Movement Of Substances Into and Out Of Cells Flashcards
What is diffusion?
Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration
Molecules move down a concentration gradient.
Diffusion helps living organisms to: ?
Gain nutrients and oxygen
Remove waste products
Examples of diffusion in living organisms
Small intestine - Digested food products (glucose, amino acids) move from the lumen of the small intestine to the blood. Leaf - oxygen moves from air spaces between mesophyll cells to mitochondria in all cells.
What is osmosis?
Osmosis is the movement of water molecules from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution), through a partially permeable membrane
Osmosis is the diffusion of water, as the water is moving down its concentration gradient.
Osmosis in Animal Cells
Animal cells lose and gain water as a result of osmosis
As animal cells do not have a supporting cell wall the results of osmosis can be severe:
If an animal cell is placed into a strong sugar solution (with a lower water concentration than the cell), it will lose water by osmosis and become crenated (shrivelled up)
If an animal cell is placed into distilled water (with a higher water concentration than the cell), it will gain water by osmosis as it has no cell wall to create turgor pressure
It will continue to gain water until the cell membrane is stretched too far and it bursts.
Osmosis in Plant Cells
Plant cells lose or gain water as a result of osmosis
As plant cells have a supporting cell wall, they are protected from cell lysis:
If a plant cell is placed into a strong sugar solution (with a lower water concentration than the cell), it will lose water by osmosis
The vacuole gets smaller and the cell membrane shrinks away from the cell wall
The cell becomes flaccid or plasmolysed
If a plant cell is placed into distilled water (with a higher water concentration than the cell), it will gain water by osmosis
The vacuole gets bigger, pushing the cell membrane against the cell wall
The plant cell is described as being turgid
Turgid cells provide structural support for plants, preventing them from wilting.
What is active transport?
The movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration. Energy is needed for active transport because particles are being moved against a concentration gradient
Energy is released during cellular respiration
Active transport across the cell membrane involves protein carrier molecules embedded in the cell membrane
Active transport in animals
Food molecules (such as the sugar glucose) can be absorbed across the wall of the small intestine by diffusion, but this is dependent on a concentration gradient existing between the lumen of the intestine and the bloodstream
Active transport allows molecules such as glucose to be transported into the bloodstream from the lumen of the small intestine (the gut) when the concentration of sugar molecules in the blood is higher
The active uptake of glucose by epithelial cells in kidney tubules in the kidney nephron allows for the reabsorption of glucose back into the blood so that none is lost in the urine
Sugar molecules are used in respiration to release energy for cells to function.
Active transport in plants
Root hair cells lining the surface of plant roots need to move minerals such as magnesium ions from a region of lower concentration (the very dilute solution of minerals in the soil surrounding the roots) to a region of higher concentration (inside the cytoplasm of the cell)
Mineral ions are needed by plants to function
Magnesium ions are required to make chlorophyll
Nitrate ions are needed to make amino acids for protein synthesis (and subsequently growth).
One factor that influences diffusion is…
Surface area to volume ratio
The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface
Many cells which are adapted for diffusion have increased surface area in some way - e.g. root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion).
Another factor that influences diffusion is…
Diffusion distance
The smaller the distance molecules have to travel the faster transport will occur
This is why blood capillaries and alveoli have walls which are only one cell thick, ensure the rate of diffusion across them is as fast as possible.
Another factor that influences diffusion is…
Temperature
The higher the temperature, the faster molecules move as they have more energy
This results in more collisions against the cell membrane and therefore a faster rate of movement across them.
Another factor that influences diffusion is…
Concentration gradient
The greater the difference in concentration on either side of the membrane, the faster movement across it will occur
This is because on the side with the higher concentration, more random collisions against the membrane will occur.
