Transport across membrane
Selectively permeable
The cell membrane is describes as selectively permeable as only certain materials are transported between the cell and the extracellular space. Certain materials diffuse through the membrane more readily than others. The selective permeability is attributed to the lipid bilayer and to the presence of transport proteins that transport some materials across the membrane but not others.
Diffusion
The net movement of molecules or ions down a concentration gradient.
Particles move passively with energy input from a region of higher concentration to a region of lower concentration until the concentration gradient is zero.
The rate of diffusion is dependent on the concentration gradient, the thickness and surface area of the membrane and the solubility of the materials in the lipid bilayer
Osmosis
Osmosis is the passive diffusion of water across a selectively permeable membrane
Occurs from a region of higher water concentration to lower concentration
The movement of water is also affected by the solute concentrations in the extracellular environment.
The extracellular environment
Isotonic: Solute concentration outside the cell is equal to the solute concentration inside the cell.
Hypotonic: Solute concentration outside the cell is lower than the solute concentration inside the cell
Hypertonic: Solute concentration outside the cell is higher than the solute concentration inside the cell
Water diffuses passively from a region of lower solute concentration to a region of higher solute concentration
The cell volume increases when water diffuses into a cell from a hypotonic solution. Palisade mesophyll have a cell wall that maintains the cell structure as the volume increases, whereas red blood cells lack a cell wall and may burst when placed in a hypotonic solution. The cell volume decreases when water diffuses out of a cell and into a hypertonic solution. The decrease in volume of a palisade mesophyll cell and a red blood cell can inhibit life processes including metabolism, respiration and growth
Facilitated diffusion
passive movement of a solute down its concentration gradient and is facilitated by integral proteins in the membrane
Two types of transport protein: channel and carrier protein
Channel proteins
facilitate the diffusion of ions and other hydrophilic materials
Channel proteins have pores spanning the membrane which allows million of ions to diffuse across the membrane every second
Carrier proteins
Selectively bind to substrate in the cell or the 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
The movement of water through the membrane is too slow so protein channel called aquaporins are used.
Active transport
Active transport uses carrier proteins to facilitate the movement of materials across the membrane in the opposite direction to the concentration gradient
Requires an input of ATP
Active process
Bulk Transport
Active transport of large molecules between the cell and the extracellular.
Two form: Endocytosis and Exocytosis
Endocytosis
bulk transport of material
To forms: phagocytosis and pinocytosis
Phagocytosis: transport of solid materials. In phagocytosis, the cell membrane produces two extensions called pseudopodia that surround the solid materials to be transported into the cell. The pseudopodia fuse and create a vesicle around the particle
Pinocytosis: transport of fluid. In pinocytosis, the cell membrane invaginates which allows the fluid to flow inwards. the liquid enclosed in vesicle
Exocytosis
bulk transport of materials from the cell to the extracellular environment
The contents of the vesicle are released into the extracellular environment as the vesicle fuses with the cell membrane
Surface Area to volume Ratio
Materials such as nutrients, waste products and gases are exchanged across the cell membrane and the rate of exchange is proportional to a cell's surface area while the amount of exchange is proportional to its volume.
Complex multicellular organisms such as animals and plants exhibit structure that increase the surface area to volume ratio for the exchange of materials. Such structure occur inside and outside the body of an organism
Cells with a greater surface area to volume ratio are more effective at sustaining metabolism as the rate of diffusion of raw and waste materials are equal to or greater than the rate of metabolism
Cells with a smaller surface area to volume ratio are unlikely to survive as they are unable to supply raw materials and remove waste materials at a rate that sustains metabolism. Therefore growing cells tend to divide when they reach a certain size to maintain a surface area to volume ratio that sustains the rate of metabolism