4.1 Transport across cell membranes Flashcards
What are plasma membranes
The membranes around cells, which have the same basic structure
What is the cell surface membrane
The name given to the plasma membrane that surrounds cells and forms the boundary between cell cytoplasm and the environment
What is the function of the cell surface membrane
It allows for there to be different conditions inside and outside of cells.
It controls the movement of substances in and out of the cell
Why are phospholipids important components of cell surface membranes
Clue: Hydrophillic heads and hydrophobic tails
. They have hydrophillic heads and hydrophobic tails
The hydrophobic heads of both phospholipid layers point to the outside of the cell surface membrane, as they’re attracted by water
The hydrophobic tails of both phospholipid layers point into the centre of the cell membrane, as they’re repelled by water on both sides
What are the 3 functions of the phospholipid membrane
. Allow lipid soluble substances to enter and leave the cell
. Prevent water soluble substances entering and leaving the cell
. Make the membrane flexiable and self-sealing
What do peripheral proteins embedded in the cell surface membrane that never extend completely across it do
. Some proteins occur in the surface of the phospholipid 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 do integral proteins that span the phospholipid bilayer from one side to the other do
. The proteins that span the phospholipid bilayer can be two things
. Protein channels: Which form water filled tubes to allow water soluble ions to diffuse across the membrane
. Others are carrier proteins that bind to ions or molecules eg glucose and amino acids , then change shape in order to move these molecules across the membrane
What are the 5 functions of 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 and hormones
. Help cells adhere together
How do cholesterol molecules occur in the phospholipid bilayer of the cell surface membrane
. They are very hydrophobic so play an important role in preventing loss of water and dissolved ions from the cell
. They add strength to the membranes
. They pull together the fatty acid tails of the phospholipid molecules, limiting their movement and that of other molecules , but without making the membrane as a whole too rigid
3 Functions 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 glycoproteins
. Carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane.
. These glycoproteins also act as cell surface receptors, more specifically for hormones
What are the three functions of glycoproteins in the cell membrane
. Act as recognition sites
. Help cells to attach to one another and so form tissues
. Allow cells to recognise one another eg lymphocytes can recognise an organisms own cells
Why do most molecules not freely diffuse across the cell membrane
Many are..
. Not soluble in lipids and therefore cannot pass through phospholipid layer
. Too large to pass through channels in the membrane
. They are of the same charge as the charge on the protein channels so they repel even if they’re small enough to pass through
. They are polar and therefore have difficulty passing through the non polar hydrophobic tails in the phospholipid bilayer
What are the 5 functions of membranes within cells
. Control entry and exit of materials in discrete organelles such as mitochondria and chloroplasts
. Separate organelles from cytoplasm so that the metabolic reactions can take place within them
. Provide an internal transport system eg endoplasmic reticulum
. Isolate enzymes that might damage the cell eg lysosomes
. Provide surfaces on which reactions can occur eg protein synthesis using ribosomes on rough endoplasmic reticulum
Describe the fluid mosaic model of the cell surface membrane
. It is fluid because the individual phospholipid molecules can move relative to one another.
This gives the membrane a flexible structure that is constantly changing in shape
Mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern in the same way the tiles on a mosaic do
