Cell Structure
FoundationThe cell membrane is a fluid phospholipid bilayer studded with proteins that controls what enters and leaves; the nucleus, mitochondria and ribosomes each have their own job — and the differences between animal, plant, prokaryotic and eukaryotic cells are the heart of this lesson.
- The cell membrane is a fluid phospholipid bilayer with embedded proteins, controlling the movement of substances.
- Chromosomes in the nucleus carry genetic information; mitochondria supply energy; ribosomes build proteins; the endoplasmic reticulum transports materials.
- Plant cells alone have a cell wall, chloroplasts and a large central vacuole; animal cells have none of these.
文體小知識：科學知識說明文——按流動鑲嵌模型講解細胞膜，逐一認識各種細胞器的功能，並比較動物細胞、植物細胞與細菌細胞的異同。
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1The cell is the basic unit of structure and function in all living things. Under a light microscope we can already see the cell membrane, cytoplasm and nucleus; the electron microscope resolves far finer detail, revealing the subcellular structures called organelles. Cell theory states that all organisms are made of cells and that cells arise only from other cells — the starting point of this lesson.
2The cell membrane follows the fluid mosaic model: phospholipids form a bilayer that is the membrane's skeleton, while protein molecules stud the layers like mosaic tiles and can drift about, making the membrane flexible. The membrane separates the cell's contents from the outside world and is selectively permeable, controlling what enters and leaves to keep the internal environment stable.
3The nucleus, enclosed by a nuclear membrane, holds the chromosomes — DNA and proteins carrying the genetic instructions that direct the cell's activities. The mitochondrion is the site of aerobic respiration, supplying the cell with energy. Ribosomes join amino acids into proteins. The endoplasmic reticulum is a network of membranes that transports materials; rough ER carries ribosomes on its surface.
4Plant cells have three extra structures: a cellulose cell wall for support, chloroplasts containing chlorophyll for photosynthesis, and a large central vacuole holding cell sap that keeps the cell firm. Prokaryotic cells such as bacteria have no membrane-bound nucleus and none of the membrane-bound organelles — a far simpler design.
1Summary 重點摘要
The cell membrane is a fluid phospholipid bilayer studded with proteins that controls what enters and leaves; the nucleus, mitochondria and ribosomes each have their own job — and the differences between animal, plant, prokaryotic and eukaryotic cells are the heart of this lesson.
2Vocabulary cards Vocabulary
The boundary of every cell; a selectively permeable membrane of phospholipid bilayer and proteins that controls the movement of substances.
The model of membrane structure: a fluid phospholipid bilayer with proteins embedded like mosaic tiles.
A thread of DNA and proteins in the nucleus; it carries the genetic information.
The organelle where aerobic respiration takes place, releasing energy as ATP.
The tiny particle that assembles amino acids into proteins, either free in the cytoplasm or on the endoplasmic reticulum.
A network of membranes in the cytoplasm that transports materials; rough ER has ribosomes on its surface.
The plant organelle containing chlorophyll, where photosynthesis takes place.
A fluid-filled sac; the large central vacuole of plant cells stores cell sap that keeps the cell firm.
The tough outer layer of plant cells, made of cellulose, providing support and protection.
A cell without a membrane-bound nucleus (such as bacteria); its genetic material lies free in the cytoplasm and it lacks other membrane-bound organelles.
3Step by step Steps
- Step 1: The cell membrane — fluid mosaic modelThe cell membrane separates the cell from its surroundings and controls what passes through. In the fluid mosaic model, phospholipids form a bilayer that is the skeleton, while proteins are scattered across it like mosaic tiles and can drift, giving the membrane flexibility. For example, white blood cells can change shape and squeeze through vessel walls precisely because the membrane is fluid rather than rigid. The same structure explains its selective permeability: small, fat-soluble molecules pass easily, while large molecules and ions are restricted.
- Step 2: Nucleus and chromosomesThe nucleus, enclosed by a nuclear membrane, is the control centre of the cell. Its chromosomes are made of DNA and proteins; the DNA stores the instructions for everything the cell does — which proteins to make, when to divide. For example, if the nucleus is removed from an amoeba, the cell soon stops growing and dividing, showing how vital the nucleus is. The nucleus is not a static storehouse: its instructions are constantly copied out to the cytoplasm to direct protein synthesis.
- Step 3: Mitochondrion — the cell's power stationThe mitochondrion is the site of aerobic respiration, oxidising glucose and other nutrients step by step to release energy that is captured as ATP for the cell's activities. Organelle numbers match cell function. For example, muscle cells contract constantly and demand huge amounts of energy, so they contain many mitochondria, while fat cells, which mainly store energy, have fewer. Structure matching function is a principle you will use in every comparison question.
- Step 4: Ribosomes and endoplasmic reticulumRibosomes are tiny particles that join amino acids in the order dictated by the genetic code, building proteins; some float free in the cytoplasm, others stud the rough endoplasmic reticulum. The endoplasmic reticulum is a membrane network threading through the cytoplasm that transports proteins and other materials; with ribosomes on its surface it is called rough ER, without them, smooth ER. For example, the digestive gland cells of the pancreas secrete large amounts of enzymes (proteins), so their ribosomes and endoplasmic reticulum are especially well developed.
- Step 5: The plant cell's three extrasPlant cells have three structures animal cells lack. Outside the membrane sits a cellulose cell wall, tough yet slightly elastic, giving support and protection. Chloroplasts contain chlorophyll and carry out photosynthesis. A large central vacuole stores cell sap — water, sugars and salts — and when full presses the cytoplasm against the wall, keeping the cell firm. For example, vegetables go limp when their vacuoles lose water, and firm up again after a soak in fresh water.
- Step 6: Animal cells versus plant cellsBoth cell types share the membrane, cytoplasm, nucleus, mitochondria, ribosomes and endoplasmic reticulum. Only plant cells have a cell wall, chloroplasts and a large central vacuole; animal cells have none of these, and at most small vacuoles. In comparison questions, answer item by item and watch for exceptions. For example, onion epidermal cells are plant cells that grow underground and never see light, so they contain no chloroplasts — another case of structure matching function.
- Step 7: Prokaryotic and eukaryotic cellsProkaryotic cells, represented by bacteria, are simple: they have a cell wall, membrane, cytoplasm and ribosomes, while their genetic material is a circular strand of DNA lying free in the cytoplasm with no nuclear membrane, and they lack mitochondria, endoplasmic reticulum and other membrane-bound organelles. Eukaryotic cells keep their genetic material inside a true nucleus and divide the work among membrane-bound organelles. For example, under the electron microscope E. coli shows no nuclear membrane — direct evidence of a prokaryotic cell.
4Diagrams Diagrams
5Worked example Example
A micrograph shows three cells: a human cheek epithelial cell, an onion epidermal cell and a bacterial cell. List the structures present in each and state the key features used to tell them apart.
- The cheek cell is an animal cell: it has a membrane, cytoplasm, nucleus, mitochondria, ribosomes and endoplasmic reticulum, but no cell wall, chloroplasts or large vacuole.
- The onion epidermal cell is a plant cell: it also has a cell wall and a large central vacuole; chloroplasts are usually found in illuminated leaf cells, and onion epidermis, growing underground, has none.
- The bacterial cell is prokaryotic: it has a cell wall, membrane, cytoplasm and ribosomes, but its circular DNA lies free in the cytoplasm without a nuclear membrane, and it has no mitochondria or other membrane-bound organelles.
- Key distinctions: a cell wall and large vacuole indicate a plant cell; no nuclear membrane indicates a prokaryote; neither feature indicates an animal cell.
- Answer: Animal and plant cells share the membrane, cytoplasm, nucleus and mitochondria, while plant cells alone have a cell wall and a large vacuole (chloroplasts appear in illuminated cells); bacteria have no membrane-bound nucleus or membrane-bound organelles and are prokaryotic. In comparison questions, list the structures item by item instead of generalising.
6Common mistakes Mistakes
7Quick check Quick check
Q1According to the fluid mosaic model, what forms the skeleton of the cell membrane?
Q2What are the functions of the mitochondrion and the ribosome?
Q3Name three structures a plant cell has but an animal cell does not.
Q4What is the main difference between prokaryotic and eukaryotic cells?
Q5Why do muscle cells contain far more mitochondria than fat cells?
8Recap Recap
- The cell membrane is a fluid phospholipid bilayer with embedded proteins, controlling the movement of substances.
- Chromosomes in the nucleus carry genetic information; mitochondria supply energy; ribosomes build proteins; the endoplasmic reticulum transports materials.
- Plant cells alone have a cell wall, chloroplasts and a large central vacuole; animal cells have none of these.
- Prokaryotic cells (such as bacteria) lack a nuclear membrane and membrane-bound organelles; only eukaryotic cells have a true nucleus.
- Organelle numbers match function: energy-hungry cells have many mitochondria; protein-secreting cells have many ribosomes.