Chapter 4.2 Flashcards
diffusion
-Diffusion is a type of transportation that occurs across the cell membrane -It can be defined as: -The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.
Diffusion Factors Table
causes of diffusion
-The molecules or ions move down a concentration gradient -The random movement is caused by the natural kinetic energy of the molecules or ions -As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space -The rate at which a substance diffuses across a membrane depends on several factors
Facilitated diffusion
-Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include: -Large polar molecules such as glucose and amino acids Ions such as sodium ions (Na+) and chloride ions (Cl–) -These substances can only cross the phospholipid bilayer with the help of certain proteins
There are two types of proteins that enable facilitated diffusion
-Channel proteins -Carrier proteins They are highly specific (they only allow one type of molecule or ion to pass though)
Channel proteins
-Channel proteins are water-filled pores -They allow charged substances (eg. ions) to diffuse through the cell membrane -The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore -This allows the channel protein to control the exchange of ions
Carrier proteins
carrier proteins can switch between two shapes -This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape -The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane -Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
The Process of Osmosis
-All cells are surrounded by a cell membrane which is partially permeable -Water can move in and out of cells by osmosis -Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane
The water potential of pure water (without any solutes) at atmospheric pressure
is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)
osmosis
Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane
Active transport is important in:
-Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules -Absorption of some products of digestion from the digestive tract -Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant Loading inorganic ions from the soil into root hairs
how osmosis works
The Process of Active Transport
-Active transport is the movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration -Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion) -Although facilitated diffusion also uses carrier protein, active transport is different as it requires energy -The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane -The energy required is provided by ATP (adenosine triphosphate) produced during respiration
Endocytosis
Endocytosis is the process by which the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it There are two forms of endocytosis
There are two forms of endocytosis
~Phagocytosis: -This is the bulk intake of solid material by a cell -Cells that specialise in this process are called phagocytes -The vacuoles formed are called phagocytic vacuoles -An example is the engulfing of bacteria by phagocytic white blood cells ~Pinocytosis: -This is the bulk intake of liquids -If the vacuole (or vesicle) that is formed is extremely small then the process is called micropinocytosis
Exocytosis features
-The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body -These vesicles then travel to the cell surface membrane -Here they fuse with the cell membrane and release their contents outside of the cell -An example is the secretion of digestive enzymes from pancreatic cells
exocytosis meaning
–Exocytosis is the process by which materials are removed from, or transported out of, cells (the reverse of endocytosis)
Surface Area to Volume Ratios
-Surface area and volume are both very important factors in the exchange of materials in organisms -The surface area refers to the total area of the organism that is exposed to the external environment -The volume refers to the total internal volume of the organism (total amount of space inside the organism) -As the surface area and volume of an organism increase (and therefore the overall ‘size’ of the organism increases), the surface area : volume ratio decreases -This is because volume increases much more rapidly than surface area as size increases
Estimating Water Potential in Plant Tissue
-It is possible to investigate the effects of immersing plant tissue in solutions of different water potentials and then use the results to estimate the water potential of the plant tissue itself -The most common osmosis practical of this kind involves cutting cylinders of potato and placing them into solutions with a range of different water potentials (usually sucrose solutions of increasing concentration – at least 5 different concentrations are usually required)
Osmosis: Plant Cells
-If a plant cell is placed in pure water or a dilute solution, water will enter the plant cell through its partially permeable cell surface membrane by osmosis,
-If plants do not receive enough water
the cells cannot remain rigid and firm (turgid) and the plant wilts
as the pure water or dilute solution has a higher water potential than the plant cell
-water enters the vacuole of the plant cell, the volume of the plant cell increases -The expanding protoplast (living part of the cell inside the cell wall) pushes against the cell wall and pressure builds up inside the cell – the inelastic cell wall prevents the cell from bursting -The pressure created by the cell wall also stops too much water entering and this also helps to prevent the cell from bursting
If a plant cell is placed in a solution with a lower water potential than the plant cell
water will leave the plant cell through its partially permeable cell surface membrane by osmosis As water leaves the vacuole of the plant cell, the volume of the plant cell decreases The protoplast gradually shrinks and no longer exerts pressure on the cell wall As the protoplast continues to shrink, it begins to pull away from the cell wall This process is known as plasmolysis – the plant cell is plasmolysed
Plasmolysis of a plant cell that has been placed in a solution with a lower water potential than the cell itself
turgidity
-When a plant cell is fully inflated with water and has become rigid and firm, it is described as fully turgid -This turgidity is important for plants as the effect of all the cells in a plant being firm is to provide support and strength for the plant – making the plant stand upright with its leaves held out to catch sunlight
Osmosis: Animal Cells
Like plant cells, animal cells can also lose and gain water as a result of osmosis As animal cells do not have a supporting cell wall (unlike plant cells), the results of this loss or gain of water on the cell are more severe
Effect of osmosis on animal cells
osmosis: animals vs plants
