cell membranes and transport

fluid mosaic model of membranes

• The basic structure of all cell membranes is the same

• This includes the cell surface membrane and the membranes surrounding eukaryotic organelles (e.g. nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplasts, lysosomes)

  • These membranes:

    • Are composed of a phospholipid bilayer

    • Contain intrinsic and extrinsic proteins

    • May include cholesterol (in animal cells), glycoproteins, and glycolipids

• The fluid mosaic model describes how the molecules are arranged within cell membranes

  • The term "fluid" refers to the lateral movement of phospholipids and some proteins, giving the membrane flexibility

  • The term "mosaic" reflects the scattered arrangement of proteins within the bilayer

what does the fluid mosaic model explain

• Partially permeable

• Sites for cell signalling, recognition, and communication

• Responsible for controlling the exchange of substances across compartments

structural components of cell membranes - phospholipid bilayer

• Cell membranes are primarily made of a phospholipid bilayer with two layers of phospholipid molecules

• Each phospholipid has two regions:

  • A phosphate head that is polar (hydrophilic) and therefore soluble in water

  • Two fatty acid tails that are non-polar (hydrophobic) and insoluble in water

• The bilayer arranges so that the hydrophobic tails face inward, forming a hydrophobic core, while the hydrophilic heads face outward towards aqueous environments

• This structure forms a selectively permeable barrier, preventing most polar or water-soluble substances (e.g. ions, glucose, amino acids) from freely crossing the membrane

• Phospholipids can be chemically modified to act as signalling molecules by:

  • Moving within the bilayer to activate other molecules (eg. enzymes)

  • Being hydrolysed which releases smaller water-soluble molecules that bind to specific receptors in the cytoplasm

structural components of cell membranes - cholesterol

• Cholesterol regulates the fluidity of the membrane

• Cholesterol molecules sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing.

• Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid

  • Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together

• It also makes the membrane less permeable to small charged particles (like ions) and strengthens the membrane so that the cell doesn't burst

structural components of cell membranes - glycolipids and glycoproteins

• Glycolipids and glycoproteins contain carbohydrate chains that exist on the surface (the periphery/extrinsically), which enables them to act as receptor molecules

• This allows glycolipids and glycoproteins to bind with certain substances at the cell’s surface

• There are three main receptor types:

  • Signalling receptors for hormones and neurotransmitters

  • Receptors involved in endocytosis

  • Receptors involved in cell adhesion and stabilisation (as the carbohydrate part can form hydrogen bonds with water molecules surrounding the cell

• Some act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)

structural components of cell membranes - proteins

• Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane. There are two types:

  • channel (pore) proteins

  • carrier proteins

• Each transport protein is specific to a particular ion or molecule

• Transport proteins allow the cell to control which substances enter or leave

diffusion

• Diffusion is a type of transportation that occurs across the cell membrane

• It can be defined as:

The net movement of molecules or ions from a region of higher concentration to a region of lower concentration

• The molecules or ions move down a concentration gradient

• Movement is random and is caused by the natural kinetic energy of the molecules or ions

• As a result of diffusion, molecules or ions tend to reach an equilibrium situation (given sufficient time), where they are evenly spread within a given volume of space

factors affecting the rate of diffusion - steepness of concentration gradient

• A greater difference in concentration means more molecules move from high to low concentration

• This increases the net rate of diffusion across the membrane

factors affecting the rate of diffusion - temperature

• Higher temperatures give molecules more kinetic energy, so they move faster

• This results in a higher rate of diffusion

factors affecting the rate of diffusion - surface area

• A larger surface area allows more molecules to diffuse at once

• Folding (e.g. microvilli, cristae) increases surface area

• In larger cells, a lower surface area to volume ratio slows diffusion

factors affecting the rate of diffusion - properties of molecules or ions

• Large molecules diffuse more slowly as they need more energy

• Uncharged, non-polar molecules diffuse directly through the bilayer

• Non-polar molecules diffuse faster than polar ones

facilitated diffusion

• Certain substances cannot diffuse through the phospholipid bilayer of cell membranes. These include:

  • Large polar molecules such as glucose and amino acids

  • Ions such as sodium ions (Na⁺) and chloride ions (Cl^-)

• These substances can only cross the phospholipid bilayer with the help of certain proteins

• This form of diffusion is known as facilitated diffusion

• There are two types of proteins that enable facilitated diffusion:

  • Channel proteins

  • Carrier proteins

• They are highly specific (they only allow one type of molecule or ion to pass through)

channel proteins

• Channel proteins are water-filled pores

• They allow charged substances (eg. ions) to diffuse through the cell membrane

• The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore

• This allows the channel protein to control the exchange of ions

carrier proteins

• Unlike channel proteins which have a fixed shape, carrier proteins can switch between two shapes

• This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape

• Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)

osmosis

Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

water potential

• A dilute solution has a high water potential (the left-hand side of the diagram below)

• A concentrated solution has a low water potential (the right-hand side of the diagram below)

• The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa;

  • Any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)

  • The more negative a water potential value, the lower the water potential is said to be

osmosis in plant cells

• Plant and animal cells are affected differently by osmosis, as plant cells have a cell wall

• In a hypertonic solution, water leaves the plant cell by osmosis, the protoplast shrinks and pulls away from the cell wall — this is called plasmolysis

  • Without enough water, cells lose turgor and the plant wilts

• In a hypotonic solution, water enters the plant cell, the vacuole expands, and the cell becomes turgid — the cell wall prevents bursting

  • Turgidity supports the plant, helping it stay upright and catch sunlight

• In an isotonic solution, water moves in and out equally, so there is no net change, and the cell is neither turgid nor plasmolysed

osmosis in animal cells

• Animal cells, like plant cells, gain or lose water by osmosis, but the effects are more severe as they lack a cell wall

  • In a hypertonic solution, water leaves the cell, causing it to shrink and shrivel

  • In a hypotonic solution, water enters the cell, which may swell and burst (cytolysis)

  • In an isotonic solution, water moves in and out equally, so there is no net change to the cell

• Maintaining a stable water potential in animal tissue fluid is essential to prevent cell damage

active transport

• Active transport is defined as:

The movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, using energy from respiration

• Active transport requires carrier proteins (each carrier protein being specific for a particular type of molecule or ion)

• The energy is required to make the carrier protein change shape, allowing it to transfer the molecules or ions across the cell membrane

• Energy is provided by the hydrolysis of ATP (adenosine triphosphate) into ADP and inorganic phosphate

processes that use active transport

• Active transport is important in:

  • Reabsorption of useful molecules and ions into the blood after filtration into the kidney tubules

  • Absorption of some products of digestion from the digestive tract

  • Loading sugar from the photosynthesising cells of leaves into the phloem tissue for transport around the plant

  • Loading inorganic ions from the soil into root hairs

co-tranport

• Co-transport is the coupled movement of two substances across a membrane via a carrier protein

• One moves down its concentration gradient, allowing the other to move against its gradient

processes that use co-transport

• In the mammalian ileum, co-transport absorbs glucose and sodium ions:

  1. Active transport moves Na⁺ from the epithelial cell into the blood, creating a Na⁺ gradient.

  2. Na⁺ then diffuses in from the ileum, carrying glucose via a co-transporter.

  3. Glucose moves into the blood by facilitated diffusion

factors affecting the rate of transport

• temperature

• surface area of the exchange surface

• concentration gradient across the membrane

• thickness (or diffusion distance) of the exchange surface

• number of protein channels or carrier proteins

• availability of ATP (for active transport)

adaptions for rapid transport - increased surface area

More membrane surface allows more substances to cross simultaneously

adaptions for rapid transport - more channel proteins

Allows faster facilitated diffusion of specific ions or polar molecules

adaptions for rapid transport - more carrier proteins

Speeds up facilitated diffusion and active transport of larger molecules

adaptions for rapid transport - thin exchange surface

Reduces diffusion distance, speeding up the rate of diffusion

adaptions for rapid transport - rich blood supply

Maintains a steep concentration gradient by constantly removing or supplying substances

adaptions for rapid transport - ventilation or flow of surrounding medium

Replaces substances to maintain high/low external concentrations, sustaining a gradient

adaptions for rapid transport - many mitochondria

Provides more ATP for active transport, supporting uptake against a concentration gradient

example of transport in specialised cells - root hair cells

• Adapted for the absorption of water and mineral ions from the soil

• They have long ‘hair-like’ projections

  • This increases the surface area, boosting the rate of osmosis and active transport

• A thin cell wall

  • This gives a shorter diffusion distance for water

• The permanent vacuole stores water and mineral ions as they enter the cell

  • This helps to maintain a steep water potential gradient

example of transport in specialised cells - epithelial cells of the small intestine

• Adapted for the absorption of digested food molecules

• They have microvilli on the surface

  • This provides a large surface area for increased diffusion

• A rich capillary network continually transports the products of digestion away from the epithelial cells

  • This ensures a steep concentration gradient

• Many co-transport proteins

  • This facilitates active uptake of glucose and amino acids

example of transport in specialised cells - cells in the collecting duct of the kidney

• Adapted for the uptake of water

• These cells have membranes that contain a very high number of aquaporins

  • Aquaporins are special channel proteins that allow the facilitated diffusion of water through cell membranes

• This allows these kidney cells to reabsorb water

example of transport in specialised cells - neurones and muscle cells

• Adapted for the transport of sodium, potassium and calcium across the membrane

  necessary for the transmission of electrical impulses around the body

• Cell membranes in these cells have channel proteins for sodium, potassium and calcium ions

• The opening and closing of ion channel proteins, and the number of channels present, affect how quickly ions move by facilitated diffusion

  • This directly influences the speed of electrical transmission:

    • Along neurone membranes during nerve impulses

    • Across muscle cell membranes during muscle contraction