2.1 Cell structure Flashcards
1
Q
What is the equation for magnification
A
Magnification = image size/ actual size
2
Q
What is cell fractionation and what are the 2 main steps
A
- The process where cells are broken up and their organelles are separated so they can be studied in detail.
- The 2 main steps are homogenisation and ultracentrifugation.
3
Q
Describe the process of homogenisation
A
- The cells are broken up by a homogeniser to release organelles from the cell.
- This forms the resultant fluid called the homogenate. This contains the organelles, cell membrane and cells which were not broken up.
- The homogenate is then filtered to remove complete cells which were not broken, as well as large cell debris.
4
Q
Describe the process of ultracentrifugation
A
- The filtered homogenate is spun in the ultracentrifuge to seperate out the fragments at a very high speed.
- This forms a sediment pellet at the bottom containing the heaviest organelles, e.g. nuclei, and a fluid called a supernatant.
- The supernatant is then transferred to another tube and spun faster to create a new pellet.
5
Q
what are the conditions necessary for cell fractionation
A
A cold, buffered solution with the same water potential as the cells.
6
Q
Why must the solution be cold
A
- Causes the enzymes present in the cell to work slower.
- This prevents them from breaking down organelles.
7
Q
Why must the solution be buffered
A
- To keep a constant pH so that enzymes and other proteins in the cell don’t denature.
- pH interferes with bonds in proteins tertiary structure.
8
Q
Why must the solution have the same water potential as the cells
A
- Same water potential (isotonic) means no osmosis will occur.
- This will prevent the organelles from shrivelling or bursting under osmotic pressure.
9
Q
What is the order of fractination in cell fractination (order of organelles in pellets)
A
- Nucleus
- Chloroplasts
- Mitochondria
- Lysosomes
- Endoplasmic reticulum
- Ribosomes
10
Q
What is contrast
A
The difference between the lightest and darkest parts of an image. It can be increases by staining.
11
Q
What is magnification
A
The number of times an object has been enlarged.
12
Q
What is resolution
A
The minimum distance apart that 2 objects can be in order for them to appear as separate items.
13
Q
How do light/optical microscopes work
A
They work by passing light through a specimen and focusing the light on an eyepiece or camera which magnifies the image.
14
Q
What are some advantages of light microscopes
A
- The specimen can be living.
- Simple preparation of specimen, so unlikely to distort cell structure.
15
Q
What are some disadvantages of light microscopes
A
- Lower magnification (than electron)
- Visible light has a longer wavelength than electrons, creating a lower resolution.
16
Q
How do Transmission Electron Microscopes (TEM’s) work
A
- A beam of electrons passes through a thin section of a specimen.
- Areas that absorb the electrons (denser areas) appear darker on the electron micrograph that is produced (as the electrons don’t pass through)
- The electron beam is focused by electromagnets inside a vacuum environment.
17
Q
How do Scanning Electron Microscopes (SEM’s) work
A
- A beam of electrons passes across the surface and scatter.
- The pattern of scattering builds up a 3D image depending on the contours of the specimen.
18
Q
What are some advantages of electron microscopes
A
- Higher magnification and resolution than light microscopes (electrons have a shorter wavelength).
- Can therefore see smaller structures like ribosomes.
- SEM produces a 3D image.
19
Q
What are some disadvantages of electron microscopes
A
- Whole system must be in a vacuum so living specimen cannot be observed.
- A complex staining process is required which could introduce artefacts into the image.
- Specimen must be very thin, especially for TEM, so that electrons can pass through.
20
Q
What are 5 key features of eukaryotic cells
A
- DNA contained in a membrane-bound nucleus (surrounded by a membrane).
- DNA tightly wrapped around proteins called histones. The DNA and histone proteins form chromosomes.
- DNA is a linear molecule ( ends of chromosomes not joined in a loop).
- Contain membrane-bound organelles, e.g. mitochondria
- Contain 80s ribosomes.
21
Q
Describe the structure of the nucleus
A
- Surrounded by the nuclear envelope - a double membrane which controls the entry/ exit of materials and contains reactions within the nucleus.
- The nuclear envelope has nuclear pores - allows the passage of large molecules.
- Bulk of the nucleus is made of nucleoplasm - a granular, jelly-like material.
- Nucleolus in the centre - manufactures ribosomal RNA (rRNA) and assembles ribosomes.
22
Q
What are the functions of the nucleus
A
- Controlling the cell’s activities - the DNA contains instructions to produce proteins.
- Synthesis of ribosomes - the nucleolus makes ribosomal RNA.
- Exchange between nucleus and cytoplasm - Substances can enter or leave the nucleus via the nuclear pores.
23
Q
Describe the structure of the cell-surface membrane
A
- Phospholipid bilayer.
- Mainly made up of lipids and proteins.
24
Q
What are the functions of the cell-surface membrane
A
- Controlls movement of substances into and out of the cell - the membrane is partially permeable.
- Cell signalling - receptors can detect signals from other cells.
25
Describe the structure of mitochondria
- Double membrane - controls entry and exit of materials. Inner membrane folds to form cristae.
- Cristae - Provide large surface area for the attatchement of enzymes and other proteins involved in respiration.
- Matrix - contains proteins, lipids, ribosomes and DNA (mtDNA).
26
What are the functions of mitochondria
Site of aerobic respiration - produces ATP from respiratory substrates, e.g. glucose.
27
Describe the structure of a ribosome
- Made of proteins and rRNA (ribosomal RNA).
- Consist of a large and a small subunit.
- Not surrounded by a membrane.
- 80S found in eukaryotic cells.
- 70S found in prokaryotic cells, chloroplasts and mitochondria (slightly smaller).
28
What is the function of ribosomes
Site of protein synthesis - involved in the process of translation.
29
Describe the structure of the Golgi apparatus
- Contain fluid-filled, membrane-bound sacs called cisternae.
- Contain smaller vesicles (Golgi vesicles).
30
What are the functions of the Golgi apparatus
- Process and package lipids and proteins - carried out by the cisternae.
- Store and transport lipids and proteins - Carried out by the vesicles.
- Synthesise lysosomes (specialised vesicles).
- Combines triglycerides with proteins.
31
Describe the structure of the rough endoplasmic reticulum (REM)
- Contains a network of membranes enclosing a fluid-filled space, known as cisternae.
- The surface of the cisternae is covered with ribosomes.
32
What is the function of the Rough endoplasmic reticulum (RER)
Folds and processes proteins made on the ribosomes.
33
Describe the structure of the smooth endoplasmic reticulum (SER)
A system of membrane bound sacs (same as rough endoplasmic reticulum but without the ribosomes).
34
What is the function of the SER
Synthesises, stores and transports lipids and carbohydrates.
35
Describe the structure of lysosomes
- Type of vesicle.
- Contain hydrolytic/ digestive enzymes.
- Surrounded by a membrane to keep enzymes separate from the cytoplasm of the cell.
- Formed when vesicles produced by golgi contain enzymes.
36
What are the functions of lysosomes
- Use enzymes to hydrolyse material ingested by phagocytic cells.
- Digest worn out organelles.
- Break down cells after they have died (autolysis).
37
Describe the structure of the cell wall
- Made of cellulose in plants, cellulose/glycoprotein in algae and chitin in fungi).
- Contain pores within the walls called 'plasmodesmata' which allows for the exchange and transport of substances between 2 cells.
38
What are the functions of the cell wall
- Supports the cell - contents of the cell press against the cell wall, making it rigid.
- Provides mechanical strength which prevents the cell from bursting due to osmotic pressure.
- Allows exchange of substances between cells - plasmodesmata connect neighbouring cells.
39
Describe the structure of a chloroplast
- Surrounded by a double membrane, enclosing a fluid known as stroma (site of stage 2 of photosynthesis).
- Contain grana - stacks of disc like structures called thylakoids containing chlorophyll.
- Contain their own DNA and ribosomes.
40
What is the function of chloroplasts
Site of photosynthesis - these reactions take place in the grana and stroma.
41
How are chloroplasts adapted for photosynthesis
- Granal membranes provide a large surface area for the attatchement of chlorophyll.
- Stroma fluid contains all the enzymes needed for photosynthesis.
- Chloroplasts contain both DNA and ribosomes, so they can synthesise proteins needed for photosynthesis.
42
Describe the structure of the vacoule
- Contains cell sap.
- Surrounded by a single selectively permeable mebrane called a tonoplast.
43
What are the functions of the vacoule
- Maintains pressure within the cell (supports plant by making them turgid).
- Temporary food store (contains sugars).
- Contains pigments (e.g. to colour petals).
44
What are 5 differences between prokaryotic and eukaryotic cells
- Differences in DNA
- Prokaryotic cells have no membrane-bound organelles
- 70s (smaller) ribosomes (80s in eukaryotic)
- No nucleus in prokaryotic cells
- Cell wall in prokaryotic cells made of murein (a glycoprotein , made of cellulose in plant cell walls)
45
Describe 4 differences in DNA between prokaryotic and eukaryotic cells
- Longer in eukaryotic cells
- Associated with histones in eukaryotic DNA
- DNA linear in eukaryotic, circular in prokaryotic
- Found in the nucleus in eukaryotic, free in the cytoplasm in prokaryotic.
46
What are plasmids
- A smaller loop of DNA that can replicate independently of the chromosomal DNA.
- They may contain genes that benefit the cell further, e.g. antibiotic resistance.
47
What is the role of the cell wall in prokaryotic cells
- Made of murein (a glycoprotein)
- Excludes against certain substances, protects against mechanical damage and osmotic lysis.
48
What is the role of the capsule in prokaryotic cells
- Secretes mucus slime to protect it from phagocytosis
- Helps groups of bacteria to stick together
