Cellular Biology Concepts

• Life begins with cells and their chemical constituents and the levels of biological organization.

Cell Theory

• In 1665, Robert Hooke discovered cells using a compound microscope while studying cork.
• In 1702, Van Leeuwenhoek described many protists.
• In 1838, Schleiden stated that all plants consist of cells.
• In 1839, Schwann stated that all animals consist of cells.
• In 1855, Virchow proposed that all cells arise from pre-existing cells.
• All organisms consist of cells
• Cells serve as the structural and functional units of life.
• Cells originate from the division of pre-existing cells.
• Cells contain all the information needed for growth, development, and functioning.
• Cells exist independently under suitable conditions.

Chemical Constituents of Cells

Carbohydrates

• Carbohydrates consist of carbon (C), hydrogen (H), and oxygen (O).
• The ratio of hydrogen to oxygen (H:O) in carbohydrates is 2:1.
• The general formula for carbohydrates is Cx(H2O)y.

Monosaccharides/Simple Sugars

• Monosaccharides represent the simplest form of carbohydrates.
• Simple sugars dissolve in water.
• These are used as fuels for respiration.
• Examples include glucose and fructose.

Disaccharides

• Disaccharides form when two monosaccharide molecules join.
• Disaccharides dissolve in water.
• Used for carbohydrate storage in plants.
• Examples include sucrose and maltose.

Polysaccharides

• Numerous monosaccharide units link to form polysaccharides.
• The three common types of polysaccharides are cellulose, starch, and glycogen.
• Cellulose forms from glucose, is water-insoluble, and is a component of plant cell walls.
• Starch forms from glucose, it is slightly soluble in water, but soluble in hot water, and functions as carbohydrate storage in plants.
• Glycogen forms from glucose, is water-insoluble, and functions as a carbohydrate storage in animals, typically in the muscles and liver.

Lipids

• Lipids consist of carbon, hydrogen, and oxygen.
• The hydrogen to oxygen ratio (H:O) varies.
• Lipids do not dissolve in water, but they do dissolve in organic solvents like ethanol, chloroform, ether, and acetone.
• Lipids are divided into triglycerides (fats), phospholipids, and steroids.

Triglycerides/Fats

• Fats form when glycerol and fatty acids link together.

Phospholipids

• When a phosphate group replaces one fatty acid in a triglyceride, a phospholipid forms.
• The phosphate group ionizes in aqueous medium, forming the phospholipid molecule's polar/hydrophilic head.
• The two hydrocarbon chains of the fatty acids form the non-polar/hydrophobic tail.
• Uses of fats and phospholipids:
• Respiratory substrate for energy release in respiration.
• Energy reserve in animals, fruits, and seeds in plants.
• Heat insulator that keeps the body warm (subcutaneous fat layer).
• Electrical insulator as part of the myelin sheath of neurons.
• Serves as a structural component of the cell membrane.

Steroids

• Steroids consist of four carbon atom rings.
• It forms the component of cell membranes in animal cells and can be used to form hormones and vitamin D.

Proteins

Amino Acids

• Proteins consist of carbon, hydrogen, oxygen, and nitrogen and some contain sulfur.
• The basic protein unit is amino acid.
• Each amino acid molecule contains an amino group (-NH2) and a carboxyl group (-COOH).
• The R group differs for different amino acids.

Formation of Dipeptide

• Two amino acids can be joined by a peptide bond, forming a dipeptide.

Protein (Polypeptide)

• Numerous linked amino acids form a polypeptide/protein.
• Every polypeptide chain has a free amino group and a free carboxyl group at its terminals.
• Different numbers and sequences of amino acids in a polypeptide result in different proteins.
• Uses of proteins:
• Structural materials in the cell membrane and cytoplasm. Catalysts such as enzymes.
• Maintaining cell shape, allowing cell movement via microtubules and microfilaments.
• Fuel for respiration.
• Hormones that regulate organism activities.

Nucleic Acids

• A nucleic acid (polynucleotide) is a long-chain molecule made up of subunits called nucleotides.
• Nucleotides consist of a pentose (sugar), phosphate, and nitrogenous organic bases linked together.
• The two types of nucleic acids are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA).
• DNA resides inside the cell nucleus and functions as the cell's genetic material.
• RNA is concerned with synthesizing proteins in the cell.
• Watson & Crick model characteristics:
• Different organisms have different DNA compositions, but the total concentration of pyrimidines always equals the total concentration of purines.
• The two polynucleotide strands coil in opposite directions to form a double helix (antiparallel pattern).
• The two strands are held together by hydrogen bonds between bases, forming complementary base pairs.
• Adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).
• The resulting base pairs stack between the two chains, perpendicular to the molecule's axis.

Water

• Water is necessary for all living organisms.
• It constitutes 65-95% of an organism's mass.
• Uses:
• Component of cytoplasm.
• Medium for chemical reactions inside a cell.
• Reactant in certain chemical reactions within a cell (e.g., photosynthesis in plant cells).

Inorganic Ions

• Examples of inorganic ions found in cells:
• Sodium ion (Na+), potassium ion (K+), calcium ion (Ca2+), magnesium ion (Mg2+), chloride ion (Cl-), phosphate ion (PO43-), and hydrogen carbonate ion (HCO3-).
• Uses of inorganic ions within cells:
• Some ions are necessary for enzyme activity (e.g., zinc, magnesium, iron). To maintain anion-cation equilibrium (e.g., in neurons).
• Form compounds essential for body structural components or metabolism (e.g., PO43- in phospholipids in lipids and magnesium in chlorophyll found in plant cells).
• Iron in hemoglobin, found in red blood cells of vertebrates.
• Act as buffers to stabilize pH (e.g., HCO3).

Cell Structure

• Organelles are functional structures inside a cell.

Cell Membrane/Plasma Membrane

• Composed of lipid and protein.
• The cell membrane consists of a lipid bilayer interspersed with protein molecules.
• Proteins (integral proteins) attach to the inner/outer surface, embed within, or extend across the membrane.
• The cell membrane encloses the protoplasm, forming a barrier between the cell and the external environment.
• The cell membrane controls material passage in or out of the cell and is semi-permeable.
• Small non-polar molecules (e.g., oxygen) can freely pass.

Fluid Mosaic Model

• Phospholipid molecules move sideways within the membrane.
• Proteins do not form a continuous layer and are not fixed.
• Proteins shift position in the membrane, like icebergs floating on a phospholipid sea.
• Cholesterol, only found in animal cells, reducing fluidity.

Functions of Membrane-Bound Proteins

• Channels allow ions to pass.
• Pumps/carriers transport ions across the membrane. Enzymes catalyze chemical reactions at the membrane surface.
• Proteins attached with carbohydrates form glycoproteins, serving as cell markers to recognize cell types.
• Receptors bind to hormones or neurotransmitters to regulate cellular metabolism.
• Protoplasm: fluid inside the cell and provides the medium for metabolic reactions.
• Protoplasm = cytoplasm + nucleoplasm
• Cytoplasm: Fluid surrounds the cell membrane outside the nucleus and is the most metabolically active site.
• Nucleoplasm: Fluid inside the nucleus.

Mitochondria

• Round, oval or sausage-shaped organelle surrounded by a double membrane.
• The outer membrane is smooth
• The inner membrane folds to form finger-like structures (cristae) which increase surface area for respiration processes.
• Has its own DNA, different from nucleus
• Carries out aerobic respiration to release energy for cellular activities.

Endoplasmic Reticulum (ER)

• Network of folded membranes forming sheets, tubes, or flattened sacs in the cytoplasm.
• Transports large molecules inside the cell and facilitates its metabolism, in addition to providing a structural skeleton.
• Rough ER (RER) is attached with ribosomes on its surfaces and is the site of protein synthesis. Cells with a lot of protein synthesis contain a lot of RER.
• Smooth ER (SER) is not coated with ribosomes, looks like a tubular structure, is concerned with lipid metabolism. It is abundant in cells of steroid-synthesizing tissues.

Golgi Apparatus/Golgi Body

• Flattened membranous sacs with vesicles bud off from its margin.
• Proteins and lipids made in ER are collected, modified, and packaged into vesicles.
• Vesicles move to other parts of the cell for secretion or deposition.

Lysosome

• Vesicles containing hydrolytic enzymes that break down cellular debris and worn-out organelles.
• Hydrolytic enzymes break down the organelle and recover useful biochemicals.

Centrioles

• Pair of rod-like structures arranged at right angles.
• Spindle fibres (microtubules/protein filaments) are formed here during cell division.
• Found only in animal cells.

Nucleus

• Contains genetic materials to control cell activities.
• Genetic materials are diffused as deeply stained chromatin.
• Nucleolus:
• A heavily stained round body inside the nucleus which varies in number.
• Composed of RNA and protein and an internal protein synthesis site.
• Nuclear envelope:
• A double membrane that surrounds the nucleus.
• There are pores on the nuclear envelope which allow passage of large molecules (e.g. RNA).

Chloroplast

• A type of plastid which is a type of organelle in plant cells that produces and stores food materials.
• Oval in shape and bounded by a double membrane.
• Contains chlorophyll for photosynthesis.
• Has own DNA, different from nucleus
• It is found in plant cells.

Vacuole

• Fluid-filled space surrounded by a semi-permeable membrane known as the tonoplast.
• The fluid inside is known as cell sap (a solution of sugar, salts, wastes & pigments).
• The vacuole increases cell surface area for storage.
• It is present in plant cells, while animal cells do not have vacuoles or have very small vacuoles.

Cell Wall

• It is present in plant cells for support and protection.
• The cell wall is made of cellulose.
• It is freely permeable.

Microtubules & Microfilaments

• Protein fibres inside cells that provide support, allow the transport of materials, and the movement of the cell.

Comparison Between Plant and Animal Cells

• Cell wall
• Animal cell: absent
• Plant cell: present
• Chloroplast
• Animal cell: absent
• Plant cell: present
• Vacuole
• Animal cell: usually absent, very small if present.
• Plant cell: present, usually a large central vacuole.
• Centrioles
• Animal cell: present
• Plant cell: absent
• Shape
• Animal cell: able to change (e.g. amoeba, white blood cells).
• Plant cell: fixed because the cell wall is rigid.

Prokaryotic vs Eukaryotic Cells

• Prokaryotic Cells
• Cells without nucleus are known as prokaryotic cells (e.g bacteria)
• No membranous organelles within the cell
• Prokaryotic Cell features
• Capsule
• Cell wall
• Food reserve
• Cell membrane
• Mesosome
• Cytoplasm
• Photosynthetic lamellae
• Flagellum
• Plasmid
• Nuclear material
• Ribosome
• Pili.
• Eukaryotic Cells
• With nucleus (genetic materials are surrounded by nuclear membrane) are known as eukaryotic cells (e.g. fungi, algae, protozoans, plants, animals).
• Eukaryotic cells have membranous organelles.

Comparison of Features

• Nucleus
• Prokaryotic: No true nucleus, no nuclear envelope.
• Eukaryotic: Enclosed in nuclear envelope.
• Genetic Material
• Prokaryotic: DNA not associated with protein, in ring form
• Eukaryotic: DNA associated with protein called histones forming chromosomes.
• Cell Wall
• Prokaryotic: Present except in the smallest prokaryotes.
• Eukaryotic: Variable.
• Cell Membrane
• Prokaryotic: Present
• Eukaryotic: Present
• ER
• Prokaryotic: Absent
• Eukaryotic: Universally present
• Organelles
• Prokaryotic: Membrane-bounded organelles absent
• Eukaryotic: Present
• Photosynthetic Apparatus
• Prokaryotic: Contains chlorophyll, not surrounded by membrane..
• Eukaryotic: Present as chloroplast
• Ribosomes
• Prokaryotic: Free in cytoplasm..
• Eukaryotic: Free in cytoplasm or associated with ER.
• Centrioles
• Prokaryotic: Absent
• Eukaryotic: Present only in animal cells
• Size Range
• Prokaryotic: 100 - 2,000 nm.
• Eukaryotic: 10,000 - 100,000 nm.

Levels of Organization

• Cells: Organisms are composed of cells, which are their structural and functional units (e.g., blood cells, bone cells, muscle cells)
• Tissues: Made up of groups of cells having a similar shape and size that work together and have a particular function (e.g., animal bones, muscles, or plant epidermis, mesophyll)
• Organs: Different tissues that are grouped together to form a functional unit are called an organ (e.g., animal heart, eye, leaf, or flower)
• Organ Systems: Related organs coordinate to form a system. (e.g., human digestive, respiratory, or excretory system).
• Organisms: Individuals that carry out all basic life processes. Made up of organ systems but may be unicellular (e.g., protists and bacteria)
• Organization Levels in Living Systems: cells → tissues → organs → systems → organism

Organ System Examples

Excretory System

• This consists of;
• A pair of kidneys: located at the back of the abdominal cavity for blood filtering to maintain a constant potential for water.
• A pair of ureters: Connect the kidneys to the urinary bladder where urine is drained
• A urinary bladder to temporarily store urine.
• A urethra allows the bladder to drain urine outside the body.
• The renal arteries carry blood to the kidneys where filtering by structures known as nephrons takes place.
• Water content can be adjusted by the kidneys, metabolic wastes such as removed urea.

Respiratory System

• In humans, gas exchange occurs in the lungs.

Explore cell biology with these questions, covering topics from water's role to organelle functions. Test your knowledge of cell membranes, enzymes, and division. Understand key processes and components within cells.