded by a membrane.

Function

- Site where proteins are made.

Description

- System of membranes enclosing a fluid-filled space.

- Surface is covered with ribosomes.

Function

- Folds and processes proteins that have been made by the ribosomes.

Description

- Similar to Rough Endoplasmic Reticulum, but with no ribosomes.

Function

- Synthesises and proceses lipids.

Description

- Rigid structure that surrounds cells in plants, algae and fungi.

- Made mainly of cellulose in plants and algae, made of chitin in fungi.

Function

- Supports cells and prevents them from changing shape.

- Cells become specialised in eukaryotic, multicellular organisms to carry out specific functions.

- A cells structure helps it carry out it's function so depending on it's job, a specialised cell can look very different to other cells.

- E.g. If a cell uses a lot of energy, it will need a lot of mitochondria. If it makes proteins, it will need a lot of ribosomes.

Example:Epithelial cells in the small intestine are specialised to absorb food efficiently.

1) The walls of the small intestine have lots of finger-like projections called villi - increase surface area for absorption.

2) Epithelial cells on villi have folds in their cell-surface membrane called microvilli - increase surface area even more.

3) Have lots of mitochondria to provide energy for the transport of digested food molecules in the cell.

- In multicellular eukaryotic organisms, specialised cells group together to form tissues.

- A tissue is a group of cells working together to perform a particular function.

- Different tissues work together to form organs and different organs make up a organ system.

Example

- Epithelial cells make up epithelial tissue.

- Epithelial tissue, muscular tissue and glandular tissue (secretes chemicals) work together to form the stomach - an organ.

- The stomach is part of the digestive system - an organ system made up of all of the organs involved in the digestion and absorption of food.

- Viruses are nucleic acids surrounded by protein.

- Viruses contain a core of genetic material - DNA or RNA.

- Protein coat around the core is called the capsid.

- Attachment proteins stick out of the capsid.

- Attachment proteins allow the virus cling to a suitable host cell.

1) The circular DNA and plasmids replicate. Main DNA loop is replicated once but plasmids can replicate more than once.

2) The cells get bigger and the DNA loops move to opposite ends of the cell.

3) The cytoplasm begins to divide and new cell walls begin to form.

4) The cytoplam divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA but can have a variable number of the plasmids.

1) Viruses use their attachment proteins to bind to complementary receptor proteins on the surface of host cells.

2) Different virues have different attachment proteins so they require different receptor proteins on the host cells. This means that some viruses can only infect one type of cells and others can infect lots of different cells. 

3) Viruses are not alive so they don't undergo cell division. They inject their DNA or RNA into the host cells and then the host cell uses it's own 'machinery' (e.g. enzymes, ribosomes) to replicate the viral particles.

- Magnification is how much bigger the image is than the specimen. Calculated by:

Magnification = size of image

                         size of real object

Example: If you have a magnified image that is 5mm wide and your specimen is 0.05mm wide, the magnification is 5 / 0.05 = x100.

- Resolution is how detailed the image is. It is how well a microscope distinguishes between two points that are close together. 

- If a microscope can't separate two objects, increasing the magnification won't help.

Optical (light) microscopes

- Use light to form an image.

- Maximum resolution of 0.2 micrometres (um).Can't use a light microscope to view organelles smallers that 0.2um. This includes ribosomes, andoplasmic reticulum and lysosomes. 

- Can see nucleus and mitochondria, but not in perfect detail. 

- Maximum useful magnification is x1500. 

Electron microscopes

- Use electrons to form images. 

- Higher resolution than optical microscopes so give a more detailed image and used to see more organelles. 

- Maximum resolution of 0.0002um - 1000 times higher than optical microscope.

- Maximum useful magnification is x 1500000

Transmission electron microscopes (TEM)

- Use electromagnets to focus a beam of electrons, which is transmitted through the specimen.

- Denser parts of the specimen absorb more electrons, which makes them look denser on the image you end up with. 

- Give high resolution images and shows the internal structure of organelles like chloroplasts. 

- Only be used on thin specimens. 

Scanning electron microscopes (SEM)

- Scan a beam of electrons across the specimen. This knocks off electrons from the specimen, which are gathered in a cathode ray tube to form an image.

- Images show the surface of the specimen and they can be 3D.

- Can be used on thick specimens.

- Give lower resolution images then TEMs. 

Preparing a 'temporary mount' of a specimen on a slide

- Pipette a small water drop onto the slide (a ***** of clear glass or plastic).

- Then use tweezers to place a thin section of your specimen on the water drop.

- Add a drop of stain to highlight objects in a cell. For example, eosin is sued to show the cytoplasm and iodine in potassium iodide is used to stain starch grains in plant cells.

- Finally, add the cover slip (a square of clear plastic that protects the specimen). Stand the slip upright on the slide, next to the water droplet. Then tilt and lower it carefully so it covers the specimen.

- Try not to get any air bubbles under the cover slip because air bubbles will obstruct your view of the specimen.

Homegenisation - Breaking up the cells

- Can be done in different ways, e.g. by vibrating the cells or grinding the cells in a blender. 

- Breaks up plasma membrane and releases the organelles into solution. 

- Solution must be ice-cold to reduce the enzyme activity.

- Solution must be isotonic so that there is the same concentration of chemicals as the cells being broken down to prevent damage to the cells by osmosis. 

- A buffer solution will maintain the pH.

Filtration - Getting rid of the big bits

- The homogenised cell solution is filtered through a gauze to separate any large cell debris or tissue debris.

- Organelles are smaller than this debris so they pass through the gauze. 

Ultracentrifugation - Separating the organelles

1) The cells fragments are put into a tube. The tube is put into a centrifuge and is spun at a low speed. The heaviest organelles, like the nuclei, sink to the bottom of the tube and form a thick sediment - the pellet. The rest of the organelles stay suspended in the fluid above the sediment - the supernatant. 

2) The supernatant is drained off, poured into another tube and spun at a higher speed. The heviest organelles sink to the bottom again, this time the mitochondria, and form a pellet at the bottom of the tube. The supernatant is drained off and spun at an even higher speed. 

3) This process is repeated at higher and higher speeds until all the organelles are separated out. Each time the pellet at the bottom is made of lighter organelles. 

- In mitosis a parent cell divides to produce two genetically identical daughter cells. 

- Mitosis is needed for the growth of multicellular organisms and for repairing damaged tissue. Not all cells have the ability to divide. The ones that do follow a cell cycle, which mitosis is part of: 

- The cell cycle consists of a period of cell growth and DNA replication called interphase. Mitosis happens after interphase. Interphase is divided into 3 growth stages called G1, S and G2. 

- Chromosomes condense and get shorter and fatter.

- Centrioles (tiny bundles of protein) start moving to opposite ends of the cell, forming a network of proteins called the spindle.

- The nuclear envelope breaks down and chromosomes lie free in the cytoplasm.

- The chromosomes each have two chromatids, joined by a centromere, line up along the middle of the cell.

- The spindle attaches to the chromosomes by their centromere.

- The centromeres divide and separate each pair of the sister chromatids.

- The spindles contract, pulling the chromatids to each opposite poles of the spindles, centromere first.

- This makes the chromatids appear v-shaped.

- The chromatids reach opposite poles of the spindle.

- They uncoil and become long and thin again. - Called chromosomes again.

- A nuclear envelope forms around each group of chromosomes so there are now two nuclei.

- The cytoplasm divides (cytokenesis) and there are two daughter cells that are genetically identical to the parent cell and to each other.

- Mitosis is finished and the daughter cell is in interphase, getting ready for the next round of mitosis.

Example

A scientist observes a section of growing tissue under the microscope. He counts 100 cells undergoing mitosis. Of those, 10 cells are in metaphase. One complete cell cycle of the tissue lasts 15 hours. How long do the cells spend in metaphase? Give your answer in minutes.

1) The scientist has observed that 10 cells out of 100 are in metaphase. This suggests that the proportion of the time the cells spend in metaphase much be 10/100th of the cell cycle.

2) You're told the cell cylce lasts 15 hours. That's 15 x 60 = 900 minutes.

3) So the cells spend: 10/100 x 900 = 90 minutes in metaphase.

- Mitosis and the cell cycle are controlled by genes.

- When cells have divided enough times to make new cells, they stop.

- If there is a mutation in a gene that controls cell division, the cells can grow out of control.

- The cells keep dividing to make more and more cells, which form a tumour.

- Cancer is a tumour that invades surrounding tissue.

- Some treatments for cancer are designed to control the rate of cell division in tumour cells by disrupting the cell cycle. This kills the tumour cells.

- These treatements don't distinguish between tumour cells and normal cells though, so they also kill normal cells that are invading.

- Tumour cells divide much more frequently than normal cells, so the treatments are more likely to kill tumour cells. Some cell cycle target of cancer treatments include:

1) G1 (cell growth and protein production) - Some chemical drugs (chemotherapy) prevent the synthesis of enzymes needed for DNA replication. If these aren't produced, the cell can't enter the synthsis phase (S), disrupting the cell cycle and forcing the cell to kill itself.

2) S phase (DNA replication) - Radiation and some drugs damage DNA. At several points in the cell cycle (including just before and during S phase) the DNA is checked for damage. If severe damage is detected, the cell will kill itself, preventing further tumour growth.

1) Cut 1cm from a growing root. It needs to be a root because that is where growth takes place and where mitosis occurs.

2) Prepare a boiling tube with 1M hydrochloric acid and put it in a water bath at 60 degrees.

3) Place the route tip in the boiling tube and incubate for 5 minutes.

4) Rinse the root tip well with cold water, using a pipette, and leave to dry on a paper towel.

5) Place the route tip on a microscopic slide and cut 2mm from the very tip of it. 

6) Use a mounted needle to break the tip open and spread the cells out thinly. 

7) Add a few drops of stain. The stain will make the chromosomes seen more clearly.

8) Place a cover slip on the cells and push down firmly - make the tissue thinner and allows light pass through. Don't smear the slip sideways because the chromosomes will get damaged. 

- Start by clipping the prepared slide on the stage. 

- Select the lowest-powered lens - lowest magnification.

- Use the course adjustment knob to move the objective lens down to just above the slide. 

- Look down the eyepiece and adjust the focus with a fine adjustment knob until you get a clear image. 

- If you need to see the slide with a higher magnification, use a higher-powered objective lens and refocus. 

Mitotic index = number of cells with visible chromosomes

                       total number of cells observed

- This lets you work out how mny cells are undergoing mitosis and if there is anything weird going on. 

- A plant root tip is constantly growing so you'd expect a high mitotic index. 

- In other tissue samples, a high mitotic index could mean that the tissue is being repaired or there is cancerous growth in the tissue.

- Artefacts are things that you can see down the microscope that aren't part of the cell or specimen you're looking at. 

- These could be dust, bubbles, fingerprints and air or inaccuracies caused by squashing or staining your sample. 

- Artefacts are usually made during the preparation of slides and shouldn't be there at all.

- Artefacts are especially common in electron micrographs because specimens need a lot of preparation before you view them under an electron microscope. 

- The first scientists to use these microscopes could only distinguish between artefacts and organelles by repeatedly preparing specimens in different ways. 

- If an object could be seen with on preperation but not another, it was more likely to be an artefact than an organelle.