Download MOVEMENT OF SUBSTANCES ACROSS THE PLASMA MEMBRANE

MOVEMENT OF SUBSTANCES ACROSS THE
PLASMA MEMBRANE
Substances Required by Cells and Substances
Eliminated from Cells
1. Metabolism consists of two processes:
a) Anabolism, which is the building up of molecules. Examples are the syntheses of
proteins and ATPs, which a cell needs.
b) Catabolism, which is the breaking down of large molecules to smaller, simpler
molecules. An example is the oxidation of glucose in cell respiration.
2. A cell takes in raw substances for anabolism from external environment.
3. Wastes from catabolism and excess substances are removed from the cell.
4. Also, water and various salts are either brought into, or are taken out of the
cell.
Substances that leave the cell
Substance
Carbon dioxide (animal cell)
Oxygen (plant cell)
Secretions
Nitrogenous waste
Excess water
Reason of Removal
A waste product of respiration
A waste product of photosynthesis
Are cellular products for use in other parts of
the body
Are waste products from breakdown of excess
proteins
Osmoregulation
Substances that enter the cell
Substance
Oxygen (animal cell)
Carbon dioxide (plant cell)
Glucose
Amino acids
Ionic salts
Reason of intake
For cell respiration
As a raw material in photosynthesis
For cell respiration
As raw materials for protein synthesis
For cell metabolism and osmoregulation
The Necessity for Movement of Substances
Across the Plasma Membrane
(a) Cells need nutrients and oxygen. These pass through the plasma membrane
from the external environment.
(b) Cells produce waste products which exit through the plasma membrane.
(c) The plasma membrane can control the types and the amounts of subtances
needed by the cell at any time.
The Structure of the Plasma Membrane
1. Figure below is an electron micrograph of two plasma membranes.
2. Each plasma membrane looks like a railway track.
3. Singer and Nicholson proposed the fluid mosaic model in 1972 to explain the
structure of the plasma membrane.
4. The fluid mosaic model is the currently accepted model of the cell membrane.
Epidermal cells
(a)
Epidermal cells,
(b) Electron micrograph showing two plasma membranes of
adjacent epidermal cells
5. To understand the concept of the plasma membrane, we shall look at the:
(a) basic unit of the plasma membrane
(b) formation of the plasma membrane
(c) other molecules present in the plasma 'membrane
6. The basic unit of the plasma membrane is the phospholipid molecule.
7. The phospholipid molecule consists of:
(a) a polar phosphate molecule head which is hydrophilic, and
(b) two non-polar fatty acid tails which are hydrophobic
8. The meanings of the three technical words:
(a) A polar molecule carries an unequal distribution of electric charges. This
unequal distribution of electrical charges produces a polar molecule which
can attract other polar molecules, such as water molecules.
(b) Hydrophilic means "water-loving" or attracted to water molecules.
(c) Hydrophobic means "water-hating," or repelling water molecules.
9. Formation of the plasma membrane.
(a) Phospholipid units (polar heads) attract each other, grouping together side by
side to form a layer of phospholipids
(b) One layer of phospholipids forms over another to produce a phospholipid
bilayer
(c) In this phospholipid bilayer the:
 hydrophilic heads point outwards facing water molecules on both sides (th
external environment, and the aquah cytoplasm).
 hydrophobic tails point inwards, away fron water molecules.
(d) This phospholipid bilayer forms the frame work of the plasma membrane.
10. Other molecules present in the plasma membran are:
(a) Cholesterol molecules which fit in between the phospholipids molecules,
making the plasma membrane more rigid and stable.
(b) Carrier proteins and channel protein which assist and control the
movement of water-soluble ions and certain molecule across the membrane.
(c) Glycolipids which are combinations of lipid and polysaccharides, help cells
to recognise each other.
(d) Glycoproteins which are combinations of proteins and polysaccharides,
also hel cells to recognise each other.
11. Based on Figure below, the meaning of the "fIuid mosaic model” is clear
because:
(a) fIuid refers to the free movement of protein and phospholipids within the
membrane, and
(b) mosaic refers to the array of different protein embedded in the phospholipid
bilayer.
glycoprotein
polysaccharide
protein
Permeability of the Plasma Membrane
Permeability of the Phospholipid Bilayer
1. Permeable means "allowing something to pass through".
2. The proteins and the phospholipids of the plasma membrane affect the
permeability of the plasma membrane.
3. The plasma membrane is selectively permeable or semi-permeable as it allows
only certain substances to pass through it but not others.
4. The plasma membrane is only permeable to small molecules such as water,
oxygen and carbon .dioxide.
5. The phospholipid bilayer is permeable to:
a) small non-polar (hydrophobic) molecules that are lipid-soluble, such as:
fatty acids, glycerol, steroids, vitamins A, D, E and K.
b) small uncharged molecules, such as: water, oxygen and carbon dioxide.
These molecules are small enough to squeeze through between the
phospholipid gaps by simple diffusion or osmosis (for water) down their
respective concentration gradients.
6. The phospholipid bilayer is not permeable to:
a) large polar molecules that are not soluble in lipid, such as glucose, amino
acids, nucleic acids and polysaccharides
b) ions (charged), regardless of size, such as:
H+, Na+, HC03-, K+, Ca2+, Cl-, and Mg2+.
7. To summarise, the phospholipid bilayer:
a) is semi-permeable, and
b) allows the entry of small uncharged or hydrophobic molecules, but
c) prevents the entry of larger polar or even small
charged substances.
8. The three characteristics of a molecule that determine its permeability across the
plasma membrane are its:
a) polarity - (hydrophobic as compared to hydrophilic)
b) charge - (charged as compared to uncharged)
c) size - (large as compared to small)
Types of Transport Across the Plasma Membrane
Solutes move across the plasma membrane by two main processes:
(a) passive transport - which does not require a cell to use energy, and
(b) active transport - which requires a cell to use energy to move molecules through its cell
membrane
1)The Movement of Soluble Substances Across the Plasma
Membrane Through the Process of Passive Transport
A) Simple Diffusion
1. Simple diffusion is the random movement of ions or molecules from a region of
their high concentration to a region of their low concentration down a
concentration gradient until an equilibrium is achieved.
2. Molecules have kinetic energy, move randomly, and collide with each other.
3. There are more collisions in a region of high concentration than in a region of
low concentration.
4. Random collisions of molecules spread the molecules out, down the
concentration gradient (difference in concentration of a particular substance in one region
compared to another region.)
5. Diffusion achieves a dynamic equilibrium when the concentration is the same in
all regions.
6. The structure of the plasma membrane determines the types of molecules or ions
that can cross it.
7. Molecules that can cross are (see Figure 3.4): (a) small non-polar (hydrophobic)
molecules (b) small uncharged molecules
8. These molecules squeeze between the polar phospholipids heads, then dissolve in
the lipid layers and come out on the other side.
9. Factors affecting the rate of diffusion:
Factor
Diffusion gradient
Size of molecules
/ions
Temperature
Diffusion medium
Surface area
Effect on the rate of simple diffusion
The steeper the diffusion gradient, the higher the rate
The smaller the size, the higher the rate
The higher the temperature, the higher the rate
Rate in gas>rate in liquid> rate in solid
The larger the surface area, the higher the rate
Examples of simple diffusion in biology:
(i) Gaseous exchange in the alveoli: oxygen from air to blood; carbon dioxide from
blood to air .
(ii) Gaseous exchange in photosynthesis: carbon dioxide from air into the leaf;
oxygen from leaf to air.
(iii) Gaseous exchange in respiration: oxygen frOIL blood to tissue cells; carbon
dioxide in the opposite direction.
(iv) Osmosis: the diffusion of water through the semi-permeable membrane of the
root hair cell .
(v) The absorption of digested food through the villi of the small intestine.
(iv) The absorption of oxygen by unicellular organisms.
B) Facilitated Diffusion
1. Facilitated diffusion is the movement of specific molecules(or ions) across the
plasma membrane
2. Facilitated diffusion is assisted either by pore proteins or by carrier proteins, and
the direction of movement is down the concentration gradient of the molecules
concerned.
3. No energy is required in facilitated diffusion
4. The functions of pore proteins and carrier proteins:
a) Pore proteins (channel proteins)
i. Charged ions (such as Na+, K+, Ca2+, and Cl-) cannot diffuse across the nonpole: centre of the phospholipids bilayer.
ii. Pore proteins open up pores or channels across the membrane to allow entry
or exit.
iii. Each pore or channel is specific and will only allow one particular type of ion
through.
iv. Pores can open or close, acting as gates, to cater to the needs of the cells.
b) Carrier protein
i. They allow larger polar molecules (such as sugars and amino acids) to cross.
ii. A particular protein attaches itself to the binding site of a carrier protein.
iii Then, the carrier protein changes shape and delivers the molecule across the
plasma membrane.
C) Osmosis
1. Osmosis is the movement of water molecules from a region of low solute
concentration (high water concentration) to a region of high solute concentration
(low water concentration) through a semi-permeable membrane.
2. The semi-permeable membrane allows water molecules, but not the solute
molecules to pass through.
3. Water continues to flow from the region of low solute concentration to the region
of high solute concentration until the solute concentrations in both regions are
the same
4. Examples of osmosis in biology
a) Absorption of water by root hairs.
b) Movement of water from one cell to another
c) Absorption of water in the alimentary canal-stomach, small intestine and colon
2)The Movement of Soluble Substances Across the Plasma
Membrane Through the Process of Active Transport
1. Active transport is the movement of substances across the plasma membrane
from a region of low concentration to a region of high concentration.
2. In active transport, the substances move across a membrane against the
concentration gradient. This transport requires work, therefore the cell must
expend its own metabolic energy.
3. The active transport of substances against the
concentration gradient is performed by specific protein
molecules embedded in the plasma membrane.
4. These transport proteins which function as carrier proteins require energy to
change the shape of the protein such that the substance can be pumped across
the membrane.
5. The energy required for active transport is supplied by ATP (Adenosine
Triphosphate).
6. All living cells can carry out active transport.
7. An example of active transport is the pumping of sodium ions (Na+) out of the
cell
8. An example of active transport is the intake of mineral ions by the
root hairs of a plant.
9. In the soil, the mineral salts dissolve in water to form mineral
ions.
10.The concentration of ions in the root hairs is higher than its
concentration in the soil.
11. As the plant needs mineral, it has to pump the ions across the membrane of
the root hairs againts the concentratin gratient. Energy is expended in the active
transport of mineral ions into the root hairs of the plant.
COMPARING AND CONTRASTING PASSIVE TRANSPORT AND ACTIVE TRANSPORT
ASPECT
PASSIVE TRANSPORT
OSMOSIS
DIFFUSION
Movement
Direction of
movement
Requirement of
energy
Requirement of
special
membrane
proteins
Selective of
molecules
Control by cell
Example of
molecules
transported
ACTIVE TRANSPORT
FACILITATED
TRANSPORT
Random movement of
Random movement of
Random movement of
molecules
water molecules
molecules (ions)
From higher to lower concentration
(Down the concentration gradient)
From lower to higher concentration (Against the
concentration gradient)
No energy is required from the cell
Uses energy from the cell (ATP molecules)
No
No
Pore/channel proteins
and carrier proteins
Not selective
Only water molecules
Yes, very specific
No
No
Yes
Lipid-soluble molecules,
gases, water
Water
Glucose and amino
acids
Selective movement of molecules (ions)
Carrier proteins (also called "pumps")
. Yes, very specific
Yes
Mineral salts, amino acids
Hypotonic, Hypertonic and Isotonic Solutions
Solute, Solvent, and Solution
A solution is made up of two parts, the solute and the solvent.
The solvent dissolves the solute.
If you dissolve salt in water, you make a salt solution.
The salt is the solute and the water is the solvent.
The concentration of a solution refers to the quantity of solute dissolved in a unit
volume of solvent.
6. The units of solution concentration are either of these units: grams per litre, grams per
cubic decimetre, molarity or percentage concentration.
7. A concentrated solution has a lot of solute dissolved in the solvent.
8. A dilute solution has a small amount of solute dissolved in the solvent.
1.
2.
3.
4.
5.
Hypotonic, Hypertonic and Isotonic Solutions
1. a) “Iso" means the same as, and "tonicity" refers to the strength (concentration
of solute) of the solution.
b) Two solutions are isotonic if they have the same solute concentrations.
2. a) “Hype”,' means more than.
b) Solution A is hypertonic to solution B if solution A has a higher solute
concentration than solution B.
3. a) “ Hypo" means less than.
b) Solution A is hypotonic to solution B if solution A has a lower solute
concentration than solution B.
The Effects of Hypotonic, Hypertonic and
Isotonic Solutions on Plant Cells d Animal Cells
A) The effects of Hypotonic, Hypertonic and isotonic Solutions on Plant Cells
1) When a plant cell is in an isotonic solution:
a) The concentration of solute in the external solution is equal to the
concentration of solute in the cell sap.
b) Also, the concentration of water molecules in the external solution is equal to
the concentration of water molecules in the cell sap.
c) Therefore, water diffuses into and out of the cell at equal rates.
d) The net movement of water across the plasma membrane is zero.
e) There is no change in the original size of the cell.
2. When a plant cell is in a hypotonic solution:
a) The concentration of solute in the external solution is less than the
concentration of solute in the cell sap.
b) Also, the concentration of water molecules in the external solution is greater
than the concentration of water molecules in the cell sap.
c) Therefore, water molecules move into the cell by osmosis, pushing the cell
contents outwards, against the cellulose cell wall and the cell becomes turgid.
d) The strong and rigid cellulose cell wall pushes inwards with an equal force and
prevents the cell from rupturing or bursting.
e) When the cell is turgid, water molecules will diffusese into and out of the cell
at equal rates (dynamic equilibrium).
3. When a plant cell is in a hypertonic solution:
(a) The concentration of solute in the external solution is greater than the
concentration of solute in the cell sap.
(b) Also, the concentration of water molecules in the external solution is less
than the concentration of water in the cell sap.
(c) Therefore, water molecules diffuse out of the cell vacuole by osmosis and the
cytoplasm shrinks away from the cell walL
(d) Plasmolysis is the separation of plant cell cytoplasm from the cell wall as a
result of water loss.
(e) The plant cell becomes flaccid (soft and weak).
(f) If the plasmolysed cell is placed in a hypotonic solution, it absorbs water
causing the cytoplasm to expand and comes into contact with the cell wall
again. This is called deplasmolysis.
B) The Effects of Hypotonic, Hypertonic and Isotonic Solutions on Animal
cells
1.When an animal cell is put in an isotonic solution:
(a) The concentration of solute in the external solution is equal to the
concentration of solute in the cell.
(b) Also, the concentration of water molecules in the external solution is equal to
the concentration of water molecules in the cell. (c) Therefore, water diffuses
into and out of the cell at equal rates.
(d) The net movement of water is zero.
(e) There is no change in the size of the cell
2.When an animal cell is in a hypotonic solution:
(a) The concentration of solute in the external solution is less than the
concentration of the solute in the cell.
(b) Also, the concentration of water molecules in the external solution is greater
than the concentration of water molecules in the cell. (c) Therefore water
molecules move into the cell by osmosis, inflating and finally rupturing the
cell - this is called cell lysis.
(d) Figure below shows the process of haemolysis, which is the rupturing of red
blood cells in a hypotonic solution.
3.When an animal cell is in a hypertonic solution:
(a) The concentration of solute in
the external solution is greater than the concentration of solute in the cell
(b) Also, the concentration of water molecules in the external solution is less
than the concentration of water in the cell
(c) Therefore, water molecules diffuse out of the cell by osmosis and the cell
shrinks - this is called crenation (Figure below).
The Effects of Hypotonic, Hypertonic and Isotonic Solutions on Plant
Cells and Animal Cells