Unit 4 Cell structure and specialisation
Key unit competence: Describe the structure and function of cells in an organism.
Key ideas
- Cell theory: all living things are made of one or more cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells (Schleiden, Schwann, Virchow).
- Under the light microscope plant and animal cells show a cell membrane, cytoplasm and nucleus; plant cells also show a cell wall, chloroplasts and a large vacuole. The electron microscope reveals the ultrastructure: organelles such as ER, Golgi, ribosomes, mitochondria, lysosomes and centrioles.
- Nucleus: double membrane (nuclear envelope) with pores; contains chromatin (DNA + proteins) and a nucleolus (makes ribosomes). Controls cell activities by controlling protein synthesis and carries hereditary information.
- Rough ER (with ribosomes) makes and transports proteins; smooth ER (no ribosomes) makes lipids and steroids and detoxifies. Ribosomes (80S in eukaryotes, 70S in prokaryotes, mitochondria and chloroplasts) are sites of protein synthesis. Golgi apparatus modifies, packages and sends out proteins in vesicles; makes lysosomes.
- Mitochondria: double membrane, inner membrane folded into cristae, matrix; site of aerobic respiration and ATP production; many in active cells (muscle, sperm, liver). Chloroplasts: double membrane, thylakoids stacked into grana, stroma; site of photosynthesis. Both contain their own circular DNA and 70S ribosomes.
- Lysosomes: vesicles of hydrolytic enzymes that digest worn-out organelles and engulfed bacteria. Centrioles: pairs of microtubule cylinders in animal cells that form the spindle in cell division. Vacuole: large permanent vacuole in plant cells with cell sap, surrounded by the tonoplast; maintains turgor and stores substances.
- Fluid mosaic model of the cell membrane (Singer and Nicolson): a phospholipid bilayer (hydrophilic heads outside, hydrophobic tails inside) with proteins floating in it (channel and carrier proteins for transport, receptor proteins, enzymes), cholesterol (regulates fluidity and stability) and glycoproteins and glycolipids (cell recognition, receptors). It is partially permeable.
- Prokaryotic cells (bacteria) have no nucleus or membrane-bound organelles, circular DNA, plasmids, 70S ribosomes and a peptidoglycan wall; eukaryotic cells have a nucleus, membrane-bound organelles, linear chromosomes and 80S ribosomes. Eukaryotic cells are usually larger (10–100 µm vs 1–5 µm).
- Specialised cells are adapted to their functions: red blood cell (biconcave, no nucleus, haemoglobin), sperm (tail, acrosome, mitochondria), nerve cell (long axon, myelin), muscle cell (contractile proteins, many mitochondria), ciliated epithelium (cilia move mucus), root hair cell (long extension, large surface area), palisade cell (many chloroplasts), guard cells (control stomata), xylem vessels (dead, lignified, hollow), phloem sieve tubes (sieve plates, companion cells).
Common misconceptions
- "Plant cells have no mitochondria because they have chloroplasts." No: plant cells have both; they respire all the time.
- "Ribosomes are found only on the rough ER." No: ribosomes are also free in the cytoplasm, and inside mitochondria and chloroplasts.
- "The cell membrane is a rigid layer." No: it is fluid; phospholipids and proteins move within it.
- "Prokaryotes have no DNA." No: they have DNA as a circular chromosome in the cytoplasm, not in a nucleus.
Worked examples
Example 1. Explain how a cell that secretes digestive enzymes, such as a pancreatic cell, uses its organelles.
Solution
Genes in the nucleus are copied; ribosomes on the rough ER make the enzyme proteins; the ER transports them in vesicles to the Golgi apparatus, which modifies and packages them; secretory vesicles move to the membrane and release the enzymes by exocytosis. Mitochondria supply the ATP.
Example 2. Explain the role of cholesterol and glycoproteins in the cell membrane.
Solution
Cholesterol sits between phospholipid tails and regulates fluidity, making the membrane stable over a range of temperatures. Glycoproteins have carbohydrate chains on the outside that act as receptors (e.g. for hormones) and in cell recognition (antigens).
Practice questions
State the three principles of the cell theory.
Show answer
All living things are made of cells; the cell is the basic unit of structure and function; all cells come from pre-existing cells.
Common mistake: Saying cells come from non-living matter.
State the functions of: (a) rough ER; (b) Golgi apparatus; (c) lysosome; (d) mitochondrion.
Show answer
(a) Synthesis and transport of proteins. (b) Modifies, packages and secretes proteins; makes lysosomes. (c) Digests worn-out organelles and pathogens. (d) Aerobic respiration, ATP production.
Common mistake: Saying the Golgi makes energy.
Describe the fluid mosaic model of the cell membrane.
Show answer
A phospholipid bilayer with hydrophilic heads outward and hydrophobic tails inward; proteins (channel, carrier, receptor) embedded or attached; cholesterol between phospholipids; glycoproteins and glycolipids on the outer surface; components move laterally (fluid).
Common mistake: Leaving out proteins.
Give four differences between prokaryotic and eukaryotic cells.
Show answer
Prokaryotic: no nucleus, no membrane-bound organelles, circular DNA, 70S ribosomes, smaller, peptidoglycan wall. Eukaryotic: nucleus, membrane-bound organelles, linear chromosomes, 80S ribosomes, larger.
Common mistake: Saying prokaryotes have no ribosomes.
Compare the structure of mitochondria and chloroplasts.
Show answer
Both have double membranes, own circular DNA and 70S ribosomes. Mitochondria: inner membrane folded into cristae, matrix, respiration. Chloroplasts: thylakoids in grana, stroma, chlorophyll, photosynthesis.
Common mistake: Saying mitochondria contain chlorophyll.
Explain how three animal cells are adapted to their functions.
Show answer
Red blood cell: biconcave, no nucleus, haemoglobin for oxygen transport. Sperm: tail, many mitochondria, acrosome. Nerve cell: long axon and myelin for fast impulses. Ciliated cell: cilia sweep mucus.
Common mistake: Listing features without functions.
Explain how two plant cells are adapted to their functions.
Show answer
Root hair cell: long extension gives large surface area for water and mineral uptake. Palisade cell: many chloroplasts near the upper surface for photosynthesis. Guard cells: uneven walls open and close stomata.
Common mistake: Saying root hair cells photosynthesise.
State the functions of cholesterol, glycoproteins and carrier proteins in the membrane.
Show answer
Cholesterol: regulates fluidity and stability. Glycoproteins: cell recognition, receptors for hormones. Carrier proteins: transport specific substances across the membrane (facilitated diffusion and active transport).
Common mistake: Saying cholesterol transports glucose.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Stacks of thylakoids in chloroplasts are called: a. ribosomes 2. The organelle that packages proteins into vesicles is the: b. Mitochondrion 3. Which organelle contains its own circular DNA? c. grana 4. The nucleolus makes: d. Golgi apparatus Show answer
1 → c (grana) · 2 → d (Golgi apparatus) · 3 → b (Mitochondrion) · 4 → a (ribosomes)
- Light reactions occur there.
- It also makes lysosomes.
- Chloroplasts also do.
- It is inside the nucleus.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
forming the spindle in cell division · aerobic respiration of glucose · pre-existing cells · cell recognition and as receptors- Centrioles are involved in .
- Glycoproteins on the membrane surface act in .
- The cell theory states that all cells arise from .
Show answer
1: forming the spindle in cell division · 2: cell recognition and as receptors · 3: pre-existing cells
- They occur in animal cells.
- They have carbohydrate chains.
- Proposed by Virchow.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The main lipid in the cell membrane is: a. very active, like a muscle cell 2. A cell with many mitochondria is probably: b. tonoplast 3. The membrane around the plant vacuole is the: c. ribosomes 4. Proteins are synthesised on: d. phospholipid Show answer
1 → d (phospholipid) · 2 → a (very active, like a muscle cell) · 3 → b (tonoplast) · 4 → c (ribosomes)
- It forms a bilayer.
- Mitochondria supply ATP.
- It controls movement into the vacuole.
- Ribosomes read mRNA.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
vacuole · nucleus · 80S · cristae- The folded inner membranes of mitochondria are .
- The structure that controls the activities of the cell is the .
- Eukaryotic ribosomes are .
Show answer
1: cristae · 2: nucleus · 3: 80S
- They increase surface area for respiration.
- It contains DNA.
- Prokaryotes have 70S.