Lesson 10: Cell and Cell Organelles

In the figure : In the figure a plant cell has a strong cell wall of cellulose and pectin outside its plasma membrane. An animal cell has no cell wall.

Rule 1: Cell wall — It is found in algae, fungi and plants, not in animals. It is elastic and strong, mainly made of carbohydrates like cellulose and pectin. Lignin, suberin and cutin are added later as needed. It supports and protects the cell and prevents too much water from entering.

• Easy : A wooden log is hard because its cell walls are strong (lignin).
• Medium : Animal cells lack a cell wall, so animals need a skeleton or other body support.
• Exam-type : A plant cell does not burst even in plain water because the strong cell wall resists the extra water.

In the figure : In the figure the plasma membrane has protein molecules in two layers of phospholipids and is selectively permeable: oxygen enters and carbon dioxide leaves.

Rule 2: Plasma membrane — It is thin, fragile and elastic and has protein molecules embedded in two layers of phospholipids. It is selectively permeable: useful molecules of water, salt and oxygen enter, and CO₂ leaves. Even if outside conditions change, the cell's inner environment stays stable: homeostasis.

• Easy : Oxygen enters the cell; CO₂ leaves it.
• Medium : Like a security guard at a gate, the membrane lets in only what the cell needs.
• Exam-type : Our body temperature stays about 37 °C in summer or winter: a body-level example of homeostasis.

Spot the difference

In the figure : In the figure diffusion and osmosis need no cell energy, while endocytosis (taking in) and exocytosis (sending out) use it.

Rule 1: Movement with and without energy — Using cell energy: endocytosis (engulfing food and other substances from outside) and exocytosis (sending waste out). Without cell energy: diffusion (small molecules like O₂ and CO₂) and osmosis (a physical process).

• Easy : Perfume spreading in a room is diffusion.
• Medium : O₂ moving into a cell is diffusion; no energy is used.
• Exam-type : An amoeba engulfing a food particle uses energy: endocytosis.

In the figure : In the figure water enters a cell in a hypotonic solution, leaves it in a hypertonic solution and does not move overall in an isotonic solution.

Rule 2: Three kinds of solutions — Isotonic: same water inside and outside, so no net movement. Hypotonic: the cell has less water than outside, so water enters (endosmosis); raisins in water swell and become turgid. Hypertonic: the cell has more water than outside, so water leaves (exosmosis); fruit pieces in thick sugar solution shrink.

• Easy : A raisin in plain water swells: hypotonic outside.
• Medium : Fruit pieces in thick sugar syrup shrink: hypertonic outside.
• Exam-type : Pickled mango releases water in salt: water leaves the cells (exosmosis).

In the figure : In the figure a plant cell turns turgid when water enters it. In a thick solution its cytoplasm shrinks away from the wall, which is plasmolysis.

Rule 3: Turgid and plasmolysed — In water a plant cell becomes turgid. In a thick solution the cytoplasm contracts away from the wall: plasmolysis (cytoplasm contraction).

• Easy : Fresh leaves are crisp because their cells are turgid.
• Medium : Wilted leafy vegetables revive when kept in water, because water enters the cells.
• Exam-type : Excess fertilizer in soil can plasmolyse root cells, which is why plants droop.

Spot the difference

In the figure : In the figure an animal cell has a plasma membrane and a nucleus. Organelles such as the ER, Golgi complex, mitochondria and lysosomes float in the cytoplasm.

Rule 1: Cytoplasm — Many organelles are suspended in the cytoplasm and chemical reactions occur in it. The part other than organelles is the cytosol, which stores amino acids, glucose and vitamins. It is granular and dense in animal cells, and thin and pushed to the edge by a large central vacuole in plant cells. Except the nucleus and chloroplast, all organelles are seen only with an electron microscope.

• Easy : Chemical reactions of the cell occur in the cytoplasm.
• Medium : In a mature plant cell the large central vacuole pushes the cytoplasm to the edges.
• Exam-type : Of the organelles, only the nucleus and chloroplast can be seen with a light microscope.

In the figure : In the figure the nucleus has a double membrane with pores, a nucleolus and chromatin fibres. The ER is joined to it and rough ER carries ribosome granules.

Rule 2: Nucleus and ER — The nucleus has a double membrane with small pores, one round nucleolus and a network of chromatin fibres that condense into chromosomes at cell division; functional segments of chromosomes are genes. It controls all metabolic activity and cell division and passes on hereditary characters. RBCs lose the nucleus to carry more haemoglobin. The endoplasmic reticulum (ER) is a net of tubes and sheets joined to the nucleus and plasma membrane; rough ER has ribosome granules. It supports the cell, conducts proteins and makes toxins water soluble so they are flushed out.

• Easy : The nucleus controls the cell.
• Medium : Genes are functional segments of chromosomes.
• Exam-type : Mature RBCs have no nucleus so that more haemoglobin can fit.

Spot the difference

In the figure : In the figure vesicles from the ER join the Golgi sacs at the formation face, and the finished packets leave from the maturation face.

Rule 1: Golgi complex — It has 5-8 flat hollow sacs (cisternae) with enzymes. Vesicles with proteins from the ER fuse at the formation face and leave from the maturation face, like a packing department. It is the secretory organ; it modifies, sorts and packs materials, produces vacuoles and secretory vesicles and helps form the cell wall, plasma membrane and lysosomes. Camillo Golgi first described it and won the 1906 Nobel prize with Santiago Cajal.

• Easy : The Golgi complex works like a packing department.
• Medium : Golgi's Nobel prize was in 1906, which is 2026 − 1906 = 120 years ago.
• Exam-type : If one Golgi complex has 8 sacs, 3 complexes have 3 × 8 = 24 sacs.

In the figure : In the figure a mitochondrion has a porous outer membrane and an inner membrane folded into cristae around the matrix, and it stores energy as ATP.

Rule 2: Lysosomes and mitochondria — Lysosomes are single-membrane sacs of digestive enzymes: the immune system (destroy viruses and bacteria), demolition squads (clear worn-out organelles), suicide bags (burst when the cell is old or damaged), and they digest stored proteins and fats in starvation. Mitochondria have a double membrane, outer porous and inner folded into cristae; the matrix has ribosomes, phosphate granules and DNA. They oxidise carbohydrates and fats and store energy as ATP; they are absent in RBCs.

• Easy : Mitochondria store energy as ATP.
• Medium : Lysosomes are called suicide bags because they burst when the cell is old or damaged.
• Exam-type : Muscle cells need much energy, so they have many mitochondria; RBCs have none.

In the figure : In the figure the Golgi complex packs materials, lysosomes digest and clear away waste, mitochondria make ATP energy and vacuoles store substances.

Rule 3: Vacuoles — Vacuoles are single-membrane storage sacs of no fixed shape. They maintain osmotic pressure and store by-products, waste and food (an amoeba stores food before digestion). In plant cells they are full of cell sap and give turgidity and rigidity.

• Easy : The plant vacuole is full of cell sap.
• Medium : An amoeba stores food in vacuoles before digestion.
• Exam-type : A large vacuole keeps a plant cell firm; when it loses water the plant wilts.

Spot the difference

In the figure : In the figure plastids carry pigments: chlorophyll is green, carotene orange, xanthophyll yellow, anthocyanin purple or blue and betalain dark pink. Leucoplasts are colourless.

Rule 1: Types of plastids — Leucoplasts are white or colourless and make and store starch, oils and proteins. Chromoplasts are coloured. Pigments: chlorophyll (green), carotene (orange, in carrot), xanthophyll (yellow), anthocyanin (purple, blue) and betalains (dark pink, in beet). Raw green tomatoes turn red as chlorophyll breaks down and lycopene is made.

• Easy : Carrot is orange due to carotene.
• Medium : Beetroot is dark pink due to betalains.
• Exam-type : A potato stores starch in leucoplasts, so it is colourless inside.

In the figure : In the figure of a chloroplast the chlorophyll traps sunlight for photosynthesis, and the stroma holds enzymes, DNA, ribosomes and carbohydrates.

Rule 2: Chloroplast — Chlorophyll traps solar energy and converts it into chemical energy for photosynthesis. The stroma has enzymes, DNA, ribosomes and carbohydrates. Mitochondria and plastids can make proteins and replicate because they have their own DNA and ribosomes.

• Easy : Chloroplasts do photosynthesis.
• Medium : Mitochondria and plastids have their own DNA.
• Exam-type : Leaves look green because chloroplasts are full of chlorophyll.

In the figure : In the figure eukaryotic cells (5 to 100 micrometre) have a true nucleus. Prokaryotic cells (1 to 10 micrometre) have only a nucleoid and no membrane-bound organelles.

Rule 3: Eukaryotic and prokaryotic cells — Eukaryotic cell: 5-100 µm, more than one chromosome, true nucleus with nuclear membrane, nucleolus and nucleoplasm, mitochondria and plastids. Prokaryotic cell (bacteria): 1-10 µm, one chromosome, nucleoid, no membrane-bound organelles. NCCS, Pune does research in cytology and cancer treatment.

• Easy : Bacteria have prokaryotic cells.
• Medium : A 50 µm eukaryotic cell is 50 ÷ 5 = 10 times larger than a 5 µm... but a typical bacterium is only 1-10 µm.
• Exam-type : 100 µm = 0.1 mm, the upper size limit of a eukaryotic cell.

Spot the difference