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.
What type of molecules pass through the cell membrane and how?
???????
Hypertonic
Concentrated
Hypotonic
Dilute
Passive movement
Without energy form the cell. Slower than active movement. Used to transport substances down/along the concentration gradient. High to low. Hypertonic to hypotonic to isotonic.
Active movement
Active means energy is expended. With energy from the cell (ATP). Faster than passive movement. Can be used to transport substances against the concentration gradient. Low to high. Hypotonic to hypertonic.
Passive movement across membranes.
Diffusion
Osmosis
Facilitated diffusion
Isotonic
Balanced
Active movement across membranes.
Active transport
Exocytosis
Endocytosis
What is the concentration gradient?
It is the difference in concentration between two solutions.
The larger the concentration gradient….
The more rapid the rate of diffusion.
What is diffusion?
Passive movement of a substance from a region of high concentration (hypertonic) to one of a low concentration (hypotonic).
The substance moves down its concentration gradient.
Slow process.
Eg: the solute molecules move from the area of high concentration to the area of low solute concentration.
Why is diffusion a passive process?
Because there is a concentration difference and doesn’t need an input of energy.
What is osmosis?
Passive movement of water from a dilute solution (hypotonic), to a more concentrated one (hypertonic).
A special type of diffusion incorporating water.
Solutes attract free water molecules so that they are no longer free.
As a result, there are more free water molecules in a dilute solution than in a concentrated one.
Hypotonic= lots of water, therefore not much concentration. Hypertonic= lots of concentration, therefore not much water.
What happens to a red blood cell undertaking osmosis?
It swells up and then bursts as it has no cell wall to prevent it from bursting.
What happens to a plant cell during osmosis?
It swells up until the turgor pressure is exactly the same inside, as it is outside of the cell. It does not burst as the rigid cell wall prevents it.
What are cells with high internal fluid pressures said to be?
Turgid
What is plasmolysis?
When a cell is placed in a more concentrated solution than that of the cytoplasm, water leaves the cell, causing it to shrink.
What are cells that have loss mass amounts of water said to be….
Flaccid.
What is facilitated diffusion?
Passive movement of a substance (particular ions and molecules) down the concentration gradient (high to low), aided by protein transporters in the cell membrane.
Diffusion involving a carrier system (channel proteins or carrier proteins) but without any energy expenditure.
What are some characteristics/properties of facilitated diffusion?
Transport is more rapid than by simple diffusion.
The channels are specific for particular molecules, so transport is selective- some substances are transported and others are not.
The channels can become saturated (fully occupied) as the concentration of the transported substances increases.
The transport of one substance may be inhibited by the presence of another substance that uses the same channel.
No energy is required, and substances can only be moved down a concentration gradient
What is active transport?
Movement of a substance from a region of low concentration (hypotonic) to one of a high concentration (hypertonic).
This process requires energy from the cell in the form of ATP.
It requires energy as it is moving large substance against the concentration gradient.
From an area of low concentration to an area of already high concentration.
How does active transport occur?
It occurs through channel proteins whereby ATP attaches to a channel proteins and changes the proteins shape, allowing the substance to move through the cell membrane.
What are some characteristics/properties of active transport?
Transport is more rapid than passive.
The channels are specific for particular molecules, so transport is selective- some substances are transported and others are not.
The channels can become saturated (fully occupied) as the concentration of the transported substances increases.
The transport of one substance may be inhibited by the presence of another substance that uses the same channel.
Substances can be moved against the concentration gradient.
What is exocytosis?
The movement of substances out of a cell by the use of vesicles that fuse with the cells membrane.
Substances that cross the cell membrane by this method are often large molecules.
Eg: wastes or substances for secretion
Requires energy.
Which unicellular organism is an example of this?
Amoeba. Removes digestive waste this way.
What is an example of exocytosis?
Insulin (proteins) are made at the ribosomes in a cell. They are packaged by the Golgi into a vesicles. The vehicle merges with the cell membrane to export the insulin to other cells.
What is endocytosis?
Movement of external material into a cell by enclosing it in plasma membrane, which pinches off within the cell to form a vesicles: includes phagocytosis (entry of solids) and pinocytosis (entry of liquid).
Substances entering a cell.
An example of endocytosis is…?
When white blood cells engulf and destroy microorganisms by phagocytosis.
What is phagocytosis?
The entry of solids through endocytosis.
Water potential
The tendency of water to migrate across a semipermeable membrane.
Increases when solute concentration is high.
Osmotic pressure
Pressure of the solution needed to stop the passage of water molecules into a solution during osmosis.
Increases when solute concentration is low.
What is pinocytosis?
The entry of liquid droplets through endocytosis.
