2.5 Biological membranes Flashcards
What are the roles of the cell surface membrane?
- separates the inside from the outside
- regulates transport in and out
- may contain enzymes involved in specific metabolic pathways
- has antigens for the immune system
- release chemical signals to other cells
- site of chemical reactions
What is the fluid mosaic model?
The model which aims to outline the structure and function of the membrane
Describe the fluid mosaic model:
- phospholipid bilayer (7nm)
- carrier proteins
- channel proteins
- glycoproteins
- peripheral proteins
What is the purpose of cholesterol in eukaryotic membranes?
To regulate membrane fluidity
How is a neuron a specialized cell?
- carrier and channel proteins span the long axis to allow electrical impulses
- myelin sheath is 20% protein and 76% lipid
What difference do root hair cells have that make them optimal?
They have lots of carrier and channel proteins to allow easy uptake of ions from soil
What is diffusion?
the movement of molecules down a concentration gradient
How do steroid hormones move in and out of the cell?
Using simple diffusion to dissolve in the bilayer
What are specific water channel proteins called?
Aquarporins, to allow the water to move across the membrane despite being a polar molecule
How is the concentration gradient maintained?
- oxygen is used in mitochondria during aerobic respiration
- carbon dioxide used in the chloroplasts during photosynthesis
What are the factors that affect rate of diffusion?
- Temperature
- Distance (of diffusion)
- surface area
- size of diffusing molecule
- concentration gradient
What molecules cannot pass through the membrane?
charged molecules interact with the hydrophobic tails badly, some molecules are just too large.
How do molecules, that cannot pass through the membrane via simple diffusion, enter the cell?
Facilitated diffusion - using carrier proteins across the membrane to be transported
What and why are the neuron and epithelial channels specific?
They are specific to sodium or potassium ions which is crucial within the transmission of electrical impulses
They are specific to chloride ions to maintain mucus composition
What is the definition of osmosis?
the net movement of water from a high water potential to a low water potential through a partially (semi) permeable membrane
What liquid has the highest water potential?
distilled water
What is the correlation between solute molecules and water potential?
more solute molecules = lower water potential
What is water potential measured in?
kilopascals (kPa), starting at 0 at the highest and decreasing to negative for lower water potential
What is the measurement for the water potential in distilled water?
0 kPa
What happens to cells placed in a solution of higher water potential?
The wp in the cell is lower than the solution, so the water moves down the water potential gradient by osmosis through a semi-permeable membrane into the cell, gaining mass
What is it called when an animal cell bursts?
cytolysis
What happens when lots of water enters a plant cell?
The cell becomes turgid but due to the strong nature of the cell wall it does not burst
What happens to cells placed in a solution of lower water potential?
The water potential in the cell is higher, so the water moves down the water potential gradient into the solution, losing mass
What is it called when animal cells shrivel?
crenation
What happens when a plant cell loses water?
The cytoplasm shrivels, pulling the cell membrane away from the cell wall, known as plasmolysation
What is a plant tissue with plasmolysed cells called?
flaccid
Why can’t metabolism take place in plasmolysed cells?
They are too dehydrated, the enzyme-catalysed reactions need a solution to work in
What are key characteristics of active transport?
- Requires ATP
- Moves against the concentration gradient
What are the main active processes of substance movement?
- active transport
- carrier proteins
- bulk transport
What are the 2 types of bulk transport?
endocytosis and exocytosis
What is endocytosis?
The intake of a substance, also known as phagocytosis for solid matter, and pino(endo)cytosis for liquids
Why does endocytosis require ATP?
To form vesicles and transport using motor proteins
What is exocytosis?
The vesicle fuses with the plasma membrane
What is an example of exocytosis?
Synapses use exocytosis by fusing the vesicle with the plasma membrane to release the chemicals into the synaptic cleft
Why is ATP needed in exocytosis?
One molecule of ATP is created for each step of the motor proteins on the cytoskeleton. ATP is also used for fusing the vesicle with the membrane
What happens to unsaturated fatty acids at lower temperatures?
The acids become compressed and the kinks in their tails push the phospholipid molecules away, maintaining membrane fluidity.
How does cholesterol help maintain membrane fluidity at low temperatures?
the molecules fill the gap between phospholipids so prevent total compression
What happens to a membrane when the temperature increases?
A higher temp means more kinetic energy so the permeability of the membrane increases. Phagocytosis and cell signals will be affected.
What does temperature do to the overall structure of the membrane?
hydrogen and ionic bonds break at high temperatures due to vibrations. Tertiary structure is affected so cells become denatured.
