Cell Structure
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課程筆記
Cell Theory
- Cell theory states that all living organisms are made up of one or more cells.
- Cells are the basic functional unit (basic unit of structure and organisation) in living organisms.
- New cells are produced from pre-existing cells by division.
- Robert Hooke coined the term 'cells' in the 1660s after examining cork.
- Matthias Schleiden and Theodor Schwann proposed in 1837 that all living organisms are made of cells.
- All cells, despite varying in size and shape, are surrounded by a membrane, contain genetic material, and have enzyme-catalysed chemical reactions.
- Cell theory is a unifying concept in biology, meaning it is universally accepted.
- Deductive reasoning can be used to predict that a newly discovered organism will consist of one or more cells.
Hooke's microscope and cork cells
Microscopy Skills
- Magnification is how many times bigger the image is compared to the actual size; it has no units.
- Resolution is the ability to distinguish between two objects that are very close together.
- The magnification equation is: magnification = image size ÷ actual size.
- When preparing a temporary mount, gently press the coverslip to remove air bubbles.
- Start with the low power objective lens to find the specimen and prevent damage; use the coarse focus, then fine focus.
- At higher objective powers, only use the fine focusing knob.
- A graticule is a small disc with an engraved scale placed in the eyepiece; it must be calibrated using a stage micrometer.
- A scale bar represents the actual size before enlargement and can be used to calculate magnification.
Parts of a light microscope
Microscopes: Light vs Electron
- Optical (light) microscopes use light to form an image and have a maximum resolution of about 0.2 μm (200 nm).
- The maximum useful magnification of a light microscope is about ×1500.
- Electron microscopes use electrons, giving a much higher resolution of about 0.0002 μm (0.2 nm).
- The maximum useful magnification of an electron microscope is about ×1,500,000.
- Transmission electron microscopes (TEMs) pass electrons through thin specimens, showing internal structures in high detail.
- Scanning electron microscopes (SEMs) scan electrons across the surface, producing 3-D images of the surface.
- Electron microscopes require specimens to be dead and do not produce colour images.
- Light microscopes can observe living specimens and produce colour images.
Magnification equation triangle
General Cell Structure
- All cells share common features: DNA as genetic material, cytoplasm, and a plasma membrane.
- DNA stores genetic material, allows new cells to form from old cells, and controls production of enzymes and proteins.
- Cytoplasm is mainly water with dissolved substances; the fluid is called cytosol and many cellular reactions occur here.
- The plasma membrane is a bilayer of lipids that controls interactions between the cell's interior and exterior.
- Membrane proteins have functions including cell recognition, cell communication, and transport into and out of the cell.
A typical animal cell
Prokaryotic Cell Structure
- Prokaryotes are the simplest cells, ranging from 0.1 μm to 5.0 μm, and lack a nucleus.
- They are classified into two domains: Bacteria (Eubacteria) and Archaebacteria (Archaea).
- Prokaryotic cytoplasm lacks membrane-bound organelles; common structures include 70S ribosomes, circular DNA, cytoplasm, plasma membrane, and cell wall.
- The 70S ribosomes bind and read mRNA during translation to produce proteins.
- Genetic material is a naked single circular DNA molecule (not associated with proteins) in the nucleoid, plus smaller loops called plasmids.
- Plasmids contain genes that can be passed between prokaryotes, e.g. antibiotic resistance.
- Most prokaryotes have a cell wall containing murein/peptidoglycan, which protects, maintains shape, and prevents bursting.
- Additional structures include capsules (protect from drying and immune attack), flagella (movement), and pili (adhesion, conjugation).
A typical bacterial cell
Eukaryotic Cell Structure
- Eukaryotic cells have membrane-bound organelles, dividing the cytoplasm into compartments.
- Compartmentalisation allows enzymes and substrates to be at higher concentrations, separates damaging substances, and maintains optimal conditions.
- The nucleus is surrounded by a double membrane (nuclear envelope) with pores; it contains chromatin (DNA wound around histones).
- The nucleolus is the site of ribosome production.
- Rough endoplasmic reticulum (RER) is covered in 80S ribosomes and processes proteins made by the ribosomes.
- Ribosomes are complexes of rRNA and protein, constructed in the nucleolus, and are the site of translation.
- Mitochondria are the site of aerobic respiration; they have a double membrane with inner folds called cristae, and contain their own DNA and ribosomes.
- The Golgi apparatus modifies proteins and lipids, packaging them into vesicles for export, lysosomes, or delivery to organelles.
Plant Cell Structures
- Chloroplasts are larger than mitochondria, surrounded by a double membrane, and contain thylakoids stacked into grana joined by lamellae.
- The light-dependent stage of photosynthesis occurs in the thylakoids; the light-independent stage (Calvin Cycle) occurs in the stroma.
- Chloroplasts contain small circular DNA and ribosomes for protein synthesis.
- Large permanent vacuoles are surrounded by the tonoplast, a selectively permeable membrane; animal cells have small, temporary vacuoles.
- The cell wall is found in plant cells but not animal cells, formed outside the cell membrane, and provides structural support.
- Plant cell walls are made of cellulose; bacterial cell walls contain peptidoglycan.
- Plasmodesmata are narrow threads of cytoplasm connecting neighbouring plant cells.
- The cell wall is freely permeable to most substances, unlike the plasma membrane.
A typical plant cell
Functions of Life
- All organisms must carry out: metabolism, reproduction, homeostasis, growth, response, excretion, and nutrition.
- Metabolism includes all enzyme-catalysed reactions in a cell, including cell respiration.
- Homeostasis is maintaining and regulating internal conditions within tolerable limits.
- Response (sensitivity) is the ability to respond to external or internal changes (stimuli).
- Excretion is the disposal of metabolic waste products, including carbon dioxide.
- Nutrition is the acquisition of energy and nutrients, either by absorbing organic matter or by synthesising organic molecules.
- Unicellular organisms have adapted unique ways to carry out these functions, e.g. using cilia or flagella for movement.
Eukaryotic Comparisons & Atypical Examples
- Eukaryotic cells exist in four kingdoms: animal, plant, fungal, and protist.
- Cell walls: animal cells lack a cell wall; plant cell walls are cellulose; fungal cell walls are mainly glucans, chitin, and glycoproteins.
- Vacuoles: plant cells have large permanent vacuoles; animal and fungal cells have small, temporary vacuoles.
- Chloroplasts: present in plant cells, absent in animal and fungal cells.
- Centrioles: present in animal cells, absent in plant and fungal cells.
- Cilia and flagella: animal cells can have them; plant and true fungal cells do not.
- Animal and fungal cells store carbohydrates as glycogen; plants store starch.
- Atypical examples that challenge cell theory include striated muscle fibres (long, multinucleated), aseptate fungal hyphae (multinucleated, continuous cytoplasm), red blood cells, and phloem sieve tubes.
投影片
練習題
免費預覽——58 題中的 8 題。註冊即可查看全部。
1.Which structure is found in both an animal cell and a bacterial cell?
Easy- ACell wall
- BRibosome
- CChloroplast
- DNucleus
2.Which of the following cannot be viewed using a light microscope?
Easy- ANucleus
- BChloroplast
- CRibosome
- DCell wall
3.A prokaryotic cell has a diameter of 1 μm. It is magnified ×50,000 by an electron microscope. What is the diameter of the cell in the electron micrograph?
Easy- A5 × 10⁰ mm
- B5 × 10⁻¹ mm
- C5 × 10² mm
- D5 × 10¹ mm
4.Erythrocytes (red blood cells) have a diameter of 7,000 nm. Pancreatic cells have a diameter of 35 μm. Which statement about their relative sizes is correct?
Medium- AThe erythrocytes are 5 times smaller
- BThe erythrocytes are 50 times smaller
- CThe erythrocytes are 5 times larger
- DThe erythrocytes are 50 times larger
5.Which row correctly describes the features of prokaryotic chromosomal DNA?
Medium- AAssociated with histone proteins and circular
- BNot associated with histone proteins and linear
- CNot associated with histone proteins and circular
- DContains a few hundred genes and circular
6.Which of the following ideas are part of cell theory? I. Cells are the smallest unit of life. II. Cells show great variety in shape and structure. III. Cells are derived from pre-existing cells by division.
Medium- AI only
- BII only
- CI and III only
- DII and III only
7.Which of the following organelles does not directly help an exocrine gland cell in the pancreas to secrete digestive enzymes?
Medium- ARough endoplasmic reticulum
- BGolgi apparatus
- CLysosomes
- DMitochondria
8.Which of the following statements about eukaryotic chromosomes are correct? I. Chromosomes are long, linear molecules that can be condensed for compact storage. II. After DNA replication, the two strands of DNA are held together at the centromere. III. DNA wraps around histone proteins to form chromatids.
Medium- AI and II only
- BII only
- CII and III only
- DI, II and III
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