Cell membrane

Question 1

function of cell membrane

Answer 1

1. definition of cell’s boundaries
   - cell surface membrane keeps interior of cell physically separated from the surrounding enviornment
   - phospholipid bilayer is selectively permeable, desirable substances kept within, undesirable substances kept out of cell
2. organisation and localisation of function
   - structures with specific functions are embedded in membranes or localised within organelles -> **organise and compartmentalise functions **
3. regulation of cell’s contents
   - proteins and other components of cell membrane regulate transport of substances into and out of the cell and its organelles
   - take up useful substances (water, ions, glucose), remove metabolic waste products, confine certain chemicals within specific regions of the cell
4. signal transduction
   - specific protein receptors on outer surface of cell membrane plays a key role in the detection of specific signals, triggering specific responses within the cell
5. cell-to-cell communication
   - cell membrane has membrane proteins that bind the extracellular matrix or cell surface constituents to mediate adhesion and communication between adjacent cells

Question 2

what is the fluid mosaic model

Answer 2

viewed as
- a mosiac or collage of proteins randomly distributed in or loosely attached to
- a fluid phospholipic bilayer which is free to move about laterally

Question 3

components of cell membrane

Answer 3

1. phospholipids
2. cholesterol
3. proteins
4. carbohydrates

Question 3

characteristics of fluid mosaic model

Answer 3

1. The fluid layer is asymmetrical
   * The two lipid layers (bilayer) may differ in composition and arrangement of proteins and
   lipids
   * The three major types of membrane lipids are phospholipids, cholesterol and glycolipids
2. The phospholipid bilayer is** fluid or mobile**, lateral movement of phospholipids is possible.
3. The unit membrane is a dynamic structure, where the embedded proteins can float, some moving freely while others are fixed in positions by microfilaments on the cytoplasmic face.
4. Membranes are amphipathic.
   * The hydrophilic phosphate head of the phospholipids face outwards into the aqueous
   environment both inside and outside of the cell;
   * The hydrophobic hydrocarbon tails face inwards and create a hydrophobic core.

Question 4

phospholipid structure

Answer 4

structure
1. a glycerol backbone (a three-carbon molecule) with three hydroxyl (-OH) groups
2. two fatty acid chains – contributing the hydrophobic hydrocarbon “tails”
3. a negatively charged phosphate group
4. additional small, charged molecules which may be linked to the phosphate group

• hydrophillic head = negatively charged phosphate group + small charged molecules linked to phosphate group
• hydrophobic tail
• amphipatic, formation of phospholipic bilayers in aq environment
  **- long hydrocarbon chains of fatty acids form a hydrophobic barrier against polar and charged solutes **

Question 5

how do membranes move

Answer 5

• The membrane comprises of
  phospholipid molecules which are held together primarily by hydrophobic interactions between the hydrophobic fatty acid tails.
• hydrophobic interactions are weak, phospholipid molecules are free to move about laterally
• rare for the molecules to flip transversely across the membrane, switching from one phosphoplipid layer to the other as hydrophilic part of the molecule must cross the hydrophobic core of the membrane

Question 6

factors affectiing membrane fluidity

Answer 6

1. temperature
2. length of fatty acid chains
3. degree of saturation of fatty acid chains
4. amount of cholesterol
   - Cholesterol increases the stability and regulates the fluidity of membranes in animal cells.

Question 7

how does temp affect membrane fluidity

Answer 7

• As temperature decreases, membrane fluidity decreases, and vice versa
• as temp decreases, membranes remain fluid -> phase transition (at phase transition temp) -> phospholipids settle into a closely-packed arrangement and the membrane solidifies

At low temperature,
- the kinetic energy of the hydrocarbon chains decreases and the hydrocarbon chains are tightly packed
- results in increased hydrophobic interactions between phospholipid molecules -> their motion is restricted.
- the bilayer exists in a semisolid state, i.e. membrane is less fluid

At high temperature,
- the kinetic energy and thus motion of the hydrocarbon chains increases,
- this increase allows for increased lateral movements of individual molecules, flexing of the chains and transverse flipping,
- thus overcoming hydrophobic interactions between phospholipids, resulting in increased space between adjacent phospholipid molecules.
- the bilayer exists in a fluid state, membrane is more fluid

Question 8

how does length of fatty acid chains affect membrane fluidity

Answer 8

• As length of fatty acid chains increases, membrane fluidity decreases.
• longer hydrocarbon chains -> increased hydrophobic interactions between hydrocarbon chains -> higher the melting point (phase transition temperature)

Question 9

how does degree of saturation of fatty acid chains affect membrane fluidity

Answer 9

• As degree of saturation of fatty acid chains increases, membrane fluidity decreases and vice versa.
• Saturated lipids have long, straight hydrocarbon chains, which allows for
  close packing and thus enhances membrane solidification
• Unsaturated lipids have kinks, which prevents the hydrocarbon chains
  from packing closely together thus enhances membrane fluidity
• Mixtures of phospholipids broaden the temperature range of the transition
  phase considerably.

Question 10

cholesterol structure, position in membrane and effect on membrane

Answer 10

structure
- steroids found wedged btw phospholipid molecules in the cell membranes of animal cells.

effects on cell membrane
1. membrane stability
- Cholesterol molecules are usually found in both layers of the cell membrane, intercalated into the lipid monolayers
- Its rigid steroid ring interferes with the motions of the hydrocarbon chains of phospholipids, thus enhancing the mechanical stability of the membrane.
2. membrane fluidity
- dual effects on the fluidity of the
membrane; resisting changes in membrane fluidity that can be caused by changes in temperature, acting as a “temperature buffer” for the membrane.
high temperature:
- cholesterol interfers with the motions of the hydrocarbon chains -> restrains the movements of phospholipids
- decreased membrane fluidity

low temperature:
- cholesterol prevents the hydrocarbon chains from packing closely together, thus decreasing the tendency of the membrane to freeze upon,
- resulting in increased membrane fluidity.

3. membrane permeability
   - cholesterol fills in spaces between hydrocarbon chains of phospholipids -> plugging transient gaps through which ions and small molecules might otherwise pass –> decreases the permeability of a lipid bilayer to ions and small polar molecules.

Question 11

proteins structure + differences n type of proteins

Answer 11

2 types of membrane proteins
1. integral proteins (intrinsic proteins)
2. peripheral proteins (extrinsic proteins)

(REFER TO BOOK 1 / CONSOLIDATED NOTES FOR DIFFERENCES AND COMPARISON)

Question 12

functions of membrane proteins

Answer 12

1. anchorage
   - Anchoring proteins attach the cell membrane to other substances, stabilise the position of the cell membrane and can help maintain cell shape.
   - Anchoring proteins attached to the extracellular matrix can coordinate
   extracellular and intracellular changes
   - On the cytoplasmic side, they are bound to microfilaments of the cytoskeleton.
   - On the exterior side, they may attach the cell to fibres of the extracellular matrix.
2. transport
   Carrier proteins bind solutes and transport them across the membrane
   - This transport process involves a conformational change of the protein when solute binding occurs, and a return to its original form when the solute
   is released.
   - Energy in the form of ATP may or may not be required depending on transport method (facilitated diffusion, active transport)

channel proteins
- Some integral proteins contain a water-filled central pore, or hydrophilic channel that forms a passageway to permit the movement of water, ions and small hydrophilic solutes across the cell membrane.

i. Leak channels
- Permit movement of water at all times, e.g. aquaporins
- Permit movement of ions at all times (rate may vary), e.g. Na+ or K+ leak channels
ii. Gated channels, which can open or close to regulate ion passage e.g. voltage-gated Na+ or K+ channels

3. enzymatic activity
   - catalyse reactions in the extracellular fluid or within the cytosol, depending on locaiton of active site
   - several enzyems can be grouped together to carry out sequential steps in a metabolic pathway
4. signal transduction
   - these proteins have specific 3D conformations, ideal as receptor molecules for chemical signalling between cells
   - cell membrans diff in type and number of receptor protiens they contain -> hence diff sensitivities to hormones, neurotransmitters
   (cell signalling - binding of ligand to receptor protein which triggers changes in cell)
5. cell-to-cell recognition
   - recognition proteins are usually glycoproteins
   - wide array of shapes to carbohydrate side chains, each cell type has their own specific markers -> enables cells to recognise other cells + foreign markers to be recognised and attacked by immune system
6. intercellular joining
   - membrnae proteins of adjacent cells adhere tgt in various kinds of intercellular junctions

Question 13

carbohydrates

Answer 13

structure
- short branched chains of fewer than 15 sugar units
- some covalently bonded to polar ends of phospholipids molecules in outer lipid layer, fomring glycolipids
- some covalently bonded to membrane proteins, forming glycoproteins

function of carbohydrates
- carbohydrates are highly hydrophilic, hence glycolipids and glycoproteins are kept in contact with external aq environment, unlikely to rotate towards interior to diffuse transversely -> maintain orientation of glycoproteins and glycolipids within membrane

functions of glycolipids and glycoproteins
- important recognition components, involved in
1. sorting of cells into tissues and organs in animal embryos
2. binding extracellular signal molecules in antibody-antigen reactions
3. intercellular adhesion to form tissues
4. cell-to-cell recognition,
- ability of a cell to distinguish one cell from another

Question 14

differences between active vs passive processes

Answer 14

passive processes (simple diffusion, facilitated diffusion, osmosis)
1. conc gradient: occurs down a concentration gradient, substance moves from region of higher concentration to region of lower concentration
2. energy requirement: no cellular energy expenditure (ATP reuqired), concentration grad provides driving force for movement across membrane

**active proesses ** (active transport, endocytosis, exocytosis)
1. conc gradient: occurs against a concentration gradient, substances moves from region of lower concentration to rgion of higher concentration
2. energy requirement: cellular energy expenditure required usually in form of ATP

Question 14

why is transport across membrane necssary

Answer 14

1. maintain suitable pH and ionic concentration within cell for enzyme activity
2. to obtain food supplies for energy and raw materials
3. excrete toxic substances
4. secrete useful substances;
5. generate the ionic gradients essential for nervous and muscular activity.

Question 15

what is diffusion + the two types of diffusion

Answer 15

• net movement of a substance from a region of higher concentration to a region of lower concentration, down a concentration gradient
• continues until dynamic equilibrium is reached
• simple and facilitated diffusion

Question 16

when does simple diffusion occur + how does it occur

Answer 16

occurs for molecules that are able to cross the phospholipid bilayer directly:
1. molecules that have a small molecular weight
2. readily soluble in the lipid bilayer (hydrophobic molecules)

• Diffusion occurs directly across the plasma membrane without any need for the aid of channel or carrier proteins.
• Diffusion can occur in either direction, from within the cell to outside the cell, or vice versa, depending on the concentration gradient.
• Dynamic equilibrium is reached when concentrations of the diffusing substances are equal on both sides of the membrane -> no net movement of substances
• Energy from ATP is not needed.

Question 16

how does facilitated diffusion occur

Answer 16

• transport of substances occurs down a concentration gradient without the use of ATP (i.e. passive), until equilibrium is reached.
• channel/carrier mediated transport: transport protein in the membrane is used to enhance / increase the rate of transport of the substance across the membrane.
• Facilitated diffusion is used for larger, hydrophilic substances, such as glucose, amino acids and ions.
• transport protein is specific to substance being tranpsorted

carrier vs channel proteins
1. carrier proteins possesses binding site for solute molecules but channel proteins possesses central hydrophillic core that allows free movement of transported substance across membrane
2. carrier proteins undergoes conformational changes to transport a substance across the membrane, whilst channel proteins does not undergo conformational changes to transport a substance across the membrane medium

Question 17

factors affecting rate of diffusion

Answer 17

1. Concentration gradient:
   - The steeper the concentration gradient, the faster the diffusion process.
2. Distance over which diffusion occurs
   - The shorter the distance over which diffusion occurs, the faster the diffusion process.
   - Hence the thinner the cell membrane, the greater the number of molecules that can diffuse across per unit time.
3. Area across which diffusion occurs
   - the larger the surface area, the greater the number of molecules that can diffuse across per unit time, and hence the faster the diffusion process.
4. Structure through which diffusion occurs
   - The presence of transient gaps in the cell membrane may enhance diffusion
   - type and number of transport proteins present per unit surface area of membrane will also affect the diffusion rate.

5 Size and type of diffusing molecule
- The smaller the molecules, the faster they can diffuse across the cell membrane.

6. Temperature
   - The higher the temperature, the faster the diffusion process.

Question 18

osmosis

Answer 18

net movement of freely moving water molecules from region of less negative water potential to more negative water potential through selectivley permeable membrane

Question 19

water potential

Answer 19

measure of the tendency for water to move from one region to another.

• pure water has water potential of 0 -> highest possible value
• presence of solutes will make the water potential more negative (any solution will have -ve water potential.)
• Water always moves from a region of less negative water potential to a region of more negative water potential.

Question 20

what factors affect water potential in plant and animal cells

Answer 20

plant cells:
1. solute concentration - expressed as solute potential, Ws, -ve
2. pressure exerted by cell wall on its contents, which is generated when water enters the cell – expressed as the pressure potential, Wp (which is positive)

water potential of a plant cell = solute potential + pressure potential

animal cells:
- water potential of a cell is determined primarily by solute potential as it has no cell wall

Question 20

solute potential

Answer 20

• solute potential: measure of the ability of a solute to make the water potential more negative.
• dissolving solute molecules in pure water reduces number of free (unbound) water molecules as solute molecules bind to water molecules -> water potential of the solution
  more negative (The amount of lowering is known as the solute potential)
• solute poential is always negative, more solute molecules, more negative the solute potential
• more solute molecules -> more -ve solute potential -> more -ve water potential

Question 21

incipient plasmolysis

Answer 21

- point at which plasmolysis bgins - at incipient plasmolysis, **protoplast just ceased to exert any pressure against the cell** wall as cell membrane starts to pull away from cell wall, cell is **flaccid** - water continues to leave till contents in protoplast are of same water potential as external environment, no further shrinkage occurs in plant cells: incipient plasmolysis defined as point where 50% of plant cells are plasmolysed

Question 21

pressure potential

Answer 21

- measure of the pressure exerted by the cell wall on its contents (not applicable for animal cells as they lack cell walls) - pressure is applied to pure water or a solution -> pressure applied which forces water to move from 1 place to another -> water potential becomes less negative - Pressure potential increases as the cell absorbs water and increases in volume. It is a positive value, since it tends to move water out of the cell as opposed to solute potential which tends to move water into the cell

Question 22

effects of osmosis on plant cells

Answer 22

plant cells 1. in **solution of less negative water potential** - Water potential in cell is more negative than that of solution. - Water enters cell by osmosis from the solution - Cell **swells** and become **turgid**. 2. In a solution of equal water potential - Water potential in cell is **equal** to that of solution. - **No net movement** of water. - **No change** in cell volume. 3. In a solution of more negative water potential - Water potential in cell is less negative than that of solution. - Water leaves the cell by osmosis. Water is lost first from the cytoplasm through the cell membrane, and then from the vacuole through the tonoplast. - The **protoplast**, which is the living contents of the cell surrounded by the cell wall, **shrinks and eventually pulls away from the cell wall**. This process is known as **plasmolysis**, and the cell is said to be plasmolysed. | Both cell membrane and tonoplast are selectively permeable.

Question 23

effects of osmosis on animal cells

Answer 23

1. Water potential of solution is less negative than that of cell (**hypotonic** solution) * Water potential in cell is more negative than that of solution. * Water enters cell by **osmosis** from the solution. * Cell **swells and lyses** as it lacks a cell wall. 2. Water potential of solution is equivalent to that of cell (**isotonic** solution) - Water potential in cell is equal to that of solution. - **No net movement of water.** - No change in cell volume. 3. Water potential of solution is more negative than that of cell (**hypertonic** solution) - Water potential in cell is less negative than that of solution. - Water leaves the cell by osmosis. - Cell becomes **shrivelled**.

Question 23

bulk transport + endocytosis vs exocytosis (process, function)

Answer 23

bulk transport: Endocytosis and exocytosis are active processes that require ATP expenditure for the transport of macromolecules such as proteins, polysaccharides, enzyme complexes into or out of the cell endocytosis: A process where the cell takes in macromolecules by **invagination of the plasma membrane to form vesicles.** - The vesicles form from a localised region of plasma membrane that invaginates, and then pinches off the cytoplasm. - function: process is used by cells to **incorporate extracellular substances.** exocytosis: A process where the cell secretes macromolecules by **fusion of vesicles with the plasma membrane.** - The vesicles usually bud off from the endoplasmic reticulum (ER) or Golgi apparatus (GA) and migrate to the plasma membrane where they fuse with it. - function: used by secretory cells for export of products or removal of waste materials

Question 24

primary active transport

Answer 24

Primary active transport: **direct use of ATP by a carrier protein to perform transport.** 3 types of active transport carriers exist: 1. **Uniport** – Only one substance is transported across the membrane at a time 2. **Symport** – Two substances are transported across the membrane **together in the same direction** 3. **Antiport** – Two substances are transported across the membrane together in **opposite directions** (eg sodium potassium pump, transports 3 NA+ ions out for every 2K+ ions in)

Question 25

active transport

Answer 25

Active transport: movement of substances from a region of lower concentration to a region of higher concentration, against the concentration gradient. The energy required is provided in the form of ATP. - Active transport occurs for ions and small hydrophilic molecules. - The transport proteins that move solutes against a concentration gradient are all **carrier proteins** - As movement occurs against a concentration gradient, **conformational change in the transport protein is necessary** to ensure that there is no “leakage” of solutes back across the membrane down the concentration gradient. - function: enables a cell to **maintain internal concentrations of molecules that are much higher or lower than those in the extracellular environment.** -> allows cell or cell organelles to store useful substances and keep out harmful ones. how a solute is actively transported across a membrane via a carrier protein: 1. Solutes on the cytoplasmic side of the plasma membrane **bind to a specific binding sites on the transport protein.** 2. **ATP** then transfers a **phosphate group** to the transport protein. 3. This causes the **protein to change its conformation** in such a way that the solute is released on the other side of the membrane. 4. This **phosphate group detaches** and the transport protein returns to its **original conformation.**

Question 26

types of endocytosis

Answer 26

1. Phagocytosis - Large solid particles such as food particles, bacteria etc, are taken into the cell - **Pseudopodia** extend outwards from the cell to enclose the solid particle - The **vesicles formed usually fuse with lysosomes which contain hydrolytic enzymes to digest these macromolecules.** 2. pinocytosis - **droplets of extracellular fluid are incorporated** into small vesicles, process is **non-specific**, cell takes in all solutes dissolved in droplet 3. Receptor-mediated endocytosis - coated pits form vesicles when **specific molecules bind to receptor proteins on the cell surface.** - Coated pits are **reinforced on their cytoplasmic side** by the protein **clathrin**. - The vesicles formed are known as **clathrin-coated vesicles** - a **selective form of endocytosis** which allows the cell to take up only specific substances from the extracellular fluid.