Cell Biology: Structure, Function, and Theory

• Cell Biology studies cells in terms of structure, function, and chemistry.

Cell Structure

• Cells are made of molecules.
• Individual cells cooperate and interact to form an organism.
• Molecules interact to synthesize complex molecules for life and subcellular processes.

Importance of Cells

• Cells are the basic unit of life.
• All living things are made of one or more cells.
• Cells are the smallest functional unit of all living things.
• All functions of living things occur at the cellular level.

Modern Cell Theory

• All known living organisms are made of cells.
• Unicellular organisms include prokaryotes (e.g., bacteria) and some eukaryotes (e.g., protozoa).
• Multicellular organisms are mostly eukaryotes (fungi, plants, and animals).
• Fungi can be unicellular or multicellular.
• The cell is the structural and functional unit of all living things.
• All body parts are formed from cells: organism → organs → tissues → cells.
• Cell structure relates to the organism's function.
• All cells are basically the same in chemical composition.
• Cells come from pre-existing cells by division; spontaneous generation does not occur.
• Cells contain hereditary material passed during cell division.
• All energy pathways occur within cells.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells are single cells without a nucleus.
• Eukaryotic cells have organized nucleus.
• Nuclear material in prokaryotes is present without a nuclear membrane ("Naked DNA").
• Eukaryotic DNA is linear.
• Prokaryotes lack membrane-bound organelles.
• Eukaryotes contain membrane-bound organelles.
• Prokaryotes are smaller in size (0.1-5 µm).
• Eukaryotes are larger (10-100 µm).
• Examples of prokaryotes: bacteria, mycoplasma, and cyanobacteria.
• Examples of eukaryotes: protozoa, fungi, plant, and animal cells.
• Cells are diverse in size, shape, and type, influenced by their roles and functions.
• Cell shape aids their function.
• Nerve cells extended for conducting impulses
• Epithelial cells are flat to protect and cover body surfaces
• Smooth muscle cells have a spindle shape to contract and relax
• Sperm cells have tails for motility.

Functions of Cells

• Metabolism: converting nutrients into energy.
• Growth and Repair: dividing and differentiating to replace damaged cells.
• Homeostasis: maintaining a stable internal environment.
• Communication: using chemical signals and electrical impulses.
• Immune Response: defending against pathogens with cells like white blood cells.
• Transport: facilitating the movement of substances.
• Synthesis: creating biological molecules like proteins and lipids.
• Reproduction: Some cells are involved in reproduction.
• Storage: storing substances like fats and carbohydrates.
• Specialization: performing specific functions in organs and systems.

Composition of Cells

• Human cells have complex structures with various components.
• Cell Membrane: A protective barrier to regulate the cells in and out.
• Cytoplasm: This is the substance that fills the cell, containing many molecules and organelles

Organelles

• Nucleus: the control center of the cell that holds DNA and controls gene expression and division.
• Mitochondria: generate ATP (energy) through cellular respiration.
• Ribosomes: the sites of protein synthesis
• Endoplasmic Reticulum (ER): involved in synthesizing, processing lipids and detoxification.
• Golgi Apparatus: modifies, sorts, and packages proteins and lipids.
• Lysosomes: break down waste materials and cellular debris using digestive enzymes.
• Peroxisomes: metabolize fatty acids and detoxify harmful substances.
• Cytoskeleton: provides structural support and facilitates movement.
• Centrioles: involved in cell division and the formation of cilia and flagella.
• Vesicles: transport materials within the cell.

The Cell Membrane

• A biological membrane that separates the cell's interior from the external environment.
• Animal cells solely rely on this to separate the cell, while plant, bacteria and fungi use cell walls on top for protection.

Functions of the Cell Membrane

• Protects the cells by acting as a barrier.
• Regulates movement in and out of the cell via a "semi-permeable barrier."
• Provides shape to the cell.
• Receives chemical messengers for cell communication.
• Attaches the cell to other cells to form tissues.

Constituents of the Cell Membrane

• Lipids: Include and cholesterol, and phospholipids.
• Proteins: help give it shape

Phospholipids

• They form a bilayer.
• Each one has a hydrophilic head (attracts water) and hydrophobic tails (repels water).

Other components

• Carbohydrates: attach to proteins or lipids and play role in the signal recognition, signaling and adhesion.
• Cytoskeletal Components: They maintain shape and help facilitate the cells movement.
• Cholesterol: stabilizes membrane fluidity.

Membrane Fluidity

• Lipids and proteins move in the plane of the membrane.
• Fluidity is important for changing cell shape.

Factors Affecting Membrane Fluidity

• Increased cholesterol decreases fluidity.
• Saturated fatty acids decrease fluidity; unsaturated fatty acids increase fluidity.

Proteins in cell membranes

• Integral Proteins: Embedded within the lipid bilayer and acts a channel to transport molecules
• Peripheral Proteins: Loosely attached to the membrane's exterior or interior surfaces and helps it communicate and keep its shape.