Transport across cell membranes Flashcards
What is the function of the cell surface membrane?
It is the plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and the environment.
It allows different conditions to be established inside and outside the cell.
It controls the movement of substances in and out of the cell.
Which molecules make up the cell-surface membrane?
Phospholipids
Proteins
Cholesterol
Glycolipids
Glycoproteins
What is the function of phospholipids?
Allows 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.
What is the structure of phospholipids?
The hydrophilic heads of both phospholipid layers point to the outside of the cell-surface membrane attracted by water on both sides.
The hydrophobic tails point into the centre of the cell membrane, repelled by the water on both sides.
What is the function of the proteins in the membrane?
Provide 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 for identifying cells.
Help cells adhere together.
Act as receptors, for example for hormones.
How are proteins embedded in the phospholipid bilayer?
There are peripheral proteins and transmembrane proteins.
What are peripheral proteins?
Some proteins occur in the surface of the bilayer and never extend completely across it.
They act either to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones.
What are transmembrane proteins?
Other proteins completely span the phospholipid bilayer from one side to the other.
There are protein channels and carrier proteins.
What are protein channels?
A type of transmembrane protein that forms water-filled tubes to allow water-soluble ions to diffuse across the membrane.
What are carrier proteins?
A type of transmembrane protein that bind to ions or molecules like glucose and amino acids, then change shape in order to move these molecules across the membrane.
What is cholesterol?
They occur within the phospholipid bilayer of the cell-surface membrane.
They add strength to the membrane without making it rigid.
Cholesterol molecules are very hydrophobic so help to prevent loss of water and dissolved ions from the cell.
They also pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules without making the membrane rigid.
What is the function of cholesterol in the membrane?
Reduce lateral movement of other molecules including phospholipids.
Make the membrane less fluid at high temperatures.
Prevent leakage of water and dissolved ions from the cell.
What are glycolipids?
Glycolipids are made up of a carbohydrate covalently bonded with a lipid.
The carbohydrate portion extends from the phospholipid bilayer into the watery environment outside the cell where it acts as a cell-surface receptor for specific chemicals, for example the human ABO blood system operates as a result of glycolipids.
What is the function of glycolipids?
Act as recognition sites.
Help maintain the stability of the membrane.
Help cells to attach to one another and so form tissues.
What are glycoproteins?
Carbohydrate chains are attached to extrinsic proteins on the outer surface of the cell membrane.
These glycoproteins also act as cell-surface receptors, more specifically for hormones and neurotransmitters.
What is the function of glycoproteins?
Act as recognition sites.
Help cells to attach to one another and so form tissues.
Allows cells to recognise one another, for example lymphocytes can recognise an organism’s own cells.
Why do most molecules not freely diffuse across the membrane?
They are:
Not soluble in lipids and 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 are repelled.
Electrically charged (polar) and therefore have difficulty passing through the non-polar hydrophobic tails in the bilayer.
What is the function of the membranes within cells?
Control the entry and exit of materials in discrete organelles such as mitochondria and chloroplasts.
Separate organelles from cytoplasm so that specific metabolic reactions can take place within them.
Provide an internal transport system (endoplasmic reticulum).
Isolate enzymes that might damage the cell e.g. lysosomes.
Provide surfaces on which reactions can occur, e.g. protein synthesis using ribosomes on RER.
What is the fluid in the fluid-mosaic model?
The individual phospholipid molecules can move relative to one another.
This gives the membrane a flexible structure that is constantly changing in shape.
What is the mosaic in the fluid-mosaic model?
The proteins embedded in the phospholipid bilayer vary in shape, size and pattern in the same way as the stones or tiles of a mosaic.
How are substances exchanged?
The exchange of substances between cells and the environment occurs either through active transport or by passive transport.
Active transport requires metabolic energy.
Passive transport doesn’t, for example, diffusion.
What is diffusion?
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.
