Cell membrane

The Cell Membrane and The Cell Membrane and Transport SystemsTransport Systems

The Cell Membrane The boundary of all living cells is a cell

membrane which controls the entry of dissolved substances into and out of the cell.

A cell membrane consists of both lipid and protein.

The membrane is formed of a double – layer of lipids.

Proteins are embedded in this layer forming channels that allow water – soluble substances pass through

The Cell Membrane Phospholipid bi-layer

The Cell Membrane

Crossing The Membrane All cells must be able to take in &

expel various substances in order to survive, grow & reproduce.

As a cell membrane only allows some dissolved substances to cross it, it is described as being semi – permeable OR partially – permeable.

Main Types of Transport Passive Transport: diffusion & osmosis Active Transport Endocytosis and Exocytosis

Passive – movement across membranes that does not require any energy

Active – movement across membranes that does require energy

Diffusion Net movement of particles from a

region of high concentration to a region of low concentration.

Diffusion Difference in concentration between

two regions = concentration gradient Diffusion occurs when a gradient

exists and continues until equilibrium Factors can effect the rate of diffusion

when the concentration gradient is large when heat is applied when molecules are smaller when movement occurs through a

gaseous membrane

Diffusion Water, carbon dioxide and other

small uncharged particles

Osmosis Net movement of a solvent,

(usually water) across a differentially permeable membrane from a region of low solute concentration to high solute concentration.

That is, water molecules move from a region of high solvent concentration to low concentration.

Osmosis

Osmosis in Animal Cells Isotonic – when the extracellular fluid and

intracellular fluids are at equal concentrations.

Hypotonic – when the extracellular fluid has a higher water concentration than the intracellular fluid.

Hypertonic – when the intracellular fluid has a higher water concentration than the extracellular fluid.

Osmosis

Partially permeable membrane

Water molecules Solute molecules cannot pass through the partially permeable membrane

There is a net movement of water molecules towards the less concentrated side (in terms

of water molecules) until water concentrations equalize.

High Water Concentration

means the solution is Hypotonic

Loses water by osmosis

Low water concentration means

the solution isHypertonic

Gains water by osmosis

Osmosis in Animal Cells

Osmosis in Animal Cells

Isotonic solutionNo net movement of solvent = equilibrium

Red blood cell = 0.9% saline

Solution = 0.9% saline

Osmosis in Animal Cells

Hypotonic solution Solvent moves into the cell

Red blood cell = 0.9% saline

Solution = 0.4% saline

Osmosis in Animal Cells

Hypertonic solution Solvent moves out of the cell into the solution

Red blood cell = 0.9% saline

Solution = 1.2% saline

Osmosis in Plant Cells Different to animal cells because of the cell wall

Osmosis in Plant Cells

Isotonic solutionNo net movement of solvent = equilibrium

Cell placed in solution that has the same concentration as the cell sap

Osmosis in Plant Cells

Hypotonic solutionPlant cell placed in solution that has a lower concentration than the cell sap

•Water enters the vacuole

•The vacuole expands

•Cytoplasm is pushed outwards

•‘Turgid’ cell

Osmosis in Plant Cells

Hypertonic solutionPlant cell placed in solution that has a higher concentration than the cell sap

•Water exits the vacuole

•Vacuole shrinks

•Cytoplasm moves inwards

•‘Plasmolysis’

Active Transport Diffusion is very slow & it moves substances

down a concentration gradient.

Many substances are needed by organisms in much greater amounts than what can be provided by diffusion alone. Additionally, some substances need to be accumulated in cells against the concentration gradient.

In order for this to occur, energy must be used to move the needed substances across the cell membrane.

Active TransportActive transport occurs through protein channels.