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AS Level Biology 9700

4. Cell membranes and transport

Written by: Rhia Sakthivel
Formatted by: Pranav I

Index

4.1 Fluid mosaic membranes
4.2 Roles of the molecules found in the membranes
4.3 Cell signaling 
4.4 Movement into and out of cells 

4.1 Fluid mosaic membranes

  • Phospholipids form membranes around cells and organelles
  • Phospholipids have hydrophilic (polar) heads and hydrophobic (non-polar) tails
  • In water, phospholipids form:
    • Micelles: Hydrophilic heads face outward, shielding hydrophobic tails
    • Bilayers: Hydrophobic tails are shielded by hydrophilic heads, forming a membrane
  • The phospholipid bilayer is the basic structure of membranes, about 7 nm wide
  • Membranes also contain proteins, visible in electron micrographs
Fig 4.1 A single layer of phospholipids and a micelle
Fig 4.2 A phospholipid bilayer

Fluid mosaic model

  • Fluid: phospholipids and proteins move by diffusion (similar to the fluidity of olive oil)
    • Phospholipid molecules move sideways in their layers
    • Some proteins move within the bilayer, while others remain fixed to structures
  • Mosaic: describes the pattern formed by scattered protein molecules when viewed from above

Cholesterol, glycolipids and glycoproteins

  • The membrane consists of a bilayer of phospholipids with hydrophobic tails facing inward and hydrophilic heads facing outward into the aqueous environment
  • Proteins:
    • Found in the inner or outer layer or spanning the whole membrane (transmembrane proteins)
    • Proteins have both hydrophobic (non-polar) and hydrophilic (polar) regions
    • Hydrophobic regions interact with phospholipid tails while hydrophilic regions interact with the aqueous environment or form pores for the passage of polar substances
  • Glycoproteins and glycolipids:
    • Many proteins and lipids have short carbohydrate chains
    • Found on the outer surface of cell membranes
    • Made in the Golgi body
  • Cholesterol is also present in the membrane
Fig 4.3 Molecules found in the cell membrane

4.2 Roles of the molecules found in the membranes

Phospholipids

  • Form a bilayer, which is the basic structure of the membrane
  • Fluidity:
    • Affected by the saturation of phospholipid tails (unsaturated tails increase fluidity)
    • Longer tails reduce fluidity
    • Temperature decrease makes membranes less fluid as phospholipid molecules get close together
      • Organisms like bacteria and yeasts respond by increasing unsaturated fatty acids in their membranes
  • Hydrophobic tails:
    • Create a hydrophobic core
    • Make it difficult for polar molecules or ions to pass through the membrane
    • Membranes act as a barrier to most water-soluble substances
      • Preventing leakage of sugars, amino acids, and proteins out of the cell
      • Blocking unwanted molecules from entering
  • Some phospholipids can be modified to act as signaling molecules

Cholesterol

  • A small molecule with hydrophilic heads and hydrophobic tails
  • Fits between phospholipid molecules, with heads at the membrane surface
  • Found in animal cell membranes in amounts similar to phospholipids
  • Less common in plant cell membranes and absent in prokaryotes (where similar compounds serve the same function)
  • Functions:
    • Provides mechanical stability to membranes by reducing fluidity
    • Prevents membranes from breaking and cells from bursting
    • The hydrophobic regions help block ions and polar molecules from passing through
    • Important in the myelin sheath of nerve cells to prevent ion leakage, ensuring efficient nerve impulse transmission
    • At low temperatures, it prevents phospholipid tails from packing too closely, maintaining correct membrane fluidity for cell survival in cold conditions

Glycolipids, glycoproteins and proteins

  • Receptor molecules:
    • Carbohydrate chains help glycoproteins and glycolipids act as receptors
    • Bind with specific substances (such as hormones or neurotransmitters) to trigger internal cell reactions
    • Example: glucagon receptor in liver cells
  • Cell-to-cell recognition:
    • Glycolipids and glycoproteins serve as cell markers or antigens
    • Enable cells to recognize each other (e.g. ABO blood group antigens)
  • Transport proteins:
    • Provide channels or passageways for ions and polar molecules to pass through
    • Include channel proteins and carrier proteins, specific for different ions or molecules
  • Enzymes:
    • Some membrane proteins act as enzymes (e.g. digestive enzymes in small intestine cells)
    • Catalyze reactions (hydrolysis of disaccharides)
  • Cytoskeleton:
    • Proteins attached to cytoskeleton filaments help maintain cell shape and assist in movement
  • Other roles:
    • In organelles like mitochondria and chloroplasts, membrane proteins are involved in respiration and photosynthesis

4.3 Cell signaling

✅ Signaling (definition)

The process of transmitting a message from one place to another in an organism

  • It is essential for coordinating cellular activities and responding to environmental stimuli
  • Signaling pathways can be electrical (e.g. nervous system) or chemical (e.g. hormone system)

Chemical signaling pathways

  1. Stimulus: a stimulus causes cells to secrete a specific chemical, called a ligand (e.g. glucagon in response to low blood sugar)
  2. Transport: the ligand is transported to the target cells, typically through the bloodstream
  3. Binding: the ligand binds to specific cell surface receptors on the target cells (glycolipids, glycoproteins or proteins)
  4. Transduction: the binding changes the shape of the receptor, triggering a cascade of reactions inside the cell
Fig 4.4 The cell signaling cascade

G proteins and second messengers

  • The receptor interacts with a G protein, which activates the release of a second messenger
  • Amplification: one receptor activation leads to the production of many second messenger molecules, amplifying the signal
  • Enzyme activation: second messengers activate enzymes, which further propagate the signal, eventually leading to changes in cell metabolism
  • This series of events is called a signaling cascade

Alternative pathways for receptors

  • Ion channel: some receptors open ion channels, altering the membrane potential
  • Enzyme: some receptors act as membrane-bound enzymes, triggering metabolic changes
  • Intracellular receptor: some signals (e.g. steroid hormones) pass through the membrane and bind to intracellular receptors, directly altering gene expression

🚨 Some signaling molecules are hydrophobic and can diffuse across the cell membrane

🚨 Cells can signal each other through direct contact (e.g. lymphocytes recognizing foreign antigens)

4.4 Movement into and out of cells

Simple diffusion

✅ Diffusion (definition)

The net movement of molecules or ions from a region of higher concentration to a region of lower concentration due to random molecular motion

  • Molecules move down a concentration gradient until equilibrium is reached (when molecules are evenly spread)
  • Random movement occurs due to the kinetic energy of molecules or ions
  • Molecules that can diffuse through cell membranes:
    • Respiratory gases (oxygen and carbon dioxide) → uncharged and non-polar
    • Water molecules → polar but small enough to diffuse rapidly across the bilayer
    • Hydrophobic molecules → pass through the membrane’s hydrophobic interior
  • Factors affecting diffusion rate:
    • Steepness of the concentration gradient
    • Temperature
    • Nature of molecules or ions (e.g. size, polarity)
    • Surface area of the membrane

Steepness of the concentratin gradient

  • The gradient is the difference in substance concentration across the membrane
  • A steeper gradient results in a faster diffusion rate
  • Net movement occurs
    • Molecules move in both directions, but more move from higher to lower concentration
    • The direction of net movement depends on the concentration gradient

Temperature

  • High temperatures increase the kinetic energy of molecules and ions
  • Faster-moving molecules and ions result in a higher rate of diffusion

Nature of molecules or ions

  • Size
    • Large molecules diffuse more slowly as they require more energy to move
    • Small molecules diffuse faster due to lower energy requirements
  • Polarity
    • Non-polar molecules (e.g. glycerol, alcohol, steroid hormones) diffuse easily through cell membranes as they are soluble in the non-polar interior
    • Polar molecules diffuse less easily through cell membranes

Surface area of the membrane

  • A larger surface area allows more molecules or ions to cross at once, increasing the rate of diffusion
  • Cell membranes increase surface area through folding (e.g. microvilli in intestinal cells, cristae in mitochondria)
  • Volume increases more rapidly than surface area as size increases
    • Larger cells have a smaller surface area-to-volume ratio
  • Diffusion is effective only over short distances, limiting cell size
    • Molecules like amino acids diffuse micrometers in seconds but take hours to diffuse centimeters
    • Most eukaryotic cells are under 50 micrometers in diameter; prokaryotic cells are even smaller
    • Large aerobic cells would quickly deplete oxygen and die due to inefficient diffusion

Facilitated diffusion

✅ Facilitated diffusion (definition)

The diffusion of a substance through a transport protein (carrier protein or channel protein) across a cell membrane 

  • Large polar molecules (e.g. glucose, amino acids) cannot diffuse through the phospholipid bilayer
  • Ions (e.g. Na⁺ and Cl⁻) also cannot pass through the bilayer
  • Facilitated diffusion allows these molecules to cross the membrane with the help of transport proteins
  • DOES NOT require energy from ATP
  • Channel and carrier proteins are the two types of transport proteins involved in facilitated diffusion
  • Each transport protein is highly specific, allowing only one type of molecule or ion to pass through
Fig 4.5 A channel protein
Fig 4.5 A carrier protein

Channel proteins

  • Have a fixed shape
  • Have water-filled pores, enabling charged substances like ions to diffuse through the membrane
  • Gated → they can open or close to regulate ion exchange
  • Gated proteins control ion movement
    • Sodium ion (Na⁺) entry during action potential production
    • Potassium ion (K⁺) exit during repolarization
  • Some channels are formed by a single protein, while others are made from multiple proteins combined
  • Channel proteins involved in facilitated diffusion do not require energy from ATP

Carrier proteins

  • Can flip between two shapes
  • Alternate the binding site’s orientation to either side of the membrane, enabling molecule or ion transport
  • Carrier proteins involved in facilitated diffusion change shape spontaneously
    • Pumps (a type of carrier protein) that require energy (ATP) and are involved in active transport

Rate of diffusion through channel and carrier proteins

  • The rate of facilitated diffusion depends on the number of channel or carrier proteins in the membrane
  • For channel proteins, the rate is also influenced by whether they are open or closed

Osmosis

✅ Osmosis (definition)

The net diffusion of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane

  • Osmosis is a type of diffusion that involves only water molecules
  • Solute + solvent = solution
  • A partially permeable membrane allows only certain molecules (like water) to pass through
  • In a comparison of two solutions separated by a membrane:
    • The solution with a higher solute concentration is more concentrated
    • The solution with a lower solute concentration is more dilute
  • Without the membrane, solute and water molecules would move randomly, spreading evenly
  • With the membrane, solute molecules cannot pass through, only water molecules can
  • Water molecules move from a region of higher water potential to a region of lower water potential
  • Over time, water molecules spread out evenly, causing the volume of the more concentrated solution to increase
  • This process is called osmosis

Water potential

  • Water potential (ψ) refers to the tendency of water to move from one place to another
  • Water always moves from a region of higher water potential to a region of lower water potential, down a water potential gradient
  • Water potential reaches equilibrium when it is the same throughout the system
  • Water potential depends on two factors:
    • Concentration of the solution
    • Pressure applied to the solution
  • A dilute solution has a higher water potential than a concentrated solution
  • Applying pressure to a solution increases its water potential, making it higher than the same solution with no pressure
Measuring water potential
  • Water potential is measured in kilopascals (kPa)
  • The water potential of pure water is 0 kPa
  • Solutions have a lower water potential than pure water, so their water potential is negative
  • A dilute solution has a less negative water potential than a concentrated solution
  • E.g. a solution with a water potential of -10 kPa has a higher water potential than one with -20 kPa

Osmosis in animal cells

  • Red blood cells are commonly used to study osmosis in animal cells
  • If the water potential of the surrounding solution is too high, the cell swells and bursts
  • If the water potential is too low, the cell shrinks
  • Maintaining a constant water potential inside animal bodies is important to prevent these effects
Fig 4.5 The effects of osmosis on red blood cells

Osmosis in plant cells

  • Plant cells have cell walls that are rigid and prevent bursting
  • When placed in a solution with higher water potential, water enters the cell by osmosis, causing the protoplast to expand
    • The cell wall resists expansion, building up internal pressure, and increasing the water potential until equilibrium is reached
    • A fully inflated plant cell is described as turgid
  • In a solution with lower water potential, water leaves the plant cell, causing the protoplast to shrink
    • As the protoplast shrinks, it pulls away from the cell wall, a process called plasmolysis
    • The point at which plasmolysis begins is called incipient plasmolysis, where the protoplast exerts no pressure on the cell wall
    • Equilibrium is reached when the water potential of the cell equals that of the external solution
    • Plasmolysis can be observed with a microscope using epidermal strips from plants like rhubarb or onion bulbs
Fig 4.6 The effects of osmosis on plant cells

Active transport

✅ Active transport (definition)

The movement of molecules or ions through transport proteins across a cell membrane, against their concentration gradient, using energy from ATP

  • Some ions (like potassium and chloride) are more concentrated inside cells than outside
  • Active transport is responsible for maintaining this concentration gradient by moving ions against the gradient
  • Active transport requires energy (usually provided by ATP) to move ions or molecules from low to high concentration
  • Pumps (carrier proteins) are used for active transport and are specific to certain molecules or ions
    • E.g. sodium-potassium pump → pumps 3 sodium ions out and 2 potassium ions in per ATP molecule used
  • Active transport plays a role in kidney reabsorption, digestion absorption, and transport in plants
  • Active transport requires ATP from cell respiration and can occur either into or out of the cell

Endocytosis

  • Requires energy in the form of ATP
  • Larger scale than previous mechanisms → bulk transport
  • In endocytosis, the cell surface membrane engulfs material to form a small sac
  • Phagocytosis (cell eating): this is the bulk uptake of solid material
    • Phagocytes are cells that ingest and destroy pathogens or damaged body cells
    • The vacuoles formed are called phagocytic vacuoles
  • Pinocytosis (cell drinking): this is the bulk uptake of liquid
    • The vacuoles (or vesicles) formed are often extremely small
    • In this case, the process is called micropinocytosis

Exocytosis

  • Exocytosis is the reverse of endocytosis and involves the removal of materials from cells
    • E.g. secretion of digestive enzymes from pancreatic cells
  • Requires energy in the form of ATP
  • It is a form of bulk transport
  • Secretory vesicles pinch off from the Golgi apparatus and carry materials through the cytoplasm to the cell surface for release
  • The vesicles fuse with the cell membrane and then open up so that the materials are released out of the cell
  • Plant cells use exocytosis to transport cell wall-building materials outside the cell
Fig 4.7 The steps involved in exocytosis
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