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• All cells are surrounded by a cell surface membrane or plasma membrane.
• The principal biochemical components are phospholipid and protein molecules.
• The phospholipid molecules are arranged as a BILAYER, and appear as a double line under the electron microscope .
On this electron micrograph, there are two membranes visible.
Each membrane is seen as 2 lines 7-8 nm a part
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7-8nm
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Components of the membrane:
1. Phospholipids
Draw and label a phospholipid molecule:
Phospholipids form bilayers with hydrophilic heads pointing outwards interacting with the aqueous tissue fluid/ blood plasma that surrounds the cell and the aqueous cytoplasm inside the cell. The hydrophobic tails of both layers point towards the of the membrane. Explain why::
2. Proteins
All proteins in the membrane are GLOBULAR. They can be found on the surface of the bilayer, or partly embedded (EXTRINSIC) or they extend completely across it
(INTRINSIC).
Describe and explain the position of the charged, or hydrophilic, parts of the protein, in relation to the phospholipids.
3. Carbohydrates
These are found only pointing out of the outside of the cell, attached to either proteins (as glycoproteins ) or attached to the phospholipids ( glycolipids )
These are involved in cell signalling and cell recognition.
4. Cholesterol present within the bilayer controls membrane__________
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The model is called the fluid mosaic because:
FLUID:
MOSAIC:
:
1.
Transports certain substances into /out of cells:
• Taking up nutrients
• Secretion of chemicals
• Obtain oxygen, remove carbon dioxide
• Maintain water potential gradient.
2. Cell-cell recognition and surface recognition by hormones, enzymes and antibodies
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The cell membrane is selectively permeable to water and some solutes.
Small uncharged molecules such as ____, _____ freely diffuse through the membrane.
Non polar molecules such as __________, ____________ can dissolve in lipids and diffuse very quickly through cell membranes.
Polar molecules, such as ________, ____________, as well as ions, e.g._____ pass more slowly through membranes. Their charge makes them less soluble in lipid, but they can pass through some intrinsic proteins.
• Temperature -
• Organic solvents e.g. ethanol -
1.
Diffusion
2.
Facilitated diffusion
3.
Active transport
4.
Osmosis
5.
Bulk transport
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Random movement of molecules and ions from a _______ to a _______ concentration until is reached. It does not require ________
(passive) and the membrane has no control over this movement.
•
•
•
•
The rate of diffusion through the cell membrane is affected by:
Describe and explain this graph:
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•Diffusion through a channel or carrier ____________ down a ___________ gradient.
•For charged particles or ions e.g insoluble in lipid.
•Does not require __________ (passive). and molecules such as glucose that are
•Facilitated diffusion is more rapid than simple diffusion.
•The rate of facilitated diffusion through the membrane is affected by:
•
•
•
The two types of proteins that allow molecules to pass through the membrane are:
Channel Proteins:
•Consist of water filled pores lined with polar groups allowing charged ions to pass through.
•Each channel is for one type of ion.
•They can open and depending on the needs of the cell.
Carrier Proteins:
•Allow diffusion of larger polar molecules such as and amino acids.
•A molecule attaches to a and causes the protein to change its site releasing the molecule through to the other side of the membrane. 8

• Transport against the _______________ gradient.
• Requires _____________ in the form of _________, which is produced by respiration.
• If respiration is inhibited e.g. by cyanide poison, soisactive transport.
• Requires a carrier ____________ as in facilitated _________ except because it’s against the concentration gradient it requires __________ and the carrier acts as a pump.
• Only charged particles or ions e.g and molecules such as glucose that are insoluble in lipid, can be actively transported.
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The below graphs show the effect of an increasing concentration on the rate of uptake of substances across the cell membrane. The effect of adding a respiratory inhibitor is also shown.
For each graph state the type of uptake involved and explain your answer.
Type of uptake:
Reason for choice:
Type of uptake:
Reason for choice:
Type of uptake:
Reason for choice:
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Endocytosis and Exocytosis are the processes where the cell transports materials in bulk into or out of the cell (without actually having to cross the membrane).
•A Vesicle is produced in the cytoplasm (e.g. budding off at one end of Golgi body.)
•The vesicle migrates to plasma membrane, fuses with it and secretes its contents to the outside of the cell.
•e.g. the hormone insulin or digestive enzymes,
•
What happens to the surface area of the cell after exocytosis?
•The cell __________________folds around the particle.
•The folding closes off the link to the outside the cell so now the particle is fully trapped inside the cell in a vesicle or vacuole.
•Solids – phagocytosis e.g. ___________________
•Liquids – pinocytosis e.g. ____________________
Draw a diagram to show phagocytosis:
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Transport method
Diffusion
ATP used?
Proteins used?
Properties of substances
Examples of substances
Facilitated
Diffusion
Active Transport
Osmosis
Exocytosis
Endocytosis
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• Osmosis is the diffusion of water from an area of _________water potential to an area of ________water potential across a partially permeable _______________.
•It does not require __________.
• Water potential ( Ψ) is the potential energy of water to move in relation to pure water and is measured in KPa. (The greater the number of free water molecules in a solution, the higher the water potential).
• Pure water has the greatest potential energy to move and it is therefore given the highest value of 0 kPa.
•As you add a solute (i.e. a sugar or salt), the number of free water molecules is decreased, the potential energy of the water decreases and therefore the water potential becomes more negative e.g. a dilute solution may have a water potential of
-50kPa, whereas a more concentrated solution may have a water potential of -100kPa.
•Hypotonic
•Hypertonic
•Isotonic
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Water moves from a high water potential to a lower or more negative water potential.
Show the movement of water between the neighbouring cells:
KPa KPa
KP a
In plant cells the following equation is used to describe the relationship between the forces involved in water potential.
Ψ = Ψ s
+ Ψ p
Give a definition of:
• Ψ s
(solute potential)
• Ψ p
(pressure potential)
As water enters the cell, the rigid cell wall develops an opposing pressure potential
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HYPOTONIC SOLUTION
If a plant cell is placed in a hypotonic solution then water will move into the cell. This causes a plant cells to become TURGID
Why don’t plant cells burst?
HYPERTONIC SOLUTION
If a plant cell is placed in a hypertonic solution water will move out of the cell into the surrounding solution. The vacuole shrinks and the cytoplasm will draw away from the cell wall. This causes the plant cell to become PLASMOLYSED.
Label the plasmolysed cell:
A and B have the same water potential.
Explain why:
A
B
ISOTONIC SOLUTION
The theoretical definition for plant cells at this point is : the point at which the cell membrane is just about to come away from the cell wall. It is known as INCIPIENT
PLASMOLYSIS. Experimentally, incipient plasmolysis is the point where 50% of the cells in a sample are plasmolysed.
At incipient plasmolysis what would Ψ p be ?
Complete the equation for Ψ at incipient plasmolysis:
Ψ =
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Label the following diagram with hypertonic, isotonic and hypotonic:
Why do animal cells burst in a hypotonic solution?
Describe and explain the above graph:
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