Cell Transport Mechanisms

Movement into and out of Cells

• Molecules can cross the cell membrane through four ways: diffusion, osmosis, active transport, and cytosis.

Diffusion

• Diffusion is the random movement of substances from a high concentration to a low concentration area until equilibrium is established.
• It is a passive process, meaning it does not require energy.
• Molecules that commonly diffuse across the cell membrane include oxygen and carbon dioxide during gas exchange in the lungs and during photosynthesis.
• The concentration gradient is the distribution of particles from high to low concentration, and molecules always move down the concentration gradient to create an equilibrium.

Facilitated Diffusion

• Facilitated diffusion is a form of passive transport that uses transport proteins to move large molecules across the cell membrane.
• It occurs down the concentration gradient, hence it does not require energy.
• Example: transport of large molecules like glucose across the cell membrane.

Osmosis

• Osmosis is a special case of diffusion, involving water only.
• It is the movement of water across a selectively permeable membrane from a region of low solute concentration to a region of high solute concentration.

Active Transport

• Active transport is a process that uses membrane proteins to move substances across the membrane against the concentration gradient.
• It requires energy, which is obtained from ATP.
• Transport proteins involved in this process are called protein pumps/carrier proteins.

Cytosis

• Cytosis is an energy-requiring process that involves the movement of substances into cells that are too large to cross the membrane.
• There are two types of cytosis: endocytosis and exocytosis.

Endocytosis

• Endocytosis involves the movement of large substances into the cell.
• The cell membrane folds inward to form a vesicle that engulfs the substance to be transported.
• Types of endocytosis include phagocytosis (engulfing solids) and pinocytosis (engulfing liquids).

Exocytosis

• Exocytosis is the movement of a large substance out of the cell.
• Vesicles from within the cell fuse with the membrane and release their contents into the extracellular environment.
• Example: hormones are packaged into a vesicle by the Golgi body and removed from the cell by exocytosis.

Types of Microorganisms

• Cellular (living) microorganisms include parasites, fungi, and bacteria.
• Acellular (non-living) microorganisms include viruses and prions.

Bacteria

• Bacteria are possibly the most numerous types of organisms on the planet.
• They are very small, smaller than eukaryotic cells.
• Bacteria can be harmful, but they are also needed to keep us healthy.
• The bacteria on our skin, in our airways, and in our digestive system are the first line of defense against foreign pathogens that can cause infection and other problems.