Transport Across the Cell Membrane

Cell Membrane Structure in Relation to Transport

The cell membrane is a phospholipid bilayer, with hydrophobic (water repelling) tails facing inwards and hydrophilic (water attracting) heads facing outwards.
Peripheral proteins positioned on the surface, and integral proteins embedded in the bilayer contribute to functionality, including transport.
The feature of the cell membrane as selectively permeable allows it to control the passage of substances across it.
Cholesterol molecules within the cell membrane maintain membrane fluidity and stability.

Types of Transport Across Cell Membranes

Passive transport involves the movement of molecules down their concentration gradient without the need for energy.
Three main types of passive transport include diffusion, facilitated diffusion, and osmosis.
Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.
In facilitated diffusion, carrier proteins or channel proteins aid diffusion of larger or charged molecules.
Active transport uses energy (ATP) to move substances against their concentration gradient, often using protein pumps.

Role of Carrier and Channel Proteins in Transport

Channel proteins form hydrophilic channels in the membrane for specific molecules (usually ions) to pass through by diffusion.
Carrier proteins combine with a specific molecule before changing shape to allow movement across the membrane, these proteins are used in both passive (facilitated diffusion) and active transport.
Protein pumps involved in active transport are carrier proteins that utilise energy to move molecules against their concentration gradient, including the sodium-potassium pump.

Importance of Membrane Transport in Cellular Function

The transport of substances across the cell membrane is vital for nutrient uptake, waste elimination, maintaining ion concentration balance and responding to changes in the cell environment.
Active transport is crucial in muscle contraction, nerve impulse transmission, and maintaining cell volume among other cellular activities.
Facilitated diffusion is important in glucose uptake in cells, as seen in the use of glucose transport proteins.
The inability of certain solutes to cross the cell membrane assists in creating and maintaining cell potential.

Endocytosis and Exocytosis in Cellular Transport

Endocytosis is a form of active transport where the cell membrane invaginates (folds in) to form a vesicle and engulf substances into the cell.
In phagocytosis, a form of endocytosis, the cell engulfs large particles or even whole cells.
Pinocytosis is another form of endocytosis where the cell engulfs extracellular fluid along with its dissolved substances.
Exocytosis is the opposite process, where substances are expelled from the cell via vesicles that merge with the cell membrane.
Both endocytosis and exocytosis are critical in immune response, cell growth, and neurotransmitter release.