Definition of fluid mosaic model
Theory of cell membrane structure with proteins embedded in a sea of phospholipids
Definition of glycolipid
Lipid/phospholipid with chain of carbohydrate molecules attached
Definition of glycoprotein
Protein with chain of carbohydrate molecules attached
Definition of plasma membrane
Cell surface membrane
Definition of diffusion
Movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached; may or may not be across a membrane; not involving metabolic energy
Definition of facilitated diffusion
Movement of molecules from an area of high concentration to an area of low concentration until equilibrium is reached, across a partially permeable membrane via protein channels or carriers;not involving metabolic energy
Definition of osmosis
Passage of water molecules down their water potential gradient, from an area of high water potential to an area of low water potential across a partially permeable membrane
Definition of water potential
Measure of tendency of water molecules to diffuse from one region to another
Definition of active transport
Movement of substances against their concentration gradient (from an area of high to low) across a cell membrane using ATP and carrier proteins
Definition of endocytosis
Bulk transport of molecules, too big to pass through cell membrane even via channel or carrier proteins, into a cell
Definition of exocytosis
Bulk transport of molecules, too large to pass through cell membrane even via chanel or carrier proteins, out of a cell
What does amphipathic mean?
Both polar (water soluble) and non-polar (non-water soluble)
What do cell membranes do?
Form barriers and seperate cell contents from cells exterior environment and seperates organelles from cytoplasm
What does permeability refer to?
Ability to let substances pass through
Small moleculs …
Diffuse straight through
Some substances dissolve …
In lipid bilayer and pass through
Some substances pass through…
Protein channels or carried by carrier proteins
What are the roles of membranes at the surface of the cell?
Plasma membrane
Seperates cell contents from external environment
Regulates transport of materials in & out of cell
May contain enzymes involved in metabolic pathways
Has antigens so immune system recognises cell as 'self' and doesn't attack it
May release chemicals to signal to other cells
May be sight of chemical reactions
Contains receptors for signalling so sight for cell communication
At the surface of the cell, membranes seperate …
Cell contents from external environment
At the surface of the cell, membranes regulate …
Transport of materials in and out of cell
At the surface of the cell, membranes may contain…
Enzymes involved in metabolic pathways
At the surface of the cell, membranes have antigens …
So immune system recognises cell as 'self' and doesn't attack it
At the surface of the cell, membranes may release …
Chemicals to signal to other cells
At the surface of the cell, membranes may be
Site of chemical reactions
At the surface of the cell, membranes contain …
Receptors for signalling so it's site for call communication
What are the roles of membranes within cells?
Seperates organelles from cytoplasm to enable it to function
Some organelles metabolic processes occur on their membranes
What's the purpose of mitochondrias cristae?
Increase SA for aerobic respiration and localise enzymes needed for respiration to occur
What's the purpose of a chloroplasts thylakoid?
Houses chlorophyll
Photosynthesis reaction occurs
Whats the purpose of the epithelial cells surfacing the small intestine?
Contains digestive enzymes to catalyse the final stages in breakdown of certain sugars
What effects the permeability of a membrane?
Temperature
What does increasing temperature do?
Gives all molecules more KE. As result, molecules move faster
What does decreasing the temperature do?
Reduces KE of all molecules, causing them to move more slowly
Where are betacyanin pigments found?
Beetroot
What do betacyanin pigments do and why?
Help lower blood pressure as they're nitrogenous compound's and stimulate the linings of your arteries to produce nitric acid which dilates arteries and reduces blood pressure
What are the effects of organic solvents on phospholipids?
e.g acetone/ethanol will damage cell membranes as they dissolve lipids
What do the membranes in some plants act as?
Heat sensor
Heat-shock pathway:
What happens when proteins in plant membrane begin to denature?
Triggers expression of genes to make heat-shock proteins
What are heat-shock proteins?
Chaperone proteins that bind to misshapen proteins, preventing further unfolding
What triggers the heat-signalling pathway?
Influx of calcium ions caused by opening of calcium ion channels in membranes when cells subjected to mild heat
What does a calcium ion influx do to bacterias membranes?
Increases fluidity
What fatty acids are used to make membranes?
Unsaturated
What happens to saturated fatty acids when the temperature decreases?
Become compressed
What happens to unsaturated fatty acids when temp drops?
Become compressed and their kinks push adjacent phospholipid molecules away, maintaining membrane fluidity
What determines the fluidity of of cell membranes at cold temps?
Proportions of unsaturated and saturated fatty acids in membrane
What does the cholesterol in the membrane do when temp drops?
Buffers effects of lowered temp, to prevent reduction in membranes fluidity
How does cholesterol buffer the effects of a lowered temp?
By preventing the phospholipid molecules from packing too closely together as cholesterol molecules are between phospholipid groups
How does an increase in temperature affect phospholipid in the membrane?
Aquire more energy, move around more randomly, increases the fluidity
Permeability increases
What does cholesterol do when temperature increases?
Buffers effects by reducing increase in fluidity (solidifies membrane)
How does an increase in temp affect the proteins in the membrane?
Molecules vibrate breaking hyd and ionic bonds holding tertiary structure (unfold and denature)
Membrane starts to fall apart, becoming more permeable as holes appear
If denature, cease to function, rate of reaction decreases
Passive processes
Diffusion, facilitated diffusion, osmosis
Active processes
Active transport, endocytosis, exocytosis
What's the difference between passive and active processes?
Active processes require ATP wheras passive require no energy
What's a solvent?
Liquid in which solute molecules are dissolved in
What's the solvent of an aqueous solution?
Water
Can water molecules pass directly through phospholipid bilayer?
Yes
How do H2O molecules move?
Randomly
What do all molecules have?
Kinetic energy
What happens when solute molecules are added to water?
The relative number of water molecules in resulting solution is changed
What happens if the solute molecules added to water dissociation into charged ions and why?
Exert more effect on the relative number of water molecules than larger, non-polar molecules
This is because as it dissociates the number of particles in the solution doubles
What are prokaryotic cells subject to?
Osmosis
What happens if prokaryotic cells lose enough water?
Metabolism can't proceed and can't reproduce
Why do we add sugar or salt to food?
To preserve
How does adding sugar or salt preserve food?
It prevents spoilage bacteria respire, grow or multiply as water is lost from the bacterial cells by osmosis
What does antibiotic penicillin do to prokaryotic cells?
Prevent some bacteria from growing by synthesizing their peptidoglycan wall, making them vulnerable to effects of osmosis as if swell, they will burst
What has the highest water potential?
Pure water
What decreases water potential?
Adding more solute molecules
More solute molecules =
Lower water potential
What happens if two aqueous solutions are separated by a partially permeable membrane?
The h 2 o molecules removed from solution with higher potential to one with lower
What happens when the water potential on both sides is equal?
There's no net osmosis but the water molecules continue to move randomly
What does isotonic mean?
A solution with the same water potential as another
What does hypertonic mean?
The more concentrated of two solutions resulting in a lower water potential
What does hypotonic mean?
The less concentrated of two solutions resulting in a higher water potential
What's water potential measured in?
Kilopascals
What's the water potential of pure water?
0 kilopascals
How is a lower water potential represented?
A more negative value
Is the water potential inside cells lower or higher than that of pure water and why?
Lower as they are also used in the solution in cytoplasm and large vacuole
What happens when cells are placed in a solution of higher water potential?
Water molecules move by osmosis down the water potential gradient across the plasma membrane into the cell
What happens to a plant cell when placed in pure water?
Water molecules move into the cell by osmosis down the water potential gradient
The cell wall prevents bursting and the membrane pushes against the wall and becomes turgid
What happens when an animal cell is placed in pure
Water molecules move in osmosis down the water potential gradient
The animal cell burst open and the plasma membrane breaks (cytolysed)
What happens when a cell is placed in a solution of lower water potential?
H2O molecules leave by osmosis across the partially permeable membrane down the water potential gradient
What happens when a plant cell is placed in a more concentrated solution?
Water moves out of the cell by osmosis down the water potential gradient
The membrane pulls away from the wall as the water leaves (plasmolysed)
What happens to an animal cell when placed in a more concentrated solution?
The cell shrinks and appears wrinkled (crenated)
When is plant tissue described as flaccid?
When it contains plasmolysed cells
What do cells that are plasmolysed suffer from and what does this mean?
Dehydration and it means the metabolism can't proceed as enzyme-catalysed reactions need to be in the solution
What does active transport require?
More energy than the kinetic energy of the molecules
How is the energy for active transport provided?
By the hydrolysis of ATP
Where do you find carrier proteins?
In the membrane
When are carrier proteins used?
In active transport
How do carrier proteins work?
They can bind reversibly with only certain solute molecules/ions and change shape to allow the molecules through the membrane
What do carrier proteins have a region for?
A region that binds to and allows hydrolysis of ATP to release energy (in this way act as enzymes)
What does the energy help a carrier protein do?
Change it confirmation and in doing so carries the ion from one side of the cell membrane to the other
Describe the way in which guard cells act as carrier proteins?
Guard cells use ATP made by chloroplasts to provide energy to actively transport potassium ions from surrounding cells into guard cells
The influx of ions lowers the water potential in the guard cells and does the guard cell swells, their tips bulge opening stomata between them
What does bulk transport require?
Energy from ATP
What's bulk transport used for?
Transporting molecules and particles that are too large to pass through the plasma membrane in or out
Give two types of bulk transport
Endocytosis
Exocytosis
What's endocytosis?
Movement of large particles into a cell by active means
Give an example of endocytosis
Phagocytosis
What pinocytosis?
Cells ingesting liquid by endocytosis
How are large particles brought into the cell by endocytosis?
They don't pass through the membrane, instead a segment of membrane surrounds and encloses the particles and brings them into the cell enclosed in a vesicle
Describe the process of phagocytosis
The prokaryotic cell approaches the bacterium
The cell extends to surround the bacterium
The bacterium encloses in the phagocytic vesicle, a phagosome
Why is ATP needed for endocytosis?
To provide energy to form the vesicles and move them using molecular motor proteins along the cytoskeleton threads into the cells interior
What's exocytosis?
The bulk movement of large molecules out of the cell by active means
How to large molecules get exported out of the cell through exocytosis?
They don't pass through the membrane, instead a vesicle containing them is moved towards then fuses with the plasma membrane
Give an example of exocytosis
Synapses
Describe how synapses work
Chemicals in vesicles are moved by motor proteins moving along cytoskeleton threads to presynaptic membrane. Here, the vesicle membranes and the plasma membranes fuse and the neurotransmitter chemicals are released into the synaptic cleft
Describe the process of exocytosis
The membrane-bound vesicle containing the substance to be secreted is moved towards the cell surface membrane
The cell surface membrane and the vesicle membrane fuse together
Refused site opens released in the contents of of secretory vesicle
Why is ATP needed for exocytosis?
Defuse the membranes together and for moving the vesicle
How is the ATP hydrolysed in exocytosis?
A molecule of ATP is hydrolysed for every step the motor protein takes along the cytoskeleton thread as it drags the vesicle
Why do factors altering kinetic energy affect the rate of diffusion?
Simple diffusion relies on molecules own kinetic energy
What factors affect the rate of simple
Diffusion distance
Surface area
Size of diffusion molecule
Concentration gradient
How does temperature affect the rate of simple diffusion?
As temperature increases the phospholipid molecules gain more kinetic energy so the rate increases
As they lose heat the rate decreases and the membrane becomes more rigid
How does diffusion distance affect the rate of simple diffusion?
Thicker membranes decrease the rate of diffusion
How does surface area affect the rate of simple diffusion?
More diffusion can take place across a larger surface area
How does the size of the diffusing molecule affect the rate of simple diffusion?
Small ions or molecules increase the rate
How does the concentration gradient affect the rate of simple diffusion?
A steeper gradient would have a faster diffusion to the side of fewer molecules down the gradient
What does fick's law show?
Rate of diffusion is proportional to some factors
How is the concentration gradient maintained within cells?
Most molecules entering the cell are used for metabolic reactions
How is the concentration gradient O2 maintained within cells?
O2 diffusing into the cytoplasm of respiring cells then diffuses to the mitochondria and is used for aerobic respiration
How is the concentration gradient of CO2 maintained within cells?
The CO2 diffusing into palisade mesophyll cells of plant leaf then diffuses is into chloroplasts and is used for photosynthesis
When are aquaporins needed in membranes?
When there's a very high rate of water movement required
What happened once molecules have moved down the concentration gradient?
They will continue to move randomly but remain evenly dispersed
There's no net diffusion as it's reached equilibrium
What does facilitated diffusion involve?
Proteins in the membrane
What can't pass through the membrane?
Small, polar molecules
What can't small, polar molecules pass through the membrane?
They're insoluble in lipids so cant interact with the tails of the lipids bilayer
How do small, polar molecules pass through the membrane?
Diffuse through water-filled protein channels embedded in membrane
How big are the water-filled channels in the membrane?
0.8 nm diameter
What do cholesterol molecules in the membrane do?
Reduce/increase permeability membrane
How do glucose molecules too large to diffuse through protein channels pass through the membrane?
Bind to transmembrane carrier proteins which opens to allow glucose to pass
What channels do the membranes of epithelial cells in the airway contain and why?
Chloride channels to regulate composition of mucus to trap particles and pathogens
What channels do neurone plasma membranes contain and why?
Channels specific for sodium and potassium. Diffusion of these ions crucial for conduction of nerve impulses
What happens to the membrane when temp increases?
Phospholipids acquire more KE and move around more, increasing fluidity, increasing permeability
How does an increase in temp affect membrane embedded proteins?
Way they're positioned and function - if proteins that act as enzymes in membrane drift sideways, could alter rate of reactions they catalyse
What could be affected when the fluidity of the membrane increases as a result of increase in temp?
Affect infolding of plasma membrane during phagocytosis
May change ability of cells to signal to others by releasing chemicals by exocytosis
What does the presence of cholesterol molecules do when temp increases?
Buffers effects of increasing heat as reduced increase in fluidity
What happens to the proteins in the membrane when the temp increases?
Causes atoms within their large molecules to vibrate, breaking the hydrogen and ionic bonds holding structure
Tertiary structure changes and can't change back when cools (denatures)
When will the plasma membrane begin to fall apart?
When the membrane-embedded proteins and cytoskeleton threads denature
What happens when plasma membrane begins to fall apart?
Becomes more permeable as holes appear
What happens when the membrane-embedded enzymes start to seize/denature as a result of an increase in temp?
Active site changes, enzymes move in membrane, rate of reaction they catalyse will reduce
Who came up with the fluid mosaic model and when?
Singer and Nicolson 1972
What does the fluid mosaic model explain?
How membranes could be more dynamic and interactional with the cells environment
What did singer and Nicolson propose?
The membrane fabric assisted of phospholipid bilayer with proteins floating in it creating a mosaic pattern
What did singer and Nicolson propose about the lipid molecules in the membrane?
They can change places and some proteins move giving fluidity
What makes up the main part of the phospholipid bilayer?
Hydrophilic phosphate heads and hydrophobic tails
What are integral proteins?
The channel proteins and carrier proteins which span the whole bilayer
What do the carbohydrate molecules in the bilayer do?
The very hydrophobic can attract water molecules with dissolved solutes helping cell interact with watery environment and obtain dissolved substances
How wide is the bilayer?
7 NM
What do extrinsic proteins do?
They provide mechanical support or act with glycolipids as receptors for complementary shapes signalling
What do intrinsic proteins do?
Span bilayer and act as carriers or enzymes
What do glycoproteins and glycolipids do in the bilayer?
Stabilise membrane by forming hydrogen bonds with surrounding H2O molecules
Actors receptors in cell signalling or antigens for cell recognition
Allow cells to attach to one another to form tissues
What do peripheral proteins do?
Functions of enzymes and genes or receptor sites for complementary shaped signalling chemicals
In which type of cell would you find cholesterol in the bilayer?
Eukaryotic
What does cholesterol do in the membrane?
Regulate fluidity
Stability
Resist effects of temperature change
Prevents leakage of water and ions
What's the composition of the membrane of root hair cells?
Contain carrier proteins to actively transport nitrate ions from soil cells
What's the composition of membranes in cristae?
76% protein and 24% lipid due to containing an electron carriers made of protein and hydrogen ion channels associated with ATP synthase enzymes
What's the composition of membranes in white blood cells?
A special protein receptors that enable them to recognise antigens on foreign cells
What's the composition of the membrane on neurones?
Have protein channels and carriers on long axon to allow entry and exit of ions to bring about conduction of electrical impulses along their length
What gives neurones multiple layers of membrane?
Myelin sheath
What forms the myelin sheath?
Flattened cells
How much protein and how much lipid forms the membrane for the myelin sheath?
76% lipid 20% protein
Phospholipid
Glycolipid
Channel
Carrier
Glycoprotein
Peripheral
Integral protein
Cholesterol
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