Unit 3 AOS 1: Plasma Membrane Flashcards
Phospholipids
- When placed in water, phospholipids will spontaneously form a lipid bilayer
- Hydrophobic tails face inwards, away from watery solution; hydrophilic heads face outwards, towards a watery solution
Fluid Mosaic Model
Is the theory of how the plasma membrane is structured
● The plasma membrane is fluid because it’s main
component, phospholipids continually move laterally (side to side) in the membrane
➔ Occasionally, phospholipids may ‘flip-flop’ between the two layers of the plasma membrane
● The ‘mosaic’ component of the model comes from the proteins and carbohydrates embedded in the membrane
➔ These molecules can move about the bilayer, like ice floating on water
Glycoproteins
markers for cell-cell communication
Semipermeable
(selectively permeable): only selected substances can pass through while others stay out.
Glycolipids
maintain the stability of the cell and assist cellular recognition
Transmembrane protein
A functional protein, often composed of more than one polypeptide molecule, which spans the entire thickness of the plasma membrane.
Integral proteins
- Embedded in bilayer
- “Transmembrane”
- Hard to be separated from PM
- Often have glycoproteins attached
Peripheral proteins
- Exterior only
- Can be easily separated from PM
Cholesterol
A substance found in cell membranes; in cold temperatures, it makes the PM more fluid and vice versa during hot temperatures (i.e. keeps the PM at a similar
fluidity level regardless of external temperatures that would otherwise affect it)
Hydrophobic
Repelled by water; non-polar
Hydrophilic
Attracted to water; polar
Factors affecting permeability
Temperature
Phospholipid composition and structure
Cholesterol
4 ways molecules can cross the PM
- Diffusion
- Facilitated Diffusion
- Osmosis
- Active Transport
Characteristics of molecules can cross the PM
Method of transport depends on:
- Size of the molecule (bigger is harder)
- Charge of the molecule (charged is harder)
- Polar (hydrophilic) or non-polar (hydrophobic) (non-polar (lipid-soluble) is easier)
- Concentration gradient (easier if conc. gradient is high)
- Small uncharged polar molecules can cross
- Small hydrophobic molecules can cross
Distinguish simple and facilitated diffusion
Simple diffusion: The passive net movement of a solute from a region of high solute concentration to low solute concentration across a semi-permeable membrane
- Passive form of transport
- Small, lipid-soluble (non-polar), uncharged molecules (including osmosis)
Facilitated diffusion: The passive net movement of a substance from an area of high concentration to an area of low concentration via a membrane protein
- Passive transport
- Assistance required for molecules to pass through
- Uses protein transporters
> protein carriers: molecule binds, protein carrier changes shape, moves the substance down and out
> protein channels: doesn’t change shape, acts like a pore. Transports substances faster than diffusion.
> e.g. aquaporins – enables water to enter and exit quickly
> e.g. beta-barrels – enables water to enter and exit quickly because of internal
polar amino acids and external non-polar amino acids
Osmosis
Net movement of water across a semi-permeable membrane from an area of low solute concentration to high solute concentration
- (solute = substance e.g. salt, sugar)
- Type of simple diffusion related to water (therefore is passive)
Cytosol
the liquid part of the cell that suspends the organelles.
Cytoplasm
Includes cytosol and all organelles except for the nucleus.
Mitochondria
Organelle where majority of ATP synthesis occurs – this ATP is needed for exocytosis to occur. Double- membrane.
Ribosomes
An organelle made of protein and ribosomal RNA (rRNA) – the site where polypeptides are made
Endoplasmic reticulum
System of membrane-bound channels. Allows things to channel throughout the cells (the rough ER has ribosomes embedded in its surface, the smooth ER does not). Double-membrane. Transports proteins around the cell via transport vesicles. Manufactures lipids.
Golgi apparatus
Proteins are packaged into secretory vesicles ready for export (exocytosis) from the cell. The Golgi apparatus is a stack of flattened membrane-bound sacs. Post-translational modification occurs here
Vesicles
A small sac, made of a phospholipid bilayer in the Golgi.
Hypertonic
Having a higher osmotic pressure than a particular fluid, typically a body fluid or intracellular fluid.
Hypotonic
Having a lower osmotic pressure than a particular fluid, typically a body fluid or intracellular fluid.
Isotonic
A solution is isotonic when its effective osmole concentration is the same as that of another solution.
How do hypertonic solutions effect plant cells?
Hypertonic solutions have a higher solute concentration. When plant cells are placed in such solutions, water will move from inside the plant cell to the outside of the cell, resulting in the shrinking of the cell (the cell is said to be plasmolyzed).
This occurs because of osmosis. When there are solutes on two sides of a membrane, a balance of solute on the two sides of the membrane will be attempted. The molecules on both sides of the membrane will try to move across the membrane, but the net movement will be down the concentration gradient (from high to low concentration).
In a hypertonic solution, there is less water outside than inside the plant cell, so the water within the plant will try to diffuse outside in order to achieve equilibrium.
Effect of hypotonic solutions on animal and plant cells
In hypotonic solutions, animal cells swell up and explode as they cannot become turgid because there is no cell wall to prevent the cell from bursting. When the cell is in danger of bursting, organelles called contractile vacuoles will pump water out of the cell to prevent this.
Plant cells have a cell wall around the outside than stops them from bursting, so a plant cell will swell up in a hypotonic solution, but will not burst.
Effect of isotonic solutions on animal and plant cells
An isotonic solution is a solution, which contains the same concentration of solute as in a cell. If animal and plant cells are kept in isotonic solution then cells will not swell or shrink. Hence, there will not be any change in cells.
Active transport
The movement of molecules, generally from a region of low concentration to high concentration, requiring the use of energy in the form of ATP
- Goes against conc. gradient, thus requires energy
Endocytosis (Phagocytosis,Pinocytosis) vs Exocytosis
Endo: Moving materials into the cell
> phagocytosis: moving solids in and out of the cell
> forms vesicles for large molecules to fuse and form phagosome.
> pinocytosis: moving liquids in and out of the cell
Exo: Moving of materials out of the cell
Hierarchy of folding proteins
Primary
Secondary
Tertiary
Quaternary
Primary structure of proteins
A sequence of amino acids. Covalent peptide bonds.
Monomers are joined by anabolic reactions which require energy.
Secondary structure of proteins
- Gives proteins their properties; consists of alpha helixes, beta-pleated sheets and random coils.
- Hydrogen bonds form between carboxyl and amine groups.
Tertiary structure of proteins
The specific 3D shape of the protein that consists of a secondary structure folded – the 3D shape of the protein determines its function. R groups interact with chemical bonds, hydrophobic interactions. Disulphide bonds also contribute.
Quaternary structure of proteins
Made up of 2 or more polypeptide chains joined together to form a functional protein. Note: this is different from two tertiary structures joined together.
Enzymes
A protein that catalyses a biological reaction, lowering the activation energy required; it is not used up in the reaction
Structure of Enzyme
Enzymes structure are made up of α amino acids which are linked together via amide (peptide) bonds in a linear chain. This is the primary structure. The resulting amino acid chain is called a polypeptide or protein. The specific order of amino acids in the protein is encoded by the DNA sequence of the corresponding gene.
Enzyme conformational change
The theory of induced fit predicts that enzymes undergo conformational changes as they bind their substrate. Similar with the lock-and-key process
Distinguish the lock-and-key process and induced fit model
> Induced fit model: enzyme changes shape slightly when it bonds with substrate/s which stresses chemical bonds and catalyses a chemical reaction.
Lock and key model: substrate/s fit perfectly into the active site of an enzyme, like how a key will fit into a lock. The shape is fixed.
Enzyme denaturation
Amino acid bonds break, preventing normal chemical functioning.
Enzyme structures unfold (denature) when heated or exposed to chemical denaturants and this disruption to the structure typically causes a loss of activity. Protein folding is key to whether a globular protein or a membrane protein can do its job correctly. It must be folded into the right shape to function.
Enzyme - temperature
- Every enzyme has an optimal temp.
- Below this temp, it works slowly. Less collisions occur due to less heat energy.
- Above this temp, it can become denatured and metabolic reactions cannot occur fully fast enough to be functional.
- If the temperature gets too high, the bonds holding the protein in its tertiary (3D) shape break apart. Without its tertiary shape the enzyme cannot function correctly due to its active site’s shape-changing.
Enzyme - pH
- Like temperature, every enzyme has an optimal pH level
- If the pH goes above or below optimal, the rate of reaction will slow (it is very unlikely the enzyme will become denatured though)
- The rate of reaction will increase with the more substrate that is added, until all enzymes are working at maximum speed (i.e. when they become ‘saturated’)
- The more enzymes that are present the faster the rate of reaction, until enzyme concentration is no longer the limiting factor
Enzyme - concentration (substrate, enzyme)
- Higher enzyme concentration increases number of available active sites, increasing rate of reaction.
- Higher substrate concentration increases rate of reaction but plateaus once all
available active sites are taken.
Anabolic vs Catabolic
> Catabolic reaction: Lower energy level products. One substrate is broken down into two (or more) products, releasing energy.
> Anabolic reaction: Higher energy level products (energy is added). Two (or more) substrates are built up into one product, using energy.
Endergonic vs Exergonic
> Endergonic: reaction that requires energy
> Exergonic: reaction that produces energy
Inhibition
Competitive vs Non-competitive
➢ Competitive inhibition: inhibitor that binds to the active site of an enzyme and prevents the substrate from binding; can be dislodged by changing environmental conditions
➢ Non-competitive inhibition: inhibitor that binds to an allosteric site (site other than the active site) and in doing so changes the shape of the active site, preventing substrates from binding; can’t be dislodged by changing environmental conditions
Cofactors vs Coenzymes
- Cofactors: inorganic; helps substrate fit better in the active site of enzyme; increases the rate of reaction
- Coenzyme: organic non-protein; damaged in the reaction. A new molecule is needed for each subsequent reaction. Assists enzymatic reaction by binding to the active site, transforming into a catalytic molecule capable of binding to the substrate.
Plasma Membrane
is the selectively permeable barrier between the cell and it’s environment.
Its structure is described by the fluid mosaic model, in which molecules within the membrane can move around and it is embedded with a myriad of proteins and other molecules.
Passive transport
Passive transport is the movement of molecules across a membrane without the use of energy.
Diffusion, facilitated diffusion, and osmosis are the three types of passive transport
Passive transport
Passive transport is the movement of molecules across a membrane without the use of energy.
Diffusion, facilitated diffusion, and osmosis are the three types of passive transport
Diffusion
- The tendency of particles of gases, liquids and solutes to disperse randomly and fill available space.
- The rate of diffusion increases with increased temperature
- From high to low concentration
A pathway for the production of protein for these junctions is:
N R ER V G
Nucleus – ribosome – endoplasmic reticulum – vesicle – Golgi apparatus
