Cell Structure and Function

Cells: Introduction

• Cells are the fundamental units of life, providing structure and function in organisms.
• A cell consists of a membrane enclosing the cytoplasm, which contains living material.
• Genetic information is stored as DNA within cells.
• Genes in DNA are transcribed into messenger RNA (mRNA).
• mRNA is then translated by ribosomes into proteins, which perform essential cellular functions, including enzymatic reactions.
• Enzymes regulate the cell's metabolism.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells are small, simple cells lacking a nucleus, the chromosome is circular.
• Prokaryotes possess circular chromosomes and may contain extra DNA in plasmids, and are found within the Bacteria and Archaea domains.
• Eukaryotic cells, found in the Eukarya domain, are larger and more complex.
• Eukaryotes have a nucleus containing multiple linear chromosomes with DNA complexed with proteins.
• Eukaryotic cells feature mitochondria and membrane-bound organelles.

Cell Size

• Cells must maintain a sufficient membrane surface area for diffusion of substances in and out.
• Small cells have a higher surface area-to-volume ratio compared to large cells.
• A cell with 1 micrometer sides has a surface area to volume ratio of 6:1
• A cell with 10 micrometer sides has a surface area to volume ratio of 0.6:1
• As cell size increases, the surface area-to-volume ratio decreases.
• Organisms adapt to increase surface area in certain tissues for diffusion through thin tissue sheets (e.g., fish gills) or folded surfaces (e.g., intestinal villi, mitochondrial membranes).
• Larger marine mammals have decreased surface area to volume ratios to minimize heat loss in cold environments.

Cellular Compartmentalization and the Endomembrane System

• Cellular compartmentalization divides the cell into distinct sections.
• Cell compartmentalization allows specialized internal chemistry, like hydrolytic enzymes in lysosomes, and provides internal surface area for membrane-bound enzymes.
• Prokaryotic cells have few compartments; eukaryotic cells are highly compartmentalized.
• The endomembrane system, found in eukaryotes, is a dynamic network of internal membranes.
• The dynamic endomembrane system includes the nuclear membrane, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles.
• Membranes and materials move between endomembrane system compartments.
• The endomembrane system membranes are dynamic, exchanging phospholipid molecules.

Origin of Mitochondria, Cellular Compartmentalization, and Chloroplasts

• Eukaryotes arose ~1.8 billion years ago through mutualistic endosymbiosis.
• An ancestral archaeal cell engulfed a bacterial cell, which evolved into the mitochondrion.
• Vesicles secreted by the engulfed bacterium formed the nuclear membrane and ER giving rise to the endomembrane system.
• Chloroplasts originated from a second endosymbiotic event when free-living cyanobacteria were engulfed by a eukaryotic cell.
• Mitochondria and chloroplasts contain their own circular DNA and replicate via binary fission, similar to bacteria.
• Mitochondria and chloroplasts use their own ribosomes to produce some proteins, resembling bacterial ribosomes.
• Mitochondria and chloroplasts possess a double-membrane structure which is evidence of endosymbiosis.

Eukaryotic Cell Parts and Functions: Nucleus

• The nucleus stores and protects genetic information as DNA in chromosomes.
• During cell division (mitosis/meiosis), DNA is organized into visible chromosomes; otherwise exists as chromatin.
• Nucleolus assembles ribosomes.
• The nuclear membrane has pores for molecule transport, like mRNA and transcription factors.

Eukaryotic Cell Parts and Functions: Ribosomes

• Ribosomes are particles of ribosomal RNA and protein composed of large and small subunits.
• The function of ribosomes is to Translate mRNA into amino acid sequences for protein synthesis.
• Ribosomes are found free in the cytoplasm or bound to the rough ER.
• Ribosomes are initially free but become bound to the ER during protein targeting if the synthesized proteins are destined for vesicles, Golgi, membranes, or lysosomes.

Eukaryotic Cell Parts and Functions: Mitochondria

• Mitochondria convert food energy into ATP.
• Key mitochondrial structures: DNA chromosome, ribosomes, inner membrane (with folds to increase surface area), matrix (containing Krebs cycle enzymes), intermembrane space, and outer membrane
• The mitochondrial inner membrane contains embedded enzymes and proteins (like ATP synthase).
• The mitochondrial matrix is the cytoplasm of the former independent cell and contains enzymes for the Krebs cycle.

Eukaryotic Cell Parts and Functions: Endoplasmic Reticulum (ER)

• The ER connects the nuclear membrane and the Golgi body.
• The two forms of the ER are rough ER and smooth ER.
• The rough ER has ribosomes for protein synthesis, destined for lysosomes, organelles, membrane, or export.
• The smooth ER lacks ribosomes but contains membrane-embedded enzymes, with functions varying by tissue which includes the synthesis of lipids, toxin conversion, and carbohydrate metabolism.

Eukaryotic Cell Parts and Functions: Golgi Complex

• The Golgi complex consists of flattened membrane-bound sacs.
• The Golgi receives vesicles from the ER, modifies their contents, and packages modified proteins into vesicles sent to organelles, membranes, or for export.

Eukaryotic Cell Parts and Functions: Lysosomes

• Lysosomes are only found in animal cells.
• Lysosomes contain hydrolytic enzymes for intracellular digestion.
• They recycle worn-out organelles and molecules and participate in apoptosis (programmed cell death).

Eukaryotic Cell Parts and Functions: Cytoskeleton, Centrosomes, and Centrioles

• The cytoskeleton is a dynamic network of protein fibers providing cell structure and movement.
• Centrosomes contain centrioles, which organize spindle fibers to separate chromosomes during cell division.

Eukaryotic Cell Parts and Functions: Plant Cell Unique Structures

• Structures only found in animal cells are lomes
• The central vacuole stores water, macromolecules, and waste in plant cells and maintains turgor pressure.
• Turgor pressure is outward pressure that keeps plant cells upright.
• Chloroplasts are derived from photosynthetic bacteria and perform photosynthesis.
• They have their own DNA, ribosomes, and double membranes.
• The cell wall is primarily composed of cellulose, providing support.
• A major function of the cell wall is to act as a pressure vessel that prevents overexpansion.
• The plant cell wall prevents overexpansion due to water inflow and is the primary component of wood and vascular tissues.

Membrane Structure, Function, and Transport

• The cell membrane separates the contents of the cell from the external environment and it is a selectively permeable boundary.
• The cell membrane controls the passage of substances in and out of the cell.
• Phospholipids have a hydrophilic polar head and hydrophobic non-polar tails.
• In water, phospholipids form a bilayer with heads interacting with water and tails forming a water-free zone.
• The phospholipid bilayer is stabilized by weak Van der Waals bonds between the tails.
• The fluid mosaic model describes the membrane as phospholipids, proteins, and cholesterol in motion.
• The membrane is fluid because components move laterally, not fixed in place.
• The membrane is mosaic due to its composition of diverse components.
• Transmembrane proteins have hydrophobic core regions and hydrophilic regions extending into the cytoplasm and cell exterior.
• Integral proteins have non-polar regions extending into hydrophobic membrane and hydrophilic regions jutting into the cytoplasm or cell exterior.
• Peripheral proteins attach to phospholipid heads on either side of the membrane.

Membrane Transport

• Osmosis is the diffusion of water.
• Things move across the cell membrane through diffusion, or passive transport.
• Diffusion moves molecules from areas of higher concentration to lower concentration.
• Diffusion occurs spontaneously based on the kinetic energy of molecules.
• Passive transport requires no cellular energy.
• Simple diffusion allows small, nonpolar molecules (O2, N2, CO2) and nonpolar substances (steroid hormones, fats) to cross the membrane.
• Facilitated diffusion is used for polar molecules and ions and requires protein channels or carriers.
• Active transport pumps molecules against its concentration gradient, from lower to higher. Active transport requires energy expenditure and converting ATP into ADP.
• Endocytosis and exocytosis are forms of bulk transport.
• Endocytosis involves membrane pinching inward to engulf particles into vesicles.
• Exocytosis involves vesicles fusing with the membrane to expel contents.
• Both endocytosis and exocytosis require energy.
• Membrane potential is an electrical charge across the membrane, creating a voltage difference.
• Membrane potential is Created by pumping ions across membranes with energy expenditure (e.g., protons pumped by mitochondria).
• Chloroplasts also pump protons.

Tonicity and Osmo regulation

• Tonicity refers to the ability of a solution to cause a cell to gain or lose water.
• Osmosis is the diffusion of water (solvent) from high to low concentration.
• Water flows from hypotonic to hypertonic regions.
• Hypotonic solutions have more water and less solute relative to another solution.
• Hypertonic solutions have less water and more solute relative to another solution.
• After osmosis, water levels equilibrate due to osmotic pressure.
• Water always flows from hypotonic to hypertonic.
• In plant cells, water flowing in creates turgor pressure, keeping cells firm.
• Plant cells rely on osmosis.
• Plasmolysis happens when the cell is hypotonic to the environment and the membrane peels away from the cell wall.
• If a cell is isotonic water enters and leaves at the same rate.
• In animal cells; if cells are hypotonic to their environement water leaves the cell and it shrivels up.
• Animal cells kept in tissue culture need to be kept in an isotonic solution.
• Cells that are hypertonic will eventually burst because there is no cell wall.
• Contractile vacuoles in freshwater protists osmo regulate by filling with water and expelling it.
• Changes in environmental tonicity causes contractile vacuoles to adapt by changing contraction rates.
• Leaf stomata are pores formed by guard cells, which regulate gas exchange and water loss.
• Guard cells regulate the opening and closing of stomata.
• When adequate water is available; Potassium ions are pumped into the guard cells, making them hypertonic to release carbon dioxide into the leaf.
• When water is scarce; the pumping stops which closes the stomata.
• Water potential (Ψ) is a measure of water's tendency to move from one location to another.
• Water flows from areas of high water potential to low water potential.
• Water Potential Formula: Ψ = ΨS + ΨP (Water potential = Solute potential + Pressure potential).
• Adding solute decreases water potential (ΨS is negative).
• Adding pressure increases water potential (ΨP).
• Water moves down its water potential gradient.