hs 1
Cells have evolved over time from bacteria and consists of two types: prokaryotic and eukaryotic. Eukaryotes have evolved by branching off from prokaryotic cells. Evolution is thought to occur by mistakes or mutation from cell division.
Bacteria comes in different shapes and sizes e.g. spherical cells, rod-shaped cells and spiral cells.
Prokaryotes:
- 1-10 micrometres.
- Anaerobic or aerobic metabolism.
- Circular DNA in cytoplasm.
- RNA and proteins synthesised in same compartment.
- Contain no cytoskeleton and no membrane bound organelles.
- Cellular organisation is mainly unicellular.
Eukaryotes:
- 10-100 micrometres.
- Aerobic metabolism.
- Linear DNA surrounded by nuclear envelope.
- RNA synthesised and processed in the nucleus. Protein synthesis in cytoplasm.
- Contain membrane bound organelles and a cytoskeleton.
- Cellular organisation is mainly multicellular.
Structure of a eukaryotic cell and its functions:
- Cytosol- metabolic pathways (e.g. protein synthesis).
- Nucleus- contains main genome (DNA and RNA synthesis).
- Nucleolus- production of ribosomal RNA.
- Rough endoplasmic reticulum (RER)- endoplasmic reticulum associated with ribosomes. Synthesis of proteins for distribution to organelles, the plasma membrane and secretion.
- Smooth endoplasmic reticulum (SER)- endoplasmic reticulum not associated with ribosomes. Involved in lipid synthesis.
- Golgi apparatus- modification, sorting and packaging of proteins (from ER) and lipids for either secretion or delivery to another organelle.
- Lysosomes- intracellular degradation.
- Endosomes- sorting of endocytosed material.
- Peroxisomes- oxidation of toxic molecules.
- Mitochondria- production of ATP via oxidative phosphorylation. Membrane bound organelle. Site of aerobic respiration.
Eukaryotic cell membranes controls what passes in and out of the cell, provides structure and allows communication with other cells. They are important as drugs often target cell membranes. They are made out of phospholipids (glycerol, 2 fatty acid chains, phosphate group) which can be saturated or unsaturated (contains C=C double bonds). The polar head is hydrophilic, and the non-polar tail is hydrophobic, allowing it to form a lipid bilayer (dynamic and selectively permeable). Water-soluble molecules cannot pass through the hydrophobic interior; only small and non-charged molecules can pass through the membrane.
Heat causes phospholipid tails to move apart whereas colder temperatures bring the phospholipid tails closer together- this causes fluidity of the membrane and gaps in the bilayer where cholesterol can be found. More cholesterol means there are less gaps within the membrane. More C=C double bonds within the phospholipid tails means more gaps within the membrane, making it more difficult to pack the lipids together (as the double bonds cause kinks) making the membrane more fluid.
Fluidity can cause things to move around in the membrane e.g. exocytosis and endocytosis.
- Exocytosis- cell shifts materials (e.g. waste products) from inside the cell to the extracellular space.
- Endocytosis- cell takes in substances from outside the cell by engulfing them in a vesicle derived from the cell membrane.
Lateral diffusion is the movement of lipids occurs readily and rapidly due to membrane fluidity. Phosphate heads are dynamic, so lipids move using flipase enzymes (inside to outside membrane) or flopase enzymes (outside to inside membrane).
The inner and outer membrane have a different composition (e.g. serine is found on the inside where choline phosphates can be found on the outside). The outer layer contains glycolipids (carbohydrate covalently linked to a protein) which act as recognition and signal molecules. The sugar attached provides a fingerprint to the cell.
Different types of proteins can be found on a membrane: integral (transmembrane) or peripheral.
- Integral- permanently embedded. Contain hydrophobic and hydrophilic areas. Function as channels, receptors or anchors- interacting directly with the hydrophobic core of the membrane.
- Peripheral- attached to surface of membrane at different times. More involved in regulating or anchoring other membrane components or processes at the membrane surface.
Structure of the nucleus:
- Nuclear envelope- consists of both an inner and outer membrane, perforated by small gaps (nuclear pores) which controls the passage of smaller molecules in and out of the nucleus.
- Nuclear lamina- mechanical support for nucleus (protein mesh).
- Chromatin- DNA complexed with histones (combine with DNA to form nucleosomes- the subunit of chromatin).
- Euchromatin- form of chromatin present during gene expression. Activated by acetylation.
- Heterochromatin- the ‘inactive’ form of DNA. Densely packed.
The cytoskeleton consists of actin filaments, microtubules and intermediate filaments. They have roles in muscle contraction (actin), holding cells together in tissues (intermediate) and moving vesicles around the cell (microtubules).