Cell Structure and Components

Cell Structure and Components

• Cells contain nucleoplasm, which lies inside the nucleus, and inclusions, which are metabolic by-products, storage forms of nutrients, and pigments.
• Three main components of cells: plasma membrane, cytoplasm, and nucleus.

Plasma Membrane

• The plasma membrane is an outer membrane that separates the cell's interior from its surrounding environment.
• It is composed of a phospholipid bilayer, proteins, and carbohydrates.
• Phospholipid bilayer: a "sandwich" made of two layers of phospholipids, with polar phosphate molecules forming the top and bottom surfaces, and nonpolar lipid tails in between.
• Proteins: scattered throughout the plasma membrane, play important roles in allowing substances to enter the cell, and come in different types (peripheral and integral proteins).
• Carbohydrates: attached to lipids (glycolipids) and proteins (glycoproteins), form specialized sites on the cell surface for cell recognition.

Plasma Membrane Functions

• The plasma membrane isolates the interior of the cell from the external environment (physical barrier).
• It allows only certain molecules and ions to enter and exit the cytoplasm freely (selectively permeable).
• Small lipid-soluble molecules, such as oxygen and carbon dioxide, can pass through the membrane easily.
• Water molecules can freely cross the membrane using protein channels called aquaporins.
• Ions and large molecules cannot cross the membrane without direct assistance.

Cytoplasm and Organelles

• Cytoplasm includes the contents of a cell between the plasma membrane and the nuclear envelope.
• It is a gel-like substance called cytosol, consisting of 70-80% water, and contains organelles and cytoskeleton.
• Many metabolic reactions, including protein synthesis, take place in the cytoplasm.
• Most organelles are surrounded by one or two lipid membranes, similar to the plasma membrane.

Nucleus

• The nucleus is a large organelle that may or may not be centrally located within the cytoplasm.
• It is enveloped in a double membrane called the nuclear envelope, with nuclear pores that control the passage of ions, molecules, and RNA.
• It stores genetic information as DNA organized into linear structures called chromosomes (chromatin).
• Most nuclei contain at least one nucleolus, which consists of ribonucleic acid (RNA) and helps in producing ribosomes.

Ribosomes

• Ribosomes are organelles composed of proteins and RNA (nucleoproteins).
• Protein synthesis occurs at the ribosomes.
• Ribosomes exist as either free ribosomes in the cytoplasm or attached ribosomes to the membrane of the endoplasmic reticulum.
• Free ribosomes synthesize proteins that aid in the control of cell function, while attached ribosomes synthesize proteins that have a different destination.

Endomembrane System

• The endomembrane system is a group of membranes and organelles in eukaryotic cells that work together to modify, package, and transport lipids and proteins.
• It includes the nuclear envelope, lysosomes, and vesicles, the endoplasmic reticulum, and Golgi apparatus.

Endoplasmic Reticulum

• The endoplasmic reticulum (ER) is a series of interconnected membranous tubules that extend from the surface of the nucleus throughout most of the cytoplasm.
• ER is a major site for vital cellular activities, including biosynthesis of proteins and lipids.
• There are two types of ER: rough ER (RER) and smooth ER (SER).
• RER has ribosomes attached to its cytoplasmic surface and is specialized for protein secretion.
• SER lacks bound ribosomes and is involved in the synthesis of carbohydrates, lipids, and steroid hormones, as well as detoxification and storage of calcium ions.

Golgi Complex

• The Golgi complex is located near the nucleus and consists of many smooth membranous saccules that contain enzymes and proteins being processed.
• The Golgi complex completes posttranslational modifications of proteins produced in the RER and then packages and addresses these proteins to their proper destinations.

Lysosomes

• Lysosomes are membranous sacs produced by the Golgi apparatus that contain hydrolytic enzymes.
• They are found in all cells of the body but are particularly numerous in white blood cells that engulf disease-causing microbes.
• Lysosomes break down engulfed substances and recycle cellular waste.

Mitochondria

• Mitochondria are oval-shaped, double-membrane organelles that are responsible for making adenosine triphosphate (ATP), the cell's main energy-carrying molecule.
• They are abundant in cells or cytoplasmic regions where large amounts of energy are expended.

Mechanisms of Transport across the Plasma Membrane

• Bulk transport (vesicular transport): uses vesicles to move large molecules across the membrane, including exocytosis and endocytosis.
• Exocytosis: a process where a vesicle fuses with the plasma membrane, releasing its contents outside the cell.
• Active transport: a process that requires a protein carrier and energy to move materials into the cell against the concentration gradient.
• Osmosis: a special case of diffusion, where water moves across a selectively permeable membrane from an area of high concentration to an area of low concentration.
• Tonicity: a measure of the concentration of solutes in a solution, which determines the direction of water diffusion.