Nucleus, Chromosmes, DNA and Transcription

nucleus:

  • largest organelle
  • takes up stains more than cytoplasm
  • nuclear envelope and nuclear pores
  • contains chromosomes

Chromosomes:

  • long molecules of DNA
  • too thin individually sotangle = chromating (darkly stained)

DNA:

  • carries instructions for making proteins
  • DNA in lighter stained parts of chromatin can be used for transcription

Transcription:

  • 1st stage of protein synthesis
  • DNA is copied onto molecules of mRNA
  • mRNA travels out of nucleus through pores into cytoplasm
  • darkest area of ucleus = nucleolous = contains DNA used to make ribosomes = tiny organelles where protein synthesis takes place
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  • Created by: Ellie
  • Created on: 03-12-13 20:54

Endoplasmic Reticulum

Network of membranes within the cytoplasm of every eukaryotic cell.  Not present in prokaryotic cells.

Rough ER:

  • some of the membranes have ribosomes attached to them giving them a rough look
  • where most protein synthesis takes place (happens on ribosomes).  As the proteins are made they collect inside the cisternae and are transported to other areas in the cell e.g. golgi apparatus
  • = a space enclosed by the membranes of the endoplasmic reticulum)

 

Smooth ER:

  • membranes with no ribosomes
  • different roles in different cells
  • in cells of ovaries and testes = site of production of steroid hormone e.g. oestrogen
  • in liver cells = where toxins are broken down and made harmless
  • mainly transport lipids
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Golgi Apparatus

  • a stack of curved membranes, enclosing a series of flattened sacks
  • some cells have several
  • not a stable structure
  • at one side tiny membrane-bound vessicles move towards the golgi apparatus and fuse together = new layer to stack
  • these vessicles have come from the ER, containing proteins, in the GA these are packaged and processed changing them into the required product
  • at other side, sacks break down = vesicles that move away from the GA
  • these vesicles pinch off and move towards the cell membrane where they fuse and release their contents by exocytosis
  • some vesicles remain in the cell - some contain proteins and work as digestive enzymes: lysosomes
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Lysosomes

  • tiny bags of digestive enzymes
  • surrounded by a single membrane
  • main funtion = fuse with other vesicles in the cell that contain something that need to be digested e.g. bacterium
  • help destroy worn-out or unwanted organelles
  • enzymes inside break down the harmful molecules
  • E.G. head of sperm contains a lysosome called acrosome - enzymes digest a pathway into the egg before fertilisation can take place
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Chloroplast

  • found in some plant cells NOT in animal cells
  • aite of photosynthesis
  • has a double membrane - envelope, isolating it's reactions from rest of cell
  • inside there are membranes called grana
  • grana form stacks = thylakoids
  • grana contains chlorophyll - where light dependant reactions of phootosynthesis take place, light energy captured by chlorophyll, splits water molecules = provides hydrogen ions used to make ATP = makes carbohydrates
  • stroma = background material of the chloroplast where light-independant reactions of phtoosynthesis take place
  • chloroplast often contains starch grains = energy store in plants
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Mitochondria

  • surrounded by envelope
  • site of aerobic respiration
  • oxygen and energy-containing molecules from glkucose produce ATP
  • cells that use up alot of energy e.g. muscle cells have a lot of mitochondria
  • inner membrane is folded to form cristae - here ATP is made
  • the background material = matrix
  • has a large surface area
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Ribosomes

  • tiny organelles
  • some are in the cytoplasm
  • some are bound to ER
  • each consits of 2 subunits
  • site of protein synthesis
  • act as an assembly line where mRNA is used to assemble proteins for amino acids
  • do not have a membrane surrounding them
  • in both prokaryotic and eukaryotic cells

 

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Vacuole

a membrane bound organelle that contains liquid

membrane = tonoplast

mature plant cells often have large vacuoles filed with sap

sap = sugars, pigments and enzymes

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Centrioles

found in animals cells

not in plant cells

make and organise microtubules (tiny structures made of tubulin - a protein)

during cell division microtubules form the spindle and move the chromosomes around in the cell e.g. pulling them to opposite ends

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cillia and flagella

long thin extentions of surface membrane

found in some animal cells and rarely in plants

cilia = short and found in large numbers

flagella =  long and found in 1's or 2's

contain microtubules

movement produced by microtubules sliding past eachother causes them to bend and straighten

cilia in a group of ciliated cells usually all move simaltaneously

movement of cilia moves fluids over surface of cell e.g. in ligning of bronchus, cilia move mucus upto throat

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Cell Signalling

signals arrive at plasma membrane from outside cell as a substance or changes in electrical potential

receptor in plasma membrane picks up these signals and brings out a response

hormones is a common substance that arrives at the plasma membrane

there are 3 types of cell signalling:

  • receptor acts as ion channel - signal/chemical attaches to protein/glycoprotein, this makes the channel open and it lets ions in
  • receptor activates G -protein - signal molecule interacts with receptor, the receptor interacts with another molecule (G-protein), which then activates it
  • receptor acts as an enzyme - the receptor is an enzyme and made of 2 parts; when signal molecule arrives it slots into both parts connecting them making them an active enzyme, the enzyme then brings out a reaction
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Movement across cell membranes

Facilitated Diffusion

  • many molecules and ions are too big to pass through membranes
  • cells provide pathways through which molecules and ions like this can pass - channel proteins
  • these proteins form a hydrophilic channel so ions can pass by diffusion down their concentraion gradient = passive
  • each channel formed by a protein can only allow 1 specific ion or molecule to pass through
  • protein channels can change shape - open and close

 Osmosis

  • water molecules are small enough to pass through membranesby diffusion
  • moves down concentration gradient through a p/p/membrane
  • water moves down its water potential gradient from where there is lots of water to where there is little
  • turgid, plasmolysed (plants) or crenates (animals)

Active Transport

  • sodium ions and potassium ions
  • most cells have higher potassium, lower sodium than the concentration outside the cell
  • to achieve this cells constantly pump sodium out and potassium in, up their concentraion gradients
  • requires energy from ATP - active transport

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endocytosis and exocytosis

Endocytosis

the movement of bulk liquids or solids into a cell, by the indentation of the plasma membrane o for vesicles containing the substance; an active process requiring ATP

Exocytosis

the movement of builk liquids or solids out of a cell, by the fusion of the vesicles containing the substance with the plasma membrane; active process requiring ATP

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animal tissue

Epithelial = surface tissue

Basement membrane = network of collagen and glycoproteins that hold epithelial cells in positiion

Squamous epithelium = covers many surfaces e.g. inner lining of cheeks, inner surface of blood vessels and inner surface of atria and ventricles of heart.  The cells are smooth , flat, thin and close together = little friction = fluids move easily over it.  The thinness also allows rapid diffusion of gases between alveoli and blood in the lungs

Ciliated Epithelium = made up of cells that possess cilia.  Lines the ends of bronchioles.  The exposed areas have cilia on them - these move in harmony to e.g. move mucus upto throat or move egg cells from the ovary along the oviduct

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Exchange Surfaces

As an organism gets bigger, both its surface area and volume increase.  However, its surface area doesnt increase as much as the volume

SO a large organism e.g. humans, must find ways of increasing surface area to provide enough surface to ensure that exchanges with the environment can take place rapidly enough to supply all of the cells with their needs

  • the exchange surface should have a large surface area.  The larger the area across which a substance can diffuse, the more substances can cros the surface in a given time
  • the exchange surface should be thin.  The shorter the distance the substance has to diffuse, the less time it takes
  • there must be a diffusion gradient across the exchange surface - the concentrations of the substance on one side of the surface must be different from the concentration on the other, so that the substance diffuses down the gradient passively
  • in a terrestrial animal (on land) the surface cells must be protected from drying out.  If wet cells are exposed to dry air, water vapour will diffuse out of them into the air.  If too much water is lost, the plasma membrane will lose structure = dead cells
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