Transport Across the Cell Membrane

• maintains homeostasis of cell

• membrane is selectively permeable – some things can pass through but others can’t

• 3 types: active transport, passive transport, and bulk transport

1. Passive Transport

• passive = no energy req’d

• move through membrane due to differences in concentration gradient

• 3 different types

1. Diffusion

• Movement of molecules from an area of high concentration to an area of low concentration across a concentration gradient

• Used transport small molecules like CO2 H2O, and O2

• Animation: How Diffusion Works

2. Osmosis

• Diffusion of waterwater from a region of high concentration to a region of low concentration

• Water can diffuse into or out of a cell, it depends on the concentration on either side of the cell membrane

• Animation: How Osmosis Works

Water potentials

• Water moves from a place with a LESS NEGATIVE (higher) water potential to a place with a MORE NEGATIVE (lower) water potential

• The water potential of pure water is 0 (zero).• Solutions have negative water potentials – the

more concentrated the solution, the more negative the water potential.

3 Types of Osmosis

1. Hypotonic • contain a low concentration of

solute relative to another solution (e.g. the cell's cytoplasm).

• cell is placed in a hypotonic solution, the water diffuses into the cell, causing the cell to swell and possibly explode.

2. Hypertonic

• contain a high concentration of solute relative to another solution (e.g. the cell's cytoplasm).

• when cell is placed in a hypertonic solution, water diffuses out of the cell, causing the cell to shrivel.

3. Isotonic• contain the same

concentration of solute as an another solution (e.g. the cell's cytoplasm).

• when cell placed in an isotonic solution, the water diffuses into and out of the cell at the same rate.

• fluid that surrounds the body

cells is isotonic.

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3. Facilitated diffusion

Key features

• how glucose/charged ions moves into cells• passive – does not require energy• uses carrier proteins• solute molecules “combine” with carrier

proteins in the membrane. • carrier molecules speed (or facilitate) the

passage of the solute molecules across the membrane.

Spot the difference!

2 kinds of proteins involved:

1.CARRIER PROTEINS

• bind to a specific type of diffusing molecule.

• have a highly specific hydrophilic region to which the solute molecule binds.

• binding cause the protein to undergo a change in shape that moves the solute across the bilayer and release it on the other side

Carrier proteins

Animation: How Facilitated Diffusion Works

Carrier proteins

2. ION CHANNELS• formed by proteins with a

central pore that is lined with charged groups.

• help the diffusion of charged particles such as Ca2+, Na+, K+, HCO3- and Cl ions.

• Some channels are gated and allow cells to regulate the flow of ions from one cell to another.

Ion channel

Factors affecting Rate of Diffusion

1. Concentration Difference• happens ONLY when a concentration gradient is

present and solute travels ALONG (down) a concentration gradient

2. Saturation• there are only a limited number of carrier

molecules per unit area of membrane.• rate of movement reaches a max. when all carrier

molecules are fully loaded with solute molecules

2. Active Transport

• the transport of molecules or ions across a membrane by carrier proteins against a concentration gradient.

• requires energy

• involves carrier proteins in the membrane.

• hydrolysis of ATP releases the energy required for active transport.

• Cells involved in active transport have a large number of mitochondria to provide the ATP required

Spot the difference

Sodium-Potassium Pump

• ex. of active transport

• Exists in most cell membranes.

• Actively removes sodium ions from the cell while actively accumulating potassium ions into them from their surroundings

• Animation: How the Sodium Potassium Pump Works

3. Bulk Transport

• used for materials to large to enter via passive or active transport

• vesicles created by folding of cell membrane onto itself to either engulf or expel materials

• 2 types: endocytosis & exocytosis

Endocytosis• the transport of large particles into the cell

in vesicles formed by folding in of the cell surface membrane

• 3 types:

1. Pinocytosis (cell drinking)• intake of small droplet of

extracellular fluid along with solute particles

• occurs in all cells often

2. Phagocytosis (cell eating)

• intake of large droplet of extracellular fluid including particulate matter (bacteria or organic matter)

• occurs only in specialised cells like amoeba or macrophages (bacteria fighting immune cells)

• Animation: Phagocytosis

3. Receptor-assisted endocytosis

• involves intake of specific molecules that attach to special protiens in cell membrane that serve as receptors

• have a unique shape that fit only to one specific molecule

• ex. animal cells use this to bring cholesterol into cell

Exocytosis• the reverse process and is used to secrete

proteins, e.g digestive enzymes, out of the cells.

• vesicle forms inside cell moves to membrane and empties contents outside of cell

• ex. pancreas secretes insulin