Transport across cell membrane Flashcards
What is the relationship between all eukaryotic cells cell membrane?
The basic structure of all cell membranes, including cell-surface membranes and the membranes around the cell organelles of eukaryotes, is the same.
What are the different structures found in the membrane? (6)
- Phospholipids
- Carrier proteins
- Channel proteins
- Cholesterol
- Glycolipids
- Glycoproteins
What is the main molecule which forms the bi layer?
Phospholipid
Describe the structure of a phospholipid (2)
1) Hydrophilic head > of both phospholipid layers point to outside of cell surface membrane > attracted by water
2) Hydrophobic tails > point to centre of cell membrane > repelled by water on both sides
What are the functions of the phospholipids? (3)
1) Allow lipid squabble substances to enter and leave the cell
2) Prevent water squabble substances entering and leaving the cell
3) Make the membrane flexible and self sealing
What are the 2 main ways proteins are embedded in the cell membrane? (2)
1) Surface of the bi layer > give mechanical support to the membrane > or in conjunction with glycolipids act as cell receptors for molecules such as hormones
2) Completely span the phospholipid bilayer > carrier and channel proteins
Describe the role of carrier proteins (2)
1) Bind to ions or molecules like glucose and amino acids
2) Change shape to move molecules across the membrane
What are the functions of proteins in the cell membrane? (6)
1) Provide structural support
2) Act as channels transporting water soluble substances across the membrane
3) Allow active transport across membrane through carrier proteins
4) Form cell surface receptors for identifying cells
5) Help cells adhere together
6) Acts as receptors > EG for hormones
Describe the structure of channel proteins relates to its function (2)
1) Form water filled tubes
2) Allows for water soluble ions to diffuse across the membrane
Where in the phospholipid bilayer is cholesterol found?
Within the bilayer
Describe and explain the functions of cholesterol (3)
1) Reduce lateral movement of other molecules > phospholipids > achieved by cholesterol pulling together the fatty acid tails of phospholipid molecules > without making membrane too rigid (ADDS STRENGTH)
2) Make the membrane less fluid at high temperatures
3) Prevents leakage of water and dissolved ions from the cell > cholesterol molecule very hydrophobic
Describe the structure of glycolipids (1)
EXTENDS from the phospholipid bilayer into watery environment outside the cell > acts as a cell surface receptor for specific chemicals
1) Carbohydrate covalently bonded with a lipid
Describe the functions of glycolipids (3)
1) Act as recognition sites
2) Helps maintain the stability of the membrane
3) Helps cells to attach to one another and form tissues
Describe the structure of glycoproteins (1)
1) Carbohydrate chains attached to many extrinsic proteins > on outer surface of membrane
Describe the functions of glycoproteins (3)
1) Acts as recognitions sites > receptors > hormones and neurotransmitters
2) Helps cells to attach to one another and form tissues
3) Allows cells to recognise one another > EG lymphocytes can recognise an organisms own cells
Describe the factors that limit the movement of substances in the membrane (4)
1) Not soluble in lipids > cannot pass through the phospholipid layer
2) Too large to pass through the channels in the membrane
3) Of the same charge as the charge on the protein channels >even if they are small enough to pass through, they arc repelled
4) Electrically charged (polar) >
have difficulty passing through the non-polar hydrophobic tails in the phospholipid bilayer.
Describe why the cell surface membrane is referred to as a fluid mosaic model (2)
1) Fluid > individual phospholipid molecules can move relative to one another > gives the membrane a flexible structure that is constantly changing in shape
2) Mosaic > proteins that are embedded in the phospholipid bilayer vary in shape
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
Describe simple diffusion
- Passive transport > energy comes from the natural, inbuilt motion of particles >rather than from some external source such as ATP
What are membranes around and within cells known as?
Plasma membranes
Describe facilitated diffusion
- Passive process
- Relies only on the inbuilt motion (kinetic energy) or diffusing molecules
- No ATP
- Occurs down a concentration gradient
- Occurs at specific points down the plasma membrane > protein molecules > carrier and channel proteins
Describe the process of facilitated diffusion within a channel protein (2)
1) Open in the presence of a specific ion > if ion not present channel remains closed > control over entry and exit of ions
2) Ions bind with protein > causing it to change shape > closes it to one side of the membrane and opens it to the other side
Describe the process of facilitated diffusion within a carrier protein (2)
1) Presence of molecule such as glucose present > is specific to protein present > binds with proteins
2) Causes change in shape > molecule is released to side of the membrane
NO external energy needed
What is osmosis?
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.
