Membranes Flashcards
How is the plasma membrane organised?
- Membranes are formed from a phospholipid bilayer
- The hydrophilic phosphate heads of the phospholipids 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
Why is the structure of the cell membrane referred to as the “fluid-mosaic model”?
The model is known as the fluid- mosaic model because the phospholipids are free to move within the layer relative to each other (they are fluid), giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size, and position (in the same way as the tiles of a mosaic)
What is the model of the membrane referred to as?
The fluid mosaic model
What are the two types of proteins found in the plasma membrane?
Intrinsic and extrinsic proteins
What is an intrinsic protein?
- Intrisic proteins, or integral proteins, are transmembrane proteins that are embedded through both layers of a membrane
- They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place
What are the two types of intrinsic proteins?
Carrier and channel proteins
What is a carrier protein?
- Carrier proteins have an important role in both passive transport and active transport into cells
- This often involves the shape of the protein changing
What is a channel protein?
- Channel proteins provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient through membranes
- They are held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins
What is an extrinsic protein?
- Extrinsic proteins or peripheral proteins are present in one side of the bilayer
- They normally have hydrophilic R-groups on their outer surfaces and intract with the polar heads of the phospholipids or with intrinsic proteins
- They can be present in either layer and some move between layers
What are glycoproteins and glycolipids referred to as?
Glycocalyx
What are the roles of the glycocalyx (glycolipids and glycoproteins)?
- Cell-cell recognition
- Cell-cell signalling
- Cell-cell adhesion
- Site of attachment for the cytoskeleton
What are glycoproteins and what role do they play?
- Glycoproteins are intrinsic proteins
- They are embedded in the cell-surface membrane with carbohydrate (sugar) chains of varying lengths and shapes attached to them
- Glycoproteins play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and as receptors for chemical signals
What happens when a chemical binds to a receptor on a cell, and what is this process called?
- The protein receptors are glycoproteins that have a complementary binding site to a specific chemcial/hormone on the extracellular side of the membrane
- When the chemical binds to the receptor, it elicits a response from the cell
- This may cause a direct response or set off a cascade of events inside the cell
- This process is known as cell communication or cell signalling
What are some examples of receptors?
- Receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone
- Receptors for peptide hormones, including insulin and glucagon, which affect the uptake and storage of glucose by cells.
What are glycolipids and what role do they play?
- They are lipids with a carbohydrate (sugar) chain attached to them
- These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self (of the organism) or non-self (of cells belonging to another organism)
How does cholesterol regulate the fluidity of the the plasma membrane?
- Cholesterol molecules are positioned between phospholipids in a membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic end interacting with the tails, pulling them together
- In this way cholesterol adds stability to membranes without making them too rigid
- The cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising
How does temperature affect membrane permeability?
- As temperature increases the phospholipids will have more inetic energy and will move more
- This makes the membrane more fluid, causing it to lose its structure (becomes disrupted or destroyed)
- If temperature continues to increase, the cell will eventually break down completely
- This loss of structure increases the permeability of the membrane, making it easier for particles to cross it
- Carrier and channel proteins in the membrane will be denatured at higher temperatures, reducing their funciton
- Because they are involved in transport across the membrane, as they denature, membrane permeability will increase
- At lower temperatures, the membrane becomes more rigid, which decreases permeability, and membrane proteins may become less active or even stop working
What is diffusion?
- The net movement of a substance down its concentration gradient (from an area of high to low concentration) until an equilibrium is reached
- Their molecules have their own kinetic energy, there is no need for an input of energy/ATP
- It is a passive process
What factors can affect the rate of diffusion?
Temperature - the higher the temperature the higher the rate of diffusion. This is because the particles have more kinetic energy and move at higher speeds
Concentration difference - the greater the difference in concentration between two regions the faster the rate of diffusion because the net movement from high to low concetration will be larger
What are the two types of diffusion?
Simple diffusion and facilitated diffusion
What is simple diffusion?
- This is when participles diffuse across the membrane by passing through the phospholipid bilayer
It includes small non-polar moelcules:
- Oxygen
- Carbon dioxide
- Lipid soluble hormones (steroid)
- Polar molecules such as water can diffuse through membrane but will only idffuse at a very slow rate (small polar moelcules pass through more easily than larger ones)
Why can’t substances such as ions pass through the membrane by simple diffusion?
The hydrophobic interior of the membrane repels substances with a positive or negative charge (ions), so they cannot easily pass through
What affects the rate at which molecules move across the membrane by simple diffusion?
Surface area - the larger the area of an exchange surface, the higher the rate of diffusion
Thickness of membrane - the thinner the exchange surface, the higher the rate of diffusion
What is facilitated diffusion?
- This is the diffusion of molecules through transmembrane proteins (carrier and channel proteins)
- Channel proteins/ion channels contain a hydrophilic channel which allows diffusion of ions across the memrbane. The channels are often gated
- Membranes with protein channels are selectively permable as most protein channels are specific to one molecule or ion
- Carrier proteins allow diffusion of small organic molecules e.g. glucose, amino acids where the conformation (shape) of the protein changes when a specfic molecule binds which allows the transport of the the molecule across the memrbane
What factors affect the rate of facilitated diffusion?
- Temperature
- Concentration gradient
- Membrane surface area
- Thickness of membrane
- Number of channel proteins present
What is active transport?
Active transport is the movement of molecules or ions against their concentration gradient (from an area of lower to higher concentration) using carrier proteins (which act as pumps) and an input of energy i.e. ATP
What is the process of active transport?
- The molecule or ion to be transported binds to receptors in the channel of the carrier protein on the outisde of the cell
- On the inside of the cell ATP binds to the carrier protein and is hydrolysded into ADP and phosphate
- Binding of the phosphate molecule to the carrier protein causes the protein to change chape - opening up to the inside of the cell
- The molecule or ion is released to the inside of the cell
- The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP
- The carrier protein returns to its original shape
Provide an example of active transport.
- One example of the sodium potassium pump
- All cells contain the sodium ion/potassium ion pump or sodium ion/potassium ion ATPase
- ATP is used as a source of energy
- Sodium ions are actively transported out of the cells
- Potassium ions are actively transported into cells
- This generation a concentration gradient for these ions which are used to drive processes such as nervous transmission, muscle contraction, hormone secretion etc
What is bulk transport?
- Bulk transport is another form of active transport
- Large molecules such as enzymes, hormones, and whole cells like bacteria are too large to move through channel or carrier proteins, so they are moved into snd out of the cell by bulk transport
What are the two types of bulk transport?
Endocytosis and exocytosis
What are the two types of endocytosis?
Phagocytosis and pinocytosis
What is pinocytosis?
- A type of endocytosis referred to as “cell drinking”
- It is the intake of fluids and solutes by the formation of small vesicles from the plasma membrane
- The plasma membrane invaginates/forms a dip or a tuck and pinches off to carry the extracellular fluid into the cell
What is phagocytosis?
- A type of endocytosis referred to as “cell eating”
- It is the intake of large particles e.g. cell debris or bacteria by the formation of vacuoles
- The plasma membrane extends outwards to form pseudopodia through the action of phagocytosis
- It is carried out by specialised cells called phagocytes or by unicellular oganisms such as amoeba
What is endocytosis?
- The reverse of exocytosis
- The substance to be carried into the cell is progressively surrouned by a small portion of the plasma membrane
- The plasma membrane pinches off to form an intracellular vesicle or vacuole
- This process requires energy
What is water potential?
- The ability/tendency of a solution/cell system to donate water to another solution/cell/system
- It is measure in kilopascals (kPa)
What is osmosis?
The net movement of water from a high water potential to a lower water potential (down a water potential gradient) across a partially permable membrane (until equilibrium is reached)
What is the thicknes of a cell surface membrane?
7nm
What is the water potential of pure water, a hypotonic and hypertonic solution?
Pure water => 0kPa
Hypotonic solution => -100kPa
Hypertonic => -1000kPa
Why are the values for water potential in a hypotonic and hypertonic solution negative?
The values are negative because they can be seen to act as a “pulling” pressure as opposed to a pushing pressure
How can the rate of osmosis across the membrane be increased?
- The use of protein channels called aquaporins which have a hydrophilic pore. These are inserted into the membrane by the exocytosis of vesciles
- The active transport of ion to lower the water potential and create a water potential gradient across the membrane
What occurs when an animal cell is in an isotonic, hypotonic and hypertonic solution?
Isotonic solution => it is normal
Hyptonic solution => becomes lysed/ruptured
Hypertonic solution => becomes crenated/shrinks
What occurs when a plant cell is in an isotonic, hypotonic and hypertonic solution?
Isotonic solution => it is flaccid, the plasma membran is just touching the cell wall
Hyptonic solution => becomes turgid, the cell wall is pushing back against the expanding cell
Hypertonic solution => becomes plasmolysed, plasma membrane has peeled away from cell wall and gap is ful of the solution from outside the cell (extracellular fluid)
What is the water potential of an animal cell equal to?
The solute potential of the cell => the value of the water poential as a consequence of only solute molecules
What is a pressure potential?
The pressure exterted by the cell wall against the expanding cell (this is equivalent to the hydrostatic pressure exterted by the expanding cell against the cell wall)
How is the water potential of a plant cell calculated?
Water potential = pressure potential (positive in cells and is the pushing pressure) + solute potential (negative and is the pulling pressure)
What does it mean if the pressure potential of a plant cell is zero?
The cell is plasmolysed (all the water has moved out)
What does it mean if the water potential of a plant cell is zero?
The pushing pressure and the pulling pressure is equal
