transport of susbstances

Cell-surface (Plasma) Membrane Definition

A partially permeable barrier that separates the cell’s internal environment from the external environment.

Cell-surface (Plasma) Membrane Functions

• Controls movement of substances into and out of the cell

• Maintains a stable internal environment

• Allows cell communication and recognition

• Forms boundaries around organelles inside eukaryotic cells

Phospholipids Structure (Cell surface membrane)

Structure:

• Hydrophilic phosphate head

• Hydrophobic fatty-acid tails

• Arranged in a bilayer in water

Phospholipids (Cell surface membrane) → why does a bilayer form

Why a bilayer forms:

• Hydrophobic tails face inward away from water

• Hydrophilic heads face outward towards water

• Bilayer forms spontaneously

• Membrane is self-sealing → allows vesicle formation and fusion

Phospholipids Function (Cell surface membrane)

Functions:

• Allow lipid-soluble substances to diffuse through

• Prevent passage of water-soluble, charged or polar molecules

• Provide membrane flexibility

Why the Membrane Controls Movement

Cells must regulate entry/exit because:

• Most substances are not lipid-soluble

• Many molecules are too large to pass through channel proteins

• Charged and polar molecules cannot pass through the hydrophobic core

• Helps maintain ion concentrations, pH, and internal conditions

Describe the fluid-mosaic model of membrane structure

●Molecules free to move laterally in phospholipid bilayer

●Many components - phospholipids, proteins,

glycoproteins and glycolipids

what are the components of a cell membrane

• phospholipids forming a bilayer

• proteins - intrinsic and extrinsic

• glycolipids (lipids with polysaccharide chains attached)

• glycoproteins (proteins with polysaccharide chains attached)

• chlolestrol (some times present)

Extrinsic (Peripheral) Proteins

Located on either surface of the membrane.

Functions:

• Provide mechanical support

• Act as enzymes

• Act as receptors for hormones or neurotransmitters

• Digestive enzymes such as maltase on intestinal epithelial cells

Intrinsic (Integral) Proteins

Span the membrane from one side to the other.

Two main types: channel and carrier proteins

Channel proteins

• Form hydrophilic channels

• Allow movement of ions and polar molecules

• Used in facilitated diffusion (passive)

Carrier proteins

• Bind specific molecules

• Change shape to transport substances

• Used in facilitated diffusion and active transport

• Active transport requires ATP

Cholesterol

• Found between phospholipid molecules.

Functions:

• Very hydrophobic, adds strength to the membrane

• Pulls fatty-acid tails together to stabilise structure

• Reduces movement of phospholipids at high temperatures

• Reduces loss of water and ions from the cell

Glycoproteins

• Proteins with carbohydrate chains on the outer surface of the membrane.

Functions:

• Act as receptors for hormones and neurotransmitters

• Act as recognition sites (cell identity)

• Allow cells to adhere to each other to form tissues

• Important in immune recognition

Glycolipids

• Lipids with carbohydrate chains on the outer surface.

Functions:

• Act as surface receptors

• Provide cell recognition

• Help cells attach to one another to form tissues

Why is it called the Fluid Mosaic Model

“Fluid”:
Phospholipids and some proteins can move laterally, giving flexibility.

“Mosaic”:
Proteins of different shapes and sizes are embedded throughout the bilayer.

Importance of Fluid Mosaic Model

Importance:

• Explains membrane flexibility

• Explains selective permeability

• Allows membranes to fuse with vesicles

• Allows receptor movement within the membrane

Suggest how cell membranes are adapted for other functions

●Phospholipid bilayer is fluid →membrane can bend for vesicle formation / phagocytosis

●Glycoproteins / glycolipids act as receptors / antigens →involved in cell signalling / recognition

Carrier proteins add. info

• may also be found in the membranes of internal organelles such as the mitochondria, chloroplasts, nucleolus, and others.​

• – carrier proteins are only open to one side of the membrane in question at a time.

channel proteins definition

proteins that open channels in the cell membrane, allowing molecules to flow in and out along their concentration

Carrier proteins definition

• are proteins that carry substances from one side of a biological membrane to the other.

Channel proteins- why are they considered to be gated

• Most channel proteins are ‘gated’. This means a cell can control the movement of molecules across the membrane by opening the gate or closing the gate.

• a water filled pore

what is carrier proteins used for

Used for facilitated diffusion and also active transport

Carrier proteins bind specific solutes and transfer them across the lipid bilayer by undergoing conformational change in shape. ​

This closes one side of the membrane and opens up the other side, releasing the molecule across.

Diffusion​

• The net movement of molecules or ions from an area of high concentration to an area of low concentration until all are evenly distributed.​across a membrane

• Molecule diffuse DOWN their concentration gradient

• It is a passive process

what are the two types of diffusion

Simple Diffusion​ + Facilitated Diffusion

Simple Diffusion​

Small, non-polar, non-charged or lipid-soluble molecules can diffuse across the phospholipid bilayer.​

e.g.​

Oxygen​

Carbon dioxide​

Steroids

Describe how movement across membranes occurs by simple diffusion

●Lipid-soluble (non-polar) or very small substances eg. O2, steroid hormones

●Move from an area of higher concentration to an area of lower conc., down a conc. gradient

●Across phospholipid bilayer

●Passive - doesn’t require energy from ATP /respiration (only kinetic energy of substances

Explain the limitations imposed by the nature of the phospholipid bilayer

●Restricts movement of water soluble (polar) & larger substances eg. Na+/ glucose

●Due to hydrophobic fatty acid tails in interior of bilayer

Facilitated Diffusion​

• The movement of molecules down a concentration gradient across a membrane via channel or carrier proteins.

• Polar or charged molecules cannot diffuse across the phospholipid bilayer. They must diffuse through intrinsic proteins, ​

e.g.​

Ions (Na+)​

Glucose​

Amino acids

Facilitated Diffusion​ how do they do it?

• Use intrinsic proteins to move across the plasma membrane.​

​

• Specific proteins for specific molecules (proteins are complementary in shape.​

• Molecules can only enter a cell if the membrane has the specific intrinsic protein.

• The higher the number of a specific carrier/channel proteins the faster or more diffusion can take place.

Describe how movement across membranes occurs by facilitated diffusion

●Water-soluble / polar / charged (or slightly larger) substances eg. glucose, amino acids

●Move down a concentration gradient

●Through specific channel / carrier proteins

●Passive - doesn’t require energy from ATP /respiration (only kinetic energy of substances

role of carrier proteins in facilitated diffusion

• Shape / charge of protein determines which substances move

●Carrier proteins facilitate diffusion of (slightly larger) substances

○Complementary substance attaches to binding site

○Protein changes shape to transport substance

role of channel proteins in facilitated diffusion

• Shape / charge of protein determines which substances move

●Channel proteins facilitate diffusion of water-soluble substances

○Hydrophilic pore filled with water

○May be gated - can open / close