2.1.5 - Biological Membranes Flashcards
What is the role of membranes within cells and at the surface of cells?
- Partially permeable barrier between the cell and its environment, between organelles and the cytoplasm and within organelles
- Sites of chemical reactions
- Sites of cell communication (cell signalling)
What is the structure of membranes?
- Membranes are made up of a phospholipid bilayer.
- The fatty acids form a hydrophobic layer sandwiched between the hydrophilic phosphate heads.
- Various proteins are associated with the bilayer.
- Carbohydrates are found attached to some lipids (glycolipids) and some proteins (glycoproteins)
- Cholesterol is found between the fatty acids
Why is the arrangement of phospholipids and proteins known as the fluid mosaic model?
- Fluid - the phospholipids move relative to each other
- Mosaic - the proteins, dotted between the phospholipids, are of various shapes and sizes, like a mosaic
Why do the phospholipids align as a bilayer?
The hydrophilic heads are attracted to water and the hydrophobic tails are repelled by water.
What is the function of phospholipids in the membrane?
- Form the basic structure of a bilayer membrane which is a partially permeable barrier
- Make the membrane flexible
- Prevent the passage of water soluble molecules
- Allow the passage of lipid soluble molecules
What is the function of intrinsic proteins in the membrane?
- Span the bilayer
- Are enzymes, carrier proteins and channel proteins
What is the function of extrinsic proteins in the membrane?
- Are found on the surface or embedded in one layer of the membrane
- Provide mechanical support
- In conjunction with glycolipids, act as cell receptors for hormones and other molecules
What is the function of glycoproteins in the membrane?
- Are receptors for chemical signals such as peptide hormones and neurotransmitters
- Act as receptors for toxins and drugs
- Role in cell adhesion in some tissues
What is the function of glycolipids in the membrane?
- Have a role in cell recognition acting as cell markers or antigens
What is the role of cholesterol in the membrane?
- Restricts movement of other membrane components making membranes less fluid providing mechanical stability
Which factors affect the membrane structure and permeability?
- Temperature
- Solvents
How does temperature affect the membrane structure and permeability?
- An increase in temperature increases the fluidity and permeability of the membrane and increases the rate of transport across a membrane as the phospholipids have more kinetic energy increasing their relative movement and making the membrane ‘leaky’
- If the temperature reaches the point at which membrane proteins start to denature a further increase in membrane permeability will occur
How do solvents affect membrane structure and permeability?
Organic solvents such as ethanol dissolve phospholipids and so will degrade the membrane eventually destroying it which allows substances to cross the membrane freely. This damage causes the fluidity of the membrane to increase and become more permeable.
What is simple diffusion?
The net movement of molecules from an area of higher concentration to an area of lower concentration until equilibrium is reached.
Is ATP required for simple diffusion?
No
What must molecules be to diffuse across the membrane?
Lipid soluble and small
Which molecules pass through the phospholipid bilayer by facilitated diffusion?
Large water soluble molecules and charged ions
How do molecules pass through the membrane by facilitated diffusion?
They use channel proteins which form pores in the membrane
Does facilitated diffusion require ATP?
No
What is osmosis?
The net movement of water molecules by diffusion from a region of higher water potential to an area of lower water potential across a partially permeable membrane
What is water potential?
A measure of the ability of water molecules to diffuse. Pure water has the highest water potential of 0kPa.
What is an isotonic solution?
When the water potential of the solution is the same as the cell within the solution
What is a hypotonic solution?
When the water potential of a solution is more positive than the cell
What is a hypertonic solution?
When the water potential of a solution is more negative than the cell
How does an isotonic solution affect animal and plant cells?
The water potential is equal between the cell and the solution so there is no net movement of water and the cells remain normal
How does a hypotonic solution affect animal cells?
Red blood cells have a lower water potential than the solution so there is a net movement of water in meaning the cells swell and may lyse
How does a hypotonic solution affect plant cells?
Plant cells have a lower water potential than the solution so there is a net movement of water. This causes the protoplast to expand and push against the cell wall so pressure builds up inside the cell and it becomes turgid
How does a hypertonic solution affect animal cells?
Red blood cells have a higher water potential than the solution so there is a net movement of water out so the cells become shrivelled
How does a hypertonic solution affect plant cells?
Plant cells have a higher water potential than the solution so there is a net movement of water out. This causes the protoplast to shrink and begin to pull away from the cell wall which is plasmolysis
What is active transport?
The movement of substances across a membrane against the concentration gradient using energy from the hydrolysis of ATP to ADP and inorganic phosphate
How do molecules pass through a membrane by active transport?
Using carrier proteins which change shape once an ion or a molecule is attached to allow the molecule through the membrane
What is endocytosis?
The cell surface membrane wraps itself around the material and bring it into the cell into a vesicle
What are the 2 main forms of endocytosis?
- Phagocytosis for solid material
- Pinocytosis for liquid material
What is exocytosis?
Where a vesicle containing enzymes, mucus or hormones fuses with the cell surface membrane to release the materials out of the cell
Do exocytosis and endocytosis require ATP?
Yes
Why does exocytosis require ATP?
To move the vesicle along the cytoskeleton
Why does endocytosis require ATP?
For the cell to engulf and change shape around the material
What do endocytosis and exocytosis do?
Transport large quantities of material into or out of the cell
