Topic 2A (cell membrane, phospholipid bilayer, transport)

Describe the structure of the cell membrane (5marks)

- The cell membrane has a phospholipid bilayer

- Each phospholipid has a hydrophilic (polar) head facing water environment and hydrophobic (non-polar) tails facing away from water environment

- membrane contains intrinsic proteins such as channel and carrier proteins, and extrinsic(peripheral) proteins

- membrane contains glycoproteins and glycolipids

(for cell recognition)

- It also contains cholesterol within the membrane

( to regulate fluidity and stability)

Compare and contrast endocytosis and exocytosis (4marks )

- both processes involve vesicles

- both need ATP energy

- in exocytosis substances leave the cell, but in endocytosis substances enter the cell

- exocytosis involve vesicles fusing with cell surface membrane but endocytosis involve formation of vesicles from cell surface

membrane.
(exo =exit by fusing with cell membrane, endo = forming vesicles from cell membrane to enter)

Describe how the mammalian lung is adapted for rapid gas exchange

(4 marks)

- large number of alveoli present which increases SA for rapid diffusion

- alveolar wall is thin as the alveolar epithelium is one cell thick, which ensures short diffusion distance

- Blood flowing in capillaries and ventillation(breathing in/out) maintains a steep concentration gradient for gas exchange
(CO2 and O2)

- moist lining which dissolves o2 for rapid diffusion

Explain why the phospholipid molecules form a bilayer (3marks)

- because phosphate heads are hydrophillic and are attracted to water

- fatty acid tails are hydrophobic and repel water

- since there is aqueous environment on both sides of the membrane, a bilayer forms

Describe the role of proteins in cell transport mechanisms

(6 marks)

- carrier proteins are involved in active transport

- (ions / AA) molecules bind to carrier protein, causing carrier p. to change shape which allows them to pass cell membrane

- against the concentration gradient, so atp is needed

- channel proteins are involved in facilitated diffusion

(btw carrier proteins also used in f. diffusion)

- (they create pathway) allows large/polar molecules to pass through cell membrane

- down the concentration gradeint

Explain the arrangement of phospholipids in liposomes

(spherical structure)

(2marks)

- the phosphate head is hydrophilic and is attracted to water molecules, thus pointing outward

- the lipid tale is hydrophobic and is repelled by water, thus pointing inward

In simple diffusion __________ / __________ molecules can directly cross the plasma membrane/ phospholipid bilayer

give examples of all molecules that can transport via simple diffusion

hydrophobic / non polar

(as phospholipid bi-layer also has hydrophobic core)

examples:

- o2, co2,

- non-polar molecules

- hydrophobic molecules such as lipids

Explain what is meant by diffusion (3marks)

- It is a passive process where simple molecules go from area of high → low concentration

- down the concentration gradient

- no channel/carrier protein needed

Describe the two types of bulk transport?

( moving large substances across cell membranes)

[2marks]

- endocytosis, large substances entering cell in the form of vesicle using energy

(pinocytosis for liquids and phagocytosis for solids)

- exocytosis, large substances exiting/leaving cell in the form of vesicle using energy

Explain what is meant by facilitated diffusion (3marks)

facilitated diffusion:

- It is a passive process where ions/ polar molecules go from area of high → low concentration

- down the concentration gradient

- channel/carrier protein needed

Which of these molecules in facilitated diffusion use channel protein and which use carrier protein:

glucose, ions, hydrophilic molecules, amino acids, polar molecule (and large molecules)

glucose, amino acid, hydrophilic molecule = carrier P

ions/polar molecules, (& large molecules) = channel P

Explain what is meant by osmosis (3marks)

osmosis:

- A passive process where water molecules go from high water potential → low water potential

- down the water potential gradient

- through a partially permeable membrane

draw a diagram to show the structure of a phospholipid molecule

(take each molecule as circle or square, tails being lines)

[] (phosphate head, hydrophilic)

|

[] (glycerol, hydrophobic)

|

| | (fatty acid tails, hydrophobic)

- FATTY ACID WILL NOT JOIN TO PHOSPHATE HEAD DIRECTLY (they repel each other)

- glycerol acts as the bridge,

(it's actually amphipathic molecule so it has both hydrophobic and hydrophilic properties.

In our syllabus more hydrophobic)

which substances and chemicals can directly pass through cell membrane (hydrophobic core)?

- only non-polar/hydrophobic can directly pass through cell membrane (hydrophobic core)

Explain what is meant by active transport (4 marks)

- movement of molecules from low → high concentration

- against the concentration gradient

- ATP is needed

- carrier protein needed

An RBC is placed in hypotonic solution. Explain what changes can be seen. (3marks)

[hypotonic -> hippo -> cell becomes big]

- In hypotonic solution, water potential inside cell is lower than outside the cell

- so water enters the cell due to osmosis

- RBC will swell up

Explain the permeability of cell membrane to each of the following: (4marks)

sodium and chloride ions

water

steroid [which is non-polar]

glucose

- Sodium and chloride ions are polar, so they pass through the channel protein via facilitated diffusion

- water is polar , so it passes membrane through channel proteins called aquaporins

(or water is polar but is small enough to pass by osmosis)

- steroids is non-polar, so it can directly pass through the membrane (by simple diffusion)

(not repelled by fatty acid tails or hydrophobic core)

- glucose is a polar molecule, so it passes through by facilitated diffusion via channel protein

An RBC is placed in hypertonic solution. Explain what changes can be seen. (3marks)

- In hypertonic solution the water potential inside cell cytoplasm is higher/greater than

outside the cell

- so water leaves the cell due to osmosis

- The RBC will become crenated, (shrink)

Give 2 ways by which phospholipids can differ from each other

- length of fatty acid tail

- whether the fatty acid tails are unsaturated or saturated

why is Singer and Nicolson's model called the fluid mosaic model?

(2marks)

- fluid because phospholipids & proteins can move within the membrane

- mosaic because proteins are randomly distributed /embedded throughout the membrane

state the property of a cell that enables it to change shape

(1mark)

- the membrane is fluid and free to move

Explain why an increase in temperature increases fluidity of membrane

(2marks)

- increase in temperature means phospholipids have more K.E

- therefore phospholipids can move around the membrane more

State Fick's Law of Diffusion

(2marks)

- rate of diffusion proportional to SA & concentration gradient

- inversely proportional to thickness/diffusion distance

Explain why a decrease in cholesterol increases the fluidity of membrane

(2marks)

- Cholesterol holds fatty acid chains together & restricts the movement phospholipids

- so a decrease in cholesterol means less restriction on phospholipid movement, thus membrane becomes more fluid

state 3 properties of a gas exchange system other than large SA

- good blood supply/steep concentration grad

- moist lining

- thin barrier/wall

Explain why multi-cellular animals require a respiratory system and a circulatory system

(4marks)

- they have high metabolic demands and small SA:V ratio

- respiratory system is present which gives large SA as diffusion is insufficient

- due to the long distance, circulatory system is present as diffusion is too slow

- it distributes O2 and other nutrients to all parts of animal

In phospholipids, fatty acid tails are formed from which elements?

- Carbon and hydrogen only

[the fatty acid tails in cell membranes primarily consist only of carbon (C) and hydrogen (H) ]