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Transport across cell membranes - Coggle Diagram
Transport across cell membranes
Structure of the cell surface membrane
Phospholipids
They are important in components in cell surface membranes as:
Phospholipids have hydrophilic heads of both phospholipid layers point to the outside of the cell surface membrane and is attracted to water.
They also have hydrophobic tails of both phospholipid bilayer point into the centre of the membrane and repels water.
The membrane allows for lipid soluble materials to pass through the membrane directly but any other materials that a polar apart from oxygen and carbon dioxide need to pass through carrier proteins.
The functions is to:
Allow lipid soluble substances to enter and leave the cell.
Prevent water soluble substances entering and leaving the cell.
Make the membrane flexible and self sealing.
Proteins
Proteins are interspersed throughout the cell surface membrane and is imbedded in the phospholipid bilayer in two main ways.
Some proteins occur in the surface of the bilayer and never extend completely across it. The proteins give mechanical support to the bilayer or in conjunction with glycoproteins, as cell receptors for molecules such as hormones.
However, other proteins span across the phospholipid bilayer in the form of protein channels which form water filled tubes to allow water soluble ions to diffuse across the membrane. Other proteins such as carrier proteins binds to molecules such as glucose and amino acids which then the protein changes shape to allow them to diffuse across the membrane.
The functions consist of provided structural support, act as channels transporting water soluble substances across the membrane.
Allow active transport across the membrane through carrier proteins.
Form cell surface receptors to identify cells.
Help cells adhere together.
Act as receptors e.g. for hormones
Cholesterol
These molecules occur within the phospholipid bilayer of the cell surface membrane and add strength to the membranes. These molecules play an important role in reduction of water loss and dissolved ions as they are very hydrophobic. These molecules also pull fatty acid tails of the phospholipid together and limit their movement and of other molecules without making the structure too rigid.
The functions of this molecules is to reduce lateral movement of other molecules including phospholipids.
Make the membrane less fluid during high temperatures.
prevent leakage of water and dissolved ions from the cell.
Glycolipids
These are made up of carbohydrates covalently bonded together with a lipid. The carbohydrate portion extends from the bilayer all the way into the watery environment where it acts as a cell surface receptor for specific chemicals e.g. human ABO blood system operates as a result of glycolipids on the cell surface membrane.
Functions of these molecules is to act as recognition sites and help maintain the stability of the membrane
Help cells to attach to one another and so form tissues
Glycoproteins
They are carbon chains attached to extrinsic proteins on the outer surface of the membrane. They also act as a cell surface receptor specifically for hormones and neurotransmitters.
Some functions of the glycoprotein is to act as recognition sites .
Help cells attach to one another so form tissues.
Allows cells to recognise one anther e.g. lymphocytes can recognise an organism's own cells.
Permeability of the cell-surface membrane
The cell surface membrane controls movement in and out of the cell of substances. Molecules don't move freely across it as:
The membrane is not soluble in lipids therefore cannot pass through the phospholipid bilayer.
Too large to pass through the channels in the membrane.
Of the same charge as the charge on the protein channels and so even if they are small enough to pass through they are repelled.
Electrically charged (in other words are polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.
Fluid mosaic model of the cell surface membrane
The structure is described as fluid mosaic due to all the way various molecules are combined into the structure of the cell surface membrane.
Its called fluid due to the individual phospholipid molecules can move relative to one another. This gives the membrane a flexible structure that is constantly changing in shape.
mosaic due to the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.
Osmosis
Osmosis can be defined as the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a selectively permeable membrane.
Solutions and water potential
A solute is something that has been dissolved in a solvent like water.
Water potential is represented by the Greek letter Psi, and is usually measure in KiloPascals kPa. Water potential is the pressure created by water molecules and under standard conditions of temperature 25 degrees is equal to 100 kPa and pure water is said to have a water potential over 0.
It follows that:
The addition of a solute to pure water will lower its water potential.
The water potential of a solution (water + solute) must always be less than zero, that is, a negative value.
The more solute that is added (i.e. the more concentrated a solution is), the lower (more negative) its water potential.
Water will move by osmosis from a region of higher (less negative) water potential (e.g. -20kPa) to one of lower (more negative) water potential (e.g. -30kPa).
Explanation of osmosis
The solution on the left has a low concentration of solute molecules while the solution on the right has a high concentration of solute molecules.
Both the solute and water molecules are in random motion due to their kinetic energy.
The selectively permeable plasma membrane, however, only allows water molecules across it and not solute molecules.
The molecules will diffuse from an area that has the higher concentration to a concentration to a lower concentration down a water potential gradient as it is a passive process, the molecules will continue to do this until a dynamic equilibrium is reached in the plasma membrane.
Understanding water potential
The higher the water potential value (closer to zero) the more pure the water is however, the more negative the water potential value the less pure the water is (further away from zero).
Osmosis and animal cells
Red blood cells when placed in water will absorb the water by osmosis as the cell has a lower water potential and has a very thin membrane of 7nm.
The cell surface membrane of the red blood cell isn't very flexible and will break once too much the cell will burst via a process called haemolysis.
To prevent haemolysis the cells live in a liquid that has the same water potential as the cells.
Diffusion
explanation of simple diffusion
All movement involves energy but can be described as passive. Passive meaning that the energy comes from the molecules that are inbuilt motion of particles rather than from a source of ATP. To understand diffusion further:
All particles are constantly in motion due to the kinetic energy that they possess.
This motion is random, with no set pattern to the way the particles move around.
Particles are constantly bouncing off one another as well as off other objects, for example, the sides of the vessel in which they are contained.
Diffusion is defined as the net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distributed.
Facilitated diffusion
Facilitated diffusion is a passive process and relies on kinetic energy of the diffusing molecules meaning there is no external output of ATP from respiration. Facilitated diffusion occurs down a concentration gradient but differs at specific point on the plasma membrane and uses protein channels and protein carriers to diffuse into the cell.
Protein channels
These are channels within the phospholipid bilayer and is specific to certain ions making them hydrophilic and allows water soluble ions through.
The channels opens depending of a particular ion such as sodium and if the sodium isn't there the channels remains closed, in this way there is control over the entry and exit of ions. The ions bind with the protein causing the protein to change shape in a way one side closes to one side of the membrane and the other opens e.g. this occurs down a concentration gradient.
Carrier proteins
This protein spans the plasma membrane and is specific to glucose which also binds to the protein causing the shape to change. This process doesn't need external energy so it just uses the energy from the cells where they move from a higher concentration to a lower concentration using there own kinetic energy of the molecules themselves.
Active and Co-transport and absorption of glucose
Active transport
What is active transport?
It is the movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.
In active transport ATP is used to:
Directly move molecules
Individually move molecules using a concentration gradient which has already been set up by (direct) active transport. This known as active transport.
It differs from passive transport as:
Metabolic energy in the form of ATP is needed.
Substances are move against concentration gradient, that is from a low concentration to a high concentration.
carrier protein molecules which act as 'pumps' are involved.
Process is selective with specific substances being transported.
Co-transport