Transport Across the Cell Membranemaintains homeostasis of cellmembrane is selectively permeable some things can pass through but others cant3 types: active transport, passive transport, and bulk transport

1. Passive Transport

passive = no energy reqd move through membrane due to differences in concentration gradient 3 different types

1. DiffusionMovement of molecules from an area of high concentration to an area of low concentration across a concentration gradientUsed transport small molecules like CO2 H2O, and O2Animation: How Diffusion Works

2. OsmosisDiffusion of water 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 membraneAnimation: How Osmosis Works

Water potentialsWater moves from a place with a LESS NEGATIVE (higher) water potential to a place with a MORE NEGATIVE (lower) water potentialThe 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 OsmosisHypotonic 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. Hypertoniccontain 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. Isotoniccontain 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 featureshow glucose/charged ions moves into cellspassive does not require energyuses carrier proteinssolute 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 proteinsAnimation: 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 DiffusionConcentration Differencehappens ONLY when a concentration gradient is present and solute travels ALONG (down) a concentration gradient2. Saturationthere 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 energyinvolves 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 Pumpex. of active transportExists in most cell membranes.Actively removes sodium ions from the cell while actively accumulating potassium ions into them from their surroundingsAnimation: How the Sodium Potassium Pump Works

3. Bulk Transportused for materials to large to enter via passive or active transportvesicles created by folding of cell membrane onto itself to either engulf or expel materials2 types: endocytosis & exocytosis

Endocytosisthe transport of large particles into the cell in vesicles formed by folding in of the cell surface membrane3 types:

Pinocytosis (cell drinking)intake of small droplet of extracellular fluid along with solute particlesoccurs 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 endocytosisinvolves intake of specific molecules that attach to special protiens in cell membrane that serve as receptorshave a unique shape that fit only to one specific moleculeex. animal cells use this to bring cholesterol into cell

Exocytosisthe 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 cellex. pancreas secretes insulin