The cells environment

• 1. All cells live in a fluid environment.– Fluid = moist or water filled.

• 2. Cells must obtain nutrients from the outside and release substances into their moist environment.

Cell membrane and transport

Plasma Membrane

• Plasma membrane- the outer boundary of a cell.

• Gatekeeper - the plasma membrane prevents certain materials from entering the cell and allows certain materials to pass into the cell.

Selectively permeable membrane

• Selectively permeable membrane- membrane allows certain structures to pass into and out of the cell while keeping others inside or outside of the cell.– Ex. Tea bag.

• SPM- maintains the organization of the cell.

Lipid Bilayer

• Lipid Bilayer- makes up the plasma membrane.– Made up of two layers of phospholipids.

• Phospholipid - composed of a phosphate head and a fatty acid tail

Phospholipid bilayer

• Hydrophilic head - the phosphorous head is attracted to water.

• Hydrophobic tails - the fatty acid tails are “afraid” of water.

• Results in a two layered (bilayer) membrane

Membrane Proteins

• Proteins are scattered about the plasma membrane.

• Proteins serve as a door in which large molecules can pass through the membrane.

• Cholesterol - (lipid) provides rigidity to the cells membrane.

Fluid Mosaic Model

• Fluid mosaic model - model for the structure of a plasma membrane in which the membrane is a phospholipid bilayer with proteins embedded within it.– Ex. Tile floor

• fig 4-8 page 95

Membrane function

• Membranes are selectively permeable.

• Random movement - “In nature, there is a tendency toward randomness. Energy is required for nonrandomness.”

• Diffusion - random movements of ions and other particles.

Diffusion

• 1. Results in movement of particles from a region of greater concentration to a region of lesser concentration.

• Dynamic equilibrium - condition in which there is a continuous movement of molecules but no overall change.

Osmosis

• Osmosis - the diffusion of water into and out of cells across a selectively permeable membrane.

• Osmotic balance - occurs when the movement of water into and out of the cell is equal. – Blood cells

– plant cells

Types of Solutions

• Isotonic Solution – equal concentration of solution and the cell. (Osmotic balance)

• Hypotonic Solution – Greater water concentration outside the cell than inside the cell (The cell bursts)

• Hypertonic Solution – Greater water concentration inside the cell than outside the cell (The cell shrinks)

Passive transport

• Passive transport - the cell plays no active role and does no work in moving the particles. Requires no Energy

• 1. Water

• 2. Lipids

• 3. Substance soluble in lipids

• 4. very small particles

Types of passive transport

• 1. Osmosis

• 2. Diffusion

• 3. Facilitated Diffusion

Facilitated Diffusion

• Many particles needed by cells must have some help getting across the cell membrane.

• Facilitated diffusion - the use of transport proteins to aid the passage of materials across the plasma membrane

Transport proteins

• 1. Channel proteins - simplest type. These are simply called pores. These serve as channels that allow ions to pass through.– Depend on size and charge.

• Even if an ion is small enough to pass through the pore, it may be prevented by its charge.

Transport proteins (continued)• Carrier proteins or GATE Proteins- more

complex, works like a gate.• 1. A particle on one side of a membrane attaches

to a protein• 2. The protein changes shape• 3. Allows the desired particle to pass into or out of

the cell.• 4. When the signal molecule is released, the gate

closes

Active Transport

• Active transport - the moving of particles from a region of lesser concentration to a region of greater concentration.– Requires Energy

• Pumps - transport proteins used to move a particle through a plasma membrane.– Requires chemical energy to alter the shape of

the protein.

Active Transport (continued)

• Examples of Active Transport

• 1. Marine Fish - actively pump out salt even though the concentration of salt is more outside their body.

Other methods of entering and exiting the cell.

• 1. Endocytosis - the process by which the mambrane engulfs and takes in a substance.– Phagocytosis - solid particles are ingested– Pinocytosis - liquid particles are ingested.

• 2. Exocytosis - cell products or wastes are released to the surroundings

• see figs 4-17, 18, 19