Knowt
1.3: Cell membranes and transport
Fluid mosaic model of cell membranes
The cell membrane is made up entirely of proteins and phospholipids, and is semi-permeable.
Phospholipids
These are important components of cell-surface membranes and form the basis of membrane structure.
They can form bilayers, with one sheet of phospholipid molecules opposite another.
The inner layer of phospholipids has its hydrophilic heads pointing inwards, towards the cell - interacting with the water in the cytoplasm.
The outer layer has its phospholipids has its hydrophilic heads pointing outwards, interacting with water surrounding the cell.
The hydrophobic heads face each other, towards the centre of the membrane.
The phospholipid component of a membrane allows lipid-soluble molecules across, but not water soluble molecules.
Proteins
Proteins are scattered throughout of the phospholipid bilayer of the membrane. There are two ways in which they are scattered.
Extrinsic proteins
These are on either surface of the bilayer - on the outside. They provide structural support and form recognition sites, by identifying cells, and receptor sites for hormone attachment. There are two types:
Glycoproteins - Used cell to cell recognition, and can recognise another cell as foreign.
Glycolipids - Used in maintaining cell stability and receptors in cell to cell communication.
Intrinsic proteins
Extend across both layers of the phospholipid bilayer. They include transport proteins, which use active or passive transport to move molecules and ions across the cell membrane.
The model
The model was proposed Singer and Nicolson in 1972. It is called the fluid mosaic model as individual phospholipid molecules can move within a layer relative to each other, and proteins embedded in the bilayer can vary in shape, size and distribution.
Plant and animal cell membranes contain glycoproteins, glycolipids and sterols. Glycolipids join directly to the lipid heads on the membrane, while glycoproteins attach to extrinsic proteins. Cholesterol is the sterol in animal cell membranes. These occurs between phospholipid molecules, making the membrane more stable at high temperatures and more fluid at low temperatures. Other sterols perform this function in plant cell membranes.
There is a carbohydrate layer around an animal cell called the glycocalyx. Some molecules in the glycocalyx have roles as hormone receptors, in cell-to-cell recognition and in cell-to-cell adhesion.
Transport across membranes
There are many different methods from molecules to enter and exit the membrane.
Diffusion
Diffusion is an example of passive transport - occurs without energy usage. It is the movement of ions and molecules from an area with high concentration to an area with low concentration until equal distribution is achieved.
While ions and particles are always in a state of random movement, they will move away from high concentration areas until there’s uniform distribution.
The rate of diffusion is affected by:
The difference in concentration - if there is high concentration gradient, diffusion speed will increase.
Distance - the shorter the distance the faster diffusion and vice versa.
Thickness of exchange surface - the thicker the molecule the slower the diffusion, and vice versa.
Surface area of the membrane - more area = more room for diffusion, therefore faster diffusion.
Temperature - higher temperature increases kinetic energy, therefore faster molecule movement.
Size of the diffusing molecules - smaller molecules diffuse faster as there is less to diffuse.
Molecule nature - those molecules that are lipid-soluble diffuse faster than water-soluble, and non-polar diffuse faster than polar.
Permeability of the membrane
Small molecules, such as oxygen and carbon dioxide can move through phospholipid molecules and easily diffuse through the membrane
Any lipid-soluble molecules, e.g vitamin A, dissolve in phospholipids and diffuse across the membrane.
However, water-soluble molecules are unable to diffuse through the membrane due to the hydrophobic tails in the centre. They require intrinsic proteins to travel.
Facilitated diffusion
As ions and molecules are unable to diffuse through the membrane directly, facilitated diffusion is required. It is a passive process that occurs when a concentration gradient arises. However, it can only occur at special sites on a membrane, which can limit the rate of this diffusion. There are two transport proteins which can be used:
Channel proteins, which are used for ions due to their pores being lined with a polar group. It is hydrophilic, so those water-soluble can pass through. They open and close according to the cell need, a gate like process.
Carrier proteins are used for the diffusion of larger polar molecules, such as glucose and amino acids. It works with a molecule attaching to the binding site, causing the carrier protein to change shape and release the molecule on the other side. It then returns to its original shape.
Active transport
Active transport requires metabolic energy, as the ions and molecules are moved against the concentration gradient.
It occurs through intrinsic proteins - specifically carrier proteins.
Processes that require this include muscle contraction and nerve impulse transmission
It occurs when the ion or molecule combining to the specific carrier protein on the outside of the membrane.
Then ATP (adenine tri-sulphide) transfers a phosphate group on the inside of the membrane. It becomes ADP (adenine di-sulphide).
The carrier protein changes shape and carries the molecule or ion across the membrane, where it is released into the cytoplasm. The phosphate is also released into the cytoplasm, where it combines to form ATP. The carrier protein returns to normal.
Active transport has some limiting factors:
The amount of carrier proteins.
The presence of respiratory inhibitors, such as cyanide, which prevent the production of ATP. Opposingly, the presence of oxygen increases the rate of active transport as it allows for more ATP production by aerobic respiration.
Co-transport
Co-transport involves facilitated, active and simple diffusion.
It occurs through special carrier proteins known as co-transporters, and through five stages:
1 - The sodium ions are transported into the blood using active transport. This is done in order to create a low concentration within the cell, so sodium ions can come in through facilitated diffusion.
2 - Sodium ions join to glucose. They travel in through a co-transporter membrane. This is done to avoid using ATP to transport in glucose.
3 - The molecules break apart, diffusing across the cell to achieve uniform distribution.
4 - Glucose diffuses into the blood using facilitated diffusion, and sodium ions are transported out using active transport. Phosphate ions move in at the same time, to keep an even amount of ions in the cell.
Osmosis
Osmosis is the term for the diffusion of water. Due to the size of water molecules, most cell membranes are permeable to them, making this a form of passive transport based off concentration gradients.
Water potential
This is the measure of free energy of water molecules in an area, or the tendency for water to move. It is measured in kilopascals (kPa).
The highest measure, 0, is pure water, as there is no tendency for water particles to move in. Additionally, they are freer to move.
The addition of a solute tends to bring water molecules in, causing an inwards negative force which gives the water potential a negative value. The higher the solute concentration, the more water particles pulled inwards, therefore the more negative the water potential. This can also be explained by water being less free to move, due to weak bonds with the solute.
Osmosis happens on a concentration gradient, however it is better phrased as ‘an area with a high water potential will move down an energy gradient to an area with low water potential‘.
This is caused by the osmotic pull, which is stronger in areas with less free molecules due to the force pulling the water inwards, preventing water molecules from moving freely.
Solute potential
This is the concentration of the solution. This measures how easily water molecules can move out of a solution.
A lower solute potential means a higher solute presence holds water tighter, decreasing how often water will leave the cell.
A higher solute potential means water is more likely to leave the area, as there is less solute.
In plant cells
In plant cells, osmosis majorly affects cell structure.
Pressure potential
As water enters the cell through osmosis, the vacuole inflates and pushes the cytoplasm against the cell wall.
The cell wall can only expand a little bit, and so outwards pressure builds up, resisting the entry of more water. This causes the cell to be turgid. This is pressure potential, and it has a positive sign as it is an outwards force.
Water potential equation
To calculate water potential, pressure potential and solute potential are added. This is because the solute potential pulls in water molecules, while water potential pushes them out.
Turgor and Plasmolysis
Hypotonic - the external solution is hypotonic when it has a higher water potential than the cell. This causes water to flow in the cell.
These cells take in water until the pressure becomes too much for the cell wall, causing the push outwards of the pressure potential to be equal and opposite to the solute potential pull inwards.
This causes the water potential to be zero, as there is no tendency for water to enter the cell.
Once this process is complete, the cell is turgid, meaning it can take in no more water. This provides structural support to a cell, holding them upright and maintaining their shape.
Hypertonic - the external solution is hypertonic when it has a lower water potential than the cell. This causes water to flow out of the cell.
This causes a process called plasmolysis; the vacuole shrinks, and cytoplasm is drawn away from the cell wall.
The beginning of this process is known as incipient plasmolysis, when the cytoplasm is beginning to be pulled from the cell wall. This leads to the cell reaching 0 pressure potential. This means the outer solute potential is the same as the cells water potential.
As this process continues, cells become flaccid, leading to the plant losing the structural support, causing the plant to wilt and eventually die.
Isotonic - the external solution has the same water potential as the cell. This causes no net water movement.
In animal cells
Animal cells have no cell wall, and therefore pressure potential is not a consideration in water potential. For them, solute potential = water potential.
Without a cell wall, cells are far more susceptible to damage from changing water potential.
If placed in a hypotonic solution, animal cells burst, a process known as haemolysis.
If placed in a hypertonic solution, they shrink, becoming crenated.
Bulk transport
Cells can transport materials in bulk in or out of the cell. When this occurs, the cell membrane is forced go change shape, which requires ATP.
For this, the cell membrane must flow, an essential property.
Endocytosis - this occurs when a material is engulfed by extensions of the cell membrane and cytoplasm. They then turn it into a vesicle. here are two types of endocytosis:
Phagocytosis - occurs when solid material is too large to be taken in by diffusion or active transport. For example, it occurs when granulocytes engulf bacteria, allowing lysosomes to fuse with the vesicle formed and enzymes to digest the cell.
Pinocytosis - the uptake of liquid, resulting in smaller vesicles.
Exocytosis - how substances leave the cell. They are transported through the cytoplasm as a vesicle which fuses through the cell membrane.