Membranes And Transport🚧 Flashcards
Functions of the plasma menbrane
- To give the cell structure
- To allow substances to exit and enter the cell
- Cell-to-cell recognition and cell-to-cell signalling
Features of a phospholipid bilayer
- Phospholipid - hydrophilic phosphate head and hydrophobic fatty acid tail
- Intrinsic and extrinsic proteins
- Cholesterol
- Glycolipids
- Glycoproteins
- Carbohydrate chains
What is the importance of the arrangement of the phospholipid molecules?
- The hydrophilic heads face out
- The hydrophobic tails face in
- This forms a hydrophobic, non-polar region in the middle
- This prevents polar molecules such as glucose from passing through
Polar
Uneven distribution of charge
Non polar
No charge
Ion
Charged particle
What is the width of the membrane under the electron microscope?
7 nm
Intrinsic proteins
Lie across both layers of the membrane.
Transport substances.
Channel or carrier protein.
Extrinsic proteins
In one layer of the membrane or in the surface.
Hormone receptors, cell recognition, enzymes.
What does a receptor protein do?
Recognises and binds with specific molecules outside the cells.
What does a glycoprotein do?
A protein with a carbohydrate attached.
Involved in cell signalling, cell recognition and binding cells together.
Some act as antigens.
What does a glycolipid do?
Lipids with a carbohydrate chain attached.
Involved in cell signalling and cell recognition.
What does a carrier protein do?
Transports larger polar molecules such as sugars and amino acids.
The molecule attaches at the binding site to a particular carrier protein and the protein changes shape to deliver the molecule.
What does a channel protein do?
Opens up pores across the membrane which are lined with polar groups. This allows charged ions to pass through.
Usually, each channel is specific.
What does the fluid mosaic model mean?
- Fluid - all parts of the membrane can move relative to each other.
- Mosaic - the proteins are dotted throughout the membrane.
Cholesterol
Can fit between the molecules.
Increases rigidity and stability of the membrane.
What affects membrane permeability?
- Low temperature
- High temperature
- Organic solvents
How does a low temperature affect permeability?
Fatty acids become compressed and so the membrane becomes less dynamic.
This decreases permeability.
How does a high temperature affect permeability?
The phospholipids acquire energy and move randomly and at a faster rate.
This increases membrane fluidity, and therefore the permeability.
How do organic solvents such as ethanol affect permeability?
Organic solvents such as ethanol can be used to dissolve the phospholipids and so increase the permeability.
What affects the rate of transport of molecules?
- Concentration gradient
- Particle size
- Thickness of membranes
- Surface area
- Distance
- Temperature
What is simple diffusion?
The movement of molecules down a concentration gradient from an area of higher concentration to and area of lower concentration.
Simple diffusion across the membrane
Only lipid soluble molecules which are non polar can pass through (also small molecules).
E.g. carbon dioxide and oxygen
Facilitated diffusion
Involves a molecule or ion moving through a carrier protein or a channel protein.
Large, water soluble, polar and charged molecules or ions.
E.g. glucose
What is co-transport?
A type of facilitated diffusion by which two substances are simultaneously transported across a membrane by a carrier protein.
Describing diffusion and facilitated diffusion graphs
- The rate of uptake increases as concentration difference increases.
- At high concentration differences, the rate of facilitated diffusion reaches maximum and levels off. This is due to saturation of the carrier or channel proteins.
- Therefore, further increases in concentration difference do not increase the rate of facilitated diffusion further.
What is active transport?
The movement of molecules or ions from a low concentration to a high concentration using energy from respiration.
ATP is required because the movement is against the concentration gradient.
Active transport in carrier proteins
ATP transfers a phosphate group to the carrier protein on the inside of the membrane.
The carrier protein changes shape and carried the molecule or ion to the other side of the membrane.
What affect can a metabolic poison such as cyanide have on active transport?
Active transport can be stopped as the poison prevents ATP from being produced.
Exocytosis
- The release of materials from the cell.
* A vesicle fuses with the membrane and the molecule is released from the vesicle outside the cell.
What affect does exocytosis have on the membrane?
It lengthens it
What affect does endocytosis have on the membrane?
It shortens it
Endocytosis
- Substances taken into the cell
- Plasma membrane folds around the molecule and engulfs it.
- The substance is then trapped in a vesicle
- Phagocytosis or pinocytosis
What is phagocytosis?
Endocytosis of large solid substances
What is pinocytosis?
Endocytosis of smaller substances such as fluids
What is osmosis?
The movement of water molecules from a high water potential to a lower water potential across a partially permeable membrane.
What is water potential?
The tendency of water molecules to enter or leave a system by osmosis.
Always zero or negative.
Why is the partially permeable membrane important in osmosis?
It ensures that the solute does not also diffuse and counteract the effect of osmosis.
What is the equation for water potential of a cell?
Water potential of a cell =
Solute potential +
Pressure potential
What is solute potential?
Generated by the solutes dissolved in the water.
Always zero or negative.
What is pressure potential?
The pressure generated by the cytoplasm pushing on the cell wall and membrane.
Always zero or positive.
What is a high water potential?
Close to zero.
Pure water is 0kPa.
What is a hypotonic solution?💧
A weak solution.
The solution with the highest water potential.
What is a hypertonic solution?🍹
A strong solution.
The solution with the lower water potential.
What is an isotonic solution?
Both solutions have the same water potential.
Water still moves but there is no net movement.
What happens to plant cells in a hypertonic solution?🌱
They lose water and become flaccid and plasmolysed.
What is plasmolysis?🌱
Where the cytoplasm shrinks and comes away form the cell wall.
This causes the plant to wilt.
What happens to plant cells in a hypotonic solution?🌱
The cells will gain water, swell and become turgid.
This ensures plants remain upright.
What happens to animal cells placed in hypertonic solution?🐯
The cells will lose water and become shrivelled.
What happens to animal cells placed in a hypotonic solution?🐯
The cells will gain water, swell and then burst (lysis).
What is incipient plasmolysis?🌱
The point at which the cytoplasm begins to come away from the cell wall.
How can you see if incipient plasmolysis has occurred?🌱
By observing the solute potential at which half of the plant cells in a sample are plasmolysed.
At this point, the solute potential is equal to the water potential.
Why is the pressure potential at 0kPa at incipient plasmolysis?
The cytoplasm is no longer pushing against the cell wall
Why does the cell membrane act as a barrier?
It is impermeable to polar molecules and ions.
