Topic 1:4b - transport across the cell membrane Flashcards
selective permeability
The cell membrane is selectively permeable.
Certain materials move through the membrane more readily than others.
The selective permeability is attributed to the lipid bilayer and the presence of transport proteins that transport some materials across the membrane but not others.
The way a substance is transported is determined by a number of factors including:
- The characteristics of the substances (polar/non polar, hydrophilic or hydrophobic, size, charge)
- The presence or absence of a concentration gradient
- The SA:Vol of the cell
Passive Transport Processes:
Do not require an additional input of energy
the rate of passive transport of substances is affected by: (5)
- Temperature
- The higher the concentration difference, the faster the rate of diffusion
- Size and charge of the molecules, smaller molecules will diffuse faster
- Distance to travel, due to the SA of the membrane and the thickness of the membrane
- The solubility of the material in the lipid bilayer (hydrophilic or hydrophobic)
define simple diffusion
Simple diffusion is the net movement of small, generally uncharged molecules across a membrane following a concentration gradient without the assistance of a transport protein.
e.g. O2, CO2, and small uncharged lipids can cross the membrane by simple diffusion as they are hydrophobic and non polar.
Small polar molecules can also cross the membrane by simple diffusion but at a slower rate.
(e.g water)
describe the movement of molecules during simple diffusion
Movement will occur from an area of high concentration to an area of low concentration until the concentration gradient is zero (equilibrium is reached) for a particular molecule.
Maintenance of a concentration gradient is achieved by:
movement or blood flow.
define facilitated diffusion
Facilitated diffusion is a passive process that transports materials across a membrane along/following the concentration gradient.
Larger, polar molecules and ions are transported by facilitated diffusion e.g. glucose and some amino acids.
proteins and facilitated diffusion:
Transport proteins imbedded into the cell membrane facilitate the diffusion of selected materials between the cell and the extracellular environment.
There are two types of transport proteins in membranes:
1. channel proteins
2. carrier proteins
define channel proteins
Channel proteins are integral membrane proteins that facilitate the diffusion of ions and other hydrophilic materials.
describe carrier proteins
Carrier proteins selectively bond to a specific material (called a substrate) in the cell or extracellular environment.
The shape or position of the carrier protein changes when the substrate bonds.
The carrier protein releases the substrate to the opposite side of the membrane.
define osmosis
Osmosis is the diffusion of water from an area with a low solute concentration (high water concentration) to an area with a high solute concentration (low water concentration) across the cell membrane until equilibrium is reached.
define a hypotonic solution
Solute concentration outside the cell is lower than the solute concentration inside the cell so water diffuses into the cell.
hypotonic solution - effect on cell volume
The cell volume increases as water flows into the cell.
-Plant cells have a cell wall that maintains the structure of a plant cell as the internal pressure increases.
-Animal cells lack a cell wall and can burst (lysis).
Animal cells become: “lysed”
Plant cells become: “turgid”
Define hypertonic solution
Solute concentration outside the cell is higher than the solute concentration inside the cell so water moves out of the cell.
hypertonic solution - effect on cell volume
The cell volume decreases as water flows out of the cell which causes the cell to shrink.
Animal cells become: “shrivelled”
Plant cells become: “plasmolysed”
Define isotonic solution
Solute concentration outside the cell is equal to the solute concentration inside the cell.
Isotonic solution - effect on cell volume
Water particles still move across the cell membrane but there is no net loss or gain of water in the cell. (No change in volume)
Animal cells remain: “normal”
Plant cells are: “flaccid”
