Rodayna Elnaggar (+201021214991) biologybel3araby@gmail.

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T4- Cell Membrane and Transport

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4.2 Structure of membranes

If phospholipids are mixed with water they form:

1. Micelles → ball like structures→ hydrophilic heads face outwards
2. Bilayers → sheet-like structures

Role of cell membrane:

1. Controls exchange of materials → nutrients & waste products between cell
& environment
2. Cell signaling

The fluid-mosaic model of membrane structure:

Fluid: The phospholipids and proteins can move around via diffusion mainly
sideways, within their own layers.
Mosaic: the scattered protein pattern within the phospholipid bilayer.

Describe the structure of phospholipid.

● Hydrophilic (polar) Phospholipid heads point outwards into the aqueous
solution that surrounds the membranes forming, stabilizing the
membrane by forming hydrogen bonds with water.

● Hydrophobic (non polar) phospholipid tails point inward facing each other
forming a hydrophobic core which acts as a barrier to hydrophilic
molecules & ions

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Factors which impact membrane fluidity:

1. Increased percentage of ● Unsaturated fatty acids have C=C double bonds which causes kinks
unsaturated fatty acids (bending) in fatty acids → they fit together more loosely
present INCREASES
membrane fluidity

2. Tail length → The ● The longer the tail, the more bond interactions between fatty acids, holding
longer the tail the less them closer, so fluidity decreases
fluid the membrane

3. Temperature ● As temperature increases fluidity increases due to phospholipid molecules

having a greater kinetic energy.

4. Cholesterol (more detail ● Fits between phospholipid molecules

soon) ● At low temperatures, cholesterol increases the membrane fluidity →
cholesterol prevents phospholipids from packing close together
● At high temperatures, cholesterol decreases the membrane fluidity.

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Membrane structure & components definitions:

1. Composed of a phospholipid bilayer Contains transport proteins → intrinsic & extrinsic

2. Hydrophilic or polar phosphate heads facing the Glycoproteins → proteins with a carbohydrate part
outside function : receptors & antigens

3. Hydrophobic or non polar phosphate heads facing Glycolipid → lipid attached to a branching
the inside carbohydrate
Function: receptors & antigens

4. 7nm thick and only seen using an electron Glycocalyx → carbohydrate coating that covers
microscope membrane
function : used in cell recognition

5. Cholesterol: for membrane support & fluidity control

** You MUST be able to draw and label the

fluid mosaic model of the cell membrane
<<

What are transmembrane proteins/ intrinsic proteins?

Proteins which span the whole membrane from outer to inner layer.

How are the transmembrane proteins embedded between the phospholipids in the membrane?
● The regions of the polypeptides with amino acids with hydrophobic R groups interact and attach with the
hydrophobic fatty acid tails.

● Regions of polypeptides with amino acids containing

hydrophilic R groups interact with the hydrophilic
phosphate head.

● The R group interactions help keep membrane stable

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Roles of molecules found in membrane:

Phospholipids ● Form a bilayer → 2 layers of phospholipid molecules

● Hydrophobic tails point to the center, hydrophilic heads point to the cytoplasm/
extracellular fluid.

Function:
● Therefore membrane acts as a barrier to ions, hydrophilic molecules
● Permeable to hydrophobic molecules

Cholesterol ● A small, lipid related molecule with a hydrophilic head and a hydrophobic tail.
● More common in animal cells than plant cells
● Fit between phospholipid molecules
● No cholesterol in prokaryotes

Function:
1. Prevent ions or polar molecules passing through due to hydrophobic regions.
2. Mechanical stability
3. Controls membrane fluidity:

At high temperatures: cholesterol decreases the fluidity by hydrophobic

interactions between fatty acid tails.

At low temperatures: cholesterol increases the fluidity of the membrane by

preventing the phospholipid molecules packing close together.

Transport 1. Control which substances enter & exit the cell by having specific binding sites to
proteins specific substances
(channel & 2. Increase permeability of the membrane to ions & polar molecules
carrier)
(more detail
later..)

glycolipids , 1. Receptor molecules ( bind with particular substances at cell membrane) & ‘signaling
glycoproteins & receptors’ are responsible for cell signaling.
proteins
2. Cell to cell recognition → some acts as antigens allowing cells to recognise each
other

3. Transport proteins: provide hydrophilic channels for ions & polar molecules, each
transport protein is specific for an ion or molecule.

4. Enzymes : some membrane proteins are enzymes (NOT glycolipids)

5. cytoskeleton : these protein filaments maintain the shape of the cell

6. From hydrogen bonds with water & stabilize the membrane

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Q: Which factors control PERMEABILITY of the membrane?

1. Percentage of saturated & unsaturated fatty acids
2. Number of transport proteins present
3. Number of Aquaporins → channel proteins responsible for transporting water through membranes.
4. Cholesterol

9700/23/m/j/15 (1d)
Common P2 Q: Describe how proteins become denatured at high temperature and explain how this
could lead to damaging cell membranes.

● Loss of tertiary/quaternary/ secondary structure & loss of globular shape

● Due to breakage of bond interactions eg ionic.
● Loss of function of membrane proteins
● Transport of polar molecules impaired
● Loss of cell signaling & cell to cell adhesion
● Loss of enzyme functions
● Membrane becomes leaky

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Cell signaling
Definition: the molecular mechanisms by which cells detect & respond to external stimuli, including
communication between cells.

Signaling pathways can be:

● electrical (nervous system)
● chemical (hormone system)

Signaling cascade steps (template answer)

1. Stimulus causes cells to secrete ligands (in PPQ you must

mention the name of cell & ligand is possible)
Ligands: a biological molecule which binds specifically to
another molecule, such as a cell surface receptor, during cell
signaling.

2. Ligands are transported to target cells → in the case of

hormones the transport medium is the blood
**Target cell specific name MUST be mentioned in PPQ

3. Ligand binds to complementary cell surface receptors on the

target cells

4. Ligand binding brings about a conformational change in the

receptor shape

5. Changing the shape of the receptor causes transduction

Transduction: is the process of converting a signal from 1
method of transmission to another.

6. G protein is activated which acts as a switch to bring about the release of a second messenger.

7. Second messenger is a small molecule which diffuses through the cell relaying the message

8. Stimulation of 1 receptor molecule results in many second messenger molecules being made →
amplification of original signal

9. A cascade of enzyme catalyzed reactions within the cell are stimulated

10. A specific cellular response is triggered → mention what it is.

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Receptors can alter the activity of a cell in other ways other than second messengers:
1. Opening an ion channel
2. Acting directly as a membrane found enzyme
3. Acting as an intracellular receptor when the initial signal passes straight through the cell surface
membrane

Intracellular receptors → non polar ligands

1. Hydrophobic signaling molecules like steroidal hormones ( estrogen & testosterone) can diffuse through
the hydrophobic core of the phospholipid bilayer.

2. Binds to receptors in the cytoplasm or the nucleus allowing gene expression.

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4.5 Movement into and out of cells

5 mechanism in which substances move into and out of cells:

1. Diffusion
2. Facilitated diffusion
3. Osmosis
4. Active transport
5. Bulk transport (endocytosis & exocytosis)

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1. Diffusion
Definition: the net movement of molecules or ions from a region of higher concentration to a region of lower
concentration down concentration gradient, as a result of random movement of particles caused by kinetic
energy of the molecules or ions.

Practical activity 4.1 : investigating simple diffusion using visking tubing

● Visking tubing is partially permeable, non living membrane made of cellulose
● Pores in this membrane are small enough to prevent the passage of large molecules (such as starch and
sucrose) but allow smaller molecules (such as glucose) to pass through by diffusion.

Process:
1. Fill a section of Visking tubing with a mixture of glucose & starch solutions
2. Suspend the visking tubing in a boiling tube full of water for a set period time
3. Test the water outside of the visking tubing (at regular intervals) for the presence of starch and
glucose to monitor whether diffusion of either substance out of the tubing has taken place.

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4. The results should indicate that only glucose diffuses out of the tubing, not starch, since only
glucose molecules are able to pass through these pores.

Practical activity 4.2 : demonstrating diffusion using plant tissue

1. Place pieces of beetroot at different temperatures or into different alcohol concentrations
2. Any damage to cell membranes results in red pigment found in the large central vacuole, leaking out of
the cells by diffusion.
3. Changes in color per time can be monitored & therefore rate of diffusion can be found.

Practical activity 4.3: effect of size on diffusion

SA: V ratio (as mentioned above)
Process:
● Time the diffusion of ions through blocks of agar of different sizes
● The larger the block, the lower the SA:V ratio, therefore diffusion takes longer.

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2. Facilitated diffusion
Definition: the diffusion of a substance through a transport protein (channel protein or a carrier protein) in a cell
membrane; the protein provides hydrophilic areas that allow the molecule or ion to pass through the membrane.

Examples of large polar molecules that require transport proteins that would otherwise not be able to enter the
cell by passing through the hydrophobic core:
● glucose, amino acids, sodium & chloride ions.

Comparing channel & carrier proteins

Channel proteins Carrier proteins

Definition: a membrane protein of a fixed shape which Definition: a membrane protein of which changes
has a water-filled pore (containing amino acids with shape to allow the passage into or out of the cell of
polar R groups) through which selected hydrophilic specific ions or molecules by facilitated diffusion or
molecules or ions can pass by facilitated diffusion. active transport.

Channel proteins are ‘gated’ → part of the protein Binding site only open to 1 side of the membrane
molecule on the inside surface of the membrane can
move to close or open the pore → channel proteins
are OPEN AT BOTH SIDES OF THE MEMBRANE

Does NOT carry out conformational change (change in Carries conformational change upon binding of polar
shape) upon the binding of polar molecules molecules

Only in facilitated diffusion Both in facilitated diffusion & active transport

Many substances are able to pass through a single Carrier proteins have a specific binding site.
channel protein

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Rate of diffusion through channel & carrier proteins depend on:

1. Direction of movement of substance depends on its relative concentration on each side of the membrane,
substances move down concentration gradient from high to low.
2. Number of channel or carrier protein molecules present on membrane
3. Weather channel proteins are open or not

3. Osmosis
Definition: net movement of water molecules from a region of higher water potential to a region of lower water
potential, through a partially permeable membrane.

Why must a partially permeable membrane be present?

● The solute molecules are too large to get through the membrane, so only water molecules can pass
through
● The number of solute molecules on each side of the membrane stay the same & water molecules move
about randomly.

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Water potential: a measure of the tendency of water to move from 1 place to another; water moves from a
solution with higher water potential to one with lower water potential until equilibrium. Water potential is
decreased by the addition of solute & increased by the application of pressure

WP units: kPA

Water potential symbol:

Terms seen in PPQ: (important)

● MORE NEGATIVE water potential = lower water potential
● LESS NEGATIVE water potential = higher water potential

Water potential depends on 2 factors: (don't need to know

it in detail)
1. Solution concentration
2. Pressure applied to it

Osmosis in animal cells Osmosis in plant cells

● If the water potential of the surrounding ● If water potential of surrounding solution is

solution is high, cell swells and bursts (lysis). high → volume of cell increase, pressure builds
up & becomes turgid & DON'T BURST
Why? Animal cells don't have cell wall
Why? Cell wall is strong & rigid, therefore able to
withstand turgor pressure

● If water potential of the surrounding solution ● If the water potential of the surrounding
is lower → cells shrink and become crenated solution is lower→
● Plasmolysed, which is: loss of water from a
plant or a prokaryote cell to a point where
protoplast shrinks away from the cell wall.

Why?
Incipient plasmolysis, the point at which plasmolysis
is about to occur, the protoplast no longer exerts
pressure on the cell wall & equilibrium is eventually
reached.

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4- Active transport
Definition: the movement of molecules or ions through carrier proteins across a
cell membrane against their concentration gradient using energy from ATP (active
process)

● Active transport is achieved by CARRIER PROTEINS called pumps

● Each carrier protein is specific for particular type of molecule or ion
● The energy from ATP is used by the carrier protein to carry out
conformational change (change shape)

Example of carrier protein: sodium-potassium pump (Na+ -K+ pump)

What is the role of the pump?

For each ATP molecule used:
1. Pump 3 sodium ions out of the cell
2. At the same time as allowing 2 potassium ions into the cell for each

What is the structure of the pump?

1. Pump has a receptor site for for atp in on its inner surface
2. Active transport can occur both into and out of the cell.

What are examples of benefits of active transport?

1. In nerve cells (action potential)
2. Kidney reabsorption
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5- Endocytosis
Definition: the BULK movement of liquids (pinocytosis) or solids (phagocytosis) into the cell by the infolding of
the cell surface membrane to form vesicles containing the substance, it required ATP

Process: cell surface membrane engulfs material to form a small sac (also known as a vesicle or a vaccule)

**Some cells need to transport substances on a larger scale than the processes mentioned above. Eg diffusion

What are 2 forms of endocytosis?

1. Phagocytosis → bulk uptake of solid material
Cells specialized for this → phagocytes → a type of cell that ingests and destroys pathogens or damaged
body cells by phagocytosis.

2. Pinocytosis → bulk uptake of liquid → very small vacuoles formed

Steps of endocytosis:

1. A particle or substance binds to receptors on the cell’s surface membrane

2. Membranes fuse, forming a phagocytic vesicle.
3. The phagocytic vesicle pinches off from the cell membrane, entering the cell.
4. The phagocytic vesicle fuses with lysosomes which destroys the vesicle’s contents and recycles them.

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6- Exocytosis
Definition: the bulk movement of liquids or solids out of a cell, by the fusion of vesicles containing the substance
with the cell surface membrane, requires ATP.

Process:
secretory vesicles from golgi body apparatus fuse with the cell membrane and contents are released

Examples of exocytosis:
● Digestive enzymes from cells of the pancreas.
● Plant cells to get cell wall building materials to the outside of the cell membrane.

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