2.2 The Transport of Substances Across a Cell Membrane
Homeostasis
⚫ Definition:
Extracellular Fluid
⚫ Fluid ______________ cells
⚫ The ecf consists of a variable mixture of __________ and __________ materials.
⚫ Some of the dissolved materials are substances needed by _____ cells.
Diffusion
⚫ The movement of molecules from a region of high concentration to a region of lower concentration.
⚫ Eg. Perfumed sprayed in one corner of the room will diffuse throughout the room.
How does diffusion work?
⚫ Molecules are in constant motion. They are constantly colliding and changing direction. (Brownian Motion)
⚫ This constant, random movement of molecules eventually causes them to spread out and fill an area
Illustrations of Diffusion:
⚫ http://highered.mcgraw-hill.com/sites/0072495855/ student_view0/chapter2/animation__how_diffusio n_works.html
Factors that Affect the Rate of
Diffusion:
a. Molecule Size
b. Molecule Polarity
Concentration Gradient
⚫ Diffusion always results in a net movement of particles from a region of __________ concentration toward a region of
_______concentration.
⚫ The ___________ in concentrations between these regions is called the “concentration
Diffusion Limits Cell Size
⚫
Most concentration gradients occur
across a _____ ________________.
⚫
The ___________ the volume, the
easier it is for diffused molecules to
reach all parts of interior of the cell.
⚫
The ___________ the surface area of
a cell relative to its volume, the
⚫ Notice that the surface area: volume ratio becomes greater as the diameter of the cell gets smaller.
Passive Transport Across Cell
Membranes by Diffusion
⚫ Cell membranes are considered to be
______-permeable because they controls the movement of certain particles across the
membrane.
⚫ All cell membranes are completely permeable to ________, ________________ and ___________. ⚫ Many compounds, such as _____________,
Passive Transport by Osmosis
⚫ Osmosis is simply the diffusion of
_________ across a ______-___________ cell membrane.
⚫ Recall: Water can move freely through a cell membrane
⚫ Water will move in such a way that its concentration on either side of the
membrane remains ____________.
⚫ As with all particles, water will diffuse from an area of _________ concentration to an
Isotonic Solutions
⚫ Isotonic solution: concentration of solutes is equal on both sides of the membrane.
Hypotonic Solutions
⚫
Hypotonic solution: Concentration of
solutes is higher on the inside of the
cell.
Turgor Pressure in Plants
⚫ Plants maintain their rigidity by ensuring that the cells are in a hypotonic solution.
Lysis in Animal Cells
⚫ If a red blood cell is placed in a hypotonic solution, water will move into the cell.
Hypertonic Solutions
⚫
Hypertonic solution: Concentration of
solutes is higher on the outside of the
cell.
Plasmolysis in Plant Cells
⚫ If a plants external environment is changed such that the concentration of solutes outside its cells is higher, water will diffuse out of the cell (called
plasmolysis)
Shrinking of Animal Cells
⚫ If animal cells are placed in a hypertonic solution, water will diffuse out causing shrinkage of the
Passive Transport by Facilitated
Diffusion
⚫
Specialized ____________ proteins
called carrier proteins help different
substances move across the membrane.
⚫
A particular protein will recognize a
particular type of particle and move it
across the membrane.
⚫
Facilitated diffusion does _____ require
any energy input from the cell and
always occurs in a direction of
Passive Transport by Carrier
Proteins and Channel Proteins
⚫
Carrier Proteins:
Active Transport: Movement
Against the Concentration Gradient
⚫ Sometimes a cell needs to maintain an intracellular environment that is
__________ from the outside of the cell.
⚫ In order to transport molecules ________ the concentration gradient, ie from _____ concentration to _______, energy is needed.
⚫ The process of moving substances across a cell membrane against the conc. gradient is called active transport.
Cells use up 40% of their energy on
active transport.
⚫
Kidney cells pump glucose and
amino acids back into the blood
⚫
Intestinal cells pump in nutrients
from the gut.
The Sodium-Potassium Pump
⚫ http://highered.mcgraw-hill.com/sites/0072495855/st udent_view0/chapter2/animation__how_the_sodium _potassium_pump_works.html
The Sodium-Potassium Pump
⚫ Pumps Na+ and K+ ___________ the concentration gradient
⚫ Used primary in neurons to create a charge difference in order that nerve impulses can be sent.
Bulk Membrane Transport:
Membrane-Assisted Transport
⚫ Some materials are too _____ or too ______ to cross through a membrane through simple diffusion, facilitated diffusion, or active
transport.
Endocytosis
⚫ Endocytosis is the process by which a cell membrane folds inward, enclosing a small amount of matter from the ecf.
⚫ Endo means within.
⚫ Two main forms of endocytosis: ⚫ Pinocytosis
⚫ Phagocytosis
Video about Bulk Membrane
Transport
⚫
http://bcs.whfreeman.com/thelifewire8e
/content/cat_040/0504003.html
⚫
http://www.youtube.com/watch?v=K7y
Pinocytosis
⚫ Means “____ __________”
⚫ Involves the intake of small amounts of
________.
⚫ The cell membrane
_______ ________and pinches off, forming a vesicle inside of the cell.
Phagocytosis
⚫ Means “____ _________”
⚫ Cell membrane folds in and surrounds particulate matter such as bacteria. Folds pinch off, forming a vacuole inside of the cell.
Exocytosis
⚫ The reverse of __________.
⚫ A vesicle from the inside of the cell ______ with the membrane and dumps its contents outside of the cell.
⚫ Many cells excrete various products and wastes in this manner.
