6. Biological Membranes Flashcards
What is the function of plasma (cell surface) membranes?
Regulates transport into and out of the cell.
Contains enzymes involved in metabolic pathways.
Contains antigens that the body recognises as ‘self’ so it does not attack.
Contains receptors for chemical signal, so is a site for cell communication.
What is the fluid mosaic model?
Proposed that the fabric of the membrane consists of a phospholipid bilayer with proteins floating in it. The lipids and some proteins can move, giving it fluidity.
What structures are present in the phospholipid bilayer?
phospholipids, glycoproteins, carrier proteins, channel proteins, cholesterol, glycolipids, protein receptor sites
What are the different proteins within membranes?
Some have pores and act as channels to allow ions to pass through.
Some proteins are carriers and, by changing their shape, carry specific molecules across the membrane.
What is the function of cholesterol in a cell surface membrane?
Helps to regulate fluidity, maintain stability and resist the effects of temperature on the membrane’s structure.
What is the total thickness of a cell membrane?
Between 5 and 10 nm
What is the distribution of proteins in neurones?
A myelin sheath covering the axon formed by flattened cells contains protein.
This allows the conduction of electrical impulses along its length.
What is the function of proteins in white blood cells?
Protein receptors in their plasma membrane enable them to recognise antigens on foreign cells.
What is the distribution of proteins in root hair cells?
They have many carrier proteins to actively transport nitrate ions from the soil into cells.
What is diffusion?
When molecules move down the concentration gradient until they reach equilibrium. It may or may not be across a membrane. It does not involve metabolic energy (ATP)
How is a concentration gradient maintained in cells?
The molecules are used for metabolic reactions which keeps more of the molecules entering the cell.
How does temperature affect the rate of diffusion?
As temperature increases, molecules have more kinetic energy, so their rate of diffusion will increase and vise versa.
How does diffusion distance affect the rate of diffusion?
The thicker the membrane across which the molecules have to diffuse, the slower the rate of diffusion.
How does surface area affect the rate of diffusion?
More diffusion can take place across a larger surface area.
How does the size of diffusing molecule affect the rate of diffusion?
Smaller ions or molecules diffuse more rapidly than larger ones.
How does concentration gradient affect the rate of diffusion?
The steeper the gradient, the faster the diffusion to the side where there are fewer molecules, down the gradient.
What is facilitated diffusion?
The movement of molecules down the concentration gradient across a partially permeable membrane via protein channels or carriers.
It does not involve metabolic energy (ATP)
What is a solution?
Solute molecules dissolved in a solvent. Water is the solvent in aqueous solutions.
What is osmosis?
The passage of water molecules down their water potential gradient, across a partially permeable membrane.
What is water potential?
Measure of the tendency of water molecules to diffuse from one region to another.
It is represented by Ψ (psi) and measured in kPa.
Pure water has the highest water potential of 0kPa.
Dissolved solutes lower the water potential.
What does hypotonic mean?
High water potential (more water than solute)
What does hypertonic mean?
Low water potential (more solute than water)
What does isotonic mean?
When a solution has the same concentration of water and solute.
What happens when animal cells are placed in a solution with a higher water potential?
Water molecules move by osmosis into the cell, the cell will swell and burst as the plasma membrane breaks.
This is cytosis.
What happens when plant cells are placed in a solution with a higher water potential?
The cellulose cell wall will prevent bursting.
The cell will swell and is described as turgid.
This helps to support non-woody plants.
What happens when plant cells are placed in a solution with a higher water potential?
The cytoplasm of plant cells shrinks and the membrane pulls away from the cellulose cell wall.
The cells are described as plasmolysed.
The plant tissue is described as flaccid.
What is active transport?
The movement of substances against their concentration gradient across a cell membrane, using ATP as protein carriers.
How do carrier proteins help active transport?
They have specific regions that combine with certain solute molecules or ions.
They have a region that allows the hydrolysis of ATP, to release energy.
The energy helps the carrier protein to change its conformation cand carry the ion across the membrane.
How does the sodium potassium pump work?
3 sodium ions bind to a specific site on the sodium-potassium pump protein.
ATP binds to its site and is hydrolysed to ADP+P releasing energy.
2 potassium ions bind to their specific site.
The energy from ATP enables the pump protein to change its shape.
The 3 sodium ions are now on the outside of the cell and the 2 potassium ions are inside.
What is endocytosis?
Bulk transport of molecules too large to pass through the cell membrane, into a cell.
A segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle.
What is endocytosis of solids called?
phagocytosis
What is endocytosis of liquids called?
pino(endo)cytosis
What is exocytosis?
Bulk transport of molecules too large to pass through the cell membrane, out of a cell.
A vesicle containing them fuses with the plasma membrane and releases them into the external environment.
Why is ATP needed in endocytosis and exocytosis?
To provide energy to form vesicles and move them using molecular motor proteins along cytoskeleton threads.
To fuse the membranes together and move the vesicles.
