Cells And Membranes Flashcards
Endocytosis
Bulk movement of material into the cell Membrane engulfs the material Membrane fuses together Vesicle is formed ATP required
Exocytosis
Bulk movement of material out of the cell
Vesicle fuses with the membrane
Substances moved out of the cell
ATP required
2 examples of bulk transport
Hormones - pancreatic cells make and package insulin into vesicle in the Golgi apparatus. When the vesicles fuse with the membrane the insulin is released into the blood stream
White blood cells - engulfing microorganisms by forming a vesicle around them and then allowing the vesicle to fuse with a lysosome in the WBC yo allow digestion of the microorganism (phagocytosis)
Plasma membranes are found
Around cells and some organelles
3 membrane facts
Partially permeable barriers
Made up of phospholipid molecules arranged in a bilayer and proteins
Under an electron microscope bilayer is seen as two dark bands and the distance across the membrane is about 7nm
Fluid mosaic model
The component molecules are not bonded together so there is some movement but it is relatively stable because of the nature of the phospholipid
Singer and Nicholson proposed it in 1972
Reason for fluid mosaic name
Fluid= phospholipids and proteins are free to move laterally (sideways
Mosaic= proteins have scattered arrangement within the phospholipid bilayer
Cell membrane diagram
LEARN
Phospholipids
PM
Make up most of plasma membrane. Arranged in a bilayer
Hydrophobic tail faces inwards
Hydrophilic head faces outwards
Act as a barrier to water soluble, polar molecules and ions
Allows lipid soluble, small, non polar substances to diffuse through
Make the membrane flexible and self sealing making endo and exocytosis possible
Cholesterol
PM
Found in eukaryotic cells for stability and fluidity
Steroid molecule that fits between fatty acid tails completing the membrane barrier to water and ions
Binds to the hydrophobic tails of the phospholipid, they prevent the membrane being too fluid at high temperatures but also stops the membrane solidifying at low temperatures
Glycolipids
PM
Phospholipids with carbohydrate attached
Inter cell signalling and recognition so immune system doesn’t kill it
Cell adhesion: helps to stick cells together and to basement membranes to form tissues
Intrinsic proteins
PM
Span entire membrane
channel or carrier proteins that allow transport of hydrophilic and large molecules like glucose through the phospholipid bilayer
Extrinsic proteins
Span half of the membrane Inter cell signalling Cell recognition Enzymes eg in mitochondria for respiration Receptors for signalling between cells Cell adhesion
Glycoprotein
Protein with carbohydrate attached
Receptor for signalling molecules
Cell signalling and recognition
Binding cells together to basement membrane making tissues cell adhesion
Resolution definition
The shortest distance between two points on a specimen that can be distinguished clearly
Magnification definition
The degree to which the size of an image is larger than the object itself
Magnification equation
I
A. M
Cell fractionation conditions
Large number of isolated but still functional organelles
Cold to reduce enzyme activity that might break down or digest organelles
Isotonic so that cells don’t burst or shrink
Buffered so PH doesn’t alter the structure of organelles or denature proteins
Homogenisation
Cells broken by homogeniser blender
Breaks plasma membrane and releases organelles
Filtered
Ultracentrifugation
Span at high speeds
Creates a centrifugal force towards the bottom of the tube
Supernatant is the fluid at the top removed and then span again
Order of organelles ultracentrifuge
Nuclei
Mitochondria
Ribosomes
Membrane
Light microscope
Magnification 1500x
Resolution 200nm
Light passes through the specimen
Can be viewed in colour
Living specimen
TEM
Electron pass through very thin section
Sample is dipped in lead or heavy metal and absorb different amounts
2D black and white
Magnification 500000x
Resolution 0.1 nm
SEM
Electron beam on surface
Bounce off and are detected by sensors
3D image black and white
Surface of specimen
Magnification 100000x
Resolution 20 nm
Microscope limitations
Living organisms can’t be observed in electron as it has to be in a vacuum
Complex straining system
Has to be very thin
May contain artefacts when it is prepared
Nucleus
5
Nuclear envelope= double membrane, contains nuclear pores, controls entry and exit of materials
Nuclear pores= allow large molecules such as mRNA
Nucleoplasm= bulk of the nucleus
Chromosomes= consist of linear DNA and histone proteins, instructions for building proteins, can be copied into mRNA
Nucleolus= makes RNA and ribosomes
Endoplasmic reticulum (2)
Series of flattened membrane bound sacs called cisternae. These are continuous with outer membrane of nucleus, and, in the case of rough endoplasmic reticulum, are studded with ribosomes.
Cells that manufacture and store large quantities of carbohydrate, proteins and lipids have a very extensive ER, e.g. Liver cells and epithelial cells that line the intestines
Rough ER
Provide a large surface area for the synthesis of proteins and glycoproteins made by the attached ribosomes using instructions encoded on molecules of mRNA. Also, provide a pathway for the transport of proteins to other parts of the cell.
Smooth ER
No ribosomes, so appears smooth, its function is to synthesise, store and transport lipids. Also stores ions in the cell. Cisternae are tubular rather than flattened.
Mitochondria
5
Spherical or sausage shaped, with a double membrane, controls the entry or exit of material.
Inner membrane is highly folded into cristae= large surface area for the attachment of enzymes involved in the later stages of aerobic respiration.
The central part is the matrix. This also contains enzymes needed for aerobic respiration together with proteins, lipids, 70S ribosomes and mitochondrial DNA that allow the mitochondria to control the production of some of their own proteins.
Synthesis of ATP in the process of aerobic respiration. This requires oxygen.
Metabolically active cells such as muscle and epithelial cells require lots of ATP, and so will contain lots of mitochondria.
Golgi (7)
Stack of membrane bound flattened sacs called cisternae with small, rounded, hollow structures called vesicles.
receives proteins and lipids from rough and smooth ER and may modify them (by chemically adding carbohydrates).
It also ‘labels’ them so that they can be be accurately sorted and sent to the correct destination. Once sorted, they are transported in a vesicle which is pinched off from the ends of the Golgi cisternae.
These vesicles may then move to the cell surface, fuse with the cell surface membrane and release the contents e.g. hormones, enzymes or antibodies to the outside (exocytosis).
Golgi apparatus also forms lysosomes.
Golgi apparatus is well developed in secretory cells such as epithelial cells that line the intestines.
Chloroplasts
6
carry out photosynthesis to make carbohydrates from carbon dioxide and water.
The chloroplast envelope is a double plasma membrane that surrounds the organelle. It controls what enters or leaves the organelle.
Within the chloroplasts are membranous sacs called thylakoids (a stack of thylakoids is a granum). This provides a large surface area.
Chlorophyll molecules (photosynthetic pigments) are on the thylakoid membranes and in intergranal lamella. This is where the first stage of photosynthesis takes place - the light dependent stage (in which light energy is absorbed and ATP and NADPH produced – more about this is year 2)
Stroma is a fluid filled matrix where the second stage of photosynthesis takes place - the light independent stage. It contains all the enzymes needed for this stage of photosynthesis. Starch granules and lipid droplets can also be found here.
Chloroplasts, like mitochondria, also contain DNA (circular) and ribosomes (70S) so they can synthesise some of the proteins needed for photosynthesis.
Lysosomes
3
Spherical sacs with single membrane.
Made by Golgi.
Contain powerful digestive enzymes/ lysozymes to break down/hydrolyse materials. E.g. White Blood Cell lysosome helps to break down micro-organisms.
Vesicles
3
Used to transport different substances around cells.
When substances are secreted by a cell, vesicles fuse with the cell surface membrane.
Movement of vesicles requires ATP.
Ribosomes
5
No membrane,
some on the surface of RER and some in cytoplasm.
They have two subunits - one large and one small. Each contains ribosomal RNA and protein.
They are the sites of protein synthesis.
mRNA (from the nucleus) is decoded by ribosomes to assemble a sequence of amino acids to synthesise a polypeptide (protein).
Permanent vacuole
2
Fluid filled sac surrounded by a single membrane called a tonoplast. Contains variable amounts of water, mineral salts, sugars, amino acids, waste and, sometimes, pigments.
It maintains cell turgidity (stiffness) by pushing cytoplasm against the cell wall. Together, lots of turgid cells help to support the whole plant. It is also involved in the isolation of unwanted chemicals.
Cell walls
4
In plants it is made of cellulose (made of polymers of β glucose)
Provide strong wall for support and strength in the plant and to prevent cells bursting.
In fungi, the wall is made of a complex polysaccharide called chitin.
In bacteria/prokaryote it is made of peptidoglycan or murein.
Cytoskeleton
4
This is a network of protein fibres which provide support and give shape to the cell
. It also holds organelles in place or allows them (e.g. vesicles) to move through the cytoplasm
. It is responsible for the movement of the chromosomes during cell division.
It is also involved in changing the shape of the cell surface membrane during endo/exocytosis.
Structure of prokaryotic cells
6
Much smaller than eukaryotic Cytoplasm with no membrane bound organelles Smaller 70S ribosomes No nucleus Free circular loop of DNA Muerin cell wall
Binary fission steps
4
- The circular DNA and plasmids replicate, the main DNA loop is only replicated once but plasmids can replicate multiple times
- The cell gets bigger and the DNA loops moved to opposite poles of the cell
- The cytoplasm begins to divide and a new cell wall begins to form
- The cytoplasm divides into two daughter cells are produced each daughter cell has one copy of the circular DNA but a variable number of copies of the plasmids
Antibiotics
5
Only work on bacteria
Prevent bacteria from making normal cell walls
Penicillin inhibits certain enzymes required for synthesis and assembly of peptide cross links in cell walls
Weakens the walls making them unable to withstand pressure
Water can the. Enter by osmosis and bursts the cell
Structure of viruses
4
Acellular
Non living
Consist of nucleic acids surrounded by protein
Don’t contain aany organelles
Viruses
4
Don’t undergo cell division
Inject DNA or RNA into host cell which then priduce the viral components
Have attachment proteins which are complimentary to the host cell
Some can only attach to one but others can attach to multiple
Roles of membranes within cells
4
:
1.Separate cell components from cytoplasm COMPARTMENTALISATION
- Holding the components of metabolic pathways in place e.g. mitochondria
- Controls what enters or leaves the organelle
- Site of attachment for enzymes/ribosomes
Roles of membranes at the surface of cells
- Separate cell contents from the outside environment
- Cell recognition and signalling
- Regulating transport of materials into or out of cells
- Create concentration gradient
Cell signalling
Membrane
Processes that lead to communication and coordination between cells so they can work together to trigger a response
Cell signalling allows cells to be recognised and prevent them being destroyed by the immune system
A receptor in the membrane detects chemical signals and brings about responses
Protein receptors on target cells have a specific shape complimentary to a specific hormone
Temperature and membrane
35-50ºC = Increase in temperature increases kinetic energy, molecules can diffuse across membrane more quickly.
.
50-60 ºC = increase in temperature denatures proteins making membrane more leaky. Also, the phospholipids melt becoming much more fluid. This causes gaps to open temporarily in the bilayer and results in ions/molecules diffusing more quickly.
60-70 ºC = Further increase in temperature causes the membrane to break down completely. Ions/molecules diffuse in first few moments of experiment and reach equilibrium quickly.
Simple diffusion
Concentration gradient
Molecules have kinetic energy
No additional energy from ATP
No specific proteins
Lipid soluble, small non polar and very small polar allowed through
Facilitated diffusion
Concentration gradient
Molecules have kinetic energy
No additional energy from ATP
Proteins channels
Channel and carrier proteins
Osmosis
Water potential gradient from high to low
Through channel proteins or directly through membrane
Water potential measure in Kpa
Pure water is 0KPa which is the highest water potential
Hypertonic
Hypotonic
Isotonic
Hyper= Higher in the cell than out so water moves out of the cell Animals= cell shrinks and crenates Plants= cell becomes flaccid membrane pulls away from cell wall and it becomes plasmalysed
Hypo= Higher outside the cell so water moves into the cell Animals= cell burst (lysis) Plant= cell becomes turgid
Isotonic= Same water potential in and out of the cell so no net movement of water
Active transport
Against a conc gradient
Energy from ATP
Specific protein carriers required
Act as pumps
Specific complimentary shape to the molecule they carry
Carry large charged molecules through the bilayer
Active transport ensuring one way flow
ATP changes the shape one side
Once energy has been used it changes shape and releases the molecule
Anything which inhibits ATP inhibits active transport
Co transporter proteins
Co transporters are a type of carrier protein. They bind two molecules at a time. The concentration gradient of one molecule is used to move the other molecule against its own concentration gradient.
Co transport in the ileum
Glucose is absorbed into the bloodstream in the small intestine – ileum (the final part of a mammal’s small intestine)
. The concentration of glucose is too low for glucose to diffuse out into the blood, so glucose is absorbed from the lumen of the ileum by co-transporters.
The epithelial cells lining the ileum have microvilli, finger like projections to increase the surface area. They also have an increased number of protein channels and carrier molecules in their membranes.
Co transport
3 steps
- Sodium ions are actively transported out by sodium potassium pump which create a concentration gradient
- This causes sodium ions to diffusefrom the lumen of the ileum into the epithelial cell down a concentration gradient via sodium glucose transporter protein Cotransporter carries glucose into the cell with the sodium as a result concentration of glucose inside the cell increases
- Glucose defuses out of the cell into the blood down its concentration gradient through protein channel by facilitated diffusion
