Cells, Membranes and Environments Flashcards
What do cell membranes do?
They control the movement of substances into and out of cells, and between different compartments within the cells. They also have important functions to do with recognition and communication.
What are cell membranes?
Cell membranes are phospholipid bilayers that enclose the cytoplasm and subdivide the cell into compartments (organelles).
What is a plasma membrane? And it’s function?
It is the phospholipid bilayer in prokaryotic and eukaryotic cells that encloses the cytoplasm.
It encloses the contents of the cell and enables the cytosol to have a different composition from the surrounding environment.
What is a non-plasma membrane? And it’s function?
It is the phospholipid bilayer only in eukaryotic cells that encloses the organelles.
They form boundaries of discrete compartments within the cell and control movement of substances between these compartments. This is why organelles contain different chemical contents from each other.
What are the functions of membranes?
Encloses the cell contents
Controls what goes in and out of the cell
Transports waste materials out of the cell
Protects the cell
- Control the type of molecules that can enter and leave a cell and cell components
- Are active environments where many essential chemical reactions of life occur
- Establish compartments within a cell that separate chemical materials
- Restricts the movement of substances between one part of a cell and another, regulation of enzymatic processes
- Have receptors involved in intercellular communication
- Are involved in the recognition of cells by other cells
- Produce electrical activity in excitable cells (never and muscle cells)
What is the plasma membrane made up of?
Phosphate heads Lipid (fatty acid) tails Integral proteins Recognition protein Channel proteins Carrier proteins Cholesterol Glycoproteins Glycolipids
Phosphate heads
Hydrophillic (water-attracting)
Makes up the phospholipid
Lipid (fatty acid) tails
Hydrophobic (water-repelling)
Makes up the phospholipid
Integral proteins
Help transport certain materials across the cell membrane.
Some integral proteins do not span the lipid bilayer.
Recognition protein
Peripheral proteins are attached to the exterior of the lipid bilayer. They are easily separable from the lipid bilayer, able to be removed without harming the bilayer in any way. Peripheral proteins are less mobile within the lipid bilayer.
Channel proteins
Channel proteins form a pore through the hydrophobic interior of the membrane to enable water soluble molecules and ions to pass by facilitated diffusion.
Carrier proteins
Carrier proteins permit the passage of specific molecules by facilitated diffusion or active transport.
Cholesterol
Cholesterol is a packing molecule and interacts with the phospholipids to regulate membrane consistency, keeping it firm, but fluid. They also reduce the permeability of the membrane to small water-soluble (polar) molecules.
What role do carbohydrates play in the plasma membrane and where are they found and what are they linked to?
They are linked to protruding proteins as they are found on the outer surface.
They play a role in recognition and adhesion between cells.
Glycoproteins
Glycoproteins are proteins with attached carbohydrate. They are important in membrane stability, in cell to cell recognition, and in the signalling, acting as receptors for hormones and neurotransmitters.
Glycolipids
Glycolipids in membranes are phospholipids with attached carbohydrate. Like glycoproteins, they are involved in cell signalling and cell to cell recognition. They also help to stabilise membrane structure.
Why are cells so small?
Cells are so small in order to have a larger surface area to volume ratio.
The the smaller the cell, the smaller the volume, but the bigger the surface area, which means nutrients and wastes can move across the membrane easily and spread throughout the cell quickly.
The size of a cell is limited by its need to exchange resources with its environment.
The rate with which a cell produced heat/waste and consumes resources (food and oxygen) is proportional to it’s volume.
The rate at which resources are taken up into the cell or heat/waste is released, is proportional to it’s surface area.
As a cell’s size increases, it’s volume increases much faster than its surface area, making it less efficient in the exchange of materials and energy.
What is surface area to volume ratio?
The relationship between surface area and volume that is important for exchange across surfaces; if an object gets larger but stays the same shape, surface : volume gets smaller.
As the size of the object increases the surface area to volume ratio …..
Decreases
What passes through cell membranes without using proteins to facilitate them?
Water molecules pass between the phospholipids molecules by osmosis.
Lipid soluble molecules eg: gases and steroids, can move through the membrane by diffusion, down their concentration gradient.
The movement of substances across the plasma membrane depends on…..?
The surface area available for exchange, the nature of the substance being exchanged, and the composition of the membrane, as well as other factors such as concentration gradients and temperature.
