Cell Theory and Cell Components

• Robert Hooke observed chambers in cork under a microscope in 1665 and called them "cells."
• In 1838, Schleiden and Schwann stated the cell theory.
• All plants and animals are composed of cells, and the cell is the basic unit of life.
• In 1858, Virchow stated that all cells arise by reproduction from previous cells ("Omnis cellula e cellula" in Latin).
• These three statements form the foundation of cell theory.

Cell Components

• Cells consist of protoplasm and a cell membrane.
• Biologists identify two basic types of cells: prokaryotic and eukaryotic.
• Prokaryotic cells are simple, lack a nuclear envelope, and are found in single-celled and some simple multicellular organisms and bacteria.
• Eukaryotic cells have a nuclear envelope and are found in most higher plants, algae, fungi, and animals.

Plant Cell Walls

• A fundamental difference between plants and animals is that each plant cell is surrounded by a rigid cell wall.
• It is a rigid structure in mature cells.
• Composed of cellulose micro-fibrils, polysaccharides, and lignin, as well as some gums, resins, silica, calcium carbonate, waxes, and cutin.
• Plant cells have two types of walls: Primary & Secondary.

Primary Cell Walls

• Typically thin (less than 1 µm) and outer.
• Characteristic of young, growing cells.

Secondary Cell Walls

• Thicker (inner) and stronger.
• Formed when most cell enlargement has ended.
• These cell walls owe their strength and toughness to lignin, a brittle, glue-like material.

Lignified Secondary Cell Walls

• Provided plants with the structural reinforcement necessary to grow vertically above the soil and to colonize the land.
• Encloses and protects the cell contents.
• Plays a vital role in cell division and expansion.
• Has pits that function in communication between cells.
• Cell wall (common in plants & fungi) surrounds the cell & limits expansion due to osmosis, since osmosis may result in uncontrollable expansion of cell.
• Cell migrations are prevented because each walled cell and its neighbor are cemented together by a middle lamella (composed of pectin).
• Plant development depends solely on patterns of cell division and cell enlargement.
• Bryophytes, which lack lignified cell walls, are unable to grow more than a few centimeters above the ground.
• Plasmodesmata are thin bridges between neighbor cells.
• Symplast is the name of continuous cytoplasm inside of cells.
• Apoplast includes cell walls and space outside the cell where communication and metabolic activity take place.

Plasma Membranes

• Perform many important tasks in cell metabolism.
• Divide the cell into numerous compartments (parts).
• Regulate the passage of molecules into and out of cells.
• Heat, cold, poisons, and alcohol can disrupt membranes, making life impossible without them.
• Composed primarily of fatty-acid lipids and proteins.

Membrane Proteins

• Two types: Extrinsic and Intrinsic.
• Extrinsic proteins are loosely attached by ionic bonds or Ca bridges to the bilayer and can also attach to intrinsic proteins.
• Intrinsic proteins are firmly embedded within phospholipids.
• Membranes actively involved in metabolism contain a higher proportion of protein.
• Lipids are two types: phosopholipids and sterols (cholestrol).
• Membrane have amphiphilic property (having dual attraction i.e containing both lipid soluble & water soluble region ) .
• Lipids are the building blocks of cellular membranes.

Membrane Properties

• Membranes can grow molecule by molecule in certain regions of the cell.
• Membrane fusion allows the transport of materials.
• Exocytosis means to excrete wastes, debris, mucilage, proteins, and polysaccharides.
• Permeability can be semi-permeable or partially permeable.
• The chemical structure of the membrane makes it flexible, the ideal boundary for rapidly growing and dividing cells.
• Membranes are formidable, allowing some dissolved substances or solutes to pass while blocking others.
• Lipid-soluble and some small molecules can cross the membrane, but other larger, water-soluble molecules and charged ions are repelled by themembrane.
• Transport of vital substances is carried out by classes of intrinsic proteins: Open channels, Facilitators, and Pumps.
• Open channels allow ions to diffuse directly into the cell.
• Facilitators help solutes diffuse the lipids.
• Pumps force solutes through the membrane.
• Particles too large are often swallowed or disgorged whole by opening and closing of the membrane.

Cytoplasm and Cytosol

• The component of the cell surrounded by the membrane.
• All organelles are present in the cytoplasm.
• Helps in large cellular activities such as glycolysis or nuclear division.
• The inner concentrated region of the cytoplasm is called endoplasm.
• The outer region of the cytoplasm is called ectoplasm.
• Cytosol (matrix of the cytoplasm) surrounds the organelles; all metabolic reactions occur here.

Vacuole

• Occupies around 30% of the cell's volume in mature cells.
• Surrounded by a single-layered membrane, the tonoplast.
• The vital function apart storage is sustain turgor pressure against the cell wall.
• Consists of cell sap, a mixture of salts, enzymes, and other substances.

Plastids

• Young cells have undifferentiated plastids (proplastids), which can multiply by simple division.
• Mature cells develop into various types classified by their primary pigment.
• Chloroplasts are the site of photosynthesis.
• It is oval-shaped and is surrounded by a double membrane.

Chloroplast Components

• Sac-like structures called thylakoids.
• A stack of thylakoids is called a granum.
• Contain green chlorophyll pigments, which with proteins use light energy to make simple sugars from CO2 and H2O.
• Also contain accessory pigments, carotenoids, and xanthophylls.
• Stroma is the liquid phase, containing proteins, DNA, RNA, oil droplets, ribosomes, and some starch grains.
• Leucoplasts are colorless plastids that contain storage products, including oils, protein, or starch.
• Chromoplasts are colored red, orange, or yellow, depending on the pigments they contain.

Mitochondria and Chloroplasts as Energy-Converting Organelles

• The energy a cell obtains from its environment is usually in the form of chemical energy in food (such as glucose) or in the form of light energy.
• Mitochondria and chloroplasts grow and reproduce themselves.
• They contain small amounts of DNA that code for proteins found in these organelles.
• These proteins are synthesized by mitochondrial or chloroplast ribosomes, and these ribosomes are similar to the ribosomes of prokaryotes.
• Originated via symbiogenesis.

Mitochondria and ATP

• All eukaryotic cells (plant, animal, fungal, and protist) contain complex organelles called mitochondria.
• The site of aerobic respiration, an oxygen-requiring process that includes most of the reactions that convert the chemical energy present in foods to ATP.
• During aerobic respiration, carbon and oxygen are removed from food molecules, such as glucose, and converted to CO2 and water.
• Is enclosed by a double membrane, which forms two different compartments within the organelle.
• The intermembrane space is the compartment formed between the outer and inner membranes.
• The matrix space is the compartment enclosed by the inner mitochondrial membrane and contains enzymes that break down food molecules.
• The outer mitochondrial membrane is smooth and allows many small molecules to pass through it.
• By contrast, the inner mitochondrial membrane has numerous folds, called cristae, which extend into the matrix and increase the surface area.