A-level Biology/Cells

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Cell Structure[edit]

Cell Organelles[edit]

Organelles are parts of cells. Each organelle has a specific function.

1. Nuclear membrane 2. Nuclear pore 3. Rough endoplasmic reticulum (REM) 4. Smooth endoplasmic reticulum 5. Ribosome attached to REM 6. Macromolecules 7. Transport vesicles 8. Golgi apparatus 9. Cis face of Golgi apparatus 10. Trans face of Golgi apparatus 11. Cisternae of Golgi apparatus 12. Secretory vesicle 13. Cell membrane 14. Fused secretory vesicle releasing contents 15. Cell cytoplasm 16. Extracellular environment

Nucleus[edit]

Description:

  • Largest organelle
  • Surrounded by a nuclear envelope, which contains pores (holes)
  • Contains chromatin and the nucleolus

Function:

  • Store the genetic material
  • Controls the cell's activities
  • Pores allow substances to move between the nucleus and the cytoplasm
  • The nucleolus makes ribosomes (see below)
The parts of a cell nucleus

Mitochondria[edit]

Description:

  • Oval shaped
  • They have a double membrane - the inner one is folded to form structures called cristae
  • Inside is the matrix, containing enzymes

Function:

  • They are the site of aerobic respiration
  • Makes energy in the form of ATP (adenosine triphosphate) as a source of energy for the cell's activities
  • Cristae give a bigger surface area so more enzymes can fit in
The diagram shows a section of a eukaryotic cell's mitochondrion.

Endoplasmic Reticulum[edit]

Description:

  • Smooth endoplasmic reticulum is a system of membranes which enclose a fluid-filled space
  • Rough endoplasmic reticulum is similar, but covered in ribosomes

Function:

  • Smooth endoplasmic reticulum synthesises and processes lipids
  • Rough endoplasmic reticulum folds and processes proteins that have been made at the ribosomes, transports proteins around the cell.

Golgi Apparatus[edit]

Description:

  • A group of fluid-filled, flattened sacs

Function:

  • Processes and packages new lipids and proteins
  • Once finished, it makes vesicles which transport the molecules to the edge of the cell for ejection
  • Makes lysosomes

Ribosome[edit]

Description:

  • Very small
  • Either floats free in the cytoplasm or is attached to rough endoplasmic reticulum

Function:

  • The site where protein synthesis takes place

Lysosome[edit]

Description:

  • Round
  • No clear internal structure

Function:

  • Contains digestive enzymes which can be used to digest invading cells or break down worn-out organelles (autolysis)

Microvilli[edit]

Description:

  • These are folds in the plasma membrane
  • Found in cells involved in absorption
  • Stereotypically found on the villi in the small intestine

Function:

  • Increase the surface area of the plasma membrane

Plasma Membrane[edit]

Found on the surface of animal cells, it's mainly made of lipids and proteins. It controls the movement of substances in and out of the cell; further explanation can be found later in this book.

Chloroplast[edit]

Description:

  • Found in plant cells only
  • Inner membrane is folded to form stacks of grana
  • Molecules of chlorophyll are on the grana

Function:

  • Chlorophyll captures photons of light used for photosynthesis
Chloroplast-new.jpg
Animal cell diagram
Plant cell diagram

Refer to the below table for the differences between plant and animal cells.

Table 1: Comparison of structures between animal and plant cells
Typical animal cell Typical plant cell
Organelles
  • Nucleus
    • Nucleolus (within nucleus)
  • Rough endoplasmic reticulum (ER)
  • Smooth ER
  • Ribosomes
  • Cytoskeleton
  • Golgi apparatus
  • Cytoplasm
  • Mitochondria
  • Vesicles
  • Lysosome
  • Centrosome/Centrioles
  • Many small vacuoles
  • Nucleus
    • Nucleolus (within nucleus)
  • Rough ER
  • Smooth ER
  • Ribosomes
  • Golgi apparatus
  • Cytoplasm
  • Mitochondria
  • Vesicles
  • Chloroplast
  • One large vacuole
Additional structures
  • Plasma membrane
  • Cilia
  • Plasma membrane
  • Cell wall
  • Plasmodesmata

Prokaryotes and Eukaryotes[edit]

Eukaryotic cells are complex, and include all animal and plant cells. Prokaryotic cells are smaller and simpler, like bacteria.

The table below is a comparison of prokaryotic and eukaryotic cells:


Table 2: Comparison of features of prokaryotic and eukaryotic cells
  Prokaryotes Eukaryotes
Typical organisms bacteria fungi, plants, animals
Typical size ~ 1-10 µm ~ 10-100 µm (sperm cells, apart from the tail, are smaller)
Type of nucleus none nucleus with double membrane
Genetic material ring of DNA, plasmids chromosomes
Ribosomes Smaller (18nm) Larger (22nm)
Cytoplasmatic structure very few structures highly structured by endomembranes and a cytoskeleton
Mitochondria none one to several thousand (though some lack mitochondria)
Chloroplasts none in algae and plants
Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells
Cell division Binary fission (simple division) Mitosis
Meiosis


Analysis of Cell Compounds[edit]

Units of Size in Microscopy[edit]

  • The basic biological unit of measure is the micrometer (µm).
  • 1000µm = 1mm.

Calculations in Microscopy[edit]

Size in real life = Size in image ÷ Magnification.
Magnification = Size in image ÷ Size in real life.

  1. Measure the size of the image in millimetres.
  2. Convert to micrometers by multiplying by 1000.

Example: A micrograph shows mitochondrion 210mm magnified 2500x. What is its size in real life?
210mm x 1000 = 210,000µm
210,000µm ÷ 2500 = 84µm

Example: An object is 130µm in real life and 52mm in an image. What is the magnification?
52mm x 1000 = 52,000µm
52,000µm ÷ 130µm = 400x

Microscopes[edit]

Light Microscopes

  • Light rays travel through the specimen and 2 lenses
  • The objective lens provides the initial magnification of the image
  • The eyepiece lens magnifies and focuses the image

Electron Microscopes

  • Transmission electron microscopes (T.E.M.) pass a beam of electrons through the specimen to produce an image on a fluorescent screen.
  • Scanning electron microscopes (S.E.M.) scan a beam of electrons over the specimen.
  • Electromagnets focus the image.
  • Electrons are produced from a tungsten filament at the top of a column.
  • The column is a vacuum. As a result, living speciments cannot be used.
  • The preparation of specimens for microscopes can be drastic, and can produce artefacts. Artefacts are things you see under a microscope, but aren't actually there in real life. This could be due to something like an air bubble.
  • Magnification refers to how much bigger the image is than the actual specimen.
  • Resolution refers to how well a microscope distinguishes two different points that are close together. If a microscope can't separate two objects, then increasing the magnification won't help.

Below is a table comparing the different types of microscope.

Table 3: Comparison of microscopes
Property Light S.E.M. T.E.M.
Resolution Low Medium High
Depth of focus Low High Medium
Field of view Good Good Limited
Ease of specimen preparation Easy Fairly skilled Skilled
Speed of specimen preparation Rapid Quite rapid Slow
Relative cost Cheap Expensive Expensive
Living specimen Yes No No

Cell Fractionation[edit]

  • Cell fractionation breaks apart cells and separates its organelles.

Step 1: Homogenisation[edit]

  • This breaks open the cells.
  • Usually done by vibrating the cells, or grinding them up in a blender.
  • It is done with a cold, isotonic buffer:
    • cold to slow down and stop organelle activity, particularly the hydrolytic enzymes in lysosomes
    • isotonic to prevent the movement of water in and out of organelles by osmosis
    • a buffer to prevent changes in pH levels.

Step 2: Filtration[edit]

  • Filter the solution to remove debris.

Step 3: Ultracentrifugation[edit]

  • Spin the solution in a centrifuge at a low speed.
  • The heaviest organelles (nuclei, chloroplasts) fall the to the bottom.
  • The rest of the organelles stay suspended in the fluid above this sediment. This is the supernatant.
  • The supernatant is drained off, poured into another tube, and spun again at a higher speed.
  • This time, organelles like mitochondria and lysosomes fall to the bottom.
  • Again, the supernatant is drained off, poured into another tube and spun at a higher speed.
  • Finally, the lightest organelles like ribosomes remain.

Plasma Membranes[edit]

Plasma membranes are located around the edge of animal cells and surround the cytoplasm and other organelles. They are made up of a phospholipid bilayer which consists of two layer of phospholipids with the hydrophobic heads on the outer layers and the hydrophylic tails on the inner layers. lipid soluble molecules can diffuse straight through the bilayer, water can osmosise through as well. The phospholipid bilayer contains intrinsic and extrinsic proteins. The intrinsic proteins go all the way through the bilayer whereas extrinsic proteins only go through the outer phospholipid layer. Extrinsic Proteins are used for recognition of the cell and usually have a glycoprotein attached for the recognition. Intrinsic Proteins are for allowing molecules through. Ways the proteins are designed to allow molecules through are: Protein pump, Protein Channel, Gated protein channel.

Cholera[edit]

Cholera is a disease commonly found in dirty water. Once ingested it sits in the endo thelial reticulum of the gut and sets up an contransporter with Na+ ions . this lowers the water potential of the gut this means thatv water moves in down the concentration gradient meaning that the person is constantly dehydrated and has wet loose faeces. These affects can be countered with oral dehydration therapy.