Cell Membrane & Transport Function

Vocabulary flashcards covering cell parts, cell membrane composition, membrane protein functions, passive and active transport mechanisms, vesicular transport, and epithelial transport pathways based on lecture notes.

Cell (Plasma) Membrane

A selectively permeable membrane that maintains an optimal intracellular environment, controls the passage of materials into and out of the cell, and enables communication with the external environment.

Smooth Endoplasmic Reticulum (SER)

An organelle responsible for synthesizing lipids and steroid hormones, as well as storing intracellular $\text{Ca}^{2+}$ in muscle cells.

Rough Endoplasmic Reticulum (RER)

An organelle studded with ribosomes that functions primarily in protein synthesis.

Golgi Apparatus

An organelle involved in post-translational processing of proteins for packaging, storage, and eventual release or secretion from the cell.

Mitochondria

Organelles that play an important role in energy ($\text{ATP}$) production for cellular processes.

Lysosomes

Membrane-bound organelles containing digestive enzymes to break down worn cellular parts or foreign extracellular substances such as bacteria.

Peroxisomes

Organelles housing enzymes that catalyze reactions such as the breakdown of hydrogen peroxide and fatty acids, as well as alcohol detoxification.

Cytoskeleton

A cellular framework that supports the cytoplasm and aids in moving substances and organelles within the cytoplasm.

Centrosome

A cellular structure that helps distribute chromosomes to new cells during cell division.

Vesicle

Contains and transports various substances

Nucleus

Holds the genetic materials that direct cell functions

Cell Membrane Structure

Composed of:

1. Lipids

2. Proteins

3. Carbohydrates

Lipids

Phospholipids: arranged in a phospholipid bilayer

Glycolipids: lipids with attached sugar groups; only on the outer membrane surface

Cholesterol

Proteins

Integral Proteins: firmly inserted into the lipid bilayer

Peripheral Proteins: loosely bound to the membrane by associations with integral membrane proteins or phospholipids

Carbohydrates

Covalently bound to the membrane lipids (glycolipids) or to proteins (glycoproteins)

Form the glycocalyx- the fuzzy, sticky, carbohydrate- rich area at the cell surface

Phospholipid Bilayer

• Each phospholipid has a polar, hydrophilic, phosphate, “head” and a nonpolar, hydrophobic, fatty acid “tail” that made of two fatty acid chains

• The heads are attracted to water, the main constituent of both the intracellular and extracellular fluids, so they lie on both the inner and outer surfaces of the membrane

• The tails, being homophobic, avoid water and line up in the center of the membrane

• Phospholipid bilayer- and a sandwich-like structure composed of two parallel sheets of phospholipid molecules lying tail to tail, with their polar heads exposed to water on either side of the membrane

• Forms a selective barrier separating the aqueous/ water environment outside and inside the cell.

Fluid Mosaic Model

A model describing the cell membrane as an exceedingly thin ($7\text{--}10\text{ nm}$) structure where floating protein molecules form a dynamic, changing pattern within a fluid lipid bilayer.

Marker Molecules

Cell membrane proteins, such as the protein portions of glycoproteins, that allow cells to identify other cells or specific molecules.

Attachment Proteins

Membrane proteins, including cadherins and integrins, that anchor cells to other cells or to extracellular molecules.

Receptor Proteins

Membrane proteins with exposure to extracellular fluid that serve as binding sites for chemical signals, triggering cellular responses.

Enzymes

Catalyse chemical reactions either inside or outside cells.

Example: Brush border enzymes of small intestine

Channel Proteins

Form passageways through the plasma membrane, allowing specific ions or molecules to enter or exit the cell.

Examples:

Leak ion channels

Gated ion channels

Leak Ion Channels (Transport Proteins- Channel Proteins)

Non-gated channel proteins that are perpetually open and determine the plasma membrane's intrinsic permeability to specific ions.

Gated Channels

Membrane channel proteins that open or close in response to specific stimuli

Ligand/ chemically-gated ion channel (response to changes in cell ligand binding)

Voltage-gated ion channel (response to membrane potential)

Mechanically-gated channel (response to mechanical stimulation, e.g., stretching, shear force)

Phosphorylation-gated-channel (response to phosphorylation of channel proteins)

Transport Proteins- Carrier Proteins (Transporter)

• Binding of the specific ion or molecule causes the carrier proteins to change shape and release the bound ion or molecule to the other side of the plasma membrane

• The carrier protein then resumes its original shape and is available to transport more ions or molecules

3 types:

Uniporter

Symporter

Antiporter

• In secondary active transport, the cotransporters couple the movement of one species of particles down the electrical/ chemical gradient to the uphill movement of another species of particles against the electrical/ chemical gradient

Uniporter

A carrier protein that transports a single specific particle type across the cell membrane down its concentration gradient.

Symporter

A carrier protein (cotransporter) that transports two different species of particles across the cell membrane in the same direction.

Antiporter

A carrier protein (cotransporter) that transports two different species of particles across the cell membrane in opposite directions.

Transport Proteins- ATP-powered Pumps (ATPases)

Require cellular energy (ATP)

Hydrolyze ATP to adenosine diphosphate (ADP) to change the shape of the protein, which moves the ions or molecules across the membrane against the concentration gradient (primary active transport)

Transport across Cell Membrane

Cell membrane is selectively permeable due to the physicochemical nature of the phospholipid bilayer

Only small (e.g., water, urea, ethanol), non-polar (e.g., carbon dioxide) and/or fat-soluble/ lipophilic (e.g., steroid) molecules can pass through

Large (e.g., proteins) and/or polar molecules (e.g., amino acids, glucose), and ions (with charges) have to rely on the use of membrane transport proteins.

Membrane Transport (Classifications)

Down concentration gradient- Passive Transport

Simple Diffusion

Facilitated Diffusion

Osmosis

Against concentration gradient-

Active Transport

• Primary active transport

• Secondary active transport

Vesicular transport (Requires ATP)

• Endocytosis (comprises of pinocytosis and phagocytosis)N

• Exocytosis

Simple Diffusion

The passive net movement of fat-soluble or small non-polar molecules directly through the phospholipid bilayer from a region of higher to lower concentration.

Osmosis

The diffusion of a solvent such as $\text{H}_2\text{O}$ across a selectively permeable membrane through the lipid bilayer or through aquaporin protein channels.

Facilitated Diffusion

• Because of their chemical structure or size, many essential molecules, e.g., amino acids, glucose (and other sugars), and ions cannot enter or exit the cell by diffusing directly through the plasma membrane. Instead, these molecules and ions cross the membrane by mediated transport.

• A mediated passive transport process that uses carrier or channel proteins to move large, polar, or charged molecules across the membrane down their concentration gradient without consuming $\text{ATP}$. This happens through:

Carrier-mediated facilitated diffusion (binds to protein carriers, hence transport rate is limited by the number of carriers present)

Channel-mediated facilitated diffusion (moves through water filled protein carriers, hence transport rate is limited by the concentration gradient)

Carrier-mediated Transport

Requires the help of membrane transport proteins: channels, carriers, and pumps

Faster than simple diffusion

Transport rate is limited by:

• Saturation

• Competition

• Specificity

Saturation (Carrier Transport)

A state where the rate of transport across the membrane reaches a maximum because all available transport proteins are bound to capacity.

Competition (Carrier Transport)

The situation in which structurally similar molecules compete for the binding site on a specific transport protein.

Specificity (Carrier Transport)

The characteristic of a membrane transport protein to bind to and transport only a single specific type of molecule or ion.

Primary Active Transport

A cellular transport mechanism that hydrolyzes $\text{ATP}$ directly to push substances across a membrane against their concentration gradient. A concentration gradient for the movement of secondary active transport is created.

Examples: Movement of various ions, sugars, and amino acids through the membranes

Secondary Active Transport

A transport process that utilizes the potential energy stored in an ion concentration gradient (established by primary active transport) to move another substance against its concentration gradient.

Example: Symport of glucose by Na+ - glucose secondary active transporters

$$\text{Na}^+/\text{K}^+\text{-ATPase}$$

An active antiporter pump that hydrolyzes one $\text{ATP}$ to pump $3\text{ Na}^+$ out of the cell and $2\text{ K}^+$ into the cell, maintaining intracellular concentration gradients.

Na+ - glucose secondary active transporter (SGLT)

Symporter

Uses the energy stored in the Na+ concentration gradient to move glucose against its concentration gradient

Vesicular Transport

Movement of larger volumes of substances across the plasma membrane through the formation of vesicles (membrane-bound sacs)

Requires energy/ATP

Vesicular transport includes:

— Endocytosis

• Phagocytosis

• Pinocytosis

• Receptor-mediated endocytosis (specific endocytosis)

—Exocytosis

Phagocytosis

A form of endocytosis ("cell eating") where amoeboid movements extend the cell membrane around a solid particle in the extracellular fluid.

Membrane pinches together to form a phagosome around the particle, which then usually fuses with a lysosome

Examples: White blood cell engulfing bacteria

Pinocytosis

A form of endocytosis ("cell drinking") in which the plasma membrane indents to form an endosome containing extracellular fluid and dissolved solutes.

Example: intestinal absorption of dissolved nutrients

Receptor-Mediated Endocytosis

A specific endocytic mechanism

membrane receptor proteins bind to target molecules

Cell membrane becomes indented, forming a vesicle

Vesicle is coated with Claritin’s on its cytosolic surface, forming coated pit

Clarithin molecules will be recycled after entering the cell

Uncoated vesicle fuses with lysosome

Examples: intake of some hormones, cholesterol, iron, and viruses

Exocytosis

A process where intracellular vesicles fuse with the plasma membrane to release their contents into the extracellular space.

Examples: secretion of neurotransmitters, hormones, mucus

Transcellular Transport

Epithelial transport of solutes across both apical and basolateral cell membranes to pass through the cell.

Involves various transport proteins

Example: glucose absorption by intestinal cells (transepithelial transport)

Na+ - glucose symporter: brings glucose into cell against its gradient using energy stored in the Na+ concentration gradient

GLUT transporter: transfers glucose to extracellular fluid by facilitated diffusion.

Na+ K+ ATPase: pumps Na+ out of the cell, keeping intracellular fluid Na+ concentration low.

Paracellular Transport

Epithelial transport where substances pass through the tight junctions and narrow spaces between adjacent cells.

Transcytosis

A mechanism for moving large molecules across an epithelium by combining endocytosis, intracellular vesicular transport, and exocytosis.