Cell Membranes in Biology

Cell Membranes (Biomembranes)

• Cell membranes are crucial parts of all cells.
• Their discovery was linked with advancements in microscopy, particularly transmission electron microscopy.
• Observations showed a typical trilaminar structure.
• Biomembranes within cells have similar structures, with minor chemical composition differences due to cellular differentiation and specialization.

Functions of Cell Membranes

• Every cell is enclosed by a cytoplasmic membrane, separating intracellular and extracellular spaces.
• The average membrane thickness is 60-100 nanometers.
• The membrane is a selective boundary, maintaining a dynamic equilibrium between the cell and its environment.
• The membrane contains enzymes, receptors, transport proteins, signaling systems, and antigens.
• It performs various functions, including substance intake, interactions with the environment, and signal recognition.
• Membranes are integral components of many cellular organelles.

Main Components of Cell Membranes

• Phospholipids are the main components.
• Phospholipid molecules have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) fatty acid tails.
• In an aqueous environment, the hydrophilic heads face outward, and the hydrophobic tails face inward, forming a bilayer.
• Phospholipids are not chemically bound to each other, allowing for lateral movement.

Other Important Components of Biological Membranes

• Proteins are also crucial components.
• Integral proteins interact with the hydrophobic parts of the phospholipid bilayer and might span it completely.
• Peripheral proteins are located outside the lipid bilayer and can be easily separated.
• The type and number of proteins in the membrane vary depending on cell differentiation and the cell cycle.
• Cells control the specific protein composition of their membranes.

Functions of Membrane Proteins

• Membrane proteins carry out varied functions, serving as structural components and contributing to the transport of ions.
• Some proteins facilitate ion transport across membranes, either via pumps or channels, following electrochemical gradients (facilitated diffusion).
• Other proteins act as receptors, binding hormones, neurotransmitters, and other signal molecules.
• Some membrane proteins function as enzymes, playing a catalytic role within the cell.
• Others contribute to the formation of antigens.

Membrane Receptors

• Membrane receptors are proteins located in the cell membrane.
• They recognize and bind to signal molecules (hormones, neurotransmitters).
• These interactions trigger processes within the cell.
• Membrane receptors are classified into different categories.

Membrane Receptors - Types

• Receptors that form part of ion channels: involved in cation and anion transport.
• Receptors with enzymatic activity: intracellular proteins that catalyze certain chemical reactions (e.g., insulin receptor).
• Receptors coupled with G proteins: the largest group of membrane receptors (five families are known).

Transport of Substances Through the Membrane

• Substances enter and exit cells through passive or active transport mechanisms.
• Passive transport doesn't require energy.
• Active transport requires energy.

Passive Transport

• Passive transport follows the concentration gradient (moving from high to low concentration).
• The speed of transport depends on the concentration gradient and the size of the substance.
• Small, non-polar molecules (water, oxygen, carbon dioxide, urea, methanol, ethanol) can be transported passively.

Diffusion

• Diffusion is a passive transport process where substances move from high to low concentration.
• Diffusion continues until the concentration is equalized on both sides of the membrane.

Osmosis

• Osmosis is the passive transport of water across a selectively permeable membrane.
• Water moves from a low solute concentration (hypotonic) to a high solute concentration (hypertonic) area to dilute the higher solute concentration.
• Cells can become shrunken (crenation) due to high solute concentration or burst (lysis) due to low solute concentration in the surrounding environment.

Facilitated Diffusion

• Facilitated diffusion is a passive transport process where substances move across the membrane via protein channels or carriers.
• The process requires no energy directly; the driving force is the concentration gradient.
• The transport protein facilitates the movement of substances without affecting the direction of transport (high to low concentration).

Active Transport

• Active transport moves substances against a concentration or electrochemical gradient.
• It requires energy in the form of ATP (adenosine triphosphate) hydrolysis.
• The transport is performed by protein complexes called pumps and channels that pass across the membrane.

Primary Active Transport

• Primary active transport directly uses ATP to move substances against their concentration gradients.
• The transport protein changes its conformational shape during transport, binding and releasing the molecule transported.
• This is the case of Na+/K+-ATPase or H+-ATPase.

Secondary Active Transport (Co-transport and Anti-port)

• Secondary active transport uses the energy from the movement of one substance down its concentration gradient to move another substance against its concentration gradient.
• Co-transport (symport) means both substance move in the same direction.
• Anti-port means both substances move in opposite directions.

Endocytosis and Exocytosis

• Endocytosis and exocytosis are bulk transport processes for substances with high molecular weights.
• Endocytosis involves substances entering the cell.
• Exocytosis involves substances leaving the cell.
• These processes involve extensive changes in the cell membrane.

Pinocytosis

• Pinocytosis is a specialized cell endocytosis process that involves the ingestion of small liquid droplets.
• The cell membrane engulfs the liquid, forming a vesicle that eventually moves inside of the cytoplasm.

Phagocytosis

• Phagocytosis is the process by which cells engulf solid particles.
• These particles are enclosed within a membrane vesicle, transported into the cytoplasm, and later digested.
• Usually seen in immune cells engulfing pathogens.

Exocytosis

• Exocytosis involves the transport of substances outside of the cell.
• Materials are packaged in vesicles that move towards and fuse with the cell membrane.
• Substances are then expelled from the cell.

Cell Organelles

• The cytoplasm comprises various organelles with specific roles and structures.
• Some organelles have a single membrane (e.g., endoplasmic reticulum, Golgi apparatus, vesicles including peroxisomes, vacuoles and lysosomes).
• Other organelles have a double membrane (e.g., mitochondria, chloroplasts).
• Organelles are involved in distinct functions, including metabolism, synthesis, secretion, and degradation.

Mitochondria

• Mitochondria generate energy for eukaryotic cells through cellular respiration.
• They possess two membranes: an outer and inner membrane.
• The inner membrane is folded into cristae, increasing its surface area for reactions.
• Mitochondria have their own DNA (circular prokaryotic-type) allowing for their self-replication.

Endoplasmic Reticulum (ER)

• ER is a network of membrane tubules and cisterns in the cell.
• It is connected with the nuclear envelope.
• The ER has smooth and rough forms; the rough ER has ribosomes on its surface and is involved in protein synthesis.
• The smooth ER synthesizes lipids, steroid hormones, and cholesterol and detoxifies harmful substances.

Golgi Apparatus

• Golgi apparatus is a system of flattened sacs that modifies, packages, and transports proteins and lipids.
• It is involved in sorting and directing proteins and lipids to their respective cellular destinations.
• It receives materials from the ER and modifies them before they are packaged into secretory vesicles.

Lysosomes and Other Vesicles

• Lysosomes are membrane-bound vesicles containing hydrolytic enzymes.
• They participate in digesting and removing unwanted materials.
• Related vesicles have different roles from digestion, in transport, and other functions.

Peroxisomes

• Peroxisomes are vesicles that contain enzymes for reactions involving oxygen. Often involved in breakdown of fatty acids and other molecules.

Ribosomes

• Ribosomes are small organelles crucial for protein synthesis.
• They are composed of ribosomal RNA (rRNA) and proteins.
• Ribosomes can be found free in the cytoplasm; some associated with the endoplasmic reticulum.

Ribosomes of Eukaryotic Cells

• Ribosomes in eukaryotic cells are larger than prokaryotic ribosomes, with a sedimentation coefficient of 80S.
• They consist of four rRNA types and 82 proteins.
• They play essential roles in the cytoplasm and on the rough endoplasmic reticulum in protein synthesis.