2.1.5 Flashcards
What is compartmentalisation?
The formation of separate membrane bound areas within a cell ; separate the contents of cells from their external environment and the different organelles within cells from the cytosol
Why is compartmentalisation important?
Metabolism includes many different and often incompatible reactions so containing these reactions allows specific conditions required for cellular reactions (chemical gradients) to be maintained
What is the plasma membrane?
The phospholipid bilayer that separates the cell from its external environment
Structure of phospholipid bilayer?
Hydrophilic phosphate heads form both the inner and outer surface of a membrane - sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane
Where are aqueous environments found and how do they matter?
Inside of cells are usually aqueous and bilayer are perfectly suited because the outer surfaces of the hydrophilic phosphate heads can interact with water
When were cell membranes first seen?
Invention of electron microscope lead to images be taken with higher magnification and resolution ; images in 1950s show cell membrane as 2 black parallel lines
When was the fluid mosaic model proposed?
1972 - Singer and Nicolson built upon earlier lipid bilayer model
Describe fluid mosaic model
Phospholipids are free to move within the layer relative to each other (they are fluid) giving the membrane flexibility and because the proteins that occupy various positions in membrane vary in shape size and position (same as mosaic)
Glycoprotein
Branched carbohydrate portion of a proteins which acts as a recognition site for chemicals (hormones)
Glycolipids
Acts as a recognition site for cholera toxins
Cholesterol
Adds stability and flexibility
What are extrinsic proteins?
Lay on surface
What are intrinsic proteins?
Span the whole phospholipid layer
Examples of intrinsic proteins
Integral proteins that are embedded through both layers of a membrane - hydrophobic r groups of their amino acids interact with hydrophobic core keeping them intact
CHANNEL AND CARRIER - BOTH TRANSPORT PROTEINS
Channel proteins
Provide a hydrophilic channel that allows passive movement of polar molecules and ions DOWN a concentration gradient ; held in place by hydrophobic interactions
Carrier proteins
Passive AND Active transport - down and against concentration gradient into cells => shape of protein changes
What are glycoproteins?
They are intrinsic proteins that are embedded in cell surface membrane with attached sugars chains ; they help in cell adhesion and act as receptors for chemical signals
Cell signalling
When chemical bonds to receptor it elicits a response - binding of neurotransmitters triggers impulse to next neurone for example
Drugs act by attaching to cell receptors (beta blockers)
What are glycolipids?
Similar to glycoproteins as they are lipids with attached sugar chains - cell markers/antigens and can be recognised by cell as self (of the organism) or non-self (belonging to another organism)
What are extrinsic proteins?
Peripheral proteins only on one side of bilayer - normally have hydrophilic R-groups on their outer surfaces and interact with polar heads of phospholipids ; EITHER LAYER AND CAN MOVE BETWEEN
Structure of cholesterol
Lipid with a hydrophilic and hydrophobic end
Role of cholesterol
Regulates fluidity of membrane by positioning itself between phospholipids with hydrophilic end interacting with PO43- heads and hydrophobic with tails - this adds stability to membrane without making them too rigid. Also prevents membranes becoming too solid by stopping the phospholipid molecules from grouping together and crystallising
Membranes as sites of chemical reactions
Proteins in membrane forming organelles have to be in particular positions for chemical reactions to take place
Requirements for the production of ATP in respiration
Electron carriers and enzyme ATP synthase have to be in the correct positions within the cristae (inner membrane of mitochondrion)
What factors affect membrane structure?
Temperature and presence of solvents
Affect of temperature on cell membrane?
Phospholipids will have more KE and will move more making membrane more fluid and making membrane lose its structure. If temperature increases too much then cell breaks down completely. Loss of structure increases permeability of the membrane making it easier for particles to cross it
Affect of temperature on proteins
Carrier and channel proteins will be denatured at higher temperatures so permeability of cell membrane affected as unable to carry molecules across
How does the cell membrane interact with water?
Non-polar hydrophobic core orientated away from water and hydrophobic heads interact with water - keeping bilayer intact
What do organic solvents do?
They are less polar than water (or non-polar) - so they dissolve membranes by disrupting cells
Why is alcohol used in antiseptic wipes?
Alcohols dissolve the membranes of bacteria in a wound, killing them and reducing infection
How does range of concentration affect cell membrane?
Very strong - destroy cells
Alcoholic drinks - cause damage but do not dissolve membrane
Non-polar alcohol MOLECULES - can pass through membrane and this disrupts membrane
What happens when cell membranes are disrupted?
More fluid + more permeable - some cells need intact membranes to perform specific functions such as transmit neuronal impulses ; this occurs in the brain explaining changes seen in people’s behaviour after consuming alcoholic drinks
LOOK AT BEETROOT PAG
LOOK AT BEETROOT PAG
Active
Required metabolic energy
How does passive movement use energy?
Utilises energy from the natural motion of particles rather than energy from another source
Define diffusion
Net overall movement of particles from an area of high concentration to an areas of low concentration down a concentration gradient - it is passive and continues until equilibrium (no difference in concentrations) is reached
Why does diffusion occur?
Particles in a gas or liquid have KE so movement is random and unequal distribution becomes an equal distribution - particles DO NOT stop moving, movement is just equal in both directions
Why does diffusion slow down over longer distances?
Rate slowly down because more collisions take place slowing down movement
Why are cells microscopic (diffusion)
Movement of particles within cell depends on diffusion and therefore short diffusion distance increases rate - supplying cells with particles needed and allowing reactions to take place quickly enough
Factors affecting rate of diffusion
Temperature and concentration difference
How does temperature affect diffusion?
Particles have higher KE move at higher speeds
How does concentration gradient affect diffusion?
Greater the difference means larger overall net movement of particles from higher concentration to lower concentration
What is simple diffusion?
Diffusion without a barrier or membrane
LOOK AT AGAR JELLY DIFFUSION PRACTICAL
LOOK AT AGAR JELLY DIFFUSION PRACTICAL
Diffusion across membranes
Can only occur if membrane is permeable ; non-polar molecules like O2 can diffuse freely DOWN a concentration gradient
Why are membranes described as partially permeable?
Hydrophobic core repels substances with positive/negative charge so they cannot pass through - polar molecules such as H2O can diffuse through SLOWLY because they are SMALL so they can pass through more easily than larger ones (partially)
Rate at which substances diffuse across membrane
Surface area - larger SA = higher rate of diffusion (more particles)
Thickness - thinner = higher rate because shorter distance
Facilitated diffusion
Membranes contain channel proteins through which polar molecules/ions can pass - diffusion across a membrane through protein channels is called facilitated diffusion.
Selectively permeable
Membranes with protein channels that are specific to one molecule or ion (partially permeable is down to size)
Describe facilitated diffusion
Can involve carrier proteins which change shape when a specific molecule binds - movement is DOWN a concentration gradient and does not require external energy
What is rate of facilitated diffusion affected by?
Thickness, SA, Temperature, Concentration gradient AND NUMBER OF CHANNEL PROTEINS - more channel proteins = higher rate of diffusion
LOOK AT DIALYSIS EXPERIMENT
LOOK AT DIALYSIS EXPERIMENT
Purpose of active transport
Maintain this concentration gradient for diffusion to take place - active transport must occur faster than rate of diffusion and this requires external metabolic ENERGY => maintains concentration gradient
Define active transport
Movement of particles into our out of a cell from a region of lower concentration to a region of higher concentration - requires energy (ATP) and carrier proteins ; particles are moved against concentration gradient
Carrier proteins act as…
Pumps (in Active Transport)
General process of active transport (outside to inside)
1) Molecule binds to receptors in the channel or carrier protein on outside
2) Triggers ATP binding to cell on inside and hydrolysing to ADP
3) Binding of phosphate molecule to carrier protein causes change of shape
4) Molecule is released to inside of cell
5) Phosphate released and recombined to ADP to form ATP
6) Carrier protein returns to original shape
Is active transport selective?
SPECIFIC SUBSTANCES ARE TRANSPORTED
What is bulk transport?
Another form of active transport for larger molecules such as enzymes/hormones that cannot move through carrier proteins so they are moved through bulk transport
Endocytosis
Bulk transport INTO cells - phagocytosis (solids) pinocytosis (liquids) - cell surface INVAGINATES (BENDS INWARDS) when it comes into contact with molecule and membrane ENFOLDS until membrane fuses forming a vesicles => PINCHES OFF AND MOVES INTO CYTOPLASM to transfer material for further processing within the cell… vesicle donating bacteria are moved towards lysosomes where bacteria are digested by enzymes
Exocytosis
OUT OF CELL - vesicles (formed by Golgi - could be secretory ) move towards and fuse with cell membrane releasing them to outside
How is energy required for bulk transport?
Energy in the form of ATP is required for movement of vesicles along the cytoskeleton - changing shape of cells to engulf materials and the fusion of cell membranes as vesicles form or as they meet the cell surface membrane
Define osmosis
It is the diffusion of water across a partially permeable membrane from an area of high water potential to an area of low water potential - it is passive and down a concentration gradient
Water potential
Pressure exerted by water molecules as they collide with a membrane or container - measured in units of pressure (Pa) and symbol for water is psi
Water potential values
Pure water - 0kPa
Highest possible value for water potential - all solutions have negative values and more solute/concentrated = more negative water potential
What happens when there are different concentrations?
Different water potentials separate by partially permeable me brand means water molecules can move between BUT SOLUTES CANNOT - net movement of water from less concentrated to more concentrated UNTIL water potential is equal on both sides (equilibrium)
What does osmosis cause?
Diffusion of water into solution increases volume of solution - if in closed system (cell) this results in increased pressure called HYDROSTATIC pressure - same units as water potential (kPa) => cellular level this is large and damaging
Hypertonic
Low water potential
Hypotonic
High water potential
Isotonic
Same water potential
If animal cell placed in a hypotonic…
Water moves in - increased hydrostatic pressure and thin cell surface membranes means they cannot stretch much and withstand pressure - causing cell to burst (cytolysis)
If animal cell placed in hypertonic solution…
Lose water by osmosis down water potential gradient and this will cause a reduction in the volume of the cell and cause cremation
How to prevent cytolysis or cremation?
Control mechanism - continuously surrounded by squeaks solutions with an equal water potential (BLOOD PLASMA)
Are plant cells able to control the water potential around them?
No - roots surrounded by almost pure water
What happens when water enters plant cells?
Increased hydrostatic pressure pushes against strong cellulose walls - this pressure is called TURGOR ; resists entry of further water and is said to be turgid
What happens when plant cells are placed in a hypertonic solution?
Water is lost therefore reduction in volume of cytoplasm which pulls away membrane from cell wall - has been PLASMOLYSED
