Cells Flashcards
What is the equation for magnification?
magnification= image size/actual size
What is cell fractionation?
In order to obtain the structure and function of the organelles that make up cells, it is necessary to obtain large numbers of isolated organelles. Cell fractionation is the process where cells are broken up into different organelles
What must the solution be for cell fractionation? (3 points)
1 - Ice cold to reduce enzyme activity
2 - Same water potential as the tissue, preventing bursting/shrinking of organelles
3 - pH buffered so pH does not fluctuate and no proteins/enzymes can be denatured
What is the first stage to cell fractionation?
Homogenation - cells are broken up by a homogeniser. This breaks the plasma membrane and releases the organelles fromthe cell. The resultant fluid (homogenate), is filtered to remove any complete cells and large pieces of debris
What is the second stage to cell fractionation?
Ultracentrifugation - process where the fragments in the homogenate are separated in a centrifuge. This spins the tubes of homogenate at very high speed in order to create a centrifugal force
- The tube of filtrate is placed in the centrifuge and is spun at slow speed
- The most dense organelles (nuclei) are forced to the bottom of the tube creating a pellet
- The fluid at the top (supernatant) is removed and is transferred into another tube where it is spunat a faster speed and for a longer time
- The mitochondria are separated next
High speed - ER/golgi
Very high speed - ribosomes
What is a light microscope?
Light passes through the specimen and is focused by glass lenses
What are the advantages and limitations of light microscopes?
Advantages:
- Magnification up to x1500
- Resolution 200nm
- Wide range of specimen can be viewed, including living organisms
Limitations:
- Can’t be used for viewing ultrastructure
What is an electron microscope?
Uses beams of electrons which are focused by magnets
What are the two types of electron microscopes?
Transmission (TEM) and scanning (SEM)
What is the difference in the two electron microscopes?
Transmission electron microscopes electron beams pass through very thin section of sample whichis dipped in heavy metals e.g. lead
Scanning electron microscopes electron beams directed on to the surface of the sample (not through)
What is the magnification and resolution of both SEMs and TEMs?
TEM:
Magnification - x500,000
Resolution - 0.1nm
SEM:
Magnification - x100,000
Resolution - 20nm
Describe TEMs
Black and white image
2D
Describe SEMs
Final image is 3D and black and white
What are the limitations of electron microscopes?
- Whole system must be in a vacuum so living organisms cannot be observed
- Complex staining process
- Specimen must be extremely thin
- May contain artefacts (e.g. dust, staining)
What is the equation for working out the size of a specimen using an eyepiece graticule and stage micrometer
Number of divisions on stage micrometer x length of one division on stage micrometer/ number of divisions on eyepiece graticule
Describe the nucleus in eukaryotes
- 10-20 micrometers in size
- Nuclear envelope (double membrane with fluid between) contains nuclear pores to allow the entrance and exit of substances
- Nuclear pores allow passage of large molecules such as mRNA
- Nucleoplasm jelly like material that makes up bulk of the nucleus.
Chromosomes consist of linear DNA and histone proteins
Nucleolus is a small dense spherical region within nucleoplasm which makes mRNA and ribosomes
Describe the endoplasmic reticulum
Series of lfattened membrane bound sacs called cisternae
- Rough endoplasmic reticulum: (studded woth ribosomes) provide large surface area for protein synthesis. Provide a pathway for the transport of proteins
- Smooth endoplasmic reticulum: No ribosomes, so appears smooth, so its function is to sysnthesise, store and transport lipids and carbohydrates
Cells that manufacture and store large quantities of carbohydrate, proteins and lipids have a very extensive ER e.g. liver cells, epithelial
Describe the mitochondria
- Spherical or sausage shaped (depending on cross section) with a double membrane that controls the entry or exit of material. Inner membrane highly folded into cristae to provide large surface area for the attachment of enzymes for respiration
- Central part is the matrix. This contains proteins, lipids, ribosomes and mitochondrial DNA
- Provides/sysnthesis of ATP by aerobic respiration which requires oxygen
Metabolically active cells such as muscle and epithelial cells require lots of ATP, and so will contain lots of mitochondria
Describe the golgi apparatus
Stack of membrane bound flattened sacs called cisternae, with small rounded hollow structures called vesicles
- The golgi receives proteins and lipids from ER and may modify them (add carbohydrates). It also labels them allowing them to be sorted then sent to the correct destination. Once sorted they are transported in a vesicle which are ‘pinched off’ from the ends of the golgi cisternae. Exocytosis may occur when the vesicles transport the material
- Golgi also forms lysosomes
Golgi apparatus is well developed in secretory cells e.g. epithelial cells that line intestines
Describe the chloroplast
- Found in plant cells and some protocists
- Double membrane
- Carry out photosynthesis to make carbohydrates from carbon dioxide and water
- Inner membrane folded into sacs called thylakoids (stack of thylakoids is a granum) Provides large SA
- Stroma is a fluid filled matrix where the second stage of photosynthesis takes place. Contains required enzymes. Starch granules can also be found here
- Chloroplasts contain DNA and ribosomes so they manufacture some of the proteins needed for photosynthesis
Describe the lysosomes
- Spherical sacs with single membrane
- Contain powerful digestive enzymes to hydrolyse materials e.g. white blood cell lysosome helps break down microorganisms
Describe the vesicles
- Membrane bound sacs
- Carry different substances around cells
Describe the ribosomes in eukaryotes
- Are size 80S in eukaryotes
- No membrane, some in ER and some in cytoplasm
- Have two subunits (one large and one small)
- Site of protein synthesis
Describe the vacuoles
- Fluid filled sac surrounded by a single membrane called a tonoplast
- Maintains cell stability by pushing cytoplasm against the wall to make cell turgid
Describe the cell walls
- In plants
- Made of cellulose (beta glucose)
- Provides strong wall for support and strength in the plant
- In fungi made of chitin
- In bacteria/prokaryotes made of peptidoglycan
Describe the cytoskeleton
- Network of protein fibres which provide support and shape to cell
- Made up of microtubules
What is division of labour?
Processes in a cell are carried out by individual organelles within the cells working together
E.g. in protein synthesis
Describe the process of protein synthesis (i.e. insulin synthesis)
1 - mRNA copy made in nucleus
2 - mRNA leaves nucleus through nuclear pore
3 - mRNA attaches to ribosome on RER - protein synthesised
4 - Proteins are ‘pinched off’ in vesicles and travel towards Golgi apparatus
5 - Vesicle fuses with golgi apparatus
6 - Golgi processes, labels and modifies protein, ready for release
7 - Packaged proteins are ‘pinched off’ in vesicles from golgi and travel to membrane
8 - Vesicle fuses with cell surface membrane
9 - Cell surface membrane opens to release proteins outside of cell
Processes 8 and 9 are exocytosis
How do cell become specialised?
By switching specific genes off - when unspecialised cells have most/all genes ‘switched on’ as they have the potential to specialise into all/most cells
What 3 ways can a cell become specialised by?
1 - Changing the shape of the cell
2 - Changing the number of specific organelles
3 - Changing the plasma membrane
Give two examples of specialised cells
- Red blood cells (erythrocytes)
- Sperm cells
Describe and explain how erythrocytes (RBCs) are specialised
- Biconcave shape - increases surface area
- No nuclei - more space inside cell for oxygen
- Presence of haemoglobin - high affinity for oxygen (aids respiration)
Describe and explain how sperm cells are specialised
- Undulipodium (tail) - controls locomotion of cell
- Lots of mitochondria - site for aerobic respiration, roduces lots of ATP (energy)
- Streamlined head - more efficient movement
- Acrosom/acrosomal cap - contains enzymes for hydrolysis of egg membrane
- Haploid nucleus - half the original number of chromosomes for fertilisation
What is a tissue? Give examples
- Collection of cells that are similar to each other and perform a similar function
- E.g. xylem tissue, nervous tissue, ciliated epithelium
What is an organ? Give examples
- Collection of tissues that work together to perform a particular function
- E.g. heart, lungs, liver, intestine
What is an organ system? Give examples
- Group of organs working together
- E.g. respiratory system, circulatory system, excretory system, digestive system
Describe a squamous epithelium cell
- Flattened cells
- Very thin
- Line the inside of blood vessels and tubes to provide smooth surface for passage of fluids
- Thin walls provide a short diffusion pathway
- E.g. alveoli for oxygen and carbon dixoide
Describe ciliated epithelial tissue
- Made up of column shaped cells
- Found on inner surface of tubes e.g. bronchi, trachea, uterus, oviducts
- Goblet cells produce and secrete mucus
- Cilia hairs can waft mucus
NOTE: Nicotine can paralyse cilia and tar can damage them so they can’t sweep mucus
What is the process of cell division in prokaryotes (bacteria)?
Binary fission
Describe the process of binary fission
1 - The circular DNA and plasmid(s) replicate. The main DNA loop is only replicated once, but plasmids can be replicated many times
2 - The cell increases in size and the DNA loops move to opposite poles of the cell
3 - The cytoplasm begins to divide (and new cells walls begin to form)
4 - The cytoplasm divides and two daughter cells are produced. Each daughter cell has one copy of the circular DNA but can have a variable number of plasmids
Explain the differences in structure of prokaryotes and eukaryotes
Prokaryotes:
- No distinct nucleus, only an area where DNA is found
- DNA not associated with histone proteins
- Circular DNA
- Presence of plasmids
- No membrane bound organelles
- Ribosomes are smaller (70S)
- Cell wall made of peptidoglycan (murein)
- May have outer capsule
Eukaryotes:
- Distinct nucleus with a nuclear envelope
- DNA is associated with histone proteins
- Linear DNA
- Plasmids not present
- Membrane bound organelles such as golgi apparatus, mitochondria etc.
- Ribosomes are larger (80S)
- Cell wall in plants made of cellulosde, in fungi made of chitin
- No capsule present
Describe the structure of viruses (akaryotes)
- Genetic material (DNA or RNA)
- Capsid (protein coat)
- Attachment points
- Do not undergo cell division
- Instead they inject their genetic material into the host cell
NOTE: Viruses are non-living (do not follow MRSGREN processes) and do not contain organelles
What are the 6 stages to cell division in eukaryotes?
- Interphase (Mitotic division) - Prophase - Metaphase - Anaphase - Telophase
- Cytokenesis
Describe the 3 parts to interphase
Growth 1
- Cell growth
- Increase in organelle number
- Proteins synthesised
- ATP synthesised
Synthesis
- DNA replication via semi-conservative replication
Growth 2
- Cell growth
- Proteins synthesised
- ATP synthesised
What is the longest phase of the cell cycle?
Interphase
Describe DNA replication during interphase
- Chromosomal DNA is replicated exactly and the two pieces of DNA are held together by the centromere
- If the copies are not done accurately mutations may occur and daughter cells will NOT receive identical genetic material
- Following mitosis the two new cells will contain one of each sister chromatids
What is mitosis (nuclear division) important for?
1 - All organisms that need to produce genetically identical daughter cells
2 - Asexual reproduction - single celled organisms divide to produce daughter cells. Multi-celled organisms may also divide asexually
3 - Growth - multi-cellular organisms grow by producing new extra cells that are genetically identical to each other and the parent cell
4 - Repair - damaged cells need to be replicated by new ones that perform the same function so must be identical
5 - Replacement - erythrocytes and skin cells need to be replaced
What are the 4 phases of mitosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
Explain what happens in prophase
- Chromosomes shorten and thicken into supercoils
- Each chromosome consists of a pair of sister chromatids
- Nuclear envelope breaks down and disappears
- Centriole divides into two
- Daughter centriole moves to opposite ends of the cell to form a spindle
Explain what happens in metaphase
Metaphase (Middle)
- Chromosomes line up down the middle of the cell
- Each chromosome is attached to a different spindle fibre by its centromere
Explain what happens in anaphase
Anaphase (Apart)
- The replicas of each chromosome are pulled apart from each other towards opposites poles of the cell
- Identical sister chromatids are pulled to different poles by the shortening of the spindle fibres
Explain what happens in telophase
Telophase (Two)
- Two new nuclei are formed
- Spindle breaks down and disappears
- Chromosomes uncoil and cannot be seen in light microscope anymore
Explain what happens in cytokenesis
- This is when the cell divides
- Occurs between telophase and interphase
- Each new daughter cell is identical to the parent cell and to each other
- In animal cells microtubules form a ‘draw string’ just inside the membrane which then fuses as it is pinched in
- In plant cells microtubules direct vesicles to the middle of the cell to form a cell plate, which forms a new cell wall. New cell surface membrane is made on either side to enclose the two cells
How does cancer occur?
A mutation in a gene that controls cell division, the cells can grow out of control which can lead to the formation of tumours. A tumour becomes cancerous if it changes from benign (contained and not spreading to other parts of the body) to malignant ( opposite of benign)
Drugs used to treat cancer usually disrupt the cell cycle by:
- Preventing DNA from replicating. Radiation and some drugs drugs damage DNA. If severe damage to DNA is detected the cell will kill itself (apoptosis)
- Inhibiting the metaphase stage if mitosis by interfering with spindle formation
- Preventing the synthesis of enzymes needed for DNA replication in growth 1
How is it possible to detect cancer cells in terms of looking at the cell cycle?
The mitotic phase of the cell cycle is longer than the whole of interphase