Download Absorption and Secretion

Absorption and
Secretion
Learning Outcomes
• Identify the structure and properties of the
cell wall.
• Identify the structure of the plasma
membrane with reference to the fluid mosaic
model.
Absorption and Secretion
• Absorption means the uptake of materials by a
cell from its external environment.
• Secretion means the discharge of useful
intracellular molecules into the surrounding
medium by a cell.
• All living cells are surrounded by a cell
membrane (plasma membrane). In addition, plant
cells have a cell wall.
The cell wall
• The cell wall is a non-living layer composed
mainly of cellulose.
• Cellulose is a complex carbohydrate that is
composed of unbranched chains of glucose
molecules grouped together as fibers.
Layer 1
Cellulose
fibres
Layer 2
Tip- Always
describe
cellulose as
fibers.
The cell wall
• The layers of cellulose fibers make the
cell wall:
–
–
–
–
–
Strong
Elastic
Hydrophilic – water is attracted to cellulose
Non-living
Freely permeable – presents no barrier to the
movement of molecules.
– Provides a continuous water conducting
channel. (allows water to move easily from
tissue to tissue without having to enter and
leave every living cell on the way)
The plasma membrane
• The plasma membrane is composed of protein and
phospholipid in an arrangement called a fluid
mosaic.
• The plasma membrane is composed of a fluid
bilayer of constantly moving phospholipid
molecules.
• The term ‘mosaic’ describes the proteins which
are embedded into the lipids in a random fashion.
Plasma Membrane
• The plasma membrane is made of:
A phospholipid bilayer
With proteins embedded in it
Phospholipid
bilayer
Protein
The Fluid
Mosaic Model
The Plasma Membrane
• One end of a phospholipid molecule is hydrophilic
(water loving), this is called the tail. The other end is
hydrophobic (water hating), this is called the head.
• The hydrophillic heads are water soluble and make
up the 2 outer surfaces, where they form bonds with
hydrogen molecules.
• The hydrophobic tails are water-insoluble and point
inwards to the centre of the bilayer since they are
attracted to those in the opposite layer.
Plasma Membrane
video
The Plasma Membrane
• The arrangement of phospholipid molecules is
fluid but at the same time forms a stable and
effective boundary around the cell.
• This allows tiny molecules like water to pass
through it rapidly.
• Larger molecules such as glucose depend upon the
protein molecules for entry or exit to the cell.
Proteins
• The protein molecules in the plasma
membrane vary in size and structure.
• Some of the proteins have narrow channels
which make the membrane porous.
Proteins
• The protein molecules vary in function:
– Provides structural support ( strength )
– Enzymes – to speed up reaction
– Receptors – for cell signalling
– Channels – for movement of larger molecules such as
glucose
– Antigenic markers for cell recognition
– Carriers – by active transport
Summary - The Fluid Mosaic Model
Carrier
Antigen marker
Channel forming protein
Receptor
Structural protein
Beetroot Experiment
• Aim: To investigate the chemical nature of the
cell membrane.
• Results: Test tube A is a control.
• In test tube B and D there was bleeding as low pH
and high temperature denatured the proteins in
the plasma membrane.
• In test tube C there was bleeding because lipids
are soluble in alcohol, so the phospholipids in the
plasma membrane dissolved.
Questions
• What is the main component of a plant cell wall?
• How does the fibrous nature of this substance
make the wall suited to its function?
• Name 2 types of molecule present in the plasma
membrane.
• Which of these would be denatured at 700C?
• State 3 functions carried out by proteins in the
cell membrane.
Learning Outcomes
• Explain the mechanisms of osmosis and
diffusion as basic cell processes.
• Define the terms hypertonic, hypotonic and
isotonic in relation to water content of cell
contents and their surroundings.
• Describe the fate of both animal and plant
cells in hypertonic, hypotonic and isotonic
solutions.
Diffusion
• Diffusion is the movement of molecules or ions from
a region of high concentration to a region of low
concentration.
• The difference in concentration that exists before
diffusion occurs is called the concentration gradient.
• During diffusion, molecules or ions always move
down a concentration gradient from high to low.
high
concentration
low
concentration
Diffusion
• Diffusion is important as it is the way that
useful substances like oxygen enter a cell
and waste substances like carbon dioxide
leave a cell.
• Diffusion is a passive process and doesn’t
require any energy.
• The cell wall is freely permeable to all
molecules in solution, so it does not act as a
barrier to diffusion.
The plasma membrane +
diffusion
• The plasma membrane is freely permeable to tiny
molecules like oxygen that are small enough to
diffuse rapidly through the bilayer.
• Molecules like urea are able to diffuse slowly through
the bilayer.
• Larger molecules like glucose depend upon protein
molecules to help them across.
• Even larger molecules like starch are unable to pass
through the membrane.
The plasma membrane is
selectively permeable.
Osmosis
• Osmosis is the movement of water from a
region of high water concentration to low
water concentration through a selectively
permeable membrane.
• Osmosis is a passive process and does not
require energy.
Water Concentration
• A solution is made when a solute is dissolved in a
solvent.
• A hypertonic solution is one that has a lower
concentration of water (solvent).
• A hypotonic solution is one that has a higher
concentration of water (solvent).
• An isotonic solution is one where the concentration
of solvent and solute are equal.
Water Concentration
• If a cell is said to be in a hypertonic solution, this
means that the water concentration of the cell
contents is higher that the water concentration of the
cell surroundings.
• If a cell is placed in a hypertonic solution, water will
move out of the cell, from high to low.
• If water moves out of an animal cell, it will shrink.
Water Concentration
• If a cell is said to be in a hypotonic solution, this
means that the water concentration of the cell
contents is lower that the water concentration of the
cell surroundings.
• If a cell is placed in a hypotonic solution, water will
move into the cell, from high to low.
• If an animal cell is placed in a hypotonic solution, it
will take on water and burst (haemolysis).
Osmosis in Animal cells
How does water move in these 3 cases?
What are the relative water concentrations?
Osmosis in Plant cells
• Osmosis in plant cells has slightly different results
because plant cells have a cell wall.
• If a plant cell is placed in a hypertonic solution, it will
lose water by osmosis and the cell contents will
shrink. The cell becomes plasmolysed.
• If a plant cell is placed in a hypotonic solution, it will
gain water by osmosis but the cell wall prevents it
from bursting. Instead it becomes turgid.
Osmosis in Plant Cells
Learning Outcomes
• Explain the basic cell process of active
transport.
• State the function of the plasma
membrane in relation to selective ion
uptake (active transport) and absorption
and release of chemicals.
Active Transport
• Active transport is the movement of
molecules and ions across the plasma
membrane from a low to a high
concentration (against a concentration
gradient).
• Active transport requires energy. This
energy is provided by ATP from respiration.
video
Role of the plasma
membrane
• Certain protein molecules present in the
plasma membrane act as carrier molecules.
• These proteins ‘recognise’ specific ions and
transfer them across the plasma membrane.
Sodium Potassium Pump
• Active transport carriers are often called
pumps.
• Some carriers have two roles: they exchange
one type of ion for another. An example of
this is the sodium potassium pump.
• The same carrier molecule actively pumps
sodium ions out of the cell and potassium
ions into the cell.
video
Sodium Potassium Pump
Conditions Required
• Factors such as temperature, availability of
oxygen and concentration of respiratory
substrate (e.g. Glucose) affect the rate of
active transport.
• Increase in temperature causes an increase
in rate of ion uptake, until at high
temperatures the enzymes needed for
respiration becomes denatured.
Conditions Required
• An increase in oxygen concentration results
in increased rate of ion uptake until some
other factor affecting the process becomes
limiting (e.g. temperature).
10 Mark Question
•
Write notes on:
a) Structure of the plasma membrane (4)
b) Function of the plasma membrane in
active transport (3)
c) Structure and function of the cell wall (3)
Structure of the plasma
membrane
1. Contains protein
2. Contains phospholipid
3. Bilayer/double layer/two layers of
phospholipids
4. Phospholipids are fluid/constantly moving
5. Protein arranged as a mosaic/scattered
6. Fluid mosaic pattern/model (only awarded if
4 or 5 not scored)
7. Has channel forming proteins/pores
Function in active
transport
• Proteins/carriers pick up/bind ions/molecules
AND move/carry/assist them across the
membrane/into/out of the cell
• Ion/molecule/substance uptake is selective
• Low to high concentration OR against
concentration gradient
• Requires energy/ATP
Structure + function of cell
wall
• Made of cellulose fibers
• Fully/freely permeable
• Provides support/rigidity for cells/plants OR
gives cells shapes (not strength or structure)
• Stops cells bursting after water uptake/when
turgid/when placed in hypotonic solution OR
allows cells to become turgid.
Learning Outcomes
• Describe the process of endocytosis.
Endocytosis
• Endocytosis is the process by which a cell
engulfs and takes in relatively large particles
or quantities of material.
• This involves the plasma membrane folding
inwards to form a ‘pouch’. The pouch then
becomes closed off and detached from the
cell membrane.
• It is now called an intracellular vesicle.
Endocytosis
• There are 2 types of endocytosis:
– Phagocytosis (cell eating)
– Pinocytosis (cell drinking)
Phagocytosis
• Phagocytosis is the engulfing of large solid
particles (eg bacteria by white blood cells).
• The contents of the vesicle are then
digested.
Pinocytosis
• Pinocytosis involves the formation of small
fluid-filled vesicles by the cell membrane.
• This is how cells take in smaller particles.
• These are then digested using enzymes.
Exocytosis
• Exocytosis is the reverse of endocytosis.
• Vesicles formed inside the cell fuse with the plasma
membrane, allowing their contents to be expelled
from the cell.
• In this way, waste and intracellular products like
enzymes and hormones can be secreted by the cell.
video