Cells Flashcards
(18 cards)
Eukaryotic cell structure and functions
-Nucleus: stores genetic info that codes for primary structure of polypeptides, site of transcription to synthesise mRNA, synthesises rRNA, site of semi conservative replication. It is made up of:
-Nuclear envelope and pores: controls entry and exit of substances in and out the nucleus.
-Chromatin wrapped around histones: linear DNA that condenses into visible chromosomes during mitosis.
-Nucleolus: manufactures rRNA and makes ribosomes.
-Mitochondria: site of ATP synthesis via aerobic respiration. Made of:
-Double membrane: controls entry and exit of substances in and out the mitochondria.
-Cristae: folded inner membrane. Provides large SA for enzymes involved in aerobic respiration.
-Matrix: liquid containing proteins, lipids, mitochondrial DNA and 70s ribosomes, which allows the mitochondria to make its own proteins for respiration, independent from the nucleus.
-Rough endoplasmic reticulum: has ribosomes on surface for protein synthesis+packages proteins into vesicles and transports to Golgi apparatus.
-Smooth endoplasmic reticulum: synthesises, stores and transports lipids+carbs to Golgi to be modified.
-Golgi apparatus: sorts, modifies, processes and packages proteins into vesicles which move towards and fuse with the csm, releasing their contents via exocytosis.
-Lysosomes: produced by Golgi, contain hydrolytic enzymes such as lysozymes. They break down old worn out organelles so they can be recycled, release enzymes outside cells to hydrolyse molecules and bacteria, and breakdown cells after they die.
-Ribosomes-site of protein synthesis, made of proteins sub units and rRNA.
Plant cells additionally contains:
Prokaryotic cell structure and functions
-Cell wall: made of meurin, provides support
-Cell membrane: controls movement of substance in and out of cells.
-Slime capsule: protects from being attacked by immune system or dehydration.
-Cytoplasm: contains enzymes and other molecules. Site of reactions. No membrane bound organelles.
-Circular DNA (nucleoid): single strand on DNA nt associated with histones, no introns, and not membrane bound.
-Plasmids: small loops of extra DNA which helps carry genes for survival e.g. antibiotic resistance genes.
-70s ribosomes: smaller than those in eukaryotes. Help carry out protein synthesis.
-Flagella: tail like structure for movement.
Light microscopes
-use visible light to illuminate specimen
-glass lenses focus the light to form a magnified image
-used to view living and dead specimens
-need to be stained with a dye to make them visible
The magnification is limited because at higher magnifications, the microscope loses resolution
The resolution is limited because the wavelength of light is relatively long, so light waves cannot resolve smaller objects.
They’re able to view cells and the nucleus, but cannot view smaller organelles or internal structures.
Transmission and scanning electron microscopes.
-uses a beam of electrons rather than light (shorter wave length, so higher resolving power) to illuminate specemin.
TEM:
-electrons pass through specimen, allowing the visualisation of internal structures such as organelles.
-specimen is thinly sliced (so electrons can pass through) and fixed in resin, so they must be dead. Denser areas of the specimen scatter more electrons, creating a contrast of the image.
-very high resolution and magnification (up to x1500000), produces a 2D black and white image.
-specimens are stained with heavy metals and must be placed in a vacuum.
SEM:
-electrons are scanned across the surface of the specimen and bounce off the surface and a detector collects the scattered electrons to produce an image.
-the resolution and magnification are lower than a TEM but higher than a light microscope. Produces a 3D image which provides excellent surface detail.
-it is in black and white, coated in a precious metal and the species must be dead and placed in a vacuum.
Both methods are expensive, require heavy equipment and training, can result in the formation of artefacts and produce black and white images and cannot visualise live specimens.
Resolution and magnification
Resolution is the ability to distinguish between objects that are close together
Magnification is how many times larger and image has been made compared to the actual object.
Eyepiece graticule and stage micrometer
Cell fractionation and ultracentrifugation
-Tissue is homogenised in a blender in an ice cold, isotonic, buffered solution to break open the cells and release their contents.
-Ice cold: reduced enzyme action that could damage organelles
-Isotonic: same WP so prevents osmosis so cells don’t burst (lysis) or shrivel
-Buffered: maintains constant pH as changes could denature proteins/enzymes
-mixture is filtered to remove large debris that isn’t homogenised, producing a solution of suspended organelles
-tube containing solution is placed in a centrifuge and spun (centrifuged) at low speed. The densest organelles (nucleus) are forced to the bottom of the tube into a pellet which is removed. The solution above the pellet is the supernatant.
-the supernatant is then centrifuged at higher speeds so that smaller, less dense organelles form a pellet.
-this process is repeated multiple times at higher speeds each time to separate all the organelles in the supernatant into pellets.
Interphase
Cells increase in mass, size and prepare for cell division by synthesising proteins and replicating DNA.
G1: cells make RNA, enzymes and other proteins required for growth. New organelles are made.
S: semi conservative replication occurs to produce DNA.
G2: cells continues to grow and checks for DNA errors.
Mitosis and cytokinesis + recognising stages from images
Cell division which produces two genetically identical daughter cells from a single parent cell.
-Prophase: chromosomes condense and supercoil and become visible. Each chromosome consisting of two sister chromatids joins at the centromere. Nuclear envelope breaks down and spindle fibres begin to form from centrioles.
Appearance: chromosomes condense and are visible.
-Metaphase: chromosomes line at the centre of cell/equator. Spindle fibres attach to centromeres.
Appearance: chromosomes lined up in the middle of cell.
-Anaphase: centromeres split, separating sister chromatids which are pulled to opposite poles by spindle fibres.
Appearance: identifiable by “V” formation. Being pulled away to opposite poles.
-Telophase: chromatids reach opposite poles and uncoil into chromatin. New nuclear envelopes form around each set of chromosomes. Spindle fibres break down.
Appearance: appear thinner as they uncoil. Nuclear envelope is reforming.
-Cytokinesis: cytoplasm divides forming two genetically identical cells.
Appearance: cleavage furrow forms.
Importance of mitosis
Growth: enables unicellular zygotes to grow into multicellular organisms.
Repair: damaged tissues are repaired, helps regenerate body parts in some animals e.g. axolotl legs.
Asexual reproduction: used by some organisms such as bacteria and plants to reproduce without a partner, producing genetically identical clone offspring.
Uncontrolled cell division and cancer
Cancers arise due to uncontrolled mitosis. This leads to cells dividing rapidly and uncontrollably, forming a tumour-an abnormal mass of cells. Cancers start when there’s a change in the genes that control cell division (mutation). Most mutations are harmless, and those that do kill cells, Those cells can be easily replaced. The mutation resulting in cancerous cells doesn’t result in early cell death, meaning it can continue to replicate and spread.
Carcinogens are any agents that cause cancer by increasing the rate of mutations (UV light, x rays, smoking etc). Benign tumours and non cancerous, and don’t spread from their site of origin. Malignant tumours are cancerous, and do spread to other tissues and destroy them.
Binary fission of prokaryotes
-The single circular DNA molecule undergoes DNA replication
-Any plasmids present undergo DNA replication
-The parent cell divides into two with the cytoplasm roughly halved between the two daughter cells.
-The two daughter cells contain a single copy of the circular DNA molecule and a variable number of plasmids.
Properties of phospholipids
They have a polar region (hydrophilic head) and non polar region (hydrophobic tail). This means the tails are repelled by water and face away from water and towards each other, whilst the heads point towards the water, resulting in the formation of a bilayer. Because of these properties, only small, non polar, lipid soluble molecules can pass through without the assistance of intrinsic and extrinsic proteins.
Fluid mosaic model/cell surface membrane
-The fluid mosaic model of the csm consists of proteins, phospholipids, carbohydrates, cholesterol and other components. The molecules that it consists of are constantly moving and so it’s named the fluid mosaic model (fluid as it has moving components and mosaic as there are many components).
Components:
-carrier proteins: have a specific tertiary binding site which aids the transport of ions and polar molecules via facilitated diffusion and active transport. Requires ATP
-channel proteins: have a specific tertiary structure. They’re filled with water and allow small water soluble ions and polar molecules to pass though.
-Enzymes e.g. membrane bound maltase in the ileum.
-Cholesterol: decreases permeability and increases stability by restricting the movement of other molecules. Causes fatty acid chains to pack together closer, increasing the membranes stability.
-Receptors: some proteins act as specific receptors for hormones with complimentary shapes e.g insulin binds to its receptor.
-glycolipids and glycoproteins: act as receptors and aid in cell recognition, sometimes acting as antigens.
Simple diffusion
-Net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient until equilibrium is reached. It is a passive process (doesn’t require ATP).
Factors affecting rate of diffusion:
-Temperature: increased kinetic energy therefore faster movement of molecules.
-Surface area: more csm for molecules to pass through therefore faster rate of diffusion e.g alveoli
-Concentration gradient: increased conc grad=faster rate of diffusion
-Diffusion distance: thinner surface means faster diffusion rate
Facilitated diffusion
Passive net movement of molecules or ions from an area of high concentration to an area of low concentration down a concentration gradient across a cell surface membrane through specific channel or carrier proteins without the use of ATP.
Polar molecules such as water cannot pass through the csm due to the hydrophobic phospholipid tails which are non polar able repel polar molecules so they have to pass through channel or carrier proteins which have a specific tertiary structure and only transport molecules with complimentary shape to the binding site.
Osmosis and water potential
Net movement of water molecules from an area of higher water potential to an area of lower water potential across a partially permeable membrane.
When water molecules collide, they exert pressure known as water potential measured in kPa. The more free water molecules, the greater the number of collisions and so the higher the WP. Pure water has a WP of 0. Adding solute (water soluble molecules) lowers WP as water is polar, so adding solute attracts the water molecules, decreasing the number of free moving water molecules.
Different solutions have different water potentials:
-Hypotonic: solution has higher WP than cell. Water moves in cell via osmosis. Cell can swell and burst (lysis).
-Hypertonic: solution has lower WP than cell. Water moves out of cell via osmosis. Cell can shrivel.
-Isotonic: equal WP in and out of cell so no net movement.
Active transport
